CuO–TiO2 samples have been prepared by impregnation of two different preformed TiO2 supports with copper nitrate, and have been characterized by X-ray diffraction (XRD), thermogravimetry–differential thermal analysis (TG–DTA), Fourier-transform infrared (FTIR), Fourier-transform far-infrared (FT-FIR) and ultraviolet–visible (UV-VIS) spectroscopies. The surface properties have been investigated through surface area and porosity measurements, and FTIR spectra of surface OH groups and of adsorbed carbon monoxide at low temperature. The surface is thought to consist of complexes of Cu2+ and Cu+, involving surface Ti–O groups of the support as the ligands. These complexes can be reduced by CO under very mild conditions with the appearance of small Cu metal clusters. Ti4+ centres are also in part exposed on the surface, but their electron-withdrawing effect is weakened by the surface copper complexes. Surface copper ions act as electron-donor centres, giving rise to absorption over in the entire visible range, with a parallel weakening of the TiO2, absorption edge. Surface copper complexes have a pronounced accelerating effect on the anatase-to-rutile phase transition. A parallelism between the CuO–TiO2 and the V2O5–TiO2 systems is proposed.
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