Infrared study of competitive crotonaldehyde and CO adsorption on Cu/TiO2

Infrared spectra of adsorbed CO have been used to monitor the surface of Cu/TiO2 which had been reduced in H-2 at 523, 623 or 723 K and was also exposed to propene, butanal or crotonaldehyde. For CO alone infrared bands at 2118-2133 cm(-1) dominated the spectra and are ascribed to CO interacting with Cu(I) sites in chains or two-dimensional rafts of Cu atoms distributed at lattice points determined by the geometric and electronic character of the underlying TiO2 surface. Three effects of crotonaldehyde on co-adsorbed CO are a geometric site blocking effect which reduces the surface concentration of sites available for CO adsorption, an electronic effect due to the electron-donating character of crotonaldehyde acting as a Lewis base and a redox effect which leads to the oxidation of crotonaldehyde to adsorbed crotonate ions and the reduction of Cu(I) to Cu(0). The enhanced selectivity, induced by TiO2 as support, towards crotyl alcohol rather than butanal formation from crotonaldehyde hydrogenation over Cu is attributed to the promotion by TiO2 of Cu(I) sites and the co-existence on catalyst surfaces of Cu(0) and Cu(I) sites which are particularly active for hydrogenation of carbonyl (C=O) bonds.