Cu/SiO2 and Cu/SiO2-TiO2 catalysts. I. TEM, DR UV-Vis-NIR, and FTIR Characterisation

Three 8 wt% copper catalysts supported on SiO2 and on two SiO2/TiO2 powders with different TiO2 content have been prepared by a "chemisorption-hydrolysis" method. Transmission electron microscopy indicated that after calcination the three catalysts contain supported particles which are small and quite homogeneous in size (mean diameter, d(m) = 3.0 nm). These particles slightly increase in size after reduction in H-2 up to 773 K (d(m) = 3.5 nm). On the basis of the diffuse reflectance UV-Vis-NIR spectra CuO and Cu2O were found to be present after calcination, while the electronic spectra were dominated by the features due to metallic Cu particles already after a mild reduction in H-2 at 523 K. FTIR spectra of CO adsorbed onto the three catalysts reduced at 523 K appeared very similar. By a spectral fitting procedure, four different carbonylic species were evidenced, three assigned to carbonylic adducts on different types of microfacets exposed at the surface of three-dimensional Cu particles and one related to CO molecules adsorbed on plate-like two-dimensional copper particles. By increasing the reduction temperature an overall decrease in intensity of the bands due to CO adsorbed on copper supported on silica-titania carriers was observed, probably because of the formation of titanium suboxides that can cover a fraction of the copper sites. Furthermore, the CO-O-2 and GO-NO reactions were studied by FTIR spectroscopy of the adsorbed species and quadrupole mass analysis of the gas phase over the catalysts. This allows the elucidation of the nature of the surface sites involved in the activation of these molecules and the nature of the intermediates present at the surface of the catalysts during the reactions. The role played in these reactions by the uncoordinated copper surface atoms exposed at the surface of the two different types of Cu particles will be discussed.