We present a mechanistic study on selective hydrogenation of acrolein over model Pd surfaces—both single crystal Pd(111) and Pd nanoparticles supported on a model oxide support. We show for the first time that selective hydrogenation of the C=O bond in acrolein to form an unsaturated alcohol is possible over Pd(111) with nearly 100% selectivity. However, this process requires a very distinct modification of the Pd(111) surface with an overlayer of oxopropyl spectator species that are formed from acrolein during the initial stages of reaction and turn the metal surface selective toward propenol formation. By applying pulsed multimolecular beam experiments and in situ infrared reflection–absorption spectroscopy, we identified the chemical nature of the spectator and the reactive surface intermediate (propenoxy species) and experimentally followed the simultaneous evolution of the reactive intermediate on the surface and formation of the product in the gas phase.

We present a mechanistic study on the interaction of water with a well-defined model Fe3O4(111) surface that was investigated by a combination of direct calorimetric measurements of adsorption energies, infrared vibrational spectroscopy, and calculations bases on density functional theory (DFT). We show that the adsorption energy of water (101 kJ mol(-1)) is considerably higher than all previously reported values obtained by indirect desorption-based methods. By employing (18)O-labeled water molecules and an Fe3 O4 substrate, we proved that the generally accepted simple model of water dissociation to form two individual OH groups per water molecule is not correct. DFT calculations suggest formation of a dimer, which consists of one water molecule dissociated into two OH groups and another non-dissociated water molecule creating a thermodynamically very stable dimer-like complex.

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Thumbnail image of graphical abstract The binding energy of oxygen on Pd nanoparticles was measured by a direct calorimetric method as a function of the particle size. The reduced dimensionality of the Pd substrate results in two counteracting trends: an increase of the Pd[BOND]O binding strength due to a change of the local configuration of the adsorption site and a strong decrease of the Pd[BOND]O bonding due to the reduction of the cluster size

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 new catalytic materials with the desired selectivity towards 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 reflectionabsorption 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 the 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 exceptionally high selectivity towards C=O bond hydrogenation over Pd (111) This process prevents formation of well-ordered overlayers of oxopropyl species required for selective C=O bond hydrogenation resulting in only minor non-selective 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 products propanal and propenol.