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

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 degrees C extracts the transition metals from the oxide, and the resulting catalysts contain metal crystallites with particle sizes of similar to 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 degrees 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 degrees 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.