Electrochemical infrared studies of monocrystalline iridium surfaces Part I: Electrooxidation of formic acid and methanol

Abstract

Voltammetry combined with parallel infrared spectroscopic measurements have been utilized to explore some kinetic and mechanistic aspects of the electrocatalytic oxidation of formic acid and methanol in acidic solution on low-index single-crystal iridium surfaces. The electrodes were ordered by means of flame annealing and subsequent cooling in H2/Ar. Similarly to other Pt-group metals, both the electrooxidation kinetics and surface speciation are sensitive to the crystallographic orientation. Extensive dissociative chemisorption of formic acid to yield carbon monoxide is seen on all three surfaces. While primarily atop (i.e., linear) adsorbed CO is observed, significant differences were found in comparison with adsorption from solution CO in that formic acid yields extensive bridged CO at low potentials on Ir(100). This behavioral difference is ascribed to an influence of other chemisorbates. More direct evidence for other chemisorbed fragments from formic acid was obtained from the appearance of a band at ca. 1340/1350 cm−1 on Ir(111) and (100), evident at potentials, above 0.3–0.4 V vs. SCE, where adsorbed CO is removed. This feature is assigned to bidentate adsorbed formate primarily by comparison with vibrational spectra in ultrahigh vacuum. Adsorbed formate is proposed to be a reaction intermediate en route to CO2 production. Comparison with the electrocatalytic potency of other monocrystalline Pt-group surfaces suggests that the low activity seen for iridium (and rhodium) is due to strong anion adsorption as well as ease of surface oxidation. The former is likely responsible for the spectral detection of adsorbed formate only on iridium, although this species may well be an important reaction intermediate on other surfaces. As for most other Pt-group metals, methanol yields lower CO coverages than those from the formic acid. Some evidence for the presence of other chemisorbed oxygenated fragments. CHvOH and COH, primarily at higher potentials was obtained from the appearance of weak infrared features at 1238 and 1383 cm−1 on Ir(110).