Construction And Design Of Cu-CHA Diesel Vehicle DeNo_x Catalysts To Meet Future Regulatory Requirements

Emissions of nitrogen oxides（NO_x）cause serious harm to human health and the global climate.Diesel vehicles are a major contributor to mobile source NO_x,and ammonia selective catalytic reduction（NH₃-SCR）is considered to be one of the most effective aftertreatment technologies available for NO_x removal,and its core is the NH₃-SCR catalyst.The commercialization of Cu-CHA small-pore zeolites catalysts（Cu-SSZ-13 and Cu-SAPO-34）is a milestone event in environmental catalysis.However,with increasingly stringent global emission regulations,such as the California Air Resources Board’s requirement to reduce NO_x from the current 0.27 g/k W-hr to an ultra-low limit of 0.027g/k W-hr by 2027,this means that the catalytic efficiency of NH₃-SCR must exceed 99%.However,Cu-CHA catalysts still face many serious challenges under actual service conditions,such as poor NO_x catalytic performance during cold start,sulfur/phosphorus/alkali metal poisoning deactivation,and high-and low-temperature hydrothermal deactivation.In order to meet the future regulatory requirements for ultra-low NO_x emissions,this dissertation designs and constructs Cu-CHA catalysts at the atomic level to develop catalytic materials with better performance.The main contents are as follows.（1）Cu-SAPO-34 catalysts with similar copper content were precisely synthesized by the"one-pot"method using different structure directing agents（diethylamine,triethylamine,propylamine,and tetraethylammonium hydroxide）,and after low-temperature hydrothermal treatment,Cu-SAPO-34 catalysts with more Si-O（H）-Al density and ZCu（OH）sites（Z represents one framework negative charge）,the Cu-SAPO-34 catalysts were more prone to low-temperature hydrothermal deactivation leading to the degradation of NH₃-SCR catalytic performance,where the Cu-SAPO-34synthesized using tetraethylammonium hydroxide exhibited the strongest water resistance and good denitrification activity.（2）The low-temperature hydrothermal stability of Cu-SAPO-34 was improved by protection strategies such as rare earth metal（Ce,La and Sm）doping,alkali metal（Na,K and Cs）exchange and NH₃ pre-sorption,but different strategies exhibited different protection effects in the low-and high-temperature regions.On the basis of revealing the mechanistic differences of these protection strategies,a combination strategy of Ce doping,Na exchange and NH₃ pre-sorption was simultaneously employed to further prevent the low-temperature hydrolytic deactivation of Si-O（H）-Al and ZCu（OH）sites,and stronger water-resistant Cu-SAPO-34 denitrification catalysts were developed.（3）The Cu-exchanged SAPO-18,SAPO-34 and SAPO-18/34 intergrowth zeolites were carefully designed by modulating the ratio of structure directing agents（N,N-diisopropylethylamine,triethylamine or their mixtures）.The results show that the Cu-SAPO-18/34 intergrowth zeolite has more Si（n Al）（n=1～4）structures compared to the pure crystalline phase Cu-SAPO-18 and Cu-SAPO-34,which leads to more and stronger acidic sites,facilitates the generation of more Cu active centers during aqueous ion exchange（AIE）and inhibits the formation of Cu O_x,thus leads to higher NH₃-SCR catalytic activity,better high-and low-temperature hydrothermal stability and resistance to SO₂ poisoning.（4）Copper exchanged SSZ-13,SAPO-34 and SAPO-18 catalysts were successfully prepared by the solid-state ion exchange（SSIE）method.Compared with the conventional AIE route,this SSIE technology can improve the ion-exchange level of copper,avoid the accumulation of Cu O_x,and improve the NH₃-SCR catalytic activity and high-temperature hydrothermal stability.In addition,the use of NH₄⁺-type zeolite carriers improves the dispersion of Cu²⁺better than H⁺-type zeolite carriers in the SSIE process,i.e.,in NH₄⁺-type zeolite carriers,at 150～250°C,Cu²⁺is solventized by NH₃ to produce Cu（NH₃）_x²⁺intermediates（x=2 or 4）,which are then dissociated and decomposed in an oxygen atmosphere at 250～400°C.The formed Cu（NH₃）_x²⁺intermediate promotes Cu²⁺mobility and lowers the reaction energy barrier to facilitate Cu²⁺occupation of ion-exchange sites,while H⁺-type zeolite carriers are unable to produce this intermediate species.