Dynamics of Oxidation of Well-Defined Adsorbed CO Phases on Pt(111) in Aqueous Acidic Electrolytes: Simultaneous in Situ Second Harmonic Generation and Differential Reflectance Spectroscopy

The oxidation dynamics of the well-defined (2 × 2)-3CO (θCO = 0.75) and √19 × √19R23.4°-13CO (θCO = 0.68) CO adlayer phases on quasi-perfect Pt(111) facets have been examined in CO-saturated 0.1 M H2SO4 aqueous solutions via a combination of simultaneous, in situ, time-resolved, second harmonic generation (SHG, λinc = 590 nm), normalized differential reflectance spectroscopy (ΔR/R, λinc = 633 nm), and potential step techniques. For both of these phases, the onset of oxidation of COads was found to be delayed with respect to the time at which the potential was stepped to values positive enough, Eox, to promote oxidation of the entire CO adlayers. This induction time, τind, was much longer for the (2 × 2)-3CO (few tens of milliseconds) compared to the √19 × √19R23.4°-13CO phase (few milliseconds) and decreased monotonically for both of the phases as Eox was increased. Furthermore, for a fixed Eox, the oxidation rates, as measured by the optical techniques, were higher for the √19 × √19R23.4°-13CO compared t...