Selective Partial Hydrogenation of Acrolein on Pd: A Mechanistic Study

Identifying the surface processes governing the selectivity in hydrogenation of α,β-unsaturated carbonyl compounds on late transition metals is crucial for the rational design of catalytic materials with the desired selectivity toward C═C or C═O bond hydrogenation. The partial selective hydrogenation of acrolein on a Pd(111) single crystal and Fe3O4-supported Pd nanoparticles under well-defined UHV conditions was investigated in the present study as a prototypical reaction. Molecular beam techniques were combined with infrared reflection–absorption spectroscopy (IRAS) and quadrupole mass spectrometry (QMS) in order to simultaneously monitor the evolution of surface species and the formation of the final gas-phase products under isothermal reaction conditions as a function of surface temperature. Over a Pd(111) single crystal, acrolein is hydrogenated at the C═O bond to form the desired reaction product propenol with nearly 100% selectivity in the temperature range between 250 and 300 K, while over Pd/Fe3O4, selective hydrogenation of the C═C bond to form propanal occurs. We found that the high selectivity toward C═O bond hydrogenation over Pd(111) is connected to the initial modification of the catalytic surface with a dense monolayer of an oxopropyl surface species. This strongly bound oxopropyl layer is formed on the pristine Pd crystal in the induction period from half-hydrogenation of the C═C bond in acrolein. Subsequently deposited acrolein molecules adsorb via the C═O bond and form a half-hydrogenated reaction intermediate propenoxy species, which is attached to Pd via a C–O single bond. The evolution of the surface concentration of the propenoxy intermediate monitored spectroscopically was found to closely follow the propenol formation rate detected in the gas phase. At temperatures higher than 300 K on Pd(111) and on Pd nanoparticles supported on Fe3O4, decarbonylation of acrolein occurs, leading to accumulation of CO and strongly dehydrogenated carbonaceous species on the surface. This process prevents formation of well-ordered overlayers of oxopropyl species required for selective C═O bond hydrogenation, resulting in only minor nonselective hydrogenation of acrolein. At temperatures below 250 K on Pd(111), only a small fraction of the initially adsorbed acrolein is converted into the oxopropyl species, yielding a partially modified surface and thus rather unselective formation of both propanal and propenol products.