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

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.