Study On The Preparation Of Wide Temperature C₃H₆-SCR Molecular Sieve Catalysts And Its Structure-Performances Relationship

Nitrogen oxides are among the major air pollutants emitted by motor vehicles.Therefore,NO_X removal is a research hotspot in the field of air pollution treatment.Many methods have been used to eliminate NO_X.Currently,the selective catalytic reduction of NO_X using C₃H₆ as a reducing agent?C₃H₆-SCR?is one of the effective methods to remove NO_X from vehicle exhaust.The key issue in NO_X removal in C₃H₆-SCR is to obtain a catalyst with low temperature activity and wide temperature range.So far,many researchers have prepared different supported wide temperature active molecular sieve catalysts and used them in C₃H₆-SCR reactions.Studies have shown that the catalytic performance of supported catalysts is not only related to the active components,but also to the structure and properties of the support.At present,the problems of low-temperature catalyst denitration activity and narrow working temperature range need to be solved urgently.Therefore,this article is aimed at simulating automobile exhaust with Co as the active component,the Co-based molecular sieve catalysts were prepared using low-temperature plasma technology.By optimizing the formulation of the catalyst,the aim is to design and synthesize Co-based catalysts with excellent low-temperature catalytic activity and wide temperature range.In this paper,the simulation of automobile exhaust gas is the research object,and different Co-based molecular sieve catalysts prepared by low-temperature plasma are developed.The effects of different preparation methods,such as impregnation method,sol-gel method and low-temperature plasma treatment combined roasting,on the low-temperature denitration activity and operating temperature window of Co-based molecular sieve catalysts,XRD,BET,TG-DTA,TPR,TPD and other physical technologies were used to characterize the structure and physical and chemical properties of the catalyst,and the specific contents of the investigation are as follows:?1?Molecular sieve catalysts doped with different contents,different active components and different auxiliaries were prepared by the impregnation method.The denitration activity of these catalysts in the C₃H₆-SCR reaction was investigated.The Co-Ce-Zr-ZSM-5 molecular sieve catalyst with optimal catalytic activity was screened out and analyzed by various characterization tests.The results show that the 5%Co-2%Ce-1.2%Zr-ZSM-5-p molecular sieve catalyst exhibits the best catalytic performance.The highest NO conversion rate at a temperature of 250?is 96%,and the operating temperature window T₅₀=185-375?.For the optimal catalyst 5%Co-2%Ce-1.2%Zr-ZSM-5-p,XRD analysis results showed that Co,Ce,and Zr in the molecular sieve catalyst are in a smaller crystalline state,and they are mainly composed of Co₂O₃ oxides and Co₃O₄ spinel crystal form,CeO₂ cubic crystal,and ZrO₂ monoclinic system are well dispersed on the surface of the molecular sieve catalyst.BET characterization analysis showed that the doped metal enlarged its pore structure,which would make the reaction gas better perform C₃H₆-SCR reaction on Co-based catalyst.The results of NH₃-TPD analysis showed that the amount of weak acids on the surface of Ce and Zr added molecular sieve catalysts was increased,thereby improving its low-temperature denitration activity.H₂-TPR characterization analysis showed that the reducing ability of Co-based molecular sieve catalysts doped with the first promoter Ce and the second promoter Zr and the interaction between Co and the support were enhanced.The TG-DTA test results showed that the low-temperature activation capacity of Co,Ce and Zr in the ternary Co-based molecular sieve catalyst is strong,which is beneficial to improve the low-temperature denitration activity of the catalyst in the C₃H₆-SCR denitration reaction.?2?On the basis of the above-mentioned Co-based molecular sieve catalyst with the best catalytic performance 5%Co-2%Ce-1.2%Zr-ZSM-5-p,in the preparation and molding process of the catalyst,low-temperature plasma treatment and joint baking are adopted.Catalysts with different treatment times,different treatment powers,different treatment atmospheres,and different treatment methods were synthesized.The effects of these treatment methods on the catalytic performance of the catalyst were investigated.The study found that the low-temperature plasma treatment time of 5%Co-2%Ce-1.2%Zr-ZSM-5-p-10min with 10min has the best NO conversion rate and operating temperature window.The maximum NO conversion of a molecular sieve catalyst with a plasma processing power of 100W is 8%higher than that of a catalyst with a processing power of50W.Catalysts treated in a plasma atmosphere with N₂ have better catalytic activity than catalysts treated in a H₂ atmosphere.100W,10min,N₂ pure plasma treatment is the best treatment among different treatments.It can replace roasting and reduce 5%Co-2%Ce-1.2%Zr-ZSM-5-p to agglomerate Co,Ce and Zr at higher temperatures,improve the interaction between Co and ZSM-5,and additives Synergistic redox capacity between Ce and Zr.?3?Compared with the optimal single metal catalyst prepared by the immersion method,the single metal optimal molecular sieve catalyst prepared by the sol-gel method,in the operating temperature range of 225-280?,the NO conversion on the single metal catalyst increased by about 5%.The binary Co-based molecular sieve catalyst with the best catalytic activity has improved its denitration activity in the temperature range of T₅₀=250-375?compared with the binary Co-based catalyst synthesized by the impregnation method.The molecular sieve catalyst 5%Co-3%Ce-0.6%Zr-ZSM-5-CA synthesized by the sol-gel method is compared with the optimal ternary Co-based catalyst prepared by the impregnation method.Improved its catalytic performance at the highest NO conversion rate temperature T_top=250?,its operating temperature window T₅₀=184-395?,the highest NO conversion rate is 97%.For the optimal molecular sieve catalyst 5%Co-3%Ce-0.6%Zr-ZSM-5-CA,XRD analysis test results showed that the catalyst synthesized by the sol-gel method has a stable ZSM-5 crystal structure;Co₃O₄ spinel crystals and ZrO₂monoclinic states are the existence states of Co and Zr in the catalyst.Ce₂O₃ oxide and CeO₂ cubic crystals exist in the form of Ce,and these three components have a good degree of dispersion.BET characterization results showed that Ce and Zr enlarge the specific surface area of Co-based catalysts.NH₃-TPD test analysis found that the molecular sieve catalyst synthesized by the sol-gel method has very good acidity,which is conducive to improving the catalyst's adsorption performance for C₃H₆ in the reaction for catalytic NO removal.H₂-TPR characterization analysis showed that Ce enhances the redox capacity of the catalyst,and Zr enhances the interaction of the components in the catalyst.The TG-DTA test results found that the thermal reaction activation process of the molecular sieve catalyst is mainly low-temperature dehydration and nitrate decomposition,and metal ion doping is beneficial to reduce the decomposition temperature of the catalyst,thereby improving its low-temperature activity.