Performance Of Manganese Oxides And Zeolites For Oxidation And Adsorption Of Nitrogen Oxides

Nitrogen oxides?NO_x?are one of the culprits of air pollution,which result in the haze,acid rain and photochemical smog.So far,the effective removal of NO_x is still a matter of concern.It is generally known that NO oxidation helps to increase the proportion of NO₂ in lean exhaust gas,which can?i?enhance the adsorption performance of NO_x storage and reduction catalysts?NSR?,?ii?improve the low temperature activity of the NO_x selective catalystic reduction?SCR?catalysts through fast SCR and?iii?promote soot combustion for the regeneration of the diesel particulate filter?DPF?.Manganese oxides are significantly potential to NO oxidation due to their low cost and environmental benefits.In addition,a new concept,Passive NO_x Adsorber?PNA?has been proposed to reduce cold-start NO_x emissions recently.In this system,NO_x can be adsorbed at low temperature temporarily and released at higher temperature at which SCR or NSR catalysts work.Pd-supported zeolite attracts considerable attention as a promising cold-start NO_x adsorber due to the high tolerance against sulfur.Herein,we prepared Mn₂O₃ nanorods?Mn₂O₃-NR?and nanoparticles?Mn₂O₃-NP?,both of which are highly active for NO oxidation.Nevertheless,Mn₂O₃-NP shows a better resistance to H₂O than that of Mn₂O₃-NR.The kinetic study and ¹⁸O isotope-labeling experiments confirm that Mn₂O₃-NR is mainly affected by O₂ adsorption,while Mn₂O₃-NP is dominated by NO adsorption.TEM shows?220?and?222?exposed surfaces for Mn₂O₃-NR and Mn₂O₃-NP,respectively.Accordingly,the adsorption of H₂O,NO,and O₂ on Mn₂O₃-NR and Mn₂O₃-NP is calculated by DFT,which suggests that?i?Mn₂O₃-NR exhibits a higher adsorption capacity for H₂O than Mn₂O₃-NP;?ii?both H₂O and O₂ are adsorbed on the Mn site of Mn₂O₃-NR and the competitive adsorption between H₂O and O₂ is generated,while H₂O and NO are adsorbed on Mn and O sites of Mn₂O₃-NP,respectively.Thus,the effect of H₂O is not so evident for Mn₂O₃-NP than for Mn₂O₃-NR,which is not reported before.However,in the presence of SO₂,the activity of Mn₂O₃-NR and Mn₂O₃-NP is sdecreased.After reactions,the sulfate species is observed by the FT-IR.Microporous zeolite SAPO-34 is prepared by a hydrothermal method.Pd is supported on SAPO-34 through incipient wetness impregnation?IM?and ion exchange?IE?.ICP and CO titration results show that the Pd content of the IM sample is approximately equal to the feeding amount,while the IE sample is significantly low.The dispersion of Pd on the IE sample is much higher than that on the IM sample.H₂-TPR indicates that two kinds of Pd species(PdO and Pd²⁺)coexist in the two samples;however,the content of PdO is much higher than that of Pd²⁺for the IM sample,while the inverse situation is observed for the IE sample.The PNA performance is measured by temperature-programmed desorption of NO_x.Both IM and IE samples show three NO_x desorption peaks,corresponding to PdO and Pd²⁺on different ion exchange sites,respectively.Furthermore,the PNA amount is proportional to the Pd content.H₂O is found to affect PdO rather than Pd²⁺.The in-situ FT-IR shows NO_x weak chemisorption.Nitro compounds is the main adsorbed species on the IM sample,while the nitrosyl complex on Pd²⁺is also observed for the IE sample.