A density functional study of adsorption structures of unsaturated aldehydes on Pt(111): A key factor for hydrogenation selectivity

Various adsorption structures of acrolein (propenal), crotonaldehyde (2-butenal), and prenal (3-methyl-2-butenal) on Pt(111) are compared by means of first-principle density functional theory calculations (DFT). Methyl substituents are added one by one at the C(1) carbon of the C=C-C=O frame and the substitution induces a general decrease in the adsorption energy. Over a large range of coverages, acrolein shows its main interaction with the surface with the C=C bond, with an eventual additional weak interaction with the oxygen atom. This could clearly result in a predominant hydrogenation of the C=C bond, as was seen experimentally. The influence of the substituents is subtle in the case of crotonaldehyde but clear for prenal. In that case, at low coverage a flat adsorption mode by both the C=C and the C=O bonds is favored. This larger implication of the C=O bond in the adsorption is a first explanation for the higher selectivity in unsaturated alcohol during the hydrogenation of prenal compared with acrolein. Additionally, if high coverage is considered, the prenal molecule is too large to keep a flat adsorption mode and it can switch to a vertical geometry, interacting with the surface by the oxygen atom, while acrolein remains bonded by the C=C bond. Such a vertical structure prevents C=C hydrogenation and could also explain the increased selectivity to unsaturated alcohol. (C) 2002 Elsevier Science (USA).