Necessity of Oxygenated Surface Species for the Electrooxidation of Methanol on Iridium

Article abstract of DOI:10.1021/jp953185i

The electrooxidation of methanol on an iridium disk in the pH range 0-14 was studied by Fourier transform infrared (FTIR) spectroscopy.The following solution were used: 1 M H2SO4, 0.1 M HClO4, 1 M H3PO4, and 1 M Na2SO4 whose pH had been adjusted to 2 with H2SO4, 0.5 M phosphate buffer of pH 7, 0.1 M borax buffer, and 1 M NaOH.Linear potential sweep (LPS) FTIR spectroscopy showed that the products of 0.1 - 1.0 M methanol electrooxidation on smooth Ir were CO2 and formic acid in acidic media and bicarbonate/carbonate and formate ions in alkaline media.Only at pH 7 and 9.2 neither formic acid nor formate ion was detected.The activity of Ir for methanol electrooxidation was very low.At all pH values dissociative chemisorption of methanol yielded chemisorbed CO, always in the linear form.At pH 14 this chemisorbed CO was completely electrooxidized by 0.6 V vs RHE, a value 0.4 V less positive than in perchloric acid and phosphoric acids of pH 1.This behavior is parallel to that of the appearance of formate/formic: at pH 14 formate is detected already at 0.4 V, while in perchloric and phosphoric acids of pH 1 formic acid is first detected at 0.9 V.This dependence on pH of the electrocatalytic activity of Ir both for the electrooxidation of chemisorbed CO and for that of methanol to formate/formic is in perfect agreement with the fact that the peak of the main Ir surface oxidation process occurs at 0.6 V vs RHE at pH 14, but at a rather more positive potential, 0.9 - 1.0 V vs RHE, in acidic media (super-Nernstian pH dependence).This agreement lends support to the hypothesis that the electrooxidation of organic compounds requires the presence of oxide, hydroxide, and/or oxyhydroxide groups on the Ir surface, which provide the O atoms necessary for the formation of CO2 and formic/formate.However, in sulfuric acid, both at pH 0 and 2, formic acid was already observed at 0.7 V, possibly because in sulfuric acid a prepeak of Ir oxidation at 0.6 V that precedes the main peak at 1.0 V is better defined.The stretching frequency of linear CO increased by up to 60 cm^-1 with increasing coverage and at the same coverage was higher in HClO4 than in H3PO4 solutions.The Stark shift varied over the range 39 - 45 cm^-1 V^-1 with pH, CO coverage, and nature of the anion in the case of acids.