NO_x Storage And Reduction Performance Of Manganese-based Mixed Metal Oxide Catalysts

Lean combustion technology is a very effective vehicle exhaust purification technology,which can reduce the emission of harmful pollutants and improve fuel economy.However,in the presence of excessive oxygen,resulting in the production and emission of high concentration of NOx.The traditional three-way catalysts cannot effectively eliminate NOx under lean burn conditions.NOx storage reduction(NSR)technology is one of the most effective ways to remove NOx from lean-bum engines.The traditional NSR catalyst Pt-BaO/Al2O3 costs expensive and has poorer water/sulfur resistance and thermal stability.Besides,it has lower NOx storage and removal efficiency in the low temperature range Therefore,it is of great significance to develop a economic and efficient NSR catalyst.In this paper,manganese-based mixed oxide catalysts Mn/MgAlOx were obtained by calcining transition metal Mn-doped hydrotalcite-like precursors,afterwards,they were modified by loading noble metal Pt and alkaline earth metal BaO.The physical and chemical properties of the catalysts were characterized by XRD,N2 adsorption/desorption,FTIR,SEM,XPS,H2-TPR techniques;the NOx storage and reduction performance of the catalysts were tested by the NOx adsorption/desorption and lean/rich cycle experiments.Furthermore,the programmed temperature oxidation experiments were used to study the soot oxidation performance of the catalysts;the in situ DRIFTS technology was used to study the species and storage path ofNOx on the surface of the catalysts.The details are as followsA series of Mn-doped hydrotalcite-like mixed oxide(Mn/MgAIOx)catalysts were synthesized using CTAB(Cetyl Trimethyl Ammonium Bromide)-assisted co-precipitation method,and their low-temperature NOx storage and release properties were studied.The researches showed that Mn doping changed the crystal phase structure of the catalysts,increased the specific surface area and pore diameter of the catalysts,and improved the redox capacity of the samples.Besides,the XPS results showed that the Mn15 and Mn20 samples had more surface Mn3+,which could promote the oxidation of NO.Besides,with the increase of Mn doping,the NOx storage capacities of the catalysts were enhanced Among them,the Mn15 catalyst(the sample containing 15wt%of Mn)had the highest NOx storage capacity.In addition,the Mn/MgAIOx catalysts exhibited high NOx storage capacity(NSC)at low temperature range(1 50-300℃),which was related to their increased surface area,improved reducibility and higher surface Mn3+content.The largest NSC measured,426μmol/g,was observed for NOx adsorption at 200℃ on Mn15 sample.The in situ DRIFTS spectra proved that the the main storage species of Mn-doped oxide catalysts at lower temperatures(<300 0C)were nitrites、while at higher temperatures(>300 0C)were mainly thermostable nitrates.Furthermore,the NSR cycling tests revealed the NOx removal rate of Mnl5 sample can reach above 50%within the temperature range of 150-250℃Moreover,the manganese-based oxide catalysts had good resistance to CO2 and soot,but they had poor resistance to SO2 and water vapor.The Mn15 sample was impregnated with different contents of noble metal Pt and alkaline earth metal BaO.The effects of noble metal and alkaline earth metal loading,pretreatment atmosphere and reaction temperatures on NOx adsorption/desorption performance of the catalysts were discussed.The studies showed that the optimal Pt loading was 0.5%.Besides,the PM(0.5%Pt/Mn15)sample showed higher NOx storage capacity after O2 atmosphere pretreatment.It may be due to the enhanced surface oxygen activity of the catalyst after O2 pretreatment.The PM sample exhibited excellent NOx storage performance in the temperature range of 250-300℃,and its active temperature window was wide.The NOx adsorption capacity(NAC)and storage capacity(NSC)of the PBM(0.5%Pt/15%BaO/Mnl5)sample were continuously improved with the increase of the BaO loading,which was related to the alkaline enhancement of the catalysts after BaO loading Taking all factors into consideration,the optimal BaO load is 15%.The PBM sample had the largest NOx storage capacity at 300℃,measured as 704μmol/g.Besides,the NOx storage of PM and PBM catalysts followed the "nitrate path",and the BaO loading was more conducive to the formation of nitrate species.In addition,the PBM smple had good resistance to sulfur,but had poor resistance to CO2,H2O and soot.In addition,the characterization results showed that the loading of Pt and BaO reduced the specific surface area,total pore volume and average pore diameter of the catalysts,which may be due to the partial blockage of the samples caused by the loading.The particle size of Pt particles was small and uniformly distributed,and Pt was well dispersed on the surface of the catalysts The above researches showed that the PBM catalyst had superior NOx absorption/desorption perfonnance.Therefore,the influences of lean/rich cycling frequency,reducing agent type and temperatures on the NSR cycle performance of PBM sample were further discussed.Moreover,the interaction between soot oxidation and NOx was also explored When the lean/rich cycling frequency was 2/1 min,the average NOx removal rate was the highest,88.3%.In addition,when H2 was used as reducing agent with optimal lean/rich cycling frequency,the removal effect of NOx was the best.The average NOx removal rate of the PBM catalyst remained above 88%in the temperature range of 300-400℃,and it increased with increasing temperature,up to 97.4%at 400℃.This could be explained by the fact that NOx was stored primarily in the form of stable nitrates at higher temperatures Moreover,the presence of soot reduced the NOx adsorption capacity and NOx storage capacity of the catalysts,which was mainly affected by weakening the oxidation activity of NO and reducing the stability of NOx adsorbed species.When soot was in tight contact with catalysts,the soot oxidation activity of the PBM sample under O2 and NO+O2 atmosphere was the best.This phenomenon indicated that the co-loading of noble metals and alkaline earth metals could promote the soot oxidation activity of the catalyst.When soot was in tight contact with the catalysts,the soot oxidation activity of the PBM sample under NO＋O2 atmosphere was significantly better than that of O2 atmosphere,which was mainly due to the promotion of the NO2-assist mechanism.Furthermore,the soot oxidation performence of the soot particles and catalysts in the loose contact mode was significantly lower than that in the tight contact mode.Furthermore,the order of soot oxidation activities of the catalysts under both O2 and NO+O2 atmosphere was:PBM>Mn15>PM,which was consistent with the results of NO-TPO.