Catalytic Desulfurization of Carbon Black on a Platinum Oxide Electrode

Organically bound sulfur is a common impurity in carbon blacks, such as vulcanized carbon and coal. Whereas this organosulfur is usually removed by heating to high temperatures, we describe a low-temperature method to desulfurize solid carbon via oxidation (ultimately to SO 4 2- ) on a Pt catalyst in oxygenated aqueous media. The reaction is monitored on Vulcan carbon, which is the favored support for the Pt catalysts used in fuel-cell electrodes but which has a high concentration (nominally 5000 ppm) of organosulfur. Vulcan carbon is electrodesulfurized by dispersing it in a heated, dilute HNO 3 electrolyte where it can physically contact a Pt mesh anode that is maintained as Pt oxide. After 40−180 min of electrolysis in oxygenated solution at +1.0 V versus Pd/H, sulfate is present in the electrolyte (as measured by precipitation of BaSO 4 ). X-ray photoelectron spectroscopy of the dried, electrodesulfurized carbon shows that ∼30% of the near-surface organosulfur is removed. The remaining sulfur presumably does not react because it does not physically contact the hydrous platinum oxide (PtO x H y ) at the electrode surface. Minimal desulfurization occurs at 0.2 V versus Pd/H, when the Pt mesh surface is composed of Pt 0 . These electrochemical results show that oxidized Pt is the catalyst necessary for oxidative desulfurization. In deoxygenated solution, electrodesulfurization still occurs for electrolyses at +1.0 V versus Pd/H, but SO 4 2- is not generated, indicating that SO 2 is the partial oxidation product. The surface area and total oxygen content of the carbon are unaffected by the electrodesulfurization process. This simple, low-temperature method for desulfurization should be applicable to other forms of solid carbon containing organosulfur moieties.