Nitrogen-doped Carbon-coated Al 2 O 3 supported Co 3 O 4 nanoparticles for Fischer-Tropsch synthesis: Boosting durability and Olefins selectivity

Nitrogen-doped Carbon-coated Al 2 O 3 supported Co 3 O 4 nanoparticles for Fischer-Tropsch synthesis: Boosting durability and Olefins selectivity | 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 Nitrogen-doped Carbon-coated Al 2 O 3 supported Co 3 O 4 nanoparticles for Fischer-Tropsch synthesis: Boosting durability and Olefins selectivity Lili Wu, Shuying Ren, Chengchao Liu This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7767967/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 12 You are reading this latest preprint version Abstract Alkali metal modification of cobalt-based Fischer-Tropsch synthesis catalysts suffers from poor stability and decreased activity. Exploring new strategies to enhance olefin selectivity and catalyst stability is of significant importance. In this work, a nitrogen-doped carbon layer-modified Co/NC-Al 2 O 3 catalyst was prepared using oleylamine as the nitrogen source and glucose as the carbon source. Under conditions of 230 ℃, 1 MPa, and 6 SL·g − 1 ·h − 1 , the Co/NC-Al 2 O 3 catalyst achieved an activity of 4.2×10 − 5 molCO·gCo − 1 s − 1 with an olefin selectivity of 34.4%, while exhibiting high stability. Characterization and performance results demonstrate that: the electron-donating effect of pyrrolic nitrogen enriches electrons in adjacent metallic Co, enhancing CO adsorption and activation while inhibiting secondary hydrogenation of α-olefins, thereby increasing olefin selectivity; The improved stability originates from the confinement effect of the carbon layer and the stabilizing role of graphitic nitrogen on Co nanoparticles. Fischer-Tropsch synthesis Co-Based Catalysts Nitrogen-doped olefin selectivity Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1 Introduction Fischer-Tropsch synthesis (FTS) is a pivotal technology for catalytically converting syngas (H 2 /CO) derived from coal, natural gas, or biomass into clean fuels and high-value chemicals, playing a critical role in energy structure transformation and carbon neutrality strategies.[ 1 – 3 ] Facing the dual challenges of global crude oil depletion and surging demand for clean chemicals, developing efficient and economical FTS catalysts and processes is imperative.[ 4 ] Cobalt-based catalysts have garnered significant attention due to their excellent intrinsic activity for CO hydrogenation, high selectivity toward heavy hydrocarbons, and low water-gas shift (WGS) activity. Their products typically consist predominantly of linear alkanes, exhibiting low olefin selectivity.[ 4 – 6 ] Olefins serve as fundamental building blocks ("cornerstones") of the chemical industry, indispensable from basic raw material production to high-value chemical manufacturing, with their economic value substantially exceeding that of alkanes. Developing novel catalysts to achieve high olefin selectivity holds significant economic importance. Among the reported strategies for enhancing olefin selectivity, the addition of alkaline promoters (e.g., K, Na) is widely adopted.[ 7 – 9 ] Eliseev et al.[ 8 ] observed that K-modified Co/Al 2 O 3 catalysts significantly increased CO adsorption capacity, leading to higher olefin content in products but reduced catalytic activity. The Xie team[ 10 ] employed Na/S co-modification on an hcp Co catalyst, achieving 54% lower olefin selectivity and 17% CH 4 selectivity at 1% CO conversion. This behavior was attributed to the electronic promoter effect weakening H 2 dissociation capability on the Co surface. Researchers propose that the electronic promoter effect of alkali metals alters the surface charge distribution of active sites,[ 11 ] suppresses hydrogen adsorption,[ 12 ] enhances CO adsorption, and promotes α-olefin desorption,[ 12 , 13 ] thereby improving catalyst olefin selectivity. Although alkali metal promotion effectively enhances olefin selectivity, it often reduces catalyst activity. Furthermore, gradual loss of alkali metals during reaction compromises selectivity stability. Consequently, exploring strategies that simultaneously enhance olefin selectivity while avoiding promoter loss and activity decline remains a key focus of current research. In recent years, strategies utilizing nitrogen-doped carbon materials as supports for metal catalysts have attracted research attention. Nitrogen-doped carbon materials can act as electron donors or acceptors to modulate the electronic structure and chemical activity of metal catalysts. Lu et al.[ 14 ] demonstrated that the anchoring effect and intrinsic basicity of nitrogen-doped carbon nanotubes (NCNTs) enabled convenient immobilization of Fe nanoparticles onto NCNTs via simple impregnation. The constructed Fe/NCNTs catalyst achieved high lower olefin (C2 = -C4 = ) selectivity up to 46.7% while exhibiting excellent stability. Rausch et al.[ 15 ] reported that co-feeding nitrogen-containing species (e.g., NH 3 , CH 3 CN) with syngas enhanced α-olefin selectivity over a 15 wt% Co/SiO 2 catalyst. This improvement was ascribed to the electron-donating effect of nitrogen species, which increased surface basicity and suppressed secondary hydrogenation of olefins. However, introducing nitrogen species into the syngas feed can induce irreversible catalyst deactivation. Our group constructed Al 2 O 3 @C-X supports with carbon layers of tailored surface properties by modulating glucose carbonization conditions on Al 2 O 3 . High-temperature carbonized layers effectively weakened the strong metal-support interaction (SMSI) between Co and Al 2 O 3 , promoted the reducibility of cobalt species, and significantly enhanced C 5+ selectivity.[ 16 ] Carbon nitride layers (TiO 2 @xCN) were formed on TiO 2 via urea pyrolysis followed by hydrothermal treatment and calcination. The interaction between carbon nitride and cobalt improved cobalt dispersion, modified electron transfer behavior between Co and TiO 2 , and inhibited TiO 2 encapsulation of Co nanoparticles during reaction. The interaction between carbon nitride and cobalt improved cobalt dispersion, modified electron transfer behavior between Co and TiO 2 , and inhibited TiO 2 encapsulation of Co nanoparticles during reaction.[ 17 ] These findings highlight the substantial potential of nitrogen as an electron donor/alkaline source in regulating both FTS stability and product selectivity. This study aims to investigate the effects of nitrogen-doped carbon layers on the product selectivity and stability of cobalt-based catalysts supported on alumina in Fischer-Tropsch synthesis. The Co/Al 2 O 3 reference catalyst was prepared by an ultrasound-assisted method; the Co/NC-Al 2 O 3 catalyst was synthesized using glucose as the carbon source and oleylamine as the nitrogen source. A comparative study was conducted to explore the influence of the nitrogen-doped carbon layer structure, formed by in-situ carbonization of oleylamine and glucose during the catalyst reduction process, on the catalytic performance. By characterizing the physicochemical properties of the catalysts and combining with the evaluation of FTS reaction performance, it was found that the nitrogen-doped carbon layer structure with oleylamine as the nitrogen source affected the electronic properties of catalytic active sites, the adsorption behavior of reactants, and the performance of secondary reactions of α-olefins. This study provides new insights for the development of cobalt-based FTS catalysts with high activity, high olefin selectivity, and high stability. 2 Experimental 2.1 Catalyst preparation Preparation of γ-Al 2 O 3 Support. The γ-Al 2 O 3 support was synthesized according to literature methods as follows:[ 18 – 20 ] 70 g of aluminum isopropoxide was dissolved in 400 mL of isopropanol. The mixture was refluxed at 80 ℃ under stirring for 1 h. Subsequently, 65 mL of acetic acid solution (1.7 wt.%) was added, and stirring was continued for 4 h. The resulting mixture was subjected to suction filtration, washed with deionized water, and dried at 60 ℃ under vacuum. Finally, the solid was calcined at 650 ℃ for 5 h to obtain the γ-Al 2 O 3 support. Preparation of Co/Al 2 O 3 Catalyst. The catalyst was prepared via an ultrasound-assisted method: Co 3 O 4 particles were mixed with γ-Al 2 O 3 in a flask containing 30 mL deionized water and 30 mL absolute ethanol. The suspension was ultrasonically dispersed for 30 min, rotary-evaporated at 50 ℃ for 30 min, and dried at 120 ℃ for 12 h. The nominal cobalt loading was 15 wt%. Preparation of Co/NC-Al 2 O 3 Catalyst. Glucose and the Co/Al 2 O 3 catalyst were uniformly mixed at a mass ratio of 1:20 (glucose:catalyst). The mixture was suspended in 30 mL deionized water and 30 mL absolute ethanol, followed by ultrasonic treatment for 30 min. After rotary evaporation at 50 ℃ for 30 min and drying at 120 ℃ for 12 h, the solid was calcined at 350 ℃ for 3 h in a tubular furnace under N 2 flow, yielding a carbon-coated precursor. 3 g of this precursor was dispersed in a solution of 30 mL ethanol and 30 mL deionized water. Oleylamine (1 mmol) was added, and the mixture was stirred in an oil bath at 80 ℃ for 8 h. The product was washed three times with ethanol, dried at 80 ℃ for 12 h, and denoted as Co/NC-Al 2 O 3 . 2.2 Catalyst characterization The functional groups and their chemical environments of the catalysts were characterized with a Nicolet IS50 fourier transform infrared spectroscopy spectrometer (FT-IR). Spectra were recorded in the 400–4000 cm -1 range at a resolution of 4 cm -1 . Data processing included baseline correction and normalization via OMNIC software, with characteristic functional groups identified by comparison to the NIST standard infrared database. The carbon content of the catalyst was determined using a NETZSCH TG 209 F3 thermogravimetric analyzer (Germany). Samples (~ 10 mg) were loaded into alumina crucibles and heated from 40 to 700 ℃ at 10 ℃·min -1 under air or N 2 flow (30 mL·min -1 ). Morphology and dispersion were analyzed using a Talos F200X scanning transmission electron microscopy (STEM). Samples were ultrasonically dispersed in ethanol for 5 min, deposited onto carbon-coated copper grids, and dried prior to imaging. The catalysts were characterized by X-ray diffraction (XRD) using a Bruker Advanced D8 diffractometer with a scanning range of 5° to 80°. Phases were identified using JCPDS reference cards. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a VG Multilab 2000 spectrometer using an Al-Kα excitation source. The energy resolution was set at 0.47 eV as measured for the Ag 3d 5/2 peak. All recorded spectra were charge-corrected by referencing the C 1s peak to a binding energy of 284.6 eV. Surface area and pore structure were determined by N 2 adsorption-desorption at -196 ℃ (ASAP 2020 HD88). Prior to analysis, samples were dried at 110 ℃ for 8 h and degassed at 200 ℃ for 6 h under vacuum. The BET method calculated surface areas; pore size distributions were derived from desorption branches using the BJH model. Reducibility was assessed by H 2 temperature-programmed reduction (H 2 -TPR) on an AMI-300 multifunctional chemisorption analyzer. Samples (100 mg) were pretreated in Ar (30 mL·min -1 ) at 150 ℃ for 1 h, cooled to 50 ℃, then exposed to 5% H 2 /Ar (30 mL·min -1 ). Temperature was ramped to 800 ℃ at 10 ℃·min -1 and held for 30 min. H 2 consumption was monitored by TCD. CO temperature-programmed desorption (CO-TPD) characterization was performed on the sample using a fully automated chemisorption analyzer (BELCAT-II). Samples (50 mg) were reduced in H 2 (30 mL·min -1 ) at 350 ℃ for 8 h, purged with He (30 mL·min -1 ) for 30 min, and cooled to 50 ℃. After CO saturation (30 mL·min -1 ), physisorbed CO was removed by Ar (30 mL·min -1 ). Desorption occurred from 50 to 350 ℃ at 10 ℃·min -1 (hold: 30 min), with signals recorded by TCD. 2.3 FTS catalytic reaction The Fischer–Tropsch synthesis (FTS) performance of the catalyst was evaluated in a micro fixed-bed reactor. The catalyst testing procedure was as follows: 0.1 g of catalyst and 0.3 g of quartz sand were weighed and uniformly mixed, then loaded into the reaction tube, with the upper and lower layers of the catalyst packed with quartz wool. After pressure testing with N 2 to 1.2 MPa for leak detection, the pipeline was purged with H 2 (purity 99.99%) for 2 h. Then, the catalyst was reduced at 350 ℃ for 8 h in an H 2 atmosphere with a flow rate of 6 SL·g -1 ·h -1 at a heating rate of 2 ℃/min. Subsequently, after cooling to room temperature, it was purged with syngas (H 2 :CO = 2:1) at a flow rate of 6 SL·g -1 ·h -1 for 2 h, then the pressure was increased to the operating pressure of 1.0 MPa, and the temperature was raised to 230 ℃ at a heating rate of 1 ℃/min to evaluate the FTS catalytic performance. The compositions of gaseous products and tail gas were analyzed online using an Agilent MicroGC 3000A gas chromatograph. Solid wax and liquid products were collected in a 100 ℃ hot trap and a 0 ℃ cold trap, respectively, and offline analysis was conducted after the reaction. The product selectivity was calculated based on the carbon mass balance. 3 Results and discussion 3.1 Chemical composition and thermal stability Fourier transform infrared spectroscopy was used to characterize the chemical composition of the catalyst surface. For the fresh catalyst (Fig. S1 a), the broad absorption band in the range of 3300–3600 cm -1 is attributed to the O-H stretching vibration of physically adsorbed water molecules on the catalyst surface.[ 17 ] The peak at 1348 cm -1 corresponds to the -NO 2 stretching vibration, indicating that oleylamine exists on the surface of the fresh Co/NC-Al 2 O 3 catalyst in the form of N-O structure, which confirms the successful modification of oleylamine on the catalyst. In addition, the peaks at 2925 cm -1 and 2851 cm -1 are assigned to the asymmetric and symmetric stretching vibrations of -CH 2 , respectively, while the peak at 1415 cm -1 is attributed to the stretching vibration of -CH 3 . These results indicate the presence of carbon-containing species on the Co/NC-Al 2 O 3 catalyst, confirming the formation of glucose-derived carbon layers. The FT-IR characterization results of the catalyst after reduction at 350 ℃ for 8 h in H 2 atmosphere are shown in Fig. 1 . The broad absorption band in the range of 3000–3700 cm -1 is ascribed to the -NH stretching vibration.[ 21 ] The peaks at 2116 cm -1 and 1316 cm -1 belong to the C = N stretching vibration and C-N stretching vibration, respectively. This suggests that oleylamine undergoes pyrolytic carbonization under reduction conditions, and nitrogen has been successfully incorporated into the carbon layer, forming nitrogen-carbon species with a C-N = C structure,[ 17 , 22 , 23 ] indicating the successful in-situ construction of the Co/NC-Al 2 O 3 catalyst with a nitrogen-doped carbon layer. Thermogravimetric (TG) characterization was conducted on the catalyst precursor and the Co/NC-Al 2 O 3 catalyst in an air atmosphere to further reveal their thermal stability and compositional characteristics. In air atmosphere, Co 3 O 4 and the inert support γ-Al 2 O 3 exhibit good thermal stability without decomposition and weight loss. The weight loss of the catalyst precursor before 200 ℃ is attributed to the vaporization of adsorbed water or hydrated water, while the weight loss above 200 ℃ mainly arises from the oxidative decomposition of the carbon layer, with a carbon weight loss rate of 5.41% (Fig. S2a). For the Co/NC-Al 2 O 3 catalyst, the weight loss above 200 ℃ is primarily due to the oxidative decomposition of the carbon layer and oleylamine (Fig. S2b), with a carbon weight loss rate of 11.97%. Compared with the catalyst precursor, the carbon weight loss percentage of the Co/NC-Al 2 O 3 catalyst increased by 6.56%, confirming the presence of oleylamine on the fresh Co/NC-Al 2 O 3 catalyst. Further analysis of the DTG curves (Fig. S2c and d) shows that the maximum weight loss rate temperatures of the catalyst precursor and the Co/NC-Al 2 O 3 catalyst are 249.8 ℃ and 294.4 ℃, respectively. The thermal decomposition of the Co/NC-Al 2 O 3 catalyst requires a higher temperature, indicating that the oleylamine-modified Co/NC-Al 2 O 3 catalyst has higher thermal stability. 3.2 Phase structure and metal particle size The phase structures of the as-prepared and reduced catalysts were characterized by XRD. The XRD patterns of the as-prepared catalysts are shown in Fig. 2 a. Both catalysts exhibit characteristic peaks of γ-Al 2 O 3 at 20.9°, 36.6°, 46.5°, 60.4°, and 66.4° (JCPDS: 29–0063). For the Co/Al 2 O 3 catalyst, the characteristic peaks at 19.0°, 31.2°, 36.8°, 44.8°, 59.3°, 65.2°, and 77.3° are attributed to the Co 3 O 4 phase (JCPDS: 73-1710). In contrast, the diffraction peaks of the Co/NC-Al 2 O 3 catalyst show obvious changes, with peaks at 36.5°, 42.2°, and 61.5° assigned to the CoO phase (JCPDS: 43-1004). This is because reductive carbon species partially reduce Co 3 O 4 to CoO during the calcination process. It is worth noting that no characteristic diffraction peaks of carbon species are observed in the XRD patterns of all as-prepared catalysts, indicating that the carbon materials exist in an amorphous structure. The XRD pattern of the reduced catalyst is shown in Fig. 2 b. The peaks at 20.5°, 45.9°, and 66.7° still correspond to γ-Al 2 O 3 (JCPDS: 29–0063), while the peaks at 36.4°, 42.3°, and 61.5° are attributed to the CoO phase (JCPDS: 71-1178), and the characteristic peak at 44.3° corresponds to the metallic Co phase (JCPDS: 15–0806). Notably, no obvious diffraction peaks of metallic Co are observed in the Co/NC-Al 2 O 3 catalyst, which may be due to the physical coverage effect of the nitrogen-doped carbon layer hindering the reduction of CoO to metallic Co, or the highly dispersed metallic Co particles being too small to be detected by XRD. The dispersion and particle size distribution of cobalt species in the catalyst after H 2 reduction were evaluated by TEM and EDS elemental mapping. As shown in Fig. S3a and Fig. S4a, the Co particles in the Co/Al 2 O 3 catalyst are uniformly dispersed, with the Co nanoparticles mainly distributed around 20.20 nm. As shown in Fig. S3 (b-h), and Fig. S4b, the Co nanoparticles in the Co/NC-Al 2 O 3 catalyst are mainly distributed around 19.87 nm, with uniform dispersion. The Co particle size is slightly reduced and the dispersion is slightly improved, which is attributed to the introduction of the nitrogen-doped carbon layer leading to the redistribution of cobalt particles. The EDS mapping results (Fig. S3 (f-h)) show that C, N, and O elements are continuously and uniformly distributed in the Co/NC-Al 2 O 3 catalyst, further confirming the successful in-situ construction of the nitrogen-doped carbon layer in the Co/NC-Al 2 O 3 catalyst. 3.3 Chemical states and reduction behavior X-ray photoelectron spectroscopy was used to analyze the surface chemical states and elemental compositions of the reduced catalysts. All catalysts were subjected to XPS measurements after being reduced in a hydrogen atmosphere at 350 ℃ for 8 hours. Table S1 presents the surface elemental compositions of the reduced Co/NC-Al 2 O 3 catalyst. The nitrogen content on the surface of the Co/NC-Al 2 O 3 catalyst is 0.73%, which further confirms that nitrogen has been successfully incorporated into the carbon layer, and is consistent with the EDS elemental mapping and FT-IR results. The Co 2p spectra (Fig. 3 a) show the presence of two spin-orbit doublets assigned to Co 2p 1/2 and Co 2p 3/2 . As shown in Table S2, the Co 2p 3/2 peak can be deconvoluted into three characteristic peaks: the peak at 779.5-780.4 eV is assigned to Co 0 , the peak at 781.0-781.3 eV is assigned to Co 3+ , and the peak at 782.7 eV is assigned to Co 2+ , indicating that the cobalt species on the surface of the reduced catalyst mainly exist in mixed valence states of Co 0 , Co 3+ , and Co 2+ . Compared with the Co/Al 2 O 3 catalyst, the Co 2p 3/2 binding energy of the Co/NC-Al 2 O 3 catalyst is decreased by 0.3–0.9 eV, indicating that the nitrogen-doped carbon layer increases the electron density on the surface of metallic cobalt through an electronic effect. The relative contents of cobalt species with different valences in the Co 2p spectra were calculated by fitting the peak areas. The content of Co 0 in the Co/NC-Al 2 O 3 catalyst decreases, which indicates that the oleylamine on the surface of the fresh catalyst is carbonized into nitrogen-doped carbon species during the reduction stage, and influences the reduction behavior of cobalt species through the electronic effect, inhibiting the complete reduction of cobalt oxide to metallic cobalt. This is consistent with the fact that no obvious Co 0 diffraction peaks were observed in XRD. Deconvolution analysis of the N 1s spectrum of the reduced Co/NC-Al 2 O 3 catalyst (Fig. 3 b and Table S3) shows that the spectrum can be deconvoluted into four characteristic peaks:[ 21 , 24 , 25 ] pyridinic nitrogen (C-N = C, 397.5 eV), metal-nitrogen bond (Co-N, 399.2 eV), pyrrolic nitrogen (-NH-, 400.6 eV), and graphitic nitrogen or quaternary nitrogen (usually associated with the π-π* transition of terminal amino groups in graphitized structures, 403.0 eV). Quantitative analysis shows that the relative contents of each nitrogen species are as follows: Co-N (22.1%), pyrrolic nitrogen (-NH-, 33.7%), pyridinic nitrogen (C-N = C, 15.9%), and graphitic nitrogen (18.5%). The reducibility of the catalysts was investigated by H 2 -TPR (Fig. S5a). The Co/Al 2 O 3 catalyst exhibited two reduction peaks at 300 ℃ and 345 ℃, corresponding to the reduction of Co 3 O 4 to CoO and the further reduction of CoO to metallic cobalt, respectively. H 2 -TPR coupled with mass spectrometry (MS) analyses were performed for both Co/Al 2 O 3 and Co/NC-Al 2 O 3 catalysts. The synchronous MS detection of the Co/NC-Al 2 O 3 catalyst (Fig. 4 b) showed that the CO and CO 2 signals detected during the reduction process originated from the thermal decomposition of carbon species and oleylamine molecules. As shown in Fig. 4 b and Fig. S5d, strong [C 3 H 4 ] + (m/z = 44) and [C 2 H 7 N] + (m/z = 45) signals were observed in the MS spectra of the Co/NC-Al 2 O 3 catalyst. This indicated that oleylamine molecules underwent thermal decomposition under the reducing atmosphere and were carbonized into nitrogen-containing carbon layers, which was consistent with the result of oleylamine carbonization observed in the FT-IR analysis. In addition, the main reduction product detected by MS was H 2 O, derived from the reduction of cobalt oxides. It is noteworthy that a negative peak appeared in the H 2 -TPR curve of the Co/NC-Al 2 O 3 catalyst during the reduction process (Fig. S5b). This was attributed to the decomposition of oleylamine molecules or their pyrolysis products during reduction, which interfered with the H 2 signal of the thermal conductivity detector (TCD). 3.4 Pore structure properties and CO adsorption behavior The pore structures of Co/Al 2 O 3 and Co/NC-Al 2 O 3 catalysts were analyzed by the N 2 adsorption-desorption method. As shown in Fig. S6, the N 2 adsorption-desorption isotherms of both catalysts exhibit H2(b)-type hysteresis loops and belong to type IV isotherms, indicating that the catalysts have a mesoporous structure. In Fischer-Tropsch synthesis, the mesoporous structure is beneficial to the diffusion of long-chain hydrocarbon products, thereby reducing the concentration gradient of reactants near the active sites and improving the selectivity of C 5+ products.[ 26 ] Oleylamine modification did not significantly alter this pore structure characteristic. Table S4 lists the data of specific surface area, pore volume, and average pore size. After modification with the nitrogen-doped carbon layer, the specific surface area of the Co/NC-Al 2 O 3 catalyst (246.0 m²/g) is slightly lower than that of Co/Al 2 O 3 (254.6 m²/g), while its average pore size (10.3 nm) is slightly larger than that of Co/Al 2 O 3 (8.1 nm). The nitrogen-doped carbon layer leads to a slight decrease in specific surface area but an increase in pore size, indicating that the nitrogen-doped carbon layer exists in the form of a thin layer rather than blocking the original mesoporous channels of the support. The results show that there is little difference in the physical structures of Co/Al 2 O 3 and Co/NC-Al 2 O 3 catalysts, which is conducive to effectively excluding the interference of pore structure differences when analyzing the influence of nitrogen elements in the subsequent series of catalysts. To further understand the surface adsorption capacity of the catalysts, the adsorption of CO probe molecules on the catalysts was characterized by CO-TPD. As shown in Fig. 5 , both Co/Al 2 O 3 and Co/NC-Al 2 O 3 catalysts exhibit an obvious desorption peak before 350 ℃, and the CO desorption peak of Co/NC-Al 2 O 3 shifts to a lower temperature. As listed in Table S5, the total CO adsorption capacities of Co/Al 2 O 3 and Co/NC-Al 2 O 3 catalysts are 553.2 and 1115.8 µmol/g, respectively. This result clearly indicates that the introduction of nitrogen-doped carbon increases the CO adsorption capacity of the Co/NC-Al 2 O 3 catalyst. 3.5 FTS performance of catalysts The FTS performance of the catalysts was evaluated using a micro fixed-bed reactor, and the activity of the catalysts in the FTS reaction was characterized by CTY. The performance data are nitrogen-doped carbon layer significantly affects the activity, stability, and product selectivity of the catalysts. Figure 6 a shows the change in CTY value with time on stream. Within the 100 h test period, the initial CTY of the Co/Al 2 O 3 catalyst is 7.1×10 − 5 molCO·gCo − 1 s − 1 , but its steady-state CTY decreases to 5.9×10 − 5 molCO·gCo − 1 s − 1 , with a deactivation rate of 16.9%. In contrast, the Co/NC-Al 2 O 3 catalyst exhibits excellent stability. Its initial CTY is 3.7×10 − 5 molCO·gCo − 1 s − 1 , and the steady-state CTY increases to 4.2×10 − 5 molCO·gCo − 1 s − 1 instead. This result indicates that although the introduction of the nitrogen-doped carbon layer reduces the catalytic activity, it can enhance the stability of the catalyst. In terms of product distribution (Table 1 ), the olefin selectivity of the Co/Al 2 O 3 catalyst is 13.0%, while that of the Co/NC-Al 2 O 3 catalyst is increased to 34.4%, with an increase of 21.4%. Figure 6 b presents the α-olefin selectivity of the catalysts, from which it can be seen that the α-olefin selectivity of the Co/NC-Al 2 O 3 catalyst is significantly improved, indicating that the nitrogen-doped carbon layer can enhance the selectivity of the Co/NC-Al 2 O 3 catalyst for olefin products. The CH 4 selectivity of Co/Al 2 O 3 is 20.0%, and the CO 2 selectivity is 1.9%; the CH 4 selectivity of the Co/NC-Al 2 O 3 catalyst is 8.9%, and the CO 2 selectivity is 0.9%, suggesting that carbon coating and the introduction of the nitrogen-doped carbon layer can inhibit the formation of by-products such as CH 4 and CO 2 . The ASF product distribution curves of the catalysts are shown in Fig. S7. Compared with the Co/Al 2 O 3 catalyst, the C 5+ selectivity of the Co/NC-Al 2 O 3 catalyst increases from 65.9% to 83.2%. The chain growth probability (α) of the Co/Al 2 O 3 catalyst is 0.78, and that of the Co/NC-Al 2 O 3 catalyst is 0.83, indicating that the nitrogen-doped carbon layer can enhance the chain growth ability and promote the formation of long-chain hydrocarbons. Compared with the reported Co-based FTS catalysts (Table S6),[ 14 , 27 – 31 ] the Co/NC-Al 2 O 3 catalyst in this study still exhibits good olefin selectivity under higher space velocity conditions, while reducing undesired by-products such as CH 4 and CO 2 , demonstrating the advantage of the nitrogen-doped carbon layer in regulating product distribution. Table 1 Fischer-Tropsch synthesis performance of catalysts Catalysts Co content (wt.%) CO 2 selectivity (%) Hydrocarbon selectivity(%) α CH 4 C 2-4 C 5+ All Olefins Co/Al 2 O 3 15.0 1.9 20.0 14.1 65.9 13.0 0.78 Co/NC-Al 2 O 3 12.2 0.9 8.9 8.0 83.2 34.4 0.83 Reduction conditions: in pure hydrogen at 350 ℃ and 1 bar for 8 h. Reaction Conditions: GHSV = 6 SL·g − 1 ·h − 1 , T = 230 ℃, P = 1.0 MPa, H 2 :CO = 2:1. Co content was determined by TG 3.6 Influence of nitrogen-doped carbon layers on FTS catalysts The Co/NC-Al 2 O 3 catalyst exhibits the characteristics of decreased activity but significantly enhanced stability. The decreased activity of the Co/NC-Al 2 O 3 catalyst is mainly attributed to the reduced accessibility of active sites, altered activation ability of reactant molecules, and decreased reducibility of the catalyst caused by the introduction of the nitrogen-doped carbon layer. Nitrogen species affect the coverage of catalytic active sites and electronic effects. FT-IR results and N 1s analysis confirm the presence of various nitrogen species on the surface of the Co/NC-Al 2 O 3 catalyst. The structures of pyrrolic nitrogen (-NH-) and pyridinic nitrogen (C-N = C) are consistent with the typical nitrogen species structures of nitrogen-doped carbon reported in the literature. The differences in the proportions of the four nitrogen species in nitrogen-doped carbon reflect the variations in coordination environments. Pyrrolic nitrogen, acting as an electron donor, is beneficial for reactant adsorption;[ 32 – 34 ] Co-N bonds are crucial to active sites; pyridinic nitrogen contributes basic sites; graphitic nitrogen is related to stability. Among them, Co-N bonds accounting for 22.1% are crucial for regulating the electron density of the metal center, but excessive formation will cover active sites and hinder the adsorption and activation of reactant molecules CO and H 2 . Meanwhile, the presence of pyridinic nitrogen (15.9%) forms Co-N sites with cobalt, reduces the electron localization of cobalt, weakens the excessive activation ability of hydrogen, and inhibits the formation of CH 4 . [ 32 – 34 ] However, the nitrogen covers part of the active metal sites and thus partially poisons the catalyst, leading to the decreased activity of the Co/NC-Al 2 O 3 catalyst. More importantly, nitrogen species (especially pyrrolic nitrogen as an electron donor) significantly regulate the electron density of cobalt active centers. XPS results confirm that the nitrogen-doped carbon layer leads to an increase in the surface electron density of cobalt species, and some literatures have pointed out that such electron-rich cobalt sites will increase the CO dissociation energy barrier.[ 35 ] This conclusion is consistent with that of CO-TPD. The increase in surface electron density of metallic cobalt species enhances the adsorption capacity for CO molecules, but the change in CO adsorption strength caused by the nitrogen-doped carbon layer and the coverage of active sites jointly affect the overall activity. Because the strength of CO adsorption has an important impact on catalyst activity. If the adsorption capacity is too strong, CO molecules will be excessively adsorbed on the catalyst surface and occupy active sites, making it impossible for other reactants to approach the active sites, thus leading to a decrease in catalyst activity.[ 36 ] Although the electron-donating effect of the nitrogen-doped carbon layer inhibits α-olefin hydrogenation, it weakens CO dissociation, resulting in a decrease in catalytic activity, which reflects the trade-off relationship between activity and selectivity. The decreased reducibility of the catalyst further exacerbates the activity decline. XRD and H₂-TPR results indicate that the reducibility of the Co/NC-Al 2 O 3 catalyst is decreased. This is because the physical coverage of the nitrogen-doped carbon layer hinders the reduction of part of cobalt oxides to cobalt, resulting in a decrease in the amount of active metallic Co 0 that can participate in the reaction. The significant enhancement in the stability of the Co/NC-Al 2 O 3 catalyst is mainly attributed to the physical confinement effect of the nitrogen-doped carbon layer and the contribution of specific nitrogen species (graphitic nitrogen). Graphitic nitrogen plays a stabilizing role. The results of N 1s show that there is 18.5% graphitic nitrogen on the surface of the Co/NC-Al 2 O 3 catalyst. Graphitic nitrogen can improve the thermal stability of carbon species themselves,[ 21 , 32 , 34 ] making the nitrogen-doped carbon layer more durable under reaction conditions, thereby indirectly enhancing the overall stability of the catalyst. The confinement effect of the carbon layer is the core reason for the improvement in stability. As a physical barrier, the carbon layer effectively inhibits the migration and agglomeration of active metal Co particles during the reaction. [ 37 ] This is confirmed by the TEM characterization of the catalyst after the reaction (Fig. S8). The Co particle size on the unmodified Co/Al 2 O 3 catalyst increases significantly and aggregates. This is because the Co/Al 2 O 3 catalyst has poor stability, and the active metals agglomerate or sinter during the reaction, which is the reason for the significant decrease in the steady-state activity of the Co/Al 2 O 3 catalyst. In contrast, the Co particles on the Co/NC-Al 2 O 3 catalyst remain uniformly dispersed without agglomeration, and nitrogen and carbon elements are uniformly distributed on the Co/NC-Al 2 O 3 catalyst, which intuitively demonstrates the role of the confinement effect. The introduction of the nitrogen-doped carbon layer precisely regulates the electronic state of active centers and the adsorption behavior of reactant molecules, and utilizes its unique pore structure, collectively promoting the selective formation of olefins and long-chain hydrocarbons. The electronic effect inhibits secondary hydrogenation reactions. It is generally believed that FTS is a continuous reaction process, where primary products are mainly linear α-olefins and a small amount of n-alkanes. These α-olefins can further undergo secondary hydrogenation to form corresponding alkanes or isoparaffins.[ 38 , 39 ] Studies have shown that in nitrogen-doped carbon supports, pyrrolic nitrogen can act as an electron donor, transferring electrons to the adjacent active metal surface to form electron-rich active sites.[ 40 ] In the Co/NC-Al 2 O 3 catalyst, pyrrolic nitrogen (-NH-, 33.7%), which accounts for the highest proportion, serves as an electron donor and transfers electrons to adjacent Co active sites through an electronic effect, forming electron-rich Co centers. XPS results confirm that nitrogen-doped carbon increases the electron density of the Co/NC-Al 2 O 3 catalyst. This electron-rich environment weakens the π-bond interaction between Co and olefins,[ 15 ] thereby inhibiting the secondary hydrogenation of α-olefins and increasing the selectivity of α-olefins. Meanwhile, the electron-rich Co also affects molecular adsorption behavior, increasing the CO adsorption capacity (consistent with the conclusion of CO-TPD), and the C/H ratio on the catalyst surface increases, which further inhibits the hydrogenation of α-olefins, leading to increased olefin selectivity of the Co/NC-Al 2 O 3 catalyst. Additionally, the probability of methyl hydrogenation to form CH 4 decreases, resulting in lower CH 4 selectivity. Combined with the basic sites provided by pyridinic nitrogen (15.9%), CO adsorption is enhanced, collectively inhibiting its secondary hydrogenation.[ 14 ] The increased C 5+ selectivity of the Co/NC-Al 2 O 3 catalyst stems from the mesoporous confinement environment formed by carbon layer coating. The mesoporous structure of the carbon layer delays the diffusion of C 5+ products, and this confined diffusion environment reduces the chances of side reactions such as cracking and hydrogenation, facilitating the continuous progress of the chain growth process. This is directly reflected in the increase in the chain growth probability α, from 0.78 for Co/Al 2 O 3 to 0.83 for Co/NC-Al 2 O 3 , ultimately promoting the enhancement of long-chain hydrocarbon selectivity. 4 Conclusions In this study, using glucose as the carbon source and oleylamine as the nitrogen source, a nitrogen-doped carbon layer modified Co/NC-Al 2 O 3 catalyst was successfully constructed through an in-situ carbonization process during the reduction activation stage. The evaluation results of FTS showed that the catalyst, while maintaining a steady-state activity of 4.2×10 − 5 molCO·gCo − 1 s − 1 , enhanced the olefin selectivity to 34.4% and exhibited excellent stability within the 100-hour test period, achieving the synergistic optimization of olefin selectivity and stability. The regulatory mechanism of catalytic performance is as follows: The decreased activity of the Co/NC-Al 2 O 3 catalyst mainly stems from the multiple effects of the nitrogen-doped carbon layer. Nitrogen species form Co-N bonds with Co, leading to reduced accessibility of active sites; the electron-donating effect of pyrrolic nitrogen increases CO adsorption capacity; and the physical coverage of the nitrogen-doped carbon layer inhibits the reduction of part of cobalt oxides. These factors collectively result in weakened activation ability of reactant molecules. In terms of selectivity, the enhancement of olefin selectivity is attributed to the electronic effect of pyrrolic nitrogen species. As electron donors, they increase the electron density on the cobalt surface, enhance CO adsorption capacity, and effectively inhibit the secondary reactions of α-olefins. In terms of stability, the physical confinement effect of the carbon layer inhibits the migration and agglomeration of cobalt particles, and combined with the thermal stabilization effect of graphitic nitrogen on carbon species, the anti-deactivation ability of the catalyst is significantly improved. This work provides new insights for the design of FTS catalysts with enhanced olefin selectivity and stability. Abbreviations α chain growth probability CTY Co time yield CO-TPD CO temperature-programmed desorption FT-IR Fourier transform infrared spectroscopy GHSV Gas Hourly Space Velocity H 2 -TPR H 2 temperature-programmed reduction STEM Scanning transmission electron microscopy TG Thermogravimetric analysis DTG Derivative Thermogravimetry XPS X-ray photoelectron spectroscopy XRD X-ray diffraction MS M ass S pectrometry Declarations Ethics and Consent to Participate Not applicable. Consent for Publication Not applicable. Competing Interest The authors declare no competing interests. Author Contribution Lili Wu: Writing – review & editing, Writing – original draft, Methodology, Investigation, Conceptualization. Shuying Ren: Writing – review & editing, Methodology, Formal analysis, Conceptualization. Chengchao Liu: Writing – review & editing, Supervision, Funding acquisition. Funding This work was supported by the National Key Research and Development Program of China (2022YFB4101200). 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ACS Catal 12(9):5316-5326. https://doi.org/10.1021/acscatal.2c00926 Additional Declarations No competing interests reported. Supplementary Files SupplementaryInformation.docx Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 09 Nov, 2025 Reviews received at journal 27 Oct, 2025 Reviews received at journal 20 Oct, 2025 Reviews received at journal 19 Oct, 2025 Reviewers agreed at journal 10 Oct, 2025 Reviewers agreed at journal 10 Oct, 2025 Reviewers agreed at journal 10 Oct, 2025 Reviewers agreed at journal 09 Oct, 2025 Reviewers invited by journal 09 Oct, 2025 Editor assigned by journal 06 Oct, 2025 Submission checks completed at journal 06 Oct, 2025 First submitted to journal 02 Oct, 2025 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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09:51:55","extension":"html","order_by":17,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":130365,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7767967/v1/1720eea1508670510cee492d.html"},{"id":94182122,"identity":"70f42dd3-65f7-40d6-a0e0-a727be20007a","added_by":"auto","created_at":"2025-10-23 09:43:55","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":37018,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFT-IR spectra of reduced catalysts\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFT-IR spectra of the reduced Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalysts. The spectra were used to characterize the functional groups on the catalyst surfaces. Pretreatment condition: H\u003csub\u003e2\u003c/sub\u003e, 350 ℃, 8 h.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7767967/v1/a4c2b858269d7718bb2fc9ae.png"},{"id":94182151,"identity":"c469116a-e3fb-4100-a95c-82cddff208f9","added_by":"auto","created_at":"2025-10-23 09:44:07","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":78260,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eXRD patterns. a fresh catalysts. b reduced catalysts\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eXRD patterns of the Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalysts. \u003cstrong\u003eA \u003c/strong\u003eXRD patterns of the fesh catalysts; \u003cstrong\u003eb\u003c/strong\u003e XRD patterns of the catalysts after reduction in H\u003csub\u003e2\u003c/sub\u003e at 350 ℃ for 8 h.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7767967/v1/7c3455a8d51f7515a1cbf33a.png"},{"id":94182378,"identity":"ab571a1c-eab5-404b-a9ff-d3bb1d06fc00","added_by":"auto","created_at":"2025-10-23 09:51:55","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":104807,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea XPS pattern of Co 2p for reduced catalysts. b Deconvoluted XPS of N 1s spectrum\u003c/strong\u003e\u003cbr\u003e\n\u003cstrong\u003ea\u003c/strong\u003e XPS spectra of Co 2p for reduced Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalysts. \u003cstrong\u003eb\u003c/strong\u003e XPS spectrum of N 1s and its corresponding peak deconvolution for the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst. Pretreatment condition: H\u003csub\u003e2\u003c/sub\u003e, 350 ℃, 8 h.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7767967/v1/7b0400b71c9876cbfd54205c.png"},{"id":94182129,"identity":"2f44f719-29d3-44a6-aa3e-ec6ebfc88ba8","added_by":"auto","created_at":"2025-10-23 09:43:55","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":97840,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMS profiles. a Co/Al\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e catalyst. b Co/NC-Al\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e catalyst\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eH\u003csub\u003e2\u003c/sub\u003e-TPR-MS profiles of the Co\u003cstrong\u003e/\u003c/strong\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003cstrong\u003e and Co/NC-\u003c/strong\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003cstrong\u003e catalysts. \u003c/strong\u003eThe profiles show the H\u003csub\u003e2\u003c/sub\u003e consumption signals monitored by mass spectrometry (MS) during the temperature-programmed reduction process.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7767967/v1/91daa0e686b1133586d68261.png"},{"id":94182126,"identity":"9629916a-faca-4f1a-99c1-15a650af9321","added_by":"auto","created_at":"2025-10-23 09:43:55","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":37024,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCO-TPD profiles of catalysts\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eCO-TPD profiles of the Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalysts.The catalysts were first reduced in a H\u003csub\u003e2\u003c/sub\u003e atmosphere (350 ℃, 8 h), purged with He for 30 minutes, and then subjected to the CO temperature-programmed desorption test.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7767967/v1/c506b7d48b73a38f85f6d1fc.png"},{"id":94182128,"identity":"85301770-012f-4d21-92c8-5733dfb21b65","added_by":"auto","created_at":"2025-10-23 09:43:55","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":46711,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea Cobalt time yield as a function of reaction time. b α-alkene selectivity of catalysts. Reduction conditions: in pure hydrogen at 350 ℃ and 1 bar for 8 h. Reaction Conditions: GHSV = 6 SL·g\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e-1\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003e·h\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e-1\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003e, T = 230 ℃, P = 1.0 MPa, H\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e:CO = 2:1. Co time yield calculated from CO steady-state conversion, space velocities, and cobalt content data\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFischer-Tropsch synthesis performance of the Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalysts. a Cobalt time yield (CTY) of the catalysts. The CTY was calculated from the CO steady-state conversion, space velocity, and cobalt content data. b α-Alkene selectivity of the catalysts. Reaction Conditions: Catalyst reduction: in pure H\u003csub\u003e2\u003c/sub\u003e at 350 ℃ and atmospheric pressure for 8 h. Performance test: GHSV = 6 SL·g\u003csup\u003e-1\u003c/sup\u003e·h\u003csup\u003e-1\u003c/sup\u003e, T = 230 °C, P = 1.0 MPa, H\u003csub\u003e2\u003c/sub\u003e/CO = 2:1.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7767967/v1/1f0dd011f744201b93b8124e.png"},{"id":94183265,"identity":"d7621a71-3c50-455d-ace1-69d6bf7d6329","added_by":"auto","created_at":"2025-10-23 10:08:00","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1498224,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7767967/v1/bab50aa3-7a3c-4055-8791-e8223a3596ed.pdf"},{"id":94182138,"identity":"62fd5b4c-f3de-44fc-a266-09049d81d075","added_by":"auto","created_at":"2025-10-23 09:43:55","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":9158396,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-7767967/v1/35fdccc8ca8605736cd9c5d7.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Nitrogen-doped Carbon-coated Al 2 O 3 supported Co 3 O 4 nanoparticles for Fischer-Tropsch synthesis: Boosting durability and Olefins selectivity","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eFischer-Tropsch synthesis (FTS) is a pivotal technology for catalytically converting syngas (H\u003csub\u003e2\u003c/sub\u003e/CO) derived from coal, natural gas, or biomass into clean fuels and high-value chemicals, playing a critical role in energy structure transformation and carbon neutrality strategies.[\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] Facing the dual challenges of global crude oil depletion and surging demand for clean chemicals, developing efficient and economical FTS catalysts and processes is imperative.[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e] Cobalt-based catalysts have garnered significant attention due to their excellent intrinsic activity for CO hydrogenation, high selectivity toward heavy hydrocarbons, and low water-gas shift (WGS) activity. Their products typically consist predominantly of linear alkanes, exhibiting low olefin selectivity.[\u003cspan additionalcitationids=\"CR5\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e] Olefins serve as fundamental building blocks (\"cornerstones\") of the chemical industry, indispensable from basic raw material production to high-value chemical manufacturing, with their economic value substantially exceeding that of alkanes. Developing novel catalysts to achieve high olefin selectivity holds significant economic importance.\u003c/p\u003e\u003cp\u003eAmong the reported strategies for enhancing olefin selectivity, the addition of alkaline promoters (e.g., K, Na) is widely adopted.[\u003cspan additionalcitationids=\"CR8\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] Eliseev et al.[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e] observed that K-modified Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalysts significantly increased CO adsorption capacity, leading to higher olefin content in products but reduced catalytic activity. The Xie team[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] employed Na/S co-modification on an hcp Co catalyst, achieving 54% lower olefin selectivity and 17% CH\u003csub\u003e4\u003c/sub\u003e selectivity at 1% CO conversion. This behavior was attributed to the electronic promoter effect weakening H\u003csub\u003e2\u003c/sub\u003e dissociation capability on the Co surface. Researchers propose that the electronic promoter effect of alkali metals alters the surface charge distribution of active sites,[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] suppresses hydrogen adsorption,[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] enhances CO adsorption, and promotes α-olefin desorption,[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] thereby improving catalyst olefin selectivity. Although alkali metal promotion effectively enhances olefin selectivity, it often reduces catalyst activity. Furthermore, gradual loss of alkali metals during reaction compromises selectivity stability. Consequently, exploring strategies that simultaneously enhance olefin selectivity while avoiding promoter loss and activity decline remains a key focus of current research.\u003c/p\u003e\u003cp\u003eIn recent years, strategies utilizing nitrogen-doped carbon materials as supports for metal catalysts have attracted research attention. Nitrogen-doped carbon materials can act as electron donors or acceptors to modulate the electronic structure and chemical activity of metal catalysts. Lu et al.[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] demonstrated that the anchoring effect and intrinsic basicity of nitrogen-doped carbon nanotubes (NCNTs) enabled convenient immobilization of Fe nanoparticles onto NCNTs via simple impregnation. The constructed Fe/NCNTs catalyst achieved high lower olefin (C2\u003csup\u003e=\u003c/sup\u003e-C4\u003csup\u003e=\u003c/sup\u003e) selectivity up to 46.7% while exhibiting excellent stability. Rausch et al.[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] reported that co-feeding nitrogen-containing species (e.g., NH\u003csub\u003e3\u003c/sub\u003e, CH\u003csub\u003e3\u003c/sub\u003eCN) with syngas enhanced α-olefin selectivity over a 15 wt% Co/SiO\u003csub\u003e2\u003c/sub\u003e catalyst. This improvement was ascribed to the electron-donating effect of nitrogen species, which increased surface basicity and suppressed secondary hydrogenation of olefins. However, introducing nitrogen species into the syngas feed can induce irreversible catalyst deactivation. Our group constructed Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@C-X supports with carbon layers of tailored surface properties by modulating glucose carbonization conditions on Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. High-temperature carbonized layers effectively weakened the strong metal-support interaction (SMSI) between Co and Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, promoted the reducibility of cobalt species, and significantly enhanced C\u003csub\u003e5+\u003c/sub\u003e selectivity.[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] Carbon nitride layers (TiO\u003csub\u003e2\u003c/sub\u003e@xCN) were formed on TiO\u003csub\u003e2\u003c/sub\u003e via urea pyrolysis followed by hydrothermal treatment and calcination. The interaction between carbon nitride and cobalt improved cobalt dispersion, modified electron transfer behavior between Co and TiO\u003csub\u003e2\u003c/sub\u003e, and inhibited TiO\u003csub\u003e2\u003c/sub\u003e encapsulation of Co nanoparticles during reaction. The interaction between carbon nitride and cobalt improved cobalt dispersion, modified electron transfer behavior between Co and TiO\u003csub\u003e2\u003c/sub\u003e, and inhibited TiO\u003csub\u003e2\u003c/sub\u003e encapsulation of Co nanoparticles during reaction.[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e] These findings highlight the substantial potential of nitrogen as an electron donor/alkaline source in regulating both FTS stability and product selectivity.\u003c/p\u003e\u003cp\u003eThis study aims to investigate the effects of nitrogen-doped carbon layers on the product selectivity and stability of cobalt-based catalysts supported on alumina in Fischer-Tropsch synthesis. The Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e reference catalyst was prepared by an ultrasound-assisted method; the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst was synthesized using glucose as the carbon source and oleylamine as the nitrogen source. A comparative study was conducted to explore the influence of the nitrogen-doped carbon layer structure, formed by in-situ carbonization of oleylamine and glucose during the catalyst reduction process, on the catalytic performance. By characterizing the physicochemical properties of the catalysts and combining with the evaluation of FTS reaction performance, it was found that the nitrogen-doped carbon layer structure with oleylamine as the nitrogen source affected the electronic properties of catalytic active sites, the adsorption behavior of reactants, and the performance of secondary reactions of α-olefins. This study provides new insights for the development of cobalt-based FTS catalysts with high activity, high olefin selectivity, and high stability.\u003c/p\u003e"},{"header":"2 Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Catalyst preparation\u003c/h2\u003e\u003cp\u003ePreparation of γ-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e Support. The γ-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e support was synthesized according to literature methods as follows:[\u003cspan additionalcitationids=\"CR19\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] 70 g of aluminum isopropoxide was dissolved in 400 mL of isopropanol. The mixture was refluxed at 80 ℃ under stirring for 1 h. Subsequently, 65 mL of acetic acid solution (1.7 wt.%) was added, and stirring was continued for 4 h. The resulting mixture was subjected to suction filtration, washed with deionized water, and dried at 60 ℃ under vacuum. Finally, the solid was calcined at 650 ℃ for 5 h to obtain the γ-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e support.\u003c/p\u003e\u003cp\u003ePreparation of Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e Catalyst. The catalyst was prepared via an ultrasound-assisted method: Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e particles were mixed with γ-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e in a flask containing 30 mL deionized water and 30 mL absolute ethanol. The suspension was ultrasonically dispersed for 30 min, rotary-evaporated at 50 ℃ for 30 min, and dried at 120 ℃ for 12 h. The nominal cobalt loading was 15 wt%.\u003c/p\u003e\u003cp\u003ePreparation of Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e Catalyst. Glucose and the Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst were uniformly mixed at a mass ratio of 1:20 (glucose:catalyst). The mixture was suspended in 30 mL deionized water and 30 mL absolute ethanol, followed by ultrasonic treatment for 30 min. After rotary evaporation at 50 ℃ for 30 min and drying at 120 ℃ for 12 h, the solid was calcined at 350 ℃ for 3 h in a tubular furnace under N\u003csub\u003e2\u003c/sub\u003e flow, yielding a carbon-coated precursor. 3 g of this precursor was dispersed in a solution of 30 mL ethanol and 30 mL deionized water. Oleylamine (1 mmol) was added, and the mixture was stirred in an oil bath at 80 ℃ for 8 h. The product was washed three times with ethanol, dried at 80 ℃ for 12 h, and denoted as Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Catalyst characterization\u003c/h2\u003e\u003cp\u003eThe functional groups and their chemical environments of the catalysts were characterized with a Nicolet IS50 fourier transform infrared spectroscopy spectrometer (FT-IR). Spectra were recorded in the 400\u0026ndash;4000 cm\u003csup\u003e-1\u003c/sup\u003e range at a resolution of 4 cm\u003csup\u003e-1\u003c/sup\u003e. Data processing included baseline correction and normalization via OMNIC software, with characteristic functional groups identified by comparison to the NIST standard infrared database.\u003c/p\u003e\u003cp\u003eThe carbon content of the catalyst was determined using a NETZSCH TG 209 F3 thermogravimetric analyzer (Germany). Samples (~\u0026thinsp;10 mg) were loaded into alumina crucibles and heated from 40 to 700 ℃ at 10 ℃\u0026middot;min\u003csup\u003e-1\u003c/sup\u003e under air or N\u003csub\u003e2\u003c/sub\u003e flow (30 mL\u0026middot;min\u003csup\u003e-1\u003c/sup\u003e).\u003c/p\u003e\u003cp\u003eMorphology and dispersion were analyzed using a Talos F200X scanning transmission electron microscopy (STEM). Samples were ultrasonically dispersed in ethanol for 5 min, deposited onto carbon-coated copper grids, and dried prior to imaging.\u003c/p\u003e\u003cp\u003eThe catalysts were characterized by X-ray diffraction (XRD) using a Bruker Advanced D8 diffractometer with a scanning range of 5\u0026deg; to 80\u0026deg;. Phases were identified using JCPDS reference cards.\u003c/p\u003e\u003cp\u003eX-ray photoelectron spectroscopy (XPS) analysis was conducted on a VG Multilab 2000 spectrometer using an Al-Kα excitation source. The energy resolution was set at 0.47 eV as measured for the Ag 3d\u003csub\u003e5/2\u003c/sub\u003e peak. All recorded spectra were charge-corrected by referencing the C 1s peak to a binding energy of 284.6 eV.\u003c/p\u003e\u003cp\u003eSurface area and pore structure were determined by N\u003csub\u003e2\u003c/sub\u003e adsorption-desorption at -196 ℃ (ASAP 2020 HD88). Prior to analysis, samples were dried at 110 ℃ for 8 h and degassed at 200 ℃ for 6 h under vacuum. The BET method calculated surface areas; pore size distributions were derived from desorption branches using the BJH model.\u003c/p\u003e\u003cp\u003eReducibility was assessed by H\u003csub\u003e2\u003c/sub\u003e temperature-programmed reduction (H\u003csub\u003e2\u003c/sub\u003e-TPR) on an AMI-300 multifunctional chemisorption analyzer. Samples (100 mg) were pretreated in Ar (30 mL\u0026middot;min\u003csup\u003e-1\u003c/sup\u003e) at 150 ℃ for 1 h, cooled to 50 ℃, then exposed to 5% H\u003csub\u003e2\u003c/sub\u003e/Ar (30 mL\u0026middot;min\u003csup\u003e-1\u003c/sup\u003e). Temperature was ramped to 800 ℃ at 10 ℃\u0026middot;min\u003csup\u003e-1\u003c/sup\u003e and held for 30 min. H\u003csub\u003e2\u003c/sub\u003e consumption was monitored by TCD.\u003c/p\u003e\u003cp\u003eCO temperature-programmed desorption (CO-TPD) characterization was performed on the sample using a fully automated chemisorption analyzer (BELCAT-II). Samples (50 mg) were reduced in H\u003csub\u003e2\u003c/sub\u003e (30 mL\u0026middot;min\u003csup\u003e-1\u003c/sup\u003e) at 350 ℃ for 8 h, purged with He (30 mL\u0026middot;min\u003csup\u003e-1\u003c/sup\u003e) for 30 min, and cooled to 50 ℃. After CO saturation (30 mL\u0026middot;min\u003csup\u003e-1\u003c/sup\u003e), physisorbed CO was removed by Ar (30 mL\u0026middot;min\u003csup\u003e-1\u003c/sup\u003e). Desorption occurred from 50 to 350 ℃ at 10 ℃\u0026middot;min\u003csup\u003e-1\u003c/sup\u003e (hold: 30 min), with signals recorded by TCD.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3 FTS catalytic reaction\u003c/h2\u003e\u003cp\u003eThe Fischer\u0026ndash;Tropsch synthesis (FTS) performance of the catalyst was evaluated in a micro fixed-bed reactor. The catalyst testing procedure was as follows: 0.1 g of catalyst and 0.3 g of quartz sand were weighed and uniformly mixed, then loaded into the reaction tube, with the upper and lower layers of the catalyst packed with quartz wool. After pressure testing with N\u003csub\u003e2\u003c/sub\u003e to 1.2 MPa for leak detection, the pipeline was purged with H\u003csub\u003e2\u003c/sub\u003e (purity 99.99%) for 2 h. Then, the catalyst was reduced at 350 ℃ for 8 h in an H\u003csub\u003e2\u003c/sub\u003e atmosphere with a flow rate of 6 SL\u0026middot;g\u003csup\u003e-1\u003c/sup\u003e\u0026middot;h\u003csup\u003e-1\u003c/sup\u003e at a heating rate of 2 ℃/min. Subsequently, after cooling to room temperature, it was purged with syngas (H\u003csub\u003e2\u003c/sub\u003e:CO\u0026thinsp;=\u0026thinsp;2:1) at a flow rate of 6 SL\u0026middot;g\u003csup\u003e-1\u003c/sup\u003e\u0026middot;h\u003csup\u003e-1\u003c/sup\u003e for 2 h, then the pressure was increased to the operating pressure of 1.0 MPa, and the temperature was raised to 230 ℃ at a heating rate of 1 ℃/min to evaluate the FTS catalytic performance. The compositions of gaseous products and tail gas were analyzed online using an Agilent MicroGC 3000A gas chromatograph. Solid wax and liquid products were collected in a 100 ℃ hot trap and a 0 ℃ cold trap, respectively, and offline analysis was conducted after the reaction. The product selectivity was calculated based on the carbon mass balance.\u003c/p\u003e\u003c/div\u003e"},{"header":"3 Results and discussion","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Chemical composition and thermal stability\u003c/h2\u003e\u003cp\u003eFourier transform infrared spectroscopy was used to characterize the chemical composition of the catalyst surface. For the fresh catalyst (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003ea), the broad absorption band in the range of 3300\u0026ndash;3600 cm\u003csup\u003e-1\u003c/sup\u003e is attributed to the O-H stretching vibration of physically adsorbed water molecules on the catalyst surface.[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e] The peak at 1348 cm\u003csup\u003e-1\u003c/sup\u003e corresponds to the -NO\u003csub\u003e2\u003c/sub\u003e stretching vibration, indicating that oleylamine exists on the surface of the fresh Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst in the form of N-O structure, which confirms the successful modification of oleylamine on the catalyst. In addition, the peaks at 2925 cm\u003csup\u003e-1\u003c/sup\u003e and 2851 cm\u003csup\u003e-1\u003c/sup\u003e are assigned to the asymmetric and symmetric stretching vibrations of -CH\u003csub\u003e2\u003c/sub\u003e, respectively, while the peak at 1415 cm\u003csup\u003e-1\u003c/sup\u003e is attributed to the stretching vibration of -CH\u003csub\u003e3\u003c/sub\u003e. These results indicate the presence of carbon-containing species on the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst, confirming the formation of glucose-derived carbon layers. The FT-IR characterization results of the catalyst after reduction at 350 ℃ for 8 h in H\u003csub\u003e2\u003c/sub\u003e atmosphere are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The broad absorption band in the range of 3000\u0026ndash;3700 cm\u003csup\u003e-1\u003c/sup\u003e is ascribed to the -NH stretching vibration.[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e] The peaks at 2116 cm\u003csup\u003e-1\u003c/sup\u003e and 1316 cm\u003csup\u003e-1\u003c/sup\u003e belong to the C\u0026thinsp;=\u0026thinsp;N stretching vibration and C-N stretching vibration, respectively. This suggests that oleylamine undergoes pyrolytic carbonization under reduction conditions, and nitrogen has been successfully incorporated into the carbon layer, forming nitrogen-carbon species with a C-N\u0026thinsp;=\u0026thinsp;C structure,[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e] indicating the successful in-situ construction of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst with a nitrogen-doped carbon layer.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThermogravimetric (TG) characterization was conducted on the catalyst precursor and the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst in an air atmosphere to further reveal their thermal stability and compositional characteristics. In air atmosphere, Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and the inert support γ-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e exhibit good thermal stability without decomposition and weight loss. The weight loss of the catalyst precursor before 200 ℃ is attributed to the vaporization of adsorbed water or hydrated water, while the weight loss above 200 ℃ mainly arises from the oxidative decomposition of the carbon layer, with a carbon weight loss rate of 5.41% (Fig. S2a). For the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst, the weight loss above 200 ℃ is primarily due to the oxidative decomposition of the carbon layer and oleylamine (Fig. S2b), with a carbon weight loss rate of 11.97%. Compared with the catalyst precursor, the carbon weight loss percentage of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst increased by 6.56%, confirming the presence of oleylamine on the fresh Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst. Further analysis of the DTG curves (Fig. S2c and d) shows that the maximum weight loss rate temperatures of the catalyst precursor and the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst are 249.8 ℃ and 294.4 ℃, respectively. The thermal decomposition of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst requires a higher temperature, indicating that the oleylamine-modified Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst has higher thermal stability.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Phase structure and metal particle size\u003c/h2\u003e\u003cp\u003eThe phase structures of the as-prepared and reduced catalysts were characterized by XRD. The XRD patterns of the as-prepared catalysts are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e2\u003c/span\u003ea. Both catalysts exhibit characteristic peaks of γ-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e at 20.9\u0026deg;, 36.6\u0026deg;, 46.5\u0026deg;, 60.4\u0026deg;, and 66.4\u0026deg; (JCPDS: 29\u0026ndash;0063). For the Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst, the characteristic peaks at 19.0\u0026deg;, 31.2\u0026deg;, 36.8\u0026deg;, 44.8\u0026deg;, 59.3\u0026deg;, 65.2\u0026deg;, and 77.3\u0026deg; are attributed to the Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e phase (JCPDS: 73-1710). In contrast, the diffraction peaks of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst show obvious changes, with peaks at 36.5\u0026deg;, 42.2\u0026deg;, and 61.5\u0026deg; assigned to the CoO phase (JCPDS: 43-1004). This is because reductive carbon species partially reduce Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e to CoO during the calcination process. It is worth noting that no characteristic diffraction peaks of carbon species are observed in the XRD patterns of all as-prepared catalysts, indicating that the carbon materials exist in an amorphous structure. The XRD pattern of the reduced catalyst is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e2\u003c/span\u003eb. The peaks at 20.5\u0026deg;, 45.9\u0026deg;, and 66.7\u0026deg; still correspond to γ-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (JCPDS: 29\u0026ndash;0063), while the peaks at 36.4\u0026deg;, 42.3\u0026deg;, and 61.5\u0026deg; are attributed to the CoO phase (JCPDS: 71-1178), and the characteristic peak at 44.3\u0026deg; corresponds to the metallic Co phase (JCPDS: 15\u0026ndash;0806). Notably, no obvious diffraction peaks of metallic Co are observed in the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst, which may be due to the physical coverage effect of the nitrogen-doped carbon layer hindering the reduction of CoO to metallic Co, or the highly dispersed metallic Co particles being too small to be detected by XRD.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe dispersion and particle size distribution of cobalt species in the catalyst after H\u003csub\u003e2\u003c/sub\u003e reduction were evaluated by TEM and EDS elemental mapping. As shown in Fig. S3a and Fig. S4a, the Co particles in the Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst are uniformly dispersed, with the Co nanoparticles mainly distributed around 20.20 nm. As shown in Fig. S3 (b-h), and Fig. S4b, the Co nanoparticles in the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst are mainly distributed around 19.87 nm, with uniform dispersion. The Co particle size is slightly reduced and the dispersion is slightly improved, which is attributed to the introduction of the nitrogen-doped carbon layer leading to the redistribution of cobalt particles. The EDS mapping results (Fig. S3 (f-h)) show that C, N, and O elements are continuously and uniformly distributed in the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst, further confirming the successful in-situ construction of the nitrogen-doped carbon layer in the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Chemical states and reduction behavior\u003c/h2\u003e\u003cp\u003eX-ray photoelectron spectroscopy was used to analyze the surface chemical states and elemental compositions of the reduced catalysts. All catalysts were subjected to XPS measurements after being reduced in a hydrogen atmosphere at 350 ℃ for 8 hours. Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e presents the surface elemental compositions of the reduced Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst. The nitrogen content on the surface of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst is 0.73%, which further confirms that nitrogen has been successfully incorporated into the carbon layer, and is consistent with the EDS elemental mapping and FT-IR results. The Co 2p spectra (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e3\u003c/span\u003ea) show the presence of two spin-orbit doublets assigned to Co 2p\u003csub\u003e1/2\u003c/sub\u003e and Co 2p\u003csub\u003e3/2\u003c/sub\u003e. As shown in Table S2, the Co 2p\u003csub\u003e3/2\u003c/sub\u003e peak can be deconvoluted into three characteristic peaks: the peak at 779.5-780.4 eV is assigned to Co\u003csup\u003e0\u003c/sup\u003e, the peak at 781.0-781.3 eV is assigned to Co\u003csup\u003e3+\u003c/sup\u003e, and the peak at 782.7 eV is assigned to Co\u003csup\u003e2+\u003c/sup\u003e, indicating that the cobalt species on the surface of the reduced catalyst mainly exist in mixed valence states of Co\u003csup\u003e0\u003c/sup\u003e, Co\u003csup\u003e3+\u003c/sup\u003e, and Co\u003csup\u003e2+\u003c/sup\u003e. Compared with the Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst, the Co 2p\u003csub\u003e3/2\u003c/sub\u003e binding energy of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst is decreased by 0.3\u0026ndash;0.9 eV, indicating that the nitrogen-doped carbon layer increases the electron density on the surface of metallic cobalt through an electronic effect. The relative contents of cobalt species with different valences in the Co 2p spectra were calculated by fitting the peak areas. The content of Co\u003csup\u003e0\u003c/sup\u003e in the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst decreases, which indicates that the oleylamine on the surface of the fresh catalyst is carbonized into nitrogen-doped carbon species during the reduction stage, and influences the reduction behavior of cobalt species through the electronic effect, inhibiting the complete reduction of cobalt oxide to metallic cobalt. This is consistent with the fact that no obvious Co\u003csup\u003e0\u003c/sup\u003e diffraction peaks were observed in XRD. Deconvolution analysis of the N 1s spectrum of the reduced Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e3\u003c/span\u003eb and Table S3) shows that the spectrum can be deconvoluted into four characteristic peaks:[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e] pyridinic nitrogen (C-N\u0026thinsp;=\u0026thinsp;C, 397.5 eV), metal-nitrogen bond (Co-N, 399.2 eV), pyrrolic nitrogen (-NH-, 400.6 eV), and graphitic nitrogen or quaternary nitrogen (usually associated with the π-π* transition of terminal amino groups in graphitized structures, 403.0 eV). Quantitative analysis shows that the relative contents of each nitrogen species are as follows: Co-N (22.1%), pyrrolic nitrogen (-NH-, 33.7%), pyridinic nitrogen (C-N\u0026thinsp;=\u0026thinsp;C, 15.9%), and graphitic nitrogen (18.5%).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe reducibility of the catalysts was investigated by H\u003csub\u003e2\u003c/sub\u003e-TPR (Fig. S5a). The Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst exhibited two reduction peaks at 300 ℃ and 345 ℃, corresponding to the reduction of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e to CoO and the further reduction of CoO to metallic cobalt, respectively. H\u003csub\u003e2\u003c/sub\u003e-TPR coupled with mass spectrometry (MS) analyses were performed for both Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalysts. The synchronous MS detection of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e4\u003c/span\u003eb) showed that the CO and CO\u003csub\u003e2\u003c/sub\u003e signals detected during the reduction process originated from the thermal decomposition of carbon species and oleylamine molecules. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e4\u003c/span\u003eb and Fig. S5d, strong [C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e+\u003c/sup\u003e (m/z\u0026thinsp;=\u0026thinsp;44) and [C\u003csub\u003e2\u003c/sub\u003eH\u003csub\u003e7\u003c/sub\u003eN]\u003csup\u003e+\u003c/sup\u003e (m/z\u0026thinsp;=\u0026thinsp;45) signals were observed in the MS spectra of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst. This indicated that oleylamine molecules underwent thermal decomposition under the reducing atmosphere and were carbonized into nitrogen-containing carbon layers, which was consistent with the result of oleylamine carbonization observed in the FT-IR analysis. In addition, the main reduction product detected by MS was H\u003csub\u003e2\u003c/sub\u003eO, derived from the reduction of cobalt oxides. It is noteworthy that a negative peak appeared in the H\u003csub\u003e2\u003c/sub\u003e-TPR curve of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst during the reduction process (Fig. S5b). This was attributed to the decomposition of oleylamine molecules or their pyrolysis products during reduction, which interfered with the H\u003csub\u003e2\u003c/sub\u003e signal of the thermal conductivity detector (TCD).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e3.4 Pore structure properties and CO adsorption behavior\u003c/h2\u003e\u003cp\u003eThe pore structures of Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalysts were analyzed by the N\u003csub\u003e2\u003c/sub\u003e adsorption-desorption method. As shown in Fig. S6, the N\u003csub\u003e2\u003c/sub\u003e adsorption-desorption isotherms of both catalysts exhibit H2(b)-type hysteresis loops and belong to type IV isotherms, indicating that the catalysts have a mesoporous structure. In Fischer-Tropsch synthesis, the mesoporous structure is beneficial to the diffusion of long-chain hydrocarbon products, thereby reducing the concentration gradient of reactants near the active sites and improving the selectivity of C\u003csub\u003e5+\u003c/sub\u003e products.[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e] Oleylamine modification did not significantly alter this pore structure characteristic. Table S4 lists the data of specific surface area, pore volume, and average pore size. After modification with the nitrogen-doped carbon layer, the specific surface area of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst (246.0 m\u0026sup2;/g) is slightly lower than that of Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (254.6 m\u0026sup2;/g), while its average pore size (10.3 nm) is slightly larger than that of Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (8.1 nm). The nitrogen-doped carbon layer leads to a slight decrease in specific surface area but an increase in pore size, indicating that the nitrogen-doped carbon layer exists in the form of a thin layer rather than blocking the original mesoporous channels of the support. The results show that there is little difference in the physical structures of Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalysts, which is conducive to effectively excluding the interference of pore structure differences when analyzing the influence of nitrogen elements in the subsequent series of catalysts.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTo further understand the surface adsorption capacity of the catalysts, the adsorption of CO probe molecules on the catalysts was characterized by CO-TPD. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e5\u003c/span\u003e, both Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalysts exhibit an obvious desorption peak before 350 ℃, and the CO desorption peak of Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e shifts to a lower temperature. As listed in Table S5, the total CO adsorption capacities of Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalysts are 553.2 and 1115.8 \u0026micro;mol/g, respectively. This result clearly indicates that the introduction of nitrogen-doped carbon increases the CO adsorption capacity of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003e3.5 FTS performance of catalysts\u003c/h2\u003e\u003cp\u003eThe FTS performance of the catalysts was evaluated using a micro fixed-bed reactor, and the activity of the catalysts in the FTS reaction was characterized by CTY. The performance data are nitrogen-doped carbon layer significantly affects the activity, stability, and product selectivity of the catalysts. Figure\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e6\u003c/span\u003ea shows the change in CTY value with time on stream. Within the 100 h test period, the initial CTY of the Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst is 7.1\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e molCO\u0026middot;gCo\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, but its steady-state CTY decreases to 5.9\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e molCO\u0026middot;gCo\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, with a deactivation rate of 16.9%. In contrast, the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst exhibits excellent stability. Its initial CTY is 3.7\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e molCO\u0026middot;gCo\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and the steady-state CTY increases to 4.2\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e molCO\u0026middot;gCo\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e instead. This result indicates that although the introduction of the nitrogen-doped carbon layer reduces the catalytic activity, it can enhance the stability of the catalyst. In terms of product distribution (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), the olefin selectivity of the Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst is 13.0%, while that of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst is increased to 34.4%, with an increase of 21.4%. Figure\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e6\u003c/span\u003eb presents the α-olefin selectivity of the catalysts, from which it can be seen that the α-olefin selectivity of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst is significantly improved, indicating that the nitrogen-doped carbon layer can enhance the selectivity of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst for olefin products. The CH\u003csub\u003e4\u003c/sub\u003e selectivity of Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e is 20.0%, and the CO\u003csub\u003e2\u003c/sub\u003e selectivity is 1.9%; the CH\u003csub\u003e4\u003c/sub\u003e selectivity of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst is 8.9%, and the CO\u003csub\u003e2\u003c/sub\u003e selectivity is 0.9%, suggesting that carbon coating and the introduction of the nitrogen-doped carbon layer can inhibit the formation of by-products such as CH\u003csub\u003e4\u003c/sub\u003e and CO\u003csub\u003e2\u003c/sub\u003e. The ASF product distribution curves of the catalysts are shown in Fig. S7. Compared with the Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst, the C\u003csub\u003e5+\u003c/sub\u003e selectivity of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst increases from 65.9% to 83.2%. The chain growth probability (α) of the Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst is 0.78, and that of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst is 0.83, indicating that the nitrogen-doped carbon layer can enhance the chain growth ability and promote the formation of long-chain hydrocarbons. Compared with the reported Co-based FTS catalysts (Table S6),[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan additionalcitationids=\"CR28 CR29 CR30\" citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e] the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst in this study still exhibits good olefin selectivity under higher space velocity conditions, while reducing undesired by-products such as CH\u003csub\u003e4\u003c/sub\u003e and CO\u003csub\u003e2\u003c/sub\u003e, demonstrating the advantage of the nitrogen-doped carbon layer in regulating product distribution.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\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\u003eFischer-Tropsch synthesis performance of catalysts\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"8\"\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\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eCatalysts\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eCo content (wt.%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e selectivity\u003c/p\u003e\u003cp\u003e(%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"4\" nameend=\"c7\" namest=\"c4\"\u003e\u003cp\u003eHydrocarbon selectivity(%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c8\"\u003e\u003cp\u003eα\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eCH\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eC\u003csub\u003e2-4\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eC\u003csub\u003e5+\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eAll Olefins\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c8\"\u003e\u0026nbsp;\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCo/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e15.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e1.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e20.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e14.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e65.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e13.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e0.78\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCo/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e12.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e8.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e8.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e83.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e34.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e0.83\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003ctfoot\u003e\u003ctr\u003e\u003ctd colspan=\"8\"\u003eReduction conditions: in pure hydrogen at 350 ℃ and 1 bar for 8 h. Reaction Conditions: GHSV\u0026thinsp;=\u0026thinsp;6 SL\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026middot;h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, T\u0026thinsp;=\u0026thinsp;230 ℃, P\u0026thinsp;=\u0026thinsp;1.0 MPa, H\u003csub\u003e2\u003c/sub\u003e:CO\u0026thinsp;=\u0026thinsp;2:1. Co content was determined by TG\u003c/td\u003e\u003c/tr\u003e\u003c/tfoot\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003e3.6 Influence of nitrogen-doped carbon layers on FTS catalysts\u003c/h2\u003e\u003cp\u003eThe Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst exhibits the characteristics of decreased activity but significantly enhanced stability. The decreased activity of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst is mainly attributed to the reduced accessibility of active sites, altered activation ability of reactant molecules, and decreased reducibility of the catalyst caused by the introduction of the nitrogen-doped carbon layer. Nitrogen species affect the coverage of catalytic active sites and electronic effects. FT-IR results and N 1s analysis confirm the presence of various nitrogen species on the surface of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst. The structures of pyrrolic nitrogen (-NH-) and pyridinic nitrogen (C-N\u0026thinsp;=\u0026thinsp;C) are consistent with the typical nitrogen species structures of nitrogen-doped carbon reported in the literature. The differences in the proportions of the four nitrogen species in nitrogen-doped carbon reflect the variations in coordination environments. Pyrrolic nitrogen, acting as an electron donor, is beneficial for reactant adsorption;[\u003cspan additionalcitationids=\"CR33\" citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e] Co-N bonds are crucial to active sites; pyridinic nitrogen contributes basic sites; graphitic nitrogen is related to stability. Among them, Co-N bonds accounting for 22.1% are crucial for regulating the electron density of the metal center, but excessive formation will cover active sites and hinder the adsorption and activation of reactant molecules CO and H\u003csub\u003e2\u003c/sub\u003e. Meanwhile, the presence of pyridinic nitrogen (15.9%) forms Co-N sites with cobalt, reduces the electron localization of cobalt, weakens the excessive activation ability of hydrogen, and inhibits the formation of CH\u003csub\u003e4\u003c/sub\u003e. [\u003cspan additionalcitationids=\"CR33\" citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e] However, the nitrogen covers part of the active metal sites and thus partially poisons the catalyst, leading to the decreased activity of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst. More importantly, nitrogen species (especially pyrrolic nitrogen as an electron donor) significantly regulate the electron density of cobalt active centers. XPS results confirm that the nitrogen-doped carbon layer leads to an increase in the surface electron density of cobalt species, and some literatures have pointed out that such electron-rich cobalt sites will increase the CO dissociation energy barrier.[\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e] This conclusion is consistent with that of CO-TPD. The increase in surface electron density of metallic cobalt species enhances the adsorption capacity for CO molecules, but the change in CO adsorption strength caused by the nitrogen-doped carbon layer and the coverage of active sites jointly affect the overall activity. Because the strength of CO adsorption has an important impact on catalyst activity. If the adsorption capacity is too strong, CO molecules will be excessively adsorbed on the catalyst surface and occupy active sites, making it impossible for other reactants to approach the active sites, thus leading to a decrease in catalyst activity.[\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e] Although the electron-donating effect of the nitrogen-doped carbon layer inhibits α-olefin hydrogenation, it weakens CO dissociation, resulting in a decrease in catalytic activity, which reflects the trade-off relationship between activity and selectivity. The decreased reducibility of the catalyst further exacerbates the activity decline. XRD and H₂-TPR results indicate that the reducibility of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst is decreased. This is because the physical coverage of the nitrogen-doped carbon layer hinders the reduction of part of cobalt oxides to cobalt, resulting in a decrease in the amount of active metallic Co\u003csup\u003e0\u003c/sup\u003e that can participate in the reaction.\u003c/p\u003e\u003cp\u003eThe significant enhancement in the stability of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst is mainly attributed to the physical confinement effect of the nitrogen-doped carbon layer and the contribution of specific nitrogen species (graphitic nitrogen). Graphitic nitrogen plays a stabilizing role. The results of N 1s show that there is 18.5% graphitic nitrogen on the surface of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst. Graphitic nitrogen can improve the thermal stability of carbon species themselves,[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e] making the nitrogen-doped carbon layer more durable under reaction conditions, thereby indirectly enhancing the overall stability of the catalyst. The confinement effect of the carbon layer is the core reason for the improvement in stability. As a physical barrier, the carbon layer effectively inhibits the migration and agglomeration of active metal Co particles during the reaction. [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e] This is confirmed by the TEM characterization of the catalyst after the reaction (Fig. S8). The Co particle size on the unmodified Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst increases significantly and aggregates. This is because the Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst has poor stability, and the active metals agglomerate or sinter during the reaction, which is the reason for the significant decrease in the steady-state activity of the Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst. In contrast, the Co particles on the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst remain uniformly dispersed without agglomeration, and nitrogen and carbon elements are uniformly distributed on the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst, which intuitively demonstrates the role of the confinement effect.\u003c/p\u003e\u003cp\u003eThe introduction of the nitrogen-doped carbon layer precisely regulates the electronic state of active centers and the adsorption behavior of reactant molecules, and utilizes its unique pore structure, collectively promoting the selective formation of olefins and long-chain hydrocarbons. The electronic effect inhibits secondary hydrogenation reactions. It is generally believed that FTS is a continuous reaction process, where primary products are mainly linear α-olefins and a small amount of n-alkanes. These α-olefins can further undergo secondary hydrogenation to form corresponding alkanes or isoparaffins.[\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e] Studies have shown that in nitrogen-doped carbon supports, pyrrolic nitrogen can act as an electron donor, transferring electrons to the adjacent active metal surface to form electron-rich active sites.[\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e] In the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst, pyrrolic nitrogen (-NH-, 33.7%), which accounts for the highest proportion, serves as an electron donor and transfers electrons to adjacent Co active sites through an electronic effect, forming electron-rich Co centers. XPS results confirm that nitrogen-doped carbon increases the electron density of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst. This electron-rich environment weakens the π-bond interaction between Co and olefins,[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] thereby inhibiting the secondary hydrogenation of α-olefins and increasing the selectivity of α-olefins. Meanwhile, the electron-rich Co also affects molecular adsorption behavior, increasing the CO adsorption capacity (consistent with the conclusion of CO-TPD), and the C/H ratio on the catalyst surface increases, which further inhibits the hydrogenation of α-olefins, leading to increased olefin selectivity of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst. Additionally, the probability of methyl hydrogenation to form CH\u003csub\u003e4\u003c/sub\u003e decreases, resulting in lower CH\u003csub\u003e4\u003c/sub\u003e selectivity. Combined with the basic sites provided by pyridinic nitrogen (15.9%), CO adsorption is enhanced, collectively inhibiting its secondary hydrogenation.[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] The increased C\u003csub\u003e5+\u003c/sub\u003e selectivity of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst stems from the mesoporous confinement environment formed by carbon layer coating. The mesoporous structure of the carbon layer delays the diffusion of C\u003csub\u003e5+\u003c/sub\u003e products, and this confined diffusion environment reduces the chances of side reactions such as cracking and hydrogenation, facilitating the continuous progress of the chain growth process. This is directly reflected in the increase in the chain growth probability α, from 0.78 for Co/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e to 0.83 for Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, ultimately promoting the enhancement of long-chain hydrocarbon selectivity.\u003c/p\u003e\u003c/div\u003e"},{"header":"4 Conclusions","content":"\u003cp\u003eIn this study, using glucose as the carbon source and oleylamine as the nitrogen source, a nitrogen-doped carbon layer modified Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst was successfully constructed through an in-situ carbonization process during the reduction activation stage. The evaluation results of FTS showed that the catalyst, while maintaining a steady-state activity of 4.2\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e molCO\u0026middot;gCo\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, enhanced the olefin selectivity to 34.4% and exhibited excellent stability within the 100-hour test period, achieving the synergistic optimization of olefin selectivity and stability. The regulatory mechanism of catalytic performance is as follows: The decreased activity of the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst mainly stems from the multiple effects of the nitrogen-doped carbon layer. Nitrogen species form Co-N bonds with Co, leading to reduced accessibility of active sites; the electron-donating effect of pyrrolic nitrogen increases CO adsorption capacity; and the physical coverage of the nitrogen-doped carbon layer inhibits the reduction of part of cobalt oxides. These factors collectively result in weakened activation ability of reactant molecules. In terms of selectivity, the enhancement of olefin selectivity is attributed to the electronic effect of pyrrolic nitrogen species. As electron donors, they increase the electron density on the cobalt surface, enhance CO adsorption capacity, and effectively inhibit the secondary reactions of α-olefins. In terms of stability, the physical confinement effect of the carbon layer inhibits the migration and agglomeration of cobalt particles, and combined with the thermal stabilization effect of graphitic nitrogen on carbon species, the anti-deactivation ability of the catalyst is significantly improved. This work provides new insights for the design of FTS catalysts with enhanced olefin selectivity and stability.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003eα\u0026nbsp; \u0026nbsp;\u0026nbsp;chain growth probability\u003c/p\u003e\n\u003cp\u003eCTY \u0026nbsp; \u0026nbsp;Co time yield\u003c/p\u003e\n\u003cp\u003eCO-TPD \u0026nbsp; \u0026nbsp;CO temperature-programmed desorption\u003c/p\u003e\n\u003cp\u003eFT-IR \u0026nbsp; \u0026nbsp;Fourier transform infrared spectroscopy\u003c/p\u003e\n\u003cp\u003eGHSV \u0026nbsp; \u0026nbsp;Gas Hourly Space Velocity\u003c/p\u003e\n\u003cp\u003eH\u003csub\u003e2\u003c/sub\u003e-TPR \u0026nbsp; \u0026nbsp;H\u003csub\u003e2\u003c/sub\u003e temperature-programmed reduction\u003c/p\u003e\n\u003cp\u003eSTEM \u0026nbsp; \u0026nbsp;Scanning transmission electron microscopy\u003c/p\u003e\n\u003cp\u003eTG \u0026nbsp; \u0026nbsp;Thermogravimetric analysis\u003c/p\u003e\n\u003cp\u003eDTG \u0026nbsp; \u0026nbsp;Derivative Thermogravimetry\u003c/p\u003e\n\u003cp\u003eXPS \u0026nbsp; \u0026nbsp;X-ray photoelectron spectroscopy\u003c/p\u003e\n\u003cp\u003eXRD \u0026nbsp; \u0026nbsp;X-ray diffraction\u003c/p\u003e\n\u003cp\u003eMS \u0026nbsp; \u0026nbsp;\u003cstrong\u003eM\u003c/strong\u003eass \u003cstrong\u003eS\u003c/strong\u003epectrometry\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics and Consent to Participate\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for Publication\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contribution\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eLili Wu: Writing – review \u0026amp; editing, Writing – original draft, Methodology, Investigation, Conceptualization. Shuying Ren: Writing – review \u0026amp; editing, Methodology, Formal analysis, Conceptualization. Chengchao Liu: Writing – review \u0026amp; editing, Supervision, Funding acquisition.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the National Key Research and Development Program of China (2022YFB4101200).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of Data and Materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis article includes the raw and processed data used to produce the results of the study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe thank the National Key Research and Development Program of China (2022YFB4101200) for financial support.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eZhou W, Cheng K, Kang J et al (2019) New horizon in C1 chemistry: breaking the selectivity limitation in transformation of syngas and hydrogenation of CO\u003csub\u003e2\u003c/sub\u003e into hydrocarbon chemicals and fuels. 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ACS Catal 12(9):5316-5326. https://doi.org/10.1021/acscatal.2c00926\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-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [Catalysis Letters](https://link.springer.com/journal/10562)","snPcode":"10562","submissionUrl":"https://submission.springernature.com/new-submission/10562/3","title":"Catalysis Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Fischer-Tropsch synthesis, Co-Based Catalysts, Nitrogen-doped, olefin selectivity","lastPublishedDoi":"10.21203/rs.3.rs-7767967/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7767967/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAlkali metal modification of cobalt-based Fischer-Tropsch synthesis catalysts suffers from poor stability and decreased activity. Exploring new strategies to enhance olefin selectivity and catalyst stability is of significant importance. In this work, a nitrogen-doped carbon layer-modified Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst was prepared using oleylamine as the nitrogen source and glucose as the carbon source. Under conditions of 230 ℃, 1 MPa, and 6 SL\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026middot;h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, the Co/NC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalyst achieved an activity of 4.2\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e molCO\u0026middot;gCo\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e with an olefin selectivity of 34.4%, while exhibiting high stability. Characterization and performance results demonstrate that: the electron-donating effect of pyrrolic nitrogen enriches electrons in adjacent metallic Co, enhancing CO adsorption and activation while inhibiting secondary hydrogenation of α-olefins, thereby increasing olefin selectivity; The improved stability originates from the confinement effect of the carbon layer and the stabilizing role of graphitic nitrogen on Co nanoparticles.\u003c/p\u003e","manuscriptTitle":"Nitrogen-doped Carbon-coated Al 2 O 3 supported Co 3 O 4 nanoparticles for Fischer-Tropsch synthesis: Boosting durability and Olefins selectivity","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-23 09:43:50","doi":"10.21203/rs.3.rs-7767967/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-11-09T18:04:40+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-10-27T09:13:15+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-10-21T02:58:45+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-10-19T13:40:01+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"207749961805025982860673548410333828473","date":"2025-10-10T15:34:27+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"114777866526812792639791034645909820468","date":"2025-10-10T11:29:15+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"148768286219420412348209885138565871195","date":"2025-10-10T06:32:19+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"248593235673270940231624928706011275183","date":"2025-10-10T01:11:44+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-10-09T16:01:41+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-10-06T13:48:55+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-10-06T13:47:46+00:00","index":"","fulltext":""},{"type":"submitted","content":"Catalysis Letters","date":"2025-10-02T16:14:01+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"catalysis-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [Catalysis Letters](https://link.springer.com/journal/10562)","snPcode":"10562","submissionUrl":"https://submission.springernature.com/new-submission/10562/3","title":"Catalysis Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"c0cfd4cd-da70-4351-a6fe-627fe2151d76","owner":[],"postedDate":"October 23rd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2025-12-10T08:39:12+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-23 09:43:50","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7767967","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7767967","identity":"rs-7767967","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}