A Study On NO_x Storage And Reduction Performance Of Pd Supported Perovskite-Based Catalysts For Lean-Burn Exhausts

NO_x storage and reduction(NSR),also named as lean NO_x trap(LNT),is an effective technique to eliminate NO_x in lean-burn exhausts,which can realize NO_xremoval under the conditions of high air-to-fuel ratios.However,the traditional Pt-based catalysts have poor thermal stability,which tend to agglomerate and deactivate at high temperatures.The perovskite-based catalysts with the formulation of ABO₃have the advantages of excellent NO oxidation ability,high thermal stability and good tunability of A/B cations,which can potentially substitute the traditional Pt-based catalysts.In this work,the De-NO_x activity of perovskite-based catalysts is further improved by loading a small amount of Pd.We comprehensively investigated the De-NO_x activity of Pd-supported perovskite catalysts in the NSR process,from the aspects of chemical states of Pd species,metal-support interactions and perovskite formulations.Pd was introduced into the perovskite-based catalysts via sol-gel and wet-impregnation methods,and we obtained the Pd-doped La_0.7Sr_0.3Co_0.97Pd_0.03O₃ and Pd-supported Pd/La_0.7Sr_0.3Co O₃ catalysts.The addition of Pd significantly improves the De-NO_x activity of the perovskite-based catalysts.The Pd-supported catalyst achieves a promising De-NO_x activity of 90.4%at 300°C,which is higher than the Pd-doped catalyst(74.4%)and pure La_0.7Sr_0.3Co O₃(51.6%).Our results show that the perovskite component is the main active site for NO oxidation and NO_x storage,while the addition of Pd enhances both the NO_x desorption and NO_x reduction in fuel-rich periods,which induces the improved NO_x trapping performance in the followed lean-burn period.The spatial location of the Pd species affects the utilization efficiency of Pd.The doped Pd species is dispersed in the bulk of perovskite and can hardly migrate to the surface to catalyze the NO_x reduction in the fuel-rich periods;while the supported Pd species is highly dispersed on the surface of perovskite and close contacts with the NO_x storage components,thus inducing the higher De-NO_x activity.We designed and synthesized the perovskite-based Pd/La_0.7Sr_0.3Mn O₃ catalysts with different degrees of metal-support interactions(MSI).An interesting self-activation behaviour of the Pd catalyst is observed in the reaction atmosphere by properly tuning MSI.The De-NO_x activity significantly increases from 56.1 to 90.1%,and N₂O production is suppressed.The XAFS and XRD results of the activated catalysts demonstrate that the self-activation phenomenon derives from the in-situ transformation of Pd²⁺into Pd⁰ in dynamically oxidizing/reducing atmospheric oscillations.The generated Pd⁰ species promotes the NO_x reduction by efficiently activating C₃H₆,and achieves an 8-fold higher TOF than Pd²⁺for NO_x reduction.Notably,excessive MSI will inhibit the in-situ generation of Pd⁰,thereby lowering the De-NO_x activity of the catalyst even with high Pd dispersion.In addition,these Pd/La0.7Sr0.3Mn O3 catalysts exhibit a much higher tolerance of H₂O,CO₂ and SO₂compared to the conventional Al₂O₃ supported catalysts.To further improve the De-NO_x performance,we prepared a series of nonstoichiometric perovskite-based Pd/La_xSr_0.3Mn O₃(x=0.5,0.7,0.9)catalysts,taking advantage of the high tunability of perovskite formulation.The A-site defective Pd/La_0.5Sr_0.3Mn O₃ catalyst exhibits a high De-NO_x activity.The A-site defects promote the Sr doping into perovskite lattice and reduce the formation of the Sr CO₃ phase.Our results demonstrate that the A-site defective perovskite can be more efficiently regenerated than the Sr CO₃ phase as NO_x storage sites due to the latter's stronger basicity,and also exhibits the higher NO oxidation ability than the A-site stoichiometric and excessive catalysts.Based on these two reasons,the A-site defects improve the NO_xstorage and De-NO_x performance of the perovskite catalyst below 300°C.Nevertheless,above 300°C,the NO_x reduction becomes the determinant of the De-NO_x activity of the perovskite-based catalysts.A-site defects can weaken the interactions between perovskite and Pd,inducing activation of Pd sites by in-situ transformation from Pd O to metallic Pd in the alternative lean-burn/fuel-rich atmospheric alternations,which boosts the De-NO_x activity of the Pd/La_0.5Sr_0.3Mn O₃ catalyst.