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 oxidation 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 Pt-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 electrocatalytic activity, moreover the current density under potentiostatic conditions undergoes an exponential decline towards zero. Other model surfaces are also found to present a continuous loss in activity during chronoamperometric 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 being caused by a second-order process. The first current decay can be repeatedly observed by re-starting the experiment after setting the potential back to the initial value, thus indicating a certain degree of reversibility. The slow loss in activity cannot be recovered at low potentials. However, the 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 optimized porous PtRu surfaces, on the other hand, do not show any current decrease during the chronoamperometric experiment.