Impact of SO₂ on elemental mercury oxidation over CeO₂–TiO₂ catalyst
2013
Li, Hailong | Wu, Chang-Yu | Li, Ying | Li, Liqing | Zhao, Yongchun | Zhang, Junying
Effect of SO₂ on elemental mercury (Hg⁰) oxidation over a highly active CeO₂–TiO₂ catalyst was systematically investigated. SO₂ was found to have different, even contrary, effects on Hg⁰ oxidation under different flue gas conditions. In pure N₂ atmosphere, SO₂ inhibited Hg⁰ oxidation. In N₂ plus O₂ atmosphere, low concentration of SO₂ promoted Hg⁰ oxidation, while high concentration of SO₂ deteriorated Hg⁰ oxidation. The promotional effect of SO₂ on Hg⁰ oxidation was probably due to SO₃ generated from SO₂ oxidation, and the inhibitive effect of SO₂ on Hg⁰ oxidation was attributed to the competitive adsorption between SO₂ and Hg⁰. The results suggest a balance point for SO₂ concentration, where the promotional effect on Hg⁰ oxidation by SO₃ originated from SO₂ is equal to the inhibitive effect of SO₂ on Hg⁰ adsorption and subsequent oxidation via the Langmuir–Hinshelwood mechanism. In the presence of NO, SO₂ with the aid of O₂ exhibited promotional effect on Hg⁰ oxidation through NO-catalyzed oxidation to form SO₃ and hence a balance point of higher SO₂ concentration. However, without O₂, SO₂ greatly limited Hg⁰ oxidation in the presence of NO. When HCl was present, most Hg⁰ oxidation was due to reactions between active chlorine species and adsorbed Hg⁰. SO₂ inhibited Hg⁰ adsorption and therefore deteriorated Hg⁰ oxidation by chlorine species. When both NO and HCl were present, NO accelerated the conversion of SO₂ to SO₃, and hence relieved the prohibitive effect of SO₂ on Hg⁰ oxidation by HCl.
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