Study On SO₂ Oxidation Mechanism On Commercial SCR Catalysts Surface

Currently,selective catalytic reduction technology（SCR）is extensively used for denitration technology in coal-fired power plants.The SO₂ in the flue gas seriously affects the denitration efficiency.After SO₂ is oxidized to SO₃,it reacts with NH₃ to produce side reaction products such as（NH₄）₂SO₄ and NH₄HSO₄,which are coated on the catalyst to cause catalyst poisoning.The interaction of SO₂ and catalyst will affect the long-term stable use of the catalyst and even causes catalyst poisoning.It is of industrial value to study the interaction between SO₂ and catalyst and the oxidation of SO₂ on the catalyst surface.Based on the commercial SCR catalyst,the oxidation conversion of SO₂ on the catalyst was studied and the oxidation mechanism was discussed.After experimental research and mechanism discussion,the following conclusions are obtained:（1）The oxidative conversion of SO₂ on commercial SCR catalyst was affected by factors such as O₂ volume concentration,SO₂ concentration,temperature and space velocity.With the increase of temperature and O₂ volume concentration,the oxidation conversion efficiency of SO₂ increased.With the increase of SO₂ initial concentration and space velocity,the oxidation conversion efficiency of SO₂ decreased.（2）The reaction between SO₂ and the catalyst increased the weak acid on the catalyst surface,and the denitration efficiency increased at low temperature.The SO₄^2-produced on the catalyst surface promoted the crystallization of anatase Ti O₂,which reduced the specific surface area and average pore diameter of the catalyst.The presence of SO₂ reduced the V⁵⁺of the catalyst surface and converted part of Mo⁶⁺to Mo⁵⁺.And the binding energy of SO₄^2-was detected on the surface of the sample after the reaction.The surface sulfate caused a decrease in chemisorbed oxygen and an increase in lattice oxygen on the catalyst surface.The sulfate material covered the surface of the sample and reduced the Ti content.（3）The mechanism of SO₂ oxidation on the catalyst was investigated by In Situ DRIFTS.SO₂ was first adsorbed on the surface of the catalyst to form a sulfur species covalent bond O=S=O,and interacted with the metal oxide to produce an intermediate metal oxide（VOSO₄）.When O₂ entered the reactor,O₂ reoxidized V⁴⁺to V⁵⁺,during which the-OH of the metal surface on the catalyst was continuously consumed.It was proved by experiments that the interaction between SO₂ and catalyst was weaker at lower temperature,the catalyst surface was more likely to accumulate weakly adsorbed sulfur species at low temperature.The catalyst surface contained more sulfate when SO₂ adsorbed on the catalyst at the high temperature,which had a greater influence on the adsorption of NH₃.The ammonia species on the catalyst surface were first consumed,followed by the generation of sulfur species on the catalyst surface,when SO₂ was adsorbed on a catalyst that pre-adsorbed ammonia.The investigation showed that NO had less influence on the sulfur species generated by the reaction between SO₂ and the catalyst.The control of ABS generation should control ammonia escape,modify the air preheater,change the arrangement of SCR equipment to purify the flue gas for reducing the SO₂ content in the flue gas.And controling the oxygen content,temperature and space velocity control the oxidation efficiency of SO₂ and the amount of SO₃.