Contribution of HRTEM to the characterization of silica-incorporated copper-oxide catalysts prepared by the sol-gel technique.

The aim of the present work is a structural characterization study of silica-incorporated copper-oxide (CuO-SiO2) catalysts using HRTEM. The catalysts were prepared by the sol-gel synthetic route. Two calcined catalysts (at 400 degrees C and 800 degrees C, respectively) were analyzed before and after the "NOx + H2" catalytic reaction. It was found that in the 400 degrees C calcined catalyst, the copper is present as CuO crystallites in a structureless form, while in the 800 degrees C calcined catalyst, it was assumed that the copper is automatically dispersed into the silica matrix since no traces of crystalline copper were observed. Under a reducing atmosphere, i.e., after reaction, the former showed large crystallites of CuO, while in the latter a segregation of colloidal crystallites, a mixture of Cu2O and metallic Cu, was observed. It is worth noting that in the case of the 400 degrees C "after-reaction" catalyst, a change in color was observed after a few minutes of air exposure. This result suggested that the reduced copper-oxide phase obtained after reduction was unstable.