VALIDITY OF DOUBLE-LAYER CHARGE-CORRECTED VOLTAMMETRY FOR ASSAYING CARBON-MONOXIDE COVERAGES ON ORDERED TRANSITION-METALS - COMPARISONS WITH ADLAYER STRUCTURES IN ELECTROCHEMICAL AND ULTRAHIGH-VACUUM ENVIRONMENTS

A coulometric procedure enabling the reliable and accurate evaluation of saturated CO coverages, theta(CO)(sat), on Pt-group transition-meta l electrodes is outlined, and applied to CO adlayers on ordered low-in dex platinum, rhodium, and iridium surfaces in acidic aqueous media. A long with voltammetric data, the method utilizes previously described measurements of the charge displaced upon CO adsorption. The reverse o f this charge, Q(dis), together with the ''background'' charge Q(b) fl owing between a suitable pair of electrode potentials in the absence o f CO, constitutes the overall ''double-layer'' correction Q(dl) to the total voltammetric charge Q(tot) measured for the electrooxidation of adsorbed CO between the same potentials. Significantly, the Q(dis) as well as the Q(b) component of Q(dl) typically constitutes moderate or even large corrections to Q(tot), so that the deduced of theta(CO)(sa t) values are noticeably (20-30%) smaller than some voltammetric-based estimates reported earlier. However, the revised coulometric theta(CO )(sat) values are in consistently good agreement with the correspondin g coverages obtained by means of an infrared spectrophotometric proced ure. These theta(CO)(sat) values are compared with adlayer structural information obtained recently from in situ scanning tunneling microsco py along with infrared spectroscopy, and also with structural data for corresponding adlayers in ultrahigh vacuum (UHV). In most cases, the electrochemical and UHV-based theta(CO)(sat) values are not greatly di fferent (within 5-10%), even though the CO binding site arrangements a re often dissimilar in these two environments. The role of the electro de potential in affecting theta(CO)(sat) under some conditions via alt erations in binding-site energetics, however, is noted for the Pt(111) /CO system. (C) 1998 Elsevier Science B.V. All rights reserved.