The gas-phase hydrogenation of acrolein over silver was studied in a broad pressure range from similar to 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 the "pressure and materials gap" in catalysis. High pressures and 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 were performed at 7.5 mbar 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 orients parallel to the surface on Ag(l 11) 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 and the possible rate-determining step in acrolein hydrogenation are discussed. (c) 2005 Elsevier Inc. All rights reserved.