Different roles of MoO3 and Nb2O5 promotion in short-chain alkane combustion over Pt/ZrO2 catalysts

Abstract

Pt/ZrO2 catalysts promoted with MoO3 and Nb2O5 were tested for the combustion of short-chain alkanes (namely, methane, ethane, propane, and n-hexane). For short-chain alkane combustion, the inhibition of MoO3 (for the methane reaction) dramatically transformed to promotion (for the ethane, propane, and n-hexane reactions) as the carbon chain length increased, whereas the remarkable promotion of Nb2O5 gradually weakened with an increase in the carbon chain length. Based on a detailed study of the oxidation reactions of methane and propane over the catalysts, the different roles of the promoters in the reactions were ascribed to differences in the acidic properties of the surface and the oxidation or reduction states of the Pt species. The MoO3 promoter could decorate the surface of the Pt species for a Pt-Mo/ZrO2 catalyst, whereas the Nb2O5 promoter on the support could be partially covered by Pt particles for a Pt-Nb/ZrO2 catalyst. The formation of accessible Pt-MoO3 interfacial sites, a high concentration of metallic Pt species, and a high surface acidity in Pt-Mo/ZrO2 were responsible for the enhanced activity for catalytic propane combustion. The lack of enough accessible Pt-Nb2O5 interfacial sites but an enhanced surface acid sites in Pt-Nb/ZrO2 explained the slight improvement in activity for catalytic propane combustion. However, the stabilized Ptn+ species in Pt-Nb/ZrO2 were responsible for the much-improved activity for methane combustion, whereas the Ptn+ species in Pt-Mo/ZrO2 could be reduced during the oxidation reaction, and the fewer exposed surface Pt species because of MoO3 decoration accounted for the inhibited activity for methane combustion. In addition, it can be concluded that MoO3 promotion is favorable for the activation of C–C bonds, whereas Nb2O5 promotion is more beneficial for the activation of C–H bonds with high energy.