COVERAGE-DEPENDENT ERRORS IN THE COULOMETRIC ASSAY OF ADSORBED REACTANTS - CARBON-MONOXIDE ON RHODIUM(100)

Values of the coverage of carbon monoxide irreversibly adsorbed on Rh(100) in aqueous 0.1M HClO4 as deduced by anodic voltammetry, theta(co)v, are compared with corresponding values, theta(co)s, determined by means of infrared spectrophotometry. The latter values were extracted from the absorbance of the 2343 cm-1 CO2 band obtained from electrooxidation of the adsorbed CO in a thin-layer spectroelectrochemical cell. Unlike the voltammetric method, the spectrophotometric assay of theta(co) is free from complications associated with coupled faradaic and nonfaradaic processes. Comparison between corresponding theta(co)v and theta(co)s values obtained simultaneously by means of coupled voltammetric-infrared measurements reveals the presence of substantial coverage-dependent systematic errors in the former. For adlayers formed at various coverages by direct dosing for various times (less than or similar to 10 min) from dilute (ca. 2 x 10(-5)M) CO solutions, the difference (theta(co)v - theta(co)s) was found to be nearly invariant with coverage for theta(co)s greater than or similar to 0.1. At saturation coverage, theta(co)s = 0.77 (+/- 0.05), whereas theta(co)v congruent-to 1.2. Comparable errors in the voltammetrically deduced coverage were also observed in 0.1M H2SO4 and in 0.1M KPF6 + 5 mM HF. The primary cause of these substantial coverage-dependent errors in the coulometric assay of theta(co) was traced to the presence of additional (probably faradaic) charge arising from the onset of surface oxidation. The background process is coupled to the occurrence of CO electrooxidation, since the presence of even low CO coverages inhibit surface oxidation. Additional errors in theta(co)v arising from nonfaradaic charges coupled to CO removal are also considered. The more general implications of these findings to the coulometric assay of adsorbates on reactive transition-metal surfaces are pointed out.