Ab initio study of CO2 hydrogenation mechanisms on inverse ZnO/Cu catalysts

Methanol formation from CO2 and molecular hydrogen on ZnO/Cu catalysts is studied by gradient corrected density functional theory. The catalytically active region is modeled as a minimum size inverse catalyst represented by ZnXOY(H) clusters of different size and a ZnO nano-ribbon on an extended Cu(1 1 1) surface. These systems are chosen as a representative of thermodynamically stable catalyst structures under typical reaction conditions. Comparison to a high level wave function method reveals that density functional theory systematically underestimates reaction barriers, but nevertheless conserves their energetic ordering. In contrast to other metal-supported oxides like ceria and zirconia, the reaction proceeds through the formation of formate on ZnOX/Cu, thus avoiding the CO intermediate. The difference between the oxides is attributed to variance in the initial activation of CO2. The energetics of the formate reaction pathway is insensitive to the exact environment of undercoordinated Zn active sites, which points to a general mechanism for Cu-Zn based catalysts.