Understanding the CO Pre-Oxidation and the Intrinsic Catalytic Activity of Step Sites in Stepped Pt Surfaces in Acidic Medium

In order to deepen the knowledge about the origin of the CO pre-oxidation process and the intrinsic catalytic activity of Pt superficial steps toward CO oxidation were performed a series of CO stripping experiments on stepped Pt electrodes in acidic medium. For the occurrence of CO pre-oxidation, it was found that it arises (reproducibly) whenever four interconnected conditions are simultaneously fulfilled: (1) CO adsorption at potentials lower than about 0.2 V; (2) on surfaces saturated with COads; (3) in presence of traces of CO in solution; (4) in presence of surface steps. If any of these four conditions is not satisfied, the CO pre-oxidation pathway does not appear, even though the steps on the electrode surface are completely covered by CO. By controlling the removal of the CO adlayer (voltammetrically), we show that once the CO adlayer has been partially oxidized, the (111) terrace sites of stepped surfaces are released earlier than the (110) step sites. Moreover, if (110) steps are selectively decorated with CO, its oxidation occurs only at potentials ~150 mV higher than the CO pre-oxidation peak. Our results systematically demonstrate that step sites are less active to oxidize CO than those ones responsible for the CO pre-oxidation process. Once the sites responsible for the CO pre-oxidation are made free, there is no apparent motion of the remaining adsorbed CO layer, suggesting that the activation of the surface controls