Cu/ZnO催化剂的表面结构及其对小分子的活化
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摘要
Cu/ZnO体系一直以来被用作从合成气制甲醇的催化剂,并且是很有潜力的合成低碳醇催化剂之一,但是,其催化机制的不明严重妨碍了对其的进一步改性。本论文工作从量子化学第一性原理出发,详细探究了催化剂的表面结构及其对H_2的活化。
由于介于离子和共价之间,且Zn和O都采用sp~3杂化,ZnO表面的很多性质都为局域性质,包括其表面弛豫重构的程度和其表面n型缺陷所定域的电子数。在H_2吸附方面,实验中报道的typeⅠ型快速可逆解离是由表面强极性的Zn-O键将H-H拉裂所致,解离位点电场越强,活化能就会越低。因此,实验中观测到的应该就是H_2在完整ZnO(1O(?)0)表面的解离吸附。对于缺陷位,由于缺乏这样的强电场,H_2的解离则需要氧空位(V_o)或者间隙锌(Zn_i)处的过剩电荷对H-H反键进行氧化加成,因此活化能较高。在吸附能上,H_2在ZnO两个完整非极性表面解离均为中等程度放热,其中在(10(?)0)表面1×1覆盖度的计算结果更接近实验值。我们考察了覆盖度对吸附热的影响,结果发现对于完整表面,覆盖度越大则吸附能反而增大,推测这是由于异裂后相邻的H~(δ+)和H~(δ-)之间库仑作用所致。对于缺陷位,H_2在V_o处的解离为吸热,推测原因是需要额外能量以破坏表面金属键。H_2在Zn_i处解离则放热较多,联系到前面所提到的活化能,实验中所描述的typeⅡ型缓慢且不可逆的解离可能与Zn_i相关。
接下来的工作是Cu在ZnO表面的沉积。我们的计算工作显示,沉积的Cu受到ZnO载体的很大影响,且Cu与ZnO之间有着很大的作用能。但是,由于Cu的d轨道几乎填满而自由价有限,Cu与ZnO的作用能以及沉积Cu之间的作用能存在竞争关系。因此,在较小单位晶胞且较高对称性的(10(?)0)表面,Cu的沉积图景依次为规则的之字形结构,重排后生成的弓字型结构,最后生成三维团簇。对于较大单位晶胞和较低对称性的(11(?)0)表面,Cu与ZnO载体会存在更强的作用,导致Cu生长的图景依次为不规则的岛状或之字形结构,二维条带,不太平整的表面Cu单层,最后生长为扁平的二维透镜状团簇。在这些吸附构型中,Cu的迁移能一般很低,这反映出Cu与ZnO之间除了共价键之外还存在无方向性的金属键,同时,这也是催化剂失活的重要原因。
Cu/ZnO is widely used as a catalyst for methanol synthesis from syngas.It is also one of candidate catalysts for higher alcohol synthesis.However,the atomic nature and catalytic mechanism of Cu/ZnO system remains obscure,which hinder further impromentation.In this thesis,by means of ab initio method,we investigated its possible surface landscape and the activation mechanism of H_2 molecule.
Firstly,we investigated the ZnO substrate,which itself is utilized as catalyst for some hydrogenation process.Because ZnO sits in the borderline between ionic and covalent substances,and because both Zn and O are hybridized as sp~3,many phenomenons are local,such as surface relaxation,surface reconstruction,and electron localization of n-type surface defects.For H_2 adsorption and subsequent dissociation,experimentally there are two types,one is fast and reversible,called typeⅠ,while another is slow and irreversible,called typeⅡ.After many calculations,we designated H_2 adsorption at perfect(10(?)0)surface as the typeⅠ. Reason for low barrier is that H-H bond is torn apart by strong electronic field near polar Zn-O dimers.Thus stronger the electronic field,lower the activation energy. For H_2 adsorption at n-type defect sites,because the lack of neighboring polar Zn-O dimers,oxidative addition of H_2 molecule onto oxygen vacancy(V_o) or zinc interstitial(Zn_i) is needed during the dissociation.Concerning energy of dissociation,we found that it is moderate exothermic for H_2 adsorption over two perfect nonpolar surfaces.An abnormal phenomenon is that this value increases as coverage,probabely due to the interaction between adjacent H~(δ+) and H~(δ-).For defect sites,dissociation of H_2 at V_o is endothermic,while at Zn_i is much exothermic. Considering the above mentioned value of reaction barrier,the dissociation at Zn_i might be designated as typeⅡ.
Our following work is copper dissociation above ZnO.It is indicated in our work that deposited copper frameworks are dictated by its ZnO substrate.And, because the limited bonding ability of copper atom,whose electronic configuration is 3d~(10)4s~1,interaction between Cu and ZnO and interaction among deposited coppers are generally competitive.Thus,above relatively small and highly symmetric(10(?)0)surfaces,copper depositions follow the order of regular zigzag structures,then undertake a rearrangement to form new zigzag structures,and finally encaged to 3D clusters.While,above relatively larger and lower symmetric (1120) surface,copper depositions follow the order of irregular islands or zigzag structures,2D streaks,2D surface layers and finally flat,disk like cages.Migration barriers of deposited coppers are generally low,indicating some metallic nature between Cu and ZnO,and,this could be the major reason for the easy deactivation of Cu/ZnO catalyst.
由于介于离子和共价之间,且Zn和O都采用sp~3杂化,ZnO表面的很多性质都为局域性质,包括其表面弛豫重构的程度和其表面n型缺陷所定域的电子数。在H_2吸附方面,实验中报道的typeⅠ型快速可逆解离是由表面强极性的Zn-O键将H-H拉裂所致,解离位点电场越强,活化能就会越低。因此,实验中观测到的应该就是H_2在完整ZnO(1O(?)0)表面的解离吸附。对于缺陷位,由于缺乏这样的强电场,H_2的解离则需要氧空位(V_o)或者间隙锌(Zn_i)处的过剩电荷对H-H反键进行氧化加成,因此活化能较高。在吸附能上,H_2在ZnO两个完整非极性表面解离均为中等程度放热,其中在(10(?)0)表面1×1覆盖度的计算结果更接近实验值。我们考察了覆盖度对吸附热的影响,结果发现对于完整表面,覆盖度越大则吸附能反而增大,推测这是由于异裂后相邻的H~(δ+)和H~(δ-)之间库仑作用所致。对于缺陷位,H_2在V_o处的解离为吸热,推测原因是需要额外能量以破坏表面金属键。H_2在Zn_i处解离则放热较多,联系到前面所提到的活化能,实验中所描述的typeⅡ型缓慢且不可逆的解离可能与Zn_i相关。
接下来的工作是Cu在ZnO表面的沉积。我们的计算工作显示,沉积的Cu受到ZnO载体的很大影响,且Cu与ZnO之间有着很大的作用能。但是,由于Cu的d轨道几乎填满而自由价有限,Cu与ZnO的作用能以及沉积Cu之间的作用能存在竞争关系。因此,在较小单位晶胞且较高对称性的(10(?)0)表面,Cu的沉积图景依次为规则的之字形结构,重排后生成的弓字型结构,最后生成三维团簇。对于较大单位晶胞和较低对称性的(11(?)0)表面,Cu与ZnO载体会存在更强的作用,导致Cu生长的图景依次为不规则的岛状或之字形结构,二维条带,不太平整的表面Cu单层,最后生长为扁平的二维透镜状团簇。在这些吸附构型中,Cu的迁移能一般很低,这反映出Cu与ZnO之间除了共价键之外还存在无方向性的金属键,同时,这也是催化剂失活的重要原因。
Cu/ZnO is widely used as a catalyst for methanol synthesis from syngas.It is also one of candidate catalysts for higher alcohol synthesis.However,the atomic nature and catalytic mechanism of Cu/ZnO system remains obscure,which hinder further impromentation.In this thesis,by means of ab initio method,we investigated its possible surface landscape and the activation mechanism of H_2 molecule.
Firstly,we investigated the ZnO substrate,which itself is utilized as catalyst for some hydrogenation process.Because ZnO sits in the borderline between ionic and covalent substances,and because both Zn and O are hybridized as sp~3,many phenomenons are local,such as surface relaxation,surface reconstruction,and electron localization of n-type surface defects.For H_2 adsorption and subsequent dissociation,experimentally there are two types,one is fast and reversible,called typeⅠ,while another is slow and irreversible,called typeⅡ.After many calculations,we designated H_2 adsorption at perfect(10(?)0)surface as the typeⅠ. Reason for low barrier is that H-H bond is torn apart by strong electronic field near polar Zn-O dimers.Thus stronger the electronic field,lower the activation energy. For H_2 adsorption at n-type defect sites,because the lack of neighboring polar Zn-O dimers,oxidative addition of H_2 molecule onto oxygen vacancy(V_o) or zinc interstitial(Zn_i) is needed during the dissociation.Concerning energy of dissociation,we found that it is moderate exothermic for H_2 adsorption over two perfect nonpolar surfaces.An abnormal phenomenon is that this value increases as coverage,probabely due to the interaction between adjacent H~(δ+) and H~(δ-).For defect sites,dissociation of H_2 at V_o is endothermic,while at Zn_i is much exothermic. Considering the above mentioned value of reaction barrier,the dissociation at Zn_i might be designated as typeⅡ.
Our following work is copper dissociation above ZnO.It is indicated in our work that deposited copper frameworks are dictated by its ZnO substrate.And, because the limited bonding ability of copper atom,whose electronic configuration is 3d~(10)4s~1,interaction between Cu and ZnO and interaction among deposited coppers are generally competitive.Thus,above relatively small and highly symmetric(10(?)0)surfaces,copper depositions follow the order of regular zigzag structures,then undertake a rearrangement to form new zigzag structures,and finally encaged to 3D clusters.While,above relatively larger and lower symmetric (1120) surface,copper depositions follow the order of irregular islands or zigzag structures,2D streaks,2D surface layers and finally flat,disk like cages.Migration barriers of deposited coppers are generally low,indicating some metallic nature between Cu and ZnO,and,this could be the major reason for the easy deactivation of Cu/ZnO catalyst.
引文
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[2]http://en.wikipedia.org/wiki/Ethanol_fuel.
[3]Herman,R.G.Catal.Today,2002,55,233.
[4]吴越,《催化化学》.科学出版社,1998.
[5]Hoflund,G.B.;Epling,W.S.;Minahan,D.M.Catal.Today,1999,52,99.
[6]Wan,Q.;Li,Q.H.;Chen,Y.J.;Wang,T.H.;He,X.L.;Li,J.P.;Lin,C.L.Appl.Phys.Lett.2004,84,3654.
[7]Campbell,C.T.Surf.Sci.Rep.1997,27,1.
[8]Waugh,K.C.Catal.Today 1992,15,51.
[9]Burch,R.;Chappell,R.J.;Appl.Catal.1988,45,131.
[10]Fleisch,T.H.;Mieville,R.L.J.Catal.1984,90,165.
[11]Chu,P.J.;Gerstein,B.C.;Sheffer,G.R.;King,T.S.J.Catal.1989,115,194.
[12]Klier,K.Adv.Catal.1982,31,243.
[13]Okamoto,Y.;Fukino,K.;Imanaka,T.;Teranishi,S.J.J..Phys.Chem.1983,87,3747.
[14]Chen,H.B.;Wang,S.J.;Liao,Y.Y.;Cai,J.;Zhang,H.B.;Tsai,K.R.Mechanism of synergistic catalysis by Cu-ZnO-Al_2O_3 catatlysts in methanol synthesis.In:Phillips,M.J.;Ternan,M.Eds.Catalysis:Theory to Practice(Proc.9~(th) ICC),Calgary,1988,2,537.
[15]胡云行 等.催化学报,1993,14,415.
[16]科顿,F.A.;威尔金森,G.;关实之等译.《高等无机化学》人民教育出版社,1981.
[17]Xu,M.;Iglesia,E.J.Catal.1999,188,125.
[18]Kohan,A.F.;Ceder,G.;Morgan,D.;van de Walle,C.G.Phys.Rev.B 2000,61,15019.
[19]Bagnall,D.M.;Chen,Y.F.;Zhu,T.;Yao,S.;Koyama,S.;Shen,M.Y.;Goto,T.Appl.Phys.Lett.1997,70,2230.
[20]Park,C.H.;Zhang,S.B.;Wei,S.Phys.Rev.B 2002,66,073202.
[21]Karazanov,S.Z.;Ravindran,P.;Kjekhus,A.;Fjellvag,H.;Grossner,U.;Svensson,B.G.J.Crys.Grow 2006,287,162.
[22]CRC Handbook of Chemistry and Physics,76th ed.;CRC Press:New York,1996.
[23]Meyer,B.;Marx,D.Phys.Rev.B 2003,67,035403.
[24]Sakarano,D.;Spoto,G.;Bodiga,S.;Zecchina,A.;Lamberti,C.Surf.Sci.1992,276,281.
[25]Bolis,V.;Fubini,B.;Giamello,E.;Relier,A.J..Chem.Soc.,Faraday Trans.1989,85,855.
[26]Roderiguez,J.A.;Campbell,C.T.J..Phys.Chem.1987,91,6648.
[27]Meyer,B.;Marx,D.Phys.Rev.B 2004,69,235420.
[28]Bromley,S.T.;French,S.A.;Sokol,A.A.;Catlow,C.R.A.;Sherwood,P.J.Phys.Chem.B 2003,107,7045.
[29]French,S.A.;Sokol,A.A.;Catlow,C.R.A.;Sherwood,P._J.Phys.Chem.C.2008,112(19),7420.
[30]Beltr(?)n,A.;Andr(?)s,J.;Calatayud,M.;Martins,J.B.L.Chem.Phys.Lett.2001,338,224.
[31](a) Payne,M.C.;Allan,D.C.;Arias,T.A.;Joannopoulos,J.D.ReV.Mod Phys.1992,64,1045.
(b) Milman,V.;Winkler,B.;White,J.A.;Pickard,C.J.;Payne,M.C.;Akhmataskaya,E.V.;Nobes,R.H.Int.J.Quantum Chem.2000,77,895.
[32]Pauling,L.The nature of the chemical bond Cornell University Press,Ithaca,New York,1960.
[33]Dronskowski,R.Computational hemistry of solid state materials.Wiley-VCH Verlag GmbH& Co.KGaA.Weinheim,2005.
[34]Schleyer,P.v.R.;Radom,L.;Hehre,W.J.;Pople,J.A.Ab initio molecular orbital theory,Wiley,1986.
[35]Hohenberg,P.;Kohn,W.phys.Rev.,1964,136,B864
[36]Kohn,W.;Sham,L.J.Phys.Rev.,1965,140,A1133
[37]Koch,W.;Holthausen,M.C.;A chemist's guide to density functional theory.Wiley-VCH:Weinheim,2001,2~(nd) ed.
[38]Anisimov,V.;Aryasetiawan,F.;Lichtenstein,A.J.Phys.Condens.Matter.1997,9,767.
[39]Monkhorst,H.J.;Pack,J.D.Phys.Rev.B 1976,13,5188.
[40]霍夫曼.R.著,郭洪猷,李静 译,王作新,郑冲 较.《固体与表面》,化学工业出版社,北京,1996.
[41]Peierls,R.E.Quantum Theory Of Solids,Oxford University Press,Oxford,1972.
[42]Jahn,H.A.;Teller,E.Proc Royal.Soc.A 1937,161,220.
[43]http://www.cryst.ehu.es/
[44]Mulliken,R.S.J..Chem.Phys.1935,3,564.
[45]Catlow,C.R.A.;French,S.A.;Sokol,A.A.;Al-Sunaidi,A.A.;Woodley,S.M.J.Comp.Chem.2008,29,2234.
[46]Carrasco,J.;Illas,F.;Bromley,S.T.;Phys.Rev.Lett.2007,99,235502.
[47]Duke,C.B.;Meyer,A.P.;Mark,P.Phys.Rev.B 1978,18,4225.
[48]Diebold,U.;Koplitz,L.V.;Dulub,O.Appl.Surf.Sci.2004,237,336.
[49]Wang,Y.;Meyer,B.;Yin,X.;Kunat,M.;Langenberg,D.;Traeger,F.;Birkner,A.;W(o｜¨)ll,C.Phys.Rev.Lett.2005,95,266104.
[50]Hammond,G.S.J.Am.Chem.Soc.,1955,77,334
[51]Lv,X.;Xu,X.;Wang,N.;Zhang,Q.;Ehara,M.;Natatsuji,H.J.Phys.Chem.B 1999,103,2689.
[52]W(o｜¨)ll,C.Prog.in Surf.Sci.2007,82,55.
[53]Kessler,J.;Thieme,F.Surf.Sci,1977,67,405.
[54]Cohen,A.J.;Mori-S(?)nchez,P.;Yang,W.Science 2008,321,792.
[55]Hammer,B.;Hansen,L.B.;NΦrskov,J.K.Phys.Rev.B,1999,59,7413.
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