Methanation of CO₂ over Ni/Al₂O₃ modified with alkaline earth metals: impacts of oxygen vacancies on catalytic activity

This study investigates the impacts of the alkaline earth metal (Mg, Ca, Sr, Ba) additives on properties and performances of nickel catalysts for CO₂ methanation. The results show that addition of Mg, Sr, and Ba creates more pores while Ca addition leads to merge of small pores. The alkalinity of the catalyst increases with the addition of Mg, Ca, Sr or Ba, however, it does not necessarily enhance the catalytic activity. The degree of reduction of nickel species is another important factor affecting catalyst activity. Mg or Ca addition promotes the reverse water gas shift reaction to form more CO but not the methanation. In converse, with the addition of Sr or Ba, the activities for methanation increased drastically, especially in the low temperature region. In situ Diffuse Reflection Infrared Fourier Transform Spectroscopy (DRIFTS) studies show that *OH, *CO₃, *CO₂, –CH_x, HCOO* *CO and H₂CO* species are main reaction intermediates. Mg or Ca promotes the carbonate formation. Sr or Ba promotes *CO and H₂CO* formation, which are the important reaction intermediates in the conversion of CO₂ to CH₄. In addition, the Electron Paramagnetic Resonance (EPR) characterization shows that the catalyst modified with Sr species generates the oxygen vacancies that prevent electrons from being paired, forming a Lewis basic position. The oxygen vacancies generated are crucial for enhancing the catalytic activities for methanation at the low reaction temperatures.