Adsorption and electrooxidation of some simple organic molecules on rhodium(111) as probed by real-time FTIR spectroscopy: comparisons with platinum (111)

Abstract

The electrooxidation pathways of seven simple organic molecules in 0.1 M HClO{sub 4} on ordered Rh(111) were examined by means of real-time sequences of FTIR spectra obtained during slow (2 mV s{sup {minus}1}) voltammetric sweeps. The reactants studied are formic acid, methanol, ethanol, acetaldehyde, ethylene glycol, glyoxal, and oxalic acid for concentrations between 0.05 and 0.5 M. High coverages ({theta} {ge} 0.4) of adsorbed CO are formed in most cases, as detected from a band at about 1880-1890 cm{sup {minus}1} at the most negative potentials ({minus}0.25 to +0.05 V vs SCE), which is replaced by a band at 2030-2040 cm{sup {minus}1} at more positive potentials. These two features are assigned to C-O vibrations, {nu}{sub CO}, associated with CO adsorbed in bridging and terminal coordination geometries, respectively. The former configuration is apparently engendered by hydrogen coadsorption. For cyclic voltammograms between {minus}0.25 and +0.6 V, the adsorbed CO formed by dissociative reactant chemisorption only undergoes complete electrooxidation during the reverse portion of the potential sweep, at ca. 0.4V. This inability to remove adsorbed CO on ordered Rh(111) appears to be at least partly responsible for the observed weakly electrocatalytic properties of this surface for organic electrooxidations. The production of CO{sub 2} evenmore » from formic acid, which occurs readily on Pt(111), does not commence in earnest on Rh(111) until CO is removed during the reverse scan. The electrooxidation of the other reactants on ordered Rh(111) is largely limited to the production of small amounts of CO{sub 2}, although some electrooxidation of acetaldehyde (but not ethanol) to acetic acid is detected. These results are compared with corresponding reactions on ordered Pt(111) and for the electrooxidation of solution CO.« less