Synthesis, characterization and activity of CeO₂-doped coprecipitated NiAlO_xcatalysts for CO₂methanation

CeO₂-doped NiAlO_x mixed oxide catalysts with molar Ce/Ni ratios of 0.05, 0.1 and 0.3 were prepared via coprecipitation and analyzed in detail regarding structure and activity in CO₂ methanation. A cerium-free NiAlO_x, a CeO₂ and a NiCe-based sample served as references. Characterization revealed a two-phase composition of the doped materials consisting of a NiAlO_x mixed oxide and a CeO₂ phase. The latter features highly reactive oxygen vacancies upon reduction with hydrogen, which provide additional binding sites for CO₂. Reduction of the CeO₂ phase (and with that the formation of oxygen vacancies in the doped catalysts) was demonstrated to be only feasible in the presence of Ni which, in this context, acts as a hydrogenation catalyst providing dissociated hydrogen. The detection of completely reversible reduction-oxidation-reduction cycles of CeO₂-doped NiAlO_x by quasi in-situ XPS suggested diminutively-sized Ni particles provided by synthesis via coprecipitation. Catalytic testing of the CeO₂-doped samples in CO₂ methanation revealed up to three times higher turnover frequencies and CH₄ weight-time yields compared to undoped NiAlO_x. An optimum in catalytic activity at Ce/Ni = 0.1 indicated the prerequisite of a balance between Ni and Ce amounts in the catalyst to avoid undesired byproduct formation.