Synthesis, spectroscopic and catalytic properties of FeO x /Al 2 O 3 nanopowders prepared by cw CO 2 laser vaporization

Synthesis, spectroscopic and catalytic properties of FeO x /Al 2 O 3 nanopowders prepared by cw CO 2 laser vaporization | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Synthesis, spectroscopic and catalytic properties of FeO x /Al 2 O 3 nanopowders prepared by cw CO 2 laser vaporization Mark G. Baronskiy, Anton I. Kostyukov, Aleksey V. Zhuzhgov, Nadezhda A. Zaitseva, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-1846170/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 7 You are reading this latest preprint version Abstract Nanostructured powders xFe/nano-Al 2 O 3 with the Fe loading of x = 0,0–5,0 wt% were obtained using laser vaporization by a cw CO 2 laser. XRF, XRD, HRTEM, PL and UV-vis DRS techniques were employed to investigate physicochemical, structural and optical properties of the synthesized nanopowders with the average particle size of 9 nm. Nanopowders xFe/nano-Al 2 O 3 as model catalysts were tested in isobutane dehydrogenation reaction. The results obtained were compared with similar data for the xFe/γ Pb -Al 2 O 3 systems synthesized by the conventional sol-gel method. According to XRD and UV-vis DRS data, in the series of xFe/nano-Al 2 O 3 samples a great part of Fe 3+ ions is in the disordered environment of subsurface layers of Al 2 O 3 nanocrystallites predominantly in tetrahedral coordination. In distinction to samples of the xFe/γ Pb -Al 2 O 3 series, in the case of nanostructured xFe/nano-Al 2 O 3 powders the formation of Fe 2 O 3 phase does not occur at any concentrations of iron or conditions of testing. The analysis of the PL spectra of xFe/nano-Al 2 O 3 powders also showed the presence of surface sites of Fe 3+ ions, which were not detected for xFe/γ Pb -Al 2 O 3 . Catalytic testing of the xFe/nano-Al 2 O 3 series samples in isobutane dehydrogenation revealed the formation of the iron active sites that ensure catalytic activity of the samples. Differences in the catalytic properties of FeO x /Al 2 O 3 samples obtained by the sol-gel method and laser vaporization are related to different states of Fe 3+ ions. Thus, the xFe/nano-Al 2 O 3 nanopowders in contrast to xFe/γ Pb -Al 2 O 3 contain a large amount of active Fe 3+ sites. These sites involved in the dehydrogenation reaction are present predominantly on the surface of the nanopowders. Nanopowders CO2 laser vaporization metal oxides alumina xFe/nano-Al2O3 systems isobutane dehydrogenation iron active sites Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. Introduction At present, mixed Al 2 O 3 -Fe 2 O 3 systems and Fe/Al 2 O 3 composites are of considerable scientific and applied interest as promising materials with the improved functional properties. Such mixed oxide systems are of great interest due to the creation of new ceramic and composite materials for practical applications in various fields and industries. For example, Al 2 O 3 -Fe 2 O 3 materials are widely employed for environmental protection as sorbents and agents in water and wastewater treatment [ 1 – 3 ]. Magnetic ferrites such as Fe 2 O 3 , Fe 3 O 4 and nanosized Al 2 O 3 -Fe-coated cenospheres are used to obtain electromagnetic wave absorbents and electrical conducting fillers in composites [ 4 – 6 ]. It was shown that Fe/Al 2 O 3 composites can be used as promising candidates for the double negative materials [ 7 , 8 ]. It is known that heterogeneous iron-based catalysts are widely used in dehydrogenation reactions, particularly, in the dehydrogenation of propane to propylene [ 9 – 11 ]. In some works, nanostructured Fe and Fe-Zn/Al 2 O 3 systems served as the catalysts for isobutane dehydrogenation [ 12 , 13 ]. In [ 14 ] it was reported that for Fe/SiO 2 catalysts, single iron sites embedded in a silica matrix were responsible for methane conversion exclusively to ethylene and aromatics. The properties of mixed iron and alumina oxide systems depend to a great extent on the methods and regimes employed for the synthesis of materials. One of the key directions in the synthesis of nanomaterials is currently the “bottom-up” approach. It consists in the stepwise formation of nanomaterials according to the conventional sequence “atoms – clusters – nanoparticles” [ 15 , 16 ]. Bright examples of the indicated approach are laser synthesis methods based on the Physical Vapor Deposition (PVD) method. Laser methods make it possible to produce nanoparticles by vaporization of the initial materials (targets) under the action of laser energy with subsequent condensation of vapor in the so-called buffer gas or liquid [ 17 – 19 ]. Physicochemical properties of the produced nanoparticles can be controlled by variation of different parameters during laser vaporization, for example, laser radiation power, composition of the buffer gas in a vaporization chamber, and gas pressure [ 20 , 21 ]. Earlier we have studied xCr/Al 2 O 3 nanopowders with a low Cr loading of 0.0–4.8 wt%, which were obtained using vaporization by a cw CO 2 laser [ 22 ]. The important features of cw CO 2 laser vaporization for the synthesis of nanopowders under consideration are the absence of chemical contaminants, monodispersity of the synthesized samples, and controlled nonstoichiometry. In the tested catalysts, two radically different types of Cr 3+ sites were revealed in the solid solution based on γ-Al 2 O 3 . One type comprises bulk Cr 3 + b sites in a strong crystal field ( Dq / B = 3.4). Another type is represented by Cr 3 + s sites located near the surface of xCr/Al 2 O 3 nanoparticles ( Dq / B = 1.4–1.8). A comparison of catalytic properties of the nanostructured system xCr/nano-Al 2 O 3 with the typical catalyst supported on γ-Al 2 O 3 (sol-gel) showed that the nanostructured system has better catalytic characteristics, which are provided by the Cr 3 + s sites formed on the surface of nanoparticles. This study was aimed to identify different states of iron on the surface and in the bulk of the tested nanostructured FeO x /Al 2 O 3 powders and to understand how the indicated iron states would manifest themselves in various physicochemical processes. This aim was accomplished by a comprehensive investigation of physicochemical properties of the nanostructured FeO x /Al 2 O 3 system with a variable iron content, which was synthesized via laser vaporization using irradiation by a cw CO 2 laser. The obtained experimental results were compared with similar data for the Fe/Al 2 O 3 system synthesized by the sol-gel method. 2. Experimental 2.1. Sample preparation Two radically different series of FeO x /Al 2 O 3 powders were synthesized: 1. FeO x /Al 2 O 3 powders with the Fe content of 0.05, 0.5, 1, 2.5 and 5 wt%, which were deposited on a single-phase γ Pb -Al 2 O 3 support; hereinafter, they are denoted as xFe/γ Pb -Al 2 O 3 , where x is the iron content in the samples (wt%). The γ Pb -Al 2 O 3 support was prepared from pseudoboehmite by the sol-gel method. The γ Pb -Al 2 O 3 sample was obtained by thermal treatment of pseudoboehmite in air at 550 °C for 4 h. In its turn, pseudoboehmite (γAlOOH·nH 2 O) was produced by simultaneous pouring of equinormal aqueous solutions of Al(NO 3 ) 3 ·9H 2 O salt and ammonium hydroxide (NH 4 OH) at a constant pH close to neutral. To obtain pseudoboehmite sediment, the mixed solutions were held at 80 °C for 2 h. The resulting sediment was separated from the aqueous solution by decantation, washed with distilled water to remove impurities, and then dried in flowing air at 150 °C for 12 h to a constant weight. During the synthesis of xFe/γ Pb -Al 2 O 3 samples, incipient wetness impregnation of the γ Pb Al 2 O 3 powder with aqueous solutions of iron(III) nitrate (Fe(NO 3 ) 3 ∙9H 2 O) was used to obtain the total concentration of Fe 3+ ions equal to 0.05, 0.5, 1, 2.5 and 5 wt%. After completion of the impregnation step, all xFe/γ Pb -Al 2 O 3 samples were dried in air at room temperature for a day. After that, the samples were dried at 110 °C for 6 h and calcined at 550 °C for 4 h in air. Along with the indicated samples, xFe/α-Al 2 O 3 powders (x[Fe 3+ ] = 0.05, 0.5, 1, 2.5, and 5 wt%) were prepared. The xFe/α-Al 2 O 3 samples with different content of iron were obtained by thermal treatment of the corresponding xFe/γ Pb -Al 2 O 3 samples at 1250 °C for 4 h in air. 2. FeO x /Al 2 O 3 powders with the iron content of 0, 0.05, 0.5, 1, 2.5 and 5 wt%, which were prepared using the nanostructured Al 2 O 3 support with a complicated phase composition and particle sizes up to 20 nm. Samples of this series are denoted as xFe/nano-Al 2 O 3 , where x is the iron content in the samples (wt%). Nanostructured xFe/nano-Al 2 O 3 samples were synthesized by laser vaporization of ceramic targets from xFe/α-Al 2 O 3 . The vaporization was carried out using a continuous CO 2 laser with the output power 100 W and power density on the target surface 10 5 W/cm 2 . Helium served as the buffer gas (99.99%). In all cases, helium pressure in the vaporization chamber was P He = 0.26 atm. To obtain xFe/nano-Al 2 O 3 nanoparticles with the specified concentrations of iron, the initial α-Al 2 O 3 targets were doped with iron at the step of target preparation via the incipient wetness impregnation of alumina (similar to the procedure described above for the synthesis of xFe/γ Pb -Al 2 O 3 by the sol-gel method). 2.2. Sample characterization Elemental analysis of the samples was made using X-ray fluorescence spectroscopy (XRF) on an ARL – Advant’x analyzer with the Rh anode of the X-ray tube. Phase composition of the samples was revealed by X-ray diffraction (XRD) analysis. X-ray diffraction patterns were recorded on a Bruker D8 diffractometer using a CuK α source and scanning over the range of reflection angles 2θ = 15–75° with a 0.05° step and accumulation time 3 s. The mean sizes of crystallites () for the 0, 1, 5 wt% Fe xFe/nano-Al 2 O 3 and 5wt% Fe xFe/γ Pb -Al 2 O 3 samples were estimated from full width at half maximum (FWHM) of the 440 peak (2θ = 67–69) using the Scherrer formula. The morphology of the synthesized samples was characterized by high-resolution transmission electron microscopy (HRTEM) on a JEM-2010 electron microscope at accelerating voltage 200 kV and resolution 1.4 Å. Samples were deposited on a copper grid by dispersing a solid phase suspension in alcohol using an ultrasonic disperser. Local analysis of elemental composition of the samples was performed on an energy-dispersive EDS spectrometer QUANTAX 200-TEM (Bruker) with a XFLASH detector at energy resolution of ca. 130 eV. Photoluminescence (PL), photoluminescence excitation (PLE) spectra and PL decay curves were measured on a CaryEclipse (Varian) spectrofluorimeter under excitation with a Xe flash lamp. All measurements were carried out at room temperature. Diffuse reflectance spectroscopy in visible and UV regions (UV-Vis DRS) of the studied samples was performed on a UV2501 PC (Shimadzu) spectrophotometer with an ISR240 A diffuse reflectance attachment. All the samples were placed in a quartz cuvette with the optical path length of 2 mm. UV-Vis DRS spectra were recorded relative to the BaSO 4 reflectance standard in a range of 190–900 nm (11000–54000 cm − 1 ). The final UVVis DRS data are represented in the coordinates: Kubelka-Munk function, F (R) versus wavenumber. Specific surface area (S sp ) of the samples was calculated according to BET using low-temperature nitrogen adsorption at 77 K on a DigiSorb-2600 (Micromeritics) automated volumetric instrument. All the samples were subjected to thermal pretreatment in vacuum at 200 °C and residual pressure 10 − 4 mm Hg for 5 h. 2.3 Isobutane dehydrogenation reaction Activity of the catalysts of both series was investigated in isobutane dehydrogenation in a fixed catalyst bed using a flow microreactor. The catalyst sample with a 0.2 or 0.35 g weight was loaded in a tubular quartz reactor with the internal diameter 14 mm and height 120 mm. The temperature in the middle of the catalyst bed was measured by a thermocouple placed in a thermocouple pocket (the external diameter of 6 mm) that was located coaxially to the reactor. Activity of the freshly synthesized catalysts was determined at atmospheric pressure and temperatures 550–620 °С. Prior to testing the catalytic properties, the fresh samples were heated in nitrogen flowing at a rate of 16.7 ml/min to the reaction temperature and held in nitrogen for 1 h to provide the desorption of water from the catalyst surface. Before testing, a part of the catalyst with the composition 5%Fe/nano-Al 2 O 3 was reduced in a hydrogen flow at 580 °C for 30 minutes. Isobutane was fed to the reactor (a flow rate of 16.7 ml/min, WHSV = 12.9 or 7.4 1/h) as a feedstock for 10 minutes. When the effect of dehydrogenation reaction time on catalytic properties of the samples was studied, the time of continuous feeding of isobutane was 150 minutes. To remove coke deposits after dehydrogenation, the catalysts were regenerated in a mixture of air and nitrogen with the volume ratio 1:1 (the total flow rate 33.3 ml/min) at the reaction temperature. Between dehydrogenation and regeneration, the reactor was purged with nitrogen. Samples of dehydrogenation products were taken for the analysis 10 minutes after the onset of isobutane feeding to the reactor; purging of the reactor with nitrogen between dehydrogenation and regeneration was performed for 10 minutes. Regeneration of the catalysts by burning off coke deposits was carried out until СО х completely disappeared from the regeneration products. In the experiments elucidating the effect of dehydrogenation time on catalytic properties of the samples, sampling from the reactor was made 5, 10, 15, 25, 35, 45, 60, 75, 105, 120 and 150 minutes after the onset of isobutane feeding to the reactor. The catalysts synthesized by the sol-gel method and laser vaporization substantially differ in their bulk density (0,75 and 0,08 g/cm 3 , respectively). So, to compare the catalytic properties of such samples, dependences of selectivity to isobutene on isobutane conversion were obtained under variation of weight hourly space velocities. Conditions of the experiments were as follows: the catalyst weight 0.2 g, the temperature 580 °C, and the volume flow rate of isobutane 1–5 L/h. Catalytic properties of the tested catalysts were compared using selectivity to isobutene at similar isobutane conversions. The initial gases and reaction products were analyzed on a CHROMOS GC-1000 chromatograph using a flame-ionization detector (FID) and a capillary column with SiO 2 having a length of 25 m. Samples for the analysis were taken on-line by autosamplers directly from the gas flow at the reactor outlet. The obtained chromatograms were processed on a computer using internal normalization. GC analysis allows determining the content of methane, ethane, ethylene, propane, propene, n-butane, isobutane, isobutene, linear butenes. The presence of other carbon-containing products in the gas stream was not observed. The analysis of H 2 , CO, and CO 2 in reaction and regeneration products in the continuous mode was performed on a TEST-1.2 gas analyzer manufactured by BonAir Ltd. (Russia) using optical absorption infrared and electrochemical sensors. The analysis of the output concentrations of CO and CO 2 by these sensors allowed calculating the amount of coke deposits. Conversion of isobutane, selectivity to isobutene and isobutene yields were calculated on a molar basis taking into account coke deposits. The catalytic experiments were carried out in the kinetic region of the process. To exclude the effect of external diffusion limitations, the experiments were performed at different linear feed velocities of isobutane and a constant contact time. On two different catalyst samples (0,2 and 0,35 g) at 580 °C and WHSV = 12,9 L/h, isobutane conversions were equal to 13,9 and 13,4, respectively. The data obtained demonstrated that external diffusion limitations were negligible. The grain size and the absence of pores in the studied catalysts exclude the occurrence of intradiffusion inhibitions. 3. Results And Discussion Iron content in the synthesized catalysts of series 1 and 2 was controlled by X-ray fluorescence analysis (see Table 1 ). The phase composition and morphology of the samples under consideration were controlled by XRD and HRTEM. According to XRD data (Fig. 1 ), the diffraction patterns of all the tested samples of both series correspond predominantly to the γ-Al 2 O 3 phase. In the diffraction patterns of xFe/nano-Al 2 O 3 samples (series 2) with different iron content, one can see that starting from the undoped nano-Al 2 O 3 sample, lattice parameters increase from a = 7.925 Å for nano-Al 2 O 3 to a = 7.931 Å for the 5%Fe/nano-Al 2 O 3 sample. With increasing iron content, the 311 peak becomes asymmetric in comparison with XRD data for nano-Al 2 O 3 sample. This indicates that an increase in the iron content in the tested samples is accompanied by incorporation of a part of Fe 3+ ions into the γAl 2 O 3 lattice, which isomorphously substitute Al 3+ ions to form the Fe 2 O 3 -γ-Al 2 O 3 solid solution. A large part of Fe 3+ ions remains in subsurface layers of Al 2 O 3 nanocrystallites (the disordered environment). In the 5%Fe/nano-Al 2 O 3 sample, an additional peak is observed in the region 2θ = 55–60° and the asymmetry of peak 311 attributed to γ-Al 2 O 3 . This asymmetry and the presence of this peak are in good agreement with the presence of the FeAl 2 O 4 phase in the sample. The phase of magnetite Fe 3 O 4 is not observed. At the same time, the diffraction pattern of the 5%Fe/γ Pb -Al 2 O 3 sample synthesized by the sol-gel method (series 1) contains narrow sharp peaks corresponding to the formation of the α-Fe 2 O 3 phase. Thus, in the 5%Fe/γ Pb -Al 2 O 3 sample, a large part of Fe 3+ ions occupies regular octahedral positions in the volume of α-Fe 2 O 3 lattice. For nano-Al 2 O 3 sample as well as for 1%Fe/nano-Al 2 O 3 , 5%Fe/nano-Al 2 O 3 and 5%Fe/γ Pb -Al 2 O 3 powders, the average particle size () was calculated by the Selyakov-Scherrer formula from XRD data using the 440 peak of the γAl 2 O 3 phase. In nanostructured samples of series 2, = 5.6, 5.5, 6.5 nm for nano-Al 2 O 3 , 1%Fe/nano-Al 2 O 3 and 5%Fe/nano-Al 2 O 3 , respectively, while for the series 1 sample 5%Fe/γ Pb -Al 2 O 3 the value is 6.2 nm. Table 1. Data on the iron and aluminum content in xFe/γ Pb -Al 2 O 3 and xFe/nano-Al 2 O 3 samples El/Ox xFe/γ Pb -Al 2 O 3 0.05 wt% Fe 0.5 wt% Fe 1.0 wt% Fe 2.5 wt% Fe 5.0 wt% Fe Al 52.77±0.01 52.40±0.03 52.01±0.03 50.79±0.05 49.05±0.07 Fe 0.09±0.01 0.58±0.03 1.07±0.04 2.68±0.07 4.99±0.09 xFe/nano-Al 2 O 3 Al 52.69±0.02 52.43±0.03 52.08±0.03 50.74±0.05 48.63±0.07 Fe 0.10±0.01 0.48±0.02 0.97±0.04 2.55±0.07 5.35±0.09 According to HRTEM data for the γ Pb -Al 2 O 3 powder prepared by the sol-gel method, it consists of mutually disordered nanocrystallites with the diameter of 10–15 nm, which form micron-sized polycrystalline aggregates (Fig. 2 a). The HRTEM images of 1%Fe/nano-Al 2 O 3 and 5%Fe/nano-Al 2 O 3 samples contain mostly the faceted 3D nanocrystals with the size from 3 to 10 nm (Fig. 2 b, с ). For 1%Fe/nano-Al 2 O 3 and 5%Fe/nano-Al 2 O 3 samples, the average particle size estimated from HRTEM images is ca. 9 nm, which is consistent with the XRD data. A minor amount of particles with the size ≥ 20 nm is also present. To reveal and identify different charge states of iron ions and their coordination in series 1 and 2 samples, optical methods were used, namely, diffuse reflectance spectroscopy (UV-vis DRS) and photoluminescence (PL) spectroscopy. PL spectra, PL excitation (PLE) spectra, PL decay curves, and UV-vis DRS spectra were obtained and analyzed, particularly, for the samples before and after catalytic dehydrogenation of isobutane. According to UV-vis DRS data, xFe/γ Pb -Al 2 O 3 samples (series 1) with the iron content from 0,05 to 5,0 wt% have poorly resolved broad absorption bands in the region above 17000 cm − 1 , which correspond to the ligand-metal charge transfer bands (CTB) of Fe 3+ ions (Fig. 3 ). Therewith, it is seen from the UV-vis DRS spectra presented in logarithmic coordinates that for the 0.05%Fe/γ Pb -Al 2 O 3 sample a symmetric low-intensity absorption band at 18700 cm − 1 can be distinguished, which is caused most likely by the d-d transition of Fe 3+ ions in the tetrahedral oxygen coordination [ 23 ]. In UV range of the UV-vis DRS spectrum, an intense absorption is observed in the region above 30000 cm − 1 , which can be attributed to the ligand-metal CTB of Fe 3+ ions in the tetrahedral and octahedral oxygen coordination. A further increase in the content of Fe 3+ ions in 2,5%Fe/γ Pb -Al 2 O 3 and 5%Fe/γ Pb -Al 2 O 3 samples leads to a higher intensity of the absorption band at 18700 cm − 1 and the ligand-metal CTB; in addition, a broad absorption band at 35300 cm − 1 appears for the 5%Fe/γ Pb -Al 2 O 3 sample. In the spectra of samples 2,5%Fe/γ Pb -Al 2 O 3 and 5%Fe/γ Pb -Al 2 O 3 , a shoulder shows up in the region of 15000 cm − 1 . For the sample with the highest iron concentration, 5%Fe/γ Pb -Al 2 O 3 , three shoulders are observed at 15000, 18700 and 21900 cm − 1 , and three broad bands at 26400, 35300 and 47400 cm − 1 . According to XRD, this sample contains the α-Fe 2 O 3 phase, in which Fe 3+ ions are stabilized in the octahedral oxygen coordination (Fe 3 + Oh ). It can be reliably concluded that the αFe 2 O 3 phase is not observed in 1%Fe/γ Pb -Al 2 O 3 and in the samples with lower iron concentration; hence it follows that Fe 3+ ions in the γ Pb Al 2 O 3 matrix are stabilized only in the tetrahedral oxygen coordination (Fe 3 + Td ). For 1%Fe/α-Al 2 O 3 and 5%Fe/α-Al 2 O 3 samples (Fig. 3 , curves 6 and 7, respectively), well resolved absorption bands are observed at 15000, 18700, 21900, 26400, 29900, 38300 and 47400 cm − 1 . Therewith, the first five absorption bands are assigned to d-d transitions of Fe 3+ cations in the octahedral oxygen coordination (Fe 3 + Oh ), while the last two more intense bands are caused by the ligand-metal CTB of Fe 3 + Oh cations. The presence of intense ligand-metal CTB in UV region of the UV-vis DRS spectra and low-intensity absorption bands in visible region, which belong to d-d transitions of Fe 3 + Oh cations, indicates that these cations are stabilized exclusively in the volume of the oxygen-containing α-Al 2 O 3 matrix. As follows from the analysis of UV-vis DRS spectra of xFe/nano-Al 2 O 3 powders with the iron content from 0,05 to 5,0 wt%, a broad intense absorption band is distinguished for all the samples of nano-series in the region of 25000–42500 cm − 1 with a maximum at 33300 cm − 1 ( Fig. 4 ). In visible region of the UV-vis DRS spectrum, a low-intensity absorption band with the maximum at 20700 cm − 1 can also be distinguished. According to the literature data, these absorption bands may be attributed to the d-d transition and ligand-metal CTB of Fe 3+ ions in the tetrahedral oxygen coordination (Fe 3 + Td ), which are stabilized in the volume of γ-Al 2 O 3 matrix. A comparison of UV-vis DRS spectra for the series of xFe/nano-Al 2 O 3 samples shows that an increase in the iron content in the samples is accompanied by a growth of intensity of the indicated absorption bands and the total background, which testifies to an increase in the content of highly dispersed phase in these samples. UV-vis DRS spectra were obtained and analyzed for 1%Fe/γ Pb -Al 2 O 3 and 5%Fe/γ Pb -Al 2 O 3 samples as well as for the xFe/nano-Al 2 O 3 series with the iron content 1 and 5 wt% after catalytic dehydrogenation of isobutane without regeneration or with subsequent regeneration. A comparison was made with the results obtained for the same samples before catalytic tests (see Fig. 5 ). Two main conclusions can be made from the analysis and comparison of the acquired data. First, the presence of the absorption band at 33500 cm − 1 in UV-vis DRS spectra (after the catalysis; without regeneration or with subsequent regeneration) of the 1%Fe/nano-Al 2 O 3 sample, and the absorption band at 35500 cm − 1 in UV-vis DRS spectra (after the catalysis; without regeneration) of the 1%Fe/γ Pb Al 2 O 3 sample indicates a difference in the strength of the crystal field generated by oxygen ligands around Fe 3+ ions in the tetrahedron. Therewith, the crystal field strength in the 1%Fe/γ Pb Al 2 O 3 sample is much greater than in 1%Fe/nano-Al 2 O 3 , which shows that in the latter case the local distortions in the tetrahedral environment of Fe 3+ ion are much more pronounced. Second, at the highest concentration of iron in the samples, UV-vis DRS spectra of 5%Fe/γ Pb Al 2 O 3 contain four absorption bands caused by stabilization of Fe 3+ ions in the Fe 2 O 3 phase on the initial γ Pb Al 2 O 3 support surface, a part of which is retained after testing in the catalytic reaction. Whereas for the 5%Fe/nano-Al 2 O 3 sample, the Fe 2 O 3 oxide is not formed at any conditions of testing. Along with UV-vis DRS spectra, for all the tested model catalysts we have obtained and analyzed PL and PLE spectra as well as PL decay curves for impurity iron ions. The luminescence measurements were made for nanostructured xFe/nano-Al 2 O 3 powders with the iron content of 0,05, 1,0 and 5,0 wt% (series 2) and for series 1 samples – γ Pb Al 2 O 3 and 0,05%Fe/γ Pb Al 2 O 3 , 1%Fe/γ Pb Al 2 O 3 and 5%Fe/γ Pb Al 2 O 3 . In addition, to identify surface sites of Fe ions in the studied xFe/nano-Al 2 O 3 samples, a comparative analysis of the luminescence data for 1%Fe/nano-Al 2 O 3 and 5%Fe/nano-Al 2 O 3 samples before and after the catalytic reaction (without regeneration) was performed. Let us consider PL, PLE spectra and PL decay curves for series 1 samples (the sol-gel method). PL spectra in this case are characterized by the presence of a broad PL band in the region of 12000–15000 cm − 1 with a maximum at λ em = 13300 − 13000 cm − 1 at excitation with λ ex = 310 nm (Fig. 6 ). The observed PL, according to the literature data, is identified as the luminescence of Fe 3+ ions that isomorphously substitute Al 3+ ions and occupy tetrahedral positions in the Al 2 O 3 structure [ 24 , 25 ]. In alumina, PL of Fe 3 + Td ions corresponds to the radiative electronic transition 4 T 1 ( 4 G) → 6 A 1 ( 6 S). In the PLE spectrum (Fig. 6 b) recorded for the PL band with λ em = 13300 cm − 1 of γ Pb Al 2 O 3 and xFe/γ Pb -Al 2 O 3 samples with the Fe content 0,05, 1,0, and 5,0 wt%, a broad band is observed in UV region with λ max = 30800–32300 cm − 1 . The excitation of PL of Fe 3 + Td ions occurs when light is absorbed in the UV region, where the ligand-metal CTB O 2− → Fe 3+ of Fe 3 + Td ions is observed, and also as a result of the intrasite transition 6 A 1 ( 6 S) → 4 E( 4 G). The PL decay curves at λ em = 13300 cm − 1 (Fig. 6 с ) for the 0.05, 1.0 and 5.0%Fe/γ Pb Al 2 O 3 samples fall in the millisecond range, which is consistent with the available literature data [26]. One can see from PL spectra that an increase in the iron content in series 1 samples, starting from the γ Pb Al 2 O 3 support to 0.05%Fe/γ Pb Al 2 O 3 , leads to a sharp growth in intensity of the broad PL band with λ em = 13300 − 13000 cm − 1 (Fe 3 + Td ). A further increase in the iron content is accompanied by a drop in the signal intensity due to the onset of concentration quenching. In PLE spectra (λ em = 755 nm) of 0.05%Fe/γ Pb Al 2 O 3 , 1.0%Fe/γ Pb Al 2 O 3 and 5.0%Fe/γ Pb Al 2 O 3 samples, a sole broad band is also observed in UV region with λ max = 30800–32300 cm − 1 . For series 2 powders xFe/nano-Al 2 O 3 obtained by laser vaporization, similar curves are observed in PL (λ ex = 310 nm (32300 cm − 1 )) and PLE (λ em = 755 nm (13300 cm − 1 )) spectra (Fig. 7 ). A broad PL band in the range of 700–850 nm with the maximum at λ em = 13300 − 13000 cm − 1 , which is observed for all the tested nanostructured powders with respective PLE spectra, is reliably identified as the spectral manifestation of Fe 3 + Td ions in the Al 2 O 3 lattice. The PL decay curves also fall in the millisecond range (Fig. 7 and Table 2 ). Table 2. Lifetimes (τ PL , ms) of the excited 4 T 1 ( 4 G) state of Fe 3+ ions for xFe/nano-Al 2 O 3 samples before and after dehydrogenation reaction Sample τ PL , ms Before dehydrogenation After dehydrogenation τ PL1 τ PL2 τ PL1 τ PL2 0.05%Fe/nano-Al 2 O 3 5.42±0.15 0.57±0.02 – – 1%Fe/nano-Al 2 O 3 4.02±0.15 0.47±0.01 3.20±0.12 0.49±0.02 5%Fe/nano-Al 2 O 3 1.58±0.05 0.60±0.02 2.54±0.08 0.40±0.02 The analysis of FWHM values of the luminescence band at λ em = 13300 − 13000 cm − 1 for xFe/nano-Al 2 O 3 samples with different iron content before catalytic dehydrogenation of isobutane shows that FWHM of the PL band increases with the iron concentration from 1612 cm − 1 for 0.05%Fe/nano-Al 2 O 3 to 1703 cm − 1 for 5%Fe/nano-Al 2 O 3 sample. An increase in FWHM of the PL band of Fe 3+ :Al 2 O 3 may indicate that a large part of Fe 3+ ions occupies nonequivalent positions (the disordered environment) in subsurface layers of Al 2 O 3 nanocrystallites. The indicated trend is not observed for xFe/γ Pb -Al 2 O 3 samples; on the contrary, FWHM values of the PL band of Fe 3+ ions in these samples decrease with the growth of iron content from the value 1583 cm − 1 for undoped γ Pb -Al 2 O 3 support to 1550 cm − 1 for 5%Fe/γ Pb -Al 2 O 3 sample. This additionally testifies that a considerable fraction of Fe 3+ ions in xFe/γ Pb -Al 2 O 3 samples may occupy regular positions in the lattice volume of γ Pb -Al 2 O 3 support. In addition, a comparison of FWHM values of the PL band of Fe 3+ ions for respective iron concentrations in series 1 and 2 samples before catalytic experiments shows that for xFe/nano-Al 2 O 3 samples these values in all cases are higher as compared to the powders based on γ Pb Al 2 O 3 ; this again testifies to a higher concentration of disordered active Fe 3+ sites in nanostructured samples obtained by laser vaporization (series 2) in comparison with the series 1 samples. Another argument in favor of the statement that changes in FWHM of the PL band, for which Fe 3+ ions in the tested samples are responsible, are related to variation in the content of surface Fe 3+ sites is a decrease in FWHM of the PL band for 1%Fe/nano-Al 2 O 3 and 5%Fe/nano-Al 2 O 3 samples after the catalytic reaction in comparison with these values before the reaction. So for 1%Fe/nano-Al 2 O 3 and 5%Fe/nano-Al 2 O 3 after the catalytic reaction the corresponding FWHM values of the PL band of Fe 3+ ions are 1569 and 1546 cm − 1 , respectively. In the course of reaction, a part of Fe 3+ ions under the action of temperature moves into the bulk and occupies regular positions in the Al 2 O 3 matrix. The estimates based on the PL decay curves demonstrate a decrease in the average lifetime ((τ PL ) av ) of the 4 T 1 ( 4 G) excited state of Fe 3+ ions in 1%Fe/nano-Al 2 O 3 and 5%Fe/nano-Al 2 O 3 samples tested in the dehydrogenation reaction. The (τ PL ) av values were obtained by the formula reported in [ 27 ]: The obtained values of (τ PL ) av were 2.90 ± 0.10 ms and 2.38 ± 0.10 ms for 1%Fe/nano-Al 2 O 3 and 0.71 ± 0.03 ms and 0.68 ± 0.03 ms for 5%Fe/nano-Al 2 O 3 , respectively, before and after the dehydrogenation reaction. Such a behavior of (τ PL ) av may be associated with a decrease in the contribution of nonradiative component, which in its turn is caused by a partial dehydroxylation of the nano-Al 2 O 3 support surface under the action of temperature during dehydrogenation, and OH groups are known to be the efficient luminescence quenchers. Another possible reason of such a behavior of (τ PL ) av may lie in the partial increase in the crystallinity of the samples as a result of the temperature effect on them during the catalytic reaction. Catalytic Characteristics Table 3 lists the results of catalytic experiments in isobutane dehydrogenation at 550 °C, which corresponds to the synthesis temperature of series 1 and 2 samples with different iron content. For the tested nano-Al 2 O 3 as well as 1%Fe/γ Pb -Al 2 O 3 and 5%Fe/γ Pb -Al 2 O 3 catalysts, specific surface area values (S sp , m 2 /g) were measured. They were 210, 194, and 171 m 2 /g for nano-Al 2 O 3 , 1%Fe/γ Pb -Al 2 O 3 , and 5%Fe/γ Pb -Al 2 O 3 , respectively. As shown by the catalytic studies in isobutane dehydrogenation, an increase in the iron content in the xFe/nano-Al 2 O 3 system is accompanied by an increase in isobutane conversion and selectivity to isobutene. This indicates that in the xFe/nano-Al 2 O 3 samples obtained by laser vaporization, necessary iron sites are formed to provide catalytic activity of the samples. The dependence of isobutylene yield on the iron concentration in the samples is virtually linear. Catalyst composition Yield of isobutylene (Y), wt% Conversion (X), % Selectivity (S), wt% Table 3 – Catalytic performance in isobutane dehydrogenation in a fixed bed of series 1 and 2 model catalysts. Conditions of testing: temperature 550 °C, atmospheric pressure, WHSV = 12.9 h − 1 . 1%Fe/nano-Al 2 O 3 2.5 4.3 58.1 2.5%Fe/nano-Al 2 O 3 3.3 5.4 61.1 5%Fe/nano-Al 2 O 3 4.2 6.3 66.7 1%Fe/γ Pb -Al 2 O 3 2.2 3.9 56.4 2.5%Fe/γ Pb -Al 2 O 3 3.0 4.4 68.0 5%Fe/γ Pb -Al 2 O 3 3.5 4.8 72.9 A comparison of the catalytic properties of series 1 and 2 samples in isobutane dehydrogenation was further made using catalysts with the iron content 5 wt%, which are the most active ones in the concentration series. Along with the catalytic activity and selectivity, we have studied the catalyst stability in a long-term (2,5 h) dehydrogenation without regeneration and its stability in the long-term testing with alternating dehydrogenation-regeneration (reduction-oxidation) cycles. For nanostructured system with the iron content 5 wt%, the effect of the thermal pretreatment medium (with reduction in hydrogen or without it) was investigated. For both series of catalysts, the dependence of catalytic activity and selectivity on the reaction temperature was explored. Due to different bulk density, it is impossible to compare directly the catalytic properties of the sol-gel synthesized catalysts and samples of the nanostructured series prepared by laser vaporization. So, their selectivities to isobutene were compared at a similar isobutane conversion using the dependences of selectivity on conversion that were obtained upon variation of the weight hourly space velocity (see Fig. 8 ). According to the data of Fig. 8 , at close isobutane conversions the selectivity to isobutene for the 5%Fe/nano-Al 2 O 3 catalyst is 3–5 mol.% higher as compared to the 5%Fe/γ Pb -Al 2 O 3 sample. We tried to elucidate the cause of differences in the catalytic behavior of series 1 and 2 catalysts by comparing the data obtained for the catalysts by physicochemical methods and their catalytic properties. The analysis of XRD data showed that for the series of xFe/nano-Al 2 O 3 samples, in distinction to xFe/γ Pb -Al 2 O 3 samples (the sol-gel method), a large part of Fe 3+ ions is in the disordered environment (evidently, in subsurface layers of Al 2 O 3 nanocrystallites). An increase in the iron concentration in xFe/γ Pb -Al 2 O 3 samples to 5 wt% is accompanied by the formation of the α-Fe 2 O 3 phase on the γ Pb -Al 2 O 3 surface, which is not observed for the samples of xFe/nano-Al 2 O 3 series. According to UV-vis DRS data for the series of xFe/γ Pb -Al 2 O 3 samples, an increase in the iron content produces typical absorption bands caused by stabilization of Fe 3+ ions in the Fe 2 O 3 phase. The observed pattern is preserved also in the samples of this series after catalytic testing, whereas for samples of the xFe/nano-Al 2 O 3 series, Fe 2 O 3 oxide does not form under any conditions. Along with isobutane dehydrogenation, side reactions of cracking and hydrogenolysis proceed, leading to the formation of С 1 -С 3 hydrocarbons, which are the precursors of coke deposits in the catalyst. Upon coking of the catalysts, active dehydrogenation sites may be blocked by coke deposits. To remove coke from the catalyst surface, the catalyst is regenerated by coke burn-off. Thus, the dehydrogenation process requires quite severe conditions of the catalyst operation – the alternation of dehydrogenation-regeneration (reduction-oxidation) cycles at high temperatures. So it is always interesting to estimate stability of the catalysts at a long-term alternation of the cycles. The stability study demonstrated that the sample of series 1 was stable for at least 20 cycles, while the sample of series 2 – for at least 31 cycles (Fig. 9 ). One of the most important characteristics of dehydrogenation catalysts is the duration of operation without regeneration, i.e. the resistance to poisoning by the coke-like deposits. We have studied changes in the catalytic properties of materials under consideration in dependence on dehydrogenation time without regeneration of the catalysts aimed to remove carbon deposits. The catalytic properties were tested for 2.5 h with a periodic analysis of the composition of reaction mixture taken from the reactor. After 2.5 h, the samples were regenerated until complete coke burn-off; the amount of evolved carbon combustion products СО х was used to estimate the yield of coke in the reaction products and the content of carbon in the catalyst. Figure 10 displays the dependence of catalytic properties of 5%Fe/γ Pb -Al 2 O 3 and 5%Fe/nano-Al 2 O 3 samples on the duration of dehydrogenation. According to the data obtained for the 5%Fe/γ Pb -Al 2 O 3 sample, the maximum values of isobutane conversion (13%) and selectivity to isobutene (70%) under the chosen conditions were reached 30–35 minutes after the onset of feeding, remained virtually constant up to 50 minutes, and then gradually decreased up to 150 minutes by 2% and 3 mol.%, respectively. The yield of coke after 150 minutes was 4,7 mol.%, and the carbon content in the catalyst – 4,9 wt%. Figure 10 b shows the dependence of catalytic properties of the 5%Fe/nano-Al 2 O 3 sample on the duration of dehydrogenation. The maximum values of isobutane conversion (16–17%) and selectivity to isobutene (72%) were observed 25–30 minutes after the onset of feeding and remained virtually constant up to the end of experiment (150 minutes). The yield of coke after 150 minutes was 7.6 mol.%, and the carbon content in the catalyst – 7.5 wt%. Data on the temperature effect on catalytic properties of the catalysts of both series with the iron content 5 wt% are listed in Table 4 . The temperature dependence is typical of both samples. Т, °C 5%Fe/γ Pb -Al 2 O 3 5%Fe/nano-Al 2 O 3 Table 4 – Effect of the reaction temperature on catalytic properties of series 1 and 2 samples with the iron content 5 wt% in isobutane dehydrogenation with a fixed catalyst bed, WHSV = 7.4 h − 1 . X, % S, mol.% S cr , mol.% С content in the catalyst, wt% X, % S, mol.% S cr , mol.% С content in the catalyst, wt% 560 7.6 68.1 31.4 0.4 580 11. 8 64.6 35.0 0.6 15.2 68.0 31.7 2.6 600 21.1 63.1 36.6 1.6 25.1 68.9 30.7 3.3 620 32.4 59.1 40.6 2.8 38.8 63.1 36.6 4.2 Thus, on the 5%Fe/γ Pb -Al 2 O 3 sample, an increase in the reaction temperature from 560 to 580 °C increased isobutane conversion from 7.6 to 11. % and decreased selectivity to isobutene by 3.5 mol.%; on the contrary, selectivity to the products of cracking reactions, which are the precursors of coke deposits, increased almost by 3 mol.%. A further temperature elevation to 620 °C resulted in a growth of isobutane conversion to 324 % and a loss in selectivity to isobutene to 59.1 mol.%, with a simultaneous growth of selectivity to the cracking products up to 40.6 mol.%. A qualitatively similar pattern was observed also for the 5%Fe/nano-Al 2 O 3 sample. 4. Conclusion A comprehensive study of physicochemical properties of the nanostructured xFe/nanoAl 2 O 3 system with the iron content 0.0–5.0 wt%, which was synthesized via laser vaporization using irradiation by a cw CO 2 laser, has been carried out. The obtained results were compared with similar data for the xFe/γ Pb -Al 2 O 3 system synthesized by the conventional sol-gel method. As a result, HRTEM data for xFe/nano-Al 2 O 3 nanopowders indicate the formation of spherical nanoparticles with the diameter of ca. 9 nm and with high crystallinity. XRD analysis shows that the phase composition of xFe/nano-Al 2 O 3 nanoparticles is represented predominantly by the low-temperature γ-Al 2 O 3 . The analysis of the obtained diffraction patterns revealed that for the series of xFe/nano-Al 2 O 3 samples, in distinction to xFe/γ Pb -Al 2 O 3 samples (the sol-gel method), a large part of Fe 3+ ions is in the disordered environment (evidently, in subsurface layers of Al 2 O 3 nanocrystallites). As the iron concentration in xFe/γ Pb -Al 2 O 3 samples is increased to 5 wt%, the α-Fe 2 O 3 phase is formed on the γ Pb -Al 2 O 3 surface, which is not observed for the series of xFe/nano-Al 2 O 3 samples. According to UV-vis DRS data for the series of xFe/γ Pb -Al 2 O 3 samples, an increase in the iron content is accompanied by the appearance of typical absorption bands caused by stabilization of Fe 3+ ions in the Fe 2 O 3 phase. The obtained pattern is preserved also in the samples of this series even after the catalytic testing, whereas for samples of the xFe/nano-Al 2 O 3 series, the Fe 2 O 3 oxide is not formed at any conditions. The analysis of PL, PLE spectra and PL decay curves for all the tested samples revealed the luminescence of Fe 3+ ions in the tetrahedral oxygen environment of (Fe 3 + Td ) that are located in the Al 2 O 3 matrix. For the series of xFe/nano-Al 2 O 3 samples, the analysis of FWHM values of this PL band of Fe 3 + Td :nano-Al 2 O 3 demonstrated that xFe/nano-Al 2 O 3 powders contain Fe 3+ ions that are in the disordered environment, which is located most likely in subsurface layers of Al 2 O 3 nanocrystallites. This is not observed for the samples of Fe/γ Pb -Al 2 O 3 series (the sol-gel method). The results of catalytic testing of the samples, particularly, the xFe/nano-Al 2 O 3 series, testify to the formation of iron sites in the xFe/nano-Al 2 O 3 samples synthesized by laser vaporization, which provide the catalytic activity of the samples. Differences in catalytic properties of the catalysts obtained by the sol-gel method and laser vaporization are based on different states of Fe 3+ ions described above. A comparison of catalytic activity of the samples with physicochemical data allowed us to conclude that a large amount of active Fe 3+ sites involved in the dehydrogenation reaction is present on the surface of nanostructured xFe/nano-Al 2 O 3 samples. Declarations Acknowledgments The authors are grateful to Vl.N. Snytnikov for laser synthesis of nanopowders, to PhD A.A. Zhdanov for elemental analysis by XRF method and to PhD A.V. Ishchenko for HRTEM images of studied nanopowders. The authors acknowledge resource center “VTAN” (Novosibirsk State University) for the access to experimental equipment (HRTEM). The study was financially supported by the Russian Science Foundation within project No. 21-19-00429. Author contributions Mark G. Baronskiy – Conceptualization, Investigation, Writing – original draft; Anton I. Kostyukov – Investigation; Aleksey V. Zhuzhgov – Investigation; Nadezhda A. Zaitseva – Investigation, Writing – original draft; Svetlana V. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-1846170","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":120160257,"identity":"ea350955-e7d3-4d4d-852d-0f7a75ddf64c","order_by":0,"name":"Mark G. 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Cherepanova","email":"","orcid":"","institution":"Boreskov Institute of Catalysis","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Svetlana","middleName":"V.","lastName":"Cherepanova","suffix":""},{"id":120160262,"identity":"dd4d8334-c13a-4c5d-81c8-f76bda58c9f9","order_by":5,"name":"Tatyana V. Larina","email":"","orcid":"","institution":"Boreskov Institute of Catalysis","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Tatyana","middleName":"V.","lastName":"Larina","suffix":""},{"id":120160263,"identity":"cdc44bb3-9de5-413d-a47a-e18194b13ff3","order_by":6,"name":"Valeriy N. Snytnikov","email":"","orcid":"","institution":"Boreskov Institute of Catalysis","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Valeriy","middleName":"N.","lastName":"Snytnikov","suffix":""}],"badges":[],"createdAt":"2022-07-11 10:14:14","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-1846170/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-1846170/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":24219406,"identity":"30fc2648-940f-4c57-9836-38c78ed573a2","added_by":"auto","created_at":"2022-07-22 20:32:27","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":120978,"visible":true,"origin":"","legend":"\u003cp\u003eDiffraction patterns of xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and xFe/γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples: 1 – nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e; 2 – 1%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e; 3 – 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e; 4 – 5%Fe/γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-1846170/v1/99df23f717d0aad120028908.png"},{"id":24219604,"identity":"abfeabab-a9e8-429e-96ae-3581a46b45de","added_by":"auto","created_at":"2022-07-22 20:37:27","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":417747,"visible":true,"origin":"","legend":"\u003cp\u003eHRTEM images: a) γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (the sol-gel method); \u003c/p\u003e\u003cp\u003eb) 1%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e; c) 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e; and EDX spectra of the indicated regions for 1%, 5% Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-1846170/v1/1f65c823fc24a31971daa26c.png"},{"id":24219603,"identity":"93459b3e-5a01-4578-b181-40637a695a39","added_by":"auto","created_at":"2022-07-22 20:37:27","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":72806,"visible":true,"origin":"","legend":"\u003cp\u003eUV-vis DRS spectra of xFe/γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples with different iron content: \u003c/p\u003e\u003cp\u003e1 – 0.05 %; 2 – 0.5 %; 3 – 1.0 %; 4 – 2.5 %; and 5 – 5.0 %; and UV-vis DRS spectra of xFe/α-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples with different iron content: 6 – 1.0 %; and 7 – 5.0 %\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-1846170/v1/e9733f47732a8f301e9ec22d.png"},{"id":24219408,"identity":"bc13630b-17ac-4edc-a52b-33c4d96b0bd8","added_by":"auto","created_at":"2022-07-22 20:32:27","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":38205,"visible":true,"origin":"","legend":"\u003cp\u003eUV-vis DRS spectra of xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples with different iron content: \u003c/p\u003e\u003cp\u003e1 – 0.05\u0026nbsp;%; 2 – 0.5\u0026nbsp;%; 3 – 1.0\u0026nbsp;%; 4 – 2.5\u0026nbsp;%; and 5 – 5.0\u0026nbsp;%\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-1846170/v1/876dcb392c87e1aca0d42508.png"},{"id":24219940,"identity":"76c04317-a28f-4f4f-98d6-87f16f13136e","added_by":"auto","created_at":"2022-07-22 20:42:27","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":37291,"visible":true,"origin":"","legend":"\u003cp\u003eUV-vis DRS spectra of 1%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples:\u003c/p\u003e\u003cp\u003e1 – before the reaction; 2 – after the reaction; 3 – after regeneration; and 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples: 4 – before the reaction; 5 – after the reaction; 6 – after regeneration\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-1846170/v1/a1bb82d928e4fef3f43148a8.png"},{"id":24219412,"identity":"741388b3-7b2e-4642-ac3d-2129ce59bb64","added_by":"auto","created_at":"2022-07-22 20:32:28","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":111742,"visible":true,"origin":"","legend":"\u003cp\u003ePL (a), PLE (b) spectra and PL decay curves (c) of γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and xFe/γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples with the iron content x= 0.05, 1.0 and 5.0 wt%.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-1846170/v1/55c85066285b7e49a75ca31c.png"},{"id":24219605,"identity":"8a4ea265-1a3e-4a85-ada5-d8422dce6aee","added_by":"auto","created_at":"2022-07-22 20:37:27","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":122645,"visible":true,"origin":"","legend":"\u003cp\u003e PL (a), PLE (b) spectra and PL decay curves (c) of xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples with the iron content 0.05, 1.0 and 5.0 wt%. The insets show PL spectra (а) and PL decay curves (c) of 1%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples after the dehydrogenation reaction, respectively.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-1846170/v1/6a92a0f94edb179b57b608b5.png"},{"id":24219410,"identity":"275c9825-26a5-4b48-96a3-f13a89478dd9","added_by":"auto","created_at":"2022-07-22 20:32:27","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":39302,"visible":true,"origin":"","legend":"\u003cp\u003e\tSelectivity to isobutene versus isobutane conversion in dehydrogenation of isobutane for 5%Fe/γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3 \u003c/sub\u003e(black) and 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (red) catalysts at 580\u0026nbsp;°C.\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-1846170/v1/5241365692dc12126b258f00.png"},{"id":24219415,"identity":"cf9f499c-c825-4060-bd91-036389b15bd8","added_by":"auto","created_at":"2022-07-22 20:32:28","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":99447,"visible":true,"origin":"","legend":"\u003cp\u003eChanges in catalytic characteristics of 5%Fe/γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (a) and 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3 \u003c/sub\u003e(b)\u003csub\u003e \u003c/sub\u003esamples from cycle to cycle. Conditions of testing: 580\u0026nbsp;°C, WHSV = 12.9 h\u003csup\u003e–1\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-1846170/v1/0d62ef60caea14db420d9169.png"},{"id":24219413,"identity":"bef30f05-da73-45a1-9f18-b7ad1fa4ceea","added_by":"auto","created_at":"2022-07-22 20:32:28","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":101067,"visible":true,"origin":"","legend":"\u003cp\u003eChanges in catalytic characteristics of 5%Fe/γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (a) and 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3 \u003c/sub\u003e(\u003c/p\u003e\u003cp\u003eb)\u003csub\u003e \u003c/sub\u003esamples in a long-term dehydrogenation of isobutane without catalyst regeneration; where X, % is the conversion of isobutane; S, mol.% – the selectivity to isobutene; Y, mol. %\u0026nbsp;– the yield of isobutene. Conditions of experiments: 580\u0026nbsp;°C, WHSV = 12.9 h\u003csup\u003e–1\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-1846170/v1/5d3af6bd3f32050ccdedad87.png"},{"id":24219941,"identity":"80f3c685-7ff8-4d41-9cd9-747c3be8b650","added_by":"auto","created_at":"2022-07-22 20:42:31","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1165252,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-1846170/v1/96979028-c13c-4d83-8840-946517b6ff17.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Synthesis, spectroscopic and catalytic properties of FeO x /Al 2 O 3 nanopowders prepared by cw CO 2 laser vaporization","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eAt present, mixed Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e systems and Fe/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e composites are of considerable scientific and applied interest as promising materials with the improved functional properties. Such mixed oxide systems are of great interest due to the creation of new ceramic and composite materials for practical applications in various fields and industries. For example, Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e materials are widely employed for environmental protection as sorbents and agents in water and wastewater treatment [\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Magnetic ferrites such as Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and nanosized Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-Fe-coated cenospheres are used to obtain electromagnetic wave absorbents and electrical conducting fillers in composites [\u003cspan additionalcitationids=\"CR5\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. It was shown that Fe/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e composites can be used as promising candidates for the double negative materials [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. It is known that heterogeneous iron-based catalysts are widely used in dehydrogenation reactions, particularly, in the dehydrogenation of propane to propylene [\u003cspan additionalcitationids=\"CR10\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. In some works, nanostructured Fe and Fe-Zn/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e systems served as the catalysts for isobutane dehydrogenation [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. In [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] it was reported that for Fe/SiO\u003csub\u003e2\u003c/sub\u003e catalysts, single iron sites embedded in a silica matrix were responsible for methane conversion exclusively to ethylene and aromatics.\u003c/p\u003e \u003cp\u003eThe properties of mixed iron and alumina oxide systems depend to a great extent on the methods and regimes employed for the synthesis of materials. One of the key directions in the synthesis of nanomaterials is currently the \u0026ldquo;bottom-up\u0026rdquo; approach. It consists in the stepwise formation of nanomaterials according to the conventional sequence \u0026ldquo;atoms \u0026ndash; clusters \u0026ndash; nanoparticles\u0026rdquo; [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Bright examples of the indicated approach are laser synthesis methods based on the Physical Vapor Deposition (PVD) method. Laser methods make it possible to produce nanoparticles by vaporization of the initial materials (targets) under the action of laser energy with subsequent condensation of vapor in the so-called buffer gas or liquid [\u003cspan additionalcitationids=\"CR18\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Physicochemical properties of the produced nanoparticles can be controlled by variation of different parameters during laser vaporization, for example, laser radiation power, composition of the buffer gas in a vaporization chamber, and gas pressure [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Earlier we have studied xCr/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e nanopowders with a low Cr loading of 0.0\u0026ndash;4.8 wt%, which were obtained using vaporization by a cw CO\u003csub\u003e2\u003c/sub\u003e laser [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. The important features of cw CO\u003csub\u003e2\u003c/sub\u003e laser vaporization for the synthesis of nanopowders under consideration are the absence of chemical contaminants, monodispersity of the synthesized samples, and controlled nonstoichiometry. In the tested catalysts, two radically different types of Cr\u003csup\u003e3+\u003c/sup\u003e sites were revealed in the solid solution based on γ-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. One type comprises bulk Cr\u003csup\u003e3\u0026thinsp;+\u003c/sup\u003e\u0026thinsp;\u003csub\u003eb\u003c/sub\u003e sites in a strong crystal field (\u003cem\u003eDq\u003c/em\u003e/\u003cem\u003eB\u003c/em\u003e\u0026thinsp;=\u0026thinsp;3.4). Another type is represented by Cr\u003csup\u003e3\u0026thinsp;+\u003c/sup\u003e\u0026thinsp;\u003csub\u003es\u003c/sub\u003e sites located near the surface of xCr/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e nanoparticles (\u003cem\u003eDq\u003c/em\u003e/\u003cem\u003eB\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1.4\u0026ndash;1.8). A comparison of catalytic properties of the nanostructured system xCr/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e with the typical catalyst supported on γ-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (sol-gel) showed that the nanostructured system has better catalytic characteristics, which are provided by the Cr\u003csup\u003e3\u0026thinsp;+\u003c/sup\u003e\u0026thinsp;\u003csub\u003es\u003c/sub\u003e sites formed on the surface of nanoparticles.\u003c/p\u003e \u003cp\u003eThis study was aimed to identify different states of iron on the surface and in the bulk of the tested nanostructured FeO\u003csub\u003ex\u003c/sub\u003e/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e powders and to understand how the indicated iron states would manifest themselves in various physicochemical processes. This aim was accomplished by a comprehensive investigation of physicochemical properties of the nanostructured FeO\u003csub\u003ex\u003c/sub\u003e/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e system with a variable iron content, which was synthesized via laser vaporization using irradiation by a cw CO\u003csub\u003e2\u003c/sub\u003e laser. The obtained experimental results were compared with similar data for the Fe/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e system synthesized by the sol-gel method.\u003c/p\u003e"},{"header":"2. Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Sample preparation\u003c/h2\u003e \u003cp\u003eTwo radically different series of FeO\u003csub\u003ex\u003c/sub\u003e/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e powders were synthesized:\u003c/p\u003e \u003cp\u003e1. FeO\u003csub\u003ex\u003c/sub\u003e/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e powders with the Fe content of 0.05, 0.5, 1, 2.5 and 5 wt%, which were deposited on a single-phase γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e support; hereinafter, they are denoted as xFe/γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, where x is the iron content in the samples (wt%). The γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e support was prepared from pseudoboehmite by the sol-gel method.\u003c/p\u003e \u003cp\u003eThe γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sample was obtained by thermal treatment of pseudoboehmite in air at 550 \u0026deg;C for 4 h. In its turn, pseudoboehmite (γAlOOH\u0026middot;nH\u003csub\u003e2\u003c/sub\u003eO) was produced by simultaneous pouring of equinormal aqueous solutions of Al(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e\u0026middot;9H\u003csub\u003e2\u003c/sub\u003eO salt and ammonium hydroxide (NH\u003csub\u003e4\u003c/sub\u003eOH) at a constant pH close to neutral. To obtain pseudoboehmite sediment, the mixed solutions were held at 80 \u0026deg;C for 2 h. The resulting sediment was separated from the aqueous solution by decantation, washed with distilled water to remove impurities, and then dried in flowing air at 150 \u0026deg;C for 12 h to a constant weight.\u003c/p\u003e \u003cp\u003eDuring the synthesis of xFe/γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples, incipient wetness impregnation of the γ\u003csub\u003ePb\u003c/sub\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e powder with aqueous solutions of iron(III) nitrate (Fe(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e∙9H\u003csub\u003e2\u003c/sub\u003eO) was used to obtain the total concentration of Fe\u003csup\u003e3+\u003c/sup\u003e ions equal to 0.05, 0.5, 1, 2.5 and 5 wt%. After completion of the impregnation step, all xFe/γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples were dried in air at room temperature for a day. After that, the samples were dried at 110 \u0026deg;C for 6 h and calcined at 550 \u0026deg;C for 4 h in air. Along with the indicated samples, xFe/α-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e powders (x[Fe\u003csup\u003e3+\u003c/sup\u003e]\u0026thinsp;=\u0026thinsp;0.05, 0.5, 1, 2.5, and 5 wt%) were prepared. The xFe/α-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples with different content of iron were obtained by thermal treatment of the corresponding xFe/γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples at 1250 \u0026deg;C for 4 h in air.\u003c/p\u003e \u003cp\u003e2. FeO\u003csub\u003ex\u003c/sub\u003e/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e powders with the iron content of 0, 0.05, 0.5, 1, 2.5 and 5 wt%, which were prepared using the nanostructured Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e support with a complicated phase composition and particle sizes up to 20 nm. Samples of this series are denoted as xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, where x is the iron content in the samples (wt%).\u003c/p\u003e \u003cp\u003eNanostructured xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples were synthesized by laser vaporization of ceramic targets from xFe/α-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. The vaporization was carried out using a continuous CO\u003csub\u003e2\u003c/sub\u003e laser with the output power 100 W and power density on the target surface 10\u003csup\u003e5\u003c/sup\u003e W/cm\u003csup\u003e2\u003c/sup\u003e. Helium served as the buffer gas (99.99%). In all cases, helium pressure in the vaporization chamber was P\u003csub\u003eHe\u003c/sub\u003e = 0.26 atm. To obtain xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e nanoparticles with the specified concentrations of iron, the initial α-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e targets were doped with iron at the step of target preparation via the incipient wetness impregnation of alumina (similar to the procedure described above for the synthesis of xFe/γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e by the sol-gel method).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Sample characterization\u003c/h2\u003e \u003cp\u003eElemental analysis of the samples was made using X-ray fluorescence spectroscopy (XRF) on an ARL \u0026ndash; Advant\u0026rsquo;x analyzer with the Rh anode of the X-ray tube.\u003c/p\u003e \u003cp\u003ePhase composition of the samples was revealed by X-ray diffraction (XRD) analysis. X-ray diffraction patterns were recorded on a Bruker D8 diffractometer using a CuK\u003csub\u003eα\u003c/sub\u003e source and scanning over the range of reflection angles 2θ\u0026thinsp;=\u0026thinsp;15\u0026ndash;75\u0026deg; with a 0.05\u0026deg; step and accumulation time 3 s. The mean sizes of crystallites (\u0026lt;\u0026thinsp;D\u0026gt;) for the 0, 1, 5 wt% Fe xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and 5wt% Fe xFe/γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples were estimated from full width at half maximum (FWHM) of the 440 peak (2θ\u0026thinsp;=\u0026thinsp;67\u0026ndash;69) using the Scherrer formula.\u003c/p\u003e \u003cp\u003eThe morphology of the synthesized samples was characterized by high-resolution transmission electron microscopy (HRTEM) on a JEM-2010 electron microscope at accelerating voltage 200 kV and resolution 1.4 \u0026Aring;. Samples were deposited on a copper grid by dispersing a solid phase suspension in alcohol using an ultrasonic disperser.\u003c/p\u003e \u003cp\u003eLocal analysis of elemental composition of the samples was performed on an energy-dispersive EDS spectrometer QUANTAX 200-TEM (Bruker) with a XFLASH detector at energy resolution of ca. 130 eV.\u003c/p\u003e \u003cp\u003ePhotoluminescence (PL), photoluminescence excitation (PLE) spectra and PL decay curves were measured on a CaryEclipse (Varian) spectrofluorimeter under excitation with a Xe flash lamp. All measurements were carried out at room temperature.\u003c/p\u003e \u003cp\u003eDiffuse reflectance spectroscopy in visible and UV regions (UV-Vis DRS) of the studied samples was performed on a UV2501 PC (Shimadzu) spectrophotometer with an ISR240 A diffuse reflectance attachment. All the samples were placed in a quartz cuvette with the optical path length of 2 mm. UV-Vis DRS spectra were recorded relative to the BaSO\u003csub\u003e4\u003c/sub\u003e reflectance standard in a range of 190\u0026ndash;900 nm (11000\u0026ndash;54000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). The final UVVis DRS data are represented in the coordinates: Kubelka-Munk function, F (R) versus wavenumber.\u003c/p\u003e \u003cp\u003eSpecific surface area (S\u003csub\u003esp\u003c/sub\u003e) of the samples was calculated according to BET using low-temperature nitrogen adsorption at 77 K on a DigiSorb-2600 (Micromeritics) automated volumetric instrument. All the samples were subjected to thermal pretreatment in vacuum at 200 \u0026deg;C and residual pressure 10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e mm Hg for 5 h.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Isobutane dehydrogenation reaction\u003c/h2\u003e \u003cp\u003eActivity of the catalysts of both series was investigated in isobutane dehydrogenation in a fixed catalyst bed using a flow microreactor. The catalyst sample with a 0.2 or 0.35 g weight was loaded in a tubular quartz reactor with the internal diameter 14 mm and height 120 mm. The temperature in the middle of the catalyst bed was measured by a thermocouple placed in a thermocouple pocket (the external diameter of 6 mm) that was located coaxially to the reactor. Activity of the freshly synthesized catalysts was determined at atmospheric pressure and temperatures 550\u0026ndash;620 \u0026deg;С. Prior to testing the catalytic properties, the fresh samples were heated in nitrogen flowing at a rate of 16.7 ml/min to the reaction temperature and held in nitrogen for 1 h to provide the desorption of water from the catalyst surface. Before testing, a part of the catalyst with the composition 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e was reduced in a hydrogen flow at 580 \u0026deg;C for 30 minutes. Isobutane was fed to the reactor (a flow rate of 16.7 ml/min, WHSV\u0026thinsp;=\u0026thinsp;12.9 or 7.4 1/h) as a feedstock for 10 minutes. When the effect of dehydrogenation reaction time on catalytic properties of the samples was studied, the time of continuous feeding of isobutane was 150 minutes. To remove coke deposits after dehydrogenation, the catalysts were regenerated in a mixture of air and nitrogen with the volume ratio 1:1 (the total flow rate 33.3 ml/min) at the reaction temperature. Between dehydrogenation and regeneration, the reactor was purged with nitrogen. Samples of dehydrogenation products were taken for the analysis 10 minutes after the onset of isobutane feeding to the reactor; purging of the reactor with nitrogen between dehydrogenation and regeneration was performed for 10 minutes. Regeneration of the catalysts by burning off coke deposits was carried out until СО\u003csub\u003eх\u003c/sub\u003e completely disappeared from the regeneration products. In the experiments elucidating the effect of dehydrogenation time on catalytic properties of the samples, sampling from the reactor was made 5, 10, 15, 25, 35, 45, 60, 75, 105, 120 and 150 minutes after the onset of isobutane feeding to the reactor.\u003c/p\u003e \u003cp\u003eThe catalysts synthesized by the sol-gel method and laser vaporization substantially differ in their bulk density (0,75 and 0,08 g/cm\u003csup\u003e3\u003c/sup\u003e, respectively). So, to compare the catalytic properties of such samples, dependences of selectivity to isobutene on isobutane conversion were obtained under variation of weight hourly space velocities. Conditions of the experiments were as follows: the catalyst weight 0.2 g, the temperature 580 \u0026deg;C, and the volume flow rate of isobutane 1\u0026ndash;5 L/h. Catalytic properties of the tested catalysts were compared using selectivity to isobutene at similar isobutane conversions.\u003c/p\u003e \u003cp\u003eThe initial gases and reaction products were analyzed on a CHROMOS GC-1000 chromatograph using a flame-ionization detector (FID) and a capillary column with SiO\u003csub\u003e2\u003c/sub\u003e having a length of 25 m. Samples for the analysis were taken on-line by autosamplers directly from the gas flow at the reactor outlet. The obtained chromatograms were processed on a computer using internal normalization. GC analysis allows determining the content of methane, ethane, ethylene, propane, propene, n-butane, isobutane, isobutene, linear butenes. The presence of other carbon-containing products in the gas stream was not observed. The analysis of H\u003csub\u003e2\u003c/sub\u003e, CO, and CO\u003csub\u003e2\u003c/sub\u003e in reaction and regeneration products in the continuous mode was performed on a TEST-1.2 gas analyzer manufactured by BonAir Ltd. (Russia) using optical absorption infrared and electrochemical sensors. The analysis of the output concentrations of CO and CO\u003csub\u003e2\u003c/sub\u003e by these sensors allowed calculating the amount of coke deposits. Conversion of isobutane, selectivity to isobutene and isobutene yields were calculated on a molar basis taking into account coke deposits.\u003c/p\u003e \u003cp\u003eThe catalytic experiments were carried out in the kinetic region of the process. To exclude the effect of external diffusion limitations, the experiments were performed at different linear feed velocities of isobutane and a constant contact time. On two different catalyst samples (0,2 and 0,35 g) at 580 \u0026deg;C and WHSV\u0026thinsp;=\u0026thinsp;12,9 L/h, isobutane conversions were equal to 13,9 and 13,4, respectively. The data obtained demonstrated that external diffusion limitations were negligible. The grain size and the absence of pores in the studied catalysts exclude the occurrence of intradiffusion inhibitions.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results And Discussion","content":"\u003cp\u003eIron content in the synthesized catalysts of series 1 and 2 was controlled by X-ray fluorescence analysis (see Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e). The phase composition and morphology of the samples under consideration were controlled by XRD and HRTEM. According to XRD data (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e), the diffraction patterns of all the tested samples of both series correspond predominantly to the \u0026gamma;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e phase. In the diffraction patterns of xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples (series 2) with different iron content, one can see that starting from the undoped nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sample, lattice parameters increase from a\u0026thinsp;=\u0026thinsp;7.925 \u0026Aring; for nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e to a\u0026thinsp;=\u0026thinsp;7.931 \u0026Aring; for the 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sample. With increasing iron content, the 311 peak becomes asymmetric in comparison with XRD data for nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sample. This indicates that an increase in the iron content in the tested samples is accompanied by incorporation of a part of Fe\u003csup\u003e3+\u003c/sup\u003e ions into the \u0026gamma;Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e lattice, which isomorphously substitute Al\u003csup\u003e3+\u003c/sup\u003e ions to form the Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-\u0026gamma;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e solid solution. A large part of Fe\u003csup\u003e3+\u003c/sup\u003e ions remains in subsurface layers of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e nanocrystallites (the disordered environment). In the 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sample, an additional peak is observed in the region 2\u0026theta;\u0026thinsp;=\u0026thinsp;55\u0026ndash;60\u0026deg; and the asymmetry of peak 311 attributed to \u0026gamma;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. This asymmetry and the presence of this peak are in good agreement with the presence of the FeAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e phase in the sample. The phase of magnetite Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e is not observed. At the same time, the diffraction pattern of the 5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sample synthesized by the sol-gel method (series 1) contains narrow sharp peaks corresponding to the formation of the \u0026alpha;-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e phase. Thus, in the 5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sample, a large part of Fe\u003csup\u003e3+\u003c/sup\u003e ions occupies regular octahedral positions in the volume of \u0026alpha;-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e lattice. For nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sample as well as for 1%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and 5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e powders, the average particle size (\u0026lt;\u0026thinsp;D\u0026gt;) was calculated by the Selyakov-Scherrer formula from XRD data using the 440 peak of the \u0026gamma;Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e phase. In nanostructured samples of series 2, \u0026lt;D\u0026thinsp;\u0026gt;\u0026thinsp;=\u0026thinsp;5.6, 5.5, 6.5 nm for nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, 1%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, respectively, while for the series 1 sample 5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e the \u0026lt;\u0026thinsp;D\u0026thinsp;\u0026gt;\u0026thinsp;value is 6.2 nm. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTable 1. Data on the iron and aluminum content in\u0026nbsp;xFe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u0026nbsp;\u003c/sub\u003esamples \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n\u003cdiv align=\"center\"\u003e\n \u003ctable border=\"0\" cellpadding=\"0\" cellspacing=\"0\" width=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd rowspan=\"2\" valign=\"bottom\" width=\"10.79734219269103%\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003eEl/Ox\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"5\" valign=\"bottom\" width=\"89.20265780730897%\"\u003e\n \u003cp\u003exFe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" width=\"19.366852886405958%\"\u003e\n \u003cp\u003e\u003cstrong\u003e0.05 wt% Fe\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"21.042830540037244%\"\u003e\n \u003cp\u003e\u003cstrong\u003e0.5 wt% Fe\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"18.24953445065177%\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.0 wt% Fe\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"22.160148975791433%\"\u003e\n \u003cp\u003e\u003cstrong\u003e2.5 wt% Fe\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"19.180633147113593%\"\u003e\n \u003cp\u003e\u003cstrong\u003e5.0 wt% Fe\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" width=\"10.79734219269103%\"\u003e\n \u003cp\u003eAl \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"17.27574750830565%\"\u003e\n \u003cp\u003e52.77\u0026plusmn;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"18.770764119601328%\"\u003e\n \u003cp\u003e52.40\u0026plusmn;0.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"16.27906976744186%\"\u003e\n \u003cp\u003e52.01\u0026plusmn;0.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"19.767441860465116%\"\u003e\n \u003cp\u003e50.79\u0026plusmn;0.05\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"17.109634551495017%\"\u003e\n \u003cp\u003e49.05\u0026plusmn;0.07\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" width=\"10.79734219269103%\"\u003e\n \u003cp\u003eFe \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"17.27574750830565%\"\u003e\n \u003cp\u003e0.09\u0026plusmn;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"18.770764119601328%\"\u003e\n \u003cp\u003e0.58\u0026plusmn;0.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"16.27906976744186%\"\u003e\n \u003cp\u003e1.07\u0026plusmn;0.04\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"19.767441860465116%\"\u003e\n \u003cp\u003e2.68\u0026plusmn;0.07\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"17.109634551495017%\"\u003e\n \u003cp\u003e4.99\u0026plusmn;0.09\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" width=\"10.79734219269103%\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd colspan=\"5\" valign=\"bottom\" width=\"89.20265780730897%\"\u003e\n \u003cp\u003exFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" width=\"10.79734219269103%\"\u003e\n \u003cp\u003eAl\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"17.27574750830565%\"\u003e\n \u003cp\u003e52.69\u0026plusmn;0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"18.770764119601328%\"\u003e\n \u003cp\u003e52.43\u0026plusmn;0.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"16.27906976744186%\"\u003e\n \u003cp\u003e52.08\u0026plusmn;0.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"19.767441860465116%\"\u003e\n \u003cp\u003e50.74\u0026plusmn;0.05\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"17.109634551495017%\"\u003e\n \u003cp\u003e48.63\u0026plusmn;0.07\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"bottom\" width=\"10.79734219269103%\"\u003e\n \u003cp\u003eFe\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"17.27574750830565%\"\u003e\n \u003cp\u003e0.10\u0026plusmn;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"18.770764119601328%\"\u003e\n \u003cp\u003e0.48\u0026plusmn;0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"16.27906976744186%\"\u003e\n \u003cp\u003e0.97\u0026plusmn;0.04\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"19.767441860465116%\"\u003e\n \u003cp\u003e2.55\u0026plusmn;0.07\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"bottom\" width=\"17.109634551495017%\"\u003e\n \u003cp\u003e5.35\u0026plusmn;0.09\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eAccording to HRTEM data for the \u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e powder prepared by the sol-gel method, it consists of mutually disordered nanocrystallites with the diameter of 10\u0026ndash;15 nm, which form micron-sized polycrystalline aggregates (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea). The HRTEM images of 1%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples contain mostly the faceted 3D nanocrystals with the size from 3 to 10 nm (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb, \u003cstrong\u003eс\u003c/strong\u003e). For 1%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples, the average particle size estimated from HRTEM images is ca. 9 nm, which is consistent with the XRD data. A minor amount of particles with the size\u0026thinsp;\u0026ge;\u0026thinsp;20 nm is also present.\u003c/p\u003e\n\u003cp\u003eTo reveal and identify different charge states of iron ions and their coordination in series 1 and 2 samples, optical methods were used, namely, diffuse reflectance spectroscopy (UV-vis DRS) and photoluminescence (PL) spectroscopy. PL spectra, PL excitation (PLE) spectra, PL decay curves, and UV-vis DRS spectra were obtained and analyzed, particularly, for the samples \u003cem\u003ebefore\u003c/em\u003e and \u003cem\u003eafter\u003c/em\u003e catalytic dehydrogenation of isobutane.\u003c/p\u003e\n\u003cp\u003eAccording to UV-vis DRS data, xFe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples (series 1) with the iron content from 0,05 to 5,0 wt% have poorly resolved broad absorption bands in the region above 17000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which correspond to the ligand-metal charge transfer bands (CTB) of Fe\u003csup\u003e3+\u003c/sup\u003e ions (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e). Therewith, it is seen from the UV-vis DRS spectra presented in logarithmic coordinates that for the 0.05%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sample a symmetric low-intensity absorption band at 18700 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e can be distinguished, which is caused most likely by the d-d transition of Fe\u003csup\u003e3+\u003c/sup\u003e ions in the tetrahedral oxygen coordination [\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e]. In UV range of the UV-vis DRS spectrum, an intense absorption is observed in the region above 30000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which can be attributed to the ligand-metal CTB of Fe\u003csup\u003e3+\u003c/sup\u003e ions in the tetrahedral and octahedral oxygen coordination. A further increase in the content of Fe\u003csup\u003e3+\u003c/sup\u003e ions in 2,5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and 5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples leads to a higher intensity of the absorption band at 18700 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and the ligand-metal CTB; in addition, a broad absorption band at 35300 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e appears for the 5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sample. In the spectra of samples 2,5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and 5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, a shoulder shows up in the region of 15000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. For the sample with the highest iron concentration, 5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, three shoulders are observed at 15000, 18700 and 21900 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and three broad bands at 26400, 35300 and 47400 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. According to XRD, this sample contains the \u0026alpha;-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e phase, in which Fe\u003csup\u003e3+\u003c/sup\u003e ions are stabilized in the octahedral oxygen coordination (Fe\u003csup\u003e3\u0026thinsp;+\u003c/sup\u003e\u0026thinsp;\u003csub\u003eOh\u003c/sub\u003e). It can be reliably concluded that the \u0026alpha;Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e phase is not observed in 1%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and in the samples with lower iron concentration; hence it follows that Fe\u003csup\u003e3+\u003c/sup\u003e ions in the \u0026gamma;\u003csub\u003ePb\u003c/sub\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e matrix are stabilized only in the tetrahedral oxygen coordination (Fe\u003csup\u003e3\u0026thinsp;+\u003c/sup\u003e\u0026thinsp;\u003csub\u003eTd\u003c/sub\u003e). For 1%Fe/\u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and 5%Fe/\u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e, curves 6 and 7, respectively), well resolved absorption bands are observed at 15000, 18700, 21900, 26400, 29900, 38300 and 47400 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Therewith, the first five absorption bands are assigned to \u003cem\u003ed-d\u003c/em\u003e transitions of Fe\u003csup\u003e3+\u003c/sup\u003e cations in the octahedral oxygen coordination (Fe\u003csup\u003e3\u0026thinsp;+\u003c/sup\u003e\u0026thinsp;\u003csub\u003eOh\u003c/sub\u003e), while the last two more intense bands are caused by the ligand-metal CTB of Fe\u003csup\u003e3\u0026thinsp;+\u003c/sup\u003e\u0026thinsp;\u003csub\u003eOh\u003c/sub\u003e cations. The presence of intense ligand-metal CTB in UV region of the UV-vis DRS spectra and low-intensity absorption bands in visible region, which belong to \u003cem\u003ed-d\u003c/em\u003e transitions of Fe\u003csup\u003e3\u0026thinsp;+\u003c/sup\u003e\u0026thinsp;\u003csub\u003eOh\u003c/sub\u003e cations, indicates that these cations are stabilized exclusively in the volume of the oxygen-containing \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e matrix.\u003c/p\u003e\n\u003cp\u003eAs follows from the analysis of UV-vis DRS spectra of xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e powders with the iron content from 0,05 to 5,0 wt%, a broad intense absorption band is distinguished for all the samples of nano-series in the region of 25000\u0026ndash;42500 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e with a maximum at 33300 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (\u003cstrong\u003eFig.\u0026nbsp;4\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003eIn visible region of the UV-vis DRS spectrum, a low-intensity absorption band with the maximum at 20700 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e can also be distinguished. According to the literature data, these absorption bands may be attributed to the \u003cem\u003ed-d\u003c/em\u003e transition and ligand-metal CTB of Fe\u003csup\u003e3+\u003c/sup\u003e ions in the tetrahedral oxygen coordination (Fe\u003csup\u003e3\u0026thinsp;+\u003c/sup\u003e\u0026thinsp;\u003csub\u003eTd\u003c/sub\u003e), which are stabilized in the volume of \u0026gamma;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e matrix. A comparison of UV-vis DRS spectra for the series of xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples shows that an increase in the iron content in the samples is accompanied by a growth of intensity of the indicated absorption bands and the total background, which testifies to an increase in the content of highly dispersed phase in these samples.\u003c/p\u003e\n\u003cp\u003eUV-vis DRS spectra were obtained and analyzed for 1%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and 5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples as well as for the xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e series with the iron content 1 and 5 wt% \u003cem\u003eafter\u003c/em\u003e catalytic dehydrogenation of isobutane without regeneration or with subsequent regeneration. A comparison was made with the results obtained for the same samples \u003cem\u003ebefore\u003c/em\u003e catalytic tests (see \u003cstrong\u003eFig.\u0026nbsp;5\u003c/strong\u003e). Two main conclusions can be made from the analysis and comparison of the acquired data.\u003c/p\u003e\n\u003cp\u003eFirst, the presence of the absorption band at 33500 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in UV-vis DRS spectra (after the catalysis; without regeneration or with subsequent regeneration) of the 1%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sample, and the absorption band at 35500 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in UV-vis DRS spectra (after the catalysis; without regeneration) of the 1%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sample indicates a difference in the strength of the crystal field generated by oxygen ligands around Fe\u003csup\u003e3+\u003c/sup\u003e ions in the tetrahedron. Therewith, the crystal field strength in the 1%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sample is much greater than in 1%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, which shows that in the latter case the local distortions in the tetrahedral environment of Fe\u003csup\u003e3+\u003c/sup\u003e ion are much more pronounced. Second, at the highest concentration of iron in the samples, UV-vis DRS spectra of 5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e contain four absorption bands caused by stabilization of Fe\u003csup\u003e3+\u003c/sup\u003e ions in the Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e phase on the initial \u0026gamma;\u003csub\u003ePb\u003c/sub\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e support surface, a part of which is retained after testing in the catalytic reaction. Whereas for the 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sample, the Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e oxide is not formed at any conditions of testing.\u003c/p\u003e\n\u003cp\u003eAlong with UV-vis DRS spectra, for all the tested model catalysts we have obtained and analyzed PL and PLE spectra as well as PL decay curves for impurity iron ions. The luminescence measurements were made for nanostructured xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e powders with the iron content of 0,05, 1,0 and 5,0 wt% (series 2) and for series 1 samples \u0026ndash; \u0026gamma;\u003csub\u003ePb\u003c/sub\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and 0,05%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, 1%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and 5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. In addition, to identify surface sites of Fe ions in the studied xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples, a comparative analysis of the luminescence data for 1%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples \u003cem\u003ebefore\u003c/em\u003e and \u003cem\u003eafter\u003c/em\u003e the catalytic reaction (without regeneration) was performed.\u003c/p\u003e\n\u003cp\u003eLet us consider PL, PLE spectra and PL decay curves for series 1 samples (the sol-gel method). PL spectra in this case are characterized by the presence of a broad PL band in the region of 12000\u0026ndash;15000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e with a maximum at \u0026lambda;\u003csub\u003eem\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;13300\u0026thinsp;\u0026minus;\u0026thinsp;13000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at excitation with \u0026lambda;\u003csub\u003eex\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;310 nm (Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eThe observed PL, according to the literature data, is identified as the luminescence of Fe\u003csup\u003e3+\u003c/sup\u003e ions that isomorphously substitute Al\u003csup\u003e3+\u003c/sup\u003e ions and occupy tetrahedral positions in the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e structure [\u003cspan class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e]. In alumina, PL of Fe\u003csup\u003e3\u0026thinsp;+\u003c/sup\u003e\u0026thinsp;\u003csub\u003eTd\u003c/sub\u003e ions corresponds to the radiative electronic transition \u003csup\u003e4\u003c/sup\u003eT\u003csub\u003e1\u003c/sub\u003e(\u003csup\u003e4\u003c/sup\u003eG) \u0026rarr; \u003csup\u003e6\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e (\u003csup\u003e6\u003c/sup\u003eS). In the PLE spectrum (Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003eb) recorded for the PL band with \u0026lambda;\u003csub\u003eem\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;13300 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of \u0026gamma;\u003csub\u003ePb\u003c/sub\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and xFe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples with the Fe content 0,05, 1,0, and 5,0 wt%, a broad band is observed in UV region with \u0026lambda;\u003csub\u003emax\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;30800\u0026ndash;32300 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The excitation of PL of Fe\u003csup\u003e3\u0026thinsp;+\u003c/sup\u003e\u0026thinsp;\u003csub\u003eTd\u003c/sub\u003e ions occurs when light is absorbed in the UV region, where the ligand-metal CTB O\u003csup\u003e2\u0026minus;\u003c/sup\u003e\u0026rarr; Fe\u003csup\u003e3+\u003c/sup\u003e of Fe\u003csup\u003e3\u0026thinsp;+\u003c/sup\u003e\u0026thinsp;\u003csub\u003eTd\u003c/sub\u003e ions is observed, and also as a result of the intrasite transition \u003csup\u003e6\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e(\u003csup\u003e6\u003c/sup\u003eS) \u0026rarr; \u003csup\u003e4\u003c/sup\u003eE(\u003csup\u003e4\u003c/sup\u003eG). The PL decay curves at \u0026lambda;\u003csub\u003eem\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;13300 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e \u003cstrong\u003eс\u003c/strong\u003e) for the 0.05, 1.0 and 5.0%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples fall in the millisecond range, which is consistent with the available literature data [26]. One can see from PL spectra that an increase in the iron content in series 1 samples, starting from the \u0026gamma;\u003csub\u003ePb\u003c/sub\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e support to 0.05%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, leads to a sharp growth in intensity of the broad PL band with \u0026lambda;\u003csub\u003eem\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;13300\u0026thinsp;\u0026minus;\u0026thinsp;13000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fe\u003csup\u003e3\u0026thinsp;+\u003c/sup\u003e\u0026thinsp;\u003csub\u003eTd\u003c/sub\u003e). A further increase in the iron content is accompanied by a drop in the signal intensity due to the onset of concentration quenching. In PLE spectra (\u0026lambda;\u003csub\u003eem\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;755 nm) of 0.05%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, 1.0%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and 5.0%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples, a sole broad band is also observed in UV region with \u0026lambda;\u003csub\u003emax\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;30800\u0026ndash;32300 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eFor series 2 powders xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e obtained by laser vaporization, similar curves are observed in PL (\u0026lambda;\u003csub\u003eex\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;310 nm (32300 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)) and PLE (\u0026lambda;\u003csub\u003eem\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;755 nm (13300 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)) spectra (Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e). A broad PL band in the range of 700\u0026ndash;850 nm with the maximum at \u0026lambda;\u003csub\u003eem\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;13300\u0026thinsp;\u0026minus;\u0026thinsp;13000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which is observed for all the tested nanostructured powders with respective PLE spectra, is reliably identified as the spectral manifestation of Fe\u003csup\u003e3\u0026thinsp;+\u003c/sup\u003e\u0026thinsp;\u003csub\u003eTd\u003c/sub\u003e ions in the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e lattice. The PL decay curves also fall in the millisecond range (Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e and Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eTable 2. Lifetimes (\u0026tau;\u003csub\u003ePL\u003c/sub\u003e, ms) of the excited \u003csup\u003e4\u003c/sup\u003eT\u003csub\u003e1\u003c/sub\u003e(\u003csup\u003e4\u003c/sup\u003eG)\u0026nbsp;state of Fe\u003csup\u003e3+\u003c/sup\u003e ions for xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u0026nbsp;\u003c/sub\u003esamples \u003cem\u003ebefore\u003c/em\u003e and \u003cem\u003eafter\u003c/em\u003e dehydrogenation reaction\u003c/p\u003e\n\u003ctable border=\"1\" cellpadding=\"0\" cellspacing=\"0\" width=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd rowspan=\"3\" valign=\"top\" width=\"27.65625%\"\u003e\n \u003cp\u003e\u0026nbsp;Sample\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"4\" valign=\"top\" width=\"72.34375%\"\u003e\n \u003cp\u003e\u0026tau;\u003csub\u003ePL\u003c/sub\u003e, ms\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd colspan=\"2\" valign=\"top\" width=\"50.9719222462203%\"\u003e\n \u003cp\u003e\u003cem\u003eBefore\u003c/em\u003e dehydrogenation\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" valign=\"top\" width=\"49.0280777537797%\"\u003e\n \u003cp\u003e\u003cem\u003eAfter\u003c/em\u003e dehydrogenation\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"26.623376623376622%\"\u003e\n \u003cp\u003e\u0026tau;\u003csub\u003ePL1\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"24.458874458874458%\"\u003e\n \u003cp\u003e\u0026tau;\u003csub\u003ePL2\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"24.458874458874458%\"\u003e\n \u003cp\u003e\u0026tau;\u003csub\u003ePL1\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"24.458874458874458%\"\u003e\n \u003cp\u003e\u0026tau;\u003csub\u003ePL2\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"27.699530516431924%\"\u003e\n \u003cp\u003e0.05%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"19.248826291079812%\"\u003e\n \u003cp\u003e5.42\u0026plusmn;0.15\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"17.683881064162755%\"\u003e\n \u003cp\u003e0.57\u0026plusmn;0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"17.683881064162755%\"\u003e\n \u003cp\u003e\u0026ndash;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"17.683881064162755%\"\u003e\n \u003cp\u003e\u0026ndash;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"27.699530516431924%\"\u003e\n \u003cp\u003e1%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"19.248826291079812%\"\u003e\n \u003cp\u003e4.02\u0026plusmn;0.15\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"17.683881064162755%\"\u003e\n \u003cp\u003e0.47\u0026plusmn;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"17.683881064162755%\"\u003e\n \u003cp\u003e3.20\u0026plusmn;0.12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"17.683881064162755%\"\u003e\n \u003cp\u003e0.49\u0026plusmn;0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"27.699530516431924%\"\u003e\n \u003cp\u003e5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"19.248826291079812%\"\u003e\n \u003cp\u003e1.58\u0026plusmn;0.05\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"17.683881064162755%\"\u003e\n \u003cp\u003e0.60\u0026plusmn;0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"17.683881064162755%\"\u003e\n \u003cp\u003e2.54\u0026plusmn;0.08\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"17.683881064162755%\"\u003e\n \u003cp\u003e0.40\u0026plusmn;0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eThe analysis of FWHM values of the luminescence band at \u0026lambda;\u003csub\u003eem\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;13300\u0026thinsp;\u0026minus;\u0026thinsp;13000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples with different iron content \u003cem\u003ebefore\u003c/em\u003e catalytic dehydrogenation of isobutane shows that FWHM of the PL band increases with the iron concentration from 1612 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for 0.05%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e to 1703 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sample. An increase in FWHM of the PL band of Fe\u003csup\u003e3+\u003c/sup\u003e:Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e may indicate that a large part of Fe\u003csup\u003e3+\u003c/sup\u003e ions occupies nonequivalent positions (the disordered environment) in subsurface layers of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e nanocrystallites. The indicated trend is not observed for xFe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples; on the contrary, FWHM values of the PL band of Fe\u003csup\u003e3+\u003c/sup\u003e ions in these samples decrease with the growth of iron content from the value 1583 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for undoped \u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e support to 1550 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for 5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sample. This additionally testifies that a considerable fraction of Fe\u003csup\u003e3+\u003c/sup\u003e ions in xFe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples may occupy regular positions in the lattice volume of \u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e support. In addition, a comparison of FWHM values of the PL band of Fe\u003csup\u003e3+\u003c/sup\u003e ions for respective iron concentrations in series 1 and 2 samples \u003cem\u003ebefore\u003c/em\u003e catalytic experiments shows that for xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples these values in all cases are higher as compared to the powders based on \u0026gamma;\u003csub\u003ePb\u003c/sub\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e; this again testifies to a higher concentration of disordered active Fe\u003csup\u003e3+\u003c/sup\u003e sites in nanostructured samples obtained by laser vaporization (series 2) in comparison with the series 1 samples. Another argument in favor of the statement that changes in FWHM of the PL band, for which Fe\u003csup\u003e3+\u003c/sup\u003e ions in the tested samples are responsible, are related to variation in the content of surface Fe\u003csup\u003e3+\u003c/sup\u003e sites is a decrease in FWHM of the PL band for 1%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples \u003cem\u003eafter\u003c/em\u003e the catalytic reaction in comparison with these values \u003cem\u003ebefore\u003c/em\u003e the reaction. So for 1%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e \u003cem\u003eafter\u003c/em\u003e the catalytic reaction the corresponding FWHM values of the PL band of Fe\u003csup\u003e3+\u003c/sup\u003e ions are 1569 and 1546 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively. In the course of reaction, a part of Fe\u003csup\u003e3+\u003c/sup\u003e ions under the action of temperature moves into the bulk and occupies regular positions in the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e matrix.\u003c/p\u003e\n\u003cp\u003eThe estimates based on the PL decay curves demonstrate a decrease in the average lifetime ((\u0026tau;\u003csub\u003ePL\u003c/sub\u003e)\u003csub\u003eav\u003c/sub\u003e) of the \u003csup\u003e4\u003c/sup\u003eT\u003csub\u003e1\u003c/sub\u003e(\u003csup\u003e4\u003c/sup\u003eG) excited state of Fe\u003csup\u003e3+\u003c/sup\u003e ions in 1%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples tested in the dehydrogenation reaction. The (\u0026tau;\u003csub\u003ePL\u003c/sub\u003e)\u003csub\u003eav\u003c/sub\u003e values were obtained by the formula reported in [\u003cspan class=\"CitationRef\"\u003e27\u003c/span\u003e]:\u003c/p\u003e\n\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\"\u003e\u003c/p\u003e\n\u003cp\u003eThe obtained values of (\u0026tau;\u003csub\u003ePL\u003c/sub\u003e)\u003csub\u003eav\u003c/sub\u003e were 2.90\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10 ms and 2.38\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10 ms for 1%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and 0.71\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03 ms and 0.68\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03 ms for 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, respectively, \u003cem\u003ebefore\u003c/em\u003e and \u003cem\u003eafter\u003c/em\u003e the dehydrogenation reaction. Such a behavior of (\u0026tau;\u003csub\u003ePL\u003c/sub\u003e)\u003csub\u003eav\u003c/sub\u003e may be associated with a decrease in the contribution of nonradiative component, which in its turn is caused by a partial dehydroxylation of the nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e support surface under the action of temperature during dehydrogenation, and OH groups are known to be the efficient luminescence quenchers. Another possible reason of such a behavior of (\u0026tau;\u003csub\u003ePL\u003c/sub\u003e)\u003csub\u003eav\u003c/sub\u003e may lie in the partial increase in the crystallinity of the samples as a result of the temperature effect on them during the catalytic reaction.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCatalytic Characteristics\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTable \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e lists the results of catalytic experiments in isobutane dehydrogenation at 550 \u0026deg;C, which corresponds to the synthesis temperature of series 1 and 2 samples with different iron content. For the tested nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e as well as 1%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and 5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalysts, specific surface area values (S\u003csub\u003esp\u003c/sub\u003e, m\u003csup\u003e2\u003c/sup\u003e/g) were measured. They were 210, 194, and 171 m\u003csup\u003e2\u003c/sup\u003e/g for nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, 1%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, and 5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, respectively.\u003c/p\u003e\n\u003cp\u003eAs shown by the catalytic studies in isobutane dehydrogenation, an increase in the iron content in the xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e system is accompanied by an increase in isobutane conversion and selectivity to isobutene. This indicates that in the xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples obtained by laser vaporization, necessary iron sites are formed to provide catalytic activity of the samples. The dependence of isobutylene yield on the iron concentration in the samples is virtually linear. \u0026nbsp;\u003c/p\u003e\n\u003ctable border=\"1\" id=\"Tab3\"\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eCatalyst composition\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eYield of isobutylene\u003c/p\u003e\n \u003cp\u003e(Y), wt%\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eConversion\u003c/p\u003e\n \u003cp\u003e(X), %\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eSelectivity\u003c/p\u003e\n \u003cp\u003e(S), wt%\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003e\u0026ndash; Catalytic performance in isobutane dehydrogenation in a fixed bed of series 1 and 2 model catalysts. Conditions of testing: temperature 550 \u0026deg;C, atmospheric pressure, WHSV\u0026thinsp;=\u0026thinsp;12.9 h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e58.1\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e5.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e61.1\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e6.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e66.7\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e56.4\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e68.0\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e72.9\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eA comparison of the catalytic properties of series 1 and 2 samples in isobutane dehydrogenation was further made using catalysts with the iron content 5 wt%, which are the most active ones in the concentration series. Along with the catalytic activity and selectivity, we have studied the catalyst stability in a long-term (2,5 h) dehydrogenation without regeneration and its stability in the long-term testing with alternating dehydrogenation-regeneration (reduction-oxidation) cycles. For nanostructured system with the iron content 5 wt%, the effect of the thermal pretreatment medium (with reduction in hydrogen or without it) was investigated. For both series of catalysts, the dependence of catalytic activity and selectivity on the reaction temperature was explored.\u003c/p\u003e\n\u003cp\u003eDue to different bulk density, it is impossible to compare directly the catalytic properties of the sol-gel synthesized catalysts and samples of the nanostructured series prepared by laser vaporization. So, their selectivities to isobutene were compared at a similar isobutane conversion using the dependences of selectivity on conversion that were obtained upon variation of the weight hourly space velocity (see Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e). According to the data of Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e, at close isobutane conversions the selectivity to isobutene for the 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst is 3\u0026ndash;5 mol.% higher as compared to the 5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sample. We tried to elucidate the cause of differences in the catalytic behavior of series 1 and 2 catalysts by comparing the data obtained for the catalysts by physicochemical methods and their catalytic properties. The analysis of XRD data showed that for the series of xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples, in distinction to xFe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples (the sol-gel method), a large part of Fe\u003csup\u003e3+\u003c/sup\u003e ions is in the disordered environment (evidently, in subsurface layers of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e nanocrystallites).\u003c/p\u003e\n\u003cp\u003eAn increase in the iron concentration in xFe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples to 5 wt% is accompanied by the formation of the \u0026alpha;-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e phase on the \u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e surface, which is not observed for the samples of xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e series. According to UV-vis DRS data for the series of xFe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples, an increase in the iron content produces typical absorption bands caused by stabilization of Fe\u003csup\u003e3+\u003c/sup\u003e ions in the Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e phase. The observed pattern is preserved also in the samples of this series after catalytic testing, whereas for samples of the xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e series, Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e oxide does not form under any conditions.\u003c/p\u003e\n\u003cp\u003eAlong with isobutane dehydrogenation, side reactions of cracking and hydrogenolysis proceed, leading to the formation of С\u003csub\u003e1\u003c/sub\u003e-С\u003csub\u003e3\u003c/sub\u003e hydrocarbons, which are the precursors of coke deposits in the catalyst. Upon coking of the catalysts, active dehydrogenation sites may be blocked by coke deposits. To remove coke from the catalyst surface, the catalyst is regenerated by coke burn-off. Thus, the dehydrogenation process requires quite severe conditions of the catalyst operation \u0026ndash; the alternation of dehydrogenation-regeneration (reduction-oxidation) cycles at high temperatures. So it is always interesting to estimate stability of the catalysts at a long-term alternation of the cycles. The stability study demonstrated that the sample of series 1 was stable for at least 20 cycles, while the sample of series 2 \u0026ndash; for at least 31 cycles (Fig. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eOne of the most important characteristics of dehydrogenation catalysts is the duration of operation without regeneration, i.e. the resistance to poisoning by the coke-like deposits. We have studied changes in the catalytic properties of materials under consideration in dependence on dehydrogenation time without regeneration of the catalysts aimed to remove carbon deposits. The catalytic properties were tested for 2.5 h with a periodic analysis of the composition of reaction mixture taken from the reactor.\u003c/p\u003e\n\u003cp\u003eAfter 2.5 h, the samples were regenerated until complete coke burn-off; the amount of evolved carbon combustion products СО\u003csub\u003eх\u003c/sub\u003e was used to estimate the yield of coke in the reaction products and the content of carbon in the catalyst. Figure \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e displays the dependence of catalytic properties of 5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples on the duration of dehydrogenation.\u003c/p\u003e\n\u003cp\u003eAccording to the data obtained for the 5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sample, the maximum values of isobutane conversion (13%) and selectivity to isobutene (70%) under the chosen conditions were reached 30\u0026ndash;35 minutes after the onset of feeding, remained virtually constant up to 50 minutes, and then gradually decreased up to 150 minutes by 2% and 3 mol.%, respectively. The yield of coke after 150 minutes was 4,7 mol.%, and the carbon content in the catalyst \u0026ndash; 4,9 wt%.\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003eb shows the dependence of catalytic properties of the 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sample on the duration of dehydrogenation. The maximum values of isobutane conversion (16\u0026ndash;17%) and selectivity to isobutene (72%) were observed 25\u0026ndash;30 minutes after the onset of feeding and remained virtually constant up to the end of experiment (150 minutes). The yield of coke after 150 minutes was 7.6 mol.%, and the carbon content in the catalyst \u0026ndash; 7.5 wt%.\u003c/p\u003e\n\u003cp\u003eData on the temperature effect on catalytic properties of the catalysts of both series with the iron content 5 wt% are listed in Table \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e. The temperature dependence is typical of both samples. \u0026nbsp;\u003c/p\u003e\n\u003ctable border=\"1\" id=\"Tab4\"\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eТ, \u0026nbsp;\u0026deg;C\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colspan=\"4\"\u003e\n \u003cp\u003e5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colspan=\"4\"\u003e\n \u003cp\u003e5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003e\u0026ndash; Effect of the reaction temperature on catalytic properties of series 1 and 2 samples with the iron content 5 wt% in isobutane dehydrogenation with a fixed catalyst bed, WHSV\u0026thinsp;=\u0026thinsp;7.4 h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eX, %\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS, mol.%\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS\u003csub\u003ecr\u003c/sub\u003e, mol.%\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eС content in the catalyst, wt%\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eX, %\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS, mol.%\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS\u003csub\u003ecr\u003c/sub\u003e, mol.%\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eС content in the catalyst, wt%\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e560\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e7.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e68.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e31.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e580\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e11. 8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e64.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e35.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e15.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e68.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e31.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.6\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e600\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e21.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e63.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e36.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e25.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e68.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e30.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.3\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e620\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e32.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e59.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e40.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e38.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e63.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e36.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4.2\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eThus, on the 5%Fe/\u0026gamma;\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sample, an increase in the reaction temperature from 560 to 580 \u0026deg;C increased isobutane conversion from 7.6 to 11. % and decreased selectivity to isobutene by 3.5 mol.%; on the contrary, selectivity to the products of cracking reactions, which are the precursors of coke deposits, increased almost by 3 mol.%. A further temperature elevation to 620 \u0026deg;C resulted in a growth of isobutane conversion to 324 % and a loss in selectivity to isobutene to 59.1 mol.%, with a simultaneous growth of selectivity to the cracking products up to 40.6 mol.%. A qualitatively similar pattern was observed also for the 5%Fe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sample.\u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eA comprehensive study of physicochemical properties of the nanostructured xFe/nanoAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e system with the iron content 0.0\u0026ndash;5.0 wt%, which was synthesized via laser vaporization using irradiation by a cw CO\u003csub\u003e2\u003c/sub\u003e laser, has been carried out. The obtained results were compared with similar data for the xFe/γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e system synthesized by the conventional sol-gel method.\u003c/p\u003e \u003cp\u003eAs a result, HRTEM data for xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e nanopowders indicate the formation of spherical nanoparticles with the diameter of ca. 9 nm and with high crystallinity. XRD analysis shows that the phase composition of xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e nanoparticles is represented predominantly by the low-temperature γ-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. The analysis of the obtained diffraction patterns revealed that for the series of xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples, in distinction to xFe/γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples (the sol-gel method), a large part of Fe\u003csup\u003e3+\u003c/sup\u003e ions is in the disordered environment (evidently, in subsurface layers of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e nanocrystallites). As the iron concentration in xFe/γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples is increased to 5 wt%, the α-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e phase is formed on the γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e surface, which is not observed for the series of xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples. According to UV-vis DRS data for the series of xFe/γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples, an increase in the iron content is accompanied by the appearance of typical absorption bands caused by stabilization of Fe\u003csup\u003e3+\u003c/sup\u003e ions in the Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e phase. The obtained pattern is preserved also in the samples of this series \u003cem\u003eeven\u003c/em\u003e after the catalytic testing, whereas for samples of the xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e series, the Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e oxide is not formed at any conditions. The analysis of PL, PLE spectra and PL decay curves for all the tested samples revealed the luminescence of Fe\u003csup\u003e3+\u003c/sup\u003e ions in the tetrahedral oxygen environment of (Fe\u003csup\u003e3\u0026thinsp;+\u003c/sup\u003e\u0026thinsp;\u003csub\u003eTd\u003c/sub\u003e) that are located in the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e matrix. For the series of xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples, the analysis of FWHM values of this PL band of Fe\u003csup\u003e3\u0026thinsp;+\u003c/sup\u003e\u0026thinsp;\u003csub\u003eTd\u003c/sub\u003e:nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e demonstrated that xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e powders contain Fe\u003csup\u003e3+\u003c/sup\u003e ions that are in the disordered environment, which is located most likely in subsurface layers of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e nanocrystallites. This is not observed for the samples of Fe/γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e series (the sol-gel method). The results of catalytic testing of the samples, particularly, the xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e series, testify to the formation of iron sites in the xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples synthesized by laser vaporization, which provide the catalytic activity of the samples. Differences in catalytic properties of the catalysts obtained by the sol-gel method and laser vaporization are based on different states of Fe\u003csup\u003e3+\u003c/sup\u003e ions described above. A comparison of catalytic activity of the samples with physicochemical data allowed us to conclude that a large amount of active Fe\u003csup\u003e3+\u003c/sup\u003e sites involved in the dehydrogenation reaction is present on the surface of nanostructured xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;The authors are grateful to Vl.N. Snytnikov for laser synthesis of nanopowders, to PhD A.A. Zhdanov for elemental analysis by XRF method and to PhD A.V. Ishchenko for HRTEM images of studied nanopowders. The authors acknowledge resource center \u0026ldquo;VTAN\u0026rdquo; (Novosibirsk State University) for the access to experimental equipment (HRTEM). The study was financially supported by the Russian Science Foundation within project No. 21-19-00429.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;Mark G. Baronskiy \u0026ndash; Conceptualization, Investigation, Writing \u0026ndash; original draft; Anton I. Kostyukov \u0026ndash; Investigation; Aleksey V. Zhuzhgov \u0026ndash; Investigation; Nadezhda A. Zaitseva \u0026ndash; Investigation, Writing \u0026ndash; original draft;\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eSvetlana V. Cherepanova \u0026ndash; Investigation, Writing \u0026ndash; original draft; Tatyana V. Larina \u0026ndash; Investigation, Writing \u0026ndash; original draft; Valeriy N. Snytnikov \u0026ndash; Conceptualization.\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eZhong Y, Yang Q, Luo K, Wu X, Li X, Liu Y, Tang W, Zeng G, Peng B (2013) Fe(II)\u0026ndash;Al(III) layered double hydroxides prepared by ultrasound-assisted co-precipitation method for the reduction of bromate. J Hazard Mater 250-251:345\u0026ndash;353. doi: 10.1016/j.jhazmat.2013.01.081 \u003c/li\u003e\n\u003cli\u003eAjouyed O, Hurel C, Ammari M, Ben Allal L, Marmiera N (2010) Sorption of Cr(VI) onto natural iron and aluminum (oxy)hydroxides: Effects of pH, ionic strength and initial concentration. 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Opt Mat 34:1553\u0026ndash;1557. doi: 10.1016/j.optmat.2012.03.029\u003c/li\u003e\n\u003cli\u003ePott GT, McNicol BD (1971) Spectroscopic study of the coordination and valence of Fe and Mn ions in and on the surface of aluminas and silicas. Discuss Faraday Soc 52:121\u0026ndash;131. doi: 10.1039/DF9715200121\u003c/li\u003e\n\u003cli\u003eKolesnikov IE, Povolotskiy AV, Mamonova DV, L\u0026auml;hderanta E, Manshina AA, Mikhailov MD (2016) Photoluminescence properties of Eu\u003csup\u003e3+\u003c/sup\u003e ions in yttrium oxide nanoparticles: defect vs. normal sites. RSC Adv 6:76533\u0026ndash;76541. doi: 10.1039/C6RA16814K\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"catalysis-surveys-from-asia","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"cats","sideBox":"Learn more about [Catalysis Surveys from Asia](http://link.springer.com/journal/10563)","snPcode":"10563","submissionUrl":"https://submission.nature.com/new-submission/10563/3","title":"Catalysis Surveys from Asia","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Nanopowders, CO2 laser vaporization, metal oxides, alumina, xFe/nano-Al2O3 systems, isobutane dehydrogenation, iron active sites","lastPublishedDoi":"10.21203/rs.3.rs-1846170/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-1846170/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eNanostructured powders xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e with the Fe loading of x\u0026thinsp;=\u0026thinsp;0,0\u0026ndash;5,0 wt% were obtained using laser vaporization by a cw CO\u003csub\u003e2\u003c/sub\u003e laser. XRF, XRD, HRTEM, PL and UV-vis DRS techniques were employed to investigate physicochemical, structural and optical properties of the synthesized nanopowders with the average particle size of 9 nm. Nanopowders xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e as model catalysts were tested in isobutane dehydrogenation reaction. The results obtained were compared with similar data for the xFe/γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e systems synthesized by the conventional sol-gel method. According to XRD and UV-vis DRS data, in the series of xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples a great part of Fe\u003csup\u003e3+\u003c/sup\u003e ions is in the disordered environment of subsurface layers of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e nanocrystallites predominantly in tetrahedral coordination. In distinction to samples of the xFe/γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e series, in the case of nanostructured xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e powders the formation of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e phase does not occur at any concentrations of iron or conditions of testing. The analysis of the PL spectra of xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e powders also showed the presence of surface sites of Fe\u003csup\u003e3+\u003c/sup\u003e ions, which were not detected for xFe/γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. Catalytic testing of the xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e series samples in isobutane dehydrogenation revealed the formation of the iron active sites that ensure catalytic activity of the samples. Differences in the catalytic properties of FeO\u003csub\u003ex\u003c/sub\u003e/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e samples obtained by the sol-gel method and laser vaporization are related to different states of Fe\u003csup\u003e3+\u003c/sup\u003e ions. Thus, the xFe/nano-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e nanopowders in contrast to xFe/γ\u003csub\u003ePb\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e contain a large amount of active Fe\u003csup\u003e3+\u003c/sup\u003e sites. These sites involved in the dehydrogenation reaction are present predominantly on the surface of the nanopowders.\u003c/p\u003e","manuscriptTitle":"Synthesis, spectroscopic and catalytic properties of FeO x /Al 2 O 3 nanopowders prepared by cw CO 2 laser vaporization","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2022-07-22 20:32:25","doi":"10.21203/rs.3.rs-1846170/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major revision","date":"2022-07-26T00:44:06+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2022-07-17T04:18:38+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"128e9b80-8ad3-4c9d-94bc-027fe16f4cb0","date":"2022-07-16T23:14:28+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2022-07-16T13:13:36+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2022-07-11T11:46:24+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2022-07-11T11:46:24+00:00","index":"","fulltext":""},{"type":"submitted","content":"Catalysis Surveys from Asia","date":"2022-07-11T10:00:53+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"catalysis-surveys-from-asia","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"cats","sideBox":"Learn more about [Catalysis Surveys from Asia](http://link.springer.com/journal/10563)","snPcode":"10563","submissionUrl":"https://submission.nature.com/new-submission/10563/3","title":"Catalysis Surveys from Asia","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"574a776b-2fe3-485f-b9c4-4cc6c91176d4","owner":[],"postedDate":"July 22nd, 2022","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2022-08-17T05:29:16+00:00","versionOfRecord":[],"versionCreatedAt":"2022-07-22 20:32:25","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-1846170","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-1846170","identity":"rs-1846170","version":["v1"]},"buildId":"FbvkV6FR0MCFSLy54lSbu","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}