Catalyst tolerance to SO2 and water vapor of Mn based bimetallic oxides for NO deep oxidation by ozone

Abstract

Improving the catalyst stabilities under different conditions (water vapor, SO₂, both water vapor and SO₂) is important for industrial applications regarding catalytic NO deep oxidation by ozone. In this paper, Ce–Mn/SA and Fe–Mn/SA catalysts were selected to investigate the stabilities. The results showed that the Ce–Mn/SA exhibited excellent stability and resistance to SO₂, while the Fe–Mn/SA only displayed excellent stability without moisture and SO₂. Almost a 50% drop in efficiency was observed after deactivation by water vapor and water vapor together with SO₂ for the two catalysts. The Fe–Mn/SA displayed inferior resistance to SO₂. After stability testing with water vapor, the surface area, pore volume, and average pore diameter all decreased. The low adsorption energy of the H₂O molecule resulted in the superior adsorption of water vapor, which occupied large amounts of active sites. XPS results showed that the ratio of Mn⁴⁺ and chemisorbed oxygen decreased after deactivation. Mn⁴⁺ favors NO oxidation, while Mn³⁺ is favorable for ozone decomposition. Therefore, better performance in NO deep oxidation by ozone requires relative balance distribution between Mn⁴⁺ and Mn³⁺. Interestingly, the TPD results showed that the NO desorption peak was unaffected and even increased a lot after water vapor stability testing. This could be attributed to the nitrates, formed by the N₂O₅ and H₂O in liquid phase, that were adsorbed on the catalyst surface prior to NO, which contributes to a bigger NO desorption peak with lower NO adsorption ability. The trace of sulfate formed after SO₂ stability testing was verified from TPD and TGA results, but it was not observed from the FTIR spectra, indicating the sulfate species formed during the ozonation process may not exist on the catalyst surface.