Resolving Sulfur Oxidation and Removal from Pt and Pt3Co Electrocatalysts Using in Situ X-ray Absorption Spectroscopy

Adsorbed sulfur is a poison to the Pt catalysts used in proton exchange membrane fuel cells, but it can be removed by potential cycling. This process is studied for Si-poisoned nanoscale Pt- and Pt3Co- on Vulcan carbon (Pt/VC and Pt3Co/VC) in perchloric acid electrolyte using the Delta mu adsorbate isolation technique for in situ X-ray absorption spectroscopy. The Delta mu technique is modified to better distinguish the Delta mu signatures for H, O, and S-x on Pt. The resulting Delta mu analysis suggests that SO2 on nanoscale Pt is oxidized to bisulfate or sulfate species in two regions, near 1.05 V on the cluster edges of the Pt nanoparticle, and at higher potentials from the Pt(111) faces where oxygen is less strongly bound. The bisulfate or sulfate species desorb from the Pt surface at high potentials due to O(OH) adsorption/replacement and at low potentials due to loss of the Coulomb attraction between the bisulfate anion and the Pt. A similar oxidation process occurs for S-x-poisoned Pt3Co/VC, but at lower potentials because a ligand effect coming from Co shifts the oxidization potential of adsorbed SO2 to lower potentials while pushing OH adsorption to higher potentials. The spectroscopic results give insights into cyclic voltammetry data and are consistent with electrochemical cycling procedures for removing the sulfur.