Study On The NO_x Adsorption-Reduction And Soot Oxidation On Cobalt Doped Hydrotalcite-based LNT Catalysts

Lean NO_X Trap?LNT?technology is one of the most important NO_X after-treatment technologies for small and medium sized diesel engines.Traditional LNT catalysts that use Al₂O₃ material as the precursor have many disadvantages,such as poor adsorption ability in low temperature,poor stability in high temperature and bad sulfur resistance,which restrict their wide application.The metal modified LNT catalysts that use hydrotalcite?HT?as the precursor have good performances on the NO_X reduction and adsorption in low temperature.Thus,the researches on such series of catalysts attract much more attention in the field of engine emission control.In this paper,series of Co modified LNT catalyst with well dispersed noble metal were prepared on the hydrotalcite by co-precipitation and impregnation methods.The crystal structure,specific surface,NO_X adsorption-reduction,NO_X purification efficiency of the prepared catalysts were investigated using XRD,SEM,in-situ DRIFTs,TPR/D,TGA techniques to filter the best Co modified hydrotalcite-based LNT catalyst.The physic-chiemical properties and NO_X adsorption-reduction of the selected catalyst doped with soot were studied as well.The hydrotalcite-based catalyst with well dispersed noble metal as well as BaO were prepared by co-precipitation and impregnation methods.The characterization results show that the Co metal is introduced and dispersed in the forms of CoO,Co₃O₄as well as CoAl₂O₄ spinel.The NO_X adsorption ability of LNT catalyst improves with the increase in the ratio of Co/Mg,and at the same time the stability of adsorbed nitrate decreases.When the ratio of Co/Mg is 0.9,the catalyst exhibits the best performance on improving NO_X desorption and widening NO_X desorption temperature.In the test temperature range,Pt/BaO/Co_2.7Mg_0.3AlO catalyst can achieve best performance on the NO_X adsorption and NO_X reduction efficiency at300?.Soot and Pt/BaO/Co_2.7Mg_0.3AlO catalyst were mixed through mechanically milling method.This mixture was heated at 500?so that the active components on the catalyst reacted with soot.This reaction decreases the intensities of XRD peaks for CoAl₂O₄ and Co₃O₄.The introduction of soot physically covers the alkaline adsorb sites and active sites on the catalyst surface,and thus reduces the adsorbed ability of the catalyst.In addition,the catalyst mixed with soot shows a better NO_X reduction efficiency than the pure catalyst at low temperature region.The Pt/BaO/Co_2.7Mg_0.3AlO catalyst involves in the soot oxidation reaction occurring in NO+O₂ atmosphere,and the NO_X species and soot can be synchronously removed in such reaction.The initial and maximum rate temperatures for soot oxidation is decreased when using the Pt/BaO/Co_2.7Mg_0.3AlO catalyst,so that the soot combustion process moves to the low temperature region by about 150?.In the soot oxidation reaction,the maxium for the CO₂ selectivity nearly reaches 90%,and at the same time a good performance on the NO_X removal is obtained.These results indicate that the Pt/BaO/Co_2.7Mg_0.3AlO catalyst has the ability of the simultaneous removal of soot and NO_X.The soot that tightly contacts with catalyst has a lower activation energy than the one that loosely contacts with catalyst.In addition,the presence of NO enhances the soot oxidation,and thereby decreases the initial temperature of soot oxidation.