PtPd(111) Surface versus PtAu(111) Surface: Which One Is More Active for Methanol Oxidation?

We have investigated the heterogeneous catalytic mechanism of methanol oxidation on the PtAu(111) and PtPd(111) surfaces. Density functional theory (DFT) calculations and microkinetics studies show that, on the PtAu(111) surface, the non-CO pathway is more favored over the CO pathway for the methanol oxidation, whereas the CO pathway is more favored on the PtPd(111) surface. This result indicates that the PtAu(111) surface apparently has higher CO-poisoning tolerance than the PtPd(111) surface since PtAu can be more effective in averting CO formation. However, our complementary experiment indicates that PtPd(111) is actually more active for the methanol oxidation despite its lower CO-poisoning tolerance in comparison to PtAu(111). To reconcile the apparent inconsistency between the computation and experiment, we have performed additional DFT calculations and found that the adsorbed CO on PtAu(111) cannot be fully removed during the methanol oxidation and thereby PtAu(111) can still be poisoned by the CO and give lower catalytic activity. In contrast, PtPd(111) entails more OH adsorption intermediates to facilitate both the oxidation and removal of adsorbed CO, thereby having a higher number of active Pt sites for methanol oxidation and giving higher catalytic activity in comparison to PtAu(111). Our finding shows the importance of OH-assisted CO removal from the PtPd(111) surface on the assessment of catalytic activity of PtPd catalysts and offers insight into the catalytic mechanism for methanol oxidation on the PtAu(111) and PtPd(111) surfaces. This comprehensive mechanistic study will benefit the future design of more efficient and stable metal alloy catalysts for direct methanol fuel cell applications.