MECHANISM OF CARBON-MONOXIDE ELECTROOXIDATION ON MONOCRYSTALLINE GOLDSURFACES - IDENTIFICATION OF A HYDROXYCARBONYL INTERMEDIATE

Kinetic data are presented for the electrooxidation of aqueous solutio n carbon monoxide to carbon dioxide on two monocrystalline gold surfac es, Au(210) and (110), with the objective of elucidating the reaction mechanism, especially regarding the nature of adsorbed intermediate(s) . Tafel plots (i.e., log rate versus electrode potential) were obtaine d by means of linear sweep voltammetry, particularly as a function of the solution reactant concentration and over a wide range (0-13.5) of the electrolyte pH. Under most conditions, the reaction order in CO wa s found to be near unity, as anticipated from the low coverages of ads orbed CO ascertained from infrared spectroscopy. Interestingly, the lo g rate-pH dependence observed on both surfaces display three distinct regions. At low (less than or equal to 2) and higher (greater than or equal to 4) pH values, essentially unit slopes were obtained (i.e., a unity reaction order in [OH-]), these regions being separated by one d isplaying apparently pH-independent kinetics. The potential region ove r which conveniently measurable electrooxidation kinetics occur lies s ubstantially (ca. 0.8 V) below the onset of gold surface oxidation thr oughout the entire pH range. The pH-dependent kinetic behavior is cons istent with a reaction pathway featuring the involvement of an adsorbe d hydroxycarbonyl intermediate. While such intermediates have been ide ntified in a number of metal complex-catalyzed CO oxidations in homoge neous solution, they apparently have not been considered previously fo r such electrocatalytic processes. The observed unity hydroxide reacti on order at higher pH values is indicative of a rate-determining step (rds) involving OH- discharge onto adsorbed CO sites to form the hydro xycarbonyl species, while the apparent transition to zero-order kineti cs at lower pH is consistent with water rather than OH- becoming the p referred reactant. This picture is supported by solvent isotope measur ements which display the onset of a substantial H/D isotope effect bel ow pH 4, signaling the occurrence of proton transfer within the rds. T he emergence of another pH-dependent reaction pathway at the lowest pH values is attributed to a rds involving hydroxycarbonyl decomposition to form CO2. The mechanistic opportunities provided by the analysis o f electrocatalytic rate-potential data over wide pH ranges are pointed out, along with the possibility that the proposed hydroxycarbonyl pat hway occurs for a wide range of related processes on transition-metal surfaces.