Mechanisms of formic acid, methanol, and carbon monoxide electrooxidation at platinum as examined by single potential alteration infrared spectroscopy

Abstract

Surface infrared spectra have been obtained as a function of potential and time during the voltammetric oxidation of formic acid and methanol on polycrystalline platinum in order to probe the possible role of adsorbed carbon monoxide in the electrooxidation mechanisms. The procedure involves obtaining a sequence of single-beam infrared spectra using an FTIR spectrometer during potential sweep or following potential-step perturbations, and referencing these to a spectrum obtained at the initial potential or after complete oxidation had occurred. Using this “single potential alteration infrared” (SPAIR) technique, individual spectra for the C-O stretch of adsorbed carbon monoxide, ν CO, as well as the O-C-O stretch for the CO 2 product could be obtained under voltammetric conditions in as little as 3.5 s, and repetitively every 7 s. Such SPAIR spectra obtained during either potential sweep or step excursions indicate that the onset of both formic acid and methanol oxidation coincides with the oxidative removal of adsorbed carbon monoxide. The electrooxidation of CO irreversibly adsorbed from solution carbon monoxide was also examined using this approach. The integrated absorbance of the ν CO band was generally found to be proportional to the CO coverage (determined from either the CO 2 band intensity or the voltammetric charge) during electrooxidation. The dependence of the ν CO peak frequency upon coverage during potentiostatic oxidation, however, is sensitive to the timescale over which the process proceeds. The adsorption kinetics of CO formed from formic acid and methanol were evaluated from the time dependence of the ν CO band intensity following suitable potential step sequences to potentials where CO electrooxidation is suitably slow. These data suggest that while adsorbed CO may act as an adsorbed intermediate for methanol oxidation, it probably acts as a chemisorbed poison for formic acid oxidation.