Surface Science 604 (2010) 1742–1751

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Reaction intermediates of methanol synthesis and the water–gas-shift reaction onthe ZnO(0001) surface

Katawut Chuasiripattana a, Oliver Warschkow a, Bernard Delley b, Cathy Stampfl a,⁎a School of Physics, The University of Sydney, Sydney, New South Wales, 2006, Australiab Paul-Scherrer-Institut, Villigen, CH-5232, Switzerland

⁎ Corresponding author.E-mail address: stampfl@physics.usyd.edu.au (C. Sta

0039-6028/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.susc.2010.06.025

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 March 2010Accepted 25 June 2010Available online 17 July 2010

Keywords:Atomic geometryAdsorptionZnO(0001) surfaceMethanol synthesisWater–gas-shift reactionVibrational frequenciesDensity functional theory

The polar Zn-ZnO(0001) surface is involved in the catalysis of methanol synthesis and the water–gas-shiftreaction. We use density functional theory calculations to explore the favorable binding geometries andenergies of adsorption of several molecular species relevant to these reactions, namely carbon monoxide(CO), carbon dioxide (CO2), water (H2O) and methanol (CH3OH). We also consider several proposed reactionintermediates, including hydroxymethyl (CH2OH), methoxyl (CH3), formaldehyde (CH2O), methyl (CH3),methylene (CH2), formic acid (HCOOH), formate (HCOO), formyl (HCO), hydroxyl (OH), oxygen (O) andhydrogen (H). For each, we identify the preferred binding geometry at a coverage of 1/4 monolayers (ML),and report calculated vibrational frequencies that could aid in the identification of these species inexperiment. We further explore the effects on the binding energy when the adsorbate coverage is lowered to1/9 and 1/16 ML.

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Methanol (CH3OH) is industrially important in the synthesis ofvarious chemicals and is also predicted to play a key role in the nextgeneration of renewable energy sources for use in fuel cells [1,2], orpotentially as a medium for hydrogen storage and transportation.Industrially, methanol is mass produced using alumina-supportedZnO catalysts [3,4]. In addition, ZnO also catalyses the forward andreverse water–gas-shift reaction [5]. As a consequence, there is muchinterest in studying the detailed chemical mechanisms of methanolsynthesis and the water–gas-shift reaction on ZnO surfaces.

In an early study, Ueno et al. [6] used infrared spectroscopy to studythe reaction intermediates of thewater–gas-shift reaction on ZnO. Theysuggested that when a mixture of carbon dioxide and hydrogen isintroduced over ZnO, the formate ion (HCOO) is formed. These reactionformates later decomposed to CO (dehydration) and CO2 (dehydroge-nation). Ueno et al. [7] also studied the reaction of methanoldecomposition on ZnO using infrared spectroscopy. By introducingmethanol over ZnO, the methoxyl ion (CH3O) and the formate ion(HCOO) are observed, while H2, CO2 and CO are evolved into the gasphase. They suggest that CO is produced mainly by formate decompo-sition while H2 and CO2 are from the reaction between methanol andformate. Bowker et al. [8] used temperature programmed desorption(TPD) and temperature programmed reaction spectroscopy (TPRS) to

study the adsorption and interaction of intermediate species such as H2,H2O, CO, CO2, formaldehyde (CH2O) and methanol (CH3OH) over ZnO.They propose that the formate ion (HCOO) is the pivotal intermediate ofZnO catalysing methanol synthesis and can decompose to CO and H2.Chadwick and Zheng [9] use TPD to study the decomposition ofmethanol on ZnO. They suggest that the decomposition products aremainly CO andH2with a small amount of CO2. In addition, the influenceof the H2O molecule on methanol decomposition is investigated. Theirresults suggest that methanol, together with coadsorbed watermolecules, enhances the formation of CO2 and H2. Bowker et al. [10]perform investigations into the crystal plane dependence of adsorptionand reaction of CO, CO2, H2 and CO2/H2 co-adsorption on ZnO. Theysuggest that these processes are confined to the polar surfaces of ZnO.Vohs and Barteau [11] also confirm that the Zn-terminated ZnO(0001)surface is active for the decomposition reactions of methanol, formal-dehyde, and formic acid.

In a more recent study, the mechanisms of methanol synthesis andthewater–gas-shift reaction on ZnOwere investigated by Tabatabaei etal. [5]. Infrared spectroscopy and TPD resolve two types of formatespecies during the coadsorption of CO2 andH2 on ZnO. For temperaturesbelow 500 K, they suggest that a bidentate formate is formed whichdecomposes to CO2 and H2. For temperatures above 500 K, amonodentate formate is present which decomposes into CO and H2.After the forward water–gas-shift reaction, only bidentate formate isidentified from the TPD analysis. Their further TPD analysis shows thatboth the reversewater–gas-shift reaction andmethanol synthesis occurconcurrently, with the existence of both types of formates. Therefore,they suggest that themonodentate formate is theprincipal intermediate

1743K. Chuasiripattana et al. / Surface Science 604 (2010) 1742–1751

for methanol synthesis while the bidentate formate is the intermediateresponsible for the forward and reverse water–gas-shift reactions.

The above-mentioned studies of methanol decomposition on ZnOsuggest that the main catalytic intermediates are the methoxyl andformate, whereas in the water–gas-shift reaction, only formateappears to be a critical intermediate. The main catalyst for thewater–gas-shift reaction and methanol synthesis is Cu/ZnO, i.e. theaddition of Cu to zinc-oxide [12–14]. At present, however, the role ofthe oxide, as well as the actual mechanism by which the copper metalactivates the reactions, is still unclear. For example, oxygen vacancieson ZnO(000 1) have recently been proposed to be the catalyticallyactive sites formethanol synthesis onpure ZnO,where the charge stateof the vacancy is also understood to affect the chemical activity [15].With regard to the (0001) Zn-terminated surface studied in thepresentwork, before the effect of copper, and other defects, on catalystactivity can be examined, the behaviour of the adsorption ofintermediates on the clean zinc-oxide surface must be established asa reference.

In the present paper, we provide insights into the preferred bindinggeometries, adsorption energies, and associated vibrational frequenciesfor various possible molecular and atomic intermediates on the ZnO(0001) surface thatmaybeof relevance for, andhelp in theunderstandingof, various heterogeneous catalytic reactions such as methanol synthesisand the water–gas-shift reaction. The gross-mechanisms involved,e.g. in methanol formation from syngas are CO+2H2↔CH3OH, CO2+3H2↔CH3OH+H2O, and CO+H2O↔CO2+H2, the latter being thewater–gas-shift reaction. The polarity of the oxide is expected to play animportant role in catalysis. For understanding the behaviour andfunction of a catalyst, it is necessary to consider all the main low energysurfaces. We choose to begin with the Zn-terminated ZnO(0001)orientation because there is recent detailed quantitative informationregarding the atomic structure for the clean surface [16] which is veryhelpful, if not mandatory, for investigation of the properties of adsorp-tion structures on a surface. There are also experimental measurements(scanning tunneling microscopy, low energy electron diffraction,ultraviolet photoelectron spectroscopy and low energy ion scattering)for the Cu/ZnO(0001) surface [17] which we are presently investigatingtheoretically.

Our results will complement and expand on earlier studies on the(0001) [18–20] and the (101

–0) surfaces [19]. Although some of the

intermediate species considered here, such as hydroxymethyl (CH2OH),methylene (CH2) and the formyl (HCO), have not beendirectly detectedin experiment, they may still occur as transient species in the catalyticprocess. The binding energies and atomic geometries of these transientsrepresent important information, allowing one to assess their possiblerole in surface-facilitated chemical transformations.

2. Computational methodology

Self-consistent total-energy calculations are performed in theframework of all-electron density functional theory (DFT) asimplemented in the DMol3 code [21]. The Kohn–Sham eigenfunctionsare expanded in terms of a numerical atom-centered basis set ofdouble numerical plus polarization (DNP) quality with a radial cutoffof 9 bohr. Exchange and correlation are treated in the generalizedgradient approximation (GGA) in the form of the Perdew, Burke, andErnzerhof (PBE) functional [22]. Scalar-relativistic corrections areexplicitly included in our treatment.

For bulk wurtzite ZnOwe use a 10×10×10 k-point grid and obtainthe following lattice parameters: a=3.302 Å, c=5.317 Å, c/a=1.610,and u=0.362. The bulk modulus is calculated to be 117 GPa. Thesevalues agree very well with the results of other ab initio calculationsby Meyer and Marx [23] also using the GGA-PBE functional; theyobtain a=3.282 Å, c=5.291 Å, c/a=1.6120, and u=0.3792, and abulk modulus of 128 GPa. The theoretical values agree well with

experimental values [24–26]: a=3.250 Å, c=5.207 Å, c/a=1.602,and u=0.3825, and bulk modulus, 143 GPa.

The ZnO(0001) surface is represented using a three-dimensionallyperiodic slab model. The supercell contains four double layers of Zn–Oand the adsorbedmolecule, and is separated from its periodic image by25 Å of vacuum. The surface of interest in this work is the Zn-terminated (0001) surface which is presented on one side of the slabonly. The other slab surface, the O-terminated (000 1) surface, iselectronically saturated using fictitious quasi-atoms with a nuclearcharge of e/2. As discussed by Meyer and Marx [23], this quenches theresidual internal electric fields in the slab. We restricted our slab tofour (double) layers due to the need to balance accuracy with theability to survey large number of configurations. Test showed thatincreasing the slab to size double layers results in an increase of thesurface energy by only 0.034 eV per (1×1) cell. Integration over theBrillouin zone is performed using a 16×16 k-point mesh [27] for a(1×1) surface unit cell. Correspondingly smaller grids are used forlarger surface unit cells. In our slab geometry optimisations, the unitcell dimensions as well as the positions of the Zn–O double layerfurthest from the (0001) surface are held fixed at bulk values.

We investigate the adsorption on ZnO(0001) for a number ofatoms and molecules relevant to the water–gas-shift reaction andmethanol synthesis. These include CH3OH, CH2OH, CH3, CH3, CH2,HCOOH, HCOO, HCO, H2O, CO2, CO, OH, O, and H. The adsorptionenergies are calculated using the following expression,

Eads = −ðEsurf +mol −Esurf −EmolÞ; ð1Þ

where Esurf+mol is the total energy of the adsorbate/substrate system,Esurf is the total energy of the clean surface and Emol is the total energyof the isolated (i.e. gas phase) molecule. As defined here, a morepositive value of Eads corresponds to a more stable configuration.

In order to ascertain that themost stable binding geometry is foundfor each molecule, we conduct an extensive survey over numerousconceivable configurations. This structure survey is carried out using a(2×2) surface unit cell (i.e. a coverage of 1/4 ML). For the case of themethanol molecule, for example, 16 configurations were tested. Theseare derived by placing the molecule in four orientations over the fourhigh-symmetry binding sites on the surface, namely, the on-top site,the face-centered cubic (fcc) site, the hexagonal close-packed (hcp)site, and the bridge site, as illustrated in Fig. 1. While not all trialstructures lead to unique or stable configurations after optimisation,this approach provides a reasonable degree of certainty that the moststable binding geometry for the molecule is found. In setting up trialstructures, we also take care not to place adsorbates on exactsymmetry positions so as to allow them to relax into other geometriesif these are preferred. For the preferred configuration at 1/4 ML,binding energies are also calculated at the lower coverages of 1/9 MLand 1/16 ML, using (3×3) and (4×4) surface unit cells.

For the most stable adsorbate configuration at 1/4 ML coverage, wealso calculate the vibrational modes. Vibrational frequencies arecalculated by diagonalizing the mass-weighted Hessian matrix, whichis computed for the adsorbate molecule and the two top-most surfaceatomic layers. The matrix elements are calculated by a three-pointnumerical differentiation of the forces usingfinite atomic displacementsof 0.01 bohr.

3. Results

3.1. The clean ZnO(0001) surface

The ZnO(0001) surface is well studied regarding its surfacestructure [16,28]. The surface has been shown to exhibit triangularpit reconstructions with the ZnO(0001) surface inside the pit. As afirst step in our investigations we consider adsorption on the ideal

Fig. 1. Four possible high-symmetry adsorption sites on the ZnO(0001) surface. Thevery small (blue) circles denote each adsorption site, i.e. on-top, fcc hollow, hcp hollowand bridge sites. The zinc atoms are represented by the large (grey) circles, while theoxygen atoms are represented by the small dark (red) circles. The same color format forthe ZnO surface is used throughout the paper. The (1×1) surface unit cell is indicatedby the dashed lines.

Table 1Calculated vibrational frequencies at the Γ point for isolated CH3OH and othermolecules. The available experimental values of vibrational frequencies are takenfrom Ref. [29]. The unit is cm−1. The bond bending and stretch modes are indicated by δand ν, respectively.

Molecule Frequencies Vibration mode

This work Exp.

CH3OH 3747 3700 νOH

3155, 3015, 2964 2950 νCH

1462, 1459, 1436 δCH1336 δCO , δOH1122 δCO1061, 1010 1050 δCO

CH2OH 3715 νOH

3257, 3087 νCH

1443 δCH1330 δCO , δOH1194, 1028 νCO

CH3O 2821, 2779 νCH

1416 δCH1311 δCH1164 δCO

CH2O 2934, 2877 2800 νCH

1808 1750 νCO

1532 1500 νCO , δCH1277 1250 δCO , δCH1213 δCO

CH3 3296, 3103 νCH

1368 δCHCH2 2912, 2816 νCH

1254 δCHHCOOH 3605 3550 νOH

3010 3000 νCH

1780 1750 νCO

1347 1350 νCO , δCH , δOH1276 1200 νCO , δCH , δOH , δCO1105 1100 νCO , δOH

HCOO 3084 νCH

1434 δCO1402, 1184 δCO , δCH

HCO 2570 νCH

1848 νCO

1109 δCO , δCHCO2 2310 2350 νCO

1300 1350 νCO

CO 2146 2100 νCO

H2O 3827, 3708 3650 νOH

1608 1550 δOHOH 3621 νOH

Fig. 2. Relaxed atomic geometry ofmethanol (CH3OH) on the ZnO(0001) surface for 1/4ML. The oxygen atomof the CH3OHmolecule adsorbs in the on-top site: (a) top view and(b) side view. The carbon atom is represented by the small pale grey (yellow) circle, thehydrogen atoms by the smallest circles (cyan), and the oxygen atoms by the small darkgrey (red) circles. The same representation is used for the other adsorbate systems in thepaper. Selected interatomic distances are given in Å.

1744 K. Chuasiripattana et al. / Surface Science 604 (2010) 1742–1751

clean surface of ZnO(0001). That is, we do not consider adsorption atthe step between the triangular pit edge and the inside of the pit.

For the Zn-terminated (1×1) ZnO(0001) surface we obtain aninward relaxation of the top layer Zn atoms of 26% and a slightexpansion of the first ZnO double layer interlayer spacing (betweenthe uppermost O layer and the Zn layer below) of 5.9%. These valuesare in very close agreement with previous ab initio calculations wherecorresponding values of 28% and 5.4% were obtained [23].

3.2. The free molecules

Wefirstly carry out calculations for the freemolecules, the obtainedtotal energies of which will be used for determining the adsorptionenergy (Eq. (1)). The effect of spin polarization on the total energy ofthe isolated atoms/molecules is also considered, and taken intoaccount in calculation of the adsorption energies in Eq. (1). We findthat for the atoms/molecules that are unstable in the gas phase, theeffect moderately lowers the total energy, specifically, CH2OH by0.33 eV, CH3O by 0.23 eV, CH3 by 0.48 eV, CH2 by 0.74 eV, HCOO by0.29 eV, HCO by 0.28 eV, OH by 0.45 eV, and O by 0.43 eV. For the otherstable species, the effect of spin polarization does not alter the totalenergies as expected. The vibrational frequencies for each molecule arecalculated from normal mode analysis using Hessian matrix fromnumerical differentiation of forces. The vibrational frequencies are listedin Table 1 and analyzed as bond bending (δ) and bond stretching (ν)modes. We also compare the calculated frequencies with availableinfrared spectroscopy data [29], namely, CH3OH, CH2O, HCOOH, CO2, COand H2O. We find that on average, a difference in frequency of up to50 cm−1 occurs between our calculated values and the experimentaldata.

3.3. Methanol (CH3OH)

We first consider adsorption of a methanol molecule (CH3OH) onZnO(0001), as mentioned above using a (2×2) surface unit cell. Fromthe calculations, we find that the lowest energy configuration consistsof the oxygen atom adsorbing in the on-top site on a zinc atom asshown in Fig. 2. In this configuration, methanol binds to the surface

with an adsorption energy of 0.60 eV. The O–Zn distance (for the Oatom of methanol) is 2.20 Å which is slightly longer than thecorresponding O–Zn distance in bulk ZnO (2.01 Å). The atomic

Fig. 3. Relaxed atomic geometry of hydroxymethyl (CH2OH) on the ZnO(0001) surfacefor 1/4 ML. The oxygen and carbon atoms of the CH2OH molecule are adsorbed in theon-top site of the surface Zn atoms: (a) top view and (b) side view. Atomic species areas in Fig. 2. Selected interatomic distances are given in Å.

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structure obtained for methanol on ZnO(0001) is similar to that foundin a reported previous theoretical studies by Casarin et al. [30,31]using DFT and the cluster approach, and also to methanol on metalsurfaces such as Cu(100) [32], Pt(111) [33], and Ni(111) [34].

The nextmost stable configuration (0.12 eV less stable) is where theO atomofmethanol also binds to the on-top Zn site, butwhere there is adifferent tilt orientation of the molecule. Other trial configurations inwhich the molecule binds to the surface through carbon or hydrogenare not stable.

The adsorption energies for the identified favorable configurationat 1/4 ML, are collected in Table 3 along with those for the lowercoverages of 1/9 ML and 1/16 ML. Table 3 also shows the C–O bondlength and vertical height of the C atom from the surface, for thelowest coverage of 1/16 ML. It can be seen that the adsorptionenergies is slightly less favorable for the lowest coverage (1/16 ML)indicating a weak attractive interaction between adsorbed methanolmolecules on ZnO(0001) for the coverages and considered.

In Table 2 the vibrational frequencies of methanol adsorbed on thesurface for coverage 1/4 ML are listed. Compared to those for the freemolecule, it can be seen for example that the O–H stretch modes arenotably reduced when the molecule is adsorbed on the surface(compare 3747 cm−1 to 3003 cm−1).

Table 2Calculated vibrational frequencies at the Γ point for the optimised structures of CH3OHand other molecules/atoms on the ZnO(0001) surface. The unit is cm−1. The bondbending and stretching modes are indicated by δ and ν, respectively.

Molecule Frequencies Vibrationmode(This work)

CH3OH 3126, 3105 νCH

3029, 3003 νCH , νOH

1532, 1471, 1441 δCH1326 δCH , δCO , δOH1196 δCH , δCO1092 δCH , δOH

CH2OH 3675 νOH

3064, 3000 νCH

1475 δCH1242, 1185 δCH , δCO , δOH1087 δCH , δOH

CH3O 3041, 3021, 2963 νCH

1491, 1461, 1428 δCH1149 δCO1030 νCO

CH2O 3022, 2958 νCH

1445, 1178, 1139 δCHCH3 3095, 3077, 2999 νCH

1459, 1431, 1170 δCHCH2 3101, 2989 νCH

1359 δCHHCOOH 3664 νOH

2988 νCH

1281, 1213 δCO1130 δCH1014 δOH , δCO

HCOO 2997 νCH

1596, 1355 δCO1352 δCH1037 δCO , δCH

HCO 2780 νCH

1427 νCO

1339 δCOCO2 1423, 1187 νCO

CO 1782 νCO

H2O 3637 νOH

2942 νOH

1540 δOHOH 3690 νOH

2276, 2255, 2254, 2252 δOHO 2290, 2262, 2261 νOZn

H 2271, 2251, 2249, 2247 νHZn

1747 δHZn

3.4. Hydroxymethyl (CH2OH)

For adsorption of hydroxymethyl (CH2OH) on ZnO(0001), thelowest energy configuration is where both the oxygen and carbonatoms of the hydroxymethyl molecule are bonded to the on-top sitesof surface Zn atoms as shown in Fig. 3. The C–Zn bond length is 2.04 Åand the O–Zn (O atom of CH2OH) bond length is 2.24 Å. These twosurface zinc atoms are significantly displaced outwards by 0.73 Å and0.32 Å, respectively, relative to the average position of the surface Znatoms not bonded to the molecule. The adsorption energy of thehydroxymethyl molecule is 2.40 eV at 1/4 ML coverage.

The next most favorable configuration is where only the C atombonds to the on-top site of a surface Zn. This structure is less stable by0.11 eV. From Table 3 it can be seen that for lower coverages of 1/9 and1/16 ML the adsorption energy increases (from 2.40 eV at 1/4 ML to2.63 eV at 1/16ML) indicating a repulsive interaction between adsorbedmolecules for this coverage range.

For the vibrational analysis, the most noticeable change for themolecule adsorbed on the surface, relative to the free molecule, is thereduction in the stretch frequencies of theO–Hbondand theC–Hbonds.

3.5. Methoxyl (CH3O)

For the adsorption of methoxyl on ZnO(0001), we find that theenergetically most stable configuration (at 1/4 ML) is where theoxygen atom of the molecule is at the fcc hollow site, three-foldcoordinated to the surface zinc atoms, with bond lengths of 2.25 Å anda vertical height of 1.33 Å, above the three surface Zn atoms. Theatomic structure is shown in Fig. 4. The three zinc atoms bonded to themolecule are relaxed outwards towards the vacuum by 0.45 Å,relative to that not bonded to the molecule. This atomic configurationis similar to that found for CH3O on the Cu(100) surface [32]. Theadsorption energy is calculated to be 3.50 eV.

For lower coverages of 1/9 ML and 1/16 ML, the adsorption energybecomes slightly more favorable (3.56 eV and 3.67 eV, respectively)indicating a slightly repulsive interaction between adsorbedmethoxylmolecules on ZnO(0001) for this coverage regime. Consistent withthis, the bond lengths of the oxygen atom of methoxyl and the threesurface Zn atoms are slightly shorter for 1/16 ML as compared to 1/4ML (2.22 Å versus 2.25 Å).

The second most stable configuration for methoxyl on ZnO(0001)(for 1/4 ML) is at the on-top site where the oxygen atom binds to asingle surface zinc atom. This structure is 0.38 eV less stable than thefavored fcc site configuration.

From the vibrational frequencies listed in Table 2 it can be seenthat on adsorption on the surface, the C–H bond stretching modes

Table 3The atomic geometries of methanol and other adsorbed atoms andmolecules on the ZnO(0001) surface as calculated in a (2×2) surface unit cell. The configurations are described bythe species adsorbed on the surface, with its binding site as a subscript, and its orientation. The adsorption energies for the coverage at 1/16ML, 1/9ML and 1/4ML are listed. For 1/16ML, the bond length is given (d) and the height (h) (along the [0001] direction perpendicular to the surface) between the adsorbed species to the surface.

Molecule Bonding Orientation Eads(eV) d h

1/16 ML 1/9 ML 1/4 ML(Å) (Å)

CH3OH η1 Otop Tilted 0.49 0.60 0.60 2.27 2.14CH2OH η2 Ctop–Otop Flat 2.63 2.50 2.40 2.03, 2.17 1.82, 2.01CH3O η3 Ofcc Upright 3.67 3.56 3.50 2.22 1.22CH2O η3 Ctop–Obridge Flat 2.02 2.12 1.35 2.04, 2.08, 2.08 1.69, 1.49CH3 η1 Ctop Upright 2.82 2.65 2.63 1.99 1.99CH2 η2 Cbridge Upright 4.73 4.64 4.13 2.01, 2.01 1.72, 1.72HCOOH η3 Ctop–Otop Flat 1.45 1.26 0.82 2.03, 1.93, 2.09 1.75, 1.81, 1.87HCOO η2 Obridge Upright 4.72 4.52 4.47 2.06, 2.06 1.91, 1.91HCO η2 Ctop–Otop Flat 2.66 2.53 2.52 2.03, 2.14 1.71, 1.93CO2 η3 Ctop–Otop Flat 1.28 1.12 0.57 2.04, 2.07 1.64, 1.84CO η2 Cbridge Upright 0.94 0.87 0.63 2.14 1.27H2O η1 Otop Tilted 0.73 0.57 0.53 2.19 2.04OH η3 Ofcc Upright 4.47 4.44 4.32 2.20 1.30O η3 Ofcc Upright 6.67 6.61 5.93 2.00 1.12H η1 Htop Upright 3.03 2.99 2.91 1.57 1.57

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increase (by an average of 208 cm−1) indicating a stabilization of themolecule.

Fig. 5. Relaxed atomic geometry of formaldehyde (CH2O) on ZnO(0001), at 1/4 ML. Thecarbon atom and the oxygen atom of CH2O are bonded to the on-top and bridge sites,respectively: (a) top view and (b) side view. Atomic species are as in Fig. 2. Selectedinteratomic distances are given in Å.

3.6. Formaldehyde (CH2O)

The most stable binding configuration for 1/4 ML formaldehyde onthe ZnO(0001) surface is shown in Fig. 5. In this structure, the CH2Omolecule is centered over the hcp hollow site such that the oxygenatom of the molecule bonds with two surface zinc atoms, and thecarbon atom forms a bond with a third zinc atom. The adsorptionenergy is 1.35 eV, which increases (it becomes more favorable) withdecreasing coverage to 2.02 eV at 1/16 ML, indicating a build-up ofa notable repulsive interaction between adsorbates for increasingcoverage.

Our atomic geometry is different to a previous theoretical studyusing a cluster approach by Jones et al. [35]. They found that theformaldehyde molecule bonds to the ZnO(0001) surface through onlythe oxygen atom of formaldehyde, where it adsorbs in the on-top site.In contrast, we find that the on-top site is not stable. The differencebetween these results is due to the cluster model used in Ref. [35]which only represents a single surface Zn atom, and can thus notdescribe the three-fold configuration found by us.

On adsorption of formaldehyde on the surface in the configurationdepicted in Fig. 5, the C–H bond stretch modes increase relative to thefree molecule by an average of 84 cm−1.

Fig. 4. Relaxed atomic geometry of methoxyl (CH3O) on the ZnO(0001) surface, ascalculated with a (2×2) surface unit cell. The oxygen atom of methoxyl adsorbs at thefcc hollow site: (a) top view and (b) side view. Atomic species are as in Fig. 2. Selectedinteratomic distances are given in Å.

3.7. Methyl (CH3)

The lowest energy configuration of CH3 on the ZnO(0001) surfacefor 1/4 ML is at the on-top site, where the carbon atom bonds to asurface zinc atom. The C–Zn bond is 1.98 Å and the bond is alignedperpendicular to the surface as shown in Fig. 6. The zinc atom towhichthe molecule is bonded exhibits a large outward relaxation, by 0.77 Årelative to the average position of the other surface Zn atoms notbonded to the molecule.

Fig. 6. Relaxed atomic geometry of the methyl radical (CH3) on ZnO(0001) at 1/4 ML.The carbon atom of CH3 adsorbs at the on-top site: (a) top view and (b) side view.Atomic species are as in Fig. 2. Selected interatomic distances are given in Å.

Fig. 8. Relaxed atomic geometry of formic acid (HCOOH) on ZnO(0001) for 1/4 ML. Thecarbon atom and the two oxygen atoms of the formic acid (HCOOH) adsorb at on-topsites of surface Zn atoms forming single bonds: (a) top view and (b) side view. Atomicspecies are as in Fig. 2. Selected interatomic distances are given in Å.

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The calculated adsorption energy of CH3 is 2.63 eV. For lowercoverages of 1/9 ML and 1/16 ML the adsorption energy becomes morefavorable (e.g. 2.82 eV at 1/16 ML) as seen from Table 3, indicating aslight repulsive interaction between adsorbates for increasing coverage.

Due to binding on the ZnO(0001) surface, the C–H bond stretchmodes of CH3 decrease by an average of 142 cm−1, indicating aweakening of these bonds.

3.8. Methylene (CH2)

For the adsorption of methylene on ZnO(0001) for 1/4 ML, thelowest energy configuration corresponds to that where the carbonatom binds at a bridge site to two surface Zn atoms, as shown in Fig. 7.The bond length between the carbon atom and the surface zinc atomsis 2.02 Å and the height of the C atom above the two surface Zn atomsto which it bonds is 1.40 Å. These zinc atoms, to which methylenebonds, relax outwards significantly by 0.80 Å, relative to the averageposition of the surface Zn atoms not bonded to the molecule. Thecorresponding adsorption energy is 4.13 eV. Similarly to the othermolecules described above, for lower coverages the adsorption energybecomes more favorable, again indicating a repulsive interactionbetween adsorbates for this coverage in the range 1/16 to 1/4 ML (seeTable 3).

On adsorption on the surface at 1/4 ML, the C–H stretch modesincrease by an average 181 cm−1.

3.9. Formic acid (HCOOH)

Formic acid (HCOOH) is considered [36] to be an importantintermediate in the water–gas-shift reaction and methanol synthesis.We find that the lowest energy configuration (see Fig. 8) consists ofthe carbon atom, and both the two oxygen atoms of the formic acidmolecule, binding to three surface zinc atoms. The C–Zn bond length is2.08 Å and the bond lengths between the oxygen atoms of HCOOHand the surface Zn atoms are 1.98 Å and 2.09 Å. The three zinc atomsto which HCOOH binds relax outwards by 0.64 Å, 0.38 Å and 0.29 Å,respectively, relative to that not bonded to the molecule. The longerO–Zn bond is the one with the oxygen atom bonded to the hydrogenatom. The calculated adsorption energy of formic acid is 0.82 eV. Forthe lower coverages of 1/9 and 1/16 ML, the adsorption energybecomes larger (i.e. 1.26 eV and 1.45 eV, respectively), indicative of arepulsive interaction between the molecules.

The second most favorable configuration for HCOOH on ZnO(0001)at 1/4ML iswhere only the carbonyl oxygen atomof themolecule bindsat an on-top site of the surface Zn atoms. This structure is less stable by0.28 eV.

On adsorption on the surface there are only moderate changes inboth the O–H and C–H stretch modes, namely an increase of 59 cm−2

and a decrease of 24 cm−2, respectively.

Fig. 7. Relaxed atomic geometry of themethylene (CH2) molecule on ZnO(0001) for 1/4ML. The carbon atom of methylene (CH2) adsorbs at the bridge site: (a) top view and(b) side view. Atomic species are as in Fig. 2. Selected interatomic distances are given inÅ.

3.10. Formate (HCOO)

Formate (HCOO) is observed in both methanol synthesis and thewater–gas-shift reaction [5] and is considered to be a pivotalintermediate for these chemical reactions [5,37,38]. It is proposed byTabatabaei et al. [5] that there are two types of formate, namelymonodentate [Fig. 9(c,d)] and bidentate [Fig. 9(e,f)] formate, which are

Fig. 9. Relaxed atomic geometry of formate (HCOO) on ZnO(0001) for 1/4 ML. Thetwo oxygen atoms of the formate molecule adsorb at on-top sites on surface Zn atoms:(a) top view and (b) side view. Also shown is the atomic structure of monodentateformate: (c) top view and (d) side view, and that of bidentate formate: (e) top view and(f) side view. Atomic species are as in Fig. 2. Selected interatomic distances are given in Å.

Fig. 11. Relaxed atomic geometry of CO2 on the ZnO(0001) surface for 1/4 ML. Thecarbon atom and the two oxygen atoms of the CO2molecule bond individually to the Znatoms of the surface: (a) top view and (b) side view. Atomic species are as in Fig. 2.Selected interatomic distances are given in Å.

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responsible for methanol synthesis and the water–gas-shift reaction,respectively. In addition, Petrie and Vohs [39] proposed that a formateion should adsorbonZnO(0001)with amonodentate configuration.Ourcalculations, however, show that neither configuration is energeticallyfavorable. The most favorable structure is that shown in Fig. 9(a) and(b). In this configuration, the two oxygen atomsof the formatemoleculeare bonded to two surface zinc atomswith bond lengths of 2.05 Å and avertical distance from these surface Zn atoms of 2.01 Å. The two zincatoms towhich themolecule bonds relaxoutwards towards thevacuumby 0.43 Å, relative to the average position of the surface Zn atoms notbonded to the molecule. The calculated adsorption energy is 4.47 eV.The second most favorable configuration is where one of the oxygenatoms of the molecule binds to an on-top site (resembling themonodentate configuration). This structure is less stable by just0.07 eV. Similarly to the other molecules discussed, for lower coveragesthere is a moderate increase (more favorable) of the adsorption energy(e.g. to 4.52 eV and 4.72 eV for coverages of 1/9 and 1/16 ML, respec-tively), again indicating a repulsive interaction between molecules.

On adsorption on the surface the calculated stretch frequency of theC–H bond decreases by 87 cm−1 from 3084 cm−1 (for the freemolecule) to 2997 cm−1.

3.11. Formyl (HCO)

For the adsorption of formyl on ZnO(0001) at 1/4 ML, the lowestenergy configuration consists of the carbon and the oxygen atoms of themolecule bonded to on-top sites of the surface zinc atoms as illustratedin Fig. 10. The C–Zn and O–Zn bond lengths are 2.04 Å and 2.16 Å,respectively. The two zinc atoms to which the molecule binds relaxoutwards to thevacuumsignificantly, by0.68 Å and0.40 Å, respectively,relative to the average of the position of the surface Zn atoms notbonded to the molecule. The carbon atom is, vertically, slightly lowerabove the surface than the oxygen atom by 0.24 Å. The calculatedadsorption energy is 2.52 eV.

The second most favorable configuration is where only the oxygenatom binds to the surface, in particular, where it forms bonds with twosurface Zn atoms (a bridge configuration). This structure is less stable byonly 0.05 eV.

Due to adsorption in the most favorable adsorption configuration,the stretch frequency of the molecular C–O bond is significantlyreduced (from 1848 cm−1 for the free molecule to 1427 cm−1)indicating a weakening of the C–O bond, while the C–H stretch modeincreases by 210 cm−1.

3.12. Carbon dioxide (CO2)

For adsorption of CO2 on the ZnO(0001) surface for 1/4 ML, thelowest energy configuration has the molecule lying flat on the surface

Fig. 10. Relaxed atomic geometry of formyl (HCO) on the ZnO(0001) surface for 1/4 ML.The carbon and the hydrogen atoms of the molecule bond to the on-top sites of surfaceZn atoms: (a) top view and (b) side view. Atomic species are as in Fig. 2. Selectedinteratomic distances are given in Å.

with the carbon and the two oxygen atoms each forming a bond witha surface zinc atom as shown in Fig. 11. The carbon atom is bonded toa zinc atomwith a bond length of 2.06 Å, while the two oxygen atoms,each have a bond length of 2.07 Å. The three zinc atoms to which CO2

bonds, relax outwards towards the vacuum by 0.55 Å (bonded tocarbon) and 0.25 Å (bonded to oxygen), relative to the surface Znatom not bonded to the molecule.

The calculated adsorption energy is relatively weak, namely0.57 eV. The second most favorable configuration is one in whichboth the oxygen atoms bind to the surface at bridge sites. Thisstructure is less favorable by 0.40 eV. For lower coverages of CO2, theadsorption energy increases (e.g. to 1.28 eV for 1/16 ML, see Table 3),indicativeof a repulsive interaction betweenmolecules for this coverageregime.

On adsorptionon the surface,with respect to the stretch (symmetric,1300 cm−1 and asymmetric 2310 cm−1) modes of the free CO2

molecule, the C–O bond stretch modes become smaller, namely, thecalculated frequencies are 1423 cm−1 and 1187 cm−1, respectively.

3.13. Carbon monoxide (CO)

For adsorption of CO on ZnO(0001) at 1/4 ML the most favorablestructure is where the COmolecule is bonded to the surface with the Catom binding to two surface Zn atoms, i.e., at a bridge site, as depictedin Fig. 12. The associated C–Zn bond length is 2.18 Å, and the C atom isvertically 1.54 Å above the Zn atoms that it is bonded to. The surface Znatoms to which it bonds are displaced outwards from the surface by0.65 Å. Interestingly, earlier theoretical studies of CO adsorptionon theZnO(0001) surface [40] suggest that the most stable configuration is

Fig. 12. Relaxed atomic geometry for CO on the ZnO(0001) for 1/4ML. The carbon atomofthe CO molecule bonds to two surface Zn atoms, i.e. at a bridge site: (a) top view and(b) side view. Atomic species are as in Fig. 2. Selected interatomic distances are given in Å.

Fig. 13. Relaxed atomic geometry of H2O on the ZnO(0001) surface for 1/4 ML. Theoxygen atom of the water molecule binds to the on-top site of surface Zn atoms: (a) topview and (b) side view. Atomic species are as in Fig. 2. Selected interatomic distancesare given in Å.

Fig. 14. Relaxed atomic geometry of OHon ZnO(0001) for 1/4ML. The oxygen atom of theOH molecule adsorbs at the fcc hollow site of the surface Zn atoms: (a) top view and(b) side view. Atomic species are as in Fig. 2. Selected interatomic distances are given in Å.

1749K. Chuasiripattana et al. / Surface Science 604 (2010) 1742–1751

the on-top site. However, the authors of this study do not consider thebridge site. We find that the bridge site is more stable than the on-topsite by 0.15 eV for 1/4ML. The adsorption energy of themost favorablebridge site is 0.63 eV. The second most favorable configuration is thatwhere the C atomalso adsorbs at the bridge site, butwhere the twoC–Znbonds are tilted. This structure is less stable by just 0.03 eV. For lowercoverages the adsorption energy increases (more energetically favor-able) to 0.87 eV and 0.94 eV at coverages 1/9 and 1/16 ML, respectively(see Table 3), indicating a build-up of repulsive interactions between COmolecules with increasing coverage. Early ultraviolet photoelectronspectroscopy studies [41] investigated the chemisorptions of CO on fourlow-index surfaces of ZnO. It was reported that despite the significantdifferences of the orientations with respect to the coordinationunsaturation, very similar bonding interactions were observed, with aninitial heat of adsorption of ~0.50 eV which decreases approximatelylinearly with increasing coverage, reaching a value of ~0.22 eV for fullmonolayer coverage.

Recent periodic DFT calculations [42] using the PBE functionalreport, for a full monolayer coverage of CO adsorbed in the top site onZn-ZnO(0001), a binding energy of 0.375 eV (36.2 kJ/mol). This valueis consistent with the experimental value, as obtained by molecularbeam investigations which could selectively measure CO adsorptionon specific ZnO crystal faces, also reported in this work, of 0.279 eV(26.9 kJ/mol). The reported DFT-PBE binding energy mentionedabove, for the top site at full monolayer coverage, agrees very wellwith our recent calculations which investigate higher coverages than1/4 ML, where we find a binding energy of 0.43 eV [43].

Earlier theoretical investigations for CO adsorption on Zn-ZnO(0001) in the top site by Casarin et al. [44] using a molecular clusterapproach, reported a binding energy of 1.13 eV (26 kcal/mol) for aneutral cluster. The greater binding energy is presumably due to theuse of the local density approximation (LDA) rather than the GGA asused in the present study. Another difference is that the authors of Ref.[44] also use a cluster to describe the ZnO(0001) surface, which maycontribute to the difference.

On adsorption on the surface in the favored bridge site (for 1/4ML)we find that the C–O bond stretch mode decreases notably, from2146 cm−1 for the free molecule, to 1782 cm−1. Experimentally[41,45,46], it has been reported that for CO adsorption on ZnO, thestretch mode increased relative to the gas phase value, not decreasedas we find. The reason for this apparent disagreement could again berelated to a possible a different local adsorption site/environmentwhere CO could adsorb at the step edges or at sites in which there is alocal oxygen excess. Indeed preliminary results [43] indicate that COadsorbed on an oxidized ZnO(0001) surface yields an increase in theCO stretch frequency consistent with experiment.

3.14. Water (H2O)

The adsorption of water (H2O) on ZnO(0001) for 1/4 ML results inan atomic configuration where the oxygen atom binds in an on-topsite of the surface Zn atoms, as shown in Fig. 13. In this structure, theoxygen atom of the water molecule binds to the surface Zn atom witha bond length of 2.22 Å. The Zn atom relaxes outwards from thesurface by 0.25 Å, relative to the position of the other surface Znatoms. The associated O–Zn bond is not up-right, but tilted by 16.6°.The associated adsorption energy is 0.53 eV. For lower coverages theadsorption energy becomes slightly more favorable, namely, 0.57 eVand 0.73 eV for 1/9 and 1/16 ML, respectively, indicating a slightlyrepulsive interaction between adsorbed water molecules.

This structure is similar to that obtained by Casarin et al. [30] whoused molecular cluster approach. Their calculated adsorption (1.69 eV)is however greater than ours; this could be due to the use of a cluster todescribe the ZnO(0001) surface and due to using the local densityapproximation for the exchange correlation functional, while in thepresent work we use the GGA.

For adsorption of water on the surface at 1/4 ML, we obtain for theantisymmetric and symmetric stretch mode values of 3637 cm−1 and2942 cm−1, respectively. Our calculated bending mode is 1540 cm−1.

3.15. Hydroxyl (OH)

For adsorption of OH on ZnO(0001) at 1/4 ML, the favored bindingsite is the fcc hollow site of the surface Zn atoms, as shown in Fig. 14.This is consistent with the study of Kresse et al. [28]. The bond lengthbetween the oxygen atom of OH and the zinc atoms to which it bindsis 2.23 Å. These three zinc atoms relax outwards from the surface by0.47 Å, relative to the non-bonded Zn surface atom. The calculatedadsorption energy is 4.32 eV, which becomes slightly more stable forthe lower coverages of 1/9 ML (4.44 eV) and 1/16 ML (4.47 eV),indicating a weak repulsive interaction between adsorbates.

The second most stable configuration is where the oxygen atom ofOH binds to the hcp hollow site on the surface. This structure is lessstable by 0.31 eV.

On adsorptionon the surface theO–Hstretchmode increases slightlycompared to in the free OH molecule, from 3621 cm−1 to 3690 cm−1.

3.16. Oxygen (O)

The lowest energy site for oxygen adsorption on the ZnO(0001)surface at 1/4 ML is the fcc hollow site of the surface Zn atoms, asdepicted in Fig. 15. The associated O–Zn bond lengths are of 2.02 Å. Thethree zinc atoms to which the adsorbed O atom binds relax outwardsfrom the surface by 0.67 Å, relative to that not bonded to Zn. Thecalculated adsorption energy is 5.93 eV (with respect to a free O atom).

Fig. 15. Relaxed atomic geometry of O on the ZnO(0001) surface for 1/4 ML. The oxygenatom adsorbs at the fcc hollow site of the surface Zn atoms: (a) top view and (b) sideview. Atomic species are as in Fig. 2. Selected interatomic distances are given in Å.

Fig. 16. Relaxed atomic geometry of H on ZnO(0001) for 1/4 ML. The hydrogen atomadsorbs at the on-top site of the surface Zn atoms: (a) top view and (b) side view.Atomic species are as in Fig. 2. Selected interatomic distances are given in Å.

1750 K. Chuasiripattana et al. / Surface Science 604 (2010) 1742–1751

For lower coverages the adsorption energy becomes more favorable,namely 6.61 eV for 1/9ML and 6.67 eV for 1/16ML, indicating a notablerepulsive interaction between adsorbed O atoms. The next mostfavorable adsorption site is the hcp site, which is only marginally lessstable. Relative toa freeO2molecule, theadsorptionenergies are 3.05 eV(the binding energy per O atom of the oxygen molecule) smaller thanthose relative to a freeatom, i.e. 2.88 eV, 3.56 eV, and3.62 eV for 1/4, 1/9,and 1/16 ML respectively. These results are consistent with those ofKresse et al. [28] who find that the fcc site is 0.2 eVmore stable than anon-top site, and that for low coverages (1/16ML) the adsorption energy

Fig. 17. Overview of adsorption energies of the considered molecules on the ZnO(0001) surSeveral metastable configurations are indicated by thin horizontal bars above.

is 0.89 eV more stable than for 1/4 ML, i.e. 3.42 eV versus 2.53 eV(relative to a free O2 molecule).

3.17. Hydrogen (H)

For the adsorption of H on the ZnO(0001) surface at 1/4 ML, thepreferred binding configuration is the on-top site as shown in Fig. 16.The calculated H–Zn bond length is 1.56 Å. The zinc atom to which Hbonds relaxes outwards fromthe surface significantly, by0.76 Å, relativeto the position of the surface Zn atoms not bonded to H. The adsorptionenergy is calculated to be 2.91 eV at 1/4 ML, which becomes slightlymore favorable for lower coverages (i.e. 2.99 eV and 3.03 eV forcoverages of 1/9 and 1/16 ML, respectively). Relative to the energy of(half) the free H2 molecule, the values of the adsorption energy are0.64 eV, 0.72 eV, and 0.76 eV, respectively for 1/4, 1/9, and 1/16ML. Thesecondmost stable configuration for1/4MLcoverage is thehcp site. Thisstructure is less stable by0.09 eV. These results are consistentwith thoseof Kresse et al. [28] who also identified the on-top adsorption site as thepreferred one for H adsorption on ZnO(0001) from DFT-GGA calcula-tions, and reported an adsorption energy relative to free H2 of ~0.5 eV inthe coverage range of 1/16 to 1/4 ML.

4. Discussion and summary

From the investigations of the adsorption of atomic and molecularspecies on the polar Zn-ZnO(0001) surface, namely, carbon monoxide(CO), carbondioxide (CO2),water (H2O)andmethanol (CH3OH), aswellas several proposed reaction intermediates, including hydroxymethyl(CH2OH), methoxyl (CH3O), formaldehyde (CH2O), methyl (CH3),methylene (CH2), formic acid (HCOOH), formate (HCOO), formyl(HCO), hydroxyl (OH), oxygen (O) and hydrogen (H), we find weakadsorption on the surface for CO, CO2, H2O and CH3OH. This can be seenfrom Fig. 17, which depicts the energetics of the most, next-most, andother less stable structures, providing an overview of the adsorptionenergies for stable and metastable structures of the various atomic andmolecular species on ZnO(0001) considered. The various consideredreaction intermediates, in particular, methylene (CH2) and formate(HCOO), bind strongly to the surface due to their electronicallyunsaturated nature in the gas phase. Thus, these intermediates aresignificantly stabilized by the ZnO catalyst. The other consideredintermediates, CH3O, CH2OH, CH3, and HCO, are also notably stabilizedby adsorption on the surface.

From Fig. 17, it can be seen that for the adsorption of a number ofatoms and molecules, the most stable site identified is only marginally

face for 1/4 ML. A thick horizontal bar indicates the lowest energy configuration found.

1751K. Chuasiripattana et al. / Surface Science 604 (2010) 1742–1751

more so than the next most energetically favorable; for example, forhydroxymethyl (CH2OH) (by 0.11 eV), formate (HCOO) (by 0.07 eV),formyl (HCO) (by 0.05 eV), and oxygen (by 0.03 eV). This implies that atelevated temperatures it is possible that these speciesmaybepresentonthe surface also in these less favorable configurations. Such metastableadsorption structures could also represent transition states that occur invarious surface chemical reactions involving these species.Wenote thatwe carriedout extensive investigations of the adsorption structures onlyat 1/4 ML, so given these very close energies, it could be possible that atlower or higher coverages than 1/4 ML, configurations other than theone most stable at 1/4 ML could become favored.

The calculated vibrational frequencies of the considered species ingas phase generally agree well with experimental data, although adifference in frequency of up to 50 cm−1 can occur. By considering alsothe vibrational frequencies of the molecules adsorbed on the surface,this represents important information which could aid in the experi-mental identification of these species. From investigation of the effect ofadsorbate coverage on the adsorption energy, by considering lowercoverages of 1/9 and 1/16ML, as well as the 14ML structures, generallywe find that the lower coverages yield more favorable adsorptionenergies indicating a repulsive interaction between the adsorbedmolecules. The only exception is the adsorption of the methanolmolecule where the adsorption energy for the low coverage of 1/16 MLis slightly less favorable (by 0.11 eV) than for the higher coverages.

5. Conclusion

Using density functional theorywe have investigated the adsorptionof atomic and molecular species on the polar Zn-ZnO(0001) surfacewhich are of relevance to the catalysis of methanol synthesis and thewater–gas-shift reaction over this surface. Through an extensive surveyof possible structures carried out for 1/4 ML, we identify theenergetically most favorable binding geometries and correspondingadsorption energies. Investigating the effect of coverage on theadsorption energy, we generally find that lower coverages (1/9 and 1/16 ML) yield somewhat more favorable energies. We also report thevibrational frequencies for the free molecules as well as when they areadsorbed on the surface. The results reported in the present paperrepresent valuable information concerning the interaction and behav-iour of these atomic and molecular species with the Zn-ZnO(0001)surface of the ZnO catalyst, whichmay aid in experimental studies, andwhich lay the foundation for further theoretical investigations towardsunderstanding the reaction energetics for methanol synthesis and thewater–gas-shift reaction over ZnO(0001).

Acknowledgement

The authors gratefully acknowledge financial support from theAustralian Research Council (grant number: DP0770631).

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