Influence Of Cl^- Modification On Structure And Catalytic Performance Of Ag/Al₂O₃ Catalysts For C₃H₆-SCR And H₂/C₃H₆-SCR Reactions

The selective catalytic reduction technology(HC-SCR)using hydrocarbons as the reducing agent is currently a widely studied technology for removing NOx from diesel vehicle exhaust.A large number of studies have shown that the Ag/Al2O3 catalyst exhibits high activity,high N2 selectivity and good catalyst stability in the HC-SCR reaction,so it may become a highly efficient HC-SCR catalyst that can be practically applied.The catalytic performance of Ag/Al2O3 catalyst in HC-SCR is closely related to the loading of Ag,and the literature reports that there is an optimal Ag doping amount of about 1%-2%.In this paper,the γ-Al2O3 before and after modification with NH4Cl was used as the carrier,and the Ag/Al2O3 and Ag/Al2O3-Cl catalysts were prepared by the impregnation method.Various structural characterizations of the catalyst were carried out,mainly including BET,EPMA,XRD,STEM/HRTEM,H2-TPR,UV-Vis DRS and XPS,and combined with the C3H6-SCR and H2/C3H6-SCR activity test,the structure-activity relationship between Ag species and catalyst activity in the HC-SCR reaction was established.In addition,this paper combines in-situ Fourier transform infrared spectroscopy(in-situ DRIFTS)and single photon ionization mass spectrometry(SVUV-PIMS)characterization to clarify the reaction mechanisms of Ag/Al2O3-catalyzed HC-SCR reaction of NOx.Structural characterization and activity test results show that the Cl-modified performance promotes the formation of silver oxide species and improves the SCR catalytic performance of the Ag/Al2O3 catalyst,especially for samples with high Ag loading.In-situ infrared spectroscopy studies have shown that γ-Al2O3 carrier(Al site)is the main active site for NO oxidation to form nitrate;silver oxide species(Agnδ+)will moderately oxidize propylene and mainly produce acetate on the surface,while metallic Silver species(Agn0)will over-oxidize propylene to mainly generate carboxylates on the surface;the promotion of H2 on the low-temperature activity of C3H6-SCR comes from the promotion of the desorption or/and dissociation of strongly adsorbed nitrate species and the conversion of intermediate to-NCO and-CN species.The gaseous intermediate acrylonitrile(CH2=CHCN)was identified in the H2/C3H6-SCR reaction using single photon ionization mass spectrometry.For the HC-SCR reaction,the silver oxide species(Agnδ+)is considered to be the active site of the SCR reaction,while the metallic silver species(Agn0)will cause the non-selective oxidation of hydrocarbons,which is unfavorable for the HC-SCR reaction.The Cl-modification of Al2O3 support can effectively adjust the existence form of silver species,promote the formation of silver oxide species,and then improve the catalytic reaction activity of HC-SCR.H2 promotes the low-temperature activity of HC-SCR,and this promotion is achieved by acting on silver oxide species(not metallic silver species).The-CN and-NCO species are regarded as the key intermediates of the HC-SCR reaction,and it is proposed that they directly react with NO+O2 in the gas phase to generate N2,which indicates that the gas phase reaction is involved in the HC-SCR reaction process.The results of SVUV-PIMS identified the gaseous intermediate acrylonitrile,which provided direct experimental evidence support for the HC-SCR gas phase reaction mechanism.These findings provide novel insights into the structure-activity relationship and reaction mechanism of the HC-SCR reaction of NO catalyzed by Ag/Al2O3.