The Investigation Of Ni/γ-Al₂O₃Catalysts For CO Methanation

The influence factors of the catalytic performance of methanation catalyst contain bothconditions of catalyst preparation and CO methanation reaction. A comprehensivethermodynamic analysis of methanation reaction under different reaction conditions isconducted using Gibbs free energy minimizing method, which is contrasted with theexperimental results. Here, we report the preparation of various Ni/Al₂O₃catalysts supportedon different commercial Al₂O₃supports and a systematical investigation on their catalyticperformance in CO methanation reaction, focusing on the effects of the Al₂O₃support, NiOloading, MgO loading, calcination temperature, as well as reaction conditions such as spacevelocity, H₂/CO ratio, reaction pressure and quartz sand/catalyst ratio on the perfornance ofCO methanation catalysts. This paper further investigates the lifetime and regenerationproperties of the Ni catalysts.A series of Ni/Al₂O₃catalysts were synthesized by using the conventional impregnationmethod, characterized by X-ray diffraction, N₂adsorption-desorption, H₂-TPR, TPO, SEM,TEM, Sulfur-Carbon element analysis and TGA.It is found that the nature of Al₂O₃support, the loading of NiO or MgO and thecalcination temperature have great influence on the type and size of NiO species therebyinfluencing the performances of the catalysts. According to the XRD and H₂-TPR results, S₄is the optimum selection due to its strong interaction with NiO particles, relatively largersurface area and low price. A moderate NiO loading （20wt%） and a relatively lowcalcination temperature （400℃） can generate a high density of reducible NiO on the supportsurface, which has an intermediate interaction with the support. The more content of β-typeNiO in the catalyst is benefit for the high activity and well thermally stable of the catalyst.After the activation, relatively small-sized and highly active Ni0particles are generated.Moreover, adding MgO into Ni/Al₂O₃catalyst can enhance the resistance to the carbondeposition thus increasing the stability of the Ni catalyst. However, Too much MgO content isbad to improve the catalytic performance due to the formation of MgNiO₂species, which canreduce the reducibility of NiO. Therefore,2wt%MgO loading is the optimum choice in ourwork. Additionally, decreasing GHSV to30000mL/g·h and increasing the reaction pressureto3MPa and H₂/CO ratio to3:1and quartz sand/catalyst ratio to5:1can lead to theenhanced CO conversion and better production of CH₄. Especially under3MPa, CO conversion can keep100%within a wide range of temperature（300⁵50℃） and CH₄selectivity increases with increasing temperature and reach the maximum of96.5%atrelatively low temperature（350℃）, which is consistent with the results from thethermodynamical analysis. The long term CO methanation reaction test shows that thesynthesized20N₂M/S₄-400catalyst is highly active, thermally stable, and resistant to carbondeposition. The CO conversion over20N₂M/S₄-400catalyst decrease only5%after196hreaction at the high temperature of400℃and the decline of CH₄selectivity was notsignificantly. The catalytic performance of20N₂M/S₄-400catalyst declines less obviouslyafter removing carbon. This work is useful for the well understanding the CO methanationprocess and ultimately ascertained the best preparation conditions of the Ni/Al₂O₃catalystsand reaction conditions of the CO methanation, as well as proposed the valid data for furtherstudying in the methanation technology.