Preparation And Modification Of Ni-based Catalysts For Methanation

Development of an advanced methanation catalyst is one of the key technologies of SNG (substitute natural gas) made from coal and carbon dioxide. The technology makes contribution to solving the shortage of natural gas and environmental pollution. In the carbon monoxide and carbon dioxide methanation catalyst systems, nickel-based catalyst has been widely studied because of its low-cost and comparable catalytic activity. Much attention was paid to improve the activity in low operate temperature and the hydrothermal stability of Ni-based catalyst. In this present, the relationship between the structure and performance of Ni/Al2O3 and the modified catalysts were investigated using the techniques such as H2-TPR, UV-DRS, XRD, TEM, XPS and insitu-DRIFTS. The main results are as follows:(1) Increasing the calcination temperature would give rise to the decrease in the surface areas and Ni dispersion. As a result, the catalytic activity of CO methanation declined. On the other hand, the interaction between nickel metal and aluminum oxide can be adjusted by calcination temperature of catalyst to obtain a better hydrothermal stability. In our investigation, Ni/Al2O3 calcined at 800℃ has a good balance for the activity and hydrothermal stability.(2) Among the ZrO2 and transition metal (Fe, Mn or Co) modified catalysts, Ni-Co/ZrO2-Al2O3 sample exhibits excellent catalytic activity and hydrothermal stability for CO methanation.These may contribute to the optimized texture, high Ni dispersion and appropriate interaction between nickel metal and support.(3) Zirconium oxide doped Ni/Al2O3 catalysts show better catalytic activity in the methanation of CO2. Especially, the 16 wt.% ZrO2-doping remarkably improves the activity. In which, the highly dispersed ZrO2 partly replaces Al2O3 that covers on the surface of nickel, inhibiting the formation of NiAl2O4 microcrystallines. It can weaken the strong interaction between nickel and support and increase the exposure area of Ni metal. On the other hand, the introduction of ZrO2 additive contributes to the dispersion of components and the decrease of nickel grain size. Both are beneficial to the increase of the number of active sites.(4) Ni-Fe/Al2O3 catalyst is likely to form Ni-Fe alloy after reduction. The ratio of Fe and Ni in the alloy phase has a significant impact on the activity for CO2 methanation. The sample with Ni/Fe molar ratio of 8:2 has the best catalytic activity.(5) The reaction mechanism for CO2 methanation over Ni-based catalysts follows Eley-Rideal (ER) mechanism. For Ni/Al2O3 and Ni/ZrO2-Al2O3 systems, the intermediate species include adsorbed CO and formate. While for Ni-Fe/Al2O3 system, the only intermediate species is adsorbed formate. The hydrogenation of formate species is rate-determining step in the methanation reaction.