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

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 CoMnxAl2-xO4 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 (CO2 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 degrees C was obtained with CoMn0.5Al1.5O4 as a precursor in line with a highly improved selectivity (S(CH4) = 97%) towards methane as compared to manganese free CoAl2O4. 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, CO2-TPD, CO2 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+/Me2+ 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 CO2 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. (C) 2020 Elsevier Inc. All rights reserved.