Studies On The NO_x Storage-reduction Catalysts Under Lean-burn Conditions

The NO_x storage-reduction (NSR) method is a very effective approach to reduce the NO_x emission from the lean-burn engines and the NSR catalysts based on Pt and Ba have been most extensively studied. However, this kind of catalyst is very sensitive to SO₂, and the presence of SO₂ in the exhaust can make the catalyst lose its activity rapidly. In this paper, instead of using BaO, MgO was selected as the storage component. A series of Pt/Mg-Al-O catalysts with different atomic ratios of Mg/Al and Pt/MgO catalyst were prepared. Their properties were characterized by XRD, N2-BET-surface area and TPD; meanwhile, their catalytic activities were evaluated by performing NO_x isothermal storage experiments and dynamic storage-reduction experiments.The results of NO_x isothermal storage experiments indicate that NO₂ storage is much more preferable than NO storage and Pt can catalyze the oxidation of NO, resulting in much higher NO* storage capacity. There was an interaction between Mg and Al; the addition of Al was advantageous to the storage of NO_x. The promoting effect of Al₂O₃ is more pronounced at low temperatures (<300℃). During low-temperature storage process, the surface species such as physically adsorbed species, weakly chemisorbed species, nitrites and nitrates were formed; while just mainly nitrites and nitrates were formed during high-temperature storage process.During the dynamic storage-reduction experiments with C₃H₆ as reductant, NO_x conversion at reduction stage increases with increasing Mg/Al atomic ratio in the catalysts; MgO can inhibit the self-poisoning of the reaction of NO_x and C₃H₆. Pt/MgO has the highest NO_x total conversion due to its best activity in the reduction of NO_x by C₃H₆ and the widest temperature span for NO removal is in the range of 350⁴50℃.With H2 as reductant, regeneration of the catalysts could be carried out at low temperatures. During the reduction process, the reaction took place in two consecutive steps. First, the nitrates were high-selectively reduced to N2, and then mainly NH3 was formed. The formation time of NH3 is dependent on the amount of NO* stored, reaction temperature and H2 concentration.When 35ppm SO2 was added to the reaction feed, the deactivation of the catalysts can be observed. The results of 100-cycles experiment showed that Pt/MgO has better resistance to SO2, comparing with Pt/Mg-Al-0 and...