Study On Mercury Removal Performance And Preparation Of Co₃O₄ Nanomaterials

Thermal power generation is the main energy source in China at this stage and for a long time to come.However,a large amount of pollutants will be emitted after coal is burned.Among them,mercury will have a huge impact on the human body and the entire ecosystem of the earth due to its high toxicity,volatility and bioaccumulation.There are three forms of mercury in coal-fired flue gas,particulate state?Hg^p?,divalent state(Hg²⁺)and zero valence state?Hg⁰?,where both particulate Hg^p and Hg²⁺can pass through existing exhaust gas treatment devices,such as bag dust device,wet desulfurization device,directly removed However,due to its stable nature,poor water solubility,Hg⁰ is difficult to remove,which is the focus and difficulty of research.At present,an effective method to remove Hg⁰ is to oxidize Hg⁰ to Hg²⁺.As the important active site of Hg⁰ catalytic oxidation reaction,the chemically adsorbed oxygen on the surface can greatly promote the oxidation process of Hg⁰.Therefore,increasing the amount of chemically adsorbed oxygen on the surface may be an effective method to improve the Hg⁰ oxidation efficiency.Metal doping is a common method to increase the chemical adsorption of oxygen on the surface.However,the interaction between the doping is too complicated,resulting in poor controllability of the hybrid doping method.Reducing the particle size is another way to enhance the chemical adsorption of the oxygen surface,which will lead to better Hg⁰ oxidation performance.Unfortunately,nanomaterials with small particle sizes are unstable and prone to agglomeration,especially at high temperatures,resulting in fewer available surface active sites.As a special microstructure,oxygen vacancies can easily absorb gaseous oxygen and form more surface active oxygen.Therefore,the content of oxygen vacancies on the surface of the catalyst is increased,which results in the generation of more chemisorbed oxygen,which in turn improves the Hg⁰ oxidation efficiency.In this paper,low-cost,high-oxidation content Co₃O₄ is used to synthesize nanomaterials with different shapes.The mercury removal performance of nanomaterials with different shapes was tested in a fixed-bed reactor,and the catalyst with the best mercury removal performance was screened for further reduction and modification and the preparation of highly dispersed catalysts.First,Co₃O₄ nanosheets were synthesized by solvothermal method,and two Co₃O₄nanoparticles were prepared by alcohol heating method and precipitation method,respectively.XPS results show that Co₃O₄ nanosheets with?2 2 0?crystal planes have a higher Co³⁺/Co²⁺ratio.DFT calculations confirmed that the?2 2 0?plane has a lower oxygen vacancy formation energy.In addition,SEM and TEM results showed that Co₃O₄ nanosheets produced more defects during the synthesis process.These results are the reason for the large number of oxygen vacancies in Co₃O₄ nanosheets.Therefore,Co₃O₄ nanosheets show the most excellent Hg⁰removal efficiency in a wide temperature range of 100-350°C at a high GHSV of 180,000 h-1.In addition,even after 80 hours of testing,the catalytic efficiency of Co₃O₄ nanosheets is still higher than 83%,and after in-situ heat treatment at 500°C for 2 hours,its Hg⁰ oxidation efficiency can be restored to its original level.The Co₃O₄ nanosheets with the most excellent mercury removal performance in the previous work were further modified.Co₃O₄ nanosheets were reduced by CO gas phase reduction method to increase the content of oxygen vacancies.The effect of different reduction temperature and reduction time on the mercury removal efficiency of the catalyst was studied.It was found that when the reduction temperature was 300°C and the reduction time was 2 h,the Hg⁰ oxidation efficiency was the highest,at 97.5%.And Raman,PL,EPR characterization indicates that Co₃O₄-Te₃₀₀Ti₂ has the best and highest oxygen vacancy content,so it can adsorb gaseous oxygen to form chemically adsorbed oxygen,thereby improving Hg⁰ oxidation efficiency.Finally,in view of the serious stacking phenomenon,low specific surface area,and insufficient exposure of active sites of Co₃O₄ nanosheets,Co₃O₄ was anchored on graphene oxide based on the original loading method and in-situ thermal synthesis.The mercury removal efficiency test was conducted.The results showed that when the loading was 50%and the effective active component weight was 0.02 g,the mercury oxidation efficiency was 92.3%,and fewer active components were used to achieve higher Hg⁰ oxidation efficiency.