Application progress of small-pore zeolites in purifying NOx from motor vehicle exhaust

Lean-burn mode results in the high NOx emission of diesel vehicles despite its high fuel economy. NOx can be effectively removed by the SCR unit during normal operation, however, elimination during the cold start period needs to be addressed soon. Passive NOx adsorber (PNA) was proposed to adsorb and store NOx emitted during the cold start period. With the increase of exhaust gas temperature during normal operation, the adsorbed NOx will be quickly released and efficiently converted to N2 when flowing through the downstream SCR catalyst. After years' investigation, small-pore zeolites such as CHA and AEI have been found to possess superior hydrothermal stability and NOx selectivity compared with medium and large pore zeolites, and the former's eight-membered ring structure effectively exclude large hydrocarbons from entering the zeolite channels, making them play a prominent role in NOx purification via both PNA and SCR units. SSZ-13 and SSZ-39 zeolites, which belong to CHA and AEI topologies respectively, has been confirmed to exhibit excellent low-temperature NOx adsorption performance and high-temperature SCR denitrification performance. In the field of purifying NOx from the diesel vehicle exhaust, the review simultaneously focused on the application of SSZ-13 and SSZ-39 zeolites in NOx purification from two aspects of PNA and SCR in recent decades. In addition, the unique advantages of their modified form of loading Pd/Co as PNAs, or loading Cu doped with Fe/Mn/Ce as SCR catalysts were systematically clarified, in order to provide some advanced guidance for the researchers in the purification of NOx from the diesel engine exhaust.