Ceria-based catalysts for the recovery of elemental sulfur from SO 2-laden gas streams

Abstract

The catalytic reduction of SO 2 to elemental sulfur by CO or CH 4 over ceria-based catalysts under demanding operating conditions is reported in this paper. Cu- or Ni-containing ceria catalysts have been shown before to be highly active and selective for SO 2 reduction by CO in dry gas streams or in the presence of low amounts of H 2O. 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 (SO 2+CO) reaction was tested in gas streams containing 10–45 mol%H 2O at high space velocities (>80,000 h −1). The addition of 5 at.% (∼2.5 wt.%) copper or nickel significantly improved the low-temperature (<500°C) 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 SO 2 and NO by CO was also studied in this work over the same catalysts. In dilute gas mixtures containing 0.1–1.0 mol%SO 2 and NO, stoichiometric amount of CO and in the presence of 40%H 2O, the NO presence in the feed gas enhances both the SO 2 conversion and the elemental sulfur yield. At 550°C, in the presence of NO, SO 2 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 CO 2 produced was the sum of the SO 2 and NO reduction reactions by CO. Ceria-based materials are also active for SO 2 reduction by methane to elemental sulfur at temperatures higher than 550°C. 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-CeO 2, dissociation of methane begins at <550°C, and side reactions favor H 2S production over elemental sulfur. However, the 5 at.%Ni-Ce(La)O x catalyst showed remarkable resistance to carbon deposition, both in the SO 2-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.