Methanol Oxidation at Platinum in Alkaline Media: A Study of the Effects of Hydroxide Concentration and of Mass Transport

• Published in: Chemphyschem 2021-07, Vol.22 (13), p.1397-1406
• Main Authors: Rus, Eric D., Wakabayashi, Ryo H., Wang, Hongsen, Abruña, Héctor D.
• Format: Article
• Language: English
• Subjects: alkaline media; Diffusion layers; Electrodes; Ion concentration; Ions; Mass transport; Methanol; methanol oxidation; microelectrode; Microelectrodes; Oxidation; platinum; Rotating disks; rotating ring-disk electrode voltammetry; Voltammetry
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.202100087

The hydroxide ion concentration dependence of the methanol oxidation reaction at Pt was studied using microelectrode voltammetry and rotating disk electrode voltammetry. Both methods suggest that the rate of methanol oxidation is limited by hydroxide mass transport at low hydroxide concentrations, while it is inhibited by hydroxide adsorption at high concentrations. It was possible to shift from the transport‐limited regime to the inhibitory regime by varying the bulk concentration of hydroxide or by varying mass transport to the electrode. Rotating ring‐disk electrode voltammetry was employed to qualitatively assess changes in the diffusion layer pH. The results indicated a decrease in the surface pH during methanol oxidation, as expected, but also that the pH reached a steady state during hydroxide transport limited methanol oxidation. The hydroxide ion plays two roles in methanol oxidation: it behaves as a reactant needed for methanol oxidation, and as a specifically adsorbing anion which inhibits methanol oxidation. At low hydroxide concentrations, the reaction can be limited by hydroxide mass transport.