Experimental Study Of NO_x Adsorption And Desorption Characteristics On Passive NO_x Adsorber Molecular Sieve Catalyst

NO_x emissions from diesel engines are one of the major emissions issues for diesel engines.Due to increasingly stringent emission regulations,for cold start and idle phases of diesel engines,NO_x emissions are receiving increasing attention when exhaust temperatures are low.At present,the main NO_x post-treatment technology route is to use SCR system to reduce NO_x in exhaust gas through NH₃.However,because the efficient active window of the catalyst in SCR system is usually above 200°C,the decomposition temperature of urea which produces NH₃ is also above 180°C.Therefore,in the stage where the exhaust temperature is low,special treatment of NO_x is required.Passive NO_x Adsorbers（PNA）technology is considered to be an effective low-temperature exhaust NO_x treatment technology.Passive NO_xAdsorbers technology completes the adsorption of NO_x at low temperature,and conduct the desorption of NO_x when the exhaust temperature rises above 180°C.The adsorption and desorption of NO_x in the entire stage is only related to the exhaust temperature.In this paper,SSZ-13molecular sieve was selected as the carrier,and the PNA characteristics of the molecular sieve catalyst supported by palladium（Pd）and cobalt（Co）as active elements were studied.The effects of silica-alumina ratio,active element loading,and different intake components on the PNA characteristics of molecular sieve catalysts during the adsorption and desorption phases were studied separately.The PNA characteristics of the catalysts after hydrothermal aging were also studied.Among Pd-SSZ-13 catalysts,reasonable silicon-aluminum ratio and Pd loading have important effects on PNA performance.This paper studies the effects of different silicon-aluminum ratios and Pd loadings on Pd-SSZ-13catalysts.The research results show that,for Pd-based PNA catalysts,divalent Pd ions play an important role.Excessively high silicon-aluminum ratio affects the distribution state of divalent Pd ions to a certain extent,thereby affecting the PNA performance of molecular sieve catalysts.The SSZ-13 carrier’s too low silicon to aluminum ratio will reduce its own crystallinity,resulting in the formation of alumina and reducing the PNA performance.For the Pd loading,when the Pd loading is too low,the active sites on the SSZ-13 support are insufficient,and the PNA performance of the catalyst is low at this time,and the excessive Pd loading will also reduce the PNA performance,which is due to the generation Excessive Pd O_xspecies cause deterioration of the pore structure parameters of SSZ-13molecules.From the perspective of atomic utilization,excessive loading will also reduce the utilization of Pd atoms.At the same time,the effect of C₃H₆ in the intake air on the performance of the catalyst was studied.The results showed that when C₃H₆ was present in the intake air,the NO_xadsorption and desorption performance of the catalyst was inhibited,and when the concentration of C₃H₆ was 200ppm,the NO_x adsorption capacity of the catalyst was completely lost.The influence of different factors on the PNA characteristics of the catalyst after hydrothermal aging was studied.In the hydrothermal aging process,the silicon-aluminum ratio has an important effect.When the silicon-aluminum ratio is too low,the Pd-SSZ-13 catalyst undergoes a severe collapse of the skeleton structure after the hydrothermal aging process,causing the catalyst decrease in NO_x adsorption and desorption performance.After hydrothermal aging of the catalyst,with the increase of the Pd loading,the PNA performance of the catalyst improves.It is considered that the hydrothermal aging conditions promote the transition of the oxidized Pd species to the ionic state to a certain extent,thereby improving the PNA performance.At the same time,the hydrothermal aging temperature has an important effect on the catalyst performance.For the Pd-SSZ-13 catalyst,when the hydrothermal aging temperature is increased to850°C,the PNA performance of the catalyst is significantly reduced.Non-precious metals are of great significance in practical applications as PNA catalysts.By using cobalt instead of palladium as the active element of molecular sieve catalysts,the factors affecting the activity of Co-based PNA catalysts are studied.The Co content has an important effect on the NO_x desorption temperature.Properly increasing the Co content can increase the NO_x adsorption amount,but too much Co will generate more stable nitrate species,which makes the NO_x desorption temperature higher.It is unfavorable for continuous PNA cycle.Compared with Pd-based molecular sieve catalysts,Co-based molecular sieve catalysts also have excellent PNA characteristics,but they are weaker than Pd-based molecular sieve catalysts after hydrothermal aging.