| Lean-burn combustion is a promising technique to increase fuel efficiency and decrease hydrocarbons (HCs) and carbon dioxide (CO2) emission. However, under lean-burn condition nitrogen oxides (NOx) can not be effectively removed by traditional three-way catalysts (TWCs). So, it is necessary to explore new catalytic technique for lean-burn NOx abatement. The NOx storage-reduction (NSR) technique is a promising solution to lean-burn NOx pollution. At present, the most widely studied catalyst system is Pt/Ba/Al2O3, which can be hardly applied extensively due to its poor sulfur-resistance. In this work, alkali metal Li and K was used as storage medium to replace Ba. The performance of Li-based catalysts supported on TiO2 and TiO2-MOx (M=Al, Zr, Si, Sn) was investigated carefully. Based on NOx storage ability and SOx-resisting performance, the TiO2-Al2O3 is selected as the support. The weight ratio of TiO2/(TiO2 + Al2O3) was optimized, meanwhile, the difference between Li-based and Ba-based catalysts was systematically investigated. In addition, the effect of La2O3 doping on the performance of Pt/K/TiO2-Al2O3 was also studied. The optimized weight ratio La2O3/(TiO2 + Al2O3 + La2O3) in the support is 3%. Besides, the effects of calcination temperature of the support and the different precursor of K were also investigated. Based on this, potential NOx storage mechanisms were proposed.A series of NSR catalysts Pt/Li/TiO2-MOx (M=Al, Zr, Si, Sn) were prepared by sequential impregnation using the supports TiO2-MOx synthesized by co-precipitation. The NOx storage capacity (NSC) of fresh Pt/Li/TiO2 is greatly improved after doped with Al2O3 or ZrO2 due to the increase of specific surface area and decrease of Pt dispersion. The regeneration of sulfated Pt/Li/TiO2-MOx strongly depends on the total acidity of the supports, including Br?nsted acid or Lewis acid. The oxidation ability of Pt/Li/TiO2-MOx is largely determined by crystallite size of Pt. Larger Pt crystallite corresponds to stronger oxidation ability. In-situ DRIFT results show that the NOx is mainly stored as nitrate at 350 oC. At this temperature, NOx is mainly stored as ionic nitrates over Pt/Li/TiO2-MOx (M=Al, Zr, Si, Sn). Sulfur poisoning of the catalysts is mainly resulted from the formation of bulk sulfates.As for the NSR catalysts supported on TiO2-Al2O3, the weight ratio of TiO2 to TiO2 + Al2O3 was optimized, and the difference between Li-based and Ba-based catalysts for NOx storage and sulfur-resistance was investigated. The doping of TiO2 into the Al2O3 could significantly improve the sulfur-resistance performance of the catalyst Pt/Li/Al2O3. Compared with those on pure TiO2, the Pt and lithium species are more highly dispersed on TiO2–Al2O3 mixed oxides, giving higher NOx storage capacity. Taking both the NOx storage capacity and the sulfur-resistance performance into account, the optimal weight ratio of TiO2/(TiO2 + Al2O3) in the catalysts is 40%. In-situ DRIFT results show that on Pt/Li/Al2O3 and Pt/Li/TiO2–Al2O3 NOx is mainly stored via bidentate nitrate intermediate at the temperature of 500 oC, while on Pt/Li/TiO2, NOx is mainly stored as ionic nitrates. The–OLi groups are regarded as the main NOx storage sites for Pt/Li/Al2O3 and Pt/Li/TiO2–Al2O3 catalysts, while lithium carbonate may be the prevailing NOx storage phase for Pt/Li/TiO2. When Pt/Li/TA(40) and Pt/Ba/TA(40) possess equal molar amounts of storage medium, they show almost the same NOx storage ability. However, the Pt/Li/TA(40) exhibits much better sulfur-resistance performance than the Ba-based NSR catalyst.To improve the thermal stability and sulfur-resistance of Pt/K/TiO2-Al2O3, the support TiO2-Al2O3 is further modified by La2O3. The La2O3 doping can obviously improve both the NOx storage capacity and the sulfur-resisting performance of Pt/K/TiO2-Al2O3. The most suitable weight ratio of La2O3/(TiO2 + Al2O3 + La2O3) is 3%. When the support was calcined at 500 oC, it exists in amorphous state and possesses large amount of acidity, with the K existing mainly as–OK groups. On the corresponding catalyst, the main NOx storage species are monodenate or bidenate nitrates. As the support was calcined at 750 oC, the surface hydroxyl groups greatly decrease and even disappear. In this case, K2CO3 is the dominating storage medium which is more efficient for NOx storage than–OK groups. In a summary, the catalyst with its support calcined at higher temperature possesses higher NSC, however, K2CO3 can react with SO2 more easily to form sulfates, decreasing its sulfur-resistance. The performance of Pt/K/TiO2-Al2O3-La2O3 is also influenced by different K precursors. When KNO3 is used, the fresh catalyst possesses the best NOx storage ability but the worst sulfur-resistance performance. On the contrary, when KCl is used, the fresh catalyst possesses the highest sulfur-resistance performance and the lowest NSC value. In-situ DRIFT results indicate that the storage mechanism is varied with different calcination temperature of the support and the different K precursors.
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