Oxidation of Aqueous Phosphorous Acid Electrolyte in Contact with Pt Studied by X-ray Photoemission Spectroscopy

The oxidation of the aqueous H₃PO₃ in contact with Pt was investigated for a fundamental understanding of the Pt/aqueous H₃PO₃ interaction with the goal of providing a comprehensive basis for the further optimization of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). Ion-exchange chromatography (IEC) experiments suggested that in ambient conditions, Pt catalyzes H₃PO₃ oxidation to H₃PO₄ with H₂O. X-ray photoelectron spectroscopy (XPS) on different substrates, including Au and Pt, previously treated in H₃PO₃ solutions was conducted to determine the catalytic abilities of selected metals toward H₃PO₃ oxidation. In situ ambient pressure hard X-ray photoelectron spectroscopy (AP-HAXPES) combined with the "dip-and-pull" method was performed to investigate the state of H₃PO₃ at the Pt|H₃PO₃ interface and in the bulk solution. It was shown that whereas H₃PO₃ remains stable in the bulk solution, the catalyzed oxidation of H₃PO₃ by H₂O to H₃PO₄ accompanied by H₂ generation occurs in contact with the Pt surface. This catalytic process likely involves H₃PO₃ adsorption at the Pt surface in a highly reactive pyramidal tautomeric configuration.