Improving electrocatalysts for O₂ reduction by fine-tuning the Pt-support interaction: Pt monolayer on the surfaces of a Pd₃Fe(111) single-crystal alloy

We improved the effectiveness of Pt monolayer electrocatalysts for the oxygen-reduction reaction (ORR) using a novel approach to fine-tuning the Pt monolayer interaction with its support, exemplified by an annealed Pd ₃Fe(111) single-crystal alloy support having a segregated Pd layer. Low-energy ion scattering and low-energy electron diffraction studies revealed that a segregated Pd layer, with the same structure as Pd (111), is formed on the surface of high-temperature-annealed Pd₃Fe(111). This Pd layer is considerably more active than Pd(111); its ORR kinetics is comparable to that of a Pt(111) surface. The enhanced catalytic activity of the segregated Pd layer compared to that of bulk Pd apparently reflects the modification of Pd surface's electronic properties by underlying Fe. The Pd₃Fe(111) suffers a large loss in ORR activity when the subsurface Fe is depleted by potential cycling (i.e., repeated excursions to high potentials in acid solutions). The Pd₃Fe(111) surface is an excellent substrate for a Pt monolayer ORR catalyst, as verified by its enhanced ORR kinetics on PT _ML/Pd/Pd₃Fe(111). Our density functional theory studies suggest that the observed enhancement of ORR activity originates mainly from the destabilization of OH binding and the decreased Pt-OH coverage on the Pt/Pd/Pd₃Fe(111) surface. The activity of Pt_ML/Pd(111) and Pt(111) is limited by OH removal, whereas the activity of Pt _ML/Pd/Pd₃Fe(111) is limited by the O-O bond scission, which places these two surfaces on the two sides of the volcano plot.