Different Apatite Supported Co₃O₄ Catalyzed Direct Decomposition Of N₂O

Ozone layer depletion and climate warming are global environmental problems that need to be solved urgently in the 21st century.N₂O is one of the culprits of these two problems.At present,the global N₂O emissions generated by human activities such as manufacturing,agriculture,industry and aerospace are increasing rapidly,posing a potential threat to the human living environment.Therefore,under the premise of ensuring high-quality economic and social development,it is urgent to develop effective technologies to reduce the N₂O content in the atmosphere,to protect the ozone layer and control the greenhouse effect.Among the many methods to eliminate N₂O,the direct catalytic decomposition of N₂O into N₂ and O₂ is one of the most promising and economical technologies.In recent years,metal oxide catalysts have been widely studied for the N₂O catalytic decomposition.Among them,Co₃O₄ spinel exhibits excellent catalytic performance during the direct decomposition of N₂O due to its unique crystal structure,excellent thermal stability and redox characteristics.However,the single Co₃O₄ catalyst is easy to be sintered during the calcination and reaction process,which results in the decrease of catalytic activity.Although the doped Co₃O₄ catalysts exhibit high catalytic activity and thermal stability due to the interaction between Co₃O₄ and doped oxides,the activation of N₂O molecules usually occurs at the gas-solid interface,which leads to the underutilization of the active phase of spinel Co₃O₄ in the catalysts.As we all know,the carrier can promote the dispersion,stability and efficient utilization of active components,and the proper texture and acidity of the carrier also have an important influence on the N₂O catalytic decomposition.Hydroxyapatite Ca₁₀（PO₄）₆（OH）₂（HAP）is widely used as a catalyst material due to its high stability,adjustable acidity and alkalinity,low cost,easy ion exchange and good recyclability.As we all know,Ca²⁺in HAP can be replaced by cations such as Zn²⁺,Mg²⁺,Sr²⁺,La²⁺,Cd²⁺,Pb²⁺,Cu²⁺and Fe²⁺,and OH-can be replaced by anions such as F^-and Cl^-,which can further adjust the thermal stability,phase stability,specific surface area and surface reactivity of HAP,thereby effectively improving the activity of various catalytic reactions.However,different apatite-supported Co₃O₄catalysts used for N₂O catalytic decomposition have rarely been reported.In this paper,calcium nitrate,diammonium hydrogen phosphate and ammonium fluoride were used as raw materials,and ammonia was used as precipitating agent,and two types of apatite including hydroxyapatite（HAP）and fluoroapatite（FAP）were prepared as carriers by co-precipitation method,and Co/HAP and Co/FAP catalysts were prepared by impregnation method,and finally investigated the N₂O catalytic performance.On this basis,Sr（NO₃）₂ is used as raw material,doped with hydroxyapatite to prepare Ca₉Sr₁,Ca₈Sr₂ and Ca₇Sr₃ carriers,and then Co/Ca₉Sr₁,Co/Ca₈Sr₂,Co/Ca₇Sr₃ catalysts are prepared by impregnation method for N₂O direct catalytic decomposition.In addition,the same method was used to prepare Co/HAP catalyst for comparison.The prepared samples were characterized by XRD,N₂-physisorption,Raman,FT-IR,H₂-TPR,XPS,O₂-TPD and CO₂-TPD.And to elucidate the influence of carriers’different anions and cations on the structure and catalytic performance of the prepared catalyst.The main conclusions obtained are as follows:1.N₂O catalytic performance of different apatite supported Co₃O₄When the feed gas composition is 0.1%N₂O and Ar,and the GHSV is10000 h^-1,the N₂O conversion rate of the Co/HAP catalyst reaches 100%at580℃,while the N₂O conversion rate of the Co/FAP catalyst is only 94%at 600℃.When 3.0 vol%O₂,3.3 vol%H₂O and 200 ppm NO were introduced into the feed gas,the catalytic activity of Co/HAP was still significantly higher than that of the Co/FAP catalyst.The characterization results show that the Co/HAP catalyst has higher specific surface area and pore volume,higher Co₃O₄ dispersion,and smaller Co₃O₄ grain size,and the Co oxide in Co/HAP has a stronger interaction with HAP.In addition,the weaker Co³⁺-O bond,more base content and higher basic site density in Co/HAP may be beneficial to the catalytic performance of De-N₂O.More importantly,More importantly,because of the strong electronegativity of F^-,the Ca-F bond in FAP is stronger than the Ca-OH bond in HAP.Therefore,the Ca²⁺in HAP can be more easily exchanged with Co²⁺,so that there is more ion exchange Co²⁺in Co/HAP,which is also one of the key factors for its higher N₂O decomposition activity.2.Sr-doped hydroxyapatite supported Co₃O₄ for N₂O decompositionWhen the feed gas composition is 0.1%N₂O and Ar,and the GHSV is10000 h^-1,the N₂O conversion rate of Co/HAP reaches 100%at 580℃.After Sr doping,the complete N₂O conversion temperature of Co/Ca₉Sr₁ and Co/Ca₈Sr₂ decreased to 540℃and 520℃,respectively,while the catalytic performance of Co/Ca₇Sr₃ was significantly reduced,and N₂O was not completely converted at 600℃.In addition,when the feed gas contains3.0 vol.%O₂,200 ppm NO or 3.3 vol.%H₂O,the N₂O catalytic activity of the Co/Ca₈Sr₂ catalyst is not significantly affected.The characterization results show that,compared with other catalysts,Co/Ca₈Sr₂ not only maintains the structure of HAP carrier well,but also has higher Co₃O₄dispersion and smaller Co₃O₄grain size.In addition,with the increase of Sr doping,the number of Co²⁺and surface oxygen vacancies in the catalyst both increase,and the number of ion-exchanged Co²⁺in Co/Ca₈Sr₂ is the largest,and oxygen species are more easily desorbed.At the same time,the surface base content and basic site density of the Co/Ca₈Sr₂ catalyst are also significantly increased,which plays an important role in the N₂O catalytic decomposition.