Adsorption and electrooxidation of carbon monoxide on polycrystalline platinum at pH 0.3–13

Abstract

Over the whole pH range 0.3–13 electrooxidation of dissolved CO on polycrystalline Pt was found to depend dramatically on the CO dosing potential, occurring at about 0.6 or 0.9 V for CO dosing potentials lower or higher, respectively, than about 0.4 V versus the reversible hydrogen electrode. Only in the former case a small anodic peak at about 0.5 V appeared in stripping voltammograms of chemisorbed CO, suggesting that dissolved CO electrooxidation at 0.6 V takes place on a small fraction of Pt sites that are free from CO. These findings are in agreement with literature reports for pH 0.3 and 14. The IR bands of both bridge and linear CO disappeared at about the same potential for each pH value, showing that at least the main peak of chemisorbed CO corresponded to both linear and bridge CO. As for the small CO stripping peak, it corresponded mostly, if not exclusively, to bridge CO. The ratio of bridge to linearly chemisorbed CO increased with increasing pH, and the stretching frequency of both bridge and linear CO decreased with increasing pH. Both trends were attributed to the exclusion by chemisorbed CO of water and its ions from the inner part of the electrochemical double layer, which would render the point of zero charge of the CO-covered metal independent of pH. At all pH values the intensity of the band of linear CO increased with increasing potential up to 0.4 V, while that of bridge CO decreased. This behaviour is in agreement with literature reports for Pt(100), Pt(111) and Rh(111), and is attributed to a decreasing back-donation from the metal d-orbitals to the antibonding 2π* MO of chemisorbed CO, which would favour a decrease of the coordination degree of CO.