Preparation Of Core-shell CuO@siO₂ Catalyst And Study On Oxidation Of AsH₃

There is a large amount of CO(>85%)in the tail gas of ore furnace,which can be used as a C1 chemical raw material gas.Meanwhile,the resource utilization of CO in the tail gas of ore furnace can also reduce a large amount of CO₂ emissions,which is conducive to the achievement of the goal of“Peak Carbon Dioxide Emissions”and“Carbon Neutrality”.However,there are also a variety of impurity gases including hydrogen arsenide(AsH₃)in the tail gas of ore furnace,which not only seriously hinder the utilization of CO resources,but also cause the corrosion of pipelines and equipment and the poisoning of catalysts.Hence,the research on the high efficiency purification of AsH₃ in the tail gas of ore furnace has been important and practically significant.Although there has been some progress in research on catalytic oxidation of AsH₃ under low temperature(<200?)and low concentration oxygen(0.5%-1.5%)condition,there are still some key problems to be solved:conventional active components load cannot make them evenly distributed and it is difficult to make full use of the target function of each component;the residual impurities in the tail gas of ore furnace also pose a challenge to the anti-toxicity of catalysts.In addition,the structure-activity relationship and catalytic reaction mechanism of catalysts also need to be further studied.Core-shell catalyst is composed of central particle and coating layer,which can integrate the properties of both shell and core components.The dispersibility,stability and toxicity resistance of the active components in the core layer can be improved effectively through the protection and confinement of the shell,and the functions of each component can be fully played.Hence,this study uses CuO@SiO₂ core-shell material as catalyst and utilizes the characteristics core-shell structure,the catalytic oxidation purification of AsH₃ in the tail gas of ore furnace under the low temperature and low concentration-oxygen condition and its mechanism were studied.The specific research contents and conclusions are as follows:(1)The CuO@SiO₂ core-shell catalyst was prepared by self-assembly method using tetraethyl orthosilicate(TEOS)as silicon source and copper nitrate(Cu(NO₃)₂·3H₂O)as precursor of CuO.Compared with the traditional supported CuO/SiO₂ catalyst,the dispersion of the active component CuO in core-shell structure is significantly better than that of supported CuO/SiO₂ catalyst due to the limited effect of SiO₂shell.In addition,due to the large contact area between SiO₂shell and CuO core,there is a strong interaction,which is mainly reflected in the coordination unsaturated Cu²⁺and activated reactive oxygen species generated by electron transfer,as well as the improvement of redox performance and acid and base sites of the catalyst.As a result,the oxidation activity of CuO@SiO₂ core-shell catalyst is significantly higher than that of supported CuO/SiO₂ catalyst.(2)The effects of several preparation conditions(hydrothermal temperature,mass ratio of PVP to copper nitrate,TEOS content,ammonia concentration and surfactant CTAB concentration)on the catalytic oxidation of AsH₃ were investigated.It was found that under the optimal conditions(hydrothermal temperature was 160?,mass ratio of PVP to copper nitrate was 1:1,TEOS content was 10 m L,ammonia concentration was0.1 mol/L and without surfactant CTAB),the CuO dispersion and mesoporous structure of CuO@SiO₂ core shell catalyst were better.The number and strength of acid and base sites were also suitable for catalytic reaction of AsH₃.The strong interaction between the core layer and shell layer leads to electron transfer,resulting in more highly active coordination unsaturated Cu²⁺and reactive oxygen species at Si-O-Cu interface,which show excellent activity in catalytic oxidation of AsH₃.(3)In order to provide some groundwork for the industrial application of the CuO@SiO₂core shell catalyst,the reaction conditions(reaction temperature,oxygen content and gas hourly space velocity)of catalytic oxidation of AsH₃ and the influence of coexistence gases(CO,H₂S,PH₃,COS and CS₂)were studied.The results indicated that the efficiency of AsH₃ oxidation over CuO@SiO₂ catalyst increases with the increase of reaction temperature.The products are also affected by the reaction temperature,the main products are As₂O₃,but low temperature(?100?)is conducive to the formation of As,high temperature(?140?)is conducive to the formation of As₂O₅.The increase of oxygen content(0?1%)could promote the catalytic oxidation reaction,excessive oxygen content(>1%)would cause a rapid accumulation of reaction products on the catalyst,thereby reducing its catalytic activity.The CuO@SiO₂ core shell catalyst has good removal performance of AsH₃ in high concentration CO system under low temperature low concentration oxygen condition.Coexistence gases H₂S,PH₃and CS₂ could occupy a part of the adsorption site/active sites in different degrees,reducing the removal efficiency of AsH₃.However,the CuO@SiO₂ catalyst has good selectivity for AsH₃ gas in the presence of coexistence gas COS.(4)The deactivation mechanism of CuO@SiO₂ core shell catalyst for AsH₃oxidation was studied.It was found that the depletion of coordination unsaturated Cu²⁺and chemisorbed oxygen,and the blocking of the As₂O₃ and As₂O₅ products are the main deactivation mechanisms of CuO@SiO₂ catalyst.Based on the above mechanism,three regeneration methods including water regeneration,alkali washing regeneration and nitrogen purge heating regeneration and their regeneration processes were studied.The results showed that the efficiency of water regeneration was low because the reaction products could not be completely removed.The regeneration efficiency of nitrogen purge heating at 500?reached 100%.During the regeneration process,the coordination unsaturated Cu²⁺and chemisorbed oxygen on the catalyst could be restored,and the products were completely removed.The regenerated catalyst with different concentration of alkali washing had higher activity than fresh catalyst,which is due to the removal of the products,the increase in the amount of AsH₃ adsorption sites(Si-OH)and the improvement of alkaline sites.In addition,through the study of alkali washing regeneration,the method of alkali modification of CuO@SiO₂ catalyst was found and the improvement of activity was also attributed to the increase of Si-OH group and the improvement of alkaline sites.In the meantime,the reusable performance of CuO@SiO₂catalyst was outstanding.Based on the modification effect of alkali washing,CuO@SiO₂ catalyst has good reusability.After 5 times of alkaline washing,the activity of catalyst is still up to 172%of that of fresh catalyst.(5)The mechanism of catalytic oxidation of AsH₃ over CuO@SiO₂ core shell catalyst was studied by molecular simulation.A heterojunction structure was constructed on?-cristobalite(001)and CuO(111)surfaces through diffuser to simulate the CuO@SiO₂ core shell catalyst.The charge transfer occurs only between layers,and the oxidation reaction of AsH₃ on the heterojunction can be regarded as the reaction on CuO(111)surface.The optimal adsorption site of AsH₃ on core shell catalyst was O active site of CuO,and the adsorption energy was-1.85 e V.The reaction path of catalytic oxidation takes place on the CuO(111)surface,and the whole process is divided into step-by-step dehydrogenation(AsH₃*?AsH₂*?AsH*?As*)and step-by-step oxidation(2As*?As*+As O*?2As O*?As₂O₃).The ration-limiting step of the whole process is the breaking of the bonds of the O₂ molecule on the CuO(111)surface.The proposed mechanism lays a theoretical foundation for the application of deep purification of AsH₃ in ore furnace gas.