Preparation Of Cu-SAPO-34 Catalysts With Enhanced Sulfur Resistance And Application For DeNO_x Of Diesel Engine Exhaust

The essive emission of nitrogen oxides（NO_x）in exhaust gas of diesel engines not only brings great pollution to the environment,but also threatens human health.Therefore,the elimination of NO_x is urgent.Nowadays,the selective catalytic reduction of NO_x with NH₃（NH₃-SCR）technology has been regarded as the most effective technology for de NO_x in diesel engine exhaust.The core of this technology is the preparation of the catalyst.The widely used V-based catalysts on the market have some disadvantages,such as poor performance of de NO_x at low temperature,and the narrow operation temperature window,the N₂ selectivity is poor will cause V loss at high temperature which makes it cannot be used for de NO_x in diesel engine exhaust.Although the metal oxide catalyst has good low-temperature activity,but the reaction activity temperature window is narrow and the catalyst is easy to sinter at high temperature,which limits its application.Zeolites-based catalysts are favored by researchers due to their excellent performance for de NO_x,broaden operating temperature window and good hydrothermal stability.However,the presence of H₂O and SO₂ in exhaust gas of diesel engines,the sulfate species will be generated in the reaction process and block the pores of the molecular sieve,result in reduced its activity and even lost its activities.Therefore,the purpose of this paper is to prepare a Cu-based molecular sieve catalyst with excellent H₂O and SO₂ resistance to be used for de NO_x in diesel engine exhaust.In the first part of this paper,a novel SAPO-34-Meso molecular sieve with hierarchical pore structure was synthesized by one-step hydrothermal synthesis via a dual templates method.Then the Cu-SAPO-34-Meso catalysts were obtained by Cu ion exchange method.The NH₃-SCR reaction activities of Cu-SAPO-34-Meso and Cu-SAPO-34-Con showed that,compared with Cu-SAPO-34-Con molecular sieves without mesopores,Cu-SAPO-34-Meso has broaden temperature window and excellent de NO_x performance,especially at low temperature range.This is mainly due to the existence of the mesoporous structure in the Cu-SAPO-34-Meso zeolites which accelerates the diffusion of the reactants,which is confirmed by diffusion calculations.Even if SO₂ is introduced into the reaction,Cu-SAPO-34-Meso showed better SO₂resistance than Cu-SAPO-34-Con.In situ diffuse reflectance infrared spectroscopy（In situ DRIFTS）proved that the enhanced sulfur resistance of Cu-SAPO-34-Meso may be attributed to the existence of its mesoporous structure,which could prevent sulfate species from blocking the pores and promote the decomposition of sulfate species.After hydrothermal treatment at high and low temperature for a long time,the catalysts could still maintain excellent NH₃-SCR reaction activities.The synthesis steps of the traditional ion exchange method are more complicated and cumbersome,so the second part of the article adopts the one-pot method to directly introduce the active components into the zeolite.The method is simple in operation and shortens the preparation period,reduces subsequent ion exchange steps,and reduces water waste and water pollution problems.In this paper,a Co element is added to the molecular sieve to enhance the sulfur resistance of the catalyst.The results showed that Cu Co-SAPO-34 catalysts haves broaden reaction temperature window compared with Cu-SAPO-34 molecular sieve.The reaction kinetics resusts of the catalysts showed that the Cu Co-SAPO-34 catalysts has a lower activation energy,indicating that the addition of Co can reduce the activation energy of the reaction and facilitate the progress of the reaction.After introducing SO₂ and H₂O to the reaction,the NO_x conversion of Cu Co-SAPO-34 catalysts was significantly higher than that of Cu-SAPO-34,indicating that CuCo-SAPO-34 catalysts has more excellent sulfur resistance.After hydrothermal aging at 700 ℃ and 80 ℃ for 16 h,the catalyst can still maintain excellent NH₃-SCR performance,indicating that the catalyst has excellent resistance to high temperature and low temperature hydrothermal aging for a long time.