Solid solutions in reductive environment – A case study on improved CO2 hydrogenation to methane on cobalt based catalysts derived from ternary mixed metal oxides by modified reducibility.

• Mn modification increases reducibility of Co-aluminates. • Mn modified catalysts allow hydrogen spillover and leads to mechanism shift. • Strongly improved CO 2 methanation activity due to increased catalytic active surface area. Mixed metal oxides as solid solutions are promising catalyst precursor species, due to their atomic dispersion of metals within an oxide matrix. Upon activation by pre-reduction highly dispersed metal nanoparticles grow on the surface of a functional mixed metal support. Herein, the impact of different amounts of structure distorting manganese that is integrated in CoMn x Al 2-x O 4 spinel phase on the reducibility as a measure for the ability for pre-activation was investigated. The reducibility of the spinel increases with increasing Mn content. By using the Sabatier reaction (CO 2 methanation) as model reaction it was shown that the activity depends on the low temperature reducibility of the spinel. The highest catalytic productivity of 0.65 mol/(mol·min) at 400 °C was obtained with CoMn 0.5 Al 1.5 O 4 as a precursor in line with a highly improved selectivity (S(CH 4) = 97%) towards methane as compared to manganese free CoAl 2 O 4. In addition, the activation energy drops from 107 kJ/mol to 69 kJ/mol upon Mn incorporation. Intense surface analysis via CO & H 2 pulsed titration, BET, CO 2 -TPD, CO 2 DRIFTS, as well as operando DRIFTS analysis revealed, that the integration of Mn into the spinel support decreases the overall surface basicity and enables, potentially due to its Mn3+/Mn2+ redox pairs, spillover of hydrogen from the metallic sites towards the surface of the support. This leads to an altered reaction mechanism via formate species without production of CO as reaction intermediates. This in combination with the ability to transfer the CO 2 conversion from the metal sites only towards the surface of the support due to hydrogen spillover leads to the observed increase in catalytic performance. This work demonstrates the high potential of specific modification of typically highly stable mixed metal oxides as valuable catalyst precursor species. [ABSTRACT FROM AUTHOR]