(598g) The Counterintuitive Dependency of Bifunctional Effect on the Degree of Ordering in Alloy Electrocatalysts for Methanol Oxidation Reaction

Bifunctional effect is commonly regarded as the reason why bimetallic catalysts outperform pure-metal catalysts in processes that include CO oxidation. It has been shown that alloy catalysts shorten the distance between sites for CO_ad and sites for OH_ad into sub-nano range and are able to oxidize CO at much lower overpotential than non-alloyed catalysts with similar compositions. To see whether the highest intimacy between Pt and secondary metal in the ordered alloy could provide the highest activity, we modify the degree of ordering of Pt₃Sn nanocubes (NCs) while maintaining the exposed {100} facets, and test them in methanol oxidation reaction and CO oxidation. Strikingly, the 60% ordered Pt₃Sn NCs outperform the 95% ordered Pt₃Sn NCs in methanol oxidation by almost 5 times in converted charge density. 60% ordered Pt₃Sn NCs also oxidize CO at lower potential in CO stripping and are more effective in bulk CO oxidation than the 95% ordered Pt₃Sn NCs and Pt NCs. Moreover, we demonstrate the outstanding CO tolerance of less ordered Pt₃Sn NCs comparing to 95% ordered Pt₃Sn NCs by saturating the electrolyte with CO during methanol oxidation. We reveal that the Sn atoms at the surface of the 60% ordered Pt₃Sn are more prone to be oxidized under oxidative potential whereas the Sn atoms at the surface of the 95% ordered Pt₃Sn NCs are resistant to oxidation. We propose that the formation of SnO_x in the vicinity of Pt sites create higher proximity between OH_ad and CO_ad in less ordered Pt₃Sn NCs, making less ordered alloys more active for methanol oxidation reactions, in which CO oxidation plays a crucial role.