Preparation And Catalytic Performance Of Co₃O₄ Based Catalysts For N₂O Catalytic Decomposition

N₂O is a micro-constituent of the troposphere and it has a long life-time（110-150 years）.Significant amounts of N₂O are produced from anthropogenic activities,such as agricultural fertilization,industrial waste gas,and automobile exhaust.As a hazardous environmental pollutant,N₂O has become one of the most serious substances to destroy the ozone layer and the third largest greenhouse gas in the world,which has a strong greenhouse effect.Moreover,overall atmospheric N₂O levels continue to grow and are on track to reach twice their current levels by 2050.Therefore,the removal of N₂O is meaningful to protect the human ecological environment.Catalytic decomposition of N₂O into harmless N₂ and O₂ has been lauded from the perspective of simplicity,high efficiency,and sustainability.This method has become one of the most promising and widely studied methods for N₂O removal.Its core lies in the selection of catalytic materials.Co₃O₄ and metal（alkali/alkaline-earth metal,noble metal,rare earth metal,and transition metal）modified Co₃O₄ have been proven to be one of the most promising contenders for de N₂O owing to its simple and easy preparation,excellent catalytic performance,and controllable active sites.However,these catalysts have many shortcomings at present in view of the practical application.For example,the comprehensive utilization of active Co₃O₄ phases of these catalysts was comparably low.Consequently,the specific activities of these catalysts were relatively low.The impurity gases in the actual N₂O emission sources will inhibit the activity of the catalyst,thus its stability and anti-impurity gas performance need to be further improved.There are also problems such as poor mechanical property and high cost.To mitigate the deficiencies mentioned above,one feasible method is Co₃O₄ supporting on an appropriate support,the other feasible method is also the doping modification of Co₃O₄.In this paper,Co₃O₄ based catalysts with excellent catalytic performance,good stability,strong resistance to impurity gases,and high cost-effectiveness were designed and synthesized by selecting appropriate supports and doping nonmetallic promoters.Specifically speaking,tetragonal zirconia and monoclinic zirconia were firstly prepared by the hydrothermal method,then Co₃O₄/Zr O₂ catalysts were obtained by an incipient wetness impregnation method.The effects of zirconia crystal phases on N₂O catalytic decomposition over Co₃O₄/Zr O₂ catalysts were investigated.On this basis,Co₃O₄/g-C₃N₄catalyst was prepared by the impregnation method with g-C₃N₄ as the support and tested for N₂O decomposition.The synergistic coupling effects of Co₃O₄and g-C₃N₄ in N₂O catalytic decomposition was investigated.Furthermore,this special material of g-C₃N₄ was used for the N source to prepare the defective N-doped Co₃O₄ catalysts via a sol-gel process and their catalytic behaviors for de N₂O reaction were evaluated.The effect of N doping on the structure and catalytic performance of Co₃O₄ was investigated.Finally,Co₃O₄/eggshell catalysts used for N₂O decomposition were prepared by using eggshell as the support,which is cost-effective and easy to use.The relationship between preparation method,structure and performance of the catalyst was investigated.The catalysts were systematically characterized by several characterization methods,and the relationship between the structure,texture,electronic properties,surface properties,and Co-O bond of the catalysts and the catalytic performance was explored.The main conclusions can be drawn as follows:1.Catalytic decomposition of N₂O over Co₃O₄ supported on Zr O₂ with different crystal phasesUnder the conditions of 1000 ppm N₂O/Ar with GHSV=10,000 h^-1,N₂O conversion of Co₃O₄/m-Zr O₂ reached nearly 80%at 400°C,while Co₃O₄/t-Zr O₂ decomposed only 51%of N₂O.Although the catalytic activity of Co₃O₄/m-Zr O₂ was similar to that of pristine Co₃O₄,the specific activity of Co₃O₄/m-Zr O₂ was much higher.Co₃O₄/m-Zr O₂ exhibited a stronger anti-contaminants resistance than Co₃O₄/t-Zr O₂.It also displayed a good catalytic stability when contaminants（O₂,H₂O,and/or NO）were added to the feed gas.Therefore,Co₃O₄/m-Zr O₂ is an excellent catalyst used for N₂O decomposition.The characterization results showed that the strong interaction between m-Zr O₂and Co₃O₄ enhanced the structural stability of Co₃O₄/m-Zr O₂,and improved its catalytic activity by enhancing the specific surface area,dispersing the Co₃O₄ nanoparticles,improving the redox property of Co²⁺/Co³⁺,and forming stable Co²⁺,along with increasing surface Co²⁺,surface oxygen vacancies,and surface basicity.2.Catalytic decomposition of N₂O over Co₃O₄ supported on g-C₃N₄Under the conditions of 1000 ppm N₂O/Ar with GHSV=10,000 h^-1,N₂O conversions over Co₃O₄/g-C₃N₄,Co₃O₄,Co₃O₄/m-Zr O₂,Co₃O₄/AC,and Co₃O₄/γ-Al₂O₃ at 400°C were 100,80,77,51,and 2%,respectively.Meanwhile,Co₃O₄/g-C₃N₄ exhibited higher specific activity and had five times the specific activity of Co₃O₄.Furthermore,Co₃O₄/g-C₃N₄ exhibited quite high activity with H₂O,O₂,and/or NO contaminants present in the feed gas.Especially,it showed a strong resistance to NO.The characterization results indicated that the redox ability of Co³⁺/Co²⁺and texture properties of Co₃O₄/g-C₃N₄ were not the key factors for determining its activity for N₂O decomposition.The enhanced catalytic efficiency of Co₃O₄/g-C₃N₄ was attributed to the increased Co²⁺concentration,massive surface oxygen species,basic sites,as well as the synergistic coupling effects among multiple active surface sites,such as Co³⁺/Co²⁺redox couple,Co-N,and nitrogen sites.This study provides a simple and practical strategy for obtaining the supported Co₃O₄ based catalysts with high performance.3.Catalytic decomposition of N₂O over N-doped Co₃O₄Under the conditions of 1000 ppm N₂O/Ar with GHSV=10,000 h^-1,pristine Co₃O₄could completely convert N₂O at 560°C.When N atoms were doped into the Co₃O₄,N-Co₃O₄ catalyst could completely convert N₂O at 380°C.Furthermore,the N-Co₃O₄ catalyst showed high catalytic activity,stability,and resistance to impurity gases under O₂,NO,and/or H₂O containing reaction conditions.The characterization results evidenced that N atoms could be incorporated into the cobalt oxide lattice by the substitution of lattice oxygen.N doping facilitated the creation of oxygen vacancy,surface Co²⁺,and basic sites on the catalyst.Furthermore,introduction of N improved the electron donation ability of Co²⁺,enlarged the specific surface area,enhanced the redox property of Co₃O₄,and weakened Co-O bond.All these factors endowed the N-Co₃O₄ catalyst with a lower Ea for decomposition of N₂O.This study confirmed the feasibility of Co₃O₄ modified by nonmetal N for N₂O catalytic decomposition.4.Catalytic decomposition of N₂O over Co₃O₄ supported on eggshellUnder the conditions of 1000 ppm N₂O/Ar with GHSV=10,000 h^-1,the Co₃O₄/eggshell catalyst prepared by deposition precipitation method could decompose N₂O completely at 400°C,while the Co₃O₄/eggshell with the same Co₃O₄ content prepared by the impregnation method and grinding method had only～10%N₂O conversion.The catalytic activity of Co₃O₄/eggshell prepared by deposition precipitation method was similar to that of pristine Co₃O₄catalyst.Compared with the pristine Co₃O₄ catalyst,the Co₃O₄/eggshell catalyst showed higher catalytic activity,stability,and resistance to impurity gases under impurity gases（O₂,NO,and/or H₂O）containing reaction conditions.The characterization results showed that compared with the pristine Co₃O₄ catalyst,Ca element in eggshell could effectively weaken the Co-O bond of Co₃O₄/eggshell catalyst,improve its redox property,and increase the strength and number of surface basic sites.It is easy to transfer electrons and activate N₂O molecular.In addition,Co₃O₄/eggshell had more oxygen vacancies than the pristine Co₃O₄.For all these reasons,Co₃O₄/eggshell catalyst showed the similar catalytic activity,high stability and resistance to impurity gases to pristine Co₃O₄ catalyst.Therefore,eggshell as the support of supported Co₃O₄ based catalyst can not only effectively improve the utilization of active components and reduce the preparation cost,but also realize the recycling of waste and decrease environmental pollution.