Influence of the preparation method on the catalytic behaviour of PtSn/TiO2 catalysts

PtSn/TiO2 catalysts containing 2 wt% Pt and a Pt:Sn atomic ratio of 2:1 and 1:1 were prepared by coimpregnation or successive impregnation method with aqueous solutions of SnCl2·2H2O and H2PtCl6·6H2O of a commercial TiO2 (P25, from Degussa). Both catalyst series, independent of the preparation method, were reduced at 473 and 773 K. XPS results show that tin was in an oxidized state after reduction at 473 K, and that a fraction was in the metallic state after reduction at 773 K. By use of in situ FTIR spectroscopy of adsorbed CO, the presence of bimetallic Pt–Sn phases was assessed after reduction at 773 K. Microcalorimetric analysis of CO adsorption enthalpy indicates that reduction at 773 K causes the appearance of a more heterogeneous distribution of active sites, as well as a loss in the amount of sites. The catalytic activity for the gas phase hydrogenation of crotonaldehyde was greatly improved when the catalysts were prepared by coimpregnation, at both reduction temperatures. The selectivity toward crotyl alcohol was higher after reduction at 773 K and independent of the preparation method, although it increased with the amount of tin, suggesting a promoting effect of tin on this reaction.