Effect Of Y₂O₃ Promoter On The Structure Of Co₃O₄ And Catalytic Performance For N₂O Decomposition

N₂O can be used as a combustion improver,rocket oxidizer and anesthetic.It has long been considered as an environmentally friendly gas.In recent years,with the deepening of understanding,it has been found that N₂ O not only destroys the ozone layer,but also has a significant greenhouse effect,and its global warming potential is 310 times and 21 times higher than that of CO₂ and CH4,respectively.On the basis of the Kyoto Protocol and the Paris Agreement concerning greenhouse gas emission limitation documents,the State Council's “13th Five-Year Plan” to control greenhouse gas emissions in2016 emphasized that the non-CO₂ greenhouse gases such as N₂ O should be further enhanced.At present,the concentration of N₂ O in the atmosphere is rising at a rate of 0.2%⁰.3% per year.Therefore,the controlled emission and elimination of N₂ O have become a serious problem that countries need to face together.For N₂ O elimination,the direct catalytic decomposition into N₂ and O₂ is the most efficient method,and the key is to choose the suitable catalyst.The N₂ O decomposition catalysts include noble metals,ion-exchanged zeolites and metal oxides.Among them,metal oxide catalysts are favored by researchers because of their simple preparation method,easy structure control,relatively good catalytic activity and low price.As the most commonly used spinel oxide Co₃O₄ catalyst,its catalytic activity for N₂ O decomposition can be significantly improved by doping alkali metals?K,Na?,alkaline earth metals?Ca,Mg?or transition metals?Ti,Ni?.Y₂O₃ is often used as support or promoter for CO oxidation and methane oxidation reactions due to its high stability and basicity,and has a positive impact on the catalytic system.However,the effect of adding Y₂O₃ on the structure and catalytic performance of Co₃O₄ for N₂ O decomposition has not been reported so far.Using Co?NO₃?₂·6H₂O aqueous solution and Y₂O₃ nitric acid solution as cobalt source and yttrium source,the Co₃O₄ and Y₂O₃-Co₃O₄ catalysts wereprepared by the precipitation and coprecipitation methods,respectively.The catalytic decomposition of N₂ O was used as a model reaction.The catalytic performance for N₂ O decomposition was tested on the fixed-bed continuous flow reactor.The catalysts were characterized by N₂-physisorption,XRD,TEM,Raman,FT-IR,H₂-TPR,XPS,O₂-TPD and CO₂-TPD.The effect of Y₂O₃ addition on the structure,texture,surface properties and the catalytic performance for N₂ O decomposition was also investigated.On this basis,the reduction-oxidation pretreatment of Y₂O₃-Co₃O₄ catalyst was carried out to explore the influence of reduction-oxidation pretreatment on the structure and catalytic performance of the catalyst.The main conclusions obtained were as follows:1.Effect of Y₂O₃ on the structure of Co₃O₄ catalyst and catalytic performance for N₂ O decompositionUnder the conditions of space velocity of 10000 h^-1 and 0.1% N₂O/Ar,the Y₂O₃-Co₃O₄ catalyst can achieve 100% N₂ O conversion at 360 ?,which is lower than that of Co₃O₄?400 ??.When 2 vol% O₂ or 2.3 vol% H₂ O is introduced into the feed gas,the catalytic activity of Y₂O₃-Co₃O₄ is still higher than that of Co₃O₄.The characterization results show that there is an interaction between Y and Co oxides at the Y₂O₃/Co₃O₄ interface,which lead to the smaller average particle size of Y₂O₃-Co₃O₄,larger BET surface area and the enhanced surface basic property.At the same time,Y₂O₃-Co₃O₄ shows stronger Co2+ reduction ability and desorption ability of surface oxygen species,so the Y₂O₃-Co₃O₄ catalyst exhibits better N₂ O catalytic decomposition activity.2.Effect of reduction-oxidation pretreatment on the structure of Y₂O₃-Co₃O₄ catalyst and catalytic performance for N₂ O decompositionWhen the composition of the reaction raw material gas is 0.1% N₂O/Ar and the GHSV is 10000 h^-1,compared with Y₂O₃-Co₃O₄,the catalytic activity for N₂ O decomposition of Y₂O₃-Co₃O₄-RO catalyst is further improved,which can reach 100% N₂ O conversion at 320 ?,lower than that of Y₂O₃-Co₃O₄?360 ??.The Y₂O₃-Co₃O₄-RO still exhibits better catalyticactivity than Y₂O₃-Co₃O₄ when 2 vol% O₂ or 2.3 vol% H₂ O is introduced into the feed gas.The characterization results show that the crystallinity and the crystallite size of Co₃O₄-RO decrease in comparison with Co₃O₄.Especially the structural reconstruction resulted from the reduction-oxidation pretreatment weakens the Co-O bond and enhances the oxygen desorption capacity on the catalyst surface,which may contributes to the excellent catalytic performance of Co₃O₄-RO for N₂ O decomposition.