Dispersion and reduction behavior of CuO/α-Fe2O3 systems

The dispersion and reduction behaviors of CuO/alpha-Fe2O3 samples have been studied by using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) and temperature programmed reduction (TPR). XRD and XPS results show that the dispersion capacity of CuO on alpha-Fe2O3 is about 13.7 Cu2+ nm(-2) (alpha-Fe2O3). At low copper loading, highly dispersed surface copper oxide is the main species, and crystalline CuO evidently appears after the Cu loading exceeds the dispersion capacity. For Cu XPS spectra, the intensity ratio of satellite peaks (I-sat) to the principal peaks (I-pp) shows that the highly dispersed Cu2+ ions simultaneously exist as tetrahedrally and octahedrally coordinated states in the low Cu loading samples, and the octahedrally coordinated surface Cu species is the predominant species in high Cu loading samples, which is in basic agreement with the prediction of the incorporation model proposed previously (Y. Chen and L. F. Zhang, Catal. Lett., 1992, 12, 51). The BET surface area of alpha-Fe2O3 support in all the samples remains constant, indicating that the calcination process has not induced an evident change to the surface of alpha-Fe2O3 support. TPR results indicate that the reduction peaks at about 473 and 493 K correspond to the reduction of the octahedrally and tetrahedrally coordinated surface copper oxide species, respectively, and the reduction peak at about 563 K is ascribed to the reduction of crystalline CuO. In addition, the influence of the different calcination temperatures on the interaction between CuO and alpha-Fe2O3 has also been investigated by XRD, TPR and XPS, and the results show that the calcination temperature can affect the extent of interaction, and a new compound, CuFe2O4, formed as the CuO/alpha-Fe2O3 sample was calcined at 1123 K.