ELECTROCHEMICAL INFRARED STUDIES OF MONOCRYSTALLINE IRIDIUM SURFACES .1. ELECTROOXIDATION OF FORMIC-ACID AND METHANOL

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 H-2 /Ar. Similarly to other Pt-group metals, both the electrooxidation kin etics and surface speciation are sensitive to the crystallographic ori entation. 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 fo und in comparison with adsorption from solution CO in that formic acid yields extensive bridged CO at low potentials on Ir(100). This behavi oral difference is ascribed to an influence of other chemisorbates. Mo re direct evidence for other chemisorbed fragments from formic acid wa s obtained from the appearance of a band at ca. 1340/1350 cm(-1) on Ir (lll) and (100), evident at potentials, above 0.3-0.4 V vs. SCE, where adsorbed CO is removed. This feature is assigned to bidentate adsorbe d formate primarily by comparison with vibrational spectra in ultrahig h vacuum. Adsorbed formate is proposed to be a reaction intermediate e n route to CO2 production. Comparison with the electrocatalytic potenc y of other monocrystalline Pt-group surfaces suggests that the low act ivity seen for iridium (and rhodium) is due to strong anion adsorption as well as ease of surface oxidation. The former is likely responsibl e for the spectral detection of adsorbed formate only on iridium, alth ough this species may well be an important reaction intermediate on ot her surfaces. As for most other Pt group metals, methanol yields lower CO coverages than those from the formic acid. Some evidence for the p resence of other chemisorbed oxygenated fragments, CHxOH and COH, prim arily at higher potentials was obtained from the appearance of weak in frared features at 1238 and 1383 cm(-1) on Ir(110). (C) 1997 Elsevier Science S.A.