Study on the Active Catalysts for the Cold Plasma Assisted NO_x Storage and Reduction Reaction

Author:Bai Zhi Feng

Supervisor:shi chuan


Degree Year:2019





NOx storage and reduction is an effective method for removing NOx from automobile exhaust.Traditional NSR catalysts of the type Pt/Ba/Al2O3 exhibit low NOx removal efficiency below 300 ℃.Based on such research background,a new way for removal of NOx emissions from lean burn engines,which combined non-thermal plasma and hetergeneous catalysis for NOx storage and reduction,were proposed.During lean phase,nitrogen oxides were stored on the catalyst;while H2-plasma was introduced in the rich phase to assist the catalyst’s regeneration in low temperature.Through such a NOx storage-cold plasma assisted regeneration cycle,higher NOx conversion could be obtained in the low temperature range(<300 ℃).The main research results are summarized as follows:To improve the NOx storage capacity(NSC)of lean NOx trap(LNT)catalysts of the Pt/BaO/Al2O3 type,the catalyst was doped with redox active transition metals of Mn,Co,or Cu,respectively.Co and Mn doped catalysts showed improved NO to NO2 oxidation ability,and therefore enhanced NOx storage capacities,which led to the increased cycle averaged NOx conversions as compared to a Pt/Ba/Al2O3 reference.However,being limited by NOx reduction in rich phase,the NOx conversions were still poor at low temperatures(<250 ℃).By employing an H2-plasma in rich phase to assist NOx reduction,NOx conversions were greatly enhanced especially for low temperatures.The best properties were achieved over Pt/Co/Ba/Al2O3 catalyst which exhibited NOx conversions higher than 80%in the whole temperature range(150-350 ℃)and best SO2 resistance abilities.A physically mixed catalyst containing LMF perovskite(with strong NO oxidation capacity)and Ba/Al2O3(with strong basicity)exhibited high NOC and NSC across a wide temperature range,but displayed poor NOx conversion under lean-rich cycling,even when H2-plasma was supplied in the rich phase.Results showed that Pt is necessary for H2 adsorption and dissociation into H atoms,the latter process being assisted by the non-thermal plasma.In the mixed catalysts tested,Pt dispersed on alumina preferentially presented the highest Pt dispersion and therefore the best regeneration behavior.LMF+BaAl+PtAl catalyst exhibited the best NOx conversion,benefitting from both its superior NSC in the lean phase and NOx reduction activity during plasma-assisted rich phase purging.CoMn-K was mechanically mixed with a Pd/Ba/Al2O3 catalyst to enhance its NOx storage and reduction properties.The Pd/Ba/Al2O3 and CoMn-K mixture showed a synergetic effect with respect to NOx storage capacity and improved NOx reduction ability.The addition of K+ions promoted the catalytic performance by increasing NO oxidation activity and therefore NOx storage via the creation of active surface oxygen species on the catalysts.The stored NOx reduction by H2 was also improved,being ascribed to the fact that Pd was mainly present as Pd0 due to the electronic interactions between Pd and K+ ions.NOx conversions and N2 selectivity at low temperatures(80-200 ℃)could be further enhanced by assistance of non-thermal plasma in rich phase.Apart from Ba as the component for NOx storage,alkali metal like K is also the most suitable elements for NOx storage.Compared to Pt/Ba/Al2O3,the Pt/K/Al2O3 catalyst exhibits the higher NO oxdation capacity and NOx storage capacity in the whole temperature(150-6000C),the effect of various reducing agents(H2,C3H6,H2+C3H6)as well as plasma on the reduction of stored NOx on catalyst is investigated.Notably,high NOx removal efficiency and N2 selectivity are obtained over wide range of temperature(150-600 ℃)using H2 mixed with C3H6 as reducing agent,or employing plasma at low temperatures(<300 ℃)in the C3H6 atmosphere.Based on in situ DRIFTS measurements combined with MS studies,the roles of H2,C3H6 and plasma in NSR are clarified in the present study.