METAL CRYSTALLINITY EFFECTS IN ELECTROCATALYSIS AS PROBED BY REAL-TIME FTIR SPECTROSCOPY - ELECTROOXIDATION OF FORMIC-ACID, METHANOL, AND ETHANOL ON ORDERED LOW-INDEX PLATINUM SURFACES

The electrooxidation kinetics of 0.05-0.25 M formic acid, methanol, and ethanol in 0.1 M HClO4 on ordered Pt(111), Pt(100), and Pt(110) surfaces were examined by means of FTIR spectra obtained during slow (2-10 mV s-1) thin-layer potential sweeps. This procedure enables the potential-dependent CO coverage, θCO, formed by dissociative reactant chemisorption to be followed in real time in relation to the appearance of CO2 and partial electrooxidation products as assayed quantitatively from their characteristic infrared bands. Terminally bonded (i.e., on-top) CO is the major form of this adsorbate detected under most conditions from the characteristic C-O stretching frequencies at ca. 2030-2060 cm-1, although bridging CO was also observed in some cases. For formic acid electrooxidation, high CO coverages (θCO ≳ 0.7) are formed on Pt(100) and Pt(110) that inhibit severely the reaction at low overpotentials during the positive-going sweep. The more facile kinetics observed on Pt(111) under these conditions are consistent with the observed low coverage of CO and other chemisorbed poisons. Only low or moderate CO coverages (θCO ≲ 0.3) were obtained on all three Pt faces from methanol or ethanol chemisorption. For the latter, direct spectroscopic evidence for chemisorbed poisons in addition to CO was obtained from their electrooxidation to CO2. On Pt(111), ethanol undergoes primarily four-electron oxidation to acetic acid. On Pt(100) and Pt(110), in contrast, acetic acid formation is inhibited almost entirely at potentials below where electrooxidation of chemisorbed poisons to CO2 occurs, two-electron ethanol oxidation to acetaldehyde dominating under these conditions. © 1990 American Chemical Society.