IN SITU TECHNIQUES AS APPLIED TO THE STUDY OF SURFACE DYNAMICS
Abstract (Summary)The in-situ normalized reflectance ?R/R vs E signal was found to vary monotonically with E within the potential region of the butterfly peak for Pt(111) microfacets in H 2SO 4. The plots of 1/R(dR/dE) vs E calculated from the R(E) vs E curves revealed a striking similarity between the bell-shaped curve of the derivatives and the cyclic voltammogram in the same region. Using Pt(poly) microelectrodes, a (semi)quantitative analysis revealed a linear correlation between ?R/R and the extent of oxidation of the Pt surface. Reflectance spectra were also collected in CO-saturated electrolyte during chronocoulometric measurements, contributions to the current derived from oxide formation during oxidation of adsorbed and bulk CO being extracted. ?R/R vs the relative oxide charge, Q n, was found to deviate from linearity in the PtOH formation region for both HClO 4and H 2SO 4solutions. Improved results were achieved upon correcting ?R/R for double-layer effects using the method originally reported in the literature but the remaining discrepancies could only be resolved by accounting for the charge due to bisulfate desorption induced by PtOH formation on the surface. After corrections, ?/R was found to be proportional to ? PtOHand independent of the applied potential for Q n< 1. Simultaneous ?R/R and SHG measurements were used to study the oxidation of CO adson a Pt(111) microfacet in acidic electrolytes. The results obtained, for potential scan experiments, are consistent with the disruption of the (?19x?19R23.4-13CO) phase prior to the oxidation of CO ads. Using potential step methods, the dynamics of the oxidation of CO adson Pt(111) revealed that the rate of oxidation, at constant E ox= 0.98 V, is significantly slower for CO adsorbed at E ads= 0.1 V ((2x2)-3CO ) compared to E ads= 0.6 V (?19x?19R23.4-13CO). Lastly, the bisulfate adsorption on Pt(111) facets was monitored using simultaneous ?R/R and SHG. The results confirmed quantitatively the linear correlations between coverage, ? HSO 4, and ?R/R using a relatively simple optical model. Moreover, the intensity of the SHG vs E, which originates primarily from the bare substrate, was shown to depend on (1 - ? HSO 4) ^2, modulated by the linearly varying dependence of the SHG signal of the bare substrate.
School Location:USA - Ohio
Source Type:Master's Thesis
Date of Publication:01/01/2007