Surface characterization of zirconia, holmium oxide/zirconia and sulfated zirconia catalysts

Zirconia-supported holmium oxide, as well as their sulfated catalysts using (NH4)2SO4 were prepared by wet impregnation and calcined at different temperatures. The samples thus obtained were subjected to bulk and surface texture analysis using X-ray powder diffractometry (XRD) and N2-sorptiometry, respectively. The surface acidic properties were characterized by in situ FT-IR spectroscopy of adsorbed pyridine molecules. The decomposition of gas phase 2-propanol molecules on the catalysts was examined at different temperatures, also using in situ FT-IR spectroscopy. The results obtained revealed that the presence of Ho2O3 on ZrO2 induces the formation of cubic and/or tetragonal ZrO2 phase at low temperatures (≤800 °C) without significant effect on the surface area. Whereas, the presence of Ho2O3 and sulfate ions on ZrO2 results in the highest formation of cubic and/or tetragonal ZrO2 phase at low temperatures and induces stabilization of the surface area. The surface of Ho2O3/ZrO2 samples exposes Lewis acid and reactive basic sites, whereas the sulfated samples show at least two types of Lewis and Brönsted acid sites with enhanced acid strength. The decomposition of 2-propanol results show that the sulfate-free samples are dual functioning catalysts at 300 °C; 2-propanol molecules decompose thereon via dehydration into propene and dehydrogenation into acetone. However, the sulfated samples are found to only catalyze the dehydration into propene at all reaction temperatures applied.