Impact of oxygen vacancies on the catalytic activity of Ni/Co3O4 for CO2 methanation

The catalytic conversion of carbon dioxide to useful products such as methane is one of the best alternatives to address the growing energy demand resulting from the depletion of fossil fuels. In this work, Co3O4 and Ni substituted Co3O4 catalysts were synthesized by a single-step solution combustion method and their ability for CO2 hydrogenation to form methane were evaluated. Ni substitution improved the methanation activity with successful substitution achieved up to 15 at% within Co3O4 lattice. Ni substitution resulted more oxygen vacancies. Subsequent reduction treatment further enhanced the oxygen vacancy concentration and facilitated CO2 activation. The physical, structural, morphological, elemental and redox properties of the catalysts were characterized by XRD, XPS, SEM, TEM, N-2 adsorption-desorption, ICP-OES, H-2-TPR, CO2-TPD, CO2-TPD-MS and H-2 pulse titration techniques. All Ni-doped catalysts offered decent performance. However, the fresh 20%Ni substituted Co3O4 catalyst demonstrated the highest performance, achieving 58% CO2 conversion and 90% CH4 selectivity at 400 degrees C. The catalyst showed significantly enhanced performance following the creation of vacancies through reduction. The CO2 conversion increased to 78% with >98% CH4 selectivity at 400 degrees C. The apparent activation energy of the reduced 20%Ni/Co3O4 was determined to be the lowest at 21.6 +/- 0.6 kJ. mol(-1). Mechanistic insights and the role of oxygen vacancies were further investigated through DFT and in situ FTIR studies. A combined formate and CO pathway was identified as the reaction mechanism over 20%Ni/Co3O4.