EFFECTS OF THIOPHENE ON CO ADSORPTION AND CROTONALDEHYDE HYDROGENATION OVER SILICA-SUPPORTED PD-CU CATALYSTS

The effects of alloying Pd and Cu on the infrared spectra of adsorbed CO are firstly, enhancement of linearly adsorbed CO on Pd at the expen se of bridged CO on Pd due to dilution of exposed ensembles of Pd by C u, secondly, reduction in the extent of dipolar coupling between adjac ent CO molecules again due to geometric dilution effect and, thirdly, an electronic effect leading to cationic Cu sites. The order of resist ance of surface sites to modification with thiophene was Pd (bridging sites) > Pd (linear sites) > Cu+ > Cu. Multibonded CO on Pd remained a fter other forms of adsorbed CO on Pd and Cu had been displaced by the thiophene. Unlike Cu/SiO2 [G.J. Hutchings, F. King, I.P. Okoye, M.B. Padley, C.H. Rochester, J. Catal. 148 (1994) 453.] the Pd-Cu/SiO2 cata lysts did not favour crotyl alcohol formation from crotonaldehyde hydr ogenation over thiophene-modified catalysts because, as demonstrated b y the infrared results, the Cu component was preferentially poisoned l eaving active Pd sites which favoured C=C bond hydrogenation. (C) 1997 Elsevier Science B.V.