Role of Surface Hydroxyl Species in Copper-Catalyzed Hydrogenation of Ketones

A comprehensive, coverage-dependent mean-field microkinetic model is developed for the hydrogenation of carbonyl compounds on Cu(111). In the model, hydrogenation by surface hydrogen, surface hydroxyl species, and adsorbed water molecules is considered, including a reaction pathway via keto–enol tautomerization. The model parameters were calculated by VdW-DF2 density functional theory and account for inter- and intraspecies repulsion. Accounting for these coverage effects changes the surface from being completely covered with 25% oxygen atoms and 75% hydroxyl groups to a surface with 65% free sites. Including coverage effects also surprisingly increases the calculated turnover frequency from 6 × 10–5 to 2 × 10–3 s–1. In the dominant reaction path, the carbonyl group is hydrogenated to an alkoxy intermediate by surface hydrogen, followed by a proton transfer from either a surface hydroxyl species or an adsorbed water molecule to form the alcohol product. The addition of small amounts of water suffices to open this pathway. The pathway in which acetone is converted to 2-hydroxypropylene via keto–enol tautomerization is kinetically irrelevant under the considered conditions. Regeneration of the hydroxyl groups is the rate-controlling step in the mechanism, suggesting an alternative role for the reducible oxide promoters which are often encountered for Cu-based carbonyl hydrogenation catalysts.