机动车污染物排放是我国大气复合污染形成的重要原因之一.尽管柴油车在我国机动车保有量中所占比例不到20%，但其排放的颗粒污染物(PM)和氮氧化物(NOx)分担率均超过60%.因此，控制柴油车尾气排放成为我国亟待解决的大气污染问题.目前，氨选择性催化还原NOx技术(NH3-SCR)已规模化应用于柴油车污染排放控制，出于安全性考虑，以尿素水溶液作为氨的来源.但NH3-SCR技术应用于柴油车尾气净化存在如下缺点：需要布建庞大的尿素添加基础设施、后处理系统复杂等.与此相反，以车载燃油为还原剂来源的HC-SCR技术可有效规避上述难题，展现了较好的应用前景.但是，直接以柴油为还原剂时， HC-SCR对NOx净化的效率还难以满足日益严格的排放法规的要求，因此需要深入研究HC选择性还原NOx的微观机制与构效关系，并以此为指导，发展以车载燃料为还原剂来源的高效净化NOx的新原理和新方法.已有的研究表明，银/氧化铝(Ag/Al2O3)具有优异的催化乙醇选择性还原NOx的能力，是最有希望应用于柴油车尾气NOx净化的催化剂-还原剂组合体系.鉴于此，本论文以Ag/Al2O3催化剂上乙醇-SCR反应为研究对象，以密度泛函理论计算方法(DFT)搭建了Ag/Al2O3催化剂的理论模型，考察了反应物乙醇(CH3CH2OH)、关键中间体(烯醇式物种CH2=CHO?和?NCO)在Ag/Al2O3催化剂上的吸附特征，采用电子态密度分析(DOS)研究了以上物种被活化的电子机制，以期甄别Ag/Al2O3催化乙醇选择性还原NOx的活性位结构，为高性能的HC-SCR催化剂设计提供指导.
依据化学态的不同， Ag/Al2O3催化剂上活性组分银可分为：高度分散的离子态(Ag+、在催化剂表面以Ag?O形式存在)、部分氧化团簇(Agnδ+)和金属颗粒银(Agn0)，其中氧化态的银是催化乙醇选择性还原NOx的活性组分. Al2O3载体的主要暴露晶面为(110)和(100)，在上述晶面上Al的配位状态存在明显差异，显著影响了银物种的锚定与分散，形成了具有不同键合特征的Ag?O?Al结构.基于对Al2O3暴露晶面上Al配位状态的分析，搭建了6种Ag?O?Al结构模型.结合Al MAS NMR对Ag/Al2O3实际催化剂的表征结果和理论模型吸附能的分析，获得了最为可能的两种Ag?O?Al结构： Ag?O?Altetra(AlO4)和Ag?O?Alocta(AlO6)；前者为AgO与Al2O3(110)面Altrip位键合形成的特征结构(Al最终为四配位)，后者系AgO锚定于Al2O3(100)面Alpenta位的能量最优结构(Al最终为六配位).
在Ag?O?Altetra上， Altetra位具有较强的酸性， Ag、Al原子轨道的杂化融合有利于电子转移；以上特性促进CH3CH2OH、CH2=CHO?、?NCO的吸附活化.在HC-SCR反应中，关键中间体?NCO通过与NOx直接反应可形成最终产物N2和CO2.可见，?NCO中N=C键的拉伸活化、断裂对上述反应的发生至关重要.由电子态密度分析可知， N=Cσ键能向Ag?O?Altetra中Altetra位转移电子，而Ag与Al的轨道融合能反馈电子到N=C π键；在这两种电子转移机制作用下，?NCO中的N=C键被最大程度弱化，有利其断裂，转化为最终产物N2和CO2.而Ag?O?Alocta上，并没有N=C键的活化拉伸，反而呈现出N=C键收缩趋势，不利于N=C键的断裂与最终产物的形成.由此推定， Ag?O?Altetra是Ag/Al2O3催化剂上HC-SCR反应的活性中心.
The adsorption of ethanol and important intermediates onto Ag/Al2O3 catalyst employed in the selective catalytic reduction of NOx by ethanol was simulated by density functional theory. Consid-ering the interaction between Ag metal and Al2O3 support, typical Ag–O–Al entities, i.e., Ag–O–Altetra and Ag–O–Alocta, (tetra =tetrahedral and octa= octahedral refer to the coordination sites of Al), were selected as potential adsorption sites on the surface of the catalyst. Ethanol, and enolic and isocyanate species were preferentially adsorbed and activated by Ag–O–Altetra entities rather than by Ag–O–Alocta entities. The strong Lewis acidity of Altetra in the Ag–O–Altetra entity was very im-portant, enabling the entity to accept an electron via forward donation from either the C–Oσbond in ethanol or the N–Cσbond in the?NCO species. Moreover, the hybridization of the Ag and Al orbitals was critical for electron back donation from the Ag–O–Altetra entity to the C–Cπbond in the enolic species or N–Cπbond in the?NCO species. The significant activation of the N–C bond in?NCO on the Ag–O–Altetra sites facilitated cleavage of?NCO to form N2. Thus, we can conclude that the acidity of the Al site and the interaction between Ag and Al play key roles in the selective cata-lytic reduction of NOx by ethanol over Ag/Al2O3.
参考文献
| [1] | Morin S;Savarino J;Frey M M;Yan N,Bekki S,Bottenheim J W,Martins J M F .[J].Science,2008,322:730. |
| [2] | He H;Ma Q X;Ma J Z;Zhang H X,Wang Y S,Ji D S,Tang G Q,Chu B W,Liu C,Hao J M .[J].Sci Rep,2014,4:4172. |
| [3] | Burch R;Breen J P;Meunier F C .[J].Appl Catal B,2002,39:283. |
| [4] | Shimizu K I;Satsuma A .[J].Phys Chem Chem Phys,2006,8:2677. |
| [5] | Liu ZM;Woo SI .Recent advances in catalytic DeNO(X) science and technology[J].Catalysis Reviews. Science and Engineering,2006(1):43-89. |
| [6] | He H;Zhang X L;Wu Q;Zhang C B,Yu Y B .[J].Catal Surv Asia,2008,12:38. |
| [7] | Granger, P.;Parvulescu, V.I. .Catalytic NO_x abatement systems for mobile sources: From three-way to lean burn after-treatment technologies[J].Chemical Reviews,2011(5):3155-3207. |
| [8] | Meunier F C;Ross J R H .[J].Appl Catal B,2000,24:23. |
| [9] | Bethke KA.;Kung HH. .SUPPORTED AG CATALYSTS FOR THE LEAN REDUCTION OF NO WITH C3H6[J].Journal of Catalysis,1997(1):93-102. |
| [10] | 贺泓,张润铎,余运波,刘俊锋.富氧条件下氮氧化物的选择性催化还原Ⅰ.Ag/Al2O3催化剂上C3H6选择性催化还原NO的性能[J].催化学报,2003(10):788-794. |
| [11] | Yu Y B;He H;Feng Q C .[J].J Phys Chem B,2003,107:13090. |
| [12] | She X;Flytzani-Stephanopoulos M .[J].J Catal,2006,237:79. |
| [13] | Zhang R D;Kaliaguine S .[J].Appl Catal B,2008,78:275. |
| [14] | Deng H;Yu Y B;Liu F D;Ma J Z,Zhang Y,He H .[J].ACS Catal,2014,4:2776. |
| [15] | Liu Z M;Ma L L;Junaid A S M .[J].J Phys Chem C,2010,114:4445. |
| [16] | Hu, C.H.;Chizallet, C.;Mager-Maury, C.;Corral-Valero, M.;Sautet, P.;Toulhoat, H.;Raybaud, P. .Modulation of catalyst particle structure upon support hydroxylation: Ab initio insights into Pd_(13) and Pt_(13)/γ-Al _2O_3[J].Journal of Catalysis,2010(1):99-110. |
| [17] | Digne M;Sautet P;Raybaud P;Euzen P,Toulhoat H .[J].J Catal,2002,211:1. |
| [18] | Digne M;Sautet P;Raybaud P;Euzen P;Toulhoat H .Use of DFT to achieve a rational understanding of acid-basic properties of gamma-alumina surfaces[J].Journal of Catalysis,2004(1):54-68. |
| [19] | Lee J H;Yezerets A;Kung M C;Kung H H.[M].Chem Commun,2001:404. |
| [20] | Yeom YH;Li MJ;Sachtler WMH;Weitz E .A study of the mechanism for NOx reduction with ethanol on gamma-alumina supported silver[J].Journal of Catalysis,2006(1):100-110. |
| [21] | Tamm S;Ingelsten H H;Palmqvist A E C .[J].J Catal,2008,255:304. |
| [22] | Yu Y B;He H;Feng Q C;Gao H W,Yang X .[J].Appl Catal B,2004,49:159. |
| [23] | Yu Y B;Gao H W;He H .[J].Catal Today,2004,93-95:805. |
| [24] | He H;Yu Y B .[J].Catal Today,2005,100:37. |
| [25] | Satsuma A;Shimizu K I .[J].Prog Energy Combus Sci,2003,29:71. |
| [26] | Sumiya S;He H;Abe A;Takezawa N,Yoshida K .[J].J Chem Soc Faraday Trans,1998,94:2217. |
| [27] | Chansai, S.;Burch, R.;Hardacre, C.;Breen, J.;Meunier, F. .Investigating the mechanism of the H_2-assisted selective catalytic reduction (SCR) of NOx with octane using fast cycling transient in situ DRIFTS-MS analysis[J].Journal of Catalysis,2010(1):49-55. |
| [28] | Chansai, S.;Burch, R.;Hardacre, C.;Breen, J.;Meunier, F. .The use of short time-on-stream in situ spectroscopic transient kinetic isotope techniques to investigate the mechanism of hydrocarbon selective catalytic reduction (HC-SCR) of NO_x at low temperatures[J].Journal of Catalysis,2011(1):98-105. |
| [29] | Gao HW;He H .Conformational analysis and comparison between theoretical and experimental vibration spectra for isocyanate species on Ag/Al2O3 catalyst[J].Spectrochimica acta, Part A. Molecular and biomolecular spectroscopy,2005(6):1233-1238. |
| [30] | Zhao S;Ren Y L;Wang J J;Yin W P .[J].J Mol Struct:THEOCHEM,2009,897:100. |
| [31] | He H;Li Y;Zhang X L;Yu Y B,Zhang C B .[J].Appl Catal A,2010,375:258. |
| [32] | Kim M K;Kim P S;Baik J H;Nam I S,Cho B K,Oh S H .[J].Appl Catal B,2011,105:1. |
| [33] | Breen J P;Burch R;Hardacre C;Hill C J .[J].J Phys Chem B,2005,109:4805. |
| [34] | Shimizu K I;Tsuzuki M;Kato K;Yokota S,Okumura K,Satsuma A .[J].J Phys Chem C,2007,111:950. |
| [35] | Korhonen S T;Beale A M;Newton M A;Weckhuysen B M .[J].J Phys Chem C,2011,115:885. |
| [36] | Yan, Y.;Yu, Y.;He, H.;Zhao, J..Intimate contact of enolic species with silver sites benefits the SCR of NOx by ethanol over Ag/Al _2O _3[J].Journal of Catalysis,2012:13-26. |
| [37] | Kwak J H;Hu J Z;Kim D H;Szanyi J S,Peden C H F .[J].J Catal,2007,251:189. |
| [38] | Kwak J H;Hu J Z;Lukaski A;Kim D H,Szanyi J S,Peden C H F .[J].J Phys Chem C,2008,112:9486. |
| [39] | Kwak JH;Mei DH;Yi CW;Kim DH;Peden CHF;Allard LF;Szanyi J .Understanding the nature of surface nitrates in BaO/gamma-Al2O3 NOx storage materials: A combined experimental and theoretical study[J].Journal of Catalysis,2009(1):17-22. |
| [40] | Kwak J H;Hu J Z;Mei D H;Yi C W,Kim D H,Peden C H F,Allard L F,Szanyi J .[J].Science,2009,325:1670. |
| [41] | Deng H;Yu Y B;He H .[J].J Phys Chem C,2015,119:3132. |
| [42] | Ukisu Y;Sato S;Muramatsu G;Yoshida K .[J].Catal Lett,1991,11:177. |
| [43] | Deng H;Yu Y B;He H.[M].Catal Today,2015 |
| [44] | Hoffman R .[J].Rev Mod Phys,1988,60:601. |
| [45] | Glassey WV.;Hoffmann R. .A molecular orbital study of surface-adsorbate interactions during the oxidation of CO on the Pt(111) surface[J].Surface Science: A Journal Devoted to the Physics and Chemistry of Interfaces,2001(1/3):47-60. |
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