Study On The Mechanisms Of Lean-burn NO_x Storage And Sulfur Tolerance Over The Third-generation NSR Catalyst Pt/K/TiO₂-ZrO₂

The NOx storage and reduction (NSR) technique proposed by Toyota provides a feasible approach to the abatement of lean-burn NOx. Up to now, the most widely studied catalyst system is Pt/Ba/Al2O3, which shows poor SO2-resisting ability. Thus, new NSR catalysts tolerant to sulfur poisoning are desiderated to be developed. In the present study, a series of NSR catalysts Pt/K/TiO2-ZrO2 with good sulfur-tolerant performance are prepared. The effect of calcination temperature of the support on storage capacity, sulfur durability and the regeneration property of the NSR catalysts are investigated in detail; meanwhile, the influences of the K loading, the kinds of storage components and the supports on the catalytic performance are also studied; in addition, the reaction routes and storage mechanisms are revealed or discussed.The results of NOx storage capacity (NSC) show that as the calcination temperature increases, the NOx storage capacities of the catalysts show volcano-type tendency, with the maximum appearing at 800oC. The results of N2 adsorption/desorption, XRD, TPD and in-situ DRIFTS show that the storage capacity is tightly related to the structures and chemical properties of the supports and the state of K speices, regardless of the specific surface areas. With the calcination temperature increasing, the structures of the supports are transformed from amorphous state at 500oC to ZrTiO4 crystalline at 650oC or above, while the total amounts of surface acidity decline evidently, accompanying with the transformation of Br?nsted acidic sites to Lewis acidic sites. The formation of -OK group arising from the interaction between the surface hydroxyl of support and the K-containing phases is not favorable to NOx storage, while the highly dispersed K2CO3 phase facilitates the NOx storage as nitrates. The sequence for the stability of nitrates formed from different K species is K2CO3> potassium oxide >-OK bond.The reduction temperature of sulfates formed on the catalysts shifts to high temperature as the calcination temperature of support increases. The results of H2-TPR reveal that the reduction of the sulfates formed on Pt/K/TiO2-ZrO2 catalyst with the support calcined at 500oC started from about 200oC and completely finished before 500oC, which is about 200oC lower than that of traditional Pt/Ba/Al2O3. The corresponding temperature for the catalyst with the support calcined at 650oC is elevated to 610oC, and at higher calcination temperature of 800oC, the major reduction peak further shifts to 625oC. The stability of sulfates is also influenced by the state of K species, the decomposition of K2CO3 has decreased the reduction temperature of the sulfates. After reduced in H2-containing atmosphere, the regenerated sample with the support calcined at 650oC shows the biggest NOx storage capacity, being about 60% of that for the fresh catalyst; and the regenerated one with the support calcined at 500oC possesses the best regeneration ability, whose storage capacity achieves 90% of that for the fresh sample; while the storage capacity for the regenerated sample with the support calcined at 800oC only reaches 20% of that for the fresh one. The catalysts Pt/K/TiO2-ZrO2 with the calcination temperature of support at 500 and 650oC possess not only high storage capacity but also novel sulfur-resisting ability.The results of NSCs show that the K loadings (5~30 wt%) are proportional to the storage capacities of the catalysts, but with the increase of K loading, the sulfates reduction shifts to higher temperature, decreasing the sulfur-resisting ability, so, the optimal loading for K2CO3 should not exceed 15 wt%. Among the different storage components of Li, K, Ba, Mg and Sr, K and Li are the best selection from the view of both the storage capacity and the sulfur resistance. The comparative studies of the support effect show that the interaction of TiO2-ZrO2 with the storage component is stronger than other supports, leading to the best SO2-resisting performance. The sulfate particles are easily agglomerated on the surface of Al2O3, which is hard to be removed, and therefore showing the worst performance for sulfur resistance.The DRIFTS results of NOx adsorption over the catalyst Pt/K/TiO2-ZrO2 indicate that the different storage mechanisms are followed for the samples with the support calcined at different temperatures. For the catalyst with the support calcined at 500oC, the reaction pathway consists of direct oxidation of NO to nitrate and the disproportionation reaction of NO2 with the formation of nitrate and NO; for the sample with the support calcined at 650oC, the main reaction routes may be the oxidation of NO to form nitrite and nitrate species; at higher calcination temperature of 800oC, the main reaction pathway is the formation of bulk nitrate species via the direct oxidation of NO and the three-dimensional transferring from surface to bulk. As the calcination temperature of support increases, the optimal temperature region for NOx storage as nitrate shifts to higher temperature direction.