基于DFT模拟不同拓扑结构分子筛的氨氧化机理
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摘要
采用铜离子交换后的分子筛对NH_3具有较好的N_2选择性和催化氧化脱除效果,但对于不同拓扑结构分子筛上氨氧化机理间的对比研究鲜有报道。研究了三种不同拓扑结构(MFI、BEA、MOR)分子筛上NH_3的催化氧化性能,并借助密度泛函理论(DFT)对反应机理进行详细阐释。三种分子筛的氨氧化机理对比研究结果表明,离子交换率相同的条件下,MFI构型的Cu-ZSM-5分子筛在300℃时,NH_3转化率高达90%,是三种分子筛中NH_3催化净化效果最好的催化剂。DFT计算结果表明,反应机理包括以下三部分:(1)吸附气态NH_3分子,随后与活性氧原子作用生成OHNH_2;(2)通过E-R机理,由生成的OHNH_2和气相的NH_3分子反应生成中间物种N_2H_4;(3)N_2H_4物种和活性氧原子继续作用,逐步脱氢,最后生成N_2和H_2O。通过反应路径活化能计算明确OHNH_2物种的生成所需能量最高,是反应的速控步骤。
After ion-exchange with Cu ion,zeolites have good efficiency on selective catalytic oxidation of NH_3,but there are few reports on the mechanism of NH_3-SCO over zeolites with different topologies.Therefore,the catalytic oxidation of ammonia over three topological zeolites(MFI, BEA, MOR) were studied and the mechanisms were investigated by density functional theory(DFT).The results show that MFI(Cu-ZSM-5) exhibits the highest NH_3conversion of 90% at 300 ℃ under the same copper exchange rate.The DFT simulation suggests that the mechanism consists of three major parts:(1)adsorption of gaseous ammonia molecules,followed by interaction with reactive oxygen atoms to form OHNH_2;(2)gas phase NH_3 molecule reacts with OHNH_2 to produce stable N_2H_4 species through E-R mechanism;(3)N_2H_4 species and active oxygen atoms continue to act,and finally generate nitrogen and water.The formation of OHNH_2 species is the rate-determining step of the reaction and energy required for it is highest.
After ion-exchange with Cu ion,zeolites have good efficiency on selective catalytic oxidation of NH_3,but there are few reports on the mechanism of NH_3-SCO over zeolites with different topologies.Therefore,the catalytic oxidation of ammonia over three topological zeolites(MFI, BEA, MOR) were studied and the mechanisms were investigated by density functional theory(DFT).The results show that MFI(Cu-ZSM-5) exhibits the highest NH_3conversion of 90% at 300 ℃ under the same copper exchange rate.The DFT simulation suggests that the mechanism consists of three major parts:(1)adsorption of gaseous ammonia molecules,followed by interaction with reactive oxygen atoms to form OHNH_2;(2)gas phase NH_3 molecule reacts with OHNH_2 to produce stable N_2H_4 species through E-R mechanism;(3)N_2H_4 species and active oxygen atoms continue to act,and finally generate nitrogen and water.The formation of OHNH_2 species is the rate-determining step of the reaction and energy required for it is highest.
引文
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[29]林涛,徐海迪,李伟,等.整体式Mn-Fe/ZrO2-TiO2催化剂的制备及在低温NH3-SCR反应中的性能[J].高等学校化学学报,2009,33(11):2240-2246.Lin Tao,Xu Haidi,Li Wei,et al.Preparation of Mn-Fe/ZrO2-TiO2 monolith catalyst and its properties for low-temperature NH3-SCR reaction[J].Chemical Journal of Chinese Universities,2009,33(11):2240-2246.
[30]Trout B L,And A K C,Bell A T.Analysis of the thermochemistry of NOx decomposition over Cu-ZSM-5 based on quantum chemical and statistical mechanical calculations[J].Journal of Physical Chemistry,1996,100(44):17582-17592.
[2]Hung C M.Fabrication,characterization and evaluation of the cytotoxicity of platinum-rhodium nanocomposite materials for use in ammonia treatment [J].Powder Technology,2011,209(1):29-34.
[3]Chmielarz L,Jabońska M.Advances in selective catalytic oxidation of ammonia to dinitrogen:a review[J].Rsc Advances,2015,5(54):43408-43431.
[4]Chenko N I.Catalytic oxidation of ammonia[J].Russian Chemical Reviews,1976,45(12):1119-1122.
[5]Zhang Q L,Wang H M,Ning P.In situ DRIFTS studies on CuO-Fe2O3 catalysts for low temperature selective catalytic oxidation of ammonia to nitrogen[J].Applied Surface Science,2017,419:733-743.
[6]Williamson W B,Flentge D R,Lunsford J H.Ammonia oxidation over Cu(Ⅱ)NaY zeolites[J].Journal of Catalysis,1975,37(2):258-266.
[7]Zawadzki J.The mechanism of ammonia oxidation and certain analogous reactions[J].Discussions of the Faraday Society,1950,8(8):140-152.
[8]SuQ Y,Feng T Z,Xie L,et al.Kinetic modelling of automotive exhaust NOx reduction over platinum[J].Topics in Catalysis,2016,59(10-12):952-960.
[9]Popa C,Santen R A V,Jansen A P J.Density-functional theory study of NHx oxidation and reverse reactions on the Rh(111) surface[J].Journal of Physical Chemistry C,2007,111(27):9839-9852.
[10]Chmielarz L,Wgrzyn A,Wojciechowska M.Selective catalytic oxidation (SCO) of ammonia to nitrogen over hydrotalcite originated Mg-Cu-Fe mixed metal oxides[J].Catalysis Letters,2011,141(9):1345-1354.
[11]Chmielarz L,Jabońska M,Strumiński A.Selective catalytic oxidation of ammonia to nitrogen over Mg-Al,Cu-Mg-Al and Fe-Mg-Al mixed metal oxides doped with noble metals[J].Applied Catalysis B:Environmental,2013,131(3):152-162.
[12]王冬东,马丽萍,唐剑骁,等.离子交换法改性ZSM-5的NH3-SCR性能研究[J].化学工程,2018,46(2):5-9.Wang Dongdong,Ma Liping,Tang Jianxiao,et al. NH3-SCR activity of modified ZSM-5 prepared by aqueous ion-exchange method[J].Chemical Engineering,2018,46(2):5-9.
[13]Liu N,Shi D J,Zhang R D,et al.Highly selective catalytic combustion of acrylonitrile towards nitrogen over Cu-modified zeolites [J].Catalysis Today,2018,(4):54-57.
[14]Wang T,Zhang X Y,Liu J,et al.Plasma-assisted catalytic conversion of NO over Cu-Fe catalysts supported on ZSM-5 and carbon nanotubes at low temperature[J].Fuel Processing Technology,2018,178:53-61.
[15]Sultana A,Nanba T,Sasaki M.Selective catalytic reduction of NOx with NH3 over different copper exchanged zeolites in the presence of decane[J].Catalysis Today,2011,164(1):495-499.
[16]刘宁,王继琼,张润铎,等.新型八元环孔结构分子筛Cu/SAPO-35 应用于NH3-SCR的研究[J].高等学校化学学报,2016,37(10):1817-1825.Liu Ning,Wang Jiqiong,Chen Biaohua,et al.Study of eight membered ring zeolitic catalyst of Cu/SAPO-35 over NH3-SCR[J]. Chemical Journal of Chinese Universities,2016,37(10):1817-1825.
[17]Pirngruber G D,Roy P K,Prins R.The role of auto reduction and of oxygen mobility in N2O decomposition over Fe-ZSM-5[J].Journal of Catalysis,2007,246(1):147-157.
[18]Zhang R,Liu N,Lei Z.Selective transformation of various nitrogen-containing exhaust gases toward N2 over zeolite catalysts[J].Chemical Reviews,2016,116(6):3658-3658.
[19]李懿, 刘宁,张润铎. NO与NH3在不同构型分子筛催化剂中吸附和扩散的分子模拟[J].工业催化,2016,24(10):33-40.Li Yi,Liu Ning,Zhang Runduo.Molecular simulations of adsorption and diffusion of NO and NH3 over zeolite catalysts with various structure configurations[J]. Industrial Catalysis,2016,24(10):33-40.
[20]徐如人, 庞文琴.分子筛与多孔材料化学[M].北京:科学出版社,2004.73-75.
[21]Parvulescu V I,Grange P,elmon B.NO decomposition over physical mixtures of Cu-ZSM-5 with zeolites or oxides[J].Applied Catalysis B:Environmental,2001,33(3):223-237.
[22]Dědeek J,Bortnovsky O,Vondrová A.Catalytic activity of Cu-Beta zeolite in NO decomposition:effect of copper and aluminum distribution[J].Journal of Catalysis,2001,200(1):160-170.
[23]Jiang H,Wang H,Liang F.Direct decomposition of nitrous oxide to nitrogen by in situ oxygen removal with a perovskite membrane[J].Angewandte Chemie International Edition,2010,48(16):2983-2986.
[24]Groothaert M,Van Bokhoven J,Battiston A.Bis(m-oxo)dicopper in Cu-ZSM-5 and its role in the decomposition of NO:a Combined in situ XAFS,UV-vis-NIR and kinetic study[J].Journal of the American Chemical Society,2003,125(25):7629-7640.
[25]Smeets P J,Groothaert M H,Teeffelen R M V.Direct and NO-assisted N2O decomposition over Cu-zeolites[J].Studies in Surface Science & Catalysis,2007,170(170):1080-1087.
[26]Frisch M J,Trucks G W,Schlegel H B,et al.Gaussian 09,Revision B.01,Gaussian,Inc:Wallingford,CT,2010.
[27]Stephens P J,Devlin F J,Ashvar C S. Theoretical calculation of vibrational circular dichroism spectra[J].Faraday Discussions,1994,99(11):103-119.
[28]Liu N,Yuan X N,Zhang R,et al.Mechanistic insight into selective combustion of acrylonitrile(C2H3CN):NCO formation and its further transformation towards N2[J].Physical Chemistry Chemical Physics,2017,19(1):7971-7977.
[29]林涛,徐海迪,李伟,等.整体式Mn-Fe/ZrO2-TiO2催化剂的制备及在低温NH3-SCR反应中的性能[J].高等学校化学学报,2009,33(11):2240-2246.Lin Tao,Xu Haidi,Li Wei,et al.Preparation of Mn-Fe/ZrO2-TiO2 monolith catalyst and its properties for low-temperature NH3-SCR reaction[J].Chemical Journal of Chinese Universities,2009,33(11):2240-2246.
[30]Trout B L,And A K C,Bell A T.Analysis of the thermochemistry of NOx decomposition over Cu-ZSM-5 based on quantum chemical and statistical mechanical calculations[J].Journal of Physical Chemistry,1996,100(44):17582-17592.