Density Functional Theory Study of Water Diffusion and Clustering on Pd(111)

25卷8期结构化学(JIEGOU HUAXUE)Vol. 25，No. 82006. 8Chinese J. Struct. Chem.909-914Density Functional Theory Study ofWater Diffusion and Clustering on Pd(111)PCHEN Jin-Wen*.C TU Xue- Yan2TIAN Kaia,b@ DAI Shu-Shan*"(School of Chemical Science and Technology, Yunnan Universiry, Kunming 650091, China)"(School of Chemistry and Biotechnology, Yunnan Nationalities Universiy, Kunming 650031, China)《(School of Chemistry and Chemical Engineering, Hunan Universityof Science and Technology, Xiangtan 411201, China)ABSTRACTThe intemal structures as well as adsorption and hopping energies of monomers,dimers, trimers, tetramers, pentamers and hexamers of water on Pd(111) have been studied bydensity functional theory (DFT) plane-wave pseudopotential method which performs the first-principles quantum-mechanical calculations to explore the properties of crystals and surfaces inmaterials. Based on the calculations, we suppose that their absorption is via one water molecule formonomers, dimmers and trimers, but three water molecules for pentamers and hexamers. Moreover,there is one water molecule bonding with Pd atom by O atom in pentamers and hexamers, whichexplains why pentamers and hexamers are stable. The binding energies of polymers may be used toexplain why the trimer comes close to two nearby monomers to form a stable pentamer instead oftetramer. And the difference of mobility of small water clusters is due to their different hoppingenergies.Keywords: density functional theory, Pd(111) surface, water diffusionand clustering, binding energy, hopping energy1 INTRODUCTIONSTMI-~12! It is well known that the water moleculeforms a singleton-like monomer or dimer moleculeThe interaction of water molecules with metal sur-on metal surfaces under low coverage at low tem-faces plays a vital role in a number of important pro-peraturesl- !. At the saturation coverage of water oncesses, such as corrosion, heterogeneous catalysis,M(111) (M = Pt, Rh, Pd, Cu, Ni) and Ru(001) atelectrochemical processes in aqueous solutions,relatively higher temperature, a hexagonal close-hydrogen production, etc." The structure and pro-packed structure extends over the surfaces~".perties of water adsorbed on well defined metal sur-Many studies have now demonstrated that DFT canfaces have been the subject of numerous experi-accurately predict the adsorption and diffusion pro- ,mental and theoretical investigations. There haveperties of atoms and molecules on metal surfaces'[13, 14been a number of experimental studies of water onFurther it has been shown that excellent agreementmetal surfaces through LEED, ESDIAD, infraredbetwe中国煤化工y determined dif-reflection absorption spectroscopy (IRAS), XAS andfusiorHCN MH C°1 Despite nume-Received 7 December 2005: acepted 18 January 2006①Supporned by the Natural Science Foundation of Yunnan Province (No.2004B0003M)②Corresponding author. Tian Kai, male, bom in 1971. lcturer, majoring in quantum chermistry. E mail: tiankai2@21cn.comCHEN J. W. et al: Density Functional Theory Study of910Water Difusion and Clustering on Pd(11)No.8rous theoretical investigations of H2O on model mne~ ractions'and thus excellent convergence of thetal surfaces', 17~21), however, the hopping and bin-plane-wave expansion with a moderate cutoff of 300ding energies for H2O diffusion and clustering on Pdev is obtained. The Pd(1I) surface is modeled bysurfaces have not been calculated.the slab supercell approach, using a slab of twoRecently, T. Mitsui et al.!21 have studied theatomic layers and (4X5) surface unit cells. Experi-adsorption, diffusion and clustering of water mole-mental values are selected for the substrate inte-cules on a Pd(11I) surface by scanning tunnelingratomic distances: 2.75 A for palladium. Themicroscopy. They found at 40 K, low-coverage wa-vacuum separation between periodically repeatedter adsorbs in the form of isolated molecules, whichslabs is at least 9.0 A. The Brillouin zone integrationdiffuse by hopping to the nearest neighboring sites.has been performed on a 3X3X2 grid for theUpon collision, they first form dimers, then trimers,structures. The absorbates are positioned on thetetramers and so on. The mobility of these speciesupper side of the slab. The geometry optimizationsincreases by several orders of magnitude whenfor minima of the potential energy surface includedimers, trimers and tetramers are formed, and thenthe H, 0 atoms and the metal atoms in the upper-decreases again when the cluster contains five ormost layer of the slab, while the bottom metallicmore molecules. The trimer comes close to twoplane is frozen in a bulk-like geometry.nearby monomers to form a stable pentamer. CyclicAccording to experimental results2-12.22 , we de-hexamers are found to be particularly stable. Theysigned the initial structures of the reactants, transi-grow with further exposure to form a commensuratetion states and products in the process of waterhexagonal, honeycomb structure relative to thediffusion and clustering for geometry optimizationsPd(11) substrate.on the two-layer slabs. The optimal structures areThe following questions inspirit us to explore: .shown in Figs. 1~6, where ,0 andHow do the adsorbed water molecules diffuse bynote O, H and Pd atoms, respectively The followinghopping to the nearest neighboring sites? Why doesequations are used to calculate the adsorption,the mobility of these species increase by severalhopping and binding energies, respectively where Eorders of magntude when dimers, trimers and tetra-dsorb is the adsorption energy per water monomer ormers form? Why does the trimer come close to twopolymer, E ou the total energy of supercell withnearby monomers to form a stable pentamer? Whywater monomer or polymer absorbed on the Pd(1I1)are the cyclic hexamers particularly stable?slab, E Ppa(11) the energy of the equivalent supercellwith clean Pd slab, E polymer the energy for the2 CALCULATION METHODSisolated water polymer, E monomer the energy for theisolated water monomer, E hopping the energy barrierAll calculations were performed with Cambridgefrom the reactant to the transition state, and E bindingSequential Total Energy. Package (CASTEP)23-25]the energy of binding to form a polymer.by using DFT plane-wave pseudopotential methodEq.1. E sdborb= E tota-E reu11)-E polymer (Ewhich performs the first principles quantum-mecha-mononner).nical calculations to explore the properties of crys-Eq.2. E bppng= E abor(the transition state) .tals and surfaces in materials, such as semicondu-E ndsorb(the reactant).ctors, ceramics, metals, minerals and zeolites. TheEq中国煤化工lymer)-nEndeorogeneralized gradient approximation proposed bymondMHCNMHGPerdew and Wang (GGA-PW91) is used for theexchange correlation encrgyl26- 28. Ultrasoft pseudo-3 RESULTS AND DISCUSSIONpotentials are used to describe the electron-ion inte-.2006 Vol. 25鲒构化学(JIEGOU HUAXUE)Chinese J. Struct. Chem.9113.1 Monomerthe bridge site with the adsorption energy of - -280.60The hopping process of water monomer on theKJ/mol. From a to b, the hopping energy is 109.13Pd(111) surface is shown in Fig. I. In a, the waterKJ/mol. The monomer frst adsorbs on Pd1 throughmolecule is adsorbed on the Pd(11I) slab via H1,the transition state, and then hops to Pd2 and Pd3 inwhere the adsorption energy is - 389.63 KJ/mol. Inturn.the transition state b, the water molecule adsorbs in->>bcFig. 1. Hopping process of the monomer of water on Pd (111)dFig 2. Hopping process of the dimer of water on Pd (11)3.2 Dimeradsorption of two water molecules on the Pd(111)The diffusion and clustering process of the dimerslab at the same time. Consequently, from d to e, theof water on Pd(11) is shown in Fig. 2. In d, the hopping enerey isnnlv 17 36 K I/mol, which is muchdimer adsorbs on the Pd surface like one water mo- lower中国煤化工That is why thelecule and the adsorption energy is -396.86 KJ/mol,dimerMYHc N M H Gmer. Eoning (dimer)lower than one of the monomer. But in the transitionis -12.16 KJ/mol. The 0 O distance in the dimer isstate e, its adsorption energy is -379.50 KJ/mol and2.76 A. The bond lengths and bond angles of twomuch lower than that of the monomer due to theintermolecular hydrogen bonds are as follows:CHEN J. w. el al: Density Functional Theory Study of912Water Diffusion and Clustering on Pd(WI)No.8d1s-o1 = 2.12 A, dH2-03 = 2.28 A, 02-H3-01= 122.04°adsorbs on P1I by H1, passes over the transitionand 0-11-02= 110.119.state h, and then the second water molecule of the3.3 Trimertrimer adsorbs on Pd2. Therefore one hoppingThe diffusion and clustering process of the trimerprocess of the trimer on the surface of palladium isof water on Pd(111) is shown in Fig.3. In g, the firstfinished. Eninding(rimer) is - 63.39 KJ/mol. The bondwater molecule of the trimer adsorbs on the Pdlengths and bond angles of three intermolecularsurface with HI, where the adsorption energy ishydrogen bonds are dH2-o3= 1.63 A, dHs-o2= 1.82 A,- -214.11 KJ/mol. The adsorption energy of thedu-01= 2.45 A, 0o2-H3-01 = 124.01°, 0o3-Hs-02 =transition state h is -157.86 KJ/mol. From g to h, the153.66, 01-12-03= 159.559, and the 0-0 distanceshopping energy is 56.25 KJ/mol, which is lower thanin the trimer are 2.58, 2.73 and 3.10 A, respectively.the corresponding energy of monomer. The trimerC>色。ghFig 3. Hopping process of the trimer of water on Pd (11)3.4 Tetramerpentamer instead of tetramer. There are only three,Fig. 4 shows the adsorption structure of tetramerthree and six intermolecular H bonds in tetramer,on the Pd (111) slab. The binding energy of thetrimer and pentamer, respectively, explaining whytetramer is- -61.56 KJ/mol, and the adsorptionFninding(etramer) > Binding(uimer)> Eninding(pentamer).energy -183.04 KJ/mol. The bond lengths and bond3.5 Pentamerangles of three intermolecular hydrogen bonds areThe adsorption structure of pentamer on Pd1I)dH1-o2= 2.07 A, du7-o3= 2.10 A, d13-o3= 2.15 A,is shown in Fig. 5. The adsorption of pentamer is via0o1-H2-02= 151.549, 0o2-H3-03= 174.45°, 04H17-03=three water molecules with the frst one adsorbed on134.16° and the 0 0 distances in the tetramer arePdl by O1 atom, the second one on Pd2 by H3 atom, .2.96, 3.12, 2.86 and 3.31 A. From our calculation,and中国煤化工HI0 atom, whichEninding(trimer) is - -63.39 KJ/mol, Eninding(tetramer)expldMHC N M H Gtable. The binding. -61.56 KJ/mol, and Eninding(pentamer) -129.97and adsorption energies of pentamer are -129.97 andKJ/mol, which may explain why the trimer comes-130.74 KJ/mol, respectively. The bond lengths andclose to two nearby monomers to form a stablebond anglcs of six intermolecular hydrogen bonds2006 Vol. 25结构化学(JIEGOU HUAXUE)Chinese J. Struct. Chem.913are dH1-o2= 1.61 A, dHs-o2= 1.97 A, du44os= 1.87A，hydrogen bonds in the hexamer, which leads thedH2-os= 2.10 A, d4r7-03= 1.75 A, dH-o4= 1.64 A，cyclic hexamer to be particularly stable. The0o1-H1-02= 166.34°, 03-H5 o2= 157.42°, 0o4-H7-03=adsorption energy of the hexamer is -109.42 KJ/mol.161.039, 0os-+19-04= 161.71°, 01-H2-05= 128.60*， The bond lengths and bond angles of the eight0-1405 =142.98*, and 'the 0-0 distances in theintermolecular hydrogen bonds are as follows:pentamer are 2.60, 2.79, 2.70, 2.62 and 2.72 A.dH3-o1= 2.47 A, dn1s-02= 1.83 A, dur-o3= 1.99 A,3.6 HexamerdH9o4= 1.76 A, du11-os= 1.82 A, dH-o6= 2.36 A,The adsorption structure of hexamer on Pd(11I) is .dH3-o6= 2.26 A, dHI- o4= 1.63 A.0o2-H-01= 111.110,shown in Fig. 6. The adsorption of hexamer is003-H5-02= 157.81", 04-17-03= 147.21", 0os-19-04=through three water molecules: one adsorbs on PdI152.829, 0H1I-o5= 154.08*, 0o1-11-06= 118.16°, ,by O1 atom, one on Pd2 by H4 atom and the last one02-113-06= 141.89*, 0-12-04= 176.57° and the 0-0on Pd3 by H8 atom, showing why the hexamer isdistances in hexamer are 2.96, 2.77, 2.87, 2.67, 2.74,stable. The binding energy of hexamer is -202.342.95 and 2.64 A.KJ/mol because there are eight intermolecularFig. 4. Adsorption of tetramer on P111) Fig. 5. Adsorption of pentaner on Pl111) fig. 6. Adsorption of hexamer on Pl111)14 CONCLUSIONclose to two nearby monomers to form a stablepentamer instead of tetramer; Secondly, the largest( 1)From the intermal structures of water clusters onbinding energy leads the cyclic hexamer to bePd(11I), we suppose that their absorption is via oneparticularly stable.water molecule for monomers, dimers and trimers,(3) There are three factors (the quantity andbut three water molecules for pentamers and hexa-intensity of intermolecular hydrogen bonds, and the'mers. Moreover, there is one water molecule bon-00 distances) affecting the binding energy. Fording with Pd by 0 atom in pentamers and hexamers,example, There are only three intermolecular hydro-which explains why they are stable.中国煤化工n trimer and six in(2) From the binding energy of forming polymers.YHCNMHGEninding(etramer) >on Pd111), we may come to the following conclu-Bninding(nmer) > binding(pentamer) in the calculationsions: Firstly,because Eninding(tetramer) > results.Ebinding(trimer) > Eoinding(pentamer), the trimer comes(4) The difference of mobility of small waterCHEN J. W. et al: Density Functional Theory Study of914Water Diffusion and Clustering on Pd(1)No.8clusters is due to their different hopping energies.of these species increases by several orders ofBased on our calculations, the hopping energies ofmagnitude when dimer and trimer are formed. Andmonomer, dimer and trimer are 109.13, 17.36 andbecause Enppin(rimer) > Enopping (dimer), the trimer56.25 KJ/mol, respectively. 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