Surface dynamics at well-defined single crystal microfacetted Pt(111) electrodes: in situ optical studies

The assembly and electrochemical oxidation of well-defined CO adlayers on Pt(111) microfacets in aqueous CO-saturated 0.1 M H2SO4 were examined by a combination of in situ, simultaneous, time-resolved, reflectance spectroscopy (RS) and second harmonic generation (SHG), and potential step and linear scan techniques. Optical transients were collected following potential steps from a value high enough for a full monolayer of bisulfate (theta = 0.2) to adsorb on the Pt(111) facet, E(ox), to potentials E(ads), at which either the c(2 x 2)-3CO or square root of 19 x square root of 19R23.4 degrees-13CO phase is expected to form once surface saturation is achieved. Similar experiments involving subsequent steps from E(ads) to E(ox) provided unambiguous evidence that the rates of oxidation of c(2 x 2)-3CO on such quasi-perfect Pt(111) facet at constant overpotential are much slower than those of square root of 19 x square root of 19R23.4 degrees -13CO, an effect attributed to the presence of intrinsic vacant sites within the latter, more dilute phase, which are required for oxidation of adsorbed CO to ensue. Furthermore, continuous CO adsorption-oxidation cycles were found to increase the rate of oxidation of the c(2 x 2)-3CO phase. This phenomenon was tentatively ascribed to the progressive emergence of defects along the edge of the facet (and/or within the facet itself) which serve as nucleation sites for the oxidation of adsorbed CO.