SO₂ adsorption products on Pt nanoparticles as a function of electrode potential and oxidative properties of carrier gas: In situ sulfur K-edge XANES approach

In situ sulfur K-edge X-ray absorption near-edge structure spectroscopy is used to determine the nature of adsorbed SO₂ species from a SO ₂/O₂ gas mixture on carbon-supported Pt nanoparticles (Pt/VC). SO₂ was adsorbed onto electrodes held at 0.1, 0.5, 0.7 and 0.9 V vs. a reversible hydrogen electrode while flowing 1000 ppm SO₂ in O₂ through the working electrode (WE) compartment. SO₂ adsorption products from SO₂/O₂ are compared to those from SO₂/N₂ gas mixtures [Baturina, et al., Langmuir, 27 (2011) 14930]. The SO₂ adsorption products are found to be essentially the same at electrodes held at 0.5, 0.7 and 0.9 V. A major difference is observed at 0.1 V, where (bi)sulfate ions are generated in the presence of SO₂ in O₂ likely due to a reaction between SO₂ and H₂O₂ formed as a byproduct of the oxygen reduction reaction in the hydrogen adsorption region. (Bi)sulfate generation on Pt/VC catalysts held at 0.1 V suggests that SO₂ may act as a peroxide radical scavenger at the PEM fuel cell cathode. Although impurities such as SO₂ and H₂S usually promote hydrogen peroxide generation at the fuel cell cathodes, their detrimental effect may be diminished by their reaction with H₂O₂.