Abstract: The ZrO2, SnO2, and SnO2-ZrO2 oxides were prepared by the urea-hydrolysis method, and the SnO2-Al2O3 was prepared by the adsorption-precipitation method. A series of catalysts with 0.5% Pd supported on these oxides were prepared by the impregnation method.X-rayphotoelectron spectroscopy, in-situ diffuse reflectance FT-IR (DRIFTS), laser Raman spectroscopy, and temperature-programmed reduction were employed to characterize the catalysts. The effect of support on the chemisorption and redox ability of the supported palladium oxide species (PdOx) was investigated and correlated with the C3H8 oxidation performance of the catalyst. The DRIFTS results indicate that the dilution effect of Pd cluster on Al2O3 by Sn in Pd/SnO2-Al2O3 catalyst increases both the dispersion of Pd and the intensity of linear CO adsorption. However, the dilution effect probably decreases the amount of Pd-PdO active site pairs and inhibits the condensation of palladium hydroxides (Pd-OH), which blocks the active sites and thus decreases the catalytic activity. In Sn0.4Zr0.6O2 support, SnO2 prevents the transformation of tetragonal phase ZrO2, and the surface area of Sn0.4Zr0.6O2 binary oxide is larger than that of SnO2 and ZrO2. As a result, the PdOx crystallites possess a moderate dispersion on Sn0.4Zr0.6O2 support. Additionally, the interaction between Pd and Sn0.4Zr0.6O2 support leads to the oxidation state of Pd between Pd0 and PdO, making the PdOx more favorable to activatingC-Hbonds. The results of activity evaluation reveal that Pd/Sn0.4Zr0.6O2 with lower surface area shows the best C3H8 oxidation activity, whereas Pd/SnO2-Al2O3 with higher surface area gives very low oxidation activity. This indicates that the highly dispersed PdOx species appear to be responsible for the detrimental effect on the C3H8 oxidation process, whereas the PdOx (x<1) crystalline with moderate dispersion and low oxidation state is the main active component.

Key words: palladium, support effect, in-situ diffuse reflectance infrared spectroscopy, Raman spectroscopy, propane, oxidation