Ceria-based catalysts for the recovery of elemental sulfur from SO2-laden gas streams

The catalytic reduction of SO2 to elemental sulfur by CO or CH4 over ceria-based catalysts under demanding operating conditions is reported in this paper. Cu- or Ni-containing ceria catalysts have been shown before to he highly active and selective for SO2 reduction by CO in dry gas streams or in the presence of low amounts of H2O. In this work, the activity/selectivity of 10 at.%La-doped ceria, Ce(La)O-x, and Cu- or Ni-containing Ce(La)O-x for the (SO2 + CO) reaction was tested in gas streams containing 10-45 mol%H2O at high space velocities (>80,000 h(-1)). The addition of 5 at.% (similar to2.5 wt.%) copper or nickel significantly improved the low-temperature (<500C) reactivity of Ce(La)O-x. This was correlated with the improved reducibility of ceria in the metal-ceria catalysts and the ensuing low-temperature activity for the water-gas-shift (WGS) reaction. The combined reduction of SO2 and NO by CO was also studied in this work over the same catalysts. In dilute gas mixtures containing 0.1-1.0 mol%SO2 and NO, stoichiometric amount of CO and in the presence of 40%H2O, the NO presence in the feed gas enhances both the SO2 conversion and the elemental sulfur yield. At 550 degreesC, in the presence of NO, SO2 conversion and sulfur yield over the 5%Ni-Ce(La)O-x catalyst were 0.94 and 0.77, respectively, the NO conversion to N-2 was complete, and the CO2 produced was the sum of the SO2 and NO reduction reactions by CO. Ceria-based materials are also active for SO2 reduction by methane to elemental sulfur at temperatures higher than 550 degreesC. The addition of Cu or Ni has a different effect on the sulfur selectivity of ceria under fuel-rich conditions. The Cu-ceria system is a complete oxidation catalyst to a much higher temperature than Ni-ceria. Over Ni-CeO2, dissociation of methane begins at <550C, and side reactions favor H2S production over elemental sulfur. However, the 5 at.%Ni-Ce(La)O-x catalyst showed remarkable resistance to carbon deposition, both in the SO2-methane reaction as well as in partial oxidation of methane by O-2 to synthesis gas with a 2:1 H-2:CO ratio. This is attributed to the high dispersion of nickel in this catalyst and the fast rate of oxygen supply from ceria to the nickel interface. Catalysts were characterized by temperature-programmed-reduction, XPS and STEM/EDS. (C) 2000 Elsevier Science B.V. All rights reserved.