Mechanism for the Electro-oxidation of Carbon Monoxide on Platinum, Including Electrode Potential Dependence

Abstract

B3LYP hybrid density functional calculations show that CO and OH bonded to Pt2Pt2 react with a low activation energy (calculated 0.44 eV) to form ‒COOH‒COOH bonded to a onefold site of Pt2.Pt2. An OH bonded to the adjacent onefold site attracts the acid group by hydrogen bonding and a low activation energy (calculated 0.23 eV) separates this complex from CO2+H2O−Pt2.CO2+H2O−Pt2. The similarity of the bond strengths between Pt2Pt2 and CO, OH, and H2OH2O to measured chemisorption bond strengths from the literature makes these results worth considering as a model for the electro-oxidation of CO(ads) by OH(ads) on platinum anodes. Calculations of activation energies for the oxidative deprotonation of Pt2‒COOHPt2‒COOH indicate that high activation energies are to be expected for the reaction ‒COOH(ads)+H2O→CO2+H3O+(aq)+e−(U)‒COOH(ads)+H2O→CO2+H3O+(aq)+e−(U) at the electrode potentials where CO(ads) is removed (∼0.6V).(∼0.6V). Therefore, the formation of OH(ads) from oxidation of H2O,H2O, shown in another study to have a low activation energy and a reversible potential of ∼0.57V,∼0.57V, is concluded to be the cause of the observed overpotential for the electro-oxidation of CO(ads). © 2002 The Electrochemical Society. All rights reserved.