Study On The Structures And Performance Of The Noble Metal Doped La_0.7Sr_0.3CoO₃ Perovskite Type NSR Catalysts

In this work, the Pt doped La_0.7Sr_0.3Co_1-xPt_xO₃ perovskite type catalysts were used to remove NO_x under lean-burn conditions by a NO_x storage-reduction （NSR） technique. A series of the La_0.7Sr_0.3Co_1-xPt_xO₃ perovskite catalysts were synthesized by a sol-gel or impregnation method. The catalyst prepared by the sol-gel method contains a lot of impurities such as carbonate, while the catalyst prepared by the impregnation method shows a relatively better perovskite structure. If the precursor, which has been impregnated by a Pt precursor solution, is calcined in flow air, the obtained catalyst will present a perfect perovskite structure with less impurity. It is a great advantage comparing with the catalyst calcined in static air atmosphere, since the heat and CO₂ produced from the complexants combustion could be well released out of the oven, which avoids the sintering of the catalyst and the formation of carbonate. XRD patterns show that the diffraction peaks belonging to the perovskite structure shift to lower diffraction angle positions after the Pt doping, demonstrating that the Pt is successfully doped into the perovskite and the size of the perovskite crystal unit cell is enlarged since Pt cations have a bigger radius than that of Co cations.The influence of the Pt proportion doped into the La0.7Sr0.3CoO₃ perovskite crystal lattice on the NO_x oxidization and storage activity was also studied. H₂-TPR results show that with the increase of the Pt proportion at the B site （0.01, 0.03 and 0.05 in atom ratio）, the amount of oxygen vacancies increased significantly, and the oxygen stored from gas phase could be reduced more easily comparing with the La0.7Sr0.3CoO₃ perovskite. From the XRD patterns, we observe a continuous decrease of the diffraction angle of the perovskite with the increase of the Pt proportion, indicating that more Pt was successfully doped into the perovskite structure. No other Pt containing species were detected. The NO_x storage capacity （NSC） measurement and NO-to-NO₂ conversion show that the perovskite La_0.7Sr_0.3Co_1-xPt_xO₃ with x=0.03 possesses the best NO_x oxidization and storage activity, i.e. 770.3μmol/g and 84.3%, respectively. Moreover, 700℃is the optimal calcination temperature during the catalyst preparation. Although the perovskite structure could also be formed with some defective structures after the precursor calcined at 600℃, a lot of the residue nitrates belonging to the raw material still existed in the catalyst. If the precursor was calcined at 800℃, the perovskite would be sintered seriously, and its BET surface area was greatly suppressed, inducing a poor catalytic activity.SO₂ poison is a big problem for the conventional NSR catalyst. Usually, the basic oxide is easily sulfated to sulfate, which is hardly reduced at low temperature; thereafter the NO_x storage sites can not be recovered under operating conditions. Herein, we evaluated the sulfur-resistance of the La_0.7Sr_0.3Co_0.97Pt_0.03O₃ perovskite catalyst by pre-treating it in a SO₂/O₂ atmosphere. The results show that this Pt doped perovskite catalyst presents a good sulfur-resistance. Its NSC only drops about 15% comparing with the fresh one. As we addressed above, the deposited sulfur species can only be reduced at high temperature, which may destroy the perovskite structure. So, we investigated the regeneration ability of the perovskite catalyst by a seriously pre-reducing and a re-oxidizing at 700℃cycle, and found that the damaged perovskite structure could be well reconstructed. Moreover, XAFS results indicate that the reduced Pt tiny particles may be located in the La₂O₃ crystal lattice, avoiding the sintering. Therefore, its damaged structure was easily regenerated since all elements were highly dispersed therein.Finally, a series of the BaFeO_3-x perovskite catalysts were synthesized by a sol-gel method using citric acid and/or EDTA as complexants with a purpose to improve their sulfur-resistance. The TG results show that almost no carbonate was formed after calcination of the xerogel precursor with the complexants’molar ratio of CA/EDTA ? 1.5, which was convinced by the in situ DRIFT spectra results during a SO₂/O₂ sorption experiment. After adding EDTA into the complexants, the metal ions of the raw material could be mixed homogeneously and react stoichiometrically by calcination at 750℃to form a uniform perovskite structure. Accordingly, the obtained BaFeO_3-x perovskite presented a performed sulfur-resistance. Moreover, the seriously damaged structure of the BaFeO_3-x perovskite by reduction could be in situ regenerated by calcination under lean conditions at 400℃, which is within the operating temperature zone of the aftertreatment system of diesel to meet the real commercial demands.