Redox chemistry over CeO2-based catalysts: SO2 reduction by CO or CH4

Abstract

The catalytic reduction of SO 2 to elemental sulfur by CO and CH 4 over Cu-modi®ed ceria catalysts is studied in this work. Doped and undoped ceria are active catalysts for the SO 2 reduction by CO or CH 4 in the temperature range 450±7508C. When CO is used as the reductant, the reaction follows the redox mechanism, and formation of surface defects (oxygen vacancies) and ceria reducibility are important for catalyst activity. SO 2 strongly adsorbs on the catalyst surface forming sulfates. Partial reduction of sulfate by CO is necessary for the reaction to light off and proceed at low temperatures. Addition of copper improves the low-temperature catalyst activity by increasing the reducibility of ceria and providing sites for CO adsorption. On the other hand, methane activation is limited by the thermal stability of surface sulfates. The activation of methane may involve surface oxygen species and partially reduced metal oxide sites at high temperature. Two independent reactions are proposed and used to explain the catalytic performance of ceria-based oxides in CH 4 SO 2 gas mixtures. One reaction leads to elemental sulfur and complete oxidation (CO 2 H 2 O) products, while the second produces H 2 S and COH 2 O under fuel-rich conditions. The addition of copper suppresses the latter, thus increasing the catalyst selectivity to elemental sulfur. The catalyst activity/selectivity studies were complemented by SO 2 uptake experiments in a TGA and reduction studies of the asprepared and pre-sulfated catalysts in CO and methane, both isothermally and in the TPR mode. #

Chemistry of SO 2 and DeSO x Processes on Oxide Nanoparticles

“…First, ceria doped with copper or other metals is able to catalyze the reduction of SO 2 by CO (14,15).…”

“…In principle, oxides that can be reduced by CO to form O vacancies, could be active catalysts for the 2CO gas + SO 2,gas 2CO 2,gas + S ads reaction. This is the case of CuO and CeO 2 (14,15,16,35). To favor the formation of O vacancies and active centers in these oxide, the reaction is usually carried out at high temperatures with a CO/SO 2 ratio in the feed ∃ 2.…”

“…To favor the formation of O vacancies and active centers in these oxide, the reaction is usually carried out at high temperatures with a CO/SO 2 ratio in the feed ∃ 2. In the case of ceria, doping with a second metal helps the formation of O vacancies (36,37) and increases catalytic activity (14,15,35,38).…”

“…For a vast number of oxide compounds, this comes out from the fact that the preparation procedures are not able to give a size distribution approaching the delta function (39 (11), and ceria based materials, useful as DeSOx catalysts (14,15).…”

“…Due to their low cost, oxides are frequently used as sorbents or scrubbers for trapping the SO 2 molecule in industrial processes (11). Also, there is a general interest (3,4,(11)(12)(13)(14)(15) in using oxides as catalysts for the Claus reaction (SO 2 + 2H 2 S → 2H 2 O + 3S n ) and the reduction of sulfur dioxide by CO (SO 2 + 2CO → 2CO 2 + S n ). In these reactions, the rupture of the S-O bonds on the oxide catalyst is one of the most difficult steps.…”