Improving electrocatalysts for O(2) reduction by fine-tuning the Pt-support interaction: Pt monolayer on the surfaces of a Pd(3)Fe(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(3)Fe(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 Pd(3)Fe(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 Pd(3)Fe(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 Pd(3)Fe(111) surface is an excellent substrate for a Pt monolayer ORR catalyst, as verified by its enhanced ORR kinetics on PT(ML)/Pd/Pd(3)Fe(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/Pd(3)Fe(111) surface. The activity of Pt(ML)/Pd(111) and Pt(111) is limited by OH removal, whereas the activity of Pt(ML)/Pd/Pd(3)Fe(111) is limited by the O-O bond scission, which places these two surfaces on the two sides of the volcano plot.