Bridging the pressure and materials gap: In-depth characterization and reaction studies of silver catalysed acrolein hydrogenation

The gas phase hydrogenation of acrolein over silver has been studied in a broad pressure range from ~2 mbar to 20 bar and with various silver materials (single crystals, sputtered silver, silica supported Ag nanoparticles) in an attempt to examine the question of "pressure and materials gap" in catalysis. High pressures as well as nanoparticles favour the formation of allyl alcohol (selectivities up to 42 %), whereas with the opposite conditions propionaldehyde is by far the main product. A critical minimum reaction pressure was identified: below ca. 100 mbar no allyl alcohol was formed. In situ-XAS measurements have been performed at 7.5 mbar in order to gain insight into the interaction of acrolein with silver samples. Despite the fact that beam-induced processes have been observed, it is concluded that at low pressures, acrolein orientates parallel to the surface on Ag(111) and is present at the surface in the form of hydrogenated propionaldehyde-like species. The influence of catalyst structure and pressure on the adsorption geometry of acrolein as well as the possible rate-determining step in acrolein hydrogenation are discussed.