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 (θ_CO = 0.75) and √19 × √19R23.4°-13CO (θ_CO = 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, λ_inc = 590 nm), normalized differential reflectance spectroscopy (R/R, λ_inc = 633 nm), and potential step techniques. For both of these phases, the onset of oxidation of CO_ads was found to be delayed with respect to the time at which the potential was stepped to values positive enough, E_ox, to promote oxidation of the entire CO adlayers. This induction time, τ_ind, 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 E_ox was increased. Furthermore, for a fixed E_ox, the oxidation rates, as measured by the optical techniques, were higher for the √19 × √19R23.4°-13CO compared to the (2 × 2)-3CO phase. In the case of the (2 × 2)-3CO phase, the transient in situ optical data collected for E_ox = 0.98 V vs RHE could be quantitatively accounted for by a model that assumes all sites on the surface are occupied either by CO or bisulfate. In fact, excellent statistical fits to the θ_CO transients for this latter phase derived from the SHG data could be obtained using Avramis nucleation and growth model. On this basis, the oxidation of the fully assembled (2 × 2)-3CO phase occurs at the edge of the domain and propagates through the entire island until the entire process is completed. Similar experiments involving CO adlayers on Pt(111) facets below saturation coverages (i.e., θ_CO < 0.75) formed by oxidative stripping of the fully assembled (2 × 2)-3CO phase yielded much longer τ_ind values than surfaces with similar θ_CO prepared by dosing the otherwise bare Pt(111) surfaces. These findings strongly suggest that the rates of oxidation of CO_ads increase with the number of non-CO-covered neighboring sites on the surface.