Direct reduction of sulfur dioxide to elemental sulfur by methane and partial oxidation of methane to syngas on ceria-based catalysts

Single-stage catalytic reduction of SO₂ to elemental sulfur is a promising technology for the treatment of SO₂-laden gas streams. In this thesis, the catalytic reduction of SO₂ to elemental sulfur by methane at atmospheric pressure was studied over ceria-based catalysts. The catalysts were prepared by urea coprecipitation/gelation and incipient wetness impregnation. H₂- and CH₄-TPR were used to characterize the catalyst reduction properties. The catalyst structure was examined by XRD, STEM/EDS and YPS. The catalyst activity/selectivity studies were complemented by SO₂ uptake experiments and by reduction of the pre-sulfated catalysts in methane, both isothermally and in the TPR mode.; Ceria-based catalysts are active for SO₂ reduction by methane to elemental sulfur in the temperature range 550–750°C. At temperatures below ∼550°C the catalyst surface is capped by SO₂. Activation of CH₄ on these surfaces is initiated after sulfate decomposition. The working catalyst surface is partially sulfated even under fuel-rich conditions. CH₄-TPR and isothermal CH₄ reduction of pre-sulfated catalysts suggest that the reaction light-off coincides with the threshold temperature for sulfate decomposition. The addition of 5 at% copper into La-doped ceria, Ce(La)Ox, improved dramatically both the wet activity of the latter as well as its selectivity to elemental sulfur under fuel-rich conditions. Nickel addition, on the other hand, favored the partial oxidation of methane, and decreased the selectivity to sulfur.; The reaction kinetics and mechanism of the SO₂ reduction by methane to elemental sulfur are discussed within the redox framework. Creation of surface oxygen vacancies by methane is followed by SO₂ giving off its oxygens to fill the vacancies and produce sulfur. The activation energy is 188 kJ/mol. The rate of partial oxidation of methane by SO₂ to H₂S over the 1wt%Cu-1wt%La/CeO₂ catalyst was also measured. The rate of H₂S formation is negligible at dry conditions, but increased at wet conditions due to the participation of the water-gas-shift reaction.; The partial oxidation of methane to syngas was also studied in this work over Ni-containing ceria with nickel content of 5, 10 and 20 at% at atmospheric pressure. All catalysts in the as-prepared state showed similar activity and CO selectivity at T ≥ 550°C. Catalyst pre-reduction was not required. Reaction mixtures were dilute, containing 3 mol% CH₄ and 1.5 mol% O₂. Methane conversion and CO selectivity approached their respective thermodynamic equilibrium values above 550°C. The H₂/CO ratio was equal to 2 at T > 600°C. Over the range 0.54 to 0.04 g.s/cm 3 (STP), contact time effects were absent in partial oxidation of methane over 5 at% Ni-Ce(La)Ox. This catalyst, comprising highly dispersed nickel oxide in ceria, showed excellent resistance to carbon deposition and stable performance during 100h-on-stream at 650°C. On the other hand, high-content (>10 at%) nickel in ceria, comprising both dispersed nickel and bulk nickel oxide particles, was unstable even after shorter time-on-stream; carbon deposition was clearly the cause of this performance instability. The high activity and stability of 5at%Ni-Ce(La)Ox for POM were correlated with the high reducibility and oxygen mobility of this material.