Structural and chemical state of doped and impregnated mesoporous Ni/CeO2 catalysts for the water-gas shift

Abstract

Mesoporous Ni/CeO2 catalysts of variable loadings were prepared using in-situ doping and impregnation synthesis techniques. The catalysts were found to exhibit activity for the water-gas shift (WGS) reaction, particularly at temperatures above 250¿°C. Structural, electronic, and surface chemical characterizations of the materials were carried out using in-situ X-ray diffraction (XRD), in-situ X-ray absorption (XANES), and in-situ infrared (DRIFTS) techniques. The effects of metal loading and preparation method on these properties were studied in order to develop a more complete understanding of the design and application of Ni-loaded mesoporous CeO2 catalysts. For WGS reaction activity, the in-situ doping method was observed to be superior, and overall activity was observed to increase with increasing metal loadings. Simple normalization of activity data to nominal nickel content revealed a trend favoring lower loadings, indicating higher activity per unit nickel. The reduction of the catalyst is observed with increasing reaction temperature (Ni2+¿¿¿Ni°, Ce4+¿¿¿Ce3+) while the active states of all catalysts were identified as a stable, partially reduced ceria fluorite lattice (Ce4+/Ce3+) with Ni2+ and Ni°. In Situ DRIFTS showed nearly identical surface chemistry for both doped and impregnated samples, likely involving an associative pathway at lower temperatures and a redox pathway at higher temperatures. Structural properties and surface chemistry were observed to depend both on metal loading and preparation method. Nickel loadings as low as 1¿wt% prepared by in-situ doping were found to display the most favorable metal-support interactions for the WGS reaction.