CO₂ hydrogenation over Co/Al₂O₃ catalysts prepared via a solid-state reaction of fine gibbsite and cobalt precursors

Abstract

The properties of Co/Al₂O₃ catalysts prepared by the solid-state reaction between gibbsite and various cobalt salts such as cobalt acetate (CoAc), cobalt acetylacetonate, cobalt chloride, and cobalt nitrate (CoNT) were investigated in the hydrogenation of carbon dioxide at 270 °C and atmospheric pressure and characterized by N₂ physisorption, X-ray diffraction, X-ray photoelectron spectroscopy, and H₂-temperature programmed reduction. Compared to the catalyst prepared by conventional impregnation of aqueous solution of cobalt nitrate on alumina (CoNT-Imp), the solid-state catalysts (CoNT and CoAc) exhibited much higher activity in the CO₂ hydrogenation with comparable CH₄ and CO selectivity. Unlike the impregnation catalysts, in which most of the Co₃O₄ particles/clusters were located deep inside the pores of alumina, the solid-state reaction resulted in the dispersion of cobalt oxides mostly on the external surface of alumina. As a consequence, CO₂ adsorption and dissociation to adsorbed CO and O (the initial steps in CO₂ hydrogenation) were not limited by the slow diffusion of CO₂ so high CO₂ hydrogenation activity was obtained. As revealed by the XRD and H₂-TPR results, the average crystallite size of Co₃O₄ and the metal-support interaction depended on the cobalt precursor used during the solid-state synthesis. Nevertheless, the solid-state reaction of gibbsite and cobalt chloride at 650 °C resulted in very poor CO₂ hydrogenation activity due to the formation of inactive cobalt aluminate.