The Investigation Of NO_x Removal At Low Temperatures Over Transition Metal-modified Chabazite Zeolites

Nitrogen oxides（NO_x）are important precursor for the formation of PM_2.5 and ground-level O₃ pollution.NO_x emissions from diesel exhausts are main sources of NO_x in atmosphere.Ammonia-assisted selective catalytic reduction（NH₃-SCR）based on small-pore copper chabazite zeolite catalyst（Cu-CHA,including Cu-SSZ-13 and Cu-SAPO-34）is a pivotal NO_xreduction technology.However,the low-temperature（below 200°C）activities of Cu-CHA catalysts are poor,resulting in low efficiency of removing NO_x emitted during the cold-start period of engine operation or under idle conditions.This thesis addresses the low-temperature efficiency problems in the reduction process of NO_x from diesel-powered heavy-duty vehicles.Experimental methods and theoretical simulations were combined to investigate the low-temperature NH₃-SCR mechanisms over Cu-CHA zeolites,revealing the important roles of key intermediates（NH₄⁺or NO⁺）and Cu mobility in the abatement of NO_x at low temperatures.Finally,new strategies were established to enhance the NH₃-SCR activities of Cu-CHA catalysts in the temperature range of 150～200°C by low-temperature pre-treatment in NH₃+NO and high-temperature pre-treatment in N₂.Furthermore,highly dispersed Pd-CHA catalysts were synthesized and applied as passive NO_x adsorber（PNA）to abate NO_x in exhausts below150°C.PNA performance in the presence of H₂O and NO_x over Pd-SSZ-13 catalysts was investigated,and the mechanism of CO-assisted NO_x removal was revealed.The poisoning mechanisms of different Pd sites(i.e.Pd²⁺and[Pd（OH）]⁺)in the presence of phosphorus were clarified,providing solid basis for the development of high-efficiency and anti-poisoning catalysts for NO_x abatement.The main contents and conclusions are as follows:（1）A series of Cu-SSZ-13 with different Cu loadings were synthesized,then the low-temperature performance and mechanisms of NH₃-SCR over the Cu-SSZ-13 catalysts were investigated.It was found that the low-temperature NO_x removal efficiencies over Cu-SSZ-13was improved after NH₃+NO pre-treatment.In situ diffused reflectance infrared fourier transform spectroscopy（DRIFTS）and density functional theory（DFT） simulations indicated that Cu²⁺was reduced to Cu⁺ in the presence of NH₃+NO when Cu loading was at a low level,generating a NH₄⁺intermediate on the Br（?）nsted acid site nearby.Subsequently,NH₄⁺interacted with Cu⁺sites to form Cu⁺/NH₄⁺species,boosting the Cu redox cycle and thus improving the low-temperature NH₃-SCR rates.（2）Impedance spectroscopy（IS）was applied to investigate the mobility of Cu ions in Cu-SAPO-34 catalyst under NH₃-SCR conditions.The results indicated that the pre-treatment in NH₃+NO at low temperatures was beneficial for Cu mobility,The activation energy of Cu motion decreased from 66.5 k J mol^-1（in NH₃ atmosphere）to 14.2 k J mol^-1（in NH₃+NO）.IS and Density-functional tight-binding（DFTB）revealed that NH₃+NO pre-treatment can result in the generation of highly mobile Cu⁺species.Combining IS with kinetic results,it was observed that the low-temperature NH₃-SCR reaction rates over Cu-SAPO-34 increased along with Cu mobility.Based on this,a new strategy was established to enhance the low-temperature NH₃-SCR activities of Cu-based zeolite catalysts（including Cu-SAPO-34 and Cu-SSZ-13）in the temperature range of 150～200°C by low-temperature NH₃+NO pre-treatment,which allowed to reinforce the Cu movement under reaction conditions.（3）By investigating the influence of pre-treatment atmosphere,i.e.O₂ or N₂,it was revealed an unexpected increase of Cu mobility in Cu-SAPO-34 catalyst after pre-treatment in N₂ at high temperatures and an incline of activation energy of Cu motion from 118.1 k J mol^-1（in O₂）to113.9 k J mol^-1（in N₂）.In the presence of NH₃,the activation energy for Cu motion was decreased even more significantly,from 70.7 k J mol^-1（in O₂）to 53.6 k J mol^-1（in N₂）.In situ DRIFTS and synchrotron X-ray absorption spectroscopy（XAS）indicated that high-temperature N₂ pre-treatment resulted in a transformation of[Cu（OH）]⁺to highly mobile Cu⁺species and an easy activation of NO to highly reactive NO⁺.The interplay of Cu⁺and NO⁺led to the formation of Cu⁺···NO⁺moieties that are highly active for reaction with NH₃ to form N₂.Based on this,a new strategy was established to enhance the low-temperature NH₃-SCR activities of Cu-based zeolite catalysts（including Cu-SAPO-34,Cu-SSZ-13 and Cu-ZSM-5）in the temperature range of 150～200°C by pre-treatment in inert atmosphere at high temperatures.（4）A series of Pd-SSZ-13 catalysts were synthesized by an ion exchange method,and the NO_x adsorption/desorption properties of the as-obtained catalysts were tested under different reaction conditions.The results suggested that the NO_x adsorption/desorption was inhibited in the presence of H₂O,and CO could alleviate largely the negative effects of H₂O.In situ DRIFTS indicated that co-adsorption of NO and CO on Pd²⁺sites resulted in the generation of stable CO-Pd²⁺-NO complexes,thus inhibiting the competitive adsorption between H₂O and NO_x on Pd²⁺sites.In addition,the P-poisoning mechanism of Pd-SSZ-13 was investigated.The results indicated that P led to the migration of Pd²⁺and[Pd（OH）]⁺to the surface of SSZ-13 zeolite,forming[Pd（OH）]⁺aggregates,which can transform to Pd O_x at high temperature.The transformation of Pd species resulted in a decline of isolated Pd²⁺in Pd-SSZ-13 systems,and in turn a degradation of NO_x adsorption/desorption performance.