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) inn-propanol containing ZrO2in the ranges 1.2–28.6 and 2.2–23.2 wt%, respectively. The samples were characterized by theSBETmethod, 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 2pindicates that ZrO2appears 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 ZrO2content (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 ZrO2content. Some Brønsted acid sites were detected on ZrO2/SiO2samples due to the slightly higher electron density on the oxygen associated with Si atoms detected by XPS.