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 SO2/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.