Co-Promoted Ni Nanocatalysts Derived from NiCoAl-LDHs for Low Temperature CO 2 Methanation

: Ni-based catalysts are prone to agglomeration and carbon deposition at high temperatures. Therefore, the development of Ni-based catalysts with high activities at low temperatures is a very urgent and challenging research topic. Herein, Ni-based nanocatalysts containing Co promoter with mosaic structure were prepared by reduction of NiCoAl-LDHs, and used for CO 2 methanation. When the reaction temperature is 250 ◦ C (0.1 MPa, GHSV = 30,000 mL · g − 1 · h − 1 ), the conversion of CO 2 on the NiCo 0.5 Al-R catalyst reaches 81%. However, under the same test conditions, the conversion of CO 2 on the NiAl-R catalyst is only 26%. The low-temperature activity is signiﬁcantly improved due to Co which can effectively control the size of the Ni particles, so that the catalyst contains more active sites. The CO 2 -TPD results show that the Co can also regulate the number of moderately basic sites in the catalyst, which is beneﬁcial to increase the amount of CO 2 adsorbed. More importantly, the NiCo 0.5 Al-R catalyst still maintains high catalytic performance after 92 h of continuous reaction. This is due to the conﬁnement effect of the AlO x substrate inhibiting the agglomeration of Ni nanoparticles. The Ni-based catalysts with high performance at low temperature and high stability prepared by the method used have broad industrial application prospects.


Introduction
The large amount of CO 2 emission has caused the greenhouse effect to be more obvious, consequently leading to serious environmental problems. Therefore, how to effectively reduce the concentration of CO 2 in the atmosphere has aroused widespread research interest [1][2][3][4][5]. At present, there have been many reports on the research in this aspect, such as the reduction of CO 2 to produce methanol [6,7], alkanes (CH 4 , C 2 H 6 ) [8][9][10][11] and alkenes (C 2 H 4 , C 3 H 6 , C 4 H 8 ) [12][13][14]. Converting CO 2 into chemical products with high added value can not only reduce the concentration of CO 2 in the atmosphere but also realize the efficient recycling of resources. Among them, the production of CH 4 with CO 2 as a raw material can also help solve the problem of insufficient supply of CH 4 in the market, so the CO 2 methanation reaction has received increasing attention.
Although precious metal-based catalysts have excellent low-temperature catalytic activity [15,16], their cost is too high to be suitable for industrial applications. Compared with precious metal catalysts, Ni-based catalysts have poor low-temperature activity, but their cost is low [17][18][19][20]. Therefore, Ni-based catalysts are currently the most widely used in industry. However, the Ni-based catalysts currently used in industrial applications require relatively high activation temperatures [21,22]. As we know, CO 2 methanation is a strongly exothermic reaction, so high temperature is not conducive to the progress of the reaction. At the same time, when reacting at high temperatures, Ni-based catalysts are prone to agglomeration, sintering and carbon deposition, which may cause catalysts deactivation [23][24][25]. At present, the commonly used methods to improve the activity and

Structural and Morphology Characterization of Samples
The XRD characterization technique was used to explore the phases of hydrothermally synthesized samples. As shown in Figure 1a, there are several diffraction peaks at 11.6 • , 22.7 • , 34.7 • , 39.0 • , 46.1 • , 60.7 • and 62.1 • , which can be indexed to the (003), (006), (009), (015), (018), (110) and (113) crystal planes of typical LDHs, respectively [41]. The XRD results indicate that NiCo x Al-LDHs (x = 0, 0.25, 0.5, 1) were successfully prepared. Compared with NiAl-LDH, the diffraction peak intensity of NiCo x Al-LDHs became stronger, indicating that the addition of Co improves the crystallinity of hydrotalcites. The NiCo x Al-LDHs precursors were directly reduced to prepare Co-promoted Ni-based catalysts, and the XRD results are shown in Figure 1b. It can be observed that the diffraction peaks of hydrotalcites disappeared, but new diffraction peaks appeared at 44.6 • , 51.9 • , 76.8 • , which correspond to the (111), (200), (220) crystal planes of Ni [42]. This indicates that the phase transformation from the hydrotalcite precursor to the nickel-based catalyst was successfully achieved after hydrogen reduction treatment. The Ni particle sizes in the NiAl-R, NiCo 0.25 Al-R, NiCo 0.5 Al-R and NiCo 1 Al-R catalysts calculated according to the Scherrer formula were 19.2, 16.6, 10.2 and 13.7 nm, respectively. This is attributed to the Co-promoter regulating effect on the size of Ni particles in the catalysts. The smaller the sizes of the Ni particles in the catalysts, the more active sites are exposed, which is crucial for improving the activity of the catalysts [43].
sizes of the Ni particles in the catalysts, the more active sites are exposed, which is c for improving the activity of the catalysts [43]. The N2 adsorption-desorption isotherms and pore size distribution curves NiCoxAl-R catalysts are shown in Figure 2. It can be seen from Figure 2a that NiAlalyst is an IV type adsorption-desorption isotherm and type H1 hysteresis loop, proves that the NiAl-R catalyst has a mesoporous structure and a narrow pore size bution (shown in Figure 2b). When Co is added as a promoter, the NiCoxAl-R cat are all type II adsorption-desorption isotherms and H3 type hysteresis loops, which cates that there is a non-uniform slit pore structure in the NiCoxAl-R catalysts. It c clearly seen from Figure 2b that adding an appropriate amount of Co as a promot significantly increase the pore size in the NiCoxAl-R catalysts, which is beneficial f entry of feed gas (CO2, H2) and the escape of products (CO2, CH4, H2). SEM characterization was used to explore the morphology of the NiCoxAl-LDH cursors and the NiCoxAl-R catalysts (shown in Figure 3). According to the SEM char ization results, it is found that the prepared NiCoxAl-LDHs precursors are all nano structures and relatively uniform in size. Among them, the nanosheets in the NiAl The N 2 adsorption-desorption isotherms and pore size distribution curves of the NiCo x Al-R catalysts are shown in Figure 2. It can be seen from Figure 2a that NiAl-R catalyst is an IV type adsorption-desorption isotherm and type H1 hysteresis loop, which proves that the NiAl-R catalyst has a mesoporous structure and a narrow pore size distribution (shown in Figure 2b). When Co is added as a promoter, the NiCo x Al-R catalysts are all type II adsorption-desorption isotherms and H3 type hysteresis loops, which indicates that there is a non-uniform slit pore structure in the NiCo x Al-R catalysts. It can be clearly seen from Figure 2b that adding an appropriate amount of Co as a promoter can significantly increase the pore size in the NiCo x Al-R catalysts, which is beneficial for the entry of feed gas (CO 2 , H 2 ) and the escape of products (CO 2 , CH 4 , H 2 ).
Catalysts 2021, 11, x FOR PEER REVIEW 3 o sizes of the Ni particles in the catalysts, the more active sites are exposed, which is cruc for improving the activity of the catalysts [43]. The N2 adsorption-desorption isotherms and pore size distribution curves of NiCoxAl-R catalysts are shown in Figure 2. It can be seen from Figure 2a that NiAl-R c alyst is an IV type adsorption-desorption isotherm and type H1 hysteresis loop, wh proves that the NiAl-R catalyst has a mesoporous structure and a narrow pore size dis bution (shown in Figure 2b). When Co is added as a promoter, the NiCoxAl-R cataly are all type II adsorption-desorption isotherms and H3 type hysteresis loops, which in cates that there is a non-uniform slit pore structure in the NiCoxAl-R catalysts. It can clearly seen from Figure 2b that adding an appropriate amount of Co as a promoter c significantly increase the pore size in the NiCoxAl-R catalysts, which is beneficial for entry of feed gas (CO2, H2) and the escape of products (CO2, CH4, H2). SEM characterization was used to explore the morphology of the NiCoxAl-LDHs p cursors and the NiCoxAl-R catalysts (shown in Figure 3). According to the SEM charact ization results, it is found that the prepared NiCoxAl-LDHs precursors are all nanosh structures and relatively uniform in size. Among them, the nanosheets in the NiAl-LD SEM characterization was used to explore the morphology of the NiCo x Al-LDHs precursors and the NiCo x Al-R catalysts (shown in Figure 3). According to the SEM characterization results, it is found that the prepared NiCo x Al-LDHs precursors are all nanosheet structures and relatively uniform in size. Among them, the nanosheets in the NiAl-LDH precursor are relatively dispersed. However, the nanosheets in the NiCo x Al-LDH precursors prepared after adding Co are intercalated and assembled to form a flower-like structure. Because the nanosheets are intercalated and stacked with each other, some irregular slit hole structures will be formed, which is consistent with the BET test results (shown  Figure 2). In addition, SEM results show that the catalysts prepared by direct reduction of NiCo x Al-LDH precursors are also a nanoplatelet structure (shown in Figure 3(a2-d2)), which is helpful for exploring the microstructure of the catalysts.
precursor are relatively dispersed. However, the nanosheets in the NiCoxAl-LDH pre sors prepared after adding Co are intercalated and assembled to form a flower-like s ture. Because the nanosheets are intercalated and stacked with each other, some irreg slit hole structures will be formed, which is consistent with the BET test results (show Figure 2). In addition, SEM results show that the catalysts prepared by direct reductio NiCoxAl-LDH precursors are also a nanoplatelet structure (shown in Figure 3a2 which is helpful for exploring the microstructure of the catalysts. The H2-TPR results of NiCoxAl-LDHs precursors are shown in Figure 4. The N LDH sample has two obvious H2 reduction signal peaks, and the corresponding ce temperatures are 397 and 633 °C, respectively. The reduction peak at low tempera (397 °C) comes from the reduction in Ni species that interact weakly with the carrier, w the reduction peak at high temperature (633 °C) corresponds to the reduction in Ni sp that interact strongly with the carrier [32]. Different from NiAl-LDH, NiCo0.25Al-L NiCo0.5Al-LDH and NiCo1Al-LDH all have three obvious reduction signal peaks, and corresponding temperature ranges are 265-385 °C, 385-433 °C and 480-750 °C, res tively. By consulting the literature, it is found that the hydrogen signal peak in the r of 265-385 °C comes from the reduction in Co species [44][45][46]. It can be found from Fi 4 that as the Co content increases, the area of the corresponding reduction peak also g ually increases. In addition, the reduction peaks in the temperature range of 385-43 and 480-750 °C are derived from the reduction in Ni species. According to the H2data, it is found that when the added amount of Co increases from 0 to 0.5 mmol reduction temperature required for Ni material decreases from 634 to 534 °C. How when the amount of Co added continues to increase to 1 mmol, the reduction tempera required for Ni species increases from 534 to 631 °C. In summary, the Co additive effectively regulate the interaction between the Ni substance and the supporter, so ad an appropriate amount of Co can significantly reduce the reduction temperature of th species. The H 2 -TPR results of NiCo x Al-LDHs precursors are shown in Figure 4. The NiAl-LDH sample has two obvious H 2 reduction signal peaks, and the corresponding center temperatures are 397 and 633 • C, respectively. The reduction peak at low temperature (397 • C) comes from the reduction in Ni species that interact weakly with the carrier, while the reduction peak at high temperature (633 • C) corresponds to the reduction in Ni species that interact strongly with the carrier [32]. Different from NiAl-LDH, NiCo 0.25 Al-LDH, NiCo 0.5 Al-LDH and NiCo 1 Al-LDH all have three obvious reduction signal peaks, and the corresponding temperature ranges are 265-385 • C, 385-433 • C and 480-750 • C, respectively. By consulting the literature, it is found that the hydrogen signal peak in the range of 265-385 • C comes from the reduction in Co species [44][45][46]. It can be found from Figure 4 that as the Co content increases, the area of the corresponding reduction peak also gradually increases. In addition, the reduction peaks in the temperature range of 385-433 • C and 480-750 • C are derived from the reduction in Ni species. According to the H 2 -TPR data, it is found that when the added amount of Co increases from 0 to 0.5 mmol, the reduction temperature required for Ni material decreases from 634 to 534 • C. However, when the amount of Co added continues to increase to 1 mmol, the reduction temperature required for Ni species increases from 534 to 631 • C. In summary, the Co additive can effectively regulate the interaction between the Ni substance and the supporter, so adding an appropriate amount of Co can significantly reduce the reduction temperature of the Ni species.
Since the basic sites on the catalyst surface play a vital role in the CO 2 hydrogenation reaction, it is necessary to study the distribution of basic sites on the catalyst surface by the CO 2 -TPD data (shown in Figure 5). CO 2 -TPD experiment results show that all catalysts contain two CO 2 desorption signal peaks; the corresponding temperature ranges are 65-195 • C and 205-450 • C, respectively. According to the reported literature, it can be observed that the two desorption signal peaks are derived from the CO 2 adsorbed at the weakly basic sites (surface OH − ) and the moderately basic sites (Lewis acid-based pairs), respectively [47]. Since the weakly basic sites have a weak adsorption force on CO 2 , the adsorbed CO 2 is desorbed from the catalyst surface before it reacts, which is not conducive to the progress of the methanation reaction. Studies have shown that moderately basic sites have a strong adsorption force for CO 2 and a large amount of adsorption, which helps to improve the catalytic activity of the catalyst [48]. According to the results of CO 2 -TPD, the relative content of intermediate basic sites in the catalysts is 68%, 80%, 88% and 77%, respectively (shown in Table 1). According to the above results, it is found that the Co promoters can significantly increase the number of moderately basic sites in the catalysts, which is very important for improving the catalytic performance of the catalysts. Since the basic sites on the catalyst surface play a vital role in the CO2 hydrogenation reaction, it is necessary to study the distribution of basic sites on the catalyst surface by the CO2-TPD data (shown in Figure 5). CO2-TPD experiment results show that all catalysts contain two CO2 desorption signal peaks; the corresponding temperature ranges are 65-195 °C and 205-450 °C, respectively. According to the reported literature, it can be observed that the two desorption signal peaks are derived from the CO2 adsorbed at the weakly basic sites (surface OH -) and the moderately basic sites (Lewis acid-based pairs), respectively [47]. Since the weakly basic sites have a weak adsorption force on CO2, the adsorbed CO2 is desorbed from the catalyst surface before it reacts, which is not conducive to the progress of the methanation reaction. Studies have shown that moderately basic sites have a strong adsorption force for CO2 and a large amount of adsorption, which helps to improve the catalytic activity of the catalyst [48]. According to the results of CO2-TPD, the relative content of intermediate basic sites in the catalysts is 68%, 80%, 88% and 77%, respectively (shown in Table 1). According to the above results, it is found that the Co promoters can significantly increase the number of moderately basic sites in the catalysts, which is very important for improving the catalytic performance of the catalysts.  The microstructure of the prepared NiCoxAl-R catalysts was further explored by TEM technology. The TEM characterization results further proved that the NiCoxAl-R catalysts have a nanosheet structure, and the Ni nanoparticles are uniformly distributed in the substrate (shown in Figure 6a-d). The histogram in Figure 6 shows the size distribution of Ni nanoparticles in the NiCoxAl-R catalysts. The average particle sizes of the Ni particles in the NiAl-R, NiCo0.25Al-R, NiCo0.5Al-R and NiCo1Al-R catalysts measured by the TEM test results are 19.3 ± 0.5 nm, 16.8 ± 0.5 nm and 10.1 ± 0.5, 13.6 ± 0.5 nm, respectively. This is consistent with the Ni particle size calculated from the XRD results (shown in Figure 1b). Figure 6e,f show high magnification TEM pictures of NiAl-R and NiCo0.5Al-R, respectively. The lattice distance is 0.203 and 0.20 nm, corresponding to the (002) crystal plane of Co and the (111) crystal plane of Ni, respectively [49]. It can be seen from Figure  6e,f that both Ni nanoparticles and Co nanoparticles are embedded in the AlOx substrate. This unique mosaic structure can effectively inhibit the migration and agglomeration of Ni nanoparticles, helping to improve the stability of the catalyst as a result.  The microstructure of the prepared NiCo x Al-R catalysts was further explored by TEM technology. The TEM characterization results further proved that the NiCo x Al-R catalysts have a nanosheet structure, and the Ni nanoparticles are uniformly distributed in the substrate (shown in Figure 6a-d). The histogram in Figure 6 shows the size distribution of Ni nanoparticles in the NiCo x Al-R catalysts. The average particle sizes of the Ni particles in the NiAl-R, NiCo 0.25 Al-R, NiCo 0.5 Al-R and NiCo 1 Al-R catalysts measured by the TEM test results are 19.3 ± 0.5 nm, 16.8 ± 0.5 nm and 10.1 ± 0.5, 13.6 ± 0.5 nm, respectively. This is consistent with the Ni particle size calculated from the XRD results (shown in Figure 1b). Figure 6e,f show high magnification TEM pictures of NiAl-R and NiCo 0.5 Al-R, respectively. The lattice distance is 0.203 and 0.20 nm, corresponding to the (002) crystal plane of Co and the (111) crystal plane of Ni, respectively [49]. It can be seen from Figure 6e,f that both Ni nanoparticles and Co nanoparticles are embedded in the AlO x substrate. This unique mosaic structure can effectively inhibit the migration and agglomeration of Ni nanoparticles, helping to improve the stability of the catalyst as a result. The spatial distribution of Ni, Co, Al and O in the NiCo0.5Al-R catalyst was further studied by TEM-EDS mapping (shown in Figure 7). It can be found from Figure 7b that Al and O elements are mainly distributed around the Ni element, which is the same as the TEM results (shown in Figure 6e-f). Furthermore, the Co-promoter is uniformly dispersed in the NiCo0.5Al-R catalyst. TEM-EDS mapping results show that the Co-promoted Nibased catalysts prepared by direct reduction of hydrotalcites have a higher degree of dispersion, which is beneficial to improve the methanation activity of the catalysts. The spatial distribution of Ni, Co, Al and O in the NiCo 0.5 Al-R catalyst was further studied by TEM-EDS mapping (shown in Figure 7). It can be found from Figure 7b that Al and O elements are mainly distributed around the Ni element, which is the same as the TEM results (shown in Figure 6e-f). Furthermore, the Co-promoter is uniformly dispersed in the NiCo 0.5 Al-R catalyst. TEM-EDS mapping results show that the Co-promoted Ni-based catalysts prepared by direct reduction of hydrotalcites have a higher degree of dispersion, which is beneficial to improve the methanation activity of the catalysts.

Catalytic Activity Tests
In order to study the effect of Co on the low-temperature catalytic activity of Ni-based catalysts, we tested the catalytic performance of NiCoxAl-R catalysts with CO2 methanation as a probe reaction (shown in Figure 8a-c, 0.1 MPa, GHSV = 30,000 mL·g −1 ·h −1 ). When the reaction temperature reaches 250 °C, the conversion of CO2 on the NiCo0.5Al-R catalyst reaches 81%, but at this time, the conversion of CO2 on the NiAl-R catalyst is only 26%. The reason why the low-temperature catalytic activity of NiCo0.5Al-R catalyst is significantly improved is that the Co-promoter reduces the size of Ni nanoparticles in the catalyst (shown in Figures 1b and 6) and increases the number of moderately basic sites (shown in Figure 5). At the same time, the results also show that the smaller the size of the Ni particles, the more active sites are exposed, and therefore, the higher the activity of the catalyst, which is consistent with the results reported in the literature [50]. It can be seen from Figure 8a that the low-temperature catalytic performance of NiCo1Al-R catalyst is lower than that of NiCo0.5Al-R catalyst, which may be caused by the excessive Co covering part of the active sites of Ni. The performance test results of the catalysts also showed that no Co-NiOx intermediate was formed during the reaction. This is inconsistent with the results reported in the literature due to the absence of oxygen in the CO2 methanation reaction [51,52]. Therefore, although the auxiliary Co is added, the Co-NiOx intermediate will not be formed during the CO2 methanation reaction. In summary, doping with an appropriate amount of Co can significantly improve the low-temperature catalytic performance of the Ni-based catalyst, thereby reducing its activation temperature in the CO2 methanation reaction.

Catalytic Activity Tests
In order to study the effect of Co on the low-temperature catalytic activity of Ni-based catalysts, we tested the catalytic performance of NiCo x Al-R catalysts with CO 2 methanation as a probe reaction (shown in Figure 8a-c, 0.1 MPa, GHSV = 30,000 mL·g −1 ·h −1 ). When the reaction temperature reaches 250 • C, the conversion of CO 2 on the NiCo 0.5 Al-R catalyst reaches 81%, but at this time, the conversion of CO 2 on the NiAl-R catalyst is only 26%. The reason why the low-temperature catalytic activity of NiCo 0.5 Al-R catalyst is significantly improved is that the Co-promoter reduces the size of Ni nanoparticles in the catalyst (shown in Figures 1b and 6) and increases the number of moderately basic sites (shown in Figure 5). At the same time, the results also show that the smaller the size of the Ni particles, the more active sites are exposed, and therefore, the higher the activity of the catalyst, which is consistent with the results reported in the literature [50]. It can be seen from Figure 8a that the low-temperature catalytic performance of NiCo 1 Al-R catalyst is lower than that of NiCo 0.5 Al-R catalyst, which may be caused by the excessive Co covering part of the active sites of Ni. The performance test results of the catalysts also showed that no Co-NiO x intermediate was formed during the reaction. This is inconsistent with the results reported in the literature due to the absence of oxygen in the CO 2 methanation reaction [51,52]. Therefore, although the auxiliary Co is added, the Co-NiO x intermediate will not be formed during the CO 2 methanation reaction. In summary, doping with an appropriate amount of Co can significantly improve the low-temperature catalytic performance of the Ni-based catalyst, thereby reducing its activation temperature in the CO 2 methanation reaction. Since the methanation of CO2 is a strong exothermic reaction, an excellent nickelbased catalyst must not only have high low-temperature catalytic activity but also high stability. In view of this, we tested the stability of the NiCo0.5Al-R catalyst at 300 °C (0.1 MPa, GHSV = 30,000 mL·g −1 ·h −1 ) (shown in Figure 9). The NiCo0.5Al-R catalyst performance degradation at the beginning of the life test experiment is because the CO2 methanation reaction has not reached a stable state. The stability test result showed that the NiCo0.5Al-R catalyst still did not deactivate after 92 h of continuous reaction, which was attributed to the confinement effect of the AlOx substrate on the Ni nanoparticles. In order to study the change of Ni nanoparticle size and carbon deposit on the surface of the catalyst after the life test, we performed XRD and Raman tests on the catalyst after the reaction (shown in Figure 10a,b). The diffraction peaks at 44.6°, 51.9° and 76.8° correspond to the (111), (200) and (220) crystal planes of Ni [42], and the weak broad diffraction peaks located at 37.1° and 63.3° are attributed to the (111) and (220) crystal planes of NiO (fcc), respectively [53]. By comparing the XRD results of the catalyst before and after the stability test, it was found that the intensity and half-width of the characteristic diffraction peaks of Ni did not change significantly. This indicates that the size of Ni nanoparticles in the NiCo0.5Al-R catalyst did not change significantly after the stability test. Additionally, through the TEM photograph of the NiCo0.5Al-R catalyst after the reaction, it can be seen more clearly that there is no agglomeration of Ni particles (shown in Figure 11a). The average size of Ni particles in the NiCo0.5Al-R catalyst after the reaction is 10.3 nm, which is basically the same as the size of Ni particles in the fresh catalyst (shown in Figure 6). Moreover, the XRD result of the NiCo0.5Al-R used does not show the diffraction peaks of carbon species, which indicates that there is no carbon deposit on the catalyst surface or the amount of carbon deposit is too small to be detected. Raman spectroscopy was used Since the methanation of CO 2 is a strong exothermic reaction, an excellent nickel-based catalyst must not only have high low-temperature catalytic activity but also high stability. In view of this, we tested the stability of the NiCo 0.5 Al-R catalyst at 300 • C (0.1 MPa, GHSV = 30,000 mL·g −1 ·h −1 ) (shown in Figure 9). The NiCo 0.5 Al-R catalyst performance degradation at the beginning of the life test experiment is because the CO 2 methanation reaction has not reached a stable state. The stability test result showed that the NiCo 0.5 Al-R catalyst still did not deactivate after 92 h of continuous reaction, which was attributed to the confinement effect of the AlO x substrate on the Ni nanoparticles. Since the methanation of CO2 is a strong exothermic reaction, an excellent nickelbased catalyst must not only have high low-temperature catalytic activity but also high stability. In view of this, we tested the stability of the NiCo0.5Al-R catalyst at 300 °C (0.1 MPa, GHSV = 30,000 mL·g −1 ·h −1 ) (shown in Figure 9). The NiCo0.5Al-R catalyst performance degradation at the beginning of the life test experiment is because the CO2 methanation reaction has not reached a stable state. The stability test result showed that the NiCo0.5Al-R catalyst still did not deactivate after 92 h of continuous reaction, which was attributed to the confinement effect of the AlOx substrate on the Ni nanoparticles. In order to study the change of Ni nanoparticle size and carbon deposit on the surface of the catalyst after the life test, we performed XRD and Raman tests on the catalyst after the reaction (shown in Figure 10a,b). The diffraction peaks at 44.6°, 51.9° and 76.8° correspond to the (111), (200) and (220) crystal planes of Ni [42], and the weak broad diffraction peaks located at 37.1° and 63.3° are attributed to the (111) and (220) crystal planes of NiO (fcc), respectively [53]. By comparing the XRD results of the catalyst before and after the stability test, it was found that the intensity and half-width of the characteristic diffraction peaks of Ni did not change significantly. This indicates that the size of Ni nanoparticles in the NiCo0.5Al-R catalyst did not change significantly after the stability test. Additionally, through the TEM photograph of the NiCo0.5Al-R catalyst after the reaction, it can be seen more clearly that there is no agglomeration of Ni particles (shown in Figure 11a). The average size of Ni particles in the NiCo0.5Al-R catalyst after the reaction is 10.3 nm, which is basically the same as the size of Ni particles in the fresh catalyst (shown in Figure 6). Moreover, the XRD result of the NiCo0.5Al-R used does not show the diffraction peaks of carbon species, which indicates that there is no carbon deposit on the catalyst surface or the amount of carbon deposit is too small to be detected. Raman spectroscopy was used In order to study the change of Ni nanoparticle size and carbon deposit on the surface of the catalyst after the life test, we performed XRD and Raman tests on the catalyst after the reaction (shown in Figure 10a,b). The diffraction peaks at 44.6 • , 51.9 • and 76.8 • correspond to the (111), (200) and (220) crystal planes of Ni [42], and the weak broad diffraction peaks located at 37.1 • and 63.3 • are attributed to the (111) and (220) crystal planes of NiO (fcc), respectively [53]. By comparing the XRD results of the catalyst before and after the stability test, it was found that the intensity and half-width of the characteristic diffraction peaks of Ni did not change significantly. This indicates that the size of Ni nanoparticles in the NiCo 0.5 Al-R catalyst did not change significantly after the stability test. Additionally, through the TEM photograph of the NiCo 0.5 Al-R catalyst after the reaction, it can be seen more clearly that there is no agglomeration of Ni particles (shown in Figure 11a). The average size of Ni particles in the NiCo 0.5 Al-R catalyst after the reaction is 10.3 nm, which is basically the same as the size of Ni particles in the fresh catalyst (shown in Figure 6). Moreover, the XRD result of the NiCo 0.5 Al-R used does not show the diffraction peaks of carbon species, which indicates that there is no carbon deposit on the catalyst surface or the amount of carbon deposit is too small to be detected. Raman spectroscopy was used to further confirm whether carbon deposits are formed. The Raman spectroscopy test results of the catalyst also do not find the signal peak of carbon material, which indicates that there is no carbon deposit on the surface of the catalyst. Combining the XRD results and Raman results of the catalyst after the stability test shows that the catalyst has high stability and carbon deposition resistance.
to further confirm whether carbon deposits are formed. The Raman spectroscopy te sults of the catalyst also do not find the signal peak of carbon material, which ind that there is no carbon deposit on the surface of the catalyst. Combining the XRD r and Raman results of the catalyst after the stability test shows that the catalyst has stability and carbon deposition resistance.

Catalyst Preparation
The different molar ratios of NiCoxAl-LDHs (x = 0, 0.25, 0.5, 1, Ni 2+ /Al 3+ = 2) obtained by the hydrothermal method. Firstly, 10 mmol Ni (NO3)2·6H2O, Co (NO3)2· and 5 mmol Al (NO3)3·9H2O were dissolved in deionized water, for which the amounts of Co were 0, 0.25, 0.5 and 1 mmol. Secondly, 50 mmol of CO(NH2)2 preci was added. Then, the suspension solution was transferred to an autoclave heating h (120 o C). After the hydrothermal reaction, the obtained samples were filtered and trifuged (8000 rpm) until the filtrate PH = 7. Finally, the samples were dried overni  to further confirm whether carbon deposits are formed. The Raman spectroscopy test results of the catalyst also do not find the signal peak of carbon material, which indicates that there is no carbon deposit on the surface of the catalyst. Combining the XRD results and Raman results of the catalyst after the stability test shows that the catalyst has high stability and carbon deposition resistance.  . TEM (a) and particle size distribution (b) of the NiCo0.5Al-R-used catalyst.

Catalyst Characterization
X-ray diffraction equipment was used to characterize the phase composition of the samples. The test voltage of the equipment was 40 kV, and the current was 40 mA. The scanning speed was 10 • ·min −1 with the angle from 5 • to 90 • (2 θ).
For N 2 physisorption measurements, a Quantachrome NOVA 3200e (Quantachrome Corporation, Boynton Beach, FL, USA) was applied to determine the adsorption and desorption isotherm results.
The H 2 -TPR and CO 2 -TPD results of the catalysts were detected by a Quantachrome automatic chemical adsorption analyzer. In order to improve the accuracy of the H 2 -TPR test results, we first heated the NiCo x Al-LDHs to 150 • C in Ar atmosphere, followed by a temperature reduction to 50 • C, and finally heated it to 800 • C in 10%-H 2 /Ar atmosphere and recorded data. For CO 2 -TPD characterization, firstly, NiCo x Al-LDHs (100 mg) were reduced in a H 2 atmosphere (600 • C, 2 h), and then the temperature dropped to 50 • C under the protection of Ar atmosphere. Immediately after the reduction, the sample was adsorbed for CO 2 . After the adsorption was saturated, the temperature was raised, and the CO 2 desorption amount was recorded with the instrument.
The morphology and microstructure of the NiCo x Al-LDHs precursors and NiCo x Al-R catalysts were characterization by SEM (JSM-7001F, INCA X-MAX, Tokyo, Japan) and TEM (JEM-2010F, Tokyo, Japan).

Catalytic Experiments
First of all, 0.05 g NiCo x Al-R catalyst with 80-120 mesh was loaded in the fixed bed equipment. Then, the reduction mixture gases comprising H 2 and Ar (H 2 /Ar = 1/9, V/V) were inserted, and the temperature was raised from 30 to 200 • C. Feed gases were then introduced, for which the composition was H 2 , CO 2 and Ar (H 2 /CO 2 /Ar = 4/1/5). The reaction condition was T = 200-395 • C, p = 0.1 MPa and GHSV = 30,000 mL·g −1 ·h −1 . The values of CO 2 and produced CO and CH 4 were measured using a gas chromatograph at each temperature after 1 h when the reaction reached steady state. The CO 2 conversion and CH 4 selectivity were calculated according to the following formula: CO 2 conversion (%) = F CO 2,in −F CO 2,out F CO 2,in ×100% CH 4 selectivity (%) = F CH 4,out F CO 2,in −F CO 2,out ×100% where F CO 2,in is the flow rate of reactant CO 2 ; F CO 2,out and F CH 4,out represent the flow rate of CO 2 and CH 4 in the outlet.

Conclusions
In summary, in order to improve the low-temperature catalytic performance of the Ni-based catalysts in the CO 2 methanation reaction, Co-promoted Ni-based catalysts were successfully prepared by in situ reduction of NiCo x Al-LDHs precursors. The results of CO 2 -TPD and TEM prove that the Co assistant can regulate the number of moderately basic sites and the size of Ni nanoparticles in the Ni-based catalyst. Furthermore, the confinement effect of the AlO x substrate effectively inhibits the migration and agglomeration of Ni particles during the reaction and improves the stability of the catalyst. The experimental results show that the NiCo 0.5 Al-R catalyst has high low temperature catalytic performance, high stability and excellent carbon deposition resistance. Therefore, it has promising industrial application prospects.