Low-Temperature Chemoselective Gold-Surface-Mediated Hydrogenation of Acetone and Propionaldehyde

Since nanoscale gold was first discovered to be catalytically active, gold-based catalysts have been studied both theoretically and experimentally in a wide range of reactions. These catalysts exhibit high activity for hydrogenation processes, in particular showing enhanced selectivity. However, there is a lack of relevant fundamental studies into these processes. Conducting hydrogenation reactions on model gold surfaces is useful for obtaining mechanistic insight and for further enhancing our understanding of the catalytic properties of supported-gold catalysts. Herein, we report the chemoselective hydrogenation of aldehydes over ketones on gold surfaces. The hydrogenation chemistry of oxygenated hydrocarbons has been studied on transition-metal surfaces for a variety of reactions that are important to the pharmaceuticaland chemical industries. For example, C=O bond hydrogenation is a key step in the catalytic conversion of cellulosic biomass. In addition, gold-based catalysts have also shown exceptional activity for the selective hydrogenation of a,b-unsaturated carbonyl compounds. Claus found that, in the production of allyl alcohol from the hydrogenation of acrolein, gold catalysts yielded about 10-times-higher selectivity for C=O bond hydrogenation than traditional platinum-based catalysts. Therefore, exploring the individual reactivity of carbonyl-hydrogenation could provide useful information for a better holistic understanding of these important catalytic reactions. By employing propionaldehyde and acetone as representative probe molecules of aldehydes and ketones, respectively, we investigated the hydrogenation of C=O bonds on a model pre-atomic-hydrogen-covered Au(111) catalyst. H atoms were used herein owing to the high energetic barrier of H2-dissociation on gold. Temperature-programmed-desorption (TPD) measurements indicated different activities for hydrogenation on gold: propionaldehyde underwent hydrogenation to afford 1-propanol on H-covered gold but acetone did not form 2propanol. Density functional theory (DFT) calculations revealed different activation energies for the reactions between a single carbonyl moiety and a H atom. First, the hydrogenation of acetone was investigated on a Au(111) surface. In a control experiment (Figure 1a), 1.62 ML (ML=monolayer) of acetone (m/z=43, the most-abundant mass fragment of acetone) was adsorbed onto a clean Au(111)