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 expense of bridged CO on Pd due to dilution of exposed ensembles of Pd by Cu, secondly, reduction in the extent of dipolar coupling between adjacent CO molecules again due to geometric dilution effect and, thirdly, an electronic effect leading to cationic Cu sites. The order of resistance of surface sites to modification with thiophene was Pd (bridging sites) > Pd (linear sites) > Cu+ > Cu. Multibonded CO on Pd remained after 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 catalysts did not favour crotyl alcohol formation from crotonaldehyde hydrogenation over thiophene-modified catalysts because, as demonstrated by the infrared results, the Cu component was preferentially poisoned leaving active Pd sites which favoured C=C bond hydrogenation. (C) 1997 Elsevier Science B.V.