Study On The Preparation And NOx Purification Mechanism Of Hydrotalcite-based LNT Catalyst

At present,after-treatment technologies have commonly adopted for the NOx reduction of diesel engines because it is very difficult to meet the emission regulations only by internal purification.Lean NOx Trap?LNT?technology is one of the most potential after-treatment technologies to reduce the NOx emission from diesel engines.The traditional LNT catalyst that use Al₂O₃ material as the coating matrix,has many disadvantages such as poor adsorption performance in low temperature,bad sulfur resistance and poor stability in high temperature,which greatly restricted its application and development.However,the metal-modified hydrotalcite-derived oxide?CuMgFeAlO?as the coating material of LNT catalyst has many excellent characteristics,showing good application prospects.In this paper,the preparation and optimization of the hydrotalcite-based catalyst has been developed and studied.The crystal structure,specific surface area,micro-morphology and NOx adsorption-reduction performance of LNT catalyst have systematically studied by means of various advanced characterization methods.Then it was applied on the engine bench to determine the optimization strategy of the post-injection of the diesel engine.This work provides high practical values for the application of LNT technology on basis of the beneficial research foundation.The main contents and results obtained in this paper are as follows:By changing the solid content?about 35%?,pH value?about 4?,the mass fraction of the binder?between 3⁶%?and adding appropriate additives,the coating slurry with stable and suitable viscosity can be prepared.Both alumina-based and hydrotalcite-based coatings have high specific surface area,and are in close contact with the support to enhance the fastness of the coating.The additions of urea or polyethylene glycol to the hydrotalcite-based slurry have little effect on the particle size distribution of the slurry,but increase the specific surface area of the hydrotalcite-based coating.The NOx adsorption process on Pt/BaO/CuMgFeAlO and Pt/BaO/Al₂O₃ catalyst follows the nitrite pathway.The modification by transient metal elements in the Pt/BaO/CuMgFeAlO catalyst can promote the transformation of nitrite species to nitrate species,and greatly increase the NOx storgage capacity of the model catalsyt.The Pt/BaO/CuMgFeAlO catalyst that possesses the best performance for NOx purification at 400?,presents good absorption and reduction performance,which is significantly better than the Pt/BaO/Al₂O₃ catalyst.During the engine bench tests,the introduction of post-injection strategy increases the concentration of CO,THC in the exhaust and reduced NOx emission to a certain extent.At the same time,the exhaust temperature was significantly increased.Thus,the NOx concentration detected after the Pt/BaO/CuMgFeAlO catalyst was significantly reduced.The best purifaction performance for NOx is realized when the post injection time was 65°CA ATDC.The model catalyst designed in the paper can effectively reduce the CO and THC produced by post-injection,and the CO and THC conversion decrease after an initial increase with the increase of the main-post interval.