Study On The Pd-modified Perovskite-type Or Hydrotalcite-based High-performance Catalysts Used For Soot Combustion

Due to the high fuel effciency, excellent durability and low CO2-emission,diesel engines operating at lean condition have been widely employed in many fields.However, the exhausts of diesel engines contain a lot of pollutants especiallysolid-state soot particulates which are serious threat to our health and environments.Nowadays, the catalytic combustion technique combined with DPF is regarded asthe most potential and efficient after-treatment technology for soot removal. Amongthe catalysts used for soot combustion, noble metal catalysts usually exhibit thehighest activity; however, they can readily sinter at relatively high temperature,resulting in significant decrease of thermal stability and catalytic activity. So thefocus of this work is to study the Pd-promoted perovskite-type or hydrotalcite-basedcatalysts which possess both good catalytic activity and high thermal stability.Firstly, a series of substituted perovskite catalysts La1-xKxCo1-yPdyO3-δ(x=0,0.1;y=0,0.05) and perovskite supported catalysts Pd/La1-xKxCoO3-δ(x=0,0.1) weresynthesized through a citrate-based sol-gel process. The influence of differentsubstitution conditions on the structures and catalytic activities of the perovskitecatalysts were investigated. The results show that the K/Pd dually substitutedperovskite catalyst exhibits the best activity for soot combustion. The catalyst duallysubstituted by K/Pd possesses the largest specific surface area, the highestredoxidibility and most active oxygen species. Besides, the valence of Pd inPd-substituted perovskites is higher than+2, which is much more active for NOoxidation and soot combustion as compared with usual bivalent Pd ions. Thesintering of Pd in the catalyst was also inhibited due to the Pd-doping into perovskitestructure. The experimental proof of NO2involved in soot combustion over thecatalyst LKCP was obtained by monitoring the concentration of NO2during reaction.Based upon these studies, a potential mechanism of soot oxidation over K/Pd duallysubstituted perovskite is proposed.Secondly, to overcome the shortcoming of the low specific surface area ofperovskites, CoAl-based hydrotalcite-derived complex oxides were prepared bypH-constant coprecipitation and used as the supports of K/Pd dually promoted sootcombustion catalysts. It is found that K/Pd simultaneously supported sample exhibitsthe highest activity. In this sample, K and Pd play their own roles in promoting the oxidation activity, meanwhile, the presence of K helps to release Pd species toparticipate in soot combustion. The functions of K are summarized in the followingaspects:(1) to promote the release of Co and Pd species and increase the active sites;(2) to improve the mobility of surface lattice oxygen;(3) to facilitate the adsorptionof NO on the surface of the catalysts. The roles of Pd could be attributed to twopoints:(1) to modify the redoxidibility of the catalysts;(2) to increase the catalyticoxidation of NO to NO2. It is revealed by in situ FT-IR that there are three reactionpathways for soot combustion: namely direct soot oxidation by surface adsorbedoxygen species, the NO2-assisted soot oxidation and nitrate-assisted soot oxidation.Finally, Mg was added to K/Pd promoted CoAl-based hydrotalcite-derivedcomplex oxides to improve their activity for soot combustion. The study found thefunctions of K and Pd are similar with the CoAl-based hydrotalcite-derived complexoxide catalysts, while the effects of Mg are entirely opposite in different catalysts.On the one hand, it can generate strong interaction with the doped Pd in Pdsubstituted catalysts to inhibit the promotion of K; on the other hand, it can providebasic sites to K/Pd simultaneously supported catalyst to accelerate soot combustion.The results of in situ FT-IR are also similar to CoAl-based hydrotalcite-derivedcomplex oxide catalysts. In addition, the thermal stability of CoAl-based andCo2.5Mg0.5Al-based catalysts are compared with the conventional supported catalystPd/Al2O3, which indicates that Pd in K/Pd supported hydrotalcite-derived complexoxide catalysts could prevent the sintering of Pd due to the strong interactionbetween Pd and other ions in the hydrotalcite-derived complex oxides.