Inverse kinetic isotope effects in the oxygen reduction reaction at platinum single crystals

Although the oxygen reduction reaction (ORR) involves multiple proton-coupled electron transfer processes, early studies reported the absence of kinetic isotope effects (KIEs) on polycrystalline platinum, probably due to the use of unpurified D₂O. Here we developed a methodology to prepare ultra-pure D₂O, which is indispensable for reliably investigating extremely surface-sensitive platinum single crystals. We find that Pt(111) exhibits much higher ORR activity in D₂O than in H₂O, with potential-dependent inverse KIEs of ~0.5, whereas Pt(100) and Pt(110) exhibit potential-independent inverse KIEs of ~0.8. Such inverse KIEs are closely correlated to the lower *OD coverage and weakened *OD binding strength relative to *OH, which, based on theoretical calculations, are attributed to the differences in their zero-point energies. This study suggests that the competing adsorption between *OH/*OD and *O₂ probably plays an important role in the ORR rate-determining steps that involve a chemical step preceding an electrochemical step (CE mechanism).