Unprecedented contributions of In2O3 promoter on ordered mesoporous Cu/Al2O3 for CO2 hydrogenation to oxygenates

Abstract

Synergy effects of In2O3 promoter on an ordered mesoporous Cu/Al2O3 structures were verified in terms of CO2 hydrogenation to oxygenates by controlling a competitive reverse water gas shift (RWGS) reaction activity resulted in altering product distributions. The contiguously interacted smaller In2O3 on the Cu-Al2O3 matrices at an optimal In/Cu ratio of ∼ 0.01 (InCu/mAl(1)) played important roles to enhance CO2 activation with its lower activation energy (∼15.6 kJ/mol) as well as to stabilize the spatially confined Cu nanoparticles by decreasing the formations of less active CuAl2O4 phases. On the optimal InCu/mAl(1), the oxygen vacant sites originated from Cu-In2O3 interfaces under reductive environments largely accelerated CO2 dissociation to form HCOO formate intermediate by suppressing RWGS reaction activity, where the formate intermediates can be further hydrogenated to oxygenates on the metallic Cu or Cu2O sites as well as dimethyl ether (DME) on the acidic sites via successive cascade reaction. However, an excess amount of In2O3 (ln/Cu ratio above 0.03) on the Cu-Al2O3 matrices decreased the catalytic activity due to severe blockages of the more active metallic Cu sites with an increased oxygenates selectivity such as methanol (CH3OH) and dimethyl ether (DME). The positive roles of oxygen-vacant Cu-In2O3 sites on the mesoporous Cu-Al2O3 structures were to effectively activate CO2 molecules to form formate and carbonate intermediates as well as to stabilize the more active partially reduced Cu phases with an enhanced hydrogenation activity to oxygenates.