Pt-Ru model catalysts for anodic methanol oxidation: Influence of structure and composition on the reactivity

The activity of different types of PtRu surfaces towards anodic methanol ox idation has been investigated. As expected the activity of Pt(111) modified with Ru and analyzed in a UHV environment depends on the total number of P t-Ru pair sites. Their population can be increased by artificially creating additional surface defects before or after ruthenium deposition. Ruthenium alloyed into smooth Pt(111) terraces in turn does not lead to comparable e lectrocatalytic activity, moreover the current density under potentiostatic conditions undergoes an exponential decline towards zero. Other model surf aces are also found to present a continuous loss in activity during chronoa mperometric tests, which consists of a fast initial current decrease during the first 5-10 min followed by a slower one over several hours. The latter decay exhibits hyperbolic behavior which we can explain kinetically as bei ng caused by a second-order process. The first current decay can be repeate dly observed by re-starting the experiment after setting the potential back to the initial value, thus indicating a certain degree of reversibility. T he slow loss in activity cannot be recovered at low potentials. However, th e original surface activity can be restored by applying a potential step to higher anodic values, e.g. up to 1.2 V for a few seconds. Structure optimi zed porous PtRu surfaces, on the other hand, do not show any current decrea se during the chronoamperometric experiment.