Characterization of Graphite-Supported Palladium–Cobalt Catalysts by Temperature-Programmed Reduction and Magnetic Measurements

Graphite-supported cobalt, palladium, and cobalt–palladium systems were prepared by a simple impregnation technique and submitted to hydrogen reduction in a temperature-programmed mode. Using X-ray diffraction to define the structure of the calcined precursors, magnetic measurements to determine the amount of metallic cobalt formed after reduction, and analysis of the gaseous medium during the reduction, a general model for the reduction of the graphite supported catalysts has been suggested. At room temperature, both pure PdO and PdO associated with Co3O4are reduced to the metallic state. In the case of bimetallic systems, a fraction of Co3O4in close proximity or interfaced with PdO can be reduced to the metallic state and to CoO species, demonstrating a strong catalytic effect of palladium on the reduction of cobalt oxides. At temperatures between 298 and 500 K, depending on the catalyst formulation, the presence of metallic palladium promotes the reduction of a large fraction of oxidized cobalt. At higher reduction temperature, at least two competitive phenomena were detected: direct reduction of the residual oxidized cobalt by the graphite, leading to carbon monoxide and dioxide formation, and hydrogasification of the graphite catalyzed by the supported metals, leading mainly to methane formation. Together with a possible short-range palladium-activated hydrogen migration, at low and moderate temperatures, long-range migration of particles is necessary to explain the observations. These migrations, in turn, favor the formation of bimetallic particles, after reduction at 773 K.