Structure–reactivity correlation in the oxygen reduction reaction: Activity of structurally well defined AuxPt1−x/Pt(111) monolayer surface alloys

The activity of structurally well defined AuxPt1−x/Pt(111) monolayer surface alloys with varying Au surface contents for the oxygen reduction reaction (ORR) was investigated by flow cell measurements under defined mass transport conditions, using a newly designed electrochemical flow cell attached to a ultra high vacuum (UHV) system. Surface alloy electrodes with well known concentrations and distributions of surface atoms were prepared under UHV conditions and characterized by scanning tunnelling microscopy (STM). Surface alloys with small amounts of Au (⩽24%) were found to exhibit reaction characteristics similar to Pt(111), transforming O2 to H2O, but did not show a measurable lowering of the ORR overpotential compared to Pt(111), while higher Au surface contents (>24%) led to an increasing shift to higher overpotentials. For Au rich surfaces (>50%), the catalytic properties of the surface alloys shift towards that of polycrystalline Au, with an increasing tendency for H2O2 formation. The kinetic currents determined experimentally are compared with simulated kinetic current – potential profiles based on the distribution of different trimer ensembles on the surface derived from STM imaging and on the adsorption energies of reaction intermediates taken from existent density functional theory (DFT) calculations (J. Greeley and J.K. Nørskov, J. Phys. Chem. 113 (2009) 4932), using different approaches for averaging over the inhomogeneous surface. It is demonstrated that the modified procedure introduced by us recently (jk averaging) yields good agreement for these inhomogeneous surfaces. Potentials and limits of this approach are discussed.