Sulfur-resistant and regenerable Ni/Co spinel-based catalysts for methane dry reforming

Abstract

Oxide-supported metal catalysts were prepared by thermal impregnation of highly crystalline and highly-faceted starting spinels of the form (M0.75Mg0.25)Al2O4, where M = Ni, Co, or Cu and mixtures thereof. In situ reduction at 900 °C extracts the transition metals from the oxide, and the resulting catalysts contain metal crystallites with particle sizes of ~100 nm and exceedingly low dispersion, but show high activity for dry reforming of methane with turnover frequencies as large as 3.9 at 850 °C. The Ni0.375Cu0.375Mg0.25Al2O4 catalyst shows stable methane conversion out to 12 hours on stream without performance-degrading coking. For the Ni/Co catalysts, the reforming activity and sulfur tolerance are both functions of the Ni/Co ratio and the synthesis temperature of the starting spinel, with Ni0.375Co0.375Mg0.25Al2O4 synthesized at 1500 °C displaying fast reaction kinetics even in the presence of 20 ppm H2S. High reforming activity is attributed to long linear lengths of high-perfection facet edges and corners on the metal crystallites. Sulfur tolerance appears to be improved by a combination of the oxygen storage capacity of the defective spinel support and its faceting that provides additional reaction sites for activation of CO2.