Different roles of MoO_(3)and Nb_(2)O_(5)promotion in short-chain alkane combustion over Pt/ZrO_(2)catalysts

Pt/ZrO_(2)catalysts promoted with MoO_(3)and Nb_(2)O_(5)were tested for the combustion of short-chain alkanes(namely,methane,ethane,propane,and n-hexane).For short-chain alkane combustion,the inhibition of MoO_(3)(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 Nb_(2)O_(5)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 MoO_(3)promoter could decorate the surface of the Pt species for a Pt-Mo/ZrO_(2)catalyst,whereas the Nb_(2)O_(5)promoter on the support could be partially covered by Pt particles for a Pt-Nb/ZrO_(2)catalyst.The formation of accessible Pt-MoO_(3)interfacial sites,a high concentration of metallic Pt species,and a high surface acidity in Pt-Mo/ZrO_(2)were responsible for the enhanced activity for catalytic propane combustion.The lack of enough accessible Pt-Nb_(2)O_(5)interfacial sites but an enhanced surface acid sites in Pt-Nb/ZrO_(2)explained the slight improvement in activity for catalytic propane combustion.However,the stabilized Pt^(n+)species in Pt-Nb/ZrO_(2)were responsible for the much-improved activity for methane combustion,whereas the Pt^(n+)species in Pt-Mo/ZrO_(2)could be reduced during the oxidation reaction,and the fewer exposed surface Pt species because of MoO_(3)decoration accounted for the inhibited activity for methane combustion.In addition,it can be concluded that MoO_(3)promotion is favorable for the activation of C-C bonds,whereas Nb_(2)O_(5)promotion is more beneficial for the activation of C-H bonds with high energy.