Highly Dispersed and Stable Ni/SiO 2 Catalysts Prepared by Urea-Assisted Impregnation Method for Reverse Water–Gas Shift Reaction

: The nickel-based catalyst was more active in the reverse water-gas shift reaction, but it is easy to sinter and deactivate in high temperature reaction ( ≥ 600 ◦ C). A urea-assisted impregnation method was utilized to create a Ni/SiO 2 -N catalyst to increase the catalytic stability of Ni-based catalysts. For at least 20 h, the Ni/SiO 2 -N catalyst in the reverse water-gas shift process at 700 ◦ C remained stable, and in the high temperature RWGS reaction, the conversion rate of CO 2 of the catalyst is close to the equilibrium conversion rate. The catalysts were characterized by BET, XRD, H 2 -TPR, and TEM, and the results demonstrate that the Ni particles had a small particle size and exhibited strong interaction with the SiO 2 support in the Ni/SiO 2 -N catalyst, which led to the catalyst’s good activity and stability. Urea-assisted impregnation is a facile method to prepare stable Ni/SiO 2 catalysts with high Ni dispersion.


Introduction
Increased CO 2 emissions in recent years, causing the serious greenhouse effect.At the same time, CO 2 is also the most abundant and cheap C 1 resource.Therefore, the conversion of CO 2 into valuable chemicals and fuels has attracted extensive attention.CO 2 hydrogenation is one of the effective methods to produce a variety of products [1], including CH 4 , CO, and C 2 H 6 O, etc.In different hydrogenation reactions, the reverse water-gas shift (RWGS: CO 2 + H 2 = CO + H 2 O, ∆H = 40.6 kJ/mol) reaction, which had been thought to be one of the most promising reactions for CO 2 conversion, can transform CO 2 into more valuable CO [2][3][4][5][6].It can be utilized to perform Fischer-Tropsch synthe-sis in the presence of H 2 to create more valuable compounds.
Nowadays, the research of RWGS reaction catalysts has attracted more and more attention, various catalysts such as copper-based catalysts [7][8][9][10][11][12], noble metals [13][14][15], and nickel-based catalysts [16,17] have been used for the study of the reverse water gas shift reaction.Copper-based catalysts have a high selectivity for CO in the RWGS reaction, but their activity is low, and copper particles are easily aggregated during hightemperature reactions.The thermal stability of the copper-based catalyst is poor, and

Catalyst Characterization
With the Autosorb-iQ analyzer (Quantachrome Instruments (USA), Boynton Beach, FL, USA), N 2 adsorp-tion-desorption isotherms were obtained at a temperature of −196 • C. First, the test tube was weighed to determine its mass (M 1 ), and then 50 mg catalyst is added.The cata-lyst is then outgassed at a temperature of 300 • C for six hours to remove any impurities.After test, the test tube's mass was measured and recorded as M 2 , and the catalyst's mass is calculated as M = M 2 − M 1 .The Barret-Joyner-Halenda (BJH) method was used to calculate the average pore diameter and pore volume, while the Brunau-er-Emmet-Teller (BET) method was utilized to calculate the specific surface area.
The powder X-ray diffraction (XRD) spectrum of the catalysts were measured us-ing a China Haoyuan Instrument DX-2700 X-ray diffractometer.All catalysts were scanned at 1.2 • /min between 10 • and 80 • .Different crystals' spectral lines correspond to particular positions, quantities, and intensities, respectively.To determine the phase information of the catalyst sample, compare the crystal spectra with the standard PDF card.Then, use the Jade software to reassess the data to determine the catalyst.The Scherrer formula is used to calculate the grain size of the particular phase in the sam-ple based on the specific lattice parameters of the sample.The Equation ( 1) is as follows D is the desired grain size; the K value is a constant related to the crystal size (generally 0.89); λ is the wavelength of the incident X-ray (λ = 0.15 nm); β is the maxi-mum value of the diffraction peak intensity 1/2 peak width; θ is the size of the diffrac-tion angle.
The H 2 temperature-programmed reduction (H 2 -TPR) experiment was performed using the TP-5080 instrument with a thermal conductivity detector (TCD).First, 50 mg catalyst was pre-treated with Ar at 400 • C for 30 min and then cooled to room temperature.After that, 5%H 2 /Ar gas was introduced for 20 min, and the catalyst was heated from room temperature to 900 • C at a ramp rate of 10 • C/min.The signal of H 2 -TPR was detected using a TCD detector.
The Ni dispersion was measured using the TP-5080 instrument through the N 2 O titration test.The catalyst of 50 mg was first pretreated with Ar gas at 400 • C for 10 min.After cooling down to room temperature, it was then exposed to 5% H 2 /Ar gas and heated from room temperature to 700 • C at a rate of 10 • C/min and kept at 700 • C for 40 min.The TCD detector was used to detect the signal, and S 1 was used to represent the reduction peak area of this process.After cooling the catalyst down to 90 • C, pure N 2 O gas was introduced for 30 min, and then the catalyst was cooled down to room temperature before exposing it again to 5% H 2 /Ar gas.It was then heated from room temperature to 700 • C at a rate of 10 • C/min for reduction.The TCD detector was used to detect the signal, and S 2 was used to represent the reduction peak area of this process.The dispersion of Ni in the sample could be calculated as D = S 2 /S 1 .
Tecnai G2 F20 microscope was used to characterize the catalyst at 200 kV accel-erating voltage, and transmission electron microscope (TEM) images were formed.Firstly, the catalysts were ultrasonically treated for 30min, then it was evenly dis-persed in acetone, and the mixture of powder and acetone was dripped into copper mesh by capillary tube, and then it was dried and analyzed.

Catalytic Test
Under the atmospheric pressure, 10 mg catalyst mixed with 100 mg quartz sand were put into a quartz tube fixed bed reactor (Φ = 8 mm).Before the RWGS reaction, the H 2 /N 2 = 1/4 mixed gas was heated to 700 • C for reduction for 40 min at a rate of 10 • C/min.
The RWGS reaction was fed with 100 mL/min of H 2 /CO 2 gas flow at a feed ratio of 1:1, the space velocity was 6 × 10 5 mL/g cat /h. and the test temperature range is 400-750 • C. Every temperature point was set at 50 • C, and each one was examined for an hour to get the average value.Under the same feed conditions, the catalyst was tested for long-term high temperature stability.The components of raw material gas and tail gas were analyzed online on the gas chromatography SP3420A equipped with a TCD detector.The CO 2 conversion and selectivity of the catalyst were calculated by Equations ( 2)-(4) to evaluate the catalyst where CO 2 in is the flow rate of CO 2 in the feed gas; CO 2 out , CO out and CH 4 out are the flows of CO 2 , CO and CH 4 in the exit airflow, respectively.

Results and Discussion
The structural characteristics of Ni/SiO 2 and Ni/SiO 2 -N catalysts and SiO 2 supports were studied by N 2 adsorption-desorption experiments.As shown in Figure 1a, all samples exhibited Type IV isotherms with H2-type hysteresis loops (IUPAC classification) at relative pressures (P/P 0 ) ranging from 0.7 to 1.0, indicating the existence of mesoporous structures in the samples [29].Figure 1b shows that the pore size distribution of all samples is around 10 nm, implying that the original mesoporous structure of the SiO 2 support was well maintained.
The RWGS reaction was fed with 100 mL/min of H2/CO2 gas flow at a feed ratio of 1:1, the space velocity was 6 × 10 5 mL/gcat/h.and the test temperature range is 400-750 °C.Every temperature point was set at 50 °C, and each one was examined for an hour to get the average value.Under the same feed conditions, the catalyst was tested for long-term high temperature stability.The components of raw material gas and tail gas were analyzed online on the gas chromatography SP3420A equipped with a TCD detector.The CO2 conversion and selectivity of the catalyst were calculated by Equations ( 2)-(4) to evaluate the catalyst where  is the flow rate of CO2 in the feed gas;  ,  and  are the flows of CO2, CO and CH4 in the exit airflow, respectively.

Results and Discussion
The structural characteristics of Ni/SiO2 and Ni/SiO2-N catalysts and SiO2 supports were studied by N2 adsorption-desorption experiments.As shown in Figure 1a, all samples exhibited Type IV isotherms with H2-type hysteresis loops (IUPAC classification) at relative pressures (P/P0) ranging from 0.7 to 1.0, indicating the existence of mesoporous structures in the samples [29].Figure 1b shows that the pore size distribution of all samples is around 10 nm, implying that the original mesoporous structure of the SiO2 support was well maintained.Table 1 provides a list of the catalysts' specific surface areas (SBET).The SiO2 support possesses the largest surface area (SBET = 367.4m 2 /g).As expected, the specific surface area of both Ni/SiO2 and Ni/SiO2-N catalysts decreased after the addition of NiO, and they are 323.3m 2 /g and 329.6 m 2 /g respectively, which was due to some NiO species being located in the pores of SiO2 or at the entrance of the pores.Moreover, the surface area and average pore size of Ni/SiO2 and Ni/SiO2-N were similar.The results show that the structural characteristics of the SiO2 support are unaffected by the addition of urea.Table 1 provides a list of the catalysts' specific surface areas (S BET ).The SiO 2 support possesses the largest surface area (S BET = 367.4m 2 /g).As expected, the specific surface area of both Ni/SiO 2 and Ni/SiO 2-N catalysts decreased after the addition of NiO, and they are 323.3m 2 /g and 329.6 m 2 /g respectively, which was due to some NiO species being located in the pores of SiO 2 or at the entrance of the pores.Moreover, the surface area and average pore size of Ni/SiO 2 and Ni/SiO 2 -N were similar.The results show that the structural characteristics of the SiO 2 support are unaffected by the addition of urea.The Ni/SiO2 and Ni/SiO2-N catalysts' XRD patterns are shown in Figure 2. Figure 2a shows the fresh catalysts.Figure 2b shows the catalysts were reduced by 50 mL/min of 20% H2/N2 mixture at 700 °C for 40 min.All samples have diffraction peaks of amorphous SiO2 at 2θ = 21.6°[30].
As shown in Figure 2a, the diffraction peaks at 2θ of 37.2°, 43.3°, and 62.9° were observed in the fresh Ni/SiO2 and Ni/SiO2-N catalysts, and they were attributed to the (111), (200), and (220) crystal planes of NiO, respectively.The X-ray diffraction pattern of reduced Ni/SiO2 shows obvious characteristic peaks of metallic Ni, which indicates that larger Ni particles are formed in the catalyst, and the size of metal Ni particles in the Ni/SiO2 catalyst is 11.9 nm (Table 1), according to the Scherrer formula.Whereas the reduced Ni/SiO2-N catalyst has weak diffraction peaks of metallic Ni, and the reduced Ni/SiO2-N catalyst has no diffraction peak at 2θ = 75.45,which means that most metallic Ni in Ni/SiO2-N catalyst is highly dispersed on the surface of SiO2 or combined with SiO2 [31].The XRD results demonstrate that the addition of urea is conducive to the formation of highly dispersed small particles of Ni-based catalyst.The H2-TPR patterns of Ni/SiO2 and Ni/SiO2-N catalysts are shown in Figure 3.There are two reduction peaks for the Ni/SiO2 catalyst.The first reduction peak is about 408 °C(α1), which is similar to the reduction temperature of pure NiO (about 403 °C) reported in the literature and corresponds to the reduction of free NiO [17,[32][33][34].The second reduction peak is at 428 °C(α2), which is higher than the reduction temperature of pure NiO and can be attributed to the reduction of NiO with weak interaction with SiO2 [30].The results indicate that in Ni/SiO2 catalyst, the interaction between large particles of NiO and SiO2 support is weak or not at all.The Ni/SiO2-N catalyst contains a smaller α2 reduction peak and a larger β reduction peak.The H2 consumption of the α2 reduction peak is 108 µmol/gcat, and that of the β reduction peak is 315 µmol/gcat.Reduction at 665 °C Peak β, indicating that the nickel-based species has a strong interaction with the support [35].The As shown in Figure 2a, the diffraction peaks at 2θ of 37.2 • , 43.3 • , and 62.9 • were observed in the fresh Ni/SiO 2 and Ni/SiO 2 -N catalysts, and they were attributed to the (111), (200), and (220) crystal planes of NiO, respectively.The X-ray diffraction pattern of reduced Ni/SiO 2 shows obvious characteristic peaks of metallic Ni, which indicates that larger Ni particles are formed in the catalyst, and the size of metal Ni particles in the Ni/SiO 2 catalyst is 11.9 nm (Table 1), according to the Scherrer formula.Whereas the reduced Ni/SiO 2 -N catalyst has weak diffraction peaks of metallic Ni, and the reduced Ni/SiO 2 -N catalyst has no diffraction peak at 2θ = 75.45,which means that most metallic Ni in Ni/SiO 2 -N catalyst is highly dispersed on the surface of SiO 2 or combined with SiO 2 [31].The XRD results demonstrate that the addition of urea is conducive to the formation of highly dispersed small particles of Ni-based catalyst.
The H 2 -TPR patterns of Ni/SiO 2 and Ni/SiO 2 -N catalysts are shown in Figure 3.There are two reduction peaks for the Ni/SiO 2 catalyst.The first reduction peak is about 408 • C(α1), which is similar to the reduction temperature of pure NiO (about 403 • C) reported in the literature and corresponds to the reduction of free NiO [17,[32][33][34].The second reduction peak is at 428 • C(α2), which is higher than the reduction temperature of pure NiO and can be attributed to the reduction of NiO with weak interaction with SiO 2 [30].The results indicate that in Ni/SiO 2 catalyst, the interaction between large particles of NiO and SiO 2 support is weak or not at all.The Ni/SiO 2 -N catalyst contains a smaller α 2 reduction peak and a larger β reduction peak.The H 2 consumption of the α 2 reduction peak is 108 µmol/g cat , and that of the β reduction peak is 315 µmol/g cat .Reduction at 665 • C Peak β, indicating that the nickel-based species has a strong interaction with the support [35].The free NiO and some NiO particles with weak interaction with SiO 2 were transformed into highly dispersed NiO particles with strong interaction with the support It may be attributed to the hydrolysis reaction (CO(NH 2 ) 2 + H 2 O → 2NH 3 + CO 2 ) in the urea-assisted impregnation process, and the [Ni(NH) 3 ) 6 ] 2+ complex formed by ammonia and Ni can avoid Ni(OH) 2 precipitation during impregnation, which is beneficial to the uniform dispersion of Ni species on the surface of SiO 2 .At the same time, the alkaline environment formed by urea can slightly dissolve the surface of silica and promote the strong interaction between Ni species and SiO 2 during the calcination process [36].
Processes 2023, 11, x FOR PEER REVIEW 6 of 12 free NiO and some NiO particles with weak interaction with SiO2 were transformed into highly dispersed NiO particles with strong interaction with the support It may be attributed to the hydrolysis reaction (CO(NH2)2 + H2O → 2NH3 + CO2) in the urea-assisted impregnation process, and the [Ni(NH)3)6] 2+ complex formed by ammonia and Ni can avoid Ni(OH)2 precipitation during impregnation, which is beneficial to the uniform dispersion of Ni species on the surface of SiO2.At the same time, the alkaline environment formed by urea can slightly dissolve the surface of silica and promote the strong interaction between Ni species and SiO2 during the calcination process [36].The results of the TEM images of the reduced Ni/SiO2 and Ni/SiO2-N catalysts, which were used to investigate the dispersion and determine the particle size of Ni, are shown in Figure 4.For the reduced Ni/SiO2 catalysts (Figure 4a-c), the Ni metal particles show obvious agglomeration, showing poor Ni dispersion.The particle size distribution diagram is shown in Figure 4b, and its distribution ranges from 6 to 24 nm, with a 14.3 nm particle size on average.The representative region in Figure 4b is extended to form the image of Figure 4c, and a lattice distance of 0.203 nm is observed, which is assigned to Ni(111), indicating the full crystal structure of Ni nanoparticles has been formed.In contrast, for the Ni/SiO2-N catalyst after H2 reduction (Figure 4d-f), the metallic Ni particles are uniformly dispersed on the SiO2 support (Figure 4e), and the particle size distribution of metallic Ni ranges from 2 to 6 nm, with 3.8 nm Ni particles on average.Similarly, as shown in Figure 4f, a lattice spacing of 0.203 nm in length can be observed, ascribed to Ni(111).The results show that the Ni/SiO2-N prepared by the urea-assisted impregnation method can effectively inhibit the agglomeration of Ni nanoparticles during the high-temperature reduction process, and formed highly dispersed Ni particles with strong interaction with the SiO2 support, which is consistent with the results of XRD and H2-TPR.4b, and its distribution ranges from 6 to 24 nm, with a 14.3 nm particle size on average.The representative region in Figure 4b is extended to form the image of Figure 4c, and a lattice distance of 0.203 nm is observed, which is assigned to Ni(111), indicating the full crystal structure of Ni nanoparticles has been formed.In contrast, for the Ni/SiO 2 -N catalyst after H 2 reduction (Figure 4d-f), the metallic Ni particles are uniformly dispersed on the SiO 2 support (Figure 4e), and the particle size distribution of metallic Ni ranges from 2 to 6 nm, with 3.8 nm Ni particles on average.Similarly, as shown in Figure 4f, a lattice spacing of 0.203 nm in length can be observed, ascribed to Ni(111).The results show that the Ni/SiO 2 -N prepared by the urea-assisted impregnation method can effectively inhibit the agglomeration of Ni nanoparticles during the high-temperature reduction process, and formed highly dispersed Ni particles with strong interaction with the SiO 2 support, which is consistent with the results of XRD and H 2 -TPR.
The Ni dispersion was measured through the N 2 O titration test.and the results are listed in Table 2.The dispersions of Ni/SiO 2 and Ni/SiO 2 -N catalysts were 7.7% and 24.7%, respectively.According to the particle size in Figure 4b,e, the dispersion of Ni/SiO 2 and Ni/SiO 2 -N catalysts was calculated to be 7.1% and 26.6%, respectively, which is close to that of the N 2 O titration test.As a result, there are a lot of active sites available for the RWGS reaction and the Ni dispersion can be improved with the addition of urea.The Ni dispersion was measured through the N2O titration test.and the results are listed in Table 2.The dispersions of Ni/SiO2 and Ni/SiO2-N catalysts were 7.7% and 24.7%, respectively.According to the particle size in Figure 4b,e, the dispersion of Ni/SiO2 and Ni/SiO2-N catalysts was calculated to be 7.1% and 26.6%, respectively, which is close to that of the N2O titration test.As a result, there are a lot of active sites available for the RWGS reaction and the Ni dispersion can be improved with the addition of urea.

Catalyst Performance Test at Different Temperature
Figure 5 shows the RWGS performance tests of catalysts at 400 °C to 750 °C.including (a) CO2 conversion and (b) selectivity of CO and CH4.The catalyst's CO2 equilibrium conversion significantly increased as the reaction temperature increased, whereas the selectivity of CH4 gradually dropped.This is because the reaction of methanation is an exothermic one that benefits from low temperatures, whereas the reverse water gas shift reaction is an endothermic reaction and benefits more from high temperatures [37].The only by-product of the Ni/SiO2 and Ni/SiO2-N catalysts is CH4.At the same temperature, the Ni/SiO2-N catalyst has a higher CO2 conversion than the Ni/SiO2 catalyst.Especially, the CO2 conversion of the Ni/SiO2 catalyst decreases gradually at the temperatures higher than 600 °C.because during the high temperature reaction process, the Ni particles in the Ni/SiO2 catalysis have an agglomeration effect, and the metal surface area becomes smaller, which leads to a decrease in the CO2 conversion of the catalyst, indicating the Ni/SiO2 catalyst was instable at high temperature.However, the difference is that with the rising temperature, Ni/SiO2-N's CO2 conversion rate gradually rises and becomes closer to the equilibrium conversion rate, and the selectivity for CO is close to 100%.Due to the highly dispersed NiO in the Ni/SiO2-N catalyst has a strong interaction with the SiO2 support, the Ni nanoparticles are not easy to agglomerate and sinter at high temperature, and the higher CO2 conversion of Ni/SiO2-N is related to highly dispersed Ni in the catalyst    1.

Catalyst Performance Test at Different Temperature
Figure 5 shows the RWGS performance tests of catalysts at 400 • C to 750 • C. including (a) CO 2 conversion and (b) selectivity of CO and CH 4 .The catalyst's CO 2 equilibrium conversion significantly increased as the reaction temperature increased, whereas the selectivity of CH 4 gradually dropped.This is because the reaction of methanation is an exothermic one that benefits from low temperatures, whereas the reverse water gas shift reaction is an endothermic reaction and benefits more from high temperatures [37].The only by-product of the Ni/SiO 2 and Ni/SiO 2 -N catalysts is CH 4. At the same temperature, the Ni/SiO 2 -N catalyst has a higher CO 2 conversion than the Ni/SiO 2 catalyst.Especially, the CO 2 conversion of the Ni/SiO 2 catalyst decreases gradually at the temperatures higher than 600 • C. because during the high temperature reaction process, the Ni particles in the Ni/SiO 2 catalysis have an agglomeration effect, and the metal surface area becomes smaller, which leads to a decrease in the CO 2 conversion of the catalyst, indicating the Ni/SiO 2 catalyst was instable at high temperature.However, the difference is that with the rising temperature, Ni/SiO 2 -N's CO 2 conversion rate gradually rises and becomes closer to the equilibrium conversion rate, and the selectivity for CO is close to 100%.Due to the highly dispersed NiO in the Ni/SiO 2 -N catalyst has a strong interaction with the SiO 2 support, the Ni nanoparticles are not easy to agglomerate and sinter at high temperature, and the higher CO 2 conversion of Ni/SiO 2 -N is related to highly dispersed Ni in the catalyst [17].According to the characterization results, most of the Ni in Ni/SiO 2 -N catalyst is widely scattered on the SiO 2 support and forms small Ni nanoparticles, which provide rich active centers for RWGS reaction and are directly connected to the high activity of the catalyst.[17].According to the characterization results, most of the Ni in Ni/SiO2-N catalyst is widely scattered on the SiO2 support and forms small Ni nanoparticles, which provide rich active centers for RWGS reaction and are directly connected to the high activity of the catalyst.

Catalyst Stability Test at High temperature
Under high temperature reaction circumstances, the stability of the catalysts is a significant concern for reverse water gas shift catalysts.To assess the stability of the catalysts, a long-term reaction test was performed at 700 °C, and the findings are shown in Figure 6.Under the condition of high temperature, the exothermic methanation reaction is inhibited, so the CO selectivity of both catalysts is close to 100%.
During 20 h reaction process, the CO2 conversion rate of the Ni/SiO2 catalyst gradually decreased from 23.4% to 13.1%, and the catalyst only kept 56% of its initial activity.The deactivation rate was 0.022 h −1 , which may be caused by the agglomeration and sintering deactivation of large Ni metal during the reaction.On the contrary, the CO2 conversion rate of the Ni/SiO2-N catalyst decreased from 40.8% to 40.2%, and the catalyst kept its initial activity of 98.5%.According to the H2-TPR characterization, it can be known that the using of urea makes the NiO species in the Ni/SiO2-N catalyst interact strongly with the support SiO2, effectively avoiding the agglomeration and sintering of Ni species at high temperatures while retaining Niʹs high degree of dispersion.Therefore, the Ni/SiO2-N catalyst demonstrates excellent stability in high temperature reaction tests.

Catalyst Stability Test at High temperature
Under high temperature reaction circumstances, the stability of the catalysts is a significant concern for reverse water gas shift catalysts.To assess the stability of the catalysts, a long-term reaction test was performed at 700 • C, and the findings are shown in Figure 6.Under the condition of high temperature, the exothermic methanation reaction is inhibited, so the CO selectivity of both catalysts is close to 100%.

Catalyst Stability Test at High temperature
Under high temperature reaction circumstances, the stability of the catalysts is a significant concern for reverse water gas shift catalysts.To assess the stability of the catalysts, a long-term reaction test was performed at 700 °C, and the findings are shown in Figure 6.Under the condition of high temperature, the exothermic methanation reaction is inhibited, so the CO selectivity of both catalysts is close to 100%.
During 20 h reaction process, the CO2 conversion rate of the Ni/SiO2 catalyst gradually decreased from 23.4% to 13.1%, and the catalyst only kept 56% of its initial activity.The deactivation rate was 0.022 h −1 , which may be caused by the agglomeration and sintering deactivation of large Ni metal during the reaction.On the contrary, the CO2 conversion rate of the Ni/SiO2-N catalyst decreased from 40.8% to 40.2%, and the catalyst kept its initial activity of 98.5%.According to the H2-TPR characterization, it can be known that the using of urea makes the NiO species in the Ni/SiO2-N catalyst interact strongly with the support SiO2, effectively avoiding the agglomeration and sintering of Ni species at high temperatures while retaining Niʹs high degree of dispersion.Therefore, the Ni/SiO2-N catalyst demonstrates excellent stability in high temperature reaction tests.During 20 h reaction process, the CO 2 conversion rate of the Ni/SiO 2 catalyst gradually decreased from 23.4% to 13.1%, and the catalyst only kept 56% of its initial activity.The deactivation rate was 0.022 h −1 , which may be caused by the agglomeration and sintering deactivation of large Ni metal during the reaction.On the contrary, the CO 2 conversion rate of the Ni/SiO 2 -N catalyst decreased from 40.8% to 40.2%, and the catalyst kept its initial activity of 98.5%.According to the H 2 -TPR characterization, it can be known that the using of urea makes the NiO species in the Ni/SiO 2 -N catalyst interact strongly with the support SiO 2 , effectively avoiding the agglomeration and sintering of Ni species at high temperatures while retaining Ni's high degree of dispersion.Therefore, the Ni/SiO 2 -N catalyst demonstrates excellent stability in high temperature reaction tests.
After the RWGS reaction was conducted at 700 • C for 20 h, the catalyst's XRD pattern is shown in Figure 7.The used Ni/SiO 2 catalyst showed the obvious characteristic peak of metallic Ni, according to the Scherrer formula, the Ni size of the used Ni/SiO 2 increased to 34.8 nm (Table 1), indicating that the Ni/SiO 2 catalyst is unstable during the long-term high temperature RWGS reaction, and the Ni particles have an aggregation effect during the reaction process, forming larger metal Ni particles.On the contrary, in the used Ni/SiO 2 -N catalyst, the weak Ni diffraction peaks were obtained, and the intensity of the diffraction peak has no obvious change, which demonstrated that the high dispersion and small Ni particle size in the catalyst of Ni/SiO 2 -N after a long-term reaction.
Processes 2023, 11, x FOR PEER REVIEW 9 of 12 After the RWGS reaction was conducted at 700 °C for 20 h, the catalyst's XRD pattern is shown in Figure 7.The used Ni/SiO2 catalyst showed the obvious characteristic peak of metallic Ni, according to the Scherrer formula, the Ni size of the used Ni/SiO2 increased to 34.8 nm (Table 1), indicating that the Ni/SiO2 catalyst is unstable during the long-term high temperature RWGS reaction, and the Ni particles have an aggregation effect during the reaction process, forming larger metal Ni particles.On the contrary, in the used Ni/SiO2-N catalyst, the weak Ni diffraction peaks were obtained, and the intensity of the diffraction peak has no obvious change, which demonstrated that the high dispersion and small Ni particle size in the catalyst of Ni/SiO2-N after a long-term reaction.Figure 8 shows the TEM images of Ni/SiO2 and Ni/SiO2-N catalysts following the stability test.Ni/SiO2 has an average particle size of 37.5 nm, as evidenced by the particle size statistics in Figure 8b., and large particles of metal Ni were formed, which is much larger than 14.3 nm before the stability test, indicating that the Ni particles are severely agglomerated.On the contrary, after the stability test, the metallic Ni nanoparticles in the Ni/SiO2-N catalyst highly dispersed on SiO2 support, which range in size from 3.0 to 6.5 nm, with just 4.6 nm being the average size.The findings indicate that the addition of urea effectively inhibits the sintering of Ni nanoparticles in high temperature reaction, due to the powerful interaction between highly dispersed Ni particles and SiO2 support.The stability of the catalyst structure is significantly influenced by the addition of urea. Figure 8 shows the TEM images of Ni/SiO 2 and Ni/SiO 2 -N catalysts following the stability test.Ni/SiO 2 has an average particle size of 37.5 nm, as evidenced by the particle size statistics in Figure 8b, and large particles of metal Ni were formed, which is much larger than 14.3 nm before the stability test, indicating that the Ni particles are severely agglomerated.On the contrary, after the stability test, the metallic Ni nanoparticles in the Ni/SiO 2 -N catalyst highly dispersed on SiO 2 support, which range in size from 3.0 to 6.5 nm, with just 4.6 nm being the average size.The findings indicate that the addition of urea effectively inhibits the sintering of Ni nanoparticles in high temperature reaction, due to the powerful interaction between highly dispersed Ni particles and SiO 2 support.The stability of the catalyst structure is significantly influenced by the addition of urea.
In order to further study the long-term high-temperature reaction stability of the Ni/SiO 2 -N catalyst, under the same high temperature test conditions, the catalyst was tested for 50 h of stability.As shown in Figure 9, after the long-term reaction test, CO 2 conversion of the Ni/SiO 2 -N catalyst dropped from 40.2% to 39.1%, and the catalyst maintained 97.2% of the initial activity.The deactivation rate of the Ni/SiO 2 -N catalyst was 0.00056 h −1 , which was only 2.5% of that of the Ni/SiO 2 catalyst (0.022 h −1 ).The results showed that the stability of the Ni/SiO 2 -N catalyst was excellent during 50 h in a high-temperature RWGS reaction.
The catalyst stability test showed that the addition of urea has a substantial effect on the Ni/SiO 2 catalyst's stability.Because of the addition of urea, the catalyst was in the form of Ni metal particles with high dispersion and small particle size, which interact strongly with the SiO 2 support.Therefore, Ni/SiO 2 -N catalyst has better thermal stability.larger than 14.3 nm before the stability test, indicating that the Ni particles are severely agglomerated.On the contrary, after the stability test, the metallic Ni nanoparticles in the Ni/SiO2-N catalyst highly dispersed on SiO2 support, which range in size from 3.0 to 6.5 nm, with just 4.6 nm being the average size.The findings indicate that the addition of urea effectively inhibits the sintering of Ni nanoparticles in high temperature reaction, due to the powerful interaction between highly dispersed Ni particles and SiO2 support.The stability of the catalyst structure is significantly influenced by the addition of urea.In order to further study the long-term high-temperature reaction stability of the Ni/SiO2-N catalyst, under the same high temperature test conditions, the catalyst was tested for 50 h of stability.As shown in Figure 9, after the long-term reaction test, CO2 conversion of the Ni/SiO2-N catalyst dropped from 40.2% to 39.1%, and the catalyst maintained 97.2% of the initial activity.The deactivation rate of the Ni/SiO2-N catalyst was 0.00056 h −1 , which was only 2.5% of that of the Ni/SiO2 catalyst (0.022 h −1 ).The results showed that the stability of the Ni/SiO2-N catalyst was excellent during 50 h in a hightemperature RWGS reaction.The catalyst stability test showed that the addition of urea has a substantial effect on the Ni/SiO2 catalyst's stability.Because of the addition of urea, the catalyst was in the form In order to further study the long-term high-temperature reaction stability of the Ni/SiO2-N catalyst, under the same high temperature test conditions, the catalyst was tested for 50 h of stability.As shown in Figure 9, after the long-term reaction test, CO2 conversion of the Ni/SiO2-N catalyst dropped from 40.2% to 39.1%, and the catalyst maintained 97.2% of the initial activity.The deactivation rate of the Ni/SiO2-N catalyst was 0.00056 h −1 , which was only 2.5% of that of the Ni/SiO2 catalyst (0.022 h −1 ).The results showed that the stability of the Ni/SiO2-N catalyst was excellent during 50 h in a hightemperature RWGS reaction.The catalyst stability test showed that the addition of urea has a substantial effect on the Ni/SiO2 catalyst's stability.Because of the addition of urea, the catalyst was in the form of Ni metal particles with high dispersion and small particle size, which interact strongly with the SiO2 support.Therefore, Ni/SiO2-N catalyst has better thermal stability.

Conclusions
In this study, the Ni/SiO2-N was prepared using a urea-assisted impregnation method and applied to the RWGS reaction.The Ni/SiO2 catalyst prepared by the conventional impregnation method, after high temperature reduction, the particle size of Ni reaches 14.3 nm, and its dispersion is only 7.1%, and the catalyst agglomerates and effects during the

Conclusions
In this study, the Ni/SiO 2 -N was prepared using a urea-assisted impregnation method and applied to the RWGS reaction.The Ni/SiO 2 catalyst prepared by the conventional impregnation method, after high temperature reduction, the particle size of Ni reaches 14.3 nm, and its dispersion is only 7.1%, and the catalyst agglomerates and effects during the high temperature reaction, resulting in a decrease in the activity of the catalyst.However, in the process of pre-paring the catalyst, adding urea to the mixture can effectively reduce

Figure 3 .
Figure 3. H 2 -TPR patterns of fresh Ni/SiO 2 and Ni/SiO 2 -N catalysts.The results of the TEM images of the reduced Ni/SiO 2 and Ni/SiO 2 -N catalysts, which were used to investigate the dispersion and determine the particle size of Ni, are shown in Figure4.For the reduced Ni/SiO 2 catalysts (Figure4a-c), the Ni metal particles show obvious agglomeration, showing poor Ni dispersion.The particle size distribution diagram is shown in Figure4b, and its distribution ranges from 6 to 24 nm, with a 14.3 nm particle size on average.The representative region in Figure4bis extended to form the image of Figure4c, and a lattice distance of 0.203 nm is observed, which is assigned to Ni(111), indicating the full crystal structure of Ni nanoparticles has been formed.In contrast, for the Ni/SiO 2 -N catalyst after H 2 reduction (Figure4d-f), the metallic Ni particles are uniformly dispersed on the SiO 2 support (Figure4e), and the particle size distribution of metallic Ni ranges from 2 to 6 nm, with 3.8 nm Ni particles on average.Similarly, as shown in Figure4f, a lattice spacing of 0.203 nm in length can be observed, ascribed to Ni(111).The results show that the Ni/SiO 2 -N prepared by the urea-assisted impregnation method can effectively inhibit the agglomeration of Ni nanoparticles during the high-temperature reduction process, and formed highly dispersed Ni particles with strong interaction with the SiO 2 support, which is consistent with the results of XRD and H 2 -TPR.The Ni dispersion was measured through the N 2 O titration test.and the results are listed in Table2.The dispersions of Ni/SiO 2 and Ni/SiO 2 -N catalysts were 7.7% and 24.7%, respectively.According to the particle size in Figure4b,e, the dispersion of Ni/SiO 2 and Ni/SiO 2 -N catalysts was calculated to be 7.1% and 26.6%, respectively, which is close to that of the N 2 O titration test.As a result, there are a lot of active sites available for the RWGS reaction and the Ni dispersion can be improved with the addition of urea.

a
Obtained by N 2 O titration test.b Determined by the expression, D Ni = 101/d Ni (%).The d Ni was measured by TEM as shown in Table

Processes 2023 , 12 Figure 8 .
Figure 8. TEM images of the catalysts after the stability test and the size distribution of Ni nanoparticles (a,b) Ni/SiO2, (c,d) Ni/SiO2-N.

Figure 8 . 12 Figure 8 .
Figure 8. TEM images of the catalysts after the stability test and the size distribution of Ni nanoparticles (a,b) Ni/SiO 2 , (c,d) Ni/SiO 2 -N.

m 2 /g) Pore Volume (cm 3 /g) Average Pore Size (nm) d NiO a (nm) d Ni (nm) By XRD By TEM
Calculated by the Scherrer formula from XRD measurements.bNisize of the reduced catalyst.cNisize of the used catalyst.dNotcalculatedduetotheweakpeak in XRD pattern.The Ni/SiO 2 and Ni/SiO 2 -N catalysts' XRD patterns are shown in Figure2.Figure2ashows the fresh catalysts.Figure2bshows the catalysts were reduced by 50 mL/min of 20% H 2 /N 2 mixture at 700 • C for 40 min.All samples have diffraction peaks of amorphous SiO 2 at 2θ = 21.6 • [30].
a a Calculated by the Scherrer formula from XRD measurements.b Ni size of the reduced catalyst.c Ni size of the used catalyst.d Not calculated due to the weak peak in XRD pattern.