The adsorption and hydrogenation of acrolein on the Ag(111) surface has been investigated by high resolution synchrotron XPS, NEXAFS, and temperature programmed reaction. The molecule adsorbs intact at all coverages and its adsorption geometry is critically important in determining chemoselectivity toward the formation of allyl alcohol, the desired but thermodynamically disfavored product. In the absence of hydrogen adatoms (H(a)), acrolein lies almost parallel to the metal surface; high coverages force the CdC bond to tilt markedly, likely rendering it less vulnerable toward reaction with hydrogen adatoms. Reaction with coadsorbed H(a) yields allyl alcohol, propionaldehyde, and propanol, consistent with the behavior of practical dispersed Ag catalysts operated at atmospheric pressure: formation of all three hydrogenation products is surface reaction rate limited. Overall chemoselectivity is strongly influenced by secondary reactions of allyl alcohol. At low H(a) coverages, the CdC bond in the newly formed allyl alcohol molecule is strongly tilted with respect to the surface, rendering it immune to attack by H(a) and leading to desorption of the unsaturated alcohol. In contrast with this, at high H(a) coverages, the CdC bond in allyl alcohol lies almost parallel to the surface, undergoes hydrogenation by H(a), and the saturated alcohol (propanol) desorbs.

The chemoselective hydrogenation of a,b-unsaturated carbonyl compounds to form unsaturated alcohols is challenging, fundamentally interesting, and important both in the research laboratory and on a technical scale, as these materials are valuable and versatile intermediates in the production of fine chemicals and pharmaceuticals. Thermodynamics favors hydrogenation of the C=C bond to form the (unwanted) saturated aldehyde or ketone as shown in Scheme 1, which refers to the particular case of crotonaldehyde. Thus chemoselective C = O hydrogenation is a demanding process that requires the manipulation of kinetic effects by means of a suitable catalyst.A variety of supported metal-particle catalysts (including Pt, Pd, Cu, Ag, Au, Ir, and Os) that exhibit a wide range of chemoselectivity have been investigated for this purpose, [1] though very little has emerged in terms of fundamental understanding. Theoretical studies [2] suggest that the adsorption geometry of the reactant molecule may be a key Scheme 1. Possible hydrogenation products of crotonaldehyde.

The chemoselective hydrogenation of crotonaldehyde to crotyl alcohol was studied by temperature programmed desorption/reaction, high resolution XPS and NEXAFS. The organic molecule adsorbed without decomposition, all three possible hydrogenation products were formed and desorbed, and the clean overall reaction led to no carbon deposition. Selectivities up to 95% were found under TPR conditions.The observed behavior corresponded well with selectivity trends previously reported for Ag/SiO 2 catalysts and the present findings permit a rationalization of the catalytic performance in terms of pronounced coverage-dependent changes in adsorption geometries of the reactant and the products.Thus at low coverages the C=O bond in crotonaldehyde lay almost parallel to the metal surface whereas the C=C was appreciably tilted, favoring hydrogenation of the former and disfavoring hydrogenation of the latter. With increasing coverage of reactants, the C=C bond was forced almost parallel to the surface, rendering it vulnerable to hydrogenation, thus markedly decreasing selectivity towards formation of crotyl alcohol. Butanol formation was the result of an overall twostep process: crotonaldehyde crotyl alcohol butanol, further hydrogenation of the desired product crotyl alcohol being promoted at high hydrogen coverage due to the C=C bond in the unsaturated alcohol being driven from a tilted to a flat-lying geometry. Finally, an explanation is offered for the strikingly different behavior of Ag(111) and Cu(111) in the chemoselective hydrogenation of crotonaldehyde in terms of the different degrees of charge transfer from metal to C=O π bond, as suggested by C 1s XPS binding energies.

Sulfur adatoms strongly activate the otherwise inert Cu(111) surface towards chemoselective hydrogenation of crotonaldehyde by electronically perturbing and strongly tilting the reactant.