Effect of Molybdenum on the Sulfur-Tolerance of Cerium–Cobalt Mixed Oxide Water–Gas Shift Catalysts

As traditional sources of energy become depleted, significant research interest has gone into conversion of biomass into renewable fuels. Biomass-derived synthesis gas typically contains concentrations ranging from ~30 to 600 ppm H₂S. H₂S is a catalyst poison which adversely impacts downstream processing of hydrogen for gas-to-liquid plants and the deactivation of water–gas shift catalysts by sulfur is typical. Novel catalysts are needed to remain active in the presence of sulfur in order to boost efficiency and mitigate costs. Previous studies have shown molybdenum to be active in concentrations of sulfur >300 ppm. Cobalt has been shown to be active as a spinel in concentrations of sulfur <240 ppm. Ceria has received attention as a catalyst due to its oxygen donating properties. In this study, mixed oxide catalysts were synthesized via Pechini’s method into various blends of metal oxide solutions. Activity testing at low steam-to-carbon ratios (1:1) produced near equilibrium conversions at a GHSV of 6,300 h⁻¹ and over a temperature range of 350–400 °C for a Ce–Co mixed oxide even after an 800 ppm sulfur treatment. The addition of molybdenum to the Ce–Co base had little effect on sulfur tolerance, but it did lead to a reduction in selectivity for methanation. Specific surface areas generally increased following the sulfur treatments and X-ray diffraction patterns confirmed that bulk sulfiding did not occur.