Nature and properties of a potassium-promoted [formula omitted] water gas shift catalyst

Abstract

A comparative study has been performed of a conventional NiMo Al 2O 3 catalyst and of this same material after impregnation with K 2CO 3. The catalysts were characterized by conventional and microprobe Raman spectroscopy, diffuse reflectance and X-ray photoelectron spectroscopy, analytical electron microscopy and temperature-programmed decomposition. The chemisorption of carbon monoxide was studied by transmission infrared spectroscopy. The results indicate a very strong interaction of K + ions with the catalyst surface. The original octahedral coordination of Mo 6+ in the NiMo Al 2O 3 catalyst is transformed into a tetrahedral coordination and the reducibility of Mo 6+ is strongly decreased in the presence of K + ions. Also the degree of sulfidation seems to be reduced in KNiMo Al 2O 3 , this phenomenon being accompanied by a stabilization of the Mo 5+ oxidation state. CO chemisorption leads to the formation of carbonates and formates on both catalysts in their oxidized and sulfided states. On reduced KNiMo Al 2O 3 CO polymer anions also seem to be formed. The effect of K + ions on the catalytic performance of the NiMo Al 2O 3 catalysts was tested for thiophene hydrodesulfurization (HDS), ethylene hydrogenation, CO methanation, and water gas shift (WGS). A significant reduction in activity was observed for HDS in the presence of K + ions, while the activity for WGS was simultaneously enhanced. An attempt is made to correlate the changes of the catalytic performance with the changes which are induced by K 2CO 3 impregnation on structure, symmetry, and valence state of the molybdenum species.