Study Of Performance And Synergistic Effect On Cu-based Catalysts For Selective Catalytic Oxidation Of Ammonia

Ammonia,as a typical air pollutant,comes from a wide range of sources.Excessive ammonia emissions bring out the threat to human health and ecological environment.Selective catalytic oxidation(SCO)of NH3 to N2 is one of the most effective technologies for abating NH3.This technology has obvious advantages,including simple operation,mild reaction conditions,less secondary pollution,consequently,it has attracted wide concern in recent years.Cu-based catalysts have potential application value due to high activity in ammonia oxidation at low reaction temperatures.However,the relationship of catalyst texture and synergistic effect of composite metal oxides needs to be clarified.Catalytic performance is difficult to improve owing to lack of efficient method to modulate synergistic effect.The impact of synergistic effect on NH3-SCO mechanism is still ambiguous.In view of the above questions,Cu-Ce-Zr,CuO@CeO2,Cu2O-CuO catalysts are synthesized to investigate synergistic effect between active component and promoter,as well as active component and active component.The catalytic performance of each catalyst is improved by controlling synergistic effect.The role of synergistic effect on activation mechanism is investigated by studying ammonia adsorption-activation process and intermediate-product formation.The main contents and achievements are as follows:(1)A series of Cu-Ce-Zr catalysts were prepared by citric acid sol-gel(SOL),homogeneous precipitation(HP)and incipient wetness impregnation(IW)methods.The influence of synergistic effect between CuO species and promoters on NH3-SCO was systematically investigated.The Cu-Ce-Zr-SOL catalyst had the best catalytic performance(T100%=230?,above 90%N2 selectivity),and the largest amount of high dispersed CuO and Cu-Ce-Zr solid solutions,which was beneficial to enhance NH3 adsorption on Lewis acid sites and NOx generation.On the contrary,the weak NH3 adsorption,a vast of NH3 on Bronsted acid sites and less NOx intermediate species over Cu-Ce-Zr-HP and Cu-Ce-Zr-IW showed remarkably negative effect for catalytic activity.Additionally,it was observed that the surface oxygen species were more active than gaseous oxygen,and it had priority to react with NHx below 190?.Above 190?,the gaseous oxygen enhanced oxidation ability of Cu-Ce-Zr catalysts and facilitated the formation of N2.(2)In order to realize the controllability of synergistic effect between CuO and CeO2,CeO2 were introduced into copper oxides directionally by template-assisted method.The generation process of CeO2 shell was controlled by the local hydrolysis reaction.Finally,the CuO@CeO2 core-shell catalyst was synthesized with a porous structure and an explicit interface between CuO and CeO2 and applied in NH3-SCO.The results showed that the synergistic effect on CuO@CeO2 facilitated formation of Cu-O-Ce solid solutions.The complete core-shell sample(CuO@CeO2-8)possessed the largest number of Cu-O-Ce solid solutions,which showed the highest catalytic performance.The full conversion temperature of NH3 was 240 C and N2 selectivity was above 85%.Additionally,it was found that the formation of Cu-O-Ce solid solution was beneficial to improve the ammonia species adsorption and activation as well as intermediate species NO transformation,which was conductive to achieve high NH3 conversion and N2 selectivity in iSCR route.(3)The uniform cubic cuprous oxide(Cu2O)sample was synthesized by liquid phase reduction method and applied in NH3-SCO.The relationship of Cu2O surface reconstruction and catalytic performance was systematically investigated by the study of Cu2O structure evolution.That result showed Cu2O surface reconstruction and formation of CuO shell were controlled by pretreatment temperature.Finally,the Cu2O-CuO mixing oxide catalyst with structure stability was synthesized.It was found that catalytic performance could be promoted by regulating synergistic effect between Cu2O and CuO.The stably appropriate Cu0-Cu2O mixing oxide(Cu2O-250),which was obtained at pretreatment temperature of 250?,exhibited best catalytic activity(100%NH3 conversion at 210? and above 88%N2 selectivity)and had excellent water resistance ability.(4)Combined with X-ray photoelectron spectroscopy(XPS),ammonia temperature programmed desorption(NH3-TPD),in situ DRIFTs and Density functional theory(DFT)calculation results,the activation process of O2 and NH3 was analyzed over different Cu2O reconstruction catalyst surface(Cu2O,Cu2O-CuO,CuO).It was demonstrated that only O2 molecular was adsorbed on Cu2O catalyst surface.And Cu2O-CuO mixed structure not only had stronger O2 adsorption ability,but also could dissociate O2 into O atom,which was more easily than that of CuO catalyst.What's more,different intermediate species(monodentate,bidentate and bridging nitrate)were observed on different Cu2O reconstruction catalyst surface.The formation of bidentate nitrate species over Cu2O-CuO mixing facilitated iSCR route,improving catalytic activity and N2 selectivity at low temperature range.Differently,the monodentate and bridging nitrate intermediate species were observed over the Cu2O and CuO catalysts,respectively,which resulted in generating a vast of by-product N2O and reducing N2 selectivity.