Impacts of La addition on formation of the reaction intermediates over alumina and silica supported nickel catalysts in methanation of CO₂

This study aimed to investigate impacts of Al₂O₃ and SiO₂, the supports of Ni catalysts with distinct properties, and the additive of La on catalytic behaviors and reaction intermediates formed during methanation of CO₂. The results showed that the addition of La to either Ni/Al₂O₃ or Ni/SiO₂ led to the reduced size of metallic nickel, the reduced reduction degree of nickel oxide, the increased alkalinity number and the increased activity for methanation of CO₂. Furthermore, the addition of La to the Ni/SiO₂ catalyst could suppress the formation of CO via the reverse water gas shift (RWGS) reaction. Ni/SiO₂ was much more active than the Ni/Al₂O₃, even though nickel size was much bigger. The in situ Diffuse Reflection Infrared Fourier Transform Spectroscopy (DRIFTS) studies showed that the addition of La to Ni/Al₂O₃ interfered with integration of hydroxyl group with *CO₂ species and formation of the bicarbonate and carbonate, while favored formation of the formate specie, enhancing the catalytic activity. For Ni/SiO₂, instead of formate, CO* became the main reaction intermediate. The strong absorption of CO* favored its further conversion and explained the low selectivity of the silica-based catalysts toward CO. The addition of La to Ni/SiO₂ catalyst facilitated further hydrogenation of CO* species to CH₄ and promoted the catalytic activity.