Determination of the nature and reactivity of copper sites in Cu-TiO2 catalysts

XPS, temperature programmed reduction (TPR) and FTIR spectroscopy of adsorbed CO and the hydrogenation of crotonaldehyde were used to study the nature of the surface species present for two Cu-TiO2 catalysts (2 and 5% Cu) subjected to reduction in hydrogen at 523, 623 and 723 K. Despite TPR and XPS evidence for complete reduction, IR studies gave clear evidence for the presence of surface Cu(II) ions and two distinct types of Cu(I) sites. The large absorption coefficient for CO on the latter meant that spectra were dominated by bands due to CO on Cu+ even though these ions made up less than an estimated 10% of the total exposed copper sites. Exposed Cu+ remained after re-oxidation treatment, at 623 K in oxygen but this treatment, when followed by 523 K reduction, did not recover Cu(0) sites lost by high temperature reduction, indicating that sintering rather than encapsulation by TiOx was responsible for loss of copper surface area.