Adsorption and Decomposition of H 2 S on MgO(100), NiMgO(100), and ZnO(0001) Surfaces: A First-Principles Density Functional Study

Adsorption and Decomposition of H2S on MgO(100), NiMgO(100), and ZnO(0001)Surfaces: A First-Principles Density Functional Study

Jose A. Rodriguez*Department of Chemistry, BrookhaVen National Laboratory, Upton, New York 11973

Amitesh MaitiMolecular Simulations Inc., 9685 Scranton Road, San Diego, California 92121

ReceiVed: January 4, 2000; In Final Form: January 28, 2000

The adsorption and dissociation of H2S on MgO(100), Ni-doped MgO(100), and ZnO(0001) was studiedusing first-principles density-functional calculations (DFT-GGA) and periodic supercells. The bonding ofH2S and its S-containing dissociated species (HS and S) is substantially stronger on ZnO(0001) than onMgO(100), making dissociation easier on zinc oxide. This behavior can be explained by the smaller ionicityin ZnO, which leads to a larger electron density around the Zn atoms and a larger reactivity toward S-containingmolecules. Replacing some of the metal centers of MgO(100) with Ni atoms enhances the binding ofS-containing species through new electronic states associated with the Ni 3d levels and located above theoccupied{O 2p+ Mg 3s} bands. In addition, structural defects, like steps, expose metal centers with lowercoordination and larger reactivity than pentacoordinated Mg atoms in MgO(100). A simple model based onperturbation theory and band-orbital mixing is able to explain the differences in the reactivity of MgO(100)and ZnO(0001) and the behavior of other oxides (Al2O3, Cr2O3, Cr3O4, Cu2O) in the presence of sulfur-containing molecules. The model predicts a negative correlation between the reactivity of the oxides and thesize of the electronic band gap, with the chemical activity of an oxide depending mainly on how well itsbands mix with the orbitals of H2S. The electrostatic interactions between the Madelung field of the oxideand the dipole moment of the molecule play only a secondary role in bonding.

I. Introduction

Sulfur-containing molecules are common impurities in fossil-derived fuels and chemical feedstocks.1,2 Today, these impuritiesconstitute a major problem in our industrial society due to theirnegative effects in environmental pollution and chemicalsprocessing.2-5 The sulfur impurities form sulfur oxides (SOx),which are major air pollutants that lead to acid rain,2 duringthe burning of fuels. In addition, sulfur impurities rapidlydeactivate or poison most metal/oxide catalysts used in thechemical or petrochemical industries and in the control of COand NOx emissions from automobile exhaust.3-6 Millions ofdollars are lost every year as a consequence of sulfur poisoning.4-6

A fundamental understanding of the interaction of sulfur-containing molecules with oxide surfaces is important for tworeasons. First, most commercial catalysts poisoned by sulfurinvolve oxides, and there is a clear need to improve theirlifetime.5,6 And, second, in many industrial operations, oxidesare used as sorbents for the removal or destruction of sulfur-containing molecules.2,7-10

On the surface of a metal oxide, sulfur can interact with ametal or an oxygen atom, producing species with very differentelectronic properties (“sulfide” versus “sulfate” formation).Experiments for the adsorption of S2, H2S, CH3SH, andthiophene on a series of oxides (Al2O3, ZnO, Cu2O, MoO2,Cr2O3, CeO2) reveal that sulfur species produced by thedissociation of these molecules mainly interact with the metalcenters of the surface.3,8a,11,12On the other hand, SO2 prefer-

entially reacts with O centers, readily forming SO3 and SO4

species.8b,9,13,14For practical reasons, it is very important toestablish which types of oxides have a high reactivity towardsulfur-containing molecules. The metal elements form oxidesthat can adopt a large diversity of crystal structures.15,16 Theseoxides can behave as semiconductors or insulators, and in somecases even exhibit metallic properties,16 leading to the possibilityof large variations in chemical reactivity.11,12 Figure 1 showsthe sulfur uptake (HS+ S) for the dissociative adsorption ofhydrogen sulfide on a series of oxides at 300 K.11,12 Oxidesthat have a large degree of ionicity like Al2O3

17 and MgO18

exhibit the lowest reactivities. A qualitative correlation isobserved between the chemical activity of an oxide and the sizeof its band gap:the smaller the band gap in an oxide, the fasterthe rate of dissociation of hydrogen sulfide. This trend can beexplained using a simple model based on the strength of themixing of the frontier orbitals of the admolecule with theconduction and valence bands of the oxide surface.11,12 Butadditional factors might also contribute to the behavior seen inFigure 1.

In this article, we use first-principles density functional theory(DFT) and periodic supercells to study the bonding interactionsof H2S, HS, and S with an insulator, MgO(100), an insulatordoped with a transition metal, NiMgO(100), and a semiconduc-tor, ZnO(0001). In previous works the chemistry of hydrogensulfide on MgO(100)12 and ZnO(0001)19,20has been examinedin detail using core and valence level photoemission. Theexperimental results show molecular adsorption of H2S on metalcenters of MgO at 80 K. The molecule dissociates into HS upon* Corresponding author. FAX: 631-344-5815. E-mail: [email protected].

3630 J. Phys. Chem. B2000,104,3630-3638

10.1021/jp000011e CCC: $19.00 © 2000 American Chemical SocietyPublished on Web 03/18/2000

heating to 300 K, and further heating to 400 K leaves atomic Sbonded to Mg on the oxide surface.12 Zinc oxide is able todissociate hydrogen sulfide at very low temperatures.19,20

Adsorption of the molecule on this oxide at∼100 K producesonly Zn-bonded HS species. These species decompose into Sadatoms at temperatures between 300 and 400 K. The reactionbetween hydrogen sulfide and zinc oxide at elevated tempera-tures, 400-500 K, produces water and zinc sulfide: H2S(gas)+ ZnO(solid)f H2O(gas)+ ZnS(solid).21

II. Theoretical Methods

All the calculations reported in section III below wereperformed using commercial versions of the density functionalprograms CASTEP22 and DMol323 available from MolecularSimulations Inc. Geometries and bonding energies of sulfur-containing species on MgO(100), NiMgO(100), and ZnO(0001)were estimated with the CASTEP code. A large body of existingwork indicates that CASTEP is excellent for predicting structuralgeometries and energy changes associated with chemicaltransformations involving oxides.24-28 In this code, the wavefunctions of valence electrons are expanded in a basis set ofplane waves with kinetic energy smaller than a specified cutoffenergyEcut. The presence of tightly bound core electrons isrepresented by nonlocal ultrasoft pseudopotentials.29 Reciprocal-space integration over the Brillouin zone is approximatedthrough a careful sampling at a finite number ofk-points usingthe Monkhorst-Pack scheme.30 The exchange-correlation con-tribution to the total electronic energy is treated in the general-ized gradient corrected (GGA)31 form of the local densityapproximation (LDA). In all calculations, the kinetic energycutoff Ecut and the density of the Monkhorst-Packk-point mesh

were chosen high enough in order to ensure convergence ofthe computed structures and energetics. Since the DFT calcula-tions were performed at the GGA level, one can expect thatthey did not overestimate the bonding energies of adsorbateson the oxide surfaces.32-35 Frequently, DFT-GGA calculationspredict adsorption energies within an accuracy of 5 kcal/mol.24,35,36 In any case, in this work our main interest is inqualitative trends in the energetics, not in absolute values.

Due to the delocalized (plane wave) nature of its basis set,CASTEP yields useful electronic information (levels and bandstructure) only in thek-space. To investigate localized chargesand localized electronic density of states around various atomsof interest, we used another DFT program from MolecularSimulations: DMol3.23 In contrast to CASTEP, DMol3 useslocalized functions to describe the atomic orbitals. Our calcula-tions employed numerical basis sets of double-ú quality pluspolarization functions to describe the valence orbitals of O, Mg,Ni, Zn, S, and H. DFT in DMol3 was performed within theGGA approximation using Becke-88 for exchange37 andPerdew-91 for correlation.38 The charge distributions wereestimated using the approach proposed by Mulliken.39,40 Theyare useful in obtaining trends and providing simple interpretationof results, but the charges must not be interpreted in absolutequantitative terms because of the uncertainty in uniquelydefining a charge-partitioning scheme.41 The Mulliken methodhas well-known shortcomings but is one of the most popularprocedures for electron population analysis.

To model the MgO(100), NiMgO(100), and ZnO(0001)surfaces, we used periodic supercells. First, the bulk MgO andZnO crystals were geometrically optimized with CASTEP inorder to determine the equilibrium atomic positions and thelattice constants. After the optimization of the bulk crystals,the appropriate surfaces (i.e. the (100) face of MgO and the(0001) face of ZnO) were cleaved, followed by the constructionof a three-dimensionally periodic supercell with a vacuum of12.5 Å on the top of the free surface. The slab models were4-6 atomic layers in depth normal to the surface plane. Thebottom 2-4 layers were frozen at the bulk crystalline spacingin order to mimic the presence of a semiinfinite crystallinematerial beneath the surface. The number of layers and theamount of vacuum chosen were expected to be sufficiently largeto yield accurate geometries and bonding energies at the oxidesurfaces.18,24,26,42

III. Results

III.1. Adsorption of H 2S, HS, and S on MgO(100).Theflat (100) face of MgO was modeled using a periodic slabcontaining four layers of Mg and O atoms arranged in the wayshown in Figure 2. Previous works have shown that slabs of3-4 layers provide a very good representation of the MgO(100)surface.18,26,42The geometry optimization of bulk MgO gave atypical rock-salt structure withao ) 4.26 Å. This value compareswell to those derived from experimental measurements (4.22Å)15,43and other theoretical calculations.26,42In the slab calcula-tions, the structural geometry of the clean or adsorbate-coveredMgO(100) was determined by relaxing the two layers near thesurface and keeping the other two layers of the slab in thegeometry of bulk MgO. The CASTEP calculations predictalmost no reconstruction of the clean MgO(100) surface withrespect to the (100) face of bulk MgO, in agreement with severalexperimental16 and theoretical42 studies.

Bulk MgO is known to be a highly ionic compound.17,18Table1 lists the Mulliken charges calculated with DMol3 formagnesium sites in a MgO(100) surface and metal centers in

Figure 1. Total coverage of sulfur (HS+ S) as a function of H2Sexposure to a series of oxides (Al2O3, MgO, Cr2O3, ZnO, Cu2O, Cr3O4)at 300 K.11,12The numbers in parentheses denote the band gap of eachoxide.

Adsorption and Decomposition of H2S on Oxides J. Phys. Chem. B, Vol. 104, No. 15, 20003631

the bulk oxide. For the pentacoordinated Mg atoms in the surface(Mg-5 in our notation) the positive charges are close to thoseseen in hexacoordinated Mg atoms in the bulk (Mg-6 in ournotation). A similar fact has been noted in previous theoreticalstudies that use periodic slabs to model the MgO(100) surface.18

H2S, HS, and S were adsorbed on top of metal centers ofMgO(100) with the bonding configurations shown in Figure 3.These bonding configurations are typical for these species onmetal oxides.11,12,44Table 2 displays adsorption geometries andenergies predicted by CASTEP. Following the usual conven-tion,24 adsorption energies (Eads) were calculated according tothe expression

whereEadsorbateis the energy of the isolated adsorbate in itsequilibrium configuration,Eslab is the total energy of the bareslab, andE(adsorbate+slab) is the total energy of the adsorbate/slabsystem. Positive adsorption energies denote an exothermicadsorption process. In Table 2, the bonding energies of theadsorbates increase in the following sequence: H2S < SH <S. The listed values are similar to those obtained in DFT-GGAcalculations for the adsorption of these species on large Mg18O17

and Mg26O25 clusters.12 The adsorption energies predicted foratomic sulfur (31-34 kcal/mol) are consistent with the behaviorseen at an experimental level for the S/MgO(100) system, wherethe S adatoms remain on the surface at temperatures well above500 K.12

As seen from Table 2, atomic S and HS form strong bondson the perfect MgO(100) surface, but the bonding interactionsof the H2S molecule are relatively weak (∼8-9 kcal/mol). Forthe dissociative chemisorption of H2S on two adjacent Mg andO sites of the flat oxide surface,

we calculate that the energy released in the reaction is close to14 kcal/mol (θH2S ) 0.25 monolayer). Thus, dissociation of themolecule is an energetically favorable process, but it may notbe easy due to the lack of strong interactions between H2S andthe flat oxide surface. The decomposition of H2S in reaction 2is helped by the formation of O-H bonds on the oxide surface.For the reactions

the CASTEP calculations predict∆E values (θHS or θS ) 0.25monolayer) of+4 and+19 kcal/mol, respectively, which meanthat one has to put a lot of energy in the system to induce thedissociation of H2S in this way (i.e. no H-Osurface bondformation).

Recent studies highlight the importance of defect sites onthe chemistry of the MgO(100) surface.13,45,46We studied theadsorption of the sulfur-containing species on an infinite slabof the type shown in Figure 4 that contains flat regions of the(100) face and steps with Mg atoms coordinated to only threeoxygen atoms. The geometry of the top two layers of the slabwas optimized during the CASTEP calculations. In the adsorbate-free slab, the tricoordinated magnesium atoms (Mg-3 in ournotation) had two “short” Mg-O distances of 2.09 Å and one“long” Mg-O distance of 2.14 Å. In contrast, pentacoordinatedmagnesium in a flat MgO(100) surface exhibited Mg-Odistances of 2.13 (oxygen underneath) and 2.14 Å (oxygenneighbors in the surface plane).

For the Mg-3 sites, DMol3 predicts a positive charge that issubstantially smaller than that of Mg-5 sites in perfect MgO(100)

Figure 2. Schematic view of the four-layer slab used to model a flatMgO(100) surface. The Mg and O atoms are represented by dark andgray octagons, respectively. Each layer of the slab contains the samenumber of Mg and O atoms. The sulfur-containing molecules wereadsorbed only on the top layer of the slab, and the geometry of thefirst two layers was optimized in the DFT calculations.

TABLE 1: Estimated Charges on Metal Sites

metal centeraMullikenchargeb metal centera

Mullikenchargeb

Mg-6 in bulk MgO 1.22 Ni-5 in MgO(100) 0.81Mg-5 in MgO(100) 1.14 Zn-4 in bulk ZnO 0.93Mg-3 defect in MgO(100) 0.86 Zn-3 in ZnO(0001) 0.75

a The number close to the element name indicates its coordinationnumber.b Here, our interest is in qualitative trends, no absolute values(see section II).

Figure 3. Bonding geometries for H2S, HS, and S on the oxidesurfaces. The sulfur-containing species were adsorbed on top of a Mg,Ni, or Zn atom (empty circle in figure) of the substrate. The molecularaxes of H2S and SH were tilted with respect to the surface normal.The metal-S and S-H bond distances, plus the metal-S-H bondangles were optimized in the DFT calculations.

Eads) Eadsorbate+ Eslab- E(adsorbate+slab) (1)

TABLE 2: DFT-GGA Results for the Adsorption of H 2S,SH, and S on MgO(100)

bond length (Å)

speciesads energy(kcal/mol) Mg-S S-Ha

On Flat Surface (Mg-5 Site)S (0.25 monolayer)b 34 2.46S (0.50 monolayer) 31 2.48HS (0.25 monolayer) 19 2.63 1.36HS (0.50 monolayer) 15 2.64 1.36H2S (0.25 monolayer) 9 2.71 1.35H2S (0.50 monolayer) 8 2.73 1.35

On Step of Surface (Mg-3 Site)S (0.25 monolayer) 48 2.34HS (0.25 monolayer) 27 2.49 1.36H2S (0.25 monolayer) 13 2.65 1.36

a For the free H2S and HS molecules the S-H distances were equalto 1.35 Å.b The numbers in parentheses denote the coverage of theadsorbate. One-quarter (0.25 monolayer) or half (0.5 monolayer) ofthe metal sites exposed in a MgO(100) surface are covered by the sulfur-containing species.

H2Sgas+ Mgsurface+ Osurfacef HS-Mgsurface+ H-Osurface

(2)

H2Sgas+ Mgsurfacef HS-Mgsurface+ 0.5H2,gas (3)

H2Sgas+ Mgsurfacef S-Mgsurface+ H2,gas (4)

3632 J. Phys. Chem. B, Vol. 104, No. 15, 2000 Rodriguez and Maiti

or Mg-6 sites in bulk MgO (see Table 1). The extra electrondensity on the Mg-3 sites probably leads to a higher chemicalactivity. For the interaction of the sulfur-containing species withthe Mg-3 sites (a-top adsorption), CASTEP predicts bondingenergies that are significantly larger than those for adsorptionon Mg-5 sites (see Table 2). This is particularly important inthe case of H2S, because in a kinetic process it will increasethe residence time of the molecule on the surface and enhancethe probability for dissociation.

III.2. Adsorption of H 2S, HS, and S on NiMgO(100).Inthis section we investigate the bonding of H2S, HS, and S toNiMgO(100) surfaces generated by replacing some of themagnesium atoms in a MgO(100) surface with nickel atoms(see Figure 5). By doing so one can transform magnesium oxidefrom an insulator into a semiconductor.16,47Compounds of theNixMg1-xO type are stable,47 and it is known that transition-metal atoms bond to magnesium vacancies present inMgO(100).16,48A nickel atom in a matrix of magnesium oxidecan be seen as a cation in NiO, a compound that also adopts arock-salt structure like MgO.15,16 Our objective here is tocompare the behavior of magnesium and a transition-metal whenboth elements are bonded to the same number of oxygen atomsand are in a similar chemical environment.

The results of geometry optimization with CASTEP indicatethat in NiMgO(100) surfaces with nickel coverages of 0.25-0.50 monolayer, the Ni atoms shift 0.1-0.15 Å upward withrespect to the plane of the Mg and O atoms. This can be

attributed to the fact that the Ni atoms have a smaller positivecharge (i.e. larger electron density) and, therefore, effectively abigger size than the Mg atoms. In Table 1, the results of DMol3indicate that a Ni atom pentacoordinated in a matrix ofMgO(100), Ni-5 in our notation (θNi ) 0.25 monolayer), has apositive charge clearly smaller than that of a Mg-5 atom in pureMgO(100). Figure 6 shows plots for the electron density aroundMg-5 and Ni-5 atoms in MgO(100) and NiMgO(100) surfaces,respectively. In the case of pure MgO(100), the total electrondensity on the magnesium atoms is small and the electron-density plot is dominated by maxima located on top of theoxygen atoms. This is not the case for NiMgO(100), where onecan see a large electron density around the nickel atoms. Aswe will see below, this has an effect on the chemical reactivityof the metal centers.

Figure 7 displays calculated density-of-states (DOS) plots forthe systems in Figures 2 and 5. The graphs were obtained usingCASTEP and include only occupied states. For MgO(100), theO 2s levels are located from-18 to -16 eV and states thatcontain a mixture of O 2p (main component) and Mg 3scharacter appear between-5 and -1.5 eV. These types offeatures are also seen in the DOS plot for NiMgO(100), and inaddition two peaks appear near the top of the valence band thatcontain Ni 3d character. There is a splitting of the 3d orbitalsof the nickel atoms due to interactions with the ligand fieldgenerated by the surrounding oxygen neighbors.16,49The Ni 3dfeatures appear at higher energy than any feature in theMgO(100) system. When comparing the properties of the Mg-5and Ni-5 atoms in these systems, one finds that the latter have

Figure 4. Schematic view of the four-layer slab used to model stepsites in a MgO(100) surface. The Mg and O atoms are represented bydark and gray octagons, respectively. In the first layer, the Mg atomshave only three nearest neighbors (two oxygen atoms in the same layerand one oxygen atom underneath).

Figure 5. Schematic view of the four-layer slab used to model a flatNiMgO(100) surface (θNi ) 0.25 monolayer). Only one Ni atom isshown in the figure. The Mg and O atoms are represented by dark andgray octagons, respectively. The sulfur-containing molecules wereadsorbed only on top of the nickel atoms, and the geometry of the firsttwo layers of the slab was optimized in the DFT calculations.

Figure 6. Calculated electron-density plots for MgO(100), top, andNiMoO(100), bottom, surfaces. The plots were obtained using CASTEP,and for simplicity only a few metal and oxygen atoms in the four-layer slabs are shown.


a larger local density of states and that many of their states areless stable in energy than the states in the Mg-5 atoms. Theelectronic differences observed in Figures 6 and 7 suggest thata NiMgO(100) surface is better suited to respond to the presenceof sulfur-containing molecules than a MgO(100) surface.

Table 3 lists structural parameters and bonding energiespredicted by CASTEP for the adsorption of H2S, HS, and S ontop of Ni sites of NiMgO(100). Again atomic S and HS formstrong bonds on the oxide surface, but now the bondinginteractions of H2S become substantial (15-17 kcal/mol). TheNi-5 sites are more chemically active than the Mg-5 or Mg-3sites in Table 2. On top of the transition metal, one sees notonly stronger metal-adsorbate bonds but also a larger elongationof the H-S bonds. Both effects should enhance the rate ofdissociation of H2S and HS with respect to that seen in the H2S/MgO(100) system.

III.3. Adsorption of H 2S, HS, and S on ZnO(0001).As anindustrial sorbent, zinc oxide is highly efficient for the trappingof H2S, SO2, disulfides, and mercaptans.6,7,8a,9,10 Previoustheoretical studies have shown that the use of infinite slabs isa valid and reliable approach for studying adsorption reactionson surfaces of zinc oxide.28,50,51 In our DFT calculations theZnO(0001) surface was modeled using an infinite slab contain-

ing three layers of zinc and three layers of oxygen atoms,arranged as shown in Figure 8. The top layer represents the(0001)-Zn face of zinc oxide. The geometry optimization forbulk ZnO gave a wurtzite hexagonal unit cell witha ) b )3.29 Å andc ) 5.25 Å (CASTEP results). These values arevery close to those derived from experimental measurements(a ) b ) 3.25 Å andc ) 5.21 Å at 298 K)52 and othertheoretical calculations (a ) b ) 3.29 Å andc ) 5.24 Å).50 Inthe slab calculations, the structural geometry of the two layersnear the surface (one Zn layer and one O layer) was relaxed,while the other four layers of the slab were kept in the geometryof bulk ZnO. In the case of clean ZnO(0001), the Zn outermostlayer moved closer to the first oxygen layer by 0.23 Å (withrespect to the separation observed for the (0001) plane of bulkZnO). A similar relaxation (0.20-0.25 Å) has also been detectedin LEED I-V experiments.16,53 This relaxation was partiallyremoved by adsorption of the sulfur-containing species.

Table 1 shows the Mulliken charges calculated with DMol3for tricoordinated zinc sites in a ZnO(0001) surface, Zn-3 inour notation, and tretracoordinated metal centers in bulk ZnO,Zn-4 in our notation. In DFT calculations for large zinc oxideclusters, Mulliken charges of 0.81-0.97e have been reportedfor the zinc cations.44a The results in Table 1 indicate that thedegree of ionicity in ZnO is significantly smaller than in MgO.This is commonly accepted: MgO is highly ionic,17,18whereasthe bonds in ZnO involve a large degree of covalency.16,54Figure9 shows a plot for the electron density of a ZnO(0001) surface.Clear maxima are seen on top of the zinc centers. This is verydifferent from the result displayed in Figure 6 for pureMgO(100), where the electron density on the metal centers islow. Thus, as in the case of NiMgO(100), ZnO(0001) is bettersuited to respond to the presence of sulfur-containing moleculesthan MgO(100).

Table 4 lists the CASTEP results for the adsorption of H2S,HS, and S on ZnO(0001). The calculated adsorption energy forH2S is 18-21 kcal/mol. DFT calculations at the LDA levelpredict a bonding energy of 32 kcal/mol for H2S on Zn atomsof zinc oxide clusters.44b The discrepancy with respect to theGGA values in Table 4 can be attributed to the fact that LDAcalculations are known to overestimate chemisorption ener-gies.24,32,33,55Calculations carried out with CASTEP at the LDAlevel give an adsorption energy of 34 kcal/mol for H2S onZnO(0001) (θH2S ) 0.25 monolayer). The values shown in Table4 for the bonding energy of atomic S (59-64 kcal/mol) areconsistent with the behavior found at an experimental level for

Figure 7. Total DOS for the occupied bands of the MgO(100) andNiMgO(100) (θNi ) 0.5 monolayer) slabs. The results were obtainedusing CASTEP. The reference for the “0” of energy is not the vacuumlevel.22

TABLE 3: DFT-GGA Results for the Adsorption of H 2S,SH, and S on NiMgO(100)

bond length (Å)

speciesads energy(kcal/mol) Ni-S S-Ha

S (0.25 monolayer)b 59 2.20S (0.50 monolayer) 55 2.23HS (0.25 monolayer) 33 2.38 1.40HS (0.50 monolayer) 32 2.37 1.39H2S (0.25 monolayer) 17 2.52 1.39H2S (0.50 monolayer 15 2.55 1.39

a For the free H2S and HS molecules the S-H distances were equalto 1.35 Å.b The numbers in parentheses denote the coverage of theadsorbate and Ni in the NiMgO(100) system. One-quarter (0.25monolayer) or half (0.50 monolayer) of the Mg atoms in a MgO(100)surface were replaced by Ni atoms.

Figure 8. Section of the six-layer slab used to model the ZnO(0001)surface. The Zn and O atoms are denoted by dark and gray octagons,respectively. In the slab, the first, third, and fifth layers contained onlyzinc atoms. The second, fourth, and sixth were oxygen layers. Thesulfur-containing species were adsorbed only on the top layer, and thegeometry of the first two layers in the slab was relaxed during theDFT calculations.


the S/ZnO20,56 and S/ZnO(0001)57 systems, in which the Sadatoms desorb from the surface at temperatures above 700 K.For the reactions

the CASTEP calculations give∆E values (θHS or θS ) 0.25monolayer) of-15 and-11 kcal/mol, respectively. Even moreexothermic decomposition reactions can be expected ifH-Osurface bonds are formed on the oxide surface. On aZnO(0001) sample this will involve O at defect sites, but insystems such as ZnO(101h0) or polycrystalline ZnO the formationof H-Osurfacebonds should be easy due to the large amount ofoxygen atoms exposed on these surfaces.

After comparing the adsorption energies in Tables 2 and 4(see Figure 10), it is clear that ZnO(0001) should be a bettersorbent for H2S than MgO(100). And indeed, the experimentalresults in Figure 1 show that at 300 K the rate of dissociationof H2S on zinc oxide is larger than on magnesium oxide.

IV. Discussion

The bonding mechanism of hydrogen sulfide on metallicsurfaces involves a transfer of electrons from the H2S(5a1,2b1)orbitals into unoccupied orbitals of the metal (σ andπ donation)and a charge transfer from the metal into the vacant H2S(3b2,6a1)orbitals (σ-back-donation).58 On metal centers of an oxide, thebonding is more complex.16 In addition to the mixing of thefrontier orbitals of the adsorbate with the conduction and valencebands of the oxide, the bonding also contains contributions fromthe interactions between the dipole of the admolecule (sulfur

negative-end down, hydrogens positive-end up)58 and theelectrostatic field generated by the charges in the oxide.16 Theelectrostatic interactions are expected to be particularly strongon oxides that have a large degree of ionicity such as Al2O3

17

and MgO.18 Interestingly, these oxides exhibit the lowestreactivities in Figure 1. For the systems in Figure 10, on theother hand, the smaller the positive charge on the metal center(Table 1), the larger its bonding ability (Tables 2 and 4). Thus,it is very difficult to explain the trends in Figures 1 and 10 interms of electrostatic bonding between H2S and the oxides.Clearly, the chemical activity of an oxide probably depends onhow well its bands mix with the orbitals of H2S, and theelectrostatic interactions with the dipole moment of H2S playonly a secondary role in bonding.

Figure 11 shows the energy positions for the valence andconduction band of a series of bulk oxides (Al2O3, MgO, ZnO,Cu2O). To generate this figure, we used values reported in theliterature for the electronic properties of the oxides,11,12,16,59

instead of results of DFT calculations. Note that for the CASTEPresults in Figure 7, the “0” of energy is not the vacuum level.22

In addition, the DFT calculations do predict that the band gapin ZnO is smaller than in MgO but the magnitude of the bandgaps is underestimated by 2-2.5 eV. This is a common problemin DFT calculations.60 For the oxides in Figure 11, when theband gap increases the valence band moves toward higherbinding energy, while at the same time there is a decrease inthe stability of the conduction band. Therefore, simple modelsbased on band-orbital mixing61-63 would predict that thesmaller the band gap in an oxide, the larger its chemicalreactivity.

For H2S, thiols (RSH) and thiophenes, the HOMOs appearat energies between-9 and-12 eV, with the LUMOs usuallylocated in the range of-1 to +5 eV.8a,11,58,64 Followingperturbation theory in combination with the Hu¨ckel and tight-binding methods,61-63 one can get an approximate expressionfor the bonding energy (Q) derived from the interaction betweenthe HOMO of a sulfur-containing molecule and the conductionband of an oxide:

whereâHOMO-conductionis the resonance integral for the interact-ing levels, andEconduction and EHOMO are the energies for thecentroid of the conduction band and HOMO, respectively. Thecorresponding expression for the bonding energy that arises from

Figure 9. Electron-density plot for the ZnO(0001) surface calculatedwith CASTEP. For simplicity only a few metal and oxygen atoms inthe six-layer slab are shown.

TABLE 4: DFT-GGA Results for the Adsorption of H 2S,SH, and S on ZnO(0001)

bond length (Å)

speciesads energy(kcal/mol) Zn-S S-Ha

S (0.25 monolayer)b 64 2.27S (0.50 monolayer) 59 2.29HS (0.25 monolayer) 38 2.40 1.40HS (0.50 monolayer) 35 2.41 1.39H2S (0.25 monolayer) 21 2.49 1.41H2S (0.25 monolayer) 18 2.52 1.39

a For the free H2S and HS molecules the S-H distances were equalto 1.35 Å.b The numbers in parentheses denote the coverage of theadsorbate. One-quarter (0.25 monolayer) or half (0.50 monolayer) ofthe metal sites exposed in a ZnO(0001) surface are covered by thesulfur-containing species.

H2Sgas+ Znsurfacef HS-Znsurface+ 0.5H2,gas (5)

H2Sgas+ Znsurfacef S-Znsurface+ H2,gas (6)

Figure 10. Calculated adsorption energies (CASTEP, DFT-GGA) forH2S and HS on MgO(100), white bars, and ZnO(0001), hatched bars.In the case of magnesium oxide, results are shown for adsorption onpenta- and tri-coordinated metal centers (Mg-5 and Mg-3, respectively).

QHOMO-conduction∝ (âHOMO-conduction)2/(Econduction- EHOMO)

(7)


the hybridization of the LUMO of the sulfur-containing moleculeand the valence band of the oxide is

where theâLUMO-valence, ELUMO, and Evalence terms have adefinition similar to that of the terms in eq 7. The magnitudeof the resonance integrals (âHOMO-conduction, âLUMO-valence)depends on the overlapping of the orbitals of the adsorbate andadsorption site.61,62The electron-density plots in Figures 6 and9 indicate that the metal centers in ZnO(0001) should overlapbetter with the orbitals in an adsorbate than the metal centersin MgO(100). In MgO and Al2O3, the cations have a very largepositive charge, which leads to orbitals that are more contracted(i.e. smaller) than the orbitals on cations of oxides that are nothighly ionic like ZnO and Cu2O. Therefore, on MgO and Al2O3,the adsorbate-surface orbital overlap is poorer and the valuesof â are smaller. After application of eqs 7 and 8 to the systemsin Figure 11, the differences in the band energies of the oxidespredict that the strength of the adsorbateToxide bondinginteractions should increase following the sequence: Al2O3 <MgO < ZnO < Cu2O, in agreement with the experimentaltrends seen in Figure 1. The qualitative correlation seen betweenthe band-gap size and reactivity in an oxidealso involves theionicity of the system. For the oxides in Figure 11, the degreeof ionicity and size of the band gap go together, and the highlyionic systems offer metal centers whose orbitals are relativelysmall and have energies that do not match well with those ofthe orbitals of the adsorbate.

The simple model of eqs 7 and 8 can be applied not only tosulfur-containing molecules but also to other types of adsorbatesin general.61-63 On oxide surfaces, a correlation between

chemical activity and band-gap size is also observed for CO,NO2, and alkali metal chemisorption. The case of CO isparticularly well-documented for adsorption of the molecule onMgO(100)18,65,66 and TiO2(110).24,55,67 CO interacts muchstronger with metal centers of TiO2(110), band gap∼2 eV 16

and not highly ionic,67 than with metal centers of MgO(100).In the CO/MgO(100) system the contributions to the chemi-sorption bond from band-orbital mixing are very weak,18,65a

but become substantial in the CO/TiO2(110) system.24,67 Forthe interaction of NO2 with the cations of ZnO(0001),28

CASTEP predicts bonding energies that are larger than thosefound on the cations of MgO(100),57 a trend that reflectsvariations in the strength of band-orbital mixing, as was seenfor the adsorption of the sulfur-containing molecules on theoxides.57 In the case of alkali metal adsorption, one finds verystrong bonding interactions on oxides that have a narrow bandgap (<4 eV).68 On the other hand, for alkali metals on wide-band-gap oxides (>5 eV), the bonding energies are weak andcontrolled by interactions of the adsorbate with vacancies anddefect sites of the oxide surface.68

According to eqs 7 and 8, one can increase the reactivity ofan oxide surface by creating metal centers which have orbitalsthat are less contracted (higher values ofâ) or have a goodmatch in energy with the orbitals of the adsorbate (no very stableoccupied states or unstable vacant states on the metal centers).This can be accomplished by introducing oxygen vacancies ona surface and reducing the positive charge on the adjacentcations. Defect sites can exhibit substantial chemical activity,as the results in Table 2 (Mg-5 vs Mg-3 adsorption energies)show. Another approach to enhance reactivity consists of dopingthe oxide surface with a second metal that has the right electronicproperties. Thus, for example, NixMg1-xO is more reactive thanMgO (Table 3 and ref 47). In previous experimental studies,8a,20

we have found that cesium enhances the efficiency of zinc oxideas a sorbent of sulfur-containing molecules. The Cs adatomshave electronic states that are located within the band gap ofZnO and are very good for bonding interactions with adsorbates.8a

V. Summary and Conclusions

The adsorption of H2S, HS, and S on MgO(100), Ni-MgO(100), and ZnO(0001) was studied using DFT calculationsand slab models. In general, the bonding energies of theadsorbates on a given surface increase in the followingsequence: H2S< HS< S. For the surfaces the chemical activityincreases in the order MgO< NiMgO < ZnO.

Atomic S and HS interact strongly with the perfect MgO(100)surface with bonding energies of 34 and 19 kcal/mol, respec-tively, at a coverage of 0.25 monolayer. The dissociation ofhydrogen sulfide (H2Sgas+ Mgsurface+ Osurfacef HS-Mgsurface

+ H-Osurface) is an energetically favorable process (∆E) -14kcal/mol atθH2S ) 0.25 monolayer), but it may not be easydue to the relatively weak interactions between the moleculeand pentacoordinated Mg atoms of the flat oxide surface (H2Sbonding energy of 9 kcal/mol). For tri-coordinated Mg atomsin step sites of MgO(100), the H2S bonding energy (13 kcal/mol) is larger, enhancing the probability for dissociation of themolecule.

When metal centers of MgO(100) are replaced with nickelatoms, new electronic states are created above the occupied{O2p+ Mg 3s} bands. In the NiMgO(100) system, there is a largeelectron density around the Ni atoms. Atomic S and HS formvery strong bonds on this oxide surface, and in addition thebonding interactions of H2S are substantial (H2S adsorption

Figure 11. Energy positions for the valence and conduction bands ofbulk Al2O3, MgO, ZnO, and Cu2O.11,12,16,59The empty and occupiedstates are indicated by dotted and solid lines, respectively. Forcomparison we also include the energies for MO’s of H2S, 5a1 and 2b1(HOMO), and other sulfur-containing molecules (thiols and thio-phenes).8a,11,58,64Not shown in the figure is the LUMO of H2S, 3b2

orbital, which is located at∼4 eV.11,58 All energies are reported withrespect to the vacuum level.

QLUMO-valence∝ (âLUMO-valence)2/(ELUMO - Evalence) (8)


energy of 17 kcal/mol atθH2S ) 0.25 monolayer) favoringdecomposition.

DFT calculations also indicate that the degree of ionicity inZnO is significantly smaller than in MgO and that the cationsin ZnO(0001) are more chemically active than penta- or tri-coordinated metal sites in magnesium oxide. On top ofZnO(0001), one sees not only stronger H2S-metal bonds (21vs 9 or 13 kcal/mol) but also a larger elongation of the H-Sbonds (0.05 vs 0.01 Å). These effects make easier the dis-sociation of H2S and HS on zinc oxide than on magnesiumoxide.

A simple model based on perturbation theory and band-orbital mixing is able to explain the differences in the reactivityof MgO(001) and ZnO(0001) and the behavior of other oxidesin the presence of sulfur-containing molecules. The reactivityof an oxide mainly depends on how well its bands mix withthe orbitals of H2S, and the electrostatic interactions with thedipole moment of H2S play only a secondary role in bonding.

Acknowledgment. The authors thank J. Z. Larese for severalthought-provoking conversations on the behavior of MgO. Theresearch carried out at Brookhaven National Laboratory wassupported by the Division of Chemical Sciences of the USDepartment of Energy (Contract DE-AC02-98CH10086).

References and Notes

(1) Speight, J. G.The Chemistry and Technology of Petroleum, 2nded.; Dekker: New York, 1991.

(2) Stern, A. C.; Boubel, R. W.; Turner, D. B.; Fox, D. L.Fundamentalsof Air Pollution, 2nd ed.; Academic: Orlando, FL, 1984.

(3) Rodriguez, J. A.; Hrbek, J.Acc. Chem. Res.1999, 32, 719.(4) Topsøe, H.; Clausen, B. S.; Massoth, F. E.Hydrotreating Catalysis;

Springer-Verlag: New York, 1996.(5) DeactiVation and Poisoning of Catalysts; Oudar, J., Wise, H., Eds.;

Dekker: New York, 1985.(6) Thomas, J. M.; Thomas, W. J.Principles and Practice of

Heterogeneous Catalysis; VCH: New York, 1997.(7) Satterfield, C. N.Heterogeneous Catalysis in Practice; McGraw-

Hill: New York, 1980.(8) (a) Jirsak, T.; Dvorak, J.; Rodriguez, J. A.J. Phys. Chem. B1999,

103, 5550. (b) Rodriguez, J. A.; Jirsak, T.; Freitag, A.; Hanson, J. C.; Larese,J. Z.; Chaturvedi, S.Catal. Lett.1999, 62, 113.

(9) (a) Chaturvedi, S.; Rodriguez, J. A.; Jirsak, T.; Hrbek, J.J. Phys.Chem. B1998, 102, 7033. (b) Rodriguez, J. A.; Jirsak, T.; Chaturvedi, S.;Kuhn, M. Surf. Sci.1999, 442, 400.

(10) Slack, A. V.; Holliden, G. A.Sulfur Dioxide RemoVal from WasteGases,2nd ed.; Noyes Data Corp.: Park Ridge, NJ, 1975

(11) Rodriguez, J. A.; Chaturvedi, S.; Kuhn, M.; Hrbek, J.J. Phys. Chem.B 1998, 102, 5511.

(12) Rodriguez, J. A.; Jirsak, T.; Chaturvedi, S.J. Chem. Phys.1999,111, 8077.

(13) Pacchioni, G.; Clotet, A.; Ricart, J. M.Surf. Sci.1994, 315, 337.(14) (a) Raza, H.; Harte, S. P.; Muryn, C. A.; Wincott, P. L.; Thorton,

G.; Casanova, R.; Rodriguez, A.Surf. Sci.1996, 366, 519. (b) Kurtz, R.L.; Henrich, V. E.Phys. ReV. B 1987, 36, 3413.

(15) Wycoff, R. W. G.Crystal Structures, 2nd ed.; Wiley: New York,1964.

(16) Henrich, V. E.; Cox, P. A.The Surface Science of Metal Oxides;Cambridge University Press: Cambridge, U. K., 1994.

(17) Sousa, C.; Illas, F.; Bo, C.; Poblet, J. M.Chem. Phys. Lett.1993,215, 97.

(18) Mejias, J. A.; Marquez, A. M.; Fernandez-Sanz, J.; Fernandez-Garcia, M.; Ricart, J. M.; Sousa, C.; Illas, F.Surf. Sci.1995, 327, 59.

(19) Lin, J.; May, J. A.; Didziulis, S. V.; Solomon, E. I.J. Am. Chem.Soc.1992, 114, 4718.

(20) Rodriguez, J. A.; Jirsak, T.; Chaturvedi, S.; Hrbek, J.Surf. Sci.1998, 407, 171.

(21) Evans, J.; Corker, J. M.; Hayter, C. E.; Oldman, R. J.; Williams,B. P. Chem. Commun.1996, 1431.

(22) Payne, M. C.; Allan, D. C.; Arias, T. A.; Johannopoulus, J. D.ReV.Mod. Phys.1992, 64, 1045.

(23) (a) Delley, B.J. Chem. Phys.1990, 92, 508. (b) Delley, B.; Schefer,J.; Woike, T.J. Chem. Phys.1997, 107, 10067. (c) Kessi, A.; Delley, B.Int. J. Quantum Chem.1998, 68, 135.

(24) Sorescu, D. C.; Yates, J. T.J. Phys. Chem. B1998, 102, 4556.(25) (a) Lindan, P. J. D.; Harrison, N. M.; Holender, J. M.; Gillan, M.

J.; Payne, M. C.Surf. Sci.1996, 364, 431. (b) Lindan, P. J. D.; Harrison,N. M.; Holender, J. M.; Gillan, M. J.Chem. Phys. Lett.1996, 261, 246.

(26) Refson, K.; Wogelius, R. A.; Fraser, D. G.; Payne, M. C.; Lee, M.H.; Milman, V. Phys. ReV. B 1995, 52, 10823.

(27) Rodriguez, J. A.; Hanson, J. C.; Chaturvedi, S.; Maiti, A.; Brito, J.L. J. Chem. Phys.2000, 112, 935.

(28) Rodriguez, J. A.; Jirsak, T.; Dvorak, J.; Sambasivan, S.; Fischer,D. J. Phys. Chem. B2000, 104, 319.

(29) Vandervilt, D.Phys. ReV. B 1990, 41, 7892(30) Monkhorst, H. J.; Pack, J. D.Phys. ReV. B 1976, 13, 5188.(31) (a) Perdew, J. P.; Wang, Y.Phys. ReV. B 1992, 46, 6671. (b) White,

J. A.; Bird, D. M. Phys. ReV. B 1994, 50, 4954(32) Ziegler, T.Chem. ReV. 1991, 91, 651(33) (a) White, J. A.; Bird, D. M.; Payne, M. C.; Stich, I.Phys. ReV.

Lett. 1994, 73, 1404. (b) Hu, P.; King, D. A.; Crampin, S.; Lee, M. H.;Payne, M. C.Chem. Phys. Lett.1994, 230, 501.

(34) Whitten, J. L.; Yang, H.Surf. Sci. Rep.1996, 24, 55.(35) van Santen, R. A.; Neurock, M.;Catal. ReV.sSci. Eng.1995, 37,

557.(36) Lopez, N.; Illas, F.; Ro¨sh, N.; Pacchioni, G.J. Chem. Phys.1999,

110, 4873.(37) Becke, A. D.Phys. ReV. A 1988, 38, 3098.(38) (a) Wang, Y.; Perdew, J. P.Phys. ReV. B 1991, 44, 13298. (b)

Perdew, J. P. InElectronic Structure of Solids ‘91; Ziesche, P., Esching,H., Eds.; Akademie: Berlin, 1991. (c) Perdew, J. P.; Chevary, J. A.; Vosko,S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C.Phys.ReV. B 1992, 46, 6671.

(39) Mulliken, R. S.J. Chem. Phys.1955, 23, 1841.(40) Szabo, A.; Ostlund, N. S.Modern Quantum Chemistry; McGraw-

Hill: New York, 1989.(41) Wiberg, K. B.; Rablen, P. R.J. Comput. Chem.1993, 14, 1504.(42) (a) Scamehorn, C. A.; Hess, A. C.; McCarthy, M. I.J. Chem. Phys.

1993, 99, 2786. (b) Ferro, Y.; Allouche, A.; Cora, F.; Pisani, C.; Girardet,C. Surf. Sci.1995, 325, 139. (c) Chen, L.; Wu, R.; Kioussis, N.; Zhang, Q.Chem. Phys. Lett.1998, 290, 255

(43) Deer, W. A.; Howie, R. A.; Zussman, J.An Introduction to theRock Forming Minerals; Longman: Essex, U.K., 1966.

(44) (a) Casarin, M.; Maccato, C.; Tondello, E.; Vittadini, A.Surf. Sci.1995, 343, 115. (b) Casarin, M.; Maccato, C.; Vitaddini, A.Surf. Sci.1997,377-379, 587.

(45) Scamehorn, C. A.; Harrison, N. M.; McCarthy, M. I.J. Chem. Phys.1994, 101, 1547.

(46) (a) Lopez, N.; Illas, F.J. Phys. Chem. B1998, 102, 1430. (b)Anchell, J. L.; Hess, A. C.J. Phys. Chem.1996, 100, 18317. (c) Johnson,M. A.; Stefanovich, E. V.; Truong, T. N.; Gu¨nster, J.; Goodman, D. W.J.Phys. Chem. B1999, 103, 3391

(47) Tomishige, K.; Chen, Y.-G.; Fujimoto, K.J. Catal.1999, 181, 91,and references cited therein.

(48) He, J. W.; Møller, P. J.Surf. Sci.1987, 180, 411.(49) Huheey, J. E.Inorganic Chemistry,3rd ed.; Harper & Row: New

York, 1983.(50) (a) Jaffe, J. E.; Harrison, N. M.; Hess, A. C.Phys. ReV. B 1994,

49, 11153. (b) Jaffe, J. E.; Hess, A. C.J. Chem. Phys.1996, 104, 3348(51) Baetzold, R. C.J. Phys. Chem.1985, 89, 4150.(52) Abrahams, S. C.; Bernstein, J. L.Acta Crystallogr. B1969, 25,

1233.(53) Duke, C. B.; Lubinsky, A. R.Surf. Sci.1975, 50, 605.(54) (a) Rodriguez, J. A.; Campbell, C. T.J. Phys. Chem.1987, 91,

6648. (b) According to the Phillips ionicity scale54c,d and Pauling’selectronegativity,54e zinc oxide is in the border of ionic and covalentcompounds. (c) Phillips, J. C.Bonds and Bands in Semiconductors;Academic: New York, 1973; Chapter 2. (d) Phillips, J. C.ReV. Mod. Phys.1970, 42, 317. (e) Pauling, L.The Nature of the Chemical Bond, 3rd ed.;Cornell University Press: New York, 1960; pp 93, 98, 246.

(55) Casarin, M.; Maccato, C.; Vittadini, A.J. Phys. Chem. B1998,102, 10745.

(56) Chaturvedi, S.; Rodriguez, J. A.; Hrbek,J. Phys. Chem. B1997,101, 10860.

(57) Rodriguez, J. A.; Jirsak, T.; Sambasivan, S.; Fischer, D.; Maiti,A. J. Chem.Phys., in press.

(58) Rodriguez, J. A.Surf. Sci.1990, 234, 421, and references citedtherein.


(59) Rodriguez, J. A.; Chaturvedi, S.; Kuhn, M.Surf. Sci.1998, 415,L1065.

(60) Parr, R. G.; Yang, W.Density-Functional Theory of Atoms andMolecules; Oxford University Press: New York, 1989.

(61) (a) Hoffmann, R.Solids and Surfaces: A Chemist’s View of Bondingin Extended Structures; VCH: New York, 1988. (b) Libit, L.; Hoffman, R.J. Am. Chem. Soc.1974, 96, 1370.

(62) Shustorovich, E.; Baetzold, R. C.Science1985, 227, 876.(63) Hammer, B.; Morikawa, Y.; Nørskov, J. K.Phys. ReV. Lett.1996,

76, 2141.(64) (a) Rodriguez, J. A.J. Phys. Chem.1997, 101, 7524. (b) Rodriguez,

J. A. Surf. Sci.1992, 278, 326.

(65) (a) Illas, F.; Pacchioni, G.; Pelmenschikov, A.; Pettersson, L. G.M.; Dovesi, R.; Pisani, C.; Neyman, K. M.; Ro¨sch, N.Chem. Phys. Lett.1999, 306, 202. (b) Wu, R.; Zhang, Q.Chem. Phys. Lett.1999, 306,205.

(66) (a) Wichtendahl, R.; Rodriguez-Rodrigo, M.; Ha¨rtel, U.; Kuhlen-beck, H.; Freund, H.-J.Surf. Sci.1999, 423, 90. (b) Freund, H.-J.; Kulenbeck,H.; Staemmler, V.Rep. Prog. Phys.1996, 59, 283.

(67) Pacchioni, G.; Ferrari, A. M.; Bagus, P. S.Surf. Sci.1996, 350,159.

(68) (a) Campbell, C. T.Surf. Sci. Rep.1997, 27, 1. (b) Madey, T.;Yakshinky, B. V.; Ageev, V. N.; Johnson, R. E.J. Geophys. Res.1998,103, 5873.


Documents

Adsorption and Decomposition of H 2 S on MgO(100), NiMgO(100), and ZnO(0001) Surfaces: A First-Principles Density Functional Study