Mechanism Of Gaseous Elemental Mercury Oxidation Over CuO/Fe-Ti Spinel

The problem of mercury pollution has aroused widespread concern in countries all over the world.China is the world’s largest mercury emitter.With the official entry into force of the Minamata Convention,China is facing a severe mercury emission reduction situation.Coal-fired power plants are an important source of man-made mercury emissions in China,so it is of great significance to control mercury pollution in coal-fired power plants.There are three forms of mercury in coal-fired flue gas:elemental mercury（Hg⁰）,oxidized mercury(Hg²⁺)and particulate mercury（Hg^p）.Because Hg⁰ has high volatility and low water solubility,it is difficult to remove by existing flue gas purification devices in coal-fired power plants.Therefore,the core of mercury pollution control in coal-fired power plants is to control Hg⁰ emissions.The use of selective catalytic reduction（SCR）catalysts to catalytically oxidize gaseous Hg⁰ in flue gas to Hg²⁺which is easily soluble in water is currently the most effective method for controlling elemental mercury pollution in coal-fired flue gas.However,the performance of commercial V₂O₅-WO₃/Ti O₂ catalysts in oxidizing gaseous Hg⁰ is average,and the flue gas components such as SO₂,NH₃,NO and H₂O seriously interfere with their performance in oxidizing gaseous Hg⁰.Therefore,the development of SCR catalysts with excellent Hg⁰oxidation performance is the current research focus of Hg⁰ pollution control in coal-fired power plants.Our previous research found that Fe-Ti spinel has excellent SCR activity,but its performance in oxidizing Hg⁰ is mediocre.Our previous research also found that CuO exhibits excellent Hg⁰ oxidation activity.Therefore,this study tried to load CuO on Fe-Ti spinel,and under the premise of ensuring the excellent SCR activity of Fe-Ti spinel,further improve its Hg⁰ oxidation performance,and developed CuO/Fe-Ti spinel that can achieve high-efficiency denitration and mercury removal SCR catalyst.At the same time,the oxidation mechanism of gaseous Hg⁰ on the surface of CuO/Fe-Ti spinel was deeply explored by means of steady-state kinetics and transient response.On this basis,it further revealed the promotion mechanism of CuO on the oxidation of gaseous Hg⁰ on the surface of Fe-Ti spinel and the effect mechanism of SO₂ on the oxidation of gaseous Hg⁰ on the surface of CuO/Fe-Ti spinel,which laid a theoretical foundation for the subsequent development of SCR catalysts with excellent Hg⁰ oxidation performance and SO₂ resistance.The main research results of this article are as follows:（1）CuO/Fe-Ti spinel has excellent SCR activity,its NO_x conversion rate reaches more than 90%at 300-400 ^oC,and the N₂O selectivity is less than 6%.At the same time,CuO/Fe-Ti spinel exhibits excellent Hg⁰ oxidation activity,and its oxidation rate of Hg⁰ reaches 6.5-8.7μg/g min at 250-450 ^oC,which is much higher than Fe-Ti spinel and V₂O₅-WO₃/Ti O₂ and other SCR catalysts.The adsorption rate of gaseous Hg⁰ on the CuO/Fe-Ti spinel surface is much smaller than the oxidation rate,so the Mars-Maessen mechanism is not dominant.The rate of CuO/Fe-Ti spinel oxidation of Hg⁰ with respect to the gaseous Hg⁰ concentration is less than 1,which is contrary to the Eley-Rideal mechanism and the Deacon mechanism.At the same time,when different concentrations of Hg⁰ are introduced into the CuO/Fe-Ti spinel pre-adsorbed HCl+O₂,the downward trend of the Hg²⁺concentration at the outlet is inconsistent,which is also contrary to the Deacon mechanism.Therefore,the oxidation of gaseous Hg⁰ on the surface of CuO/Fe-Ti spinel mainly follows the Langmuir-Hinshelwood mechanism（the adsorbed Hg⁰ reacts with active chlorine radicals to generate mercury chloride）.Although CuO slightly inhibits the physical adsorption of gaseous Hg⁰ on the surface of Fe-Ti spinel,it significantly promotes the generation of active chlorine radicals on the surface of Fe-Ti spinel,thereby greatly improving the Hg⁰ oxidation performance of Fe-Ti spinel.（2）SO₂ significantly inhibits CuO/Fe-Ti spinel from oxidizing Hg⁰,its Hg⁰ oxidation rate dropped sharply from 6.5-8.7μg/g min to 3.5-4.1μg/g min.SO₂ sulfated the surface of CuO/Fe-Ti spinel and increased the physical adsorption sites of Hg⁰ on the surface,but high concentrations of SO₂ would compete with Hg⁰ for physical adsorption sites.Therefore,SO₂significantly inhibits the physical adsorption of Hg⁰.At the same time,SO₂ sulfated the CuO/Fe-Ti spinel surface to reduce the physical adsorption sites of HCl on the surface,and SO₂ would compete with HCl for physical adsorption sites.Therefore,SO₂ severely inhibits the physical adsorption of HCl.In addition,SO₂ significantly reduces the redox ability of CuO/Fe-Ti spinel after sulfating the surface of CuO/Fe-Ti spinel,thereby inhibiting the activation of chloride ions.In addition,SO₂ will also react with the active chlorine free radicals on the CuO/Fe-Ti spinel surface to further reduce the active chlorine free radicals on the surface.Therefore,SO₂ significantly inhibits the oxidation of gaseous Hg⁰ on the CuO/Fe-Ti spinel surface.