among them also PtxAg1-x/Pt surface alloys . Using calculated O-metal bond energy on the different A3, A2B, AB2 and B3 sites as parameter -in this case threefold adsorption sites were assumed as active sites -and the abundance of the different types of threefold sites on the hexagonal surfaces as determined by statistical evaluation of atomic resolution STM images with chemical contrast , the authors could calculate relative reaction rates (exchange current densities), relative to the maximum reaction rate, for different surface compositions. The calculated volcano plot and the exchange currents calculated for the different AxB3-x ensembles (triangles) as well as the measured kinetic currents (squares) and those calculated according to equation (3) (circles) are presented in Figure . Note that the oxygen binding energies EO are given relative to that for Pt , with increasingly positive numbers indicating weaker Oad bonding. For inhomogeneous surfaces, the Oad binding energies were calculated from the weighted binding energies on the different trimer ensembles, using the trimer ensemble distribution as determined by STM. Obviously, the measured values (squares) closely follow the calculated data points (circles). Furthermore, it is clear that the enhanced activity of the surface alloys compared to Pt results from the high activity of the Ag1Pt2 trimer ensembles. For these sites, which except perhaps for the Ag7Pt93/Pt(111) surface alloy dominate the reaction, the rate determining step is given by OHad removal, as they are still on the left slope of the volcano . In contrast, for the Ag2Pt1 trimer ensembles, which is on the right hand slope of the volcano, OOHad formation would be rate limiting. Because of their much lower activity, however, the latter sites contribute little to the overall activity of the surface alloys investigated. This would be in contrast to the current mechanistic understanding that for Pt OOHad formation should be rate limiting . This and possible explanations for this discrepancy are discussed in Ref. Although this model could explain the higher ORR activity of several of the PtAg/Pt surface alloys in a very satisfactory way, a later study revealed that this approach was too simple . This study considered effects introduced by the surface composition on a larger scale, using a phase-separation Using surface unit cells with between 100 and 900 atoms they simulated PtxAg1-x/Pt(111) surfaces that agreed well in the relative contributions of the PtxAg3-x ensembles determined and used in .