Study On MnCeTiO_x Catalyst For Hg⁰ Catalytic Oxidation And NH₃ Selective Catalytic Reduction Of NO In Flue Gas

Currently,coal is still the most consumed energy source in our country,and its combustion produces nitrogen oxides（NO_x）,sulfur dioxide（SO₂）,dust,and heavy metals such as mercury,which cause serious air pollution problems.NO_x initiates many climate hazards such as acid rain,photochemical smog,and ozone layer damage.Mercury has highly toxic,migratory,and bioaccumulative,making it one of the major air pollutants of concern.The efficient removal of NO_x and Hg⁰ is one of the focused topics in the current environmental field.At present,the most practical and efficient NO_x removal technology is ammonia selective catalytic reduction（NH₃-SCR）technology,and the core of this technology is the SCR catalyst.Numerous studies presented that SCR catalysts also have the effect of catalytic oxidation of Hg⁰.The SCR catalysts can oxidize Hg⁰ to Hg²⁺.The subsequent pollutant treatment devices can remove Hg²⁺eastos to achieve the removal of Hg⁰.Therefore,it is an efficient and economical method to realize the simultaneous removal of NO_x and Hg⁰ by using the SCR denitration device.However,the existing commercial SCR catalyst has a high thigh-temperaturew.The denitration device is arranged before the desulfurization and dust removal device.The catalyst is easy to be deactivated by SO₂and dust.In addition,the oxidation of Hg⁰ by commercial SCR catalyst is significantly influenced by the concentration of chlorine.Placing the SCR operation unit after the desulfurization and dust removal unit can alleviate SO₂ poisoning considerably.However,it also reduced the temperature（below 200℃）of the gas entering the SCR operation unit.Therefore,the development of environmentally friendly catalysts with excellent NH₃-SCR and Hg⁰ oxidation activity at low temperatures is crucial for the removal of NO_x and Hg⁰ from flue gas simultaneously.Mn-based oxide catalysts have excellent low-temperature denitration activity,but their N₂selectivity and sulfur resistance performance are poor.Ce-based oxides have excellent storage and release oxygen ability.Doping Ce into Mn-based oxides can tune their redox capacity,thereby improving the N₂ selectivity and sulfur poisoning resistance of the catalysts.Ce-modified Mn-based catalysts lack systematic research on NO and Hg⁰ simultaneous removal,and the catalytic mechanism lacks in-depth research.In this thesis,the Ce-modified MnTiO_xcatalyst was taken as the research object.Its Hg⁰ removal activity and its simultaneous NO and Hg⁰ removal activity were respectively studied.According to the characterization and analysis of catalysts,the removal mechanism of NO_x and Hg⁰ on the catalyst was revealed.The main results and conclusions are as follows:（1）Ce-modified MnTiO_x catalysts were prepared by PEG-assisted coprecipitation.The effects of reaction temperature and Ce content on the Hg⁰ removal activity of the catalysts were studied,and catalysts were characterized.With the increase of Ce content,the Hg⁰ removal efficiency of the catalysts first increases and then decreases.Among them,the Mn Ce（0.1）Ti O_xcatalyst exhibits the best Hg⁰ removal performance,and Hg⁰ removal efficiency beyond 95%at 100–300°C and 400,000 h^-1.Characterization analysis results show after doping with Ce,the dispersibility of the catalyst metal oxides,the BET specific surface area,and thesurface-activee oxygen content are all improved,which are beneficial to the Hg⁰ removal of the catalyst.（2）Based on of the research on the Hg⁰ removal activity of the catalyst,taking the Mn Ce（0.1）Ti O_x catalyst as the research object,the influence of the gas composition on the Hg⁰removal activity of the catalyst was investigated,and the mechanism of Hg⁰ removal was revealed.NO and O₂ are beneficial to the Hg⁰ removal,while NH₃ inhibits the Hg⁰ removal.A low concentration of SO₂ can promote the Hg⁰ removal;a high concentration of SO₂ inhibits the Hg⁰ removal.The SCR reaction atmosphere inhibits Hg⁰ oxidation at high space velocity,which can be weakened by reducing the space velocity.Analysis using characterization shows that the Hg⁰ removal over Mn Ce Ti O_x catalyst follows the Mars–Maessen mechanism under N₂+O₂ atmosphere.（3）The performance of Ce-modified MnTiO_x catalyst for simultaneous denitration and Hg⁰ removal was explored,and its mechanism for simultaneous denitration and mercury removal was revealed.With the increase of Ce content,the NO conversion and N₂ selectivity of the catalyst gradually increased,and the Hg⁰ removal efficiency first increased and then decreased.Both SO₂ and H₂O will adversely affect the catalytic activity of the catalyst,the effect of SO₂ is irreversible,and the effect of H₂O is recoverable.The characterization and experimental results show that the denitration reaction over the Mn Ce Ti O_x catalyst follows the Langmuir–Hinshelwood mechanism and the Eley–Rideal mechanism,and the Langmuir–Hinshelwood mechanism is dominant;the Hg⁰ removal reaction follows the Mars–Maessen mechanism.