Enhancement of Oxidation of Formic Acid in Acid Medium on Zirconia-Supported Phosphotungstate-Decorated Noble Metal (Pd, Pt) Nanoparticles

Abstract

Palladium and platinum nanoparticles have been modified at sub-monolayer level with Keggin-type phosphododecatungstate (by spontaneous adsorption of H3PW12O40) and considered as catalysts for the electrooxidation of formic acid in sulfuric acid electrolyte. The presence of phosphotungstate adsorbates has been confirmed using Fourier transform infrared spectroscopy (by reflectance). The enhancement effect (described in terms of the oxidation current increases) is even more pronounced when the catalytic centres have been supported on nanostructured ZrO2. In the case of Pd catalysts, a large population of hydroxyl groups and a high mobility of interfacial protons (on zirconia) seem to favour the direct oxidation path to CO2, whereas in the case of Pt-based systems, the enhancement effect is related to the oxidative removal of otherwise passivating CO adsorbates (indirect oxidation path). The role of polytungstate adsorbates on Pd or Pt nanoparticles relates to their ability to interact competitively with the CO adsorbates at noble metal catalytic sites (‘third-body’ effect).