Impacts of alkali or alkaline earth metals addition on reaction intermediates formed in methanation of CO₂ over cobalt catalysts

Additives affect the physiochemical properties of the catalyst as well as the evolution of the reaction intermediates produced during the reaction process such as the methanation of CO₂. In this study, Co/Al₂O₃ catalysts modified with Na, K, Mg or Ca were prepared and the reaction intermediates formed during CO₂ methanation were investigated. The results showed that Na, K or Mg species reacted with alumina, forming Al(OH)₃ or MgAl₂O₄ spinel structure, leading to the re-structure of the catalysts and a remarkable decrease of the specific surface area. The increased alkalinity of the catalyst did not promote the catalytic activity for methanation but promoted CO formation. The addition of Na or K enhanced the affinity of the catalyst to the reaction intermediates of HCOO* and CO₃²⁻, slowing down their further reduction to CH₄ and leading to the lower catalytic activity. The evolution of HCOO* and CO₃²⁻ species strongly correlated with the catalytic activity, while the direct correlation between the capability for the absorbance of CO₂* as well as the C–O functionality and the catalytic activity was not found. In addition, the addition of Na or K to Co/Al₂O₃ could also induce the formation of a significant amount of the coke species in the nanotube form.