INFLUENCE OF ADSORBED CARBON-MONOXIDE ON THE ELECTROCATALYTIC OXIDATION OF SIMPLE ORGANIC-MOLECULES AT PLATINUM AND PALLADIUM ELECTRODES IN ACIDIC SOLUTION - A SURVEY USING REAL-TIME FTIR SPECTROSCOPY

The influence of adsorbed CO formed by dissociative chemisorption on the electrooxidation pathways and kinetics of 22 alcohols and aldehydes on polycrystalline platinum and palladium electrodes in relation to the adsorbed CO formed by dissociative chemisorption has been examined systematically by means of real-time sequences of FTIR spectra obtained during slow (2 mV s~x) voltammetric sweeps in 0.1 M HC104. The overall objective is*to explore the reactant structure-dependent role that adsorbed CO (and other C02-producing species) plays in the organic electrooxidation. The reactants examined include short-chain primary and secondary alcohols, aldehydes, and small bifunctional molecules (e.g., ethylene glycol) containing alcohol, aldehyde, and/or carboxylic acid units. For both primary alcohols and aldehydes, the initial coverage of adsorbed CO, 9co, diminishes progressively with increasing size of the carbon backbone, from 0.9 (methanol) to 0.25 for benzyl alcohol. The 8C0 values are derived from the intensity of the infrared C-0 stretch at 2020-2070 and 1940-1980 cm1 on platinum and palladium, respectively. Adsorbed CO appears to act primarily as an inhibitor rather than a reaction intermediate for methanol and formic acid electrooxidation on platinum on the basis of 12C/13C isotope substitution experiments. The occurrence of specific electrooxidation pathways yielding the corresponding aldehyde, carboxylic acid, and C02 was followed quantitatively under voltammetric conditions from the potential-dependent appearance of characteristic infrared bands. Generally, the onset of even partial electrooxidation of primary alcohols, forming successively aldehydes and carboxylic acids, requires electrooxidative removal of adsorbed CO. Electrooxidation of secondary alcohols to ketones proceeds readily, with no adsorbed CO being formed in most cases. The interconnection between these adsorptive and mechanistic properties and their dependence upon reactant structure are discussed. © 1990 American Chemical Society. all right reserved. © 1990, American Chemical Society. All rights reserved.