Improved oxygen reduction reactivity of platinum monolayers on transition metal surfaces

The catalytic activity of platinum monolayers supported on close-packed transition metal surfaces (Au(1 1 1), Pt(1 1 1), Pd(1 11) and Ir(1 1 1)) is investigated for the oxygen reduction reaction (ORR) by generating free energy diagrams and performing Sabatier analysis based on periodic, self-consistent density functional theory (DFT) calculations. Three different ORR mechanisms, involving direct or hydrogen-assisted activation of 02, are considered. At the ORR equilibrium potential of 1.23 V, the reactivity of all surfaces is shown to be limited by the rate of OH removal from the surface. At a cell potential of 0.80 V, the ORR reactivity of different surfaces is dictated by the strength of oxygen adsorption, with OH removal via hydrogenation and O-O bond scission in either O-2, O2H or H2O2 being the rate-limiting steps for surfaces with stronger and weaker oxygen binding, respectively. Among the surfaces studied, Pt monolayer on a Pd(1 1 1) substrate shows the highest reactivity and is more active than Pt(1 1 1). These results are in excellent agreement with our earlier experimental and theoretical work, which was based on a simpler model for the ORR. (C) 2008 Elsevier B.V. All rights reserved.