Theoretical understanding on the selectivity of acrolein hydrogenation over silver surfaces: the non-Horiuti–Polanyi mechanism is the key

Abstract

Chemoselective hydrogenation of α,β-unsaturated aldehydes to unsaturated alcohols is not only an important reaction in the chemical industry but also a good model system to understand the catalytic selectivity in heterogeneous catalysis. In the current work, the selectivity of partial hydrogenation of acrolein (C₃H₄O), the simplest α,β-unsaturated aldehyde, is investigated employing density functional theory (DFT) calculations. Two hydrogenation mechanisms, namely the Horiuti–Polanyi mechanism and the non-Horiuti–Polanyi mechanism, are employed to study the partial hydrogenation of acrolein over Ag(111), Ag(100), Ag(211) and Ag(111)-mono surfaces. It is found that the hydrogenation of C₃H₄O to C₃H₅O at the terminal carbon and oxygen atoms follows the non-Horiuti–Polanyi mechanism in which C₃H₄O reacts with hydrogen molecules directly over all the silver surfaces studied, whilst atomic hydrogen is the active hydrogen species for the hydrogenation of C₃H₅O to C₃H₆O. Subsequently, the selectivities between partial hydrogenation products, i.e. propenol, propanal and enol, over silver surfaces with different morphologies are compared by calculating the energy difference between the rate-determining transition states. We find that the selectivity of propenol formation increases with the coordination number of surface silver atoms, which is in good agreement with the trend of selectivities obtained experimentally. It is also interesting to find that the selectivity trend obtained based solely upon the Horiuti–Polanyi mechanism for the hydrogenation of C₃H₄O to C₃H₅O and C₃H₅O to C₃H₆O cannot explain the experimental results. In other words, the non-Horiuti–Polanyi mechanism is able to give a more reasonable explanation for the selectivity trend observed experimentally than the normally used Horiuti–Polanyi mechanism in heterogeneous catalysis. Our work highlights the significance of the non-Horiuti–Polanyi mechanism in understanding heterogeneous catalytic hydrogenation reactions.