To investigate the possibility of simultaneous catalytic reduction of NO and SO2 by CO, reactions of NO, NO-CO, and NO-SO2-CO were performed on γ-alumina-supported sulfides of transition metals including Co, Mo, CoMo and FeMo. NO was decomposed into N2O and N2 accompanied with the formation of SO2; this serious oxidation of lattice sulfur resulted in the deactivation of the catalysts. The addition of CO to the NO stream suppressed SO2 formation and yielded COS instead. A stoichiometric conversion of NO and CO to N2 and CO2 was observed above 350°C on the CoMo and the FeMo catalysts. Although the CO addition lengthened catalyst life, it was not enough to maintain activity. After the NO-CO reaction, an XPS analysis showed the growth of Mo6+ and SO42- peaks, especially for the sulfided FeMo/Al2O3; the FeMo catalyst underwent strong oxidation in the NO-CO reaction. The NO and the NO-CO reactions proceeded non-catalytically, consuming catalyst lattice sulfur to yield SO2 or COS. The addition of SO2 in the NO - CO system enabled in situ regeneration of the catalysts; the catalysts oxidized through abstraction of lattice sulfur experienced anew reduction and sulfurization through the SO2-CO reaction at higher temperature. NO and SO2 were completely and catalytically converted at 400°C on the sulfided CoMo/Al2O3. By contrast, the sulfided FeMo/Al2O3 was easily oxidized by NO and hardly re-sulfided under the test conditions. Oxidation states of the metals before and after the reactions were determined. Silica and titania-supported CoMo catalysts were also evaluated to study support effects.
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