| Nitrogen oxides(NOx)emitted from diesel vehicle exhausts are currently one of the main atmospheric pollutants.Ammonia selective catalytic reduction(NH3-SCR)is currently the most effective NOx control technology,and a highly efficient and stable denitration catalyst is the core of the technology.The cerium-based catalyst has the advantages of low price,adjustable storage and release of oxygen,and excellent anti-poisoning ability at medium-and high-temperature.It is a research hotspot in the field of NH3-SCR in recent years.However,the use of cerium-based catalysts in diesel vehicle denitration generally suffer from weak low-temperature reactivity,poor high temperature selectivity,and awful low-temperature anti-poisoning ability,which limits the industrial application.How to solve the above problems is the key to the development of efficient and stable NH3-SCR catalyst in wide temperature.Therefore,it is necessary to deeply understand the design principles and reaction mechanism of the wide-temperature NH3-SCR catalyst,the deactivation mechanism of the catalyst in a toxic atmosphere,and the basis for regeneration.In this study,the development of high-efficiency wide-temperature denitration catalysts for diesel vehicles has been studied.Based on the acidic site strengthening strategies i.e.,acidic site distribution regulation and catalyst crystal type regulation,a series of cerium-based NH3-SCR wide-temperature catalysts with different structures have been designed and synthesized.Comprehensive characterization tests,in-situ spectroscopy techniques and DFT theoretical calculations have been used to explore the reaction path of the cerium-based NH3-SCR wide-temperature catalyst,revealing its anti-sulfur and-water poisoning mechanism as well as the regeneration route.The details are as follows:1.Based on the acid site strengthening strategy of the acid site distribution control,the halloysite-modified ceria(CeO2/HAT)and phosphate-modified ceria(Px/CeO2)catalysts were developed.The former uses hollow tubular halloysite with rich acid sites on the inner wall as a carrier,and supports cerium oxide on the outer wall to physically separate the acid-and redox-sites of the catalyst,effectively avoiding NH3 high temperature over-oxidation and improving the medium-and high-temperature activity and selectivity.The latter uses phosphate to modify the outer surface of ceria to form a Br(?)nsted acid network of P-OH,which improves the adsorption and activation ability of NH3 on the outer surface,and thus reduces the NH3 over-oxidation,thereby improving the medium-and high-temperature selectivity.P8/CeO2 catalyst with T90 NOx conversion reaction window of 175-400?,has the most excellent denitration activity and the widest reaction temperature window.Moreover,P8/CeO2 catalyst has the lowest oxygen vacancy formation energy and the highest oxygen vacancy concentration,which solves the problem of the mismatch between the oxidation rate and the reduction rate in CeO2,and accelerates the redox cycle rate of Ce3+and Ce4+,which is beneficial to improve the low-temperature activity of NH3-SCR.Compared with the method for controlling the distribution of acid sites by physical separation,the modification of the surface acidic outer layer can improve the denitration performance of the cerium-based catalyst better.2.Based on the acid site strengthening strategy of the catalyst crystal form control,hexagonal cerium phosphate(h-CPO)and monoclinic cerium phosphate(m-CPO)with different acids were synthesized as denitration catalysts.The denitration activity of h-CPO catalyst is stronger,with T80 NOx conversion reaction window of 150-400?.The surface protonation ability of h-CPO is stronger,the content of hydroxyl groups is higher,and the special pore structure for 'water storage' which provides hydroxyl reserves for the reaction.Its surface acidity is about 4 times that of m-CPO.The structure of the h-CPO catalyst is very stable,and the redox cycle cannot be realized through Ce3+/Ce4+.Cerium within h-CPO as a lack of electricity center to adsorb NO and NO2 and forms active intermediate species to participate in the NH3-SCR reaction.3.The active species and reaction mechanism of P8/CeO2 and h-CPO acid surface enhanced catalyst were clarified.The key to improving the catalytic activity in the low temperature section is the reaction of the NH4+species formed at the Br(?)nsted acid site with the NO2(g)species adsorbed by the lack of electricity,the key to keep the catalytic activity at the high temperature section is the reaction of the-NH2 species formed in the Lewis acid site with gaseous NO.Therefore,the denitration paths of P8/CeO2 and h-CPO catalysts are all related to temperature.NOx in the low-temperature section is mainly removed through the 'Fast SCR' path in accordance with the L-H mechanism,while the 'Standard SCR' occurs in the high-temperature section following the E-R mechanism.4.The mechanism of P8/CeO2 and h-CPO catalysts against sulfur water poisoning was revealed.When both SO2 and H2O exist,due to the reductive ablity and competitive reactions,they mainly affect the formation of low-temperature active species namely NH4+and NO2(g)of P8/CeO2 and h-CPO below 200-300?.Therefore,the low-temperature activity of the catalyst was suppressed.But for high-temperatur activity,the retention of acid sites,the reducibility of SO2 and H2O slows down the inert nitrate deposition and inhibits the excessive oxidation of NH3 to improve the utilization rate,thereby broadening the reaction activity window towards high-temperature.5.The poisoning substances over the surface of P8/CeO2 and h-CPO are different.The former is Ce2(SO4)3 and the later is(NH4)2SO4/NH4HSO4.Neither P8/CeO2 nor h-CPO will be completely poisoned and inactivated,as NH3 in the reaction gas can promote the decomposition of Ce2(SO4)3 into(NH4)2SO4/NH4HSO4,while NO+O2 can promote the decomposition of(NH4)2SO4/NH4HSO4 into N2+H2O.Therefore,the formation and decomposition of Ce2(SO4)3 and(NH4)2SO4/NH4HSO4 on the surface of P8/CeO2 and h-CPO reached a dynamic balance,and the catalyst showed excellent resistance to sulfur water poisoning. |
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