Oxidation of carbon monoxide over palladium on zirconia prepared from amorphous PdZr alloy I. Bulk structural, morphological, and catalytic properties of catalyst

Abstract

Catalysts highly active for the oxidation of carbon monoxide have been prepared by oxidation of amorphous and crystalline Pd 1Zr 2 metal alloys. The oxidation was carried out in situ; i.e., the fresh metal alloys were exposed to CO-oxidation conditions at temperatures of 473–553 K. Under these conditions, the initially almost inactive metal alloys underwent a series of solid state reactions and were finally transformed into stable and highly active microporous catalysts. Bulk structural characterization using X-ray diffraction and electron microscopy revealed that the final catalysts were made up of small intergrown and poorly crystalline domains of different palladium and zirconia phases. Palladium was present in three forms in the active catalysts, as a solid solution of palladium and oxygen, as metallic palladium, and as PdO. Zirconia existed in monoclinic and tetragonal forms. Comparative kinetic measurements carried out in a differential fixed bed reactor in the temperature range 300–500 K indicated that the activity of the palladium species in the catalysts derived from the metal alloys was more than an order of magnitude higher than the activity of the palladium species in a palladium on zirconia catalyst prepared by impregnation. The comparison of the activities was based on turnover frequencies calculated on the basis of accessible palladium sites determined by CO chemisorption. The higher activity of the catalysts derived from the metal alloys is attributed to their unique structural and chemical properties leading to extremely large interfacial areas and possibly to enhanced oxygen transfer via the solid-solid interphase.