CHARACTERIZATION OF GRAPHITE-SUPPORTED PALLADIUM-COBALT CATALYSTS BY TEMPERATURE-PROGRAMMED REDUCTION AND MAGNETIC MEASUREMENTS

Graphite-supported cobalt, palladium, and cobalt-palladium systems wer e prepared by a simple impregnation technique and submitted to hydroge n reduction in a temperature-programmed mode. Using X-ray diffraction to define the structure of the calcined precursors, magnetic measureme nts to determine the amount of metallic cobalt formed after reduction, and analysis of the gaseous medium during the reduction, a general mo del for the reduction of the graphite supported catalysts has been sug gested. At room temperature, both pure PdO and PdO associated with Co3 O4 are reduced to the metallic state. In the case of bimetallic system s, a fraction of Co3O4 in close proximity or interfaced with PdO can b e reduced to the metallic state and to CoO species, demonstrating a st rong catalytic effect of palladium on the reduction of cobalt oxides. At temperatures between 298 and 500 K, depending on the catalyst formu lation, the presence of metallic palladium promotes the reduction of a large fraction of oxidized cobalt. At higher reduction temperature, a t least two competitive phenomena were detected: direct reduction of t he residual oxidized cobalt by the graphite, leading to carbon monoxid e and dioxide formation, and hydrogasification of the graphite catalyz ed by the supported metals, leading mainly to methane formation. Toget her 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, f avor the formation of bimetallic particles, after reduction at 773 K. (C) 1997 Academic Press.