| The removal of nitrogen oxides (NO and NO2, NOx) pollutants from theexhausts of fuel-efficient lean burn engines, to comply with the increasinglystringent emission regulations, is currently a major challenge for thecommercialization of fuel-efficient lean burn vehicles. Currently, platinum loadingin NSR catalyst is twice as that of three-way catalyst, because of its sulfure-resistant andsintering. So inventing a low platinum loading and anti-sintering NSR catalyst couldpromote the lean burn engines into wildly pratical.This work firstly investigates the effect of pre-calcination temperature (PCT) ofAl2O3on the NSR performance of the most wildly used Pt-BaO/Al2O3catalyst. It isfound that NSR performance of Pt-BaO/Al2O3catalysts depends strongly on the PCT,pre-calcination temperature, of Al(OH)3during preparation of the Al2O3support. Thenumbers of NOxstored and reduced during the cyclic NSR process on thePt-BaO/Al2O3(PCT) catalysts showed similar volvano-type dependences on the PCT,peaking at PCT=800oC. The gradual evolution and ordering of the γ-Al2O3phase inthe Al2O3(PCT) support on increasing the PCT up to800oC led to continued increasesin the acid site density at the support surface, which in turn resulted in improvedproximity and synergy between the Pt and BaO sites for much more efficient NSRcatalysis. These results reveal that both the crystalllinity of the γ-Al2O3phase and lewisacidity of the Al2O3support are critical to the NSR efficiency of the Pt-BaO/Al2O3catalyst.Then we investigate the platinum loading on mostly used NSR catalystPt-BaO/Al2O3to understand the determing factor to platinum loading. We fould NOxstorage capacity decrease from0.26to0.22and0.12mmol/g while N2selectivitydecrease dramtically from86%to53%and26%, when platinum loading decrease from1wt%to0.5and0.1wt%. This work showed a strong dependency of the NSR catalysison Pt loading of the reference Pt-BaO/Al2O3catalyst. Efficient NOxreduction catalysisunder the rich conditions is the key to developing innovative low platinum NSRcatalysts. The use of ZrO2-Al2O3as an alternative of the Al2O3support was effevtive inlowering the dependence on Pt laoding on the Pt-BaO for the NSR catlysis since theZrO2-Al2O3support could make, at a lower Pt loading, the catalyst highly selective for the formation of N2under the rich condition. This finding provides a viable approach inthe search for low platinum NSR catalysts.Addition of CoOxto Pt-BaO/Al2O3was also studied through varying the Coloading and calcination temperature. We found that Co promoter could enhance the NOxstorage capacity under lean conditions, while it could lower the N2selectivity of thecatalyst under rich conditions. Varying the calcination temperature of the catalyst couldgenerate the Pt-Co bimetallic, which could be benefit for the NSR performance.In real automobile application the catalysts need to be washcoated onmonolith supports. The reaction of the cations in perovskite oxides with thesurface of γ-Al2O3, which is generally the main component of a washcoat, haslong been recognized as a pitfall leading to the deterioration of catalyticperformance. The effect of perovskite oxide-Al2O3interaction on the NOxstorage performance is therefore studied in this work. It was found theinteraction between perovskite oxides and γ-Al2O3could instead be utilized inpromoting the NOxstorage process: the NSC of BaFeO3+Al2O3mechanicmixture could be enhanced more than5times on increasing the calcinationtemperature, amounting to10-time the number for the typical Pt-BaO/Al2O3catalysts. The potential of this high-NSC material in NSR application wasfurther exploited by incorporation of Ag in the material. Promising NSRperformance was registered on BaFeO3+Ag/Al2O3catalysts, demonstrating thepotential of these Pt-free catalysts based on a combination of BaFeO3andAg/Al2O3as viable alternatives to the traditional Pt-based NSR catalysts. |
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