Study On The Surface Structure Modulation Of Copper-based Supported Catalyst And Its Effect On NO+CO Reaction

With the continuous increase of car parc,mobile source has become the major contributor to urban air pollution.NO+CO is an important model reaction in NO_xelimination of motor vehicle exhausts.NO reduction by CO has attracted much attention of the researchers due to the simultaneously convert NO and CO to non-toxic N₂ and CO₂.CuO,which is generally used as active compoment,has excellent redox properties and ability to adsorb gas molecules,and exhibits superior performance in NO+CO reaction.In order to further explore the mechanism of the catalytic reaction and improve the low-temperature catalytic performance of the catalyst,a series of research has been carried out mainly on adjusting the dispersion state of Cu species on the catalyst surface,CO pretreatment temperature and concentration of catalytic active species.The specific research contents are as follows:?1?CuO/SiO₂ catalysts with vast difference in copper dispersion were prepared by impregnation?denoted as Cu Si-IM?and ammonia-evaporation?denoted as Cu Si-AE?methods and tested for NO reduction by CO.Despite the inferior dispersion of copper species in Cu Si-IM catalysts,they exhibited significantly higher NO conversion efficiency,with more than one order of activity enhancement achieved in comparison with Cu Si-AE samples.Ex situ XRD,in situ DRIFTS,and CO-TPR characterizations were carried out to study the nature of active copper species and their evolution under reaction conditions.A clear dispersion-dependent reduction behavior?for Cu Si-AE,step-wise reduction;for Cu Si-IM,one step reduction?under CO atmosphere was observed,and the generation of Cu⁰ preferentially occurred at lower temperatures for Cu Si-IM samples,which accounted for the better performance in NO+CO reaction.Lastly,NO-TPD and TPSR experiments were operated and the essential role of Cu⁰ in promoting NO dissociation was proposed.The results of present study would deepen our understanding on the reaction behaviors of copper catalysts in NO+CO reaction.?2?Based on the above research,we continued to investigate the effect of CO pretreatment?temperature modulation?on the NO+CO catalytic performance of CuO/?-Al₂O₃?Cu/Al?and CuO/Mn₂O₃/?-Al₂O₃?Cu/Mn/Al?cayalysts prepared by impregnation method.Through XRD,XPS,H₂-TPR,CO-TPR,and in situ DRIFTS characterization,we found that for CuO/?-Al₂O₃ catalyst,dispersed Cu²⁺was the predominant species.After CO pretreatment at 300 ^oC and 400 ^oC,Cu⁺and Cu⁰ were predominant species in the samples,respectively.The activity order of the three catalysts was:Cu/Al-400>Cu/Al-300>Cu/Al.However,for CuO/Mn₂O₃/?-Al₂O₃catalyst,Cu and Mn species mainly existed as Cu²⁺and Mn³⁺in the sample without CO pretreatment.After CO pretreatment at 300 ^oC,the sample mainly contained Cu⁺and Mn²⁺species.When the temperature of CO pretreatment increased to 400 ^oC,Cu⁰ and Mn²⁺were the main compoments in the sample.The activity order of the three catalysts was:Cu/Mn/Al-300>Cu/Mn/Al-400>Cu/Mn/Al.After analyzing the NO adsorption and dissociation ability of the sample by NO-TPD,it was found that Cu⁰ was the active center of catalytic reaction in CuO/?-Al₂O₃ catalyst,and Cu-?-Mn species was the active site of catalytic reaction in CuO/Mn₂O₃/?-Al₂O₃ catalyst.Kinetic tests and capture of intermediate species in the reaction by in situ DRIFTS showed that the two catalysts have different reaction mechanisms.?3?NO reduction by CO was further investigated over CO-pretreated CuO/Mn O_x/?-Al₂O₃ catalysts with different metal precursors?nitrate and acetate?in this part.It was found that the catalyst prepared from acetate salts?Cu/Mn/Al-A?exhibited significantly higher activity than counterpart catalyst from nitrate precursors?Cu/Mn/Al-N?.XRD,XPS and in situ DRIFT were carried out to approach the nature for the different catalytic performance.For both catalysts,copper mainly existed as CuO,but the status of manganese oxide was markedly different.Mn?IV?was predominant in Cu/Mn/Al-N and Mn?III?was enriched in Cu/Mn/Al-A.As a result,different dispersion behaviors of manganese oxide on?-Al₂O₃ were displayed,which induced inconsistent Cu-Mn contact.The catalyst obtained from acetate precursor exhibited enriched Cu-Mn contact and thus more Cu⁺-?-Mn^3+/2+entities would be produced after CO pretreatment,leading to promoted NO dissociation and favorable performance in NO reduction by CO.The present study sheds light on the effective tuning of Cu-O-Mn interfacial sites in CuO/Mn O_x/?-Al₂O₃ via modulating the dispersion behaviors of surface components.