Formic acid electrooxidation on platinum-group metals: Is adsorbed carbon monoxide solely a catalytic poison?

摘要

The possibility that adsorbed carbon monoxide may act as a reaction intermediate rather than merely a catalytic poison for formic acid electrooxidation on Pt-group metals in aqueous perchloric acid is explored by monitoring the time-dependent effects of reactant C-13/C-12 isotopic substitution on the adsorbate vibrational properties, as discerned from surface-enhanced Raman spectroscopy (SERS). The electrodes examined, polycrystalline rhodium and iridium (deposited as ultrathin films on gold to yield optimal SERS activity), exhibit substantial formation of adsorbed CO at potentials below and close to the onset of formic acid electrooxidation, as discerned from the well-known C-O (v(CO)) stretches at 1850-2000 cm(-1) and the metal-CO vibrations at 450-500 cm-l. As in earlier infrared studies, the former vibrations provide a sensitive means of monitoring adsorbate (CO)-C-13/(CO)-C-12 replacement from the characteristic similar to 45 cm(-1) difference in isotopic vco frequencies. These SER vibrational features are used to monitor the rates of adsorbed (CO)-C-13/(CO)-C-12 replacement triggered by abrupt switches in the reactant isotopic composition brought about either by adding a large excess of (HCOOH)-C-12 to the (HCOOH)-C-13-containing solution or by utilizing a spectroelectrochemical flow cell. At electrode potentials below the onset of formic acid electrooxidation on rhodium, only slow (CO)-C-13/(CO)-C-12 isotopic exchange was observed, requiring several minutes (or longer) for extensive substitution. The kinetics are markedly (10-100 fold) slower than those observed when using solution CO rather than formic acid. However, altering the potential to values (above 0.2 V versus SCE) beyond the onset of formic acid electrooxidation yielded relatively rapid, albeit incomplete, isotopic CO exchange. The turnover frequencies, similar to0.01-0.1 s(-1), deduced on this basis can indeed account for a significant or even substantial fraction of the electrocatalytic conversion rates of formic acid to CO2, indicating that the adsorbed CO can act as a reaction intermediate under some conditions. Qualitatively similar results were obtained for electrooxidation on iridium. The implications of these findings to the conventional "dualpathway" mechanism for such catalytic electrooxidations are briefly discussed. [References: 32]