Tuning the Catalytic Activity of NiO via Neodymium Doping for Selective Benzaldehyde Formation

Tuning the Catalytic Activity of NiO via Neodymium Doping for Selective Benzaldehyde Formation | 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 Tuning the Catalytic Activity of NiO via Neodymium Doping for Selective Benzaldehyde Formation V T Geetha, C Selvakumar, K Hema, V Selvarani, P Kumutha This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8694329/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 04 Apr, 2026 Read the published version in Journal of Inorganic and Organometallic Polymers and Materials → Version 1 posted 27 You are reading this latest preprint version Abstract Microwave-assisted method is used to synthesis Nd doped NiO and characterized by XRD, FTIR, SEM, TEM, BET, UV-Vis DRS, and PL analyses. XRD results confirmed the single-phase cubic structure of NiO, and the incorporation of Nd³⁺ ions into the Ni²⁺ lattice sites was clear from the peak shifts and decrease in crystallite size from 24.5 nm to 17.6 nm. The FTIR spectra revealed metal–oxygen vibrations (Ni–O, Nd–O–Ni) in the 450–900 cm⁻¹ region, while SEM and TEM images showed irregular nanoparticles with slight agglomeration at higher dopant concentrations. The optical band gap narrowed from 3.38 eV (pure NiO) to 2.15 eV (0.05 mol% Nd:NiO), indicating enhanced electronic transitions. BET analysis showed an increase in surface area from 41.31 to 55.23 m²/g upon Nd dopingUsing tert-butyl hydroperoxide (TBHP) as an oxidant, the samples' catalytic activity was examined for the conversion of styrene to benzaldehyde. The 0.04 mol% Nd:NiO catalyst exhibited maximum benzaldehyde selectivity (87.8%) and yield (46.9%) at 70°C with acetonitrile as the solvent. Recyclability tests confirmed the stability and reusability of the catalyst for multiple cycles. BET surface area Styrene oxidation Benzaldehyde yield Heterogeneous catalysis Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Introduction In the last ten years, semiconductor nanocrystals have been quite popular in a lot of different disciplines for research because they are a unique type of material with a huge surface area and adjustable optoelectronic properties [ 1 ]. Doping is still one of the most common ways to improve electrical conductivity and widen the range of semiconductors by lowering band-gap energy. Doped metal oxides with a chosen structure are vital not just to make them more useful in electronics and optoelectronics, but also to make them work better. An improvement in morphology could be observed; doped metal oxides should have a smaller approximate particle size (Bohr radius) than their bulk counterparts in terms of luminescence, optical characteristics, and particle size [ 2 ]. Among these methods, doping transition metals into oxide nanoparticles to customize them offers a promising way to adjust their optical, magnetic, and structural characteristics during catalytic reactions. Through an increase in electron transport from n-type to p-type, this doping also improves the optical, magnetic, and catalytic properties. To date, the inner transition-metal doped metal oxides have been prepared using several techniques. These are some of the ways to get the supplies ready. To prevent photo-generated electron/hole pair recombination, the dopant concentration in this case must undoubtedly be kept lower. Additionally, the location of the dopants inside the crystal arrangement must undoubtedly be regulated because they have the potential to significantly improve the structural, optical, and magnetic properties of the semiconductor [ 3 ]. The microwave approach is one of the simpler synthesis techniques for doped metal oxides and metal oxide nanopowders, which can be highly reactive at lower temperatures, and it also lowers the reaction temperature in which a homogeneous mixture of reagents precipitates. Experimental Procedure Materials and Synthesis Procedure The laboratory grade, 98% pure metal salt Mn(NO3)2.xH2O was bought from Sigma-Aldrich. The metal precursors were used without any further purification. Initially, 8.0 ml of urea (0.6M) was added dropwise to 6.0ml of Mn(NO 3 ) 2 .XH 2 O solution and stirring continued for 1h. This homogeneous mixture was placed inside a microwave oven and subjected to microwave irradiation for 10min at 1200W and microwave frequency of 2450MHz. After 10min, the black colour solid obtained, was washed with ethanol and dried at 120 o C for 1h. Similar procedure was followed using metal precursor such as Fe(NO 3 ) 3 .9H 2 O, Co(NO 3 ) 2 .6H 2 O, Ni(NO 3 ) 2 .6H 2 O for the preparation of the corresponding undoped metal oxides. Preparation of Nd Doped Metal (Ni) Oxides: Sigma-Aldrich provides the analytical grade, 98% pure metal salt of Ni (NO3)2.6H2O. No additional purification was done before using the metal precursors. In the synthesis process, 8.0 ml of 0.60M urea solution was added dropwise to 6.0 ml of 0.40M [Ni] metal nitrate hexahydrate solution while being continuously stirred. Neodymium (III) nitrate hexahydrate solution (0.01–0.05 mole %) was then added dropwise to the aforesaid solution, and stirring was maintained for 45 minutes. [ 4 ] The resultant mixed metal salt solution was placed inside an industrial microwave oven (Model number: 20SC2; manufacturer: IFB) and exposed to microwave energy for ten minutes at a frequency of 2450MHz and a power of 1200W. A solid was obtained after the allotted time, cleaned with ethanol, and dried for one hour at 120 degrees Celsius. Ni and Nd doped MO (0.01, 0.02, 0.03, 0.04, and 0.05 mole %) were the labels applied to the undoped and Nd doped metal oxide samples, respectively. XRD Analysis: The XRD and magnified XRD patterns of the undoped and Nd doped NiO shows in Figs. 1 and 2 , revealed the diffraction peaks at 2θ = 37.22, 45.3, 65.0 and 77.9 for the reflection planes of (111), (220), (220), (311). The XRD patterns matched well with the standard JCPDS card no. 47-1049 for NiO and thereby confirmed the single-phase formation of NiO. Moreover, additional peaks corresponding to the impurities or secondary phase formation in the Nd doped samples are absent. These observations indicate the successful incorporation of Nd 3+ ions into the Ni 2+ sites[ 5 ]. A slight shift in the 2θ peak positions and reduction in peak intensities were observed in the Nd doped NiO samples in comparison to the undoped NiO. This can be attributed to the incorporation of Nd 3+ with larger atomic radii, (1.123 Å) replacing Ni 3+ (0.740 Å ) in the NiO lattice resulting in distortion in symmetry of the host crystal lattice shown in Fig. 3 . Thus, in the Nd doped samples, the substitutions of Ni 3+ by Nd 3+ ions result in the lattice strain. Increase in the dopant concentration from 0.01 to 0.05 mole% increases the lattice strain at sub-grain levels and near grain boundaries. The increased presence of lattice strain inhibited the growth of the crystallites and thereby reduction in crystallites size from 24.51nm to 17.60 nm. This confirmed the successful incorporation of Nd 3+ into the NiO lattice. Fourier Transform Infrared Studies: Figure 4 illustrates the FTIR spectra of both undoped and Nd-doped NiO (0.01 to 0.05 mol %). The OH-stretching vibrations of free hydrogen-bonded hydroxyl groups contributed for the broad band observed at 3540 cm-1. The reformative vibrations of water molecules are linked to the second characteristic absorption band at 1600 cm-1. [ 6 , 7 ] This could be because moisture was absorbed during the manufacture of compact powder of undoped and Nd doped NiO with KBr pellets (Janetet al., 2008). The vibrations of Nd-O, Ni-O, and Nd-O-Ni bonds are linked to the stretching frequencies of metal-oxygen in the range of 450–900 cm-1 (Vijaya et al., 2008). The deformation mode of Nd-OH and Ni-OH, which are typical of this class of materials, is represented by the absorption at 2000 − 900 cm-1. In addition, the OH-deformation modes of hydrogen-bonded -OH are the primary reason for the strong multiple bands in the 2000 − 900 cm-1 region [ 8 ]. The presence of intercalated carbonate ions is responsible for the weak absorption band about 2400 − 2000 cm-1. According to Chenet al. (2002), the presence of this CO2-ion can be attributed to either airborne CO2 or CO2 within the sample grains. Morphology Analysis: SEM and TEM studies: The SEM analysis was carried out to find out the surface morphology and structure of the undoped and Nd doped NiO (0.01 to 0.05 mole %). The SEM images of undoped NiO and Nd doped NiO are presented (Fig. 5 ). The SEM images of the synthesized samples show irregular shaped particles. An increase in Nd concentration above 0.04mole % resulted in agglomeration, which may be a consequence of dipole-dipole interaction of the particles. TEM of Nd doped NiO (x = 0.01, 0.02, 0.03, 0.04 and 0.05 mole %) are presented (Fig. 6 ). TEM spectra exhibited peaks corresponding to Nd, Ni and O and also indicated the presence of Nd doped NiO (x = 0.01, 0.02, 0.03, 0.04 and 0.05 mole %) phase without any other impurities. It is in agreement with the XRD analysis. While increasing the Nd doping concentration, the decrease in oxygen content was noticed which confirmed the substitution of Ni 2+ by Nd 3+ ions in NiO crystal lattice? Uv-Vis DRS Studies: The band gap energies of NiO and Nd doped NiO (0.01, 0.02,0.03,0.04 and 0.05 mole %), were calculated to be 3.38 eV,3.20 eV, 2.95 eV, 2.82 eV, 2.20 eV and 2.15 eV respectively, which are in agreement with the values reported in the literature (Ragupathi et al. , 2014). In the same way, the doping levels can merge with the valence or conduction band edge and formband tail states at higher doping concentrations, which leads to the band gap narrowing. The band gap energy decreases when the doping concentration increases [ 9 , 10 ]. The decrease may be due to theconcentration of free charge carriers and impurities as Nd dopants. (Ichiyanagi et al ., 2004). The direct relationship between the grain size of the nanoparticles and optical band gap energy are given (Fig. 7 ). A decrease in the optical band gap energy with decreasing crystallites size indicates a weakquantum size effect (Azadmanjiri et al. , 2008). PL Spectra OF NiO AND Nd DOPED NiO: The Photoluminescence spectra of the pure NiO and Nd doped NiO samples (Fig. 8 ) revealed violet, blue and green emission bands at 420 nm, 450 nm − 480 nm and 520 nm − 560 nm, respectively, in the visible region. In the undoped and Nd doped NiO samples, the two broad peaks at 520 nm and 560 nm represent the green emissions which are attributed to the transitions from oxygen vacancies (donor)to the acceptor levels [ 11 , 12 ]. A similar observation was also noticed in the PL spectra of Ni doped ZnO prepared using precipitation method reported (Thangamani et al. (2016)). In addition, the green emission can be attributed to the presence of positively charged single ionized oxygen vacancy on the surface of the sample. The blue emissions observed at 450 nm and 480 nm arise due to the transition of charge from the metal captions vacancies to the valence bands (Tasumi et al. , 2004). The weak violet emission at 420 nm related to the near band edge emission which originate due to the interstitial defect centers of O, Ni and Nd. Similar results were also reported in the Gd doped zinc oxide [ 13 ]. (Sambasivam et al. , 2015).The above observation indicates that the synthesized samples can be used in green emission optoelectronic devices. However, a slight variation of intensity was observed with the increasing of the Nd dopant concentration which can be attributed to the quenching of the above radioactive transitions due to the higher concentration of defects in the Nd doped samples [ 14 ]. Specific Surface Area Measurements The catalytic activity of inorganic materials depends strongly on the particle size and morphology. The nitrogen adsorption/desorption isotherm values at 77 K of the undoped NiO and Nd doped NiO are presented (Table 1 ).Though, the surface area of Nd doped NiO samples was higher than the undoped NiO, the extent of increase in the surface area, upon increasing the concentration of Nd in the samples was low. This can be attributed to the agglomeration of very fine particles [ 15 ]. Table 1 BET surface area, average pore diameter and pore volume of (0.01, 0.02, 0.03, 0.04 and 0.05 mole %) NiO and Nd-doped NiO. Samples (mol %) BET surface area (m 2 /g) Average pore radius (Å) Total pore volume (cm 3 /g) Pure NiO (a ) 41.31 11.65 0.0802 0.01 Nd doped NiO (b) 43.41 10.64 0.0807 0.02 Nd doped NiO (c) 45.53 12.97 0.0811 0.03 Nd doped NiO (d) 49.87 13.67 0.0817 0.04 Nd doped NiO (e) 52.34 14.12 0.0819 0.05 Nd doped NiO (f) 55.23 15.78 0.0822 Catalytic Activity Studies: Benzaldehyde is a significant aromatic aldehyde that is primarily used as a precursor in the synthesis of industrially significant organic compounds. It is also utilized in the creation of adhesives and dyes, as well as in the processing of fragrances and pharmaceuticals. When toluene is oxidized, an intermediate organic molecule called benzoaldehyde is created. Both homogeneous and heterogeneous catalysis are used in the synthesis of benzaldehyde [ 16 , 17 ]. Transition metal oxides, zeolites, and supported metal oxide catalysts are frequently used in the heterogeneous catalysis technique. Heterogeneous catalysis has minimal drawbacks despite its advantages, including cost-effectiveness, eco-friendliness, and recyclability. Deactivation of the catalytically active sites on the surface and leaching of the active species into the reaction media during catalysis are two examples of these limits. In order to overcome those disadvantages, research on heterogeneous catalysts has drawn attention. The catalytic efficiency of Nd-doped NiO for the oxidation of styrene to benzaldehyde is assessed in this work. Further research on the catalyst's reuse occurred [ 18 ]. Catalytic Activity Studies of NiO and Nd Doped NiO: Benzaldehyde is an important aromatic aldehyde that is primarily used as a precursor in the synthesis of industrially significant organic compounds. It is also utilized in the creation of adhesives and dyes, as well as in the processing of fragrances and pharmaceuticals. When toluene is oxidized, an intermediate organic molecule called benzaldehyde is created. Both homogeneous and heterogeneous catalysis are used in the synthesis of benzaldehyde. Transition metal oxides, zeolites, and supported metal oxide catalysts are frequently used in the heterogeneous catalysis technique [ 19 ]. Heterogeneous catalysis has minimal drawbacks despite its benefits, which include cost-effectiveness, environmental friendliness, and recyclability. Deactivation of the catalytically active sites on the surface and leaching of the active species into the reaction media during catalysis are two examples of these restrictions. In order to overcome the aforementioned drawbacks, research on heterogeneous catalysts has drawn attention. The catalytic efficiency of Nd-doped NiO for the oxidation of styrene to benzaldehyde is assessed in this work. Additionally, the employed catalyst's recycling experiments were examined [ 20 , 21 ]. Table 2 Catalytic performance of (0.01, 0.02, 0.03, 0.04 and 0.05 mole %) NiO and Nd doped NiO, catalyst for styrene oxidation to benzaldehyde using TBHP as oxidant. Entry Catalyst Oxidant Benzaldehyde selectivity (%) Yield (%) 1 --- --- 8 01.1 2 0.03 Nd doped NiO --- 11 02.4 3 --- TBHP 23 08.2 4 Pure NiO TBHP 82 15.3 5 0.01 Nd doped NiO TBHP 83.5 18.8 6 0.02 Nd doped NiO TBHP 85.2 22.1 7 0.03 Nd doped NiO TBHP 87 29.9 8 0.04 Nd doped NiO TBHP 87.8 46.9 9 0.05 Nd doped NiO TBHP 88.7 38.4 Reaction conditions: Catalyst 125mg, styrene (reactant) − 5mmol, acetonitrile (solvent) − 10ml, reaction temperature − 70 o C and the reaction time 6h. Effect of the solvent: The solvent plays an important and sometimes decisive role in the catalytic behavior of a catalyst (Hulea et al ., 1998). As a result, the impact of the solvents on the yield of benzaldehyde and selectivity was examined and reported (Fig. 9 – 12 ). The solvents acetonitrile, DMSO, and hexane were utilized to determine an appropriate solvent for the styrene conversion process.Acetonitrile and DMSO has polarity indices of 5.8 and 7.2, respectively (Fig. 9 ). The conversion was found to be modest when compared to acetonitrile, despite the DMSO solvent having a greater polarity index [ 22 – 24 ]. This could be explained by the reactants' and the reaction's intermediates' decreased solubility [ 25 ]. When compared to acetonitrile and DMSO, hexane, a non-polar solvent, showed extremely low solubility. Because of its strong reactivity and selectivity, acetonitrile demonstrated the highest conversion, followed by DMSO and hexane. Acetonitrile has been selected as a solvent for further study. Effect of reaction time: Using undoped and Nd-doped metal oxides (NiO) as a catalyst, the impact of reaction duration on styrene oxidation was investigated. The reaction mixture was examined at several time intervals (i.e., from 1 to 7 h) in order to determine the impact of reaction time. Following a 6-hour reaction time, a decrease in the yield of benzaldehyde was noted [ 26 ]. Styrene oxide was the primary by-product of the reaction after six hours (Fig. 10 ). Further, as the reaction proceeded, styrene oxide partially hydrolyzed to produce a by-product, 1-phenyl-1, 2-ethanediol, which decreased the yield and selectivity of benzaldehyde. This could be explained by the reaction mixture's total depletion of H2O2 (Kumar et al., 1995). The reaction time was adjusted to 6 hours based on previous observation. Effect of catalyst amount : The reactions were conducted in acetonitrile (solvent) as reaction medium by varying the amount of catalyst loading (0 mg, 25 mg, 50 mg, 75 mg, 100 mg, 125 mg, and 150 mg) in the presence of H2O2 (oxidant) at 70 oC for 6 hours in order to examine the impact of catalyst amount during the conversion of styrene to benzaldehyde. For both undoped and Nd doped metal oxides (NiO) samples, it was found that the styrene oxidation grew as the catalyst amount increased. This is explained by the greater catalytic surface, which increases the number of accessible active sites. It was found that the styrene conversion in the Nd doped metal oxide samples increased when the catalytic quantity and doping levels increased from 0.01 to 0.04 mole percent. Increasing the catalytic activity is mostly dependent on the particle size and surface area [ 27 ]. Increasing the dopant concentration in the Nd-doped samples was shown to decrease particle size (based on SEM and TEM data) and increase surface area (based on BET measurement), which leads to the availability of additional active sites on the surface. Increased styrene oxidation to benzaldehyde results from this (Fig. 11 ). Furthermore, the benzaldehyde yield did not change noticeably with greater catalyst loading (beyond 125 mg), which can be explained by the lack of enough hydrogen peroxide because of its faster rate of breakdown. As a result, there was a decrease in the conversion of styrene to benzaldehyde (Ravindra et al., 2014). The best catalyst loading for conducting the subsequent reactions was determined to be 125 mg of Nd doped NiO based on the findings [ 28 ]. Effect of the oxidants: The reactions were carried out in the presence of oxidants such sodium hypochlorite (NaOCl), hydrogen peroxide (H2O2), and tert-butyl hydroperoxide (TBHP) in order to determine the best appropriate oxidant. The findings are shown in Fig. 12 . Since sodium chloride is the reaction's byproduct, sodium hypochlorite can be a good option when considering its price, availability, and the chemical processes' capacity for large-scale recycling [ 29 , 30 ]. However, the oxidant is less appropriate than hydrogen peroxide from an environmental perspective. Since water would be the only byproduct of the oxidation, TBHP and hydrogen peroxide (H2O2) are superior oxidants. Effect of the temperature: The effect of temperature on the oxidation of styrene was studied by varying the temperature from 40 o C to 80°C, while all other reaction parameters were kept constant (Fig. 13 ). The increase in the temperature resulted in C – C bond breaking which led to the higher benzaldehyde yield up to 70 o C. However, a larger C-O bond breaking than a C-C bond led to a poorer yield when the temperature was raised to 80oC. Styrene conversion utilizing a cobalt oxide-encapsulated zeolite catalyst yielded similar results (Jiangyong et al., 2018). At 70 degrees Celsius, the maximum H2O2 usage efficiency (48.4%) was likewise attained. The selectivities for styrene oxide and phenyl acetaldehyde were observed to decline with further temperature increases. Furthermore, there was a little improvement in the selectivity for other byproducts. Consequently, more thoroughly oxidized products were produced at temperatures higher than 70°C. As a result, 70°C was chosen as the ideal temperature for additional research [ 31 ]. Reaction Mechanism : A tentative mechanism for the selective catalytic oxidation of styrene to benzaldehyde over Nd doped metal oxides was proposed as illustrated (Fig. 14 ). The first path of the reaction is, initially, the solvent acetonitrile activate the oxidant H 2 O 2 and forms perhydroxyl anion (OOH − ), and forms peroxycarboximidic acid intermediate which act as a good oxygen transfer agent. Then, Styrene and hydrogen peroxide dissolves in solvent and gives the uniform solution and forms Nd-M-OOH • complex.Afterwards, theintermediate decomposes and forms the product as benzaldehyde (Selectivity of 81.6%, yield of 48.5%) and (32.6% selectivity, yield of 28.1%) of formaldehyde is formed. The second path of the reaction is, Major amount of formaldehyde is vaporised into the atmosphere and around 9% selectivity of formaldehyde is combined with hydrogen peroxide and forms peroxoic acid, which then combines with styrene and forms 22.9% selectivity of Styrene oxide is formed. Afterward, a meagre portion of styrene oxide undergo hydrolysis reaction and forms negligible amount of 1-phenyl-1,2- ethanediol. The third path of the reaction is, proceed after 6 h and 70 o C a trace amount of benzaldehyde undergo reaction and forms negligible amount of benzoic acid [ 32 ]. Recycling Studies: After the reaction was finished, the catalysts were filtered out of the reaction mixture, completely cleaned with ethanol, dried, and activated for three hours at 120°C in order to comprehend the impact of recycling. In order to conduct the succeeding runs under comparable reaction circumstances, the determined quantity of the same catalyst was utilized once more. The styrene to benzaldehyde formation was found to be comparable to the first run after three runs (Fig. 15 ). There was no discernible loss of Nd-doped NiO throughout the reaction in the recycle test, demonstrating the catalyst's structural and chemical stability and, thus, its recyclable nature [ 33 , 34 ]. Conclusion The observed shift in diffraction peaks and reduction in peak intensity with increasing Nd concentration indicated the successful incorporation of Nd³⁺ ions into the NiO lattice. The crystallite size decreased from 24.51 nm (pure NiO) to 17.60 nm (0.05 mol% Nd:NiO), suggesting that Nd doping induced lattice strain, inhibited grain growth, and enhanced defect formation within the crystal lattice. FTIR spectra revealed characteristic metal–oxygen vibrations (Ni–O, Nd–O, and Nd–O–Ni) in the range of 450–900 cm⁻¹, confirming the bonding interactions between the host and dopant ions. The morphological studies (SEM and TEM) showed that both pure and Nd-doped NiO nanoparticles exhibited irregular-shaped grains, with noticeable agglomeration at higher dopant levels, likely due to dipole–dipole interactions. The EDX spectra confirmed the elemental composition of Nd, Ni, and O without any impurities, validating the successful doping process. The BET surface area increased progressively from 41.31 m²/g for pure NiO to 55.23 m²/g for 0.05 mol% Nd:NiO, while maintaining moderate pore volume and pore diameter, indicating the formation of a mesoporous structure suitable for catalytic applications. The optical studies demonstrated a significant band gap narrowing from 3.38 eV to 2.15 eV with increasing Nd content. This reduction in band gap energy can be attributed to the formation of localized energy states and enhanced carrier concentration introduced by Nd³⁺ ions. The photoluminescence spectra exhibited blue and green emissions due to defect-related transitions, indicating that Nd doping enhanced defect density, which could facilitate catalytic and photonic processes. Catalytic activity studies revealed that Nd-doped NiO nanoparticles exhibit excellent performance for the selective oxidation of styrene to benzaldehyde using tert-butyl hydroperoxide (TBHP) as an oxidant in acetonitrile solvent. The 0.04 mol% Nd:NiO catalyst showed the highest catalytic efficiency with 87.8% selectivity and 46.9% yield of benzaldehyde under optimized conditions (125 mg catalyst, 70°C, 6 h). The enhanced catalytic performance can be ascribed to the smaller particle size, higher surface area, and increased concentration of oxygen vacancies facilitating improved adsorption and activation of reactant molecules. Furthermore, recycling studies demonstrated that the Nd-doped NiO catalyst retained its structural stability and catalytic activity even after three consecutive runs, confirming its excellent reusability and chemical robustness. The absence of significant structural changes after reuse indicates the strong metal–oxygen framework stability and resistance to leaching or deactivation. Overall, this study demonstrates that Nd doping significantly improves the physicochemical and catalytic properties of NiO nanoparticles. The developed Nd:NiO nanocatalyst is not only efficient and stable but also environmentally benign, making it a promising material for green catalytic applications, particularly for selective oxidation and other organic transformation reactions. Future studies may explore scaling up the synthesis, testing the catalyst under continuous flow systems, and extending its application to other oxidation and coupling reactions. Declarations Conflict of Interest: No conflict of interest in this work. Author Contribution V. T. Geetha conceptualized the study, designed the experiments, and supervised the overall research work.C. Selvakumar carried out the synthesis of materials, performed characterization studies, analyzed the data, and prepared the original draft of the manuscript.K. Hema contributed to the catalytic activity experiments and interpretation of reaction mechanism results.V. Selvarani assisted with data validation, graphical analysis, and review of experimental results.P. 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Mohd.Shkir, An Investigation on optical-nonlinear and optical limiting properties of CdS: an effect of Te doping concentrations for optoelectronic applications. J. Mater. Science: Mater. Electron. 30 , 17469–17480 (2019) Z.R. Khan, A.S. Alshammari, M. Shkir, V. Ganesh, S.AlFaify, Munirah, Enhancement in the photoluminescence, linear and third order nonlinear optical properties of nanostructured Na-CdS thin films for optoelectronic applications. J. Nanopart. Res., vo.22,77,(2020). https://doi.org/10.1007/s11051-020-4769-x G.E. Espinoza-Vazquez, M. Romero-Romo, Mild steel corrosion inhibition in HCl by di-alkyl and di-1,2,3-trizole derivatives of uracil and thymine. Mater. Chem. Phys. 145 (3), 407–417 (2014). https://doi.org/10.1016/j.matchemphys.2014.02.029 S.P.A. Demchuk, M.Stuke, Sub-picosecond UV laser ablation of metals. Appl. Phys. A, vol.61,pp.33–37,(1995), https://doi.org/10.1007/BF01538207 V.T.G.C.G. 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Cite Share Download PDF Status: Published Journal Publication published 04 Apr, 2026 Read the published version in Journal of Inorganic and Organometallic Polymers and Materials → Version 1 posted Editorial decision: Revision requested 08 Feb, 2026 Reviews received at journal 08 Feb, 2026 Reviews received at journal 08 Feb, 2026 Reviews received at journal 06 Feb, 2026 Reviews received at journal 06 Feb, 2026 Reviewers agreed at journal 06 Feb, 2026 Reviewers agreed at journal 06 Feb, 2026 Reviewers agreed at journal 02 Feb, 2026 Reviewers agreed at journal 01 Feb, 2026 Reviewers agreed at journal 01 Feb, 2026 Reviewers agreed at journal 01 Feb, 2026 Reviews received at journal 01 Feb, 2026 Reviewers agreed at journal 31 Jan, 2026 Reviewers agreed at journal 30 Jan, 2026 Reviewers agreed at journal 30 Jan, 2026 Reviewers agreed at journal 30 Jan, 2026 Reviewers agreed at journal 30 Jan, 2026 Reviewers agreed at journal 30 Jan, 2026 Reviewers agreed at journal 30 Jan, 2026 Reviewers agreed at journal 30 Jan, 2026 Reviewers agreed at journal 30 Jan, 2026 Reviewers agreed at journal 30 Jan, 2026 Reviewers agreed at journal 30 Jan, 2026 Reviewers invited by journal 30 Jan, 2026 Editor assigned by journal 28 Jan, 2026 Submission checks completed at journal 27 Jan, 2026 First submitted to journal 25 Jan, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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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-8694329","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":584114647,"identity":"48478bb9-03f9-40d3-a967-d5f8dd9e51be","order_by":0,"name":"V T Geetha","email":"","orcid":"","institution":"Sri Sai Ram Engineering College","correspondingAuthor":false,"prefix":"","firstName":"V","middleName":"T","lastName":"Geetha","suffix":""},{"id":584114650,"identity":"0b5b56cc-e638-45e3-9afc-4e0a9064cd60","order_by":1,"name":"C 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18:38:27","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8694329/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8694329/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s10904-026-04273-1","type":"published","date":"2026-04-04T15:58:51+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":101826948,"identity":"893868ae-1626-43ad-803b-a7b9f1123942","added_by":"auto","created_at":"2026-02-04 05:19:56","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":124816,"visible":true,"origin":"","legend":"\u003cp\u003eXRD pattern of (a) NiO\u003csub\u003e, \u003c/sub\u003e(b)(0.01 mole %) Nd doped NiO\u003csub\u003e,\u003c/sub\u003e(c)(0.02 mole %) Nd doped NiO, (d) (0.03 mole %) Nd doped NiO, (e) (0.04 mole %) Nd doped NiO, (f) (0.05 mole %) Nd doped NiO\u003csub\u003e.\u003c/sub\u003e\u003c/p\u003e","description":"","filename":"Onlinefloatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8694329/v1/de4435d43ede704b882a03c5.png"},{"id":101826949,"identity":"92c8fb50-c5f5-4919-b3be-17c6e561090f","added_by":"auto","created_at":"2026-02-04 05:19:56","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":111869,"visible":true,"origin":"","legend":"\u003cp\u003eMagnified XRD patterns at 2 θ value (34-43) of Pure and Nd doped NiO\u003c/p\u003e","description":"","filename":"Onlinefloatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8694329/v1/fb978547a58346b62646fdb9.png"},{"id":101881602,"identity":"3e9cdb67-c442-4db1-9e56-29af2e9d06b2","added_by":"auto","created_at":"2026-02-04 15:13:47","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":42174,"visible":true,"origin":"","legend":"\u003cp\u003eCrystallite size of Pure and Nd doped NiO\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8694329/v1/fae879ed4cd9efb04ade0f0e.png"},{"id":101826951,"identity":"3b13c101-4aad-45ff-aa55-b22631ee44fc","added_by":"auto","created_at":"2026-02-04 05:19:56","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":290194,"visible":true,"origin":"","legend":"\u003cp\u003eFT-IR spectra of (a) NiO\u003csub\u003e, \u003c/sub\u003e(b) (0.01 mole %) Nd doped NiO\u003csub\u003e,\u003c/sub\u003e(c)(0.02 mole %) Nd doped NiO, (d) (0.03 mole %) Nd doped NiO, (e) (0.04 mole %) Nd doped NiO, (f) (0.05 mole %) Nd doped NiO\u003csub\u003e.\u003c/sub\u003e\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8694329/v1/9db7a3d4234eb1c10dd08019.png"},{"id":101880994,"identity":"2ea89b85-b3b2-41fb-aa82-83f6695d37e0","added_by":"auto","created_at":"2026-02-04 15:08:46","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":436547,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of (a) NiO\u003csub\u003e, \u003c/sub\u003e(b)(0.01 mole %) Nd doped NiO\u003csub\u003e, \u003c/sub\u003e(c) (0.02 mole %) Nd doped NiO, (d) (0.03 mole %) Nd doped NiO, (e) (0.04 mole %) Nd doped NiO, (f) (0.05 mole %) Nd doped NiO\u003csub\u003e.\u003c/sub\u003e\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8694329/v1/e607f782b0ae266b47cff08d.png"},{"id":101826955,"identity":"35c34f29-b91e-4938-9ff1-87b66dddb40b","added_by":"auto","created_at":"2026-02-04 05:19:56","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":539981,"visible":true,"origin":"","legend":"\u003cp\u003eTEM images of (a) NiO\u003csub\u003e, \u003c/sub\u003e(b)(0.01 mole %) Nd doped NiO\u003csub\u003e,\u003c/sub\u003e(c)(0.02 mole %) Nd doped NiO, (d) (0.03 mole %) Nd doped NiO, (e) (0.04 mole %) Nd doped NiO, (f) (0.05 mole %) Nd doped NiO\u003csub\u003e.\u003c/sub\u003e\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8694329/v1/6668c271a5d4dee3da87ba45.png"},{"id":101826959,"identity":"2b5542fb-6325-48a9-94f4-aafa725f359b","added_by":"auto","created_at":"2026-02-04 05:19:56","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":102662,"visible":true,"origin":"","legend":"\u003cp\u003eCorrelation between crystallite size and bandgap of NiO and Nd doped NiO\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8694329/v1/c62bb0bb902621cc8ff770aa.png"},{"id":101881605,"identity":"168fe4e4-ce30-4bc9-b8b0-a7fc86c3d7bb","added_by":"auto","created_at":"2026-02-04 15:13:49","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":341962,"visible":true,"origin":"","legend":"\u003cp\u003ePL images of (a) NiO\u003csub\u003e, \u003c/sub\u003e(b)(0.01 mole %) Nd doped NiO\u003csub\u003e,\u003c/sub\u003e(c)(0.02 mole %) Nd doped NiO, (d) (0.03 mole %) Nd doped NiO, (e) (0.04 mole %) Nd doped NiO, (f) (0.05 mole %) Nd doped NiO\u003csub\u003e.\u003c/sub\u003e\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-8694329/v1/a69f13457a7a62d4c14cf0df.png"},{"id":101880995,"identity":"dbef29c8-9071-4e44-9c6b-f1daa2fe558b","added_by":"auto","created_at":"2026-02-04 15:08:47","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":39430,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of solvents on oxidation for styrene to benzaldehyde - undoped and Nd doped NiO\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-8694329/v1/2628b8d11c9275a5a9175b6d.png"},{"id":101826952,"identity":"ee16fe7e-a71b-47e7-910c-d8fbf62cbe3c","added_by":"auto","created_at":"2026-02-04 05:19:56","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":91736,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of reaction time on styrene oxidation to benzaldehyde using undoped and Nd doped NiO\u003c/p\u003e","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-8694329/v1/3d320e26e3b627aab13f9404.png"},{"id":101826954,"identity":"cd6b5b4a-3790-4054-9101-0723c9139541","added_by":"auto","created_at":"2026-02-04 05:19:56","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":94314,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of amount of catalyst undoped and Nd doped NiO on oxidation of styrene to benzaldehyde.\u003c/p\u003e","description":"","filename":"floatimage11.png","url":"https://assets-eu.researchsquare.com/files/rs-8694329/v1/a15ecfc2f135812d7eb57206.png"},{"id":101881749,"identity":"8206768b-6cd5-4b2c-a8b0-3ef2be612a7d","added_by":"auto","created_at":"2026-02-04 15:16:07","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":35891,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of oxidant on oxidation of styrene to benzaldehyde using undoped and Nd doped NiO\u003c/p\u003e","description":"","filename":"floatimage12.png","url":"https://assets-eu.researchsquare.com/files/rs-8694329/v1/a5938fd1679df66383f652ca.png"},{"id":101826962,"identity":"4242964c-97e6-4153-a5a8-ad89c4deef00","added_by":"auto","created_at":"2026-02-04 05:19:57","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":68522,"visible":true,"origin":"","legend":"\u003cp\u003eThe effect of temperature on oxidation of styrene to benzaldehyde using undoped and Nd doped NiO\u003c/p\u003e","description":"","filename":"floatimage13.png","url":"https://assets-eu.researchsquare.com/files/rs-8694329/v1/d299b8f177f2e87151484505.png"},{"id":101826961,"identity":"a98d7452-ea90-4ac8-b171-39328795d788","added_by":"auto","created_at":"2026-02-04 05:19:57","extension":"png","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":146446,"visible":true,"origin":"","legend":"\u003cp\u003eIllustration of Oxidation reaction\u003c/p\u003e","description":"","filename":"floatimage14.png","url":"https://assets-eu.researchsquare.com/files/rs-8694329/v1/6e4b2005bcd9f1e761820c89.png"},{"id":101826958,"identity":"33fe2ca7-9490-45f6-93fb-186787cb7be2","added_by":"auto","created_at":"2026-02-04 05:19:56","extension":"png","order_by":15,"title":"Figure 15","display":"","copyAsset":false,"role":"figure","size":28419,"visible":true,"origin":"","legend":"\u003cp\u003eRecycling Studies of Nd-doped NiO\u003c/p\u003e","description":"","filename":"floatimage15.png","url":"https://assets-eu.researchsquare.com/files/rs-8694329/v1/4db0275d3fbf5da957f3f282.png"},{"id":106343628,"identity":"6b32bd73-782f-4585-b54d-e711d216e811","added_by":"auto","created_at":"2026-04-07 16:07:52","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3275660,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8694329/v1/f56d1370-3b7d-4599-b123-e2e1116620f0.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Tuning the Catalytic Activity of NiO via Neodymium Doping for Selective Benzaldehyde Formation","fulltext":[{"header":"Introduction","content":"\u003cp\u003eIn the last ten years, semiconductor nanocrystals have been quite popular in a lot of different disciplines for research because they are a unique type of material with a huge surface area and adjustable optoelectronic properties [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Doping is still one of the most common ways to improve electrical conductivity and widen the range of semiconductors by lowering band-gap energy. Doped metal oxides with a chosen structure are vital not just to make them more useful in electronics and optoelectronics, but also to make them work better. An improvement in morphology could be observed; doped metal oxides should have a smaller approximate particle size (Bohr radius) than their bulk counterparts in terms of luminescence, optical characteristics, and particle size [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAmong these methods, doping transition metals into oxide nanoparticles to customize them offers a promising way to adjust their optical, magnetic, and structural characteristics during catalytic reactions. Through an increase in electron transport from n-type to p-type, this doping also improves the optical, magnetic, and catalytic properties. To date, the inner transition-metal doped metal oxides have been prepared using several techniques. These are some of the ways to get the supplies ready. To prevent photo-generated electron/hole pair recombination, the dopant concentration in this case must undoubtedly be kept lower. Additionally, the location of the dopants inside the crystal arrangement must undoubtedly be regulated because they have the potential to significantly improve the structural, optical, and magnetic properties of the semiconductor [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. The microwave approach is one of the simpler synthesis techniques for doped metal oxides and metal oxide nanopowders, which can be highly reactive at lower temperatures, and it also lowers the reaction temperature in which a homogeneous mixture of reagents precipitates.\u003c/p\u003e"},{"header":"Experimental Procedure","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eMaterials and Synthesis Procedure\u003c/h2\u003e \u003cp\u003eThe laboratory grade, 98% pure metal salt Mn(NO3)2.xH2O was bought from Sigma-Aldrich. The metal precursors were used without any further purification. Initially, 8.0 ml of urea (0.6M) was added dropwise to 6.0ml of Mn(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e.XH\u003csub\u003e2\u003c/sub\u003eO solution and stirring continued for 1h. This homogeneous mixture was placed inside a microwave oven and subjected to microwave irradiation for 10min at 1200W and microwave frequency of 2450MHz. After 10min, the black colour solid obtained, was washed with ethanol and dried at 120\u003csup\u003eo\u003c/sup\u003eC for 1h. Similar procedure was followed using metal precursor such as Fe(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e.9H\u003csub\u003e2\u003c/sub\u003eO, Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e.6H\u003csub\u003e2\u003c/sub\u003eO, Ni(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e.6H\u003csub\u003e2\u003c/sub\u003eO for the preparation of the corresponding undoped metal oxides.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003ePreparation of Nd Doped Metal (Ni) Oxides:\u003c/h3\u003e\n\u003cp\u003eSigma-Aldrich provides the analytical grade, 98% pure metal salt of Ni (NO3)2.6H2O. No additional purification was done before using the metal precursors. In the synthesis process, 8.0 ml of 0.60M urea solution was added dropwise to 6.0 ml of 0.40M [Ni] metal nitrate hexahydrate solution while being continuously stirred. Neodymium (III) nitrate hexahydrate solution (0.01\u0026ndash;0.05 mole %) was then added dropwise to the aforesaid solution, and stirring was maintained for 45 minutes. [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/p\u003e \u003cp\u003eThe resultant mixed metal salt solution was placed inside an industrial microwave oven (Model number: 20SC2; manufacturer: IFB) and exposed to microwave energy for ten minutes at a frequency of 2450MHz and a power of 1200W. A solid was obtained after the allotted time, cleaned with ethanol, and dried for one hour at 120 degrees Celsius. Ni and Nd doped MO (0.01, 0.02, 0.03, 0.04, and 0.05 mole %) were the labels applied to the undoped and Nd doped metal oxide samples, respectively.\u003c/p\u003e\n\u003ch3\u003eXRD Analysis:\u003c/h3\u003e\n\u003cp\u003eThe XRD and magnified XRD patterns of the undoped and Nd doped NiO shows in Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, revealed the diffraction peaks at 2θ\u0026thinsp;=\u0026thinsp;37.22, 45.3, 65.0 and 77.9 for the reflection planes of (111), (220), (220), (311). The XRD patterns matched well with the standard JCPDS card no. 47-1049 for NiO and thereby confirmed the single-phase formation of NiO. Moreover, additional peaks corresponding to the impurities or secondary phase formation in the Nd doped samples are absent. These observations indicate the successful incorporation of Nd\u003csup\u003e3+\u003c/sup\u003e ions into the Ni\u003csup\u003e2+\u003c/sup\u003e sites[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. A slight shift in the 2θ peak positions and reduction in peak intensities were observed in the Nd doped NiO samples in comparison to the undoped NiO. This can be attributed to the incorporation of Nd\u003csup\u003e3+\u003c/sup\u003e with larger atomic radii, (1.123 \u0026Aring;) replacing Ni\u003csup\u003e3+\u003c/sup\u003e(0.740\u003cb\u003e\u0026Aring;\u003c/b\u003e) in the NiO lattice resulting in distortion in symmetry of the host crystal lattice shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eThus, in the Nd doped samples, the substitutions of Ni\u003csup\u003e3+\u003c/sup\u003eby Nd\u003csup\u003e3+\u003c/sup\u003e ions result in the lattice strain. Increase in the dopant concentration from 0.01 to 0.05 mole% increases the lattice strain at sub-grain levels and near grain boundaries. The increased presence of lattice strain inhibited the growth of the crystallites and thereby reduction in crystallites size from 24.51nm to 17.60 nm. This confirmed the successful incorporation of Nd\u003csup\u003e3+\u003c/sup\u003e into the NiO lattice.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eFourier Transform Infrared Studies:\u003c/h3\u003e\n\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e illustrates the FTIR spectra of both undoped and Nd-doped NiO (0.01 to 0.05 mol %). The OH-stretching vibrations of free hydrogen-bonded hydroxyl groups contributed for the broad band observed at 3540 cm-1. The reformative vibrations of water molecules are linked to the second characteristic absorption band at 1600 cm-1. [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e] This could be because moisture was absorbed during the manufacture of compact powder of undoped and Nd doped NiO with KBr pellets (Janetet al., 2008).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe vibrations of Nd-O, Ni-O, and Nd-O-Ni bonds are linked to the stretching frequencies of metal-oxygen in the range of 450\u0026ndash;900 cm-1 (Vijaya et al., 2008). The deformation mode of Nd-OH and Ni-OH, which are typical of this class of materials, is represented by the absorption at 2000\u0026thinsp;\u0026minus;\u0026thinsp;900 cm-1. In addition, the OH-deformation modes of hydrogen-bonded -OH are the primary reason for the strong multiple bands in the 2000\u0026thinsp;\u0026minus;\u0026thinsp;900 cm-1 region [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. The presence of intercalated carbonate ions is responsible for the weak absorption band about 2400\u0026thinsp;\u0026minus;\u0026thinsp;2000 cm-1. According to Chenet al. (2002), the presence of this CO2-ion can be attributed to either airborne CO2 or CO2 within the sample grains.\u003c/p\u003e\n\u003ch3\u003eMorphology Analysis:\u003c/h3\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eSEM and TEM studies:\u003c/h2\u003e \u003cp\u003eThe SEM analysis was carried out to find out the surface morphology and structure of the undoped and Nd doped NiO (0.01 to 0.05 mole %). The SEM images of undoped NiO and Nd doped NiO are presented (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). The SEM images of the synthesized samples show irregular shaped particles. An increase in Nd concentration above 0.04mole % resulted in agglomeration, which may be a consequence of dipole-dipole interaction of the particles.\u003c/p\u003e \u003cp\u003eTEM of Nd doped NiO (x\u0026thinsp;=\u0026thinsp;0.01, 0.02, 0.03, 0.04 and 0.05 mole %) are presented (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). TEM spectra exhibited peaks corresponding to Nd, Ni and O and also indicated the presence of Nd doped NiO (x\u0026thinsp;=\u0026thinsp;0.01, 0.02, 0.03, 0.04 and 0.05 mole %) phase without any other impurities. It is in agreement with the XRD analysis. While increasing the Nd doping concentration, the decrease in oxygen content was noticed which confirmed the substitution of Ni\u003csup\u003e2+\u003c/sup\u003e by Nd\u003csup\u003e3+\u003c/sup\u003eions in NiO crystal lattice?\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eUv-Vis DRS Studies:\u003c/h3\u003e\n\u003cp\u003eThe band gap energies of NiO and Nd doped NiO (0.01, 0.02,0.03,0.04 and 0.05 mole %), were calculated to be 3.38 eV,3.20 eV, 2.95 eV, 2.82 eV, 2.20 eV and 2.15 eV respectively, which are in agreement with the values reported in the literature (Ragupathi\u003cem\u003eet al.\u003c/em\u003e, 2014). In the same way, the doping levels can merge with the valence or conduction band edge and formband tail states at higher doping concentrations, which leads to the band gap narrowing. The band gap energy decreases when the doping concentration increases [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. The decrease may be due to theconcentration of free charge carriers and impurities as Nd dopants. (Ichiyanagi\u003cem\u003eet al\u003c/em\u003e., 2004). The direct relationship between the grain size of the nanoparticles and optical band gap energy are given (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). A decrease in the optical band gap energy with decreasing crystallites size indicates a weakquantum size effect (Azadmanjiri\u003cem\u003eet al.\u003c/em\u003e, 2008).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003ePL Spectra OF NiO AND Nd DOPED NiO:\u003c/h3\u003e\n\u003cp\u003eThe Photoluminescence spectra of the pure NiO and Nd doped NiO samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e) revealed violet, blue and green emission bands at 420 nm, 450 nm\u0026thinsp;\u0026minus;\u0026thinsp;480 nm and 520 nm\u0026thinsp;\u0026minus;\u0026thinsp;560 nm, respectively, in the visible region. In the undoped and Nd doped NiO samples, the two broad peaks at 520 nm and 560 nm represent the green emissions which are attributed to the transitions from oxygen vacancies (donor)to the acceptor levels [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. A similar observation was also noticed in the PL spectra of Ni doped ZnO prepared using precipitation method reported (Thangamani\u003cem\u003eet al.\u003c/em\u003e (2016)). In addition, the green emission can be attributed to the presence of positively charged single ionized oxygen vacancy on the surface of the sample. The blue emissions observed at 450 nm and 480 nm arise due to the transition of charge from the metal captions vacancies to the valence bands (Tasumi \u003cem\u003eet al.\u003c/em\u003e, 2004). The weak violet emission at 420 nm related to the near band edge emission which originate due to the interstitial defect centers of O, Ni and Nd. Similar results were also reported in the Gd doped zinc oxide [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. (Sambasivam \u003cem\u003eet al.\u003c/em\u003e, 2015).The above observation indicates that the synthesized samples can be used in green emission optoelectronic devices. However, a slight variation of intensity was observed with the increasing of the Nd dopant concentration which can be attributed to the quenching of the above radioactive transitions due to the higher concentration of defects in the Nd doped samples [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e\u003cb\u003eSpecific Surface Area Measurements\u003c/b\u003e\u003c/h2\u003e \u003cp\u003eThe catalytic activity of inorganic materials depends strongly on the particle size and morphology. The nitrogen adsorption/desorption isotherm values at 77 K of the undoped NiO and Nd doped NiO are presented (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).Though, the surface area of Nd doped NiO samples was higher than the undoped NiO, the extent of increase in the surface area, upon increasing the concentration of Nd in the samples was low. This can be attributed to the agglomeration of very fine particles [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eBET surface area, average pore diameter and pore volume of (0.01, 0.02, 0.03, 0.04 and 0.05 mole %) NiO and Nd-doped NiO.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSamples (mol %)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBET surface area (m\u003csup\u003e2\u003c/sup\u003e/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAverage pore radius (\u0026Aring;)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eTotal pore volume (cm\u003csup\u003e3\u003c/sup\u003e/g)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePure NiO (a )\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e41.31\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e11.65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.0802\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0.01 Nd doped NiO (b)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e43.41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e10.64\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.0807\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0.02 Nd doped NiO (c)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e45.53\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e12.97\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.0811\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0.03 Nd doped NiO (d)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e49.87\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e13.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.0817\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0.04 Nd doped NiO (e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e52.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e14.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.0819\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0.05 Nd doped NiO (f)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e55.23\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e15.78\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.0822\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eCatalytic Activity Studies:\u003c/h2\u003e \u003cp\u003eBenzaldehyde is a significant aromatic aldehyde that is primarily used as a precursor in the synthesis of industrially significant organic compounds. It is also utilized in the creation of adhesives and dyes, as well as in the processing of fragrances and pharmaceuticals. When toluene is oxidized, an intermediate organic molecule called benzoaldehyde is created. Both homogeneous and heterogeneous catalysis are used in the synthesis of benzaldehyde [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Transition metal oxides, zeolites, and supported metal oxide catalysts are frequently used in the heterogeneous catalysis technique. Heterogeneous catalysis has minimal drawbacks despite its advantages, including cost-effectiveness, eco-friendliness, and recyclability.\u003c/p\u003e \u003cp\u003eDeactivation of the catalytically active sites on the surface and leaching of the active species into the reaction media during catalysis are two examples of these limits. In order to overcome those disadvantages, research on heterogeneous catalysts has drawn attention. The catalytic efficiency of Nd-doped NiO for the oxidation of styrene to benzaldehyde is assessed in this work. Further research on the catalyst's reuse occurred [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eCatalytic Activity Studies of NiO and Nd Doped NiO:\u003c/h2\u003e \u003cp\u003eBenzaldehyde is an important aromatic aldehyde that is primarily used as a precursor in the synthesis of industrially significant organic compounds. It is also utilized in the creation of adhesives and dyes, as well as in the processing of fragrances and pharmaceuticals. When toluene is oxidized, an intermediate organic molecule called benzaldehyde is created. Both homogeneous and heterogeneous catalysis are used in the synthesis of benzaldehyde. Transition metal oxides, zeolites, and supported metal oxide catalysts are frequently used in the heterogeneous catalysis technique [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Heterogeneous catalysis has minimal drawbacks despite its benefits, which include cost-effectiveness, environmental friendliness, and recyclability.\u003c/p\u003e \u003cp\u003eDeactivation of the catalytically active sites on the surface and leaching of the active species into the reaction media during catalysis are two examples of these restrictions. In order to overcome the aforementioned drawbacks, research on heterogeneous catalysts has drawn attention. The catalytic efficiency of Nd-doped NiO for the oxidation of styrene to benzaldehyde is assessed in this work. Additionally, the employed catalyst's recycling experiments were examined [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCatalytic performance of (0.01, 0.02, 0.03, 0.04 and 0.05 mole %) NiO and Nd doped NiO, catalyst for styrene oxidation to benzaldehyde using TBHP as oxidant.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEntry\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCatalyst\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eOxidant\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBenzaldehyde selectivity (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eYield (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e---\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e---\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e01.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.03 Nd doped NiO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e---\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e02.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e---\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTBHP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e23\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e08.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePure NiO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTBHP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e82\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e15.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.01 Nd doped NiO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTBHP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e83.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e18.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.02 Nd doped NiO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTBHP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e85.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e22.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.03 Nd doped NiO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTBHP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e87\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e29.9\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.04 Nd doped NiO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTBHP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e87.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e46.9\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.05 Nd doped NiO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTBHP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e88.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e38.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eReaction conditions:\u003c/h2\u003e \u003cp\u003eCatalyst 125mg, styrene (reactant) \u0026minus;\u0026thinsp;5mmol, acetonitrile (solvent) \u0026minus;\u0026thinsp;10ml, reaction temperature \u0026minus;\u0026thinsp;70\u003csup\u003eo\u003c/sup\u003eC and the reaction time 6h. Effect of the solvent: The solvent plays an important and sometimes decisive role in the catalytic behavior of a catalyst (Hulea \u003cem\u003eet al\u003c/em\u003e., 1998). As a result, the impact of the solvents on the yield of benzaldehyde and selectivity was examined and reported (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e). The solvents acetonitrile, DMSO, and hexane were utilized to determine an appropriate solvent for the styrene conversion process.Acetonitrile and DMSO has polarity indices of 5.8 and 7.2, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). The conversion was found to be modest when compared to acetonitrile, despite the DMSO solvent having a greater polarity index [\u003cspan additionalcitationids=\"CR23\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. This could be explained by the reactants' and the reaction's intermediates' decreased solubility [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. When compared to acetonitrile and DMSO, hexane, a non-polar solvent, showed extremely low solubility. Because of its strong reactivity and selectivity, acetonitrile demonstrated the highest conversion, followed by DMSO and hexane. Acetonitrile has been selected as a solvent for further study.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003eEffect of reaction time:\u003c/h2\u003e \u003cp\u003eUsing undoped and Nd-doped metal oxides (NiO) as a catalyst, the impact of reaction duration on styrene oxidation was investigated. The reaction mixture was examined at several time intervals (i.e., from 1 to 7 h) in order to determine the impact of reaction time. Following a 6-hour reaction time, a decrease in the yield of benzaldehyde was noted [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Styrene oxide was the primary by-product of the reaction after six hours (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e). Further, as the reaction proceeded, styrene oxide partially hydrolyzed to produce a by-product, 1-phenyl-1, 2-ethanediol, which decreased the yield and selectivity of benzaldehyde. This could be explained by the reaction mixture's total depletion of H2O2 (Kumar et al., 1995). The reaction time was adjusted to 6 hours based on previous observation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003e\u003cb\u003eEffect of catalyst amount\u003c/b\u003e:\u003c/h2\u003e \u003cp\u003eThe reactions were conducted in acetonitrile (solvent) as reaction medium by varying the amount of catalyst loading (0 mg, 25 mg, 50 mg, 75 mg, 100 mg, 125 mg, and 150 mg) in the presence of H2O2 (oxidant) at 70 oC for 6 hours in order to examine the impact of catalyst amount during the conversion of styrene to benzaldehyde. For both undoped and Nd doped metal oxides (NiO) samples, it was found that the styrene oxidation grew as the catalyst amount increased. This is explained by the greater catalytic surface, which increases the number of accessible active sites. It was found that the styrene conversion in the Nd doped metal oxide samples increased when the catalytic quantity and doping levels increased from 0.01 to 0.04 mole percent. Increasing the catalytic activity is mostly dependent on the particle size and surface area [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIncreasing the dopant concentration in the Nd-doped samples was shown to decrease particle size (based on SEM and TEM data) and increase surface area (based on BET measurement), which leads to the availability of additional active sites on the surface. Increased styrene oxidation to benzaldehyde results from this (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e). Furthermore, the benzaldehyde yield did not change noticeably with greater catalyst loading (beyond 125 mg), which can be explained by the lack of enough hydrogen peroxide because of its faster rate of breakdown. As a result, there was a decrease in the conversion of styrene to benzaldehyde (Ravindra et al., 2014). The best catalyst loading for conducting the subsequent reactions was determined to be 125 mg of Nd doped NiO based on the findings [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section2\"\u003e \u003ch2\u003eEffect of the oxidants:\u003c/h2\u003e \u003cp\u003eThe reactions were carried out in the presence of oxidants such sodium hypochlorite (NaOCl), hydrogen peroxide (H2O2), and tert-butyl hydroperoxide (TBHP) in order to determine the best appropriate oxidant. The findings are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e. Since sodium chloride is the reaction's byproduct, sodium hypochlorite can be a good option when considering its price, availability, and the chemical processes' capacity for large-scale recycling [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. However, the oxidant is less appropriate than hydrogen peroxide from an environmental perspective. Since water would be the only byproduct of the oxidation, TBHP and hydrogen peroxide (H2O2) are superior oxidants.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec18\" class=\"Section2\"\u003e \u003ch2\u003eEffect of the temperature:\u003c/h2\u003e \u003cp\u003eThe effect of temperature on the oxidation of styrene was studied by varying the temperature from 40\u003csup\u003eo\u003c/sup\u003eC to 80\u0026deg;C, while all other reaction parameters were kept constant (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e). The increase in the temperature resulted in C \u0026ndash; C bond breaking which led to the higher benzaldehyde yield up to 70\u003csup\u003eo\u003c/sup\u003eC. However, a larger C-O bond breaking than a C-C bond led to a poorer yield when the temperature was raised to 80oC. Styrene conversion utilizing a cobalt oxide-encapsulated zeolite catalyst yielded similar results (Jiangyong et al., 2018). At 70 degrees Celsius, the maximum H2O2 usage efficiency (48.4%) was likewise attained. The selectivities for styrene oxide and phenyl acetaldehyde were observed to decline with further temperature increases. Furthermore, there was a little improvement in the selectivity for other byproducts. Consequently, more thoroughly oxidized products were produced at temperatures higher than 70\u0026deg;C. As a result, 70\u0026deg;C was chosen as the ideal temperature for additional research [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec19\" class=\"Section2\"\u003e \u003ch2\u003e\u003cb\u003eReaction Mechanism\u003c/b\u003e:\u003c/h2\u003e \u003cp\u003eA tentative mechanism for the selective catalytic oxidation of styrene to benzaldehyde over Nd doped metal oxides was proposed as illustrated (Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e14\u003c/span\u003e). The first path of the reaction is, initially, the solvent acetonitrile activate the oxidant H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e and forms perhydroxyl anion (OOH\u003csup\u003e\u0026minus;\u003c/sup\u003e), and forms peroxycarboximidic acid intermediate which act as a good oxygen transfer agent. Then, Styrene and hydrogen peroxide dissolves in solvent and gives the uniform solution and forms Nd-M-OOH\u003csup\u003e\u0026bull;\u003c/sup\u003e complex.Afterwards, theintermediate decomposes and forms the product as benzaldehyde (Selectivity of 81.6%, yield of 48.5%) and (32.6% selectivity, yield of 28.1%) of formaldehyde is formed.\u003c/p\u003e \u003cp\u003eThe second path of the reaction is, Major amount of formaldehyde is vaporised into the atmosphere and around 9% selectivity of formaldehyde is combined with hydrogen peroxide and forms peroxoic acid, which then combines with styrene and forms 22.9% selectivity of Styrene oxide is formed. Afterward, a meagre portion of styrene oxide undergo hydrolysis reaction and forms negligible amount of 1-phenyl-1,2- ethanediol. The third path of the reaction is, proceed after 6 h and 70\u003csup\u003eo\u003c/sup\u003eC a trace amount of benzaldehyde undergo reaction and forms negligible amount of benzoic acid [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec20\" class=\"Section2\"\u003e \u003ch2\u003eRecycling Studies:\u003c/h2\u003e \u003cp\u003eAfter the reaction was finished, the catalysts were filtered out of the reaction mixture, completely cleaned with ethanol, dried, and activated for three hours at 120\u0026deg;C in order to comprehend the impact of recycling. In order to conduct the succeeding runs under comparable reaction circumstances, the determined quantity of the same catalyst was utilized once more. The styrene to benzaldehyde formation was found to be comparable to the first run after three runs (Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e15\u003c/span\u003e). There was no discernible loss of Nd-doped NiO throughout the reaction in the recycle test, demonstrating the catalyst's structural and chemical stability and, thus, its recyclable nature [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThe observed shift in diffraction peaks and reduction in peak intensity with increasing Nd concentration indicated the successful incorporation of Nd\u0026sup3;⁺ ions into the NiO lattice. The crystallite size decreased from 24.51 nm (pure NiO) to 17.60 nm (0.05 mol% Nd:NiO), suggesting that Nd doping induced lattice strain, inhibited grain growth, and enhanced defect formation within the crystal lattice. FTIR spectra revealed characteristic metal\u0026ndash;oxygen vibrations (Ni\u0026ndash;O, Nd\u0026ndash;O, and Nd\u0026ndash;O\u0026ndash;Ni) in the range of 450\u0026ndash;900 cm⁻\u0026sup1;, confirming the bonding interactions between the host and dopant ions.\u003c/p\u003e \u003cp\u003eThe morphological studies (SEM and TEM) showed that both pure and Nd-doped NiO nanoparticles exhibited irregular-shaped grains, with noticeable agglomeration at higher dopant levels, likely due to dipole\u0026ndash;dipole interactions. The EDX spectra confirmed the elemental composition of Nd, Ni, and O without any impurities, validating the successful doping process. The BET surface area increased progressively from 41.31 m\u0026sup2;/g for pure NiO \u003cb\u003eto\u003c/b\u003e 55.23 m\u0026sup2;/g for 0.05 mol% Nd:NiO, while maintaining moderate pore volume and pore diameter, indicating the formation of a mesoporous structure suitable for catalytic applications. The optical studies demonstrated a significant band gap narrowing from 3.38 eV to 2.15 eV with increasing Nd content. This reduction in band gap energy can be attributed to the formation of localized energy states and enhanced carrier concentration introduced by Nd\u0026sup3;⁺ ions. The photoluminescence spectra exhibited blue and green emissions due to defect-related transitions, indicating that Nd doping enhanced defect density, which could facilitate catalytic and photonic processes.\u003c/p\u003e \u003cp\u003eCatalytic activity studies revealed that Nd-doped NiO nanoparticles exhibit excellent performance for the selective oxidation of styrene to benzaldehyde using tert-butyl hydroperoxide (TBHP) as an oxidant in acetonitrile solvent. The 0.04 mol% Nd:NiO catalyst showed the highest catalytic efficiency with 87.8% selectivity and 46.9% yield of benzaldehyde under optimized conditions (125 mg catalyst, 70\u0026deg;C, 6 h). The enhanced catalytic performance can be ascribed to the smaller particle size, higher surface area, and increased concentration of oxygen vacancies facilitating improved adsorption and activation of reactant molecules. Furthermore, recycling studies demonstrated that the Nd-doped NiO catalyst retained its structural stability and catalytic activity even after three consecutive runs, confirming its excellent reusability and chemical robustness. The absence of significant structural changes after reuse indicates the strong metal\u0026ndash;oxygen framework stability and resistance to leaching or deactivation. Overall, this study demonstrates that Nd doping significantly improves the physicochemical and catalytic properties of NiO nanoparticles. The developed Nd:NiO nanocatalyst is not only efficient and stable but also environmentally benign, making it a promising material for green catalytic applications, particularly for selective oxidation and other organic transformation reactions. Future studies may explore scaling up the synthesis, testing the catalyst under continuous flow systems, and extending its application to other oxidation and coupling reactions.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eConflict of Interest:\u003c/h2\u003e \u003cp\u003eNo conflict of interest in this work.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eV. T. Geetha conceptualized the study, designed the experiments, and supervised the overall research work.C. Selvakumar carried out the synthesis of materials, performed characterization studies, analyzed the data, and prepared the original draft of the manuscript.K. Hema contributed to the catalytic activity experiments and interpretation of reaction mechanism results.V. Selvarani assisted with data validation, graphical analysis, and review of experimental results.P. Kumutha contributed to manuscript editing, revision, and final proofreading.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eA. Raghavendra Bairy, S.D. Jayarama, M.S. Kulkarni, H. Murari, Vijeth, Improved non-linear optical absorption mechanism and susceptibility (χ\u003csup\u003e(3)\u003c/sup\u003e) of CdS nanostructured thin films: Role of Zinc doping, Material Science in Semiconductor Processing, \u003cb\u003e121\u003c/b\u003e, jan2021, 105400. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.mssp.2020.105400\u003c/span\u003e\u003cspan address=\"10.1016/j.mssp.2020.105400\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eC. Selvakumar, V.T. Geetha, S.Sathiyamoorthi and, R. Sharan, Third \u0026ndash;order NLO study of manganese-doped copper sulphide nanoparticles for optical evolution. J. 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Laser Technol. \u003cb\u003e89\u003c/b\u003e, 179\u0026ndash;185 (2017). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.optlastec.2016.10.006\u003c/span\u003e\u003cspan address=\"10.1016/j.optlastec.2016.10.006\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\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":"journal-of-inorganic-and-organometallic-polymers-and-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"joip","sideBox":"Learn more about [Journal of Inorganic and Organometallic Polymers and Materials](https://www.springer.com/journal/10904)","snPcode":"10904","submissionUrl":"https://submission.nature.com/new-submission/10904/3","title":"Journal of Inorganic and Organometallic Polymers and Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"BET surface area, Styrene oxidation, Benzaldehyde yield, Heterogeneous catalysis","lastPublishedDoi":"10.21203/rs.3.rs-8694329/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8694329/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eMicrowave-assisted method is used to synthesis Nd doped NiO and characterized by XRD, FTIR, SEM, TEM, BET, UV-Vis DRS, and PL analyses. XRD results confirmed the single-phase cubic structure of NiO, and the incorporation of Nd\u0026sup3;⁺ ions into the Ni\u0026sup2;⁺ lattice sites was clear from the peak shifts and decrease in crystallite size from 24.5 nm to 17.6 nm. The FTIR spectra revealed metal\u0026ndash;oxygen vibrations (Ni\u0026ndash;O, Nd\u0026ndash;O\u0026ndash;Ni) in the 450\u0026ndash;900 cm⁻\u0026sup1; region, while SEM and TEM images showed irregular nanoparticles with slight agglomeration at higher dopant concentrations. The optical band gap narrowed from 3.38 eV (pure NiO) to 2.15 eV (0.05 mol% Nd:NiO), indicating enhanced electronic transitions. BET analysis showed an increase in surface area from 41.31 to 55.23 m\u0026sup2;/g upon Nd dopingUsing tert-butyl hydroperoxide (TBHP) as an oxidant, the samples' catalytic activity was examined for the conversion of styrene to benzaldehyde. The 0.04 mol% Nd:NiO catalyst exhibited maximum benzaldehyde selectivity (87.8%) and yield (46.9%) at 70\u0026deg;C with acetonitrile as the solvent. Recyclability tests confirmed the stability and reusability of the catalyst for multiple cycles.\u003c/p\u003e","manuscriptTitle":"Tuning the Catalytic Activity of NiO via Neodymium Doping for Selective Benzaldehyde Formation","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-04 05:19:46","doi":"10.21203/rs.3.rs-8694329/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision 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