Coverage-dependent errors in the coulometric assay of adsorbed reactants : carbon monoxide on rhodium(100)

• Published in: Journal of the Electrochemical Society Vol. 139; no. 1; pp. 147 - 152
• Main Authors: SI-CHUNG CHANG, YEUNGHAM HO, WEAVER, M. J
• Format: Journal Article
• Language: English
• Published: Pennington, NJ Electrochemical Society 1992
• Subjects: Adsorption; Chemistry; Electrochemistry; Exact sciences and technology; General and physical chemistry; Study of interfaces; Inorganic adsorbate; Fourier transformation; Potassium Hexafluophosphates; Hydrogen Fluorides; Transition metal; Crystal face; Concentration; Experimental study; Acidic solution; Sulfuric acid; Electrodes; Infrared spectrometry; Cyclic voltammetry; Adsorption; Perchloric acid; Rhodium; Adsorbed state; Oxidation; Carbon monoxide; Aqueous solution; Electrochemical reaction; Coulometry
• ISSN: 0013-4651; 1945-7111
• DOI: 10.1149/1.2069160

Values of the coverage of carbon monoxide irreversibly adsorbed on Rh(100) in aqueous 0.1M HClO sub 4 as deduced by anodic voltammetry, theta exp v sub co , are compared with corresponding values, theta exp s sub co , determined by means of infrared spectrophotometry. The latter values were extracted from the absorbance of the 2343 cm exp --1 CO sub 2 band obtained from electrooxidation of the adsorbed CO in a thin-layer spectroelectrochemical cell. Unlike the voltammetric method, the spectrophotometric assay of theta sub co is free from complications associated coupled faradaic and nonfaradaic processes. Comparison between corresponding theta exp v sub co and theta exp s sub co 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 ( < = 10 min) from dilute ( approx 2 x 10 exp --5 M ) CO solutions, the difference ( theta exp v sub co -- theta exp s sub co ) was found to be nearly invariant with coverage for theta exp s sub co = > 0.1. At saturation coverage, theta exp s sub co = 0.77 ( plus/minus 0.05), whereas theta exp v sub co approx = 1.2. Comparable errors in the voltammetrically deduced coverage were also observed in 0.1M H sub 2 SO sub 4 and in 0.1M KPF sub 6 + 5mM HF. The primary cause of these substantial coverage-dependent errors in the coulometric assay of theta sub 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 exp v sub co 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.