The stoichiometries of H 2 and CO adsorptions on cobalt: Effects of support and preparation

Abstract

Catalysts composed of unsupported cobalt and cobalt supported on silica, alumina, titania, magnesia, and carbon were prepared by thermal decomposition, impregnation, precipitation, and evaporative deposition. These catalysts were characterized by high temperature (323–423 K) hydrogen adsorption and conventional 298 K adsorptions of hydrogen and carbon monoxide. Total surface areas of unsupported cobalt catalysts were measured by BET. Metal crystallite sizes were determined independently from XRD, TEM, and hydrogen adsorption. Extents of reduction were measured by O 2 titration at 673 K. Hydrogen adsorption on cobalt is activated and reversible; extents of activation and reversibility vary with support, metal loading, and preparation. The hydrogen adsorption stoichiometry is 1.0 hydrogen atom per surface cobalt atom, if total adsorption at the temperature of maximum uptake is considered. The adsorption of CO is nonactivated and reversible; the stoichiometry for irreversible adsorption varies from 0.4 to 2.3 molecules of CO per surface cobalt atom, depending upon support, metal loading, and preparation. Cobalt dispersion and extent of reduction also vary greatly with support, metal loading, and method of preparation. Cobalt/carbon catalysts prepared by evaporative deposition have unusually high dispersions relative to other cobalt catalysts. Hydrogen adsorption is recommended as the most convenient, reliable technique for measurement of cobalt crystallite size in Co Al 2O 3 , Co SiO 2 , and Co C catalysts.