Surface characterization of zirconia-coated alumina and silica carriers

Silica- and alumina-supported zirconia samples have been prepared by impregnation of the supports with a solution of zirconium alkoxide (n-propoxide) in n-propanol containing ZrO2 in the ranges 1.2-28.6 and 2.2-23.2 wt%, respectively. The samples were characterized by the S-BET method, XRD, XPS, and FTIR. The x-ray diffraction showed that zirconia on silica was amorphous for all concentrations of ZrO2. Zirconia on alumina was amorphous up to 17.1 wt% ZrO2; beyond this value crystallites were formed. The increase in the XPS IZr 3d/ISi 2p indicates that ZrO2 appears as a monolayer on silica near the theoretical monolayer coverage (about 28.6 wt%), whereas for alumina-supported zirconia samples the monolayer is formed at lower ZrO2 content (between 12.9 and 17.1 wt%). It was observed by pyridine adsorption that the strong Lewis acid sites on alumina decreased after deposition of zirconia. However, the number of Lewis acid sites on silica-supported zirconia samples, evoked by an increase of the positive charge on Zr atoms compared to pure zirconia, increases with increasing ZrO2 content. Some Bronsted acid sites were detected on ZrO2/SiO2 samples due to the slightly higher electron density on the oxygen associated with Si atoms detected by XPS. (C) 1997 Academic Press.