Highly Dispersed Ni Nanocatalysts Derived from NiMnAl-Hydrotalcites as High-Performing Catalyst for Low-Temperature Syngas Methanation

: Increasing the low-temperature performance of nickel-based catalysts in syngas methanation is critical but very challenging, because at low temperatures there is high concentration of CO on the catalyst surface, causing formation of nickel carbonyl with metallic Ni and further catalyst deactivation. Herein, we have prepared highly dispersed Ni nanocatalysts by in situ reduction of NiMnAl-layered double hydroxides (NiMnAl-LDHs) and applied them to syngas methanation. The synthesized Ni nanocatalysts maintained the nanosheet structure of the LDHs, in which Ni particles were decorated with MnO y species and embedded in the AlO x nanosheets. It was observed that the Ni nanocatalysts exhibited markedly better low-temperature performance than commercial catalysts in the syngas methanation. At 250 ◦ C, 3.0 MPa and a high weight hourly space velocity (WHSV) of 30,000 mL · g − 1 · h − 1 , both the CO conversion and the CH 4 selectivity reached 100% over the former, while those over the commercial catalyst were only 14% and 76%, respectively. Furthermore, this NiMnAl catalyst exhibited strong anti-carbon and anti-sintering properties at high temperatures. The enhanced low-temperature performance and high-temperature stability originated from the promotion effect of MnO y and the embedding effect of AlO x in the catalyst.


Introduction
Over the last few decades, the preparation of synthetic natural gas (SNG) by coal gasification has attracted worldwide attention, particularly in the coal-rich regions, because of the requirements for efficient utilization of coal resources and for a secure supply of natural gas, which is a clean fuel [1,2]. The main technology of producing SNG from syngas is CO methanation [3][4][5]. Until now, all commercial methanation catalysts for SNG production have been Ni-based catalysts due to their high performance and low cost [6][7][8][9][10][11][12][13][14][15][16][17][18]. Nevertheless, the major weaknesses of nickel catalysts include agglomeration of Ni nanoparticles, formation of carbon deposits at higher temperatures, and poor activity at low temperatures (250 • C) for the complete methanation of syngas [19][20][21][22][23][24][25][26][27][28][29][30][31]. The increase in low temperature performance of the Ni catalysts in syngas methanation is critical. It is also very challenging because the high concentration of CO on the catalyst surface may lead to the formation of Ni(CO) 4 and further catalyst deactivation. At present, research on low-temperature methanation is mainly focused on hydrogen-rich systems and slurry-bed reactors [32][33][34][35][36][37], while that for syngas (H 2 /CO = 3/1) in fixed beds has rarely been reported.
Layered double hydroxides (LDHs) are widely used as catalysts or supports due to their unique layered structure with distribution of M 2+ (M = Mg, Co, Cu, Ni) and N 3+ (N = Al, Fe, Co, Mn) cations that are adjustable both in ratio and in type [38][39][40][41][42][43]. Ni-based hydrotalcites have been used as precursors to obtain methanation catalysts via high temperature calcination followed by reduction [13,44]. However, high temperature calcination would destroy the layered structure of LDHs and reduce the specific surface area and pores volume, which leads to poor activity of the obtained catalysts at low temperatures [45,46]. He et al. found that the Ni nanocatalysts prepared via in situ reduction of NiAl-hydrotalcites with abundant surface defects showed excellent catalytic performance in CO 2 methanation [47]. Differing from CO 2 methanation, the major challenge for low temperature methanation of high concentration CO is a catalyst deactivation phenomenon due to formation of Ni(CO) 4 . In general, low temperature and high CO partial pressure favor formation of Ni(CO) 4 at equilibrium; thus, the inlet temperature of the feed gas should be above 300 • C for SNG, at which the catalyst can convert part of CO and maintain a low CO partial pressure so that Ni(CO) 4 formation is negligible. In principle, the reaction can be operated smoothly at quite low temperatures so that the catalyst can be active enough to maintain a low CO concentration on the catalyst surface. Therefore, developing highly active catalysts for low-temperature CO methanation is possible but quite challenging.
Herein, highly dispersed Ni nanocatalysts were obtained by in situ reduction of NiMnAl-LDHs precursors and applied to CO methanation for the first time. The novel catalysts showed excellent catalytic performance at low temperatures and high stability at high temperatures. According to experimental results, the Ni nanocatalysts decorated with MnO y species and embedded in the AlO x nanosheets have the application prospect for low temperature methanation.

Structure and Morphology of LDHs and Catalysts
The X-ray diffraction (XRD) results of the NiMn x Al 10 -LDHs (x = 0, 0.5, 1, 2) are shown in Figure 1a and the inset shows the displacement of the first diffraction peak. The diffraction peaks of all the samples at 2θ = 11.6 • -11.9 • , 23.  [41]. Table 1 shows the lattice parameters of the NiMn x Al 10 -LDHs. The lattice parameter "a" is related to the radius of the cation [48]. Compared with NiAl-LDH, the values of "a" for NiMnAl-LDHs do not change after adding Mn 2+ , because the ionic radius of Ni and Mn (0.62 Å for Ni 2+ , 0.66 Å for Mn 2+ , respectively) are relatively close, while the additive amount of Mn is low. When the Mn/Al molar ratios change from 0:10 to 1:10, the lattice constant "c" gradually increases from 2.255 to 2.286 nm until "c" reaches its maximum value, which is due to the difference in bond strength of Ni-O (372 kJ/mol) and Mn-O (402 kJ/mol). When Ni is substituted by Mn, the bond strength of M-O (M-Metal cation) is increased, resulting in the weakened interaction of the metal cations with the interlayer anions as well as the water molecules, so that the interlayer spacing is increased. These results confirm that most of the added Mn 2+ entered the layered structure of the hydrotalcites and did not affect crystallinity of LDHs.
The XRD spectrums of the catalysts obtained from reducing LDHs directly are shown in Figure 1b. In comparison to Figure 1a, it can be found that the characteristic peaks of LDHs disappeared after the reduction, but some new diffraction peaks emerged at 2θ = 44.6 • , 51.9 • , 76.8 • , attributing to (111), (200), (220) crystal planes of face-centered cube (fcc) Ni, respectively. Considering the low content of added Mn 2+ that is difficult to detect, we mainly focus on the diffraction of nickel species. The weak broad diffraction peaks located at 37.1 • and 63.3 • are attributing to the (111) and (220) crystal planes of NiO (fcc), respectively [41]. The sizes of Ni particles in these NiAl-C, NiMn 0.5 Al 10 -C, NiMn 1 Al 10 -C, and NiMn 2 Al 10 -C catalysts were calculated to be 9.0 ± 0.5, 8.7 ± 0.5, 8.5 ± 0.5 and 8.6 ± 0.5 nm, respectively. This indicates that the adding of Mn has only a slight influence on the size of the Ni particles.  10 12 14 Intensity(a.u.)  The scanning electron microscope (SEM) images of NiMnxAl10-LDHs before and after the reduction are shown in Figure 2. The hydrothermally synthesized LDHs are composed of the connected nanosheets with relatively uniform size and smooth surface (Figure 2a-d). The NiMnxAl10 catalysts obtained after reduction maintain the flake-like morphology but with a rougher surface (Figure 2e-h). The SEM results demonstrate that the incorporation of Mn has no obvious effect on the morphology of LDHs crystals, and the thin layered structure was still preserved after reduction, which is helpful to investigate the change of Ni particles before and after methanation reaction.
The physical properties of the catalysts were investigated by BET ( Figure 3). As can be seen from Figure 3a, all the catalysts exhibit IV type adsorption-desorption isotherms with type H3 hysteresis loops. The specific surface areas of the catalysts are similar (shown in Table 2). Nonetheless, the pore volume of NiMn0.5Al10-C and NiMn1Al10-C are relatively larger, which is possibly caused by the formation of macropores from the packing of nanosheets (Figure 3b).  The scanning electron microscope (SEM) images of NiMn x Al 10 -LDHs before and after the reduction are shown in Figure 2. The hydrothermally synthesized LDHs are composed of the connected nanosheets with relatively uniform size and smooth surface (Figure 2a-d). The NiMn x Al 10 catalysts obtained after reduction maintain the flake-like morphology but with a rougher surface (Figure 2e-h). The SEM results demonstrate that the incorporation of Mn has no obvious effect on the morphology of LDHs crystals, and the thin layered structure was still preserved after reduction, which is helpful to investigate the change of Ni particles before and after methanation reaction.
The physical properties of the catalysts were investigated by BET ( Figure 3). As can be seen from Figure 3a, all the catalysts exhibit IV type adsorption-desorption isotherms with type H3 hysteresis loops. The specific surface areas of the catalysts are similar (shown in Table 2). Nonetheless, the pore volume of NiMn 0.5 Al 10 -C and NiMn 1 Al 10 -C are relatively larger, which is possibly caused by the formation of macropores from the packing of nanosheets (Figure 3b).   The reduction behaviors of LDHs were investigated by H2-TPR ( Figure 4a). All the samples have two distinct reduction peaks: the first one located at 300-450 °C is derived from the reduction of Ni oxide species with a weak interaction with the substrate, and the second one at 450-700 °C is attributed to the reduction of Ni oxide species, having a strong interaction with the matrix. As the    The reduction behaviors of LDHs were investigated by H2-TPR ( Figure 4a). All the samples have two distinct reduction peaks: the first one located at 300-450 °C is derived from the reduction of Ni oxide species with a weak interaction with the substrate, and the second one at 450-700 °C is attributed to the reduction of Ni oxide species, having a strong interaction with the matrix. As the  The reduction behaviors of LDHs were investigated by H 2 -TPR ( Figure 4a). All the samples have two distinct reduction peaks: the first one located at 300-450 • C is derived from the reduction of Ni oxide species with a weak interaction with the substrate, and the second one at 450-700 • C is attributed to the reduction of Ni oxide species, having a strong interaction with the matrix. As the molar ratio of Mn/Al changes from 0:10 to 1:10, the first reduction peak shifts to low temperature range, indicating that adding a proper amount of Mn can improve the reduction of nickel species [12]. However, with further increase of Mn/Al molar ratio to 2:10, the reduction temperature of the catalyst is increased again, because of the reinforced interaction between nickle oxide species and the excess of Mn oxide species [12]. Figure 4b shows the H 2 -TPD curves of the NiMn x Al 10 catalysts. Although MnO x is capable of adsorbing hydrogen, the amount of Mn is much lower than that of Ni in the catalyst, so we will mainly focus on the H 2 uptake of Ni. In addition, there is no H 2 uptake of Al 2 O 3 [11,24,28]. Therefore, the dispersion of Ni particles can be calculated from the H 2 -TPD results. All catalysts have only one H 2 desorption peak at the temperature interval of 100-200 • C, corresponding to desorption of H 2 of highly dispersed Ni nanoparticles or Ni nanoparticles with many surface defects [12], which are benefit to the diffusion and dissociation of surface hydrogen [49]. Table 2 lists the H 2 adsorption amount on a series of NiMn x Al 10 (x = 0, 0.5, 1, 2) catalysts. Among them, NiMn 1 Al 10 shows the maximum H 2 uptake of 631 µmol/g cat and highest Ni distribution of 12.8 ± 0.1%, indicating that incorporating an appropriate amount of Mn can significantly improve the H 2 adsorption capacity and the dispersion of Ni nanoparticles. However, excessive Mn will overlay fractional the active site of Ni particles and reduce the adsorption capacity for H 2 , which is very important for the performance of the catalyst.
Catalysts 2019, 9, x FOR PEER REVIEW 5 of 15 molar ratio of Mn/Al changes from 0:10 to 1:10, the first reduction peak shifts to low temperature range, indicating that adding a proper amount of Mn can improve the reduction of nickel species [12]. However, with further increase of Mn/Al molar ratio to 2:10, the reduction temperature of the catalyst is increased again, because of the reinforced interaction between nickle oxide species and the excess of Mn oxide species [12]. Figure 4b shows the H2-TPD curves of the NiMnxAl10 catalysts. Although MnOx is capable of adsorbing hydrogen, the amount of Mn is much lower than that of Ni in the catalyst, so we will mainly focus on the H 2 uptake of Ni. In addition, there is no H 2 uptake of Al 2 O 3 [11,24,28]. Therefore, the dispersion of Ni particles can be calculated from the H 2 -TPD results. All catalysts have only one H2 desorption peak at the temperature interval of 100-200 °C, corresponding to desorption of H2 of highly dispersed Ni nanoparticles or Ni nanoparticles with many surface defects [12], which are benefit to the diffusion and dissociation of surface hydrogen [49]. Table 2 lists the H2 adsorption amount on a series of NiMnxAl10 (x = 0, 0.5, 1, 2) catalysts. Among them, NiMn1Al10 shows the maximum H2 uptake of 631 μmol/gcat and highest Ni distribution of 12.8 ± 0.1%, indicating that incorporating an appropriate amount of Mn can significantly improve the H2 adsorption capacity and the dispersion of Ni nanoparticles. However, excessive Mn will overlay fractional the active site of Ni particles and reduce the adsorption capacity for H2, which is very important for the performance of the catalyst.
200 400 600 800 200 400 600 800 The dispersion of Ni nanoparticles was further investigated by transmission electron microscopy (TEM) ( Figure 5). As can be seen from TEM images (Figure 5a-d), the catalysts maintain the nanosheet morphology of the LDHs precursors. Additionally, Ni nanoparticles are highly dispersed and embedded in the AlOx matrix. The inserted histograms show the size distributions of the Ni nanoparticles. The average diameters of Ni in NiAl-C, NiMn0.5Al10-C, NiMn1Al10-C and NiMn2Al10-C are 12.6 ± 0.5, 12.3 ± 0.5, 10.6 ± 0.5 and 12.4 ± 0.5 nm (based on measurements of 150 particles), respectively. These results are very close to those calculated from the XRD results. The lattices and distribution of Ni and Mn species on the substrate can be observed from the high resolution transmission electron microscopy (HRTEM) images. The lattice distance of 0.20, 0.25 and 0.21 nm is attributed to the (111) plane of Ni (fcc) particles, the (211) plane of Mn3O4 and the (420) plane of Mn2O3, respectively. The reason why Mn exists in the form of oxides is that the reduction order of MnOy is first from MnO2/Mn2O3 to Mn3O4, and then to MnO, and the final reduction to elemental Mn requires a temperature higher than 800 °C [50]. The TEM image of NiAl-C demonstrates Ni particles are fixed by the poorly crystallized AlOx shell (Figure 5e), and the AlOx is formed during the reduction of the LDHs precursors [41]. For the Mn-added catalyst, Ni particles and the adjacent MnOy The dispersion of Ni nanoparticles was further investigated by transmission electron microscopy (TEM) ( Figure 5). As can be seen from TEM images (Figure 5a-d), the catalysts maintain the nanosheet morphology of the LDHs precursors. Additionally, Ni nanoparticles are highly dispersed and embedded in the AlO x matrix. The inserted histograms show the size distributions of the Ni nanoparticles. The average diameters of Ni in NiAl-C, NiMn 0.5 Al 10 -C, NiMn 1 Al 10 -C and NiMn 2 Al 10 -C are 12.6 ± 0.5, 12.3 ± 0.5, 10.6 ± 0.5 and 12.4 ± 0.5 nm (based on measurements of 150 particles), respectively. These results are very close to those calculated from the XRD results. The lattices and distribution of Ni and Mn species on the substrate can be observed from the high resolution transmission electron microscopy (HRTEM) images. The lattice distance of 0.20, 0.25 and 0.21 nm is attributed to the (111) plane of Ni (fcc) particles, the (211) plane of Mn 3 O 4 and the (420) plane of Mn 2 O 3 , respectively. The reason why Mn exists in the form of oxides is that the reduction order of MnO y is first from MnO 2 /Mn 2 O 3 to Mn 3 O 4 , and then to MnO, and the final reduction to elemental Mn requires a temperature higher than 800 • C [50]. The TEM image of NiAl-C demonstrates Ni particles are fixed by the poorly crystallized AlO x shell (Figure 5e), and the AlO x is formed during the reduction of the LDHs precursors [41]. For the Mn-added catalyst, Ni particles and the adjacent MnO y are collectively confined in the matrix of AlO x (Figure 5e), which is critical to improve the stability of the catalyst. The XRD, SEM, and TEM results show that the Mn incorporated LDHs maintains its structural characteristics during in situ reduction, which is beneficial to preparation Ni-based catalysts embedded in AlO x matrix with high dispersion.
Catalysts 2019, 9, x FOR PEER REVIEW 6 of 15 are collectively confined in the matrix of AlOx (Figure 5e), which is critical to improve the stability of the catalyst. The XRD, SEM, and TEM results show that the Mn incorporated LDHs maintains its structural characteristics during in situ reduction, which is beneficial to preparation Ni-based catalysts embedded in AlOx matrix with high dispersion. Energy dispersive spectroscopy (EDS) mapping was used to characterize the spatial distribution of individual elements in the catalyst ( Figure 6). Fig. 6a shows that the signal of Ni element is strong in the central part of the particles, while the Al and O elements are mainly distributed around Ni particles. The distribution of Mn element cannot be clearly seen in Figure 6a due to the small amount of Mn in the catalyst. The spatial distribution of Ni and Mn in the NiMn1Al10-C catalyst is shown in Fig. 6b. MnOy (Mn3O4 or Mn2O3) is also dispersed around the Ni particles; thus, both the AlOx and MnOy have a certain steric hindrance effect on the agglomeration of the Ni particles to increase the stability of the catalysts. In addition, it can be seen from Fig. 6b that MnOy is distributed around the Ni particles, which coincides exactly with the TEM results (Figure 5f).
The surface valence of the catalysts derived from LDHs precursors were investigated by XPS (Figure 7). The Ni 2p3/2 spectrums are shown in Figure 7A. The surface of all the catalysts is composed of a large portion of Ni 2+ (855.8 eV), and a little part of Ni 0 (853.1 eV), which is probably due to the samples being exposed to air prior to testing and the surface Ni species being oxidized [14,50]. After adding Mn in the catalyst, the binding energy of Ni 2p3/2 shifts from 855.6 to 855.9 eV, which can be attributed to the interaction between MnOy and Ni [51]. The spectrum of Mn 2p3/2 can be broken up into three peaks with centers at 647.7, 642.9, and 637.8 eV, which are corresponding to the Mn 4+ , Mn 3+ and Mn 2+ species, respectively [12,[51][52][53]. With the increase of Mn content, the Mn 4+ /Mn 3+ atomic ratios in the catalysts significantly increased ( Figure 7B and Table 3). O1s spectra are shown in Figure  7C. All the catalysts are shown a broad peak that can be fitted into two peaks by searching for the optimal combination of Gaussian bands. The obtained two peaks center at 530.5 eV and 531.8 eV are attributed to lattice oxygen (OI) and surface adsorbed oxygen (OII), respectively [53,54]. The amounts of oxygen vacancies are positively correlated with the relative percentage of adsorbed oxygen species [50,51]. The integrated area ratios of OII/(OII + OI) are listed in Table 3, indicating that the incorporated Mn can significantly increase the oxygen vacancies in the catalyst. Energy dispersive spectroscopy (EDS) mapping was used to characterize the spatial distribution of individual elements in the catalyst ( Figure 6). Fig. 6a shows that the signal of Ni element is strong in the central part of the particles, while the Al and O elements are mainly distributed around Ni particles. The distribution of Mn element cannot be clearly seen in Figure 6a due to the small amount of Mn in the catalyst. The spatial distribution of Ni and Mn in the NiMn 1 Al 10 -C catalyst is shown in Fig. 6b. MnO y (Mn 3 O 4 or Mn 2 O 3 ) is also dispersed around the Ni particles; thus, both the AlO x and MnO y have a certain steric hindrance effect on the agglomeration of the Ni particles to increase the stability of the catalysts. In addition, it can be seen from Fig. 6b that MnO y is distributed around the Ni particles, which coincides exactly with the TEM results (Figure 5f).
The surface valence of the catalysts derived from LDHs precursors were investigated by XPS (Figure 7). The Ni 2p 3/2 spectrums are shown in Figure 7A. The surface of all the catalysts is composed of a large portion of Ni 2+ (855.8 eV), and a little part of Ni 0 (853.1 eV), which is probably due to the samples being exposed to air prior to testing and the surface Ni species being oxidized [14,50]. After adding Mn in the catalyst, the binding energy of Ni 2p 3/2 shifts from 855.6 to 855.9 eV, which can be attributed to the interaction between MnO y and Ni [51]. The spectrum of Mn 2p 3/2 can be broken up into three peaks with centers at 647.7, 642.9, and 637.8 eV, which are corresponding to the Mn 4+ , Mn 3+ and Mn 2+ species, respectively [12,[51][52][53]. With the increase of Mn content, the Mn 4+ /Mn 3+ atomic ratios in the catalysts significantly increased ( Figure 7B and Table 3). O 1s spectra are shown in Figure 7C. All the catalysts are shown a broad peak that can be fitted into two peaks by searching for the optimal combination of Gaussian bands. The obtained two peaks center at 530.5 eV and 531.8 eV are attributed to lattice oxygen (O I ) and surface adsorbed oxygen (O II ), respectively [53,54].
The amounts of oxygen vacancies are positively correlated with the relative percentage of adsorbed oxygen species [50,51]. The integrated area ratios of O II /(O II + O I ) are listed in Table 3, indicating that the incorporated Mn can significantly increase the oxygen vacancies in the catalyst.     Based on the above results, it is found that both MnO y and Ni particles are fixed by the AlO x matrix, and MnO y is distributed around the Ni particles. In order to further elaborate the microstructure of the catalyst, we have drawn a schematic diagram of the structure of the catalyst (Figure 8). During the reduction process, the layered structure of LDHs is collapsed to form Ni(MnAl)O x nanosheets, and the Ni clusters decorated with MnO y species are formed, half-buried, and distributed on the nanosheets. In other words, the Ni particles are decorated with MnO y and embedded on the AlO x matrix [41]. The segregation effect of the AlO x matrix prevents the sintering of Ni particles during the methanation reaction, thereby increasing the service life of the catalyst. In summary, the nickel-based catalyst synthesized by in situ reduction of LDHs has a unique structure, which is important for enhancing the low temperature activity and high temperature stability of the catalyst.   0.50 Based on the above results, it is found that both MnOy and Ni particles are fixed by the AlOx matrix, and MnOy is distributed around the Ni particles. In order to further elaborate the microstructure of the catalyst, we have drawn a schematic diagram of the structure of the catalyst (Figure 8). During the reduction process, the layered structure of LDHs is collapsed to form Ni(MnAl)Ox nanosheets, and the Ni clusters decorated with MnOy species are formed, half-buried, and distributed on the nanosheets. In other words, the Ni particles are decorated with MnOy and embedded on the AlOx matrix [41]. The segregation effect of the AlOx matrix prevents the sintering of Ni particles during the methanation reaction, thereby increasing the service life of the catalyst. In summary, the nickel-based catalyst synthesized by in situ reduction of LDHs has a unique structure, which is important for enhancing the low temperature activity and high temperature stability of the catalyst.

Catalytic Activity
Generally, high pressure is used in the synthesis of SNG in the industry [14]. For the purpose of studying the low temperature activity of the catalysts for CO methanation under the industrial reaction conditions, the performance test was executed at 3.0 MPa and a WHSV of 30000 mL·g −1 ·h −1 (Figure 9). For NiAl-C, the CO conversion and CH4 yield at 250 °C is 62% and 59%, respectively. After adding Mn in the catalyst, the performances of NiMnxAl10-C are significantly improved (shown in Figure 9a-b). Among them, NiMn1Al10-C exhibits the highest performance at low temperature, and both CO conversion and selectivity of CH4 achieve 100% at 250 °C, owning to the high scatter of Ni nanoparticles and H2 uptake (Table 2), and appearance of a number of oxygen vacancies in the catalyst (Table 3). However, the conversion of CO on commercial catalyst (Ni-35.3 wt%) is only 14% at the same reaction condition. To the best of our knowledge, there is no reported Ni-based catalyst that can achieve 100% conversion of CO at such low temperatures and high pressure of CO with feed gas H2:CO of 3:1 [16,24,26], at which the conventional nickel-based catalysts are susceptible to inactivation owing to the formation of Ni(CO)4. In this work, the synthesized NiMnxAl10-C can convert most of the CO even at low temperatures, so that the CO concentration on the surface of the catalysts is very low, which can avoid the formation of nickel carbonyl. Therefore, the NiMnxAl10-C catalyst is promising in industrial applications of low temperature methanation.

Catalytic Activity
Generally, high pressure is used in the synthesis of SNG in the industry [14]. For the purpose of studying the low temperature activity of the catalysts for CO methanation under the industrial reaction conditions, the performance test was executed at 3.0 MPa and a WHSV of 30,000 mL·g −1 ·h −1 (Figure 9). For NiAl-C, the CO conversion and CH 4 yield at 250 • C is 62% and 59%, respectively. After adding Mn in the catalyst, the performances of NiMn x Al 10 -C are significantly improved (shown in Figure 9a-b). Among them, NiMn 1 Al 10 -C exhibits the highest performance at low temperature, and both CO conversion and selectivity of CH 4 achieve 100% at 250 • C, owning to the high scatter of Ni nanoparticles and H 2 uptake (Table 2), and appearance of a number of oxygen vacancies in the catalyst (Table 3). However, the conversion of CO on commercial catalyst (Ni-35.3 wt%) is only 14% at the same reaction condition. To the best of our knowledge, there is no reported Ni-based catalyst that can achieve 100% conversion of CO at such low temperatures and high pressure of CO with feed gas H 2 :CO of 3:1 [16,24,26], at which the conventional nickel-based catalysts are susceptible to inactivation owing to the formation of Ni(CO) 4 . In this work, the synthesized NiMn x Al 10 -C can convert most of the CO even at low temperatures, so that the CO concentration on the surface of the catalysts is very low, which can avoid the formation of nickel carbonyl. Therefore, the NiMn x Al 10 -C catalyst is promising in industrial applications of low temperature methanation. The durability test is indispensable for further investigation of the low temperature performance of the catalysts (Figure 10a). The conversion of CO of the Commercial-C decreased from 36% to 24% after 23 h of reaction at a low temperature. The amount of Ni of the fresh and used catalysts are determined by ICP; we found that the amount of Ni decreased from 35.3 wt% to 28.4 wt%, which is a result of the formation of a Ni(CO)4. However, the performance of the NiMn1Al10-C did not decrease, which was attributed to the fact that the added Mn increased the H2 uptake and the number of oxygen vacancies (Figure 4b and Figure 7C). Although Commercial-C has poor low temperature activity, its high temperature stability is good [9]. Furthermore, a good methanation catalyst should show high performance at low temperatures and strong resistance against sintering and coking at high temperatures. Therefore, the stability of NiMn1Al10-C was evaluated at 500 °C and 3.0 MPa with different space velocities. The NiMn1Al10-C is very stable at a high temperature with different space velocities: the CO conversion maintains at 100% for 180 h, and the selectivity of CH4 is slightly reduced when the space velocity is increased from 30000 mL·g −1 ·h −1 to 60000 mL·g −1 ·h −1 (Figure 10b). Compared with the supported nickel-based catalyst, the NiMnXAl10-C catalyst has high lowtemperature activity and strong anti-sintering performance [15,17,35,37]. The high stability of the NiMn1Al10-C at high temperatures is attributed to the confinement effect of AlOx and MnOy ( Figure  5f and Figure 6), which prevents the sintering of Ni particles during the reaction. The durability test is indispensable for further investigation of the low temperature performance of the catalysts (Figure 10a). The conversion of CO of the Commercial-C decreased from 36% to 24% after 23 h of reaction at a low temperature. The amount of Ni of the fresh and used catalysts are determined by ICP; we found that the amount of Ni decreased from 35.3 wt% to 28.4 wt%, which is a result of the formation of a Ni(CO) 4 . However, the performance of the NiMn 1 Al 10 -C did not decrease, which was attributed to the fact that the added Mn increased the H 2 uptake and the number of oxygen vacancies (Figures 4b and 7C). Although Commercial-C has poor low temperature activity, its high temperature stability is good [9]. Furthermore, a good methanation catalyst should show high performance at low temperatures and strong resistance against sintering and coking at high temperatures. Therefore, the stability of NiMn 1 Al 10 -C was evaluated at 500 • C and 3.0 MPa with different space velocities. The NiMn 1 Al 10 -C is very stable at a high temperature with different space velocities: the CO conversion maintains at 100% for 180 h, and the selectivity of CH 4 is slightly reduced when the space velocity is increased from 30,000 mL·g −1 ·h −1 to 60,000 mL·g −1 ·h −1 (Figure 10b). Compared with the supported nickel-based catalyst, the NiMn X Al 10 -C catalyst has high low-temperature activity and strong anti-sintering performance [15,17,35,37]. The high stability of the NiMn 1 Al 10 -C at high temperatures is attributed to the confinement effect of AlO x and MnO y ( Figures 5f and 6), which prevents the sintering of Ni particles during the reaction. The XRD results of the fresh and used NiMn1Al10-C are shown in Figure 11a, and both of them display the characteristic peaks at 2θ = 44.6°, 51.9°, 76.8°, attributing to (111), (200), (220) planes of Ni (fcc), respectively. Additionally, the peak intensities of Ni for the used catalyst almost do not change compared with that of the fresh one. The average diameters of Ni particles in the fresh and used catalysts are 8.5 ± 0.5 and 8.3 ± 0.5 nm, respectively, indicating there was no agglomeration of the Ni particles after the high temperature stability test. The result proves that the NiMn1Al10-C catalyst has a high stability at high temperatures, which was due to the confinement of the AlOx substrate ( Figure  5). The carbon deposit amount was investigated by thermogravimetric (TG) (Figure 11b). The weights of fresh and used catalysts increased at 200-400 °C is associated with the oxidation of the reduced Ni particles. The superposition of both TG curves indicates that there were almost no carbon deposits formed, implying that NiMn1Al10-C possesses excellent anti-coking properties due to the high H2 adsorption and catalyst containing a larger number of oxygen vacancies (Tables 2 and 3). By contrast, with the nickel-based catalyst synthesized by traditional means [9][10][11]15,21], the NiMn1Al10-C catalyst exhibits much higher stability due to greatly improved resistance to coking and sintering. The carbon deposit amount was further investigated by Raman spectra (Figure 11c). No peak of carbon was observed, indicating that there was no carbon deposit on the used catalyst surface.

Chemicals
Aluminum nitrate nonahydrate (Al(NO3)3·9H2O), urea (CO(NH2)2), Nickel (II) nitrate hexahydrate (Ni(NO3)2·6H2O) and manganese nitrate (Mn(NO3)2·4H2O) were purchased from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. All reagents were of analytical grade and used without further purification. The XRD results of the fresh and used NiMn 1 Al 10 -C are shown in Figure 11a, and both of them display the characteristic peaks at 2θ = 44.6 • , 51.9 • , 76.8 • , attributing to (111), (200), (220) planes of Ni (fcc), respectively. Additionally, the peak intensities of Ni for the used catalyst almost do not change compared with that of the fresh one. The average diameters of Ni particles in the fresh and used catalysts are 8.5 ± 0.5 and 8.3 ± 0.5 nm, respectively, indicating there was no agglomeration of the Ni particles after the high temperature stability test. The result proves that the NiMn 1 Al 10 -C catalyst has a high stability at high temperatures, which was due to the confinement of the AlO x substrate ( Figure 5). The carbon deposit amount was investigated by thermogravimetric (TG) (Figure 11b). The weights of fresh and used catalysts increased at 200-400 • C is associated with the oxidation of the reduced Ni particles. The superposition of both TG curves indicates that there were almost no carbon deposits formed, implying that NiMn 1 Al 10 -C possesses excellent anti-coking properties due to the high H 2 adsorption and catalyst containing a larger number of oxygen vacancies (Tables 2 and 3). By contrast, with the nickel-based catalyst synthesized by traditional means [9][10][11]15,21], the NiMn 1 Al 10 -C catalyst exhibits much higher stability due to greatly improved resistance to coking and sintering. The carbon deposit amount was further investigated by Raman spectra (Figure 11c). No peak of carbon was observed, indicating that there was no carbon deposit on the used catalyst surface. The XRD results of the fresh and used NiMn1Al10-C are shown in Figure 11a, and both of them display the characteristic peaks at 2θ = 44.6°, 51.9°, 76.8°, attributing to (111), (200), (220) planes of Ni (fcc), respectively. Additionally, the peak intensities of Ni for the used catalyst almost do not change compared with that of the fresh one. The average diameters of Ni particles in the fresh and used catalysts are 8.5 ± 0.5 and 8.3 ± 0.5 nm, respectively, indicating there was no agglomeration of the Ni particles after the high temperature stability test. The result proves that the NiMn1Al10-C catalyst has a high stability at high temperatures, which was due to the confinement of the AlOx substrate ( Figure  5). The carbon deposit amount was investigated by thermogravimetric (TG) (Figure 11b). The weights of fresh and used catalysts increased at 200-400 °C is associated with the oxidation of the reduced Ni particles. The superposition of both TG curves indicates that there were almost no carbon deposits formed, implying that NiMn1Al10-C possesses excellent anti-coking properties due to the high H2 adsorption and catalyst containing a larger number of oxygen vacancies (Tables 2 and 3). By contrast, with the nickel-based catalyst synthesized by traditional means [9][10][11]15,21], the NiMn1Al10-C catalyst exhibits much higher stability due to greatly improved resistance to coking and sintering. The carbon deposit amount was further investigated by Raman spectra (Figure 11c). No peak of carbon was observed, indicating that there was no carbon deposit on the used catalyst surface.

Catalysts Preparation
The NiAl hydrotalcite was prepared by hydrothermal method. First, 1.5 mmol of Ni(NO 3 ) 2 ·6H 2 O, 0.5 mmol of Al(NO 3 ) 3 ·9H 2 O and 10 mmol of CO(NH 2 ) 2 were dissolved in 70 mL of deionized water. Then the solution was transferred into an autoclave for a hydrothermal treatment at 120 • C for 12 h. Subsequently, the product was centrifuged and washed with deionized water. The collected product was subsequently dried at 80 • C for 15 h, which was named NiAl-LDH. The synthesis of the NiMnAl ternary hydrotalcites is similar to the above process, except that a certain amount of Mn(NO 3 ) 2 ·4H 2 O was added. The synthesized NiMnAl ternary hydrotalcites with the molar ratios of Mn/Al (Mn:Al = 0.5:10, 1:10, 2:10) are denoted as NiMn 0.5 Al 10 -LDH, NiMn 1 Al 10 -LDH, and NiMn 2 Al 10 -LDH, respectively. After reduction in hydrogen at 600 • C for 2 h (10 • C/min), the methanation catalysts are marked as NiAl-C (C stands for the catalyst), NiMn 0.5 Al 10 -C, NiMn 1 Al 10 -C, and NiMn 2 Al 10 -C accordingly. The commercial catalyst (Ni-35.3 wt%) was purchased from LiaoNing HaiTai SCI-TECH Development CO., Ltd., Fushun, China. For comparison with the commercial catalyst, nickel content of the NiMnAl catalysts was diluted to 35 wt% with α-Al 2 O 3 (commercial γ-Al 2 O 3 (GongYi HuaYu, Zhengzhou, China) calcined at 1200 • C in air for 6 h), and the α-Al 2 O 3 is inactive for CO methanation.

Catalysts Characterization
The X-ray diffraction (XRD) characterization was carried out on Cu-Ka radiation with a rate of 10 • min −1 (40 kV, 40 mA). N 2 adsorption/desorption isotherms and the specific surface area and pore size distributions of the samples were tested on the Quantachrome NOVA 3200e (FL, USA). The morphology and microscopic feature of the samples were detected by scanning electron microscope (SEM) (JSM-7001F, INCA X-MAX, Tokyo, Japan, 15 kV) and transmission electron microscopy (TEM) (JEM-2010F, JEOL, Tokyo, Japan). Hydrogen temperature programmed reduction (H 2 -TPR) and Hydrogen temperature programmed desorption (H 2 -TPD) were obtained from Quantachrome automated chemisorption analyzer. In a typical experiment of H 2 -TPR, the hydrotalcite precursor (30 mg) was sealed in the quartz U-tube reactor and purged in a He flow for 2 h at 150 • C (10 • C/min), and then reduced by injecting H 2 and Ar (10% H 2 /Ar) at a given temperature. In a typical experiment of H 2 -TPD, the hydrotalcite precursor (100 mg) firstly in situ reduced with pure H 2 , then cooled down in the N 2 atmosphere, and subsequently switched to Ar flow purging for 1.5 h to remove the physically adsorbed H 2 on the surface of the catalysts. The H 2 adsorption amounts of the catalysts were measured at a heating rate of 10 • C/min. The dispersion of Ni nanoparticles was calculated from the data of H 2 -TPD, and the calculation details can be found in our previous work [9]. The elements amount of the catalysts were obtained from Optima 7000 DV inductively coupled plasma optical emission spectrometer (ICP, MA, USA). The surface chemical compositions of the catalysts were measured by X-ray photoelectron spectroscopy (XPS) on an Esca Lab 250Xi with a monochromatic Al Kα radiation. The carbon deposits on the surface of the used catalyst were determined by thermogravimetric analyzer (TG/DTA 6300, Seiko Instruments, Tokyo, Japan) and Raman Spectrometer (Renishaw RM 2000, laser wavelength = 532 nm, London, UK).

Catalytic Performance Measurement
The methanation measurements are similar to what we have reported previously [11]. The CO methanation reaction was performed in a fixed bed reactor at 3.0 MPa, and the reaction data were recorded in the temperature interval of 250-500 • C; the spacing is 20 • C or 50 • C. In each case, the catalyst (200 mg) admixed with 5 g of quartz sand, then heated to the reaction temperature (250 • C) in a H 2 atmosphere, and finally switched to syngas (H 2 :CO:N 2 = 3:1:1) for methanation reaction. The mixed gas after the methanation reaction was first cooled down and then analyzed on line by Micro Gas chromatography (3000A; Agilent Technologies, CA, USA). The amount of H 2 , N 2 , CH 4 , and CO in the outlet gases were analyzed by a thermal conductivity detector (TCD, CA, USA) connected to a molecular sieve column. The amount of CO 2 , C 2 H 4 and C 2 H 6 in the outlet gases were detected by TCD connected to a Plot Q column (CA, USA). The stability of the catalyst is investigated at 3.0 MPa and 500 • C.

Conclusions
In summary, to enhance the low-temperature performance of the Ni catalysts for syngas methanation, the highly dispersed Ni nanocatalysts are successfully prepared by in situ reduction of NiMnAl-LDHs precursors, and Ni particles decorated with the promoter of MnO y are confined in the matrix of AlO x . The synthesized NiMnAl catalysts exhibit higher properties than the NiAl catalyst since the addition of Mn increases dispersion of Ni particles and H 2 uptake. The NiMn 1 Al 10 exhibits the best performance at low temperatures and the greatest resistance to sintering and coking due to the highest dispersion of Ni and the steric hindrance of AlO x and MnO y against aggregation of the Ni particles. This work demonstrates how to enhance the performance of the Ni catalysts in methanation by carefully adding promoter MnO y and by embedding the fine Ni particles on the AlO x nanosheets.