The effects of MoO3 impregnation order on the catalytic activity for propane combustion over Pt/ZrO2 catalysts: the crucial roles of Pt–MoO3 interfacial sites density

Abstract

MoO₃ promoted Pt/ZrO₂ catalysts were prepared by co-impregnation (Pt–Mo/ZrO₂) or stepwise impregnation (Pt/Mo/ZrO₂ and Mo/Pt/ZrO₂) for propane combustion. The impregnation order affects the density of active Pt–MoO₃ interfacial sites, in the order of: Pt–Mo/ZrO₂ (7.60 μmol g_cat⁻¹) > Pt/Mo/ZrO₂ (3.40 μmol g_cat⁻¹) > Mo/Pt/ZrO₂ (1.20 μmol g_cat⁻¹). The turnover frequency, TOF_Pt–MoO3, of Pt–MoO₃ interfacial sites is much higher than the TOF_Pt of Pt active sites (at least 50 folds). Thus, the catalytic activity for propane combustion is in the order of: Pt–Mo/ZrO₂ > Pt/Mo/ZrO₂ > Mo/Pt/ZrO₂ > Pt/ZrO₂. Kinetic results illustrate that the MoO₃ promotion significantly weakens the strong adsorption of oxygen species on the catalyst and enhances the catalytic activity. The activation energy is reduced the most over the Pt–Mo/ZrO₂ catalyst (Ea of the Pt/ZrO₂ catalyst: 102.8 ± 4.8 kJ mol⁻¹; E_a of the Pt–Mo/ZrO₂ catalyst: only 40.3 ± 1.8 kJ mol⁻¹).