Aqueous Miscible Organic LDH Derived Ni-Based Catalysts for E ﬃ cient CO 2 Methanation

: Converting CO 2 to methane via catalytic routes is an e ﬀ ective way to control the CO 2 content released in the atmosphere while producing value-added fuels and chemicals. In this study, the CO 2 methanation performance of highly dispersed Ni-based catalysts derived from aqueous miscible organic layered double hydroxides (AMO-LDHs) was investigated. The activity of the catalyst was found to be largely inﬂuenced by the chemical composition of Ni metal precursor and loading. A Ni-based catalyst derived from AMO-Ni 3 Al 1 -CO 3 LDH exhibited a maximum CO 2 conversion of 87.9% and 100% CH 4 selectivity ascribed to both the lamellar catalyst structure and the high Ni metal dispersion achieved. Moreover, due to the strong Ni metal–support interactions and abundant oxygen vacancy concentration developed, this catalyst also showed excellent resistance to carbon deposition and metal sintering. In particular, high stability was observed after 19 h in CO 2 / H 2 reaction at 360 ◦ C.


Introduction
The concentration of CO 2 in the atmosphere has risen from 270 to 385 ppm in the past 200 years which has been causing serious problems such as the greenhouse effect, global climate change, and glaciers melting [1][2][3]. Regarding these environmental issues, three ways to reduce CO 2 content in the atmosphere have been suggested [4][5][6][7], namely CO 2 emission reduction, CO 2 capture and storage (CCS), and CO 2 conversion and utilization. CO 2 is chemically stable and it needs to be activated in the presence of hydrogen in order to convert it to CH 4 . Although CO 2 activation is an endothermic process, thus requiring energy input, it can usually be integrated with fuel production from renewable energy, making CO 2 conversion in the Power-to-Gas (PtG) process a very promising green technology [4,8,9]. Due to a series of technological issues of storage and transportation, renewable energy can hardly be a primary source of energy for the society [10].
CO 2 methanation appears to be one of the largely investigated scientific topics [11,12]. Herein, CO 2 is hydrogenated to methane at relatively high temperatures, ca. 300-400 • C, known as the Sabatier reaction, which is a very promising approach for the storage of renewable energy [6]. Based on the PtG Figure 1 shows the powder XRD patterns of LDH precursors with different interlayer anions (NO 3 − and CO 3 2− ) before and after reduction. LDH precursors present typical characteristic peaks at 11.7 • , 22.8 • and 35.1 • , corresponding to the reflections of (003), (006) and (012) facets of the LDH crystal phase, respectively, indicating the successful synthesis of LDHs [41]. The peak intensity of Ni 3 Al 1 -CO 3 LDH is higher than that of Ni 3 Al 1 -NO 3 LDH, suggesting that CO 3 2− intercalated LDH is better crystallized than the NO 3 − intercalated one. The XRD patterns of reduced samples show two intensive peaks at 2θ = 44.4 and 51.8 • , corresponding to the reflections of (111) and (200) facets of metallic Ni, and no diffraction peaks due to the NiO phase could be observed [42]. This result shows that NiO is rather completely reduced into metallic Ni. However, if strong Ni support interactions prevail, NiO might not be completely reduced into metallic Ni since small NiO particles (<4 nm) cannot be detected by XRD. Moreover, the γ-Al 2 O 3 phase was not detected, result that might be related to its very low crystallinity. According to literature reports [30], the Al 2 O 3 phase derived from LDH precursors generally exists in an amorphous phase. In order to better understand the dispersion of Ni on the catalyst surface, the TEM image and EDX element mapping images of the reduced AMO-Ni 3 Al-CO 3 LDH solid are shown in Figures 2 and 3, respectively. Black dots in the TEM image represent the Ni particles, where a mean diameter of~10 nm was estimated. From Figure 3b, it can be observed that Ni is evenly dispersed in the sample. Overall, it can be concluded that highly dispersed Ni-based catalysts were successfully synthesized from Ni-Al LDH precursors.
Catalysts 2020, 10, x FOR PEER REVIEW 3 of 14 prevail, NiO might not be completely reduced into metallic Ni since small NiO particles (<4 nm) cannot be detected by XRD. Moreover, the γ-Al2O3 phase was not detected, result that might be related to its very low crystallinity. According to literature reports [30], the Al2O3 phase derived from LDH precursors generally exists in an amorphous phase. In order to better understand the dispersion of Ni on the catalyst surface, the TEM image and EDX element mapping images of the reduced AMO-Ni3Al-CO3 LDH solid are shown in Figures 2 and 3, respectively. Black dots in the TEM image represent the Ni particles, where a mean diameter of ~10 nm was estimated. From Figure 3b, it can be observed that Ni is evenly dispersed in the sample. Overall, it can be concluded that highly dispersed Ni-based catalysts were successfully synthesized from Ni-Al LDH precursors.   Catalysts 2020, 10, x FOR PEER REVIEW  3 of 14 prevail, NiO might not be completely reduced into metallic Ni since small NiO particles (<4 nm) cannot be detected by XRD. Moreover, the γ-Al2O3 phase was not detected, result that might be related to its very low crystallinity. According to literature reports [30], the Al2O3 phase derived from LDH precursors generally exists in an amorphous phase. In order to better understand the dispersion of Ni on the catalyst surface, the TEM image and EDX element mapping images of the reduced AMO-Ni3Al-CO3 LDH solid are shown in Figures 2 and 3, respectively. Black dots in the TEM image represent the Ni particles, where a mean diameter of ~10 nm was estimated. From Figure 3b, it can be observed that Ni is evenly dispersed in the sample. Overall, it can be concluded that highly dispersed Ni-based catalysts were successfully synthesized from Ni-Al LDH precursors.    The CO2 conversion and CH4 selectivity over the AMO-Ni3Al1-CO3-LDO and AMO-Ni3Al1-NO3-LDO solids were compared and results are presented in Figure 4. These two catalysts show extremely high CH4 selectivity, ca. 100% at T < 400 °C, although the conversion level is not remarkable in the low-temperature window, i.e., temperatures lower than 260 °C. Above 450 °C, the CH4 selectivity starts to decrease, due to the competition with the reverse water-gas shift reaction (RWGS) which converts CO2 to CO through hydrogenation (CO2 + H2 → CO + H2O) [43]. For both catalysts, the CO2 conversion reaches maximum value at 360 °C, namely 87.9 and 84.2% over AMO-Ni3Al1-CO3-LDO and AMO-Ni3Al1-NO3-LDO, respectively. According to our previous study [44], interlayer anions influence the morphology and the interlayer distance of LDH, and the LDH with CO3 2− interlayer has a larger SSA than the LDH with NO3 − interlayer. The values of SSA of the pre-reduced catalysts, namely 158 m 2 /g (AMO-Ni3Al1-CO3-LDO) and 134 m 2 /g (AMO-Ni3Al1-NO3-LDO) are in good agreement with the previous study. Owing to the larger SSA, Ni produced after reduction will be likely better exposed, which might be one of the intrinsic reasons for the better activity performance of AMO-Ni3Al1-CO3-LDO catalyst. The CO 2 conversion and CH 4 selectivity over the AMO-Ni 3 Al 1 -CO 3 -LDO and AMO-Ni 3 Al 1 -NO 3 -LDO solids were compared and results are presented in Figure 4. These two catalysts show extremely high CH 4 selectivity, ca. 100% at T < 400 • C, although the conversion level is not remarkable in the low-temperature window, i.e., temperatures lower than 260 • C. Above 450 • C, the CH 4 selectivity starts to decrease, due to the competition with the reverse water-gas shift reaction (RWGS) which converts CO 2 to CO through hydrogenation (CO 2 + H 2 → CO + H 2 O) [43]. For both catalysts, the CO 2 conversion reaches maximum value at 360 • C, namely 87.9 and 84.2% over AMO-Ni 3 Al 1 -CO 3 -LDO and AMO-Ni 3 Al 1 -NO 3 -LDO, respectively. According to our previous study [44], interlayer anions influence the morphology and the interlayer distance of LDH, and the LDH with CO 3 2− interlayer has a larger SSA than the LDH with NO 3 − interlayer. The values of SSA of the pre-reduced catalysts, namely 158 m 2 /g (AMO-Ni 3 Al 1 -CO 3 -LDO) and 134 m 2 /g (AMO-Ni 3 Al 1 -NO 3 -LDO) are in good agreement with the previous study. Owing to the larger SSA, Ni produced after reduction will be likely better exposed, which might be one of the intrinsic reasons for the better activity performance of AMO-Ni 3 Al 1 -CO 3 -LDO catalyst.

The Influence of Catalysts Preparation Procedure on the CO2 Methanation
The N2 adsorption-desorption isotherms were measured for the as-synthesized carbonate intercalated LDHs derived catalysts, AMO-Ni3Al1-CO3-LDO and C-Ni3Al1-CO3-LDO, by coprecipitation and AMO treatment. These pre-reduced catalysts show different SSA, namely 117 m 2 /g (C-Ni3Al1-CO3-LDO) and 158 m 2 /g (AMO-Ni3Al1-CO3-LDO). AMO-Ni3Al1-CO3-LDO, indicating that AMO treatment has a positive effect on increasing the SSA of LDH-derived solids. As previously reported [30,37], AMO treatment greatly reduces the driving force for aggregation to dense agglomerates, resulting in the increase of SSA.
The CO2 methanation catalytic performance (CO2-conversion and CH4-selectivity) of AMO-Ni3Al1-CO3-LDO and C-Ni3Al1-CO3-LDO solids is illustrated in Figure 5. There is a significant difference in the CO2-conversion (%) over the two catalysts between 250 and 350 °C. The optimal CO2 conversion of AMO-Ni3Al1-CO3-LDO is 88%, while that of C-Ni3Al1-CO3-LDO is 82.5%. As discussed above, the AMO treatment is beneficial to increase the SSA of the solid, and this could lead to better Ni dispersion and thus to an increased CO2 methanation activity per gram of solid basis.

The Influence of Catalysts Preparation Procedure on the CO 2 Methanation
The N 2 adsorption-desorption isotherms were measured for the as-synthesized carbonate intercalated LDHs derived catalysts, AMO-Ni 3 Al 1 -CO 3 -LDO and C-Ni 3 Al 1 -CO 3 -LDO, by coprecipitation and AMO treatment. These pre-reduced catalysts show different SSA, namely 117 m 2 /g (C-Ni 3 Al 1 -CO 3 -LDO) and 158 m 2 /g (AMO-Ni 3 Al 1 -CO 3 -LDO). AMO-Ni 3 Al 1 -CO 3 -LDO, indicating that AMO treatment has a positive effect on increasing the SSA of LDH-derived solids. As previously reported [30,37], AMO treatment greatly reduces the driving force for aggregation to dense agglomerates, resulting in the increase of SSA.
The CO 2 methanation catalytic performance (CO 2 -conversion and CH 4 -selectivity) of AMO-Ni 3 Al 1 -CO 3 -LDO and C-Ni 3 Al 1 -CO 3 -LDO solids is illustrated in Figure 5. There is a significant difference in the CO 2 -conversion (%) over the two catalysts between 250 and 350 • C. The optimal CO 2 conversion of AMO-Ni 3 Al 1 -CO 3 -LDO is 88%, while that of C-Ni 3 Al 1 -CO 3 -LDO is 82.5%. As discussed above, the AMO treatment is beneficial to increase the SSA of the solid, and this could lead to better Ni dispersion and thus to an increased CO 2 methanation activity per gram of solid basis.

The Influence of Catalysts Preparation Procedure on the CO2 Methanation
The N2 adsorption-desorption isotherms were measured for the as-synthesized carbonate intercalated LDHs derived catalysts, AMO-Ni3Al1-CO3-LDO and C-Ni3Al1-CO3-LDO, by coprecipitation and AMO treatment. These pre-reduced catalysts show different SSA, namely 117 m 2 /g (C-Ni3Al1-CO3-LDO) and 158 m 2 /g (AMO-Ni3Al1-CO3-LDO). AMO-Ni3Al1-CO3-LDO, indicating that AMO treatment has a positive effect on increasing the SSA of LDH-derived solids. As previously reported [30,37], AMO treatment greatly reduces the driving force for aggregation to dense agglomerates, resulting in the increase of SSA.
The CO2 methanation catalytic performance (CO2-conversion and CH4-selectivity) of AMO-Ni3Al1-CO3-LDO and C-Ni3Al1-CO3-LDO solids is illustrated in Figure 5. There is a significant difference in the CO2-conversion (%) over the two catalysts between 250 and 350 °C. The optimal CO2 conversion of AMO-Ni3Al1-CO3-LDO is 88%, while that of C-Ni3Al1-CO3-LDO is 82.5%. As discussed above, the AMO treatment is beneficial to increase the SSA of the solid, and this could lead to better Ni dispersion and thus to an increased CO2 methanation activity per gram of solid basis.

Influence of Ni/Al Ratio on Catalyst CO 2 Methanation Activity
A series of NiAl-LDHs with different Ni/Al atom ratios were synthesized and their XRD patterns are shown in Figure 6. The X-ray diffraction peaks at 11.76, 22.84 and 35.2 • prove that LDH precursors were successfully synthesized, and there are no obvious differences in the crystal structure of LDHs with different Ni/Al ratios. The precursors were then calcined and reduced (see Section 2.1) before CO 2 methanation activity tests. The CO 2 conversion (%) and CH 4 selectivity (%) vs temperature profiles obtained over these NiAl-LDHs catalysts are presented in Figure 7. AMO-Ni 2 Al 1 -CO 3 -LDO presented the lowest CO 2 conversion among all the samples, especially at low temperatures. Upon increasing of the Ni/Al ratio to 3:1, the CO 2 conversion increase. As the Ni/Al ratio continues to increase to 4:1, the CO 2 conversion levels off, and becomes almost constant. This is likely because overloaded Ni can cause aggregation of Ni particles [45]. Hence, we identify a threshold for the positive impact of increasing Ni content on the catalytic CO 2 methanation activity being the Ni/Al ratio 4:1 the experimental limit to boost conversion. As for the selectivity to CH 4 , by increasing the Ni/Al ratio no change is observed except a slight decrease at high temperatures (ca. 500 • C, Figure 7) due to the RWGS reaction.
Catalysts 2020, 10, x FOR PEER REVIEW 6 of 14 A series of NiAl-LDHs with different Ni/Al atom ratios were synthesized and their XRD patterns are shown in Figure 6. The X-ray diffraction peaks at 11.76, 22.84 and 35.2° prove that LDH precursors were successfully synthesized, and there are no obvious differences in the crystal structure of LDHs with different Ni/Al ratios. The precursors were then calcined and reduced (see Section 2.1) before CO2 methanation activity tests. The CO2 conversion (%) and CH4 selectivity (%) vs temperature profiles obtained over these NiAl-LDHs catalysts are presented in Figure 7. AMO-Ni2Al1-CO3-LDO presented the lowest CO2 conversion among all the samples, especially at low temperatures. Upon increasing of the Ni/Al ratio to 3:1, the CO2 conversion increase. As the Ni/Al ratio continues to increase to 4:1, the CO2 conversion levels off, and becomes almost constant. This is likely because overloaded Ni can cause aggregation of Ni particles [45]. Hence, we identify a threshold for the positive impact of increasing Ni content on the catalytic CO2 methanation activity being the Ni/Al ratio 4:1 the experimental limit to boost conversion. As for the selectivity to CH4, by increasing the Ni/Al ratio no change is observed except a slight decrease at high temperatures (ca. 500 °C, Figure 7) due to the RWGS reaction.   A series of NiAl-LDHs with different Ni/Al atom ratios were synthesized and their XRD patterns are shown in Figure 6. The X-ray diffraction peaks at 11.76, 22.84 and 35.2° prove that LDH precursors were successfully synthesized, and there are no obvious differences in the crystal structure of LDHs with different Ni/Al ratios. The precursors were then calcined and reduced (see Section 2.1) before CO2 methanation activity tests. The CO2 conversion (%) and CH4 selectivity (%) vs temperature profiles obtained over these NiAl-LDHs catalysts are presented in Figure 7. AMO-Ni2Al1-CO3-LDO presented the lowest CO2 conversion among all the samples, especially at low temperatures. Upon increasing of the Ni/Al ratio to 3:1, the CO2 conversion increase. As the Ni/Al ratio continues to increase to 4:1, the CO2 conversion levels off, and becomes almost constant. This is likely because overloaded Ni can cause aggregation of Ni particles [45]. Hence, we identify a threshold for the positive impact of increasing Ni content on the catalytic CO2 methanation activity being the Ni/Al ratio 4:1 the experimental limit to boost conversion. As for the selectivity to CH4, by increasing the Ni/Al ratio no change is observed except a slight decrease at high temperatures (ca. 500 °C, Figure 7) due to the RWGS reaction.

Influence of Reduction Temperature in Hydrogen on Catalyst CO 2 Methanation Activity
Since the active component of the as-synthesized CO 2 methanation catalysts is the Ni metal, NiO (formed after calcination) should be reduced to metallic Ni (H 2 is used) before catalytic activity testing. The SEM images of AMO-Ni 3 Al 1 -CO 3 LDH, AMO-Ni 3 Al 1 -CO 3 LDO, and reduced AMO-Ni 3 Al 1 -CO 3 LDO solids are presented in Figure 8. Stacked and interlayered platelet-like crystals of LDH particles of a crystal particle size of~60 nm are observed in Figure 8a, which is consistent with previous reports [46,47]. This typical platelet-like morphology remains after calcination. However, the morphology greatly changed after reduction in hydrogen.
Catalysts 2020, 10, x FOR PEER REVIEW 7 of 14 Since the active component of the as-synthesized CO2 methanation catalysts is the Ni metal, NiO (formed after calcination) should be reduced to metallic Ni (H2 is used) before catalytic activity testing. The SEM images of AMO-Ni3Al1-CO3 LDH, AMO-Ni3Al1-CO3 LDO, and reduced AMO-Ni3Al1-CO3 LDO solids are presented in Figure 8. Stacked and interlayered platelet-like crystals of LDH particles of a crystal particle size of ~60 nm are observed in Figure 8a, which is consistent with previous reports [46,47]. This typical platelet-like morphology remains after calcination. However, the morphology greatly changed after reduction in hydrogen. As shown in Figure 8c, spherical particles with a diameter of 100 ~ 200 nm are formed. The hydrogen treatment conditions had a great influence on the particle dispersion, morphology and structure, and these features would eventually affect the activity performance of the catalyst [30,48]. The powder XRD patterns of samples reduced at 500 and 600 °C are presented in Figure 9. After hydrogen treatment at 600 °C for 1 h, the sample shows relatively strong diffraction peaks at 44.48 and 51.67° corresponding to the Ni phase. The main diffraction peaks for the NiO phase (ca. 37.25 and 44.48°) were not detected, indicating that NiO has been fully reduced to Ni [49]. However, the sample exhibited relatively weak peaks of Ni phase and clear peaks of NiO phase after H2 treatment at 500 °C. The H2-TPR profile of the AMO-Ni3Al1-CO3-LDO solid ( Figure 10) shows its reducibility characteristics. It can be inferred that complete reduction of NiO cannot be accomplished at 500 °C, and it is envisaged that the full reduction temperature is at least 550 °C [50]. This is consistent with the powder XRD results that the sample reduced at 500 °C is not as complete as that reduced at 600 °C. As shown in Figure 8c, spherical particles with a diameter of 100~200 nm are formed. The hydrogen treatment conditions had a great influence on the particle dispersion, morphology and structure, and these features would eventually affect the activity performance of the catalyst [30,48]. The powder XRD patterns of samples reduced at 500 and 600 • C are presented in Figure 9. After hydrogen treatment at 600 • C for 1 h, the sample shows relatively strong diffraction peaks at 44.48 and 51.67 • corresponding to the Ni phase. The main diffraction peaks for the NiO phase (ca. 37.25 and 44.48 • ) were not detected, indicating that NiO has been fully reduced to Ni [49]. However, the sample exhibited relatively weak peaks of Ni phase and clear peaks of NiO phase after H 2 treatment at 500 • C. Since the active component of the as-synthesized CO2 methanation catalysts is the Ni metal, NiO (formed after calcination) should be reduced to metallic Ni (H2 is used) before catalytic activity testing. The SEM images of AMO-Ni3Al1-CO3 LDH, AMO-Ni3Al1-CO3 LDO, and reduced AMO-Ni3Al1-CO3 LDO solids are presented in Figure 8. Stacked and interlayered platelet-like crystals of LDH particles of a crystal particle size of ~60 nm are observed in Figure 8a, which is consistent with previous reports [46,47]. This typical platelet-like morphology remains after calcination. However, the morphology greatly changed after reduction in hydrogen. As shown in Figure 8c, spherical particles with a diameter of 100 ~ 200 nm are formed. The hydrogen treatment conditions had a great influence on the particle dispersion, morphology and structure, and these features would eventually affect the activity performance of the catalyst [30,48]. The powder XRD patterns of samples reduced at 500 and 600 °C are presented in Figure 9. After hydrogen treatment at 600 °C for 1 h, the sample shows relatively strong diffraction peaks at 44.48 and 51.67° corresponding to the Ni phase. The main diffraction peaks for the NiO phase (ca. 37.25 and 44.48°) were not detected, indicating that NiO has been fully reduced to Ni [49]. However, the sample exhibited relatively weak peaks of Ni phase and clear peaks of NiO phase after H2 treatment at 500 °C. The H2-TPR profile of the AMO-Ni3Al1-CO3-LDO solid ( Figure 10) shows its reducibility characteristics. It can be inferred that complete reduction of NiO cannot be accomplished at 500 °C, and it is envisaged that the full reduction temperature is at least 550 °C [50]. This is consistent with the powder XRD results that the sample reduced at 500 °C is not as complete as that reduced at 600 °C. The H 2 -TPR profile of the AMO-Ni 3 Al 1 -CO 3 -LDO solid ( Figure 10) shows its reducibility characteristics. It can be inferred that complete reduction of NiO cannot be accomplished at 500 • C, and it is envisaged that the full reduction temperature is at least 550 • C [50]. This is consistent with the powder XRD results that the sample reduced at 500 • C is not as complete as that reduced at 600 • C. It can be concluded that there exist strong metal-support interactions between NiO and Al 2 O 3 surface [36,[51][52][53].
Catalysts 2020, 10, x FOR PEER REVIEW 8 of 14 It can be concluded that there exist strong metal-support interactions between NiO and Al2O3 surface [36,[51][52][53]. The effect of reduction temperature on the CO2 conversion and CH4 selectivity over the AMO-Ni3Al1-CO3-LDO solid was investigated and results are presented in Figure 11. In all case, practically 100% CH4-selectivity (no CO formation) was detected below 400 °C. The CO2 conversion reaches its maximum value at 360 °C, namely 87.9%, 86.3%, 85.5%, and 77.5% for the catalysts reduced at 600, 550, 500, and 400 °C, respectively. A significant improvement of CO2 conversion can be seen after increasing the hydrogen reduction temperature from 400 to 600 °C, especially for the CO2 methanation reaction occurred at 260 °C. As discussed above, with the increase of reduction temperature, more NiO was converted into Ni, which leads to a high activity. Based on the results of powder XRD, H2-TPR and catalytic tests, it can be stated that the degree of supported Ni reduction is influenced by the hydrogen reduction temperature and it has an important impact on the CO2 conversion. It appears that 600 °C is the best reduction temperature for the present LDHs derived Nibased CO2 methanation catalysts. However, according to the H2-TPR profile, the catalyst should be reduced completely at 550 °C. Since the H2-TPR experiment is conducted dynamically, the reduction peak maximum may not appear at the exact temperature recorded. To ensure the thorough conversion of NiO into Ni, the hydrogen reduction temperature of the sample was set at 600 °C. The effect of reduction temperature on the CO 2 conversion and CH 4 selectivity over the AMO-Ni 3 Al 1 -CO 3 -LDO solid was investigated and results are presented in Figure 11. In all case, practically 100% CH 4 -selectivity (no CO formation) was detected below 400 • C. The CO 2 conversion reaches its maximum value at 360 • C, namely 87.9%, 86.3%, 85.5%, and 77.5% for the catalysts reduced at 600, 550, 500, and 400 • C, respectively. A significant improvement of CO 2 conversion can be seen after increasing the hydrogen reduction temperature from 400 to 600 • C, especially for the CO 2 methanation reaction occurred at 260 • C. As discussed above, with the increase of reduction temperature, more NiO was converted into Ni, which leads to a high activity. Based on the results of powder XRD, H 2 -TPR and catalytic tests, it can be stated that the degree of supported Ni reduction is influenced by the hydrogen reduction temperature and it has an important impact on the CO 2 conversion. It appears that 600 • C is the best reduction temperature for the present LDHs derived Ni-based CO 2 methanation catalysts. However, according to the H 2 -TPR profile, the catalyst should be reduced completely at 550 • C. Since the H 2 -TPR experiment is conducted dynamically, the reduction peak maximum may not appear at the exact temperature recorded. To ensure the thorough conversion of NiO into Ni, the hydrogen reduction temperature of the sample was set at 600 • C. Figure 11. CO2 conversion (%) and CH4 selectivity (%) vs temperature profiles for the CO2 methanation reaction over AMO-Ni3Al1-CO3-LDO reduced in hydrogen at different temperatures, ca. 400, 500, 550 and 600 °C.

Stability Performance of AMO-Ni3Al1-CO3-LDO
A stability test was performed on AMO-Ni3Al1-CO3-LDO at 360 °C for 19 h and results are shown in Figure 12. Before any measurements, the catalyst was reduced at 600 °C for 1 h. The results obtained indicate that AMO-Ni3Al1-CO3-LDO derived catalyst showed enhanced stability for long time-on-stream (TOS) since no obvious deactivation was observed ( Figure 12). The CO2 conversion remained above 87% and no CO was detected in the effluent gas stream. After the stability test, the spent catalyst was characterized by SEM. Figure 13 indicates the surface morphology of the spent catalyst. Compared with Figure 8c, the SEM images of spent catalysts show no obvious change in the particle configuration, which means that there were no structural changes in the solid catalyst. It was reported [22] that deactivation of CO2 methanation catalysts is mainly caused by possible structural changes, such as sintering of the metal, and the generation of carbon deposits. By using AMO-Ni3Al1-CO3 LDH as precursor, the obtained supported Ni nanoparticles exhibit a high degree of dispersion and strong metal-support interactions. Furthermore, the LDO support may provide oxygen vacancies [54], which are beneficial to the improvement of anti-sintering and anti-coking properties by providing an alternative route for CO2 dissociation into CO and O (lattice oxygen), the latter being able to oxidize carbon (formed on Ni) formed by the reverse Boudouard reaction [55], so as to ensure its high activity during long-term operation. Figure 11. CO 2 conversion (%) and CH 4 selectivity (%) vs temperature profiles for the CO 2 methanation reaction over AMO-Ni 3 Al 1 -CO 3 -LDO reduced in hydrogen at different temperatures, ca. 400, 500, 550 and 600 • C.

Stability Performance of AMO-Ni 3 Al 1 -CO 3 -LDO
A stability test was performed on AMO-Ni 3 Al 1 -CO 3 -LDO at 360 • C for 19 h and results are shown in Figure 12. Before any measurements, the catalyst was reduced at 600 • C for 1 h. The results obtained indicate that AMO-Ni 3 Al 1 -CO 3 -LDO derived catalyst showed enhanced stability for long time-on-stream (TOS) since no obvious deactivation was observed ( Figure 12). The CO 2 conversion remained above 87% and no CO was detected in the effluent gas stream. After the stability test, the spent catalyst was characterized by SEM. Figure 13 indicates the surface morphology of the spent catalyst. Compared with Figure 8c, the SEM images of spent catalysts show no obvious change in the particle configuration, which means that there were no structural changes in the solid catalyst. It was reported [22] that deactivation of CO 2 methanation catalysts is mainly caused by possible structural changes, such as sintering of the metal, and the generation of carbon deposits. By using AMO-Ni 3 Al 1 -CO 3 LDH as precursor, the obtained supported Ni nanoparticles exhibit a high degree of dispersion and strong metal-support interactions. Furthermore, the LDO support may provide oxygen vacancies [54], which are beneficial to the improvement of anti-sintering and anti-coking properties by providing an alternative route for CO 2 dissociation into CO and O (lattice oxygen), the latter being able to oxidize carbon (formed on Ni) formed by the reverse Boudouard reaction [55], so as to ensure its high activity during long-term operation.

Preparation of Catalysts
LDHs were first synthesized by the co-precipitation method. An aqueous solution containing nitrates of the metallic salts, ca. Ni(NO3)2·6H2O and Al(NO3)3·9H2O were added dropwise into a vigorously stirred Na2CO3 (NaNO3 for NiAl-NO3 LDH) solution. The pH of the resulting solution was kept constant at 10 by the addition of NaOH solution (4 M). The resulting slurry was stirred continuously for ~12 h at 30 °C, then filtered and washed several times with deionized water until pH reached 7, followed by drying at 60 °C in an oven. For AMO treatment, after the above-mentioned washing step, the resulting slurry was re-dispersed in ethanol for 2 h. The final solids were collected by filtration and then dried at 60 °C for further characterization. To investigate the influence of Ni content, NiAl LDHs with different Ni/Al atom ratios (x = Ni/Al) were synthesized, which were

Preparation of Catalysts
LDHs were first synthesized by the co-precipitation method. An aqueous solution containing nitrates of the metallic salts, ca. Ni(NO3)2·6H2O and Al(NO3)3·9H2O were added dropwise into a vigorously stirred Na2CO3 (NaNO3 for NiAl-NO3 LDH) solution. The pH of the resulting solution was kept constant at 10 by the addition of NaOH solution (4 M). The resulting slurry was stirred continuously for ~12 h at 30 °C, then filtered and washed several times with deionized water until pH reached 7, followed by drying at 60 °C in an oven. For AMO treatment, after the above-mentioned washing step, the resulting slurry was re-dispersed in ethanol for 2 h. The final solids were collected by filtration and then dried at 60 °C for further characterization. To investigate the influence of Ni content, NiAl LDHs with different Ni/Al atom ratios (x = Ni/Al) were synthesized, which were

Preparation of Catalysts
LDHs were first synthesized by the co-precipitation method. An aqueous solution containing nitrates of the metallic salts, ca. Ni(NO 3 ) 2 ·6H 2 O and Al(NO 3 ) 3 ·9H 2 O were added dropwise into a vigorously stirred Na 2 CO 3 (NaNO 3 for NiAl-NO 3 LDH) solution. The pH of the resulting solution was kept constant at 10 by the addition of NaOH solution (4 M). The resulting slurry was stirred continuously for~12 h at 30 • C, then filtered and washed several times with deionized water until pH reached 7, followed by drying at 60 • C in an oven. For AMO treatment, after the above-mentioned washing step, the resulting slurry was re-dispersed in ethanol for 2 h. The final solids were collected by filtration and then dried at 60 • C for further characterization. To investigate the influence of Ni content, NiAl LDHs with different Ni/Al atom ratios (x = Ni/Al) were synthesized, which were denoted as Ni x Al 1 -LDH (x = 2, 3, and 4). The obtained LDHs were calcined at 400 • C for 5 h and reduced in 10 vol% H 2 /Ar gas atmosphere (1 bar) before being used in any catalytic experiment.

Catalysts Characterization
Powder X-ray diffraction (PXRD) patterns of the solid AMO-NiAl LDOs samples were recorded using a Shimadzu XRD-7000 instrument in reflection mode and a Cu Kα radiation. The X-ray tube was operated at 40 kV and 40 mA while the accelerating voltage was set at 40 kV with 30 mA current (λ = 1.542 Å). Diffraction patterns were recorded within the range of 2θ = 5-75 • with a scanning rate of 5 • /min and a step size of 0.02 • . Textural properties of the solids were analyzed using nitrogen adsorption-desorption isotherms obtained from a physisorption analyzer (SSA-7000, Builder, Beijing, China). SSA was estimated using the Brunauer−Emmett−Teller (BET) method. Before each measurement,~0.1 g catalyst sample was degassed in a N 2 /He gas mixture at 220 • C for 4 h.
Hydrogen temperature-programmed reduction (H 2 -TPR) experiments were carried out in a multifunction chemisorption analyzer (PCA-1200, Builder, Beijing, China) equipped with a quartz U-tube reactor and a thermal conductivity detector (TCD) for gas analysis. For each test, a~0.05 g sample was utilized. Before switching to the H 2 /Ar gas stream, the sample was pretreated in Ar (40 mL/min) at 200 • C for 30 min and then cooled in Ar gas flow to 50 • C. The sample was then heated from 50 to 800 • C with a ramping rate of 10 • C/min in 5 vol% H 2 /Ar gas mixture flow (30 mL/min). A field emission scanning electron microscope (SU-8010, Hitachi, Tokyo, Japan) was used to characterize the morphology of Ni33Al LDO (secondary agglomerated particles). A carbon tape was used to stick the powder to the SEM stage. Transmission electron microscopy (TEM) images were obtained on a FEI Tecnai G2 F30 (Hillsboro, USA) which was operated at 300 kV. Energy-dispersive X-ray (EDX) mapping characterizations of the composite oxides-based catalysts was made to investigate the distribution of the elements in the samples using the same TEM instrument.

Catalytic Activity Tests
The CO 2 methanation catalytic activity tests of the synthesized AMO-NiAl LDOs were carried out at 1 atm pressure in a fixed-bed stainless-steel micro-reactor. The stainless-steel micro-reactor was installed in a vertical split-tube furnace equipped with a proportional-integral-derivative (PID) temperature controller and several mass flow controllers. Before the catalytic performance test, a catalyst amount of W cat = 0.1 g was first pretreated in 20 vol% H 2 /N 2 gas mixture (50 mL/min) at 600 • C for 1 h, and then cooled to 200 • C in N 2 gas flow. Subsequently, a gas mixture of H 2 , CO 2 and N 2 with a molar ratio of H 2 /CO 2 /Ar = 4:1:5 (ca. 40 vol% H 2 /10 vol% CO 2 /50 vol% Ar) was introduced into the reactor with a total flow rate of 80 mL/min corresponding to a space velocity of 48,000 mL g cat −1 h −1 . The methanation reaction was tested in the 200-500 • C range. After the reaction rate was stabilized at a given temperature after about 30 min, the composition of the effluent gas stream was analyzed online using a gas chromatography (SP-7890, Shandong Lunan, Zaozhuang, China) equipped with a TCD and FID detectors. The CO 2 conversion and CH 4 selectivity were estimated from the following Equations (2) In Equation (3), the [CO] concentration at the outlet of reactor refers to the non-selective CO 2 hydrogenation reaction towards CO and H 2 O products (reverse water-gas shift reaction). Also, given the CO 2 feed composition used (10 vol%), the change in the outlet total molar flow rate of the methanation reaction was small with respect to the inlet total molar flow rate, and this was considered in the estimation of CH 4 selectivity (Equation (3)).

Conclusions
Selective CO 2 methanation reaction was conducted over NiAl LDH derived catalysts which were prepared by coprecipitation and AMO treatment. AMO treated LDH exhibits higher SSA, which benefits Ni dispersion, than the conventional one. Highly dispersed Ni particles on the catalyst surface led to higher catalytic activity for CO 2 methanation. The Ni loading was found to play a key role in determining the catalytic activity by influencing the concentration of active sites along with the SSA. However, excessive Ni loading may result in Ni particle agglomeration, thus reducing the catalytic activity. As shown by the performed experiments, an optimal Ni/Al atom ratio of 3:1 was found. The catalyst reduction temperature by hydrogen was found to also play a role in its activity, which should be higher than 550 • C to ensure complete reduction of NiO to Ni. After hydrogen reduction at 600 • C, AMO-Ni 3 Al 1 -CO 3 LDH derived catalyst showed the highest CO 2 conversion (87.9%) and 100% CH 4 selectivity at 360 • C, due to the enhanced dispersion of Ni nanoparticles and SSA. Moreover, this AMO-Ni 3 Al 1 -CO 3 LDH derived catalyst shows remarkable stability in a 19-h activity test (above 87%) as the result of maintaining a highly dispersed Ni state, strong metal-support interactions, and abundant oxygen vacancy concentration.