Study On Sodium Poisoning Mechanism And Sodium Tolerance Of Cu/CHA Catalyst For NH₃-SCR On Diesel Engine

The number of automobiles in China has steadily increased year by year.To win the battle of defending the blue sky,the task of purifying and reducing automobile exhaust pollutants is arduous.More than 80%of nitrogen oxides（NOx）in automobile exhaust comes from diesel vehicles.At present,the main NOx removal technology is ammonia selective catalytic reduction（NH₃-SCR）,and the main commercial catalysts are copper-based small-pore molecular sieves,Cu/SSZ-13 and Cu/SAPO-34.Alkali metals from fuel and urea solutions deactivate the catalyst during long-term accumulation,which is an important factor affecting the service life and replacement cycle of the catalyst.In this study,the mechanism of alkali metal poisoning on Cu/SSZ-13 and Cu/SAPO-34 was compared.It was found that alkali metals seriously affected the NH₃-SCR activity of both.The collapse degree of the framework structure in Cu/SSZ-13 increases slowly with the increase of Na poisoning.The rate of decrease of NH₃-SCR activity is in proportion to the number of acid sites of the catalyst.The framework structure of Cu/SAPO-34 suddenly and completely collapsed when Na reaches 3.48 wt%.The main reason for the decreasing catalytic activity was the destruction of the framework structure.As Cu/SSZ-13 may involve different ways of introducing Na during the preparation process,this study combined experiment and DFT calculation technology to investigate the effect of residual Na ions,ion exchange Na ions and impregnated Na ions on Cu/SSZ-13 catalysts.The results showed that the residual Na ions and ion exchange Na are both at the ion exchange site of the molecular sieve,which protects the Si-OH-Al structure and improves the high-temperature hydrothermal stability.However,the residual Na ions mainly exist in the eight-membered ring（8MR）site,and ion exchange Na ions mainly exist in six-membered ring（6MR）sites,forcing more Cu ions in 6MR and 8MR respectively.Hydrothermal aging causes Cu/SSZ-13with ion-exchanged Na to generate a large number of Cu Ox species and damage the pore structure.The Cu/SSZ-13 with residual Na retains more stable 6MR Cu,and the pore structure remains intact.The impregnated Na ions caused the desilicification and the formation of Al（OH）₃（OSi）₁,which was supported by DFT calculations.The impregnated Na ions also caused serious destruction of acidic sites and significant reduction of active sites,thus SCR activity was severely affected.Aiming at solving alkali metal poisoning,this study found that Cu/SSZ-13 with larger crystal size through a simple preparation process can effectively improved the anti-Na poisoning performance of the Cu/SSZ-13.Investigations of Cu/SSZ-13 with a crystal size in the range of 0.4-2.3μm showed that the larger the crystal size,the fewer acid sites in the outer molecular sieve.Cu ions are mainly distributed on the subsurface and inner core,thus are less susceptible to Na ions.The molecular sieve on the outer surface acts as a"protective shell".