Methanation of CO₂: impacts of modifying nickel catalysts with variable-valence additives on reaction mechanism

In this study, in total 16 additives ranging from Na to Zn in the periodic table of elements were added respectively to Ni/Al₂O₃ catalyst to understand their impacts on the physiochemical properties and reaction behaviors of the catalysts in methanation of CO₂. The results showed that the additives, except Sc, Ti, V, Mn, Zr, La and Ce, all led to the enlarged pores and the decreased surface area of the catalysts, especially for Na and K. Na or K reacted with alumina, forming Al(OH)₃ phase and leading to re-structure of the catalysts. MnO₂ reacted with NiO and formed NiMnO₃ structure. The additives also significantly affected the reduction behaviors of nickel oxide and the distribution of the basic sites on surface of the catalysts. V, Cr, Mn, Fe, Co, La and Ce modified catalysts showed good activity for methanation, resulting from that the multiple valences or oxygen vacancies (verified by EPR analysis of the Fe or Ce modified catalyst) in oxide form helped to absorb/activate CO₂. The in-situ DRIFTS studies indicated that *CO₂, *CO, formate, H₂CO*, *CH₃OH, hydrogen carbonate and carbonate were important reaction intermediates. The KNiAl catalyst showed strong absorption of carbonate and H₂CO* intermediates, negatively affecting their further conversion to form methane. The strong absorption of *CO over CuNiAl also prevented the further conversion of the intermediates to the downstream products. The Cr, Mn, Fe, La and Ce modified catalysts had milder absorption of the crucial formate intermediate, and no strong absorption of *CO or H₂CO* intermediates were observed, which were responsible for their high activities for methanation.