Influence of Sn additives on the selectivity of hydrogenation of alpha-beta-unsaturated aldehydes with Pt catalysts:a density functional study of molecular adsorption

摘要

In this study,the adsorption and the reactive properties of alpha-beta-unsaturated aldehydes are studied by means of density functional calculations (DFT) on two well-defined-Pt-Sn alloy surfaces,p(2X2)Pt_3Sn(111) and (3~(1/2)X3~(1/2)) R 30 deg Pt_2Sn(111).First the electronic structure of the bulk alloys is determined:a charge transfer occurs from Sn to Pt,the work function decreases,and the d-band center is shifted away from the Fermi level.Then various adsorption structures of acrolein (propenal),crotonaldehyde (2-butenal),and prenal (3-methyl,2-butenal) are considered on the two alloys.The results are compared with those obtained on Pt(111).A large decrease of the adsorption energies is observed for the alloys.For acrolein,there is almost no change in the best adsorption modes,the adsorption through the C-C bond being predominant.This explains why the selectivity of thea crolein hydrogenation is not changed much when Pt-Sn alloys and Pt are compared.For prenal on the ocntrary,only the atop mode is stable on the alloys,whereas a structure parallel to the surface is the most stable one on Pt.This explains the modified reaction selectivity with a larger amount of unsaturated alcohol in the products in the case of the Sn alloys.