APPLICATION OF SURFACE-ENHANCED RAMAN-SPECTROSCOPY TO ORGANIC ELECTROCATALYTIC SYSTEMS - DECOMPOSITION AND ELECTROOXIDATION OF METHANOL AND FORMIC-ACID ON GOLD AND PLATINUM-FILM ELECTRODES

The nature of adsorbed species formed by decomposition of methanol and formic acid under voltammetric conditions on gold and platinum-coated gold electrodes in acidic and alkaline media has been explored by means of surface-enhanced Raman spectroscopy (SERS). Sequences of SER spectra were recorded during potential-sweep cycles to correlate the potential-dependent surface speciation with reactant electrooxidation and other voltammetric features. Despite the observed absence of methanol electrooxidation on gold in perchloric acid, decomposition to yield adsorbed -CHO, -COH, and eta2 (O,O) formate, as well as CO, was detected from the SER spectra. In contrast to the apparent inability of these species to undergo electrooxidation under such conditions, adsorbed CO formed from solution CO is seen to undergo remarkably facile electrooxidation on gold. The latter observation is apparently connected with the formation of CO bound to partially oxidized gold sites even at low potentials, as observed by SERS. Such reactive species are not formed by methanol decomposition. Formic acid, however, readily undergoes electrooxidation on gold in perchloric acid; adsorbed formate rather than CO is identified by SERS as the reaction intermediate. Methanol, but not formate, undergoes electrooxidation in 0.1 M KOH; the former yields adsorbed CO extensively at higher concentrations. Despite facile electrooxidation of methanol on the platinum films in acid, little adsorbed CO is discerned to be present from both SERS and surface infrared spectroscopy. The former technique suggests instead the predominant formation of an acetylenic species. Some virtues and limitations of SERS for deducing the nature of adsorbed species in organic electrocatalytic processes are noted in the light of these findings.