Methanol synthesis on ZnO(0001). II. Structure, energetics, and vibrationalsignature of reaction intermediates

Janos Kiss,1, 2 Johannes Frenzel,1 Bernd Meyer,1, 3, a) and Dominik Marx11)Lehrstuhl fur Theoretische Chemie, Ruhr-Universitat Bochum, 44780 Bochum,Germany2)Max-Planck-Institut fur Chemische Physik fester Stoffe, 01187 Dresden, Germany3)Interdisziplinares Zentrum fur Molekulare Materialien (ICMM) and Computer-Chemie-Centrum (CCC),Universitat Erlangen-Nurnberg, 91052 Erlangen, Germany

(Dated: June 19, 2013)

A rigorous characterization of a wealth of molecular species adsorbed at oxygen defects on ZnO(0001) is given.These defects represent the putative active sites in methanol synthesis from CO and H2. The oxidation stateof the ZnO catalyst and thus the preferred charge state and the reactivity of the oxygen vacancies depend onthe gas phase temperature and pressure conditions. Considering charge states of oxygen vacancies relevant atthe reducing conditions of the industrial process, i.e. F++/H2, F

0, F0/H2 and F−−, as well as the F++ center

which is abundant at UHV conditions and therefore important to allow for comparison with surface scienceexperiments, we have investigated the structure, energetics and vibrational frequencies of an exhaustive cata-log of reaction intermediates using electronic structure calculations. After having identified the characteristicadsorption modes of CO, formate, formic acid, hydroxymethylene, formyl, formaldehyde, dioxomethylene,hydroxymethyl, hydroxymethoxide, methoxide, as well as methanol itself, the thermodynamic stability of allspecies with respect to the charge state of the oxygen vacancy and their electronic stabilization is discussedin detail and summarized in an energy level diagram.

PACS numbers: 68.47.Gh, 68.43.Bc, 68.43.Fg, 68.43.Pq, 71.15.Mb, 82.65.+r

I. INTRODUCTION

Methanol is one of the most important bulk chemi-cals. It is produced on an industrial scale from syngas (amixture of CO, CO2 and H2) in an heterogeneously cat-alyzed process.1,2 Even with the best currently knowncatalyst, the ternary compound Cu/ZnO/Al2O3 in theICI process,1 rather severe reaction conditions have tobe employed. Thermodynamic requirements suggest touse low temperatures for a high methanol yield. How-ever, at ambient conditions the catalysts show only afeeble activity and temperatures around 550 K have tobe applied to accelerate sufficiently the reaction. Subse-quently, the pressure is raised to about 50–200 bar to en-hance the thermodynamically limited yield.1,3 Obviously,a more active low-temperature catalyst, which would notbe limited by thermodynamic equilibrium, is highly desir-able in order to lower the energy requirement for currentindustrial applications.For improving a catalyst, in particular on the basis of

a rational design strategy, a detailed knowledge of the re-action mechanism is an essential prerequisite. Althoughthe basic reactions in methanol synthesis, i.e. the step-wise hydrogenation of CO and CO2, are rather simple,and despite the industrial importance of the process, theunderlying reaction mechanism from syngas to methanolis still not fully understood. Ideally, the elucidation of areaction mechanism would comprise a full characteriza-

a)Electronic mail: bernd.meyer@chemie.uni-erlangen.de

tion of all reaction intermediates and a determination ofthe thermodynamic and kinetic parameters for each ele-mentary reaction step. Unfortunately, such experimentalstudies are rather difficult, since they are often hamperedby the complex nature of the parallel and competing re-actions steps, and due to the requirement of high pres-sure conditions which limits the applicability of manyexperimental techniques. In this situation, atomistic cal-culations can give valuable insights which complementexisting experimental data.

Most experimental and theoretical studies on thereaction mechanism of the methanol synthesis havefocused on the nowadays employed ternary catalystCu/ZnO/Al2O3. Mechanistic studies for this system,however, are quite challenging because of the complexnature of the catalyst surface structure. The Cu par-ticles and ZnO/Al2O3 support show a so-called “strongmetal-support interaction”,4,5 in which the catalyst sur-face undergoes dynamical morphological changes duringthe methanol synthesis process. In the syngas atmo-sphere ZnO gets partially reduced, Cu starts to wet theZnO surfaces, and reduced ZnO is incorporated into theboundary area of the Cu particles.6–12

In view of this complex behavior of the ternary sys-tem that makes meaningful computational modeling dif-ficult at best, we focus in the following on the Cu-freeZnO catalyst, as it has been used by BASF for decadesprior to the discovery of the ternary catalyst.1 Due to theoverwhelming importance of Cu/ZnO/Al2O3 in industrytoday, the older Cu-free ZnO catalyst became somewhatneglected. Although mechanistic studies are obviouslymuch simpler for pure ZnO, even here no agreement on

2

the precise reaction mechanism has been yet achieved!For instance, only recently it could be clarified that underindustrial relevant high pressure conditions methanol isformed from CO on the Cu-free ZnO catalyst,13 in con-trast to Cu/ZnO/Al2O3 where isotope labeling experi-ments indicate that CO2 is the primary carbon sourcefor methanol.14,15 Using a CO2–free CO/H2 mixture andnear-industrial reaction conditions Kurtz et al.13 showedthat Cu-free ZnO/Al2O3 gives about the same methanolyield as the ternary catalyst at only a 100 K higher tem-perature. Adding small traces of CO2, however, resultedin a strong decrease in activity for the Cu-free catalystbut an increase for the ternary system.

Several different reaction mechanisms for the forma-tion of methanol from CO and H2 over ZnO havebeen proposed in the past (see Fig. 1 and Ref. 3 foran overview). The first mechanism was derived basedon concepts originating from organometallic and metalcoordination chemistry of well-defined transition metalhydride and carbonyl complexes.16–18 It is suggestedthat initially metal hydrides and Zn–coordinated COmolecules are present at the surface. In the first reac-tion step CO is inserted into a metal hydride bond and aformyl intermediate is formed. Formyl is then protonatedto either hydroxymethylene or formaldehyde. In a sec-ond hydrogenation step either hydroxymethyl or methox-ide are formed, which are finally protonated to methanol(see Mechanism A in Fig. 1). For all reaction steps andintermediates analogs in organometallic complex chem-istry are known in the literature.17,18

Infrared (IR) and thermal programmed reaction spec-troscopy (TPRS) studies on ZnO powders and sin-gle crystal surfaces have shown that methanol read-ily decomposes over ZnO into CO and H2 (with tracesof CO2).

19–27 Characteristic IR bands revealed thatmethoxide and formate are intermediates in the methanoldecomposition pathway. By invoking the principle of mi-croscopic reversibility it was concluded that both speciesshould also be intermediates in the methanol synthesisreaction. In the second proposed reaction mechanism itis therefore assumed that the first reaction step consistsof an insertion of a CO molecule, which is adsorbed at aZn site, into a surface OH group to form a surface formateintermediate (see Mechanism B in Fig. 1).3 The followingsteps remain unspecific and speculative: by addition ofa H2 molecule formate could be converted to hydroxy-methoxide which, in turn, transforms into methoxide byabstraction of a water molecule and addition of anotherH2. The methoxide, finally, is protonated to methanol.

A characteristic difference between Mechanism A andB is that in A most intermediates are bound via theC atom to a surface Zn ion, while in B the bonding to thesurface is always via an O atom. The main problem withMechanism B is that ZnO would loose oxygen during thecatalytic cycle and the surface would have to be reoxi-dized, for example, via CO2 decomposition. To circum-vent this inconsistency Kung3 proposed another reactionmechanism for the methanol synthesis (see Mechanism C

in Fig. 1). He assumed that above a triangle of Zn ions(being present, for example, inside an O vacancy) COcould be more strongly bound and more activated withthe O end down than with the usual C–down orientation.In this case, the C atom would be easily accessible for astepwise hydrogenation via formyl and formaldehyde tomethoxide. The final step would be again a protonationof the methoxide to methanol.

The three proposed reaction mechanisms only focus onthe different possible reaction intermediates and whetherthey are bound to Zn or O surface sites, but the natureof these binding sites remains an open question. Surfacescience studies on well-defined single crystals have shownthat the ideal surfaces of ZnO are rather inert towardsadsorbates.28 More reactive “active sites” are required toexplain the catalytic activity of ZnO. There is ample ofevidence in the literature that the polar surfaces of ZnOare more active surface terminations than their nonpo-lar counterparts,29–32 and that the active sites involveO vacancies.13,33 These O vacancies can exist in differentcharge states depending on whether they are created byremoval of H2O molecules, OH groups or O atoms. Thus,the charge state of the O vacancies is determined by thedegree of hydroxylation of the surface, which in turn iscontrolled by the composition, temperature and pressureof the surrounding gas phase.34–36

In the preceding part I of our study “Methanol synthe-sis on ZnO(0001).I. Hydrogen coverage, charge state ofoxygen vacancies, and chemical reactivity” (see Ref. 34)we determined a phase diagram of the most stablecharge state of O vacancies on the polar O–terminatedZnO(0001) surface as function of the hydrogen chemi-cal potential of the surrounding gas phase using periodicdensity-functional theory (DFT) calculations in combina-tion with a thermodynamic analysis.36 We showed thatfor the relevant temperature and pressure conditions ofthe methanol synthesis the charge states F0, F− and F−−are most abundant, whereas the electron deficient va-cancies F++ and F+ are suppressed. F++ and F+ onlydominate at ultra-high vacuum (UHV) conditions but areeasily reduced to F0 and F− by homolytic H2 adsorptionat higher H2 partial pressure. Furthermore we foundthat O vacancies containing hydrides are very likely toexist. They are created by heterolytic dissociation ofH2. The hydride anion H− sits more or less symmet-rically above the Zn triangle in the O vacancy and theremaining proton converts a surface O2− into OH−. Wetermed these defects F++/H2 and F0/H2. They are en-ergetically slightly less favorable than their isoelectroniccounterparts F0 and F−−. However, the barrier for theheterolytic dissociative adsorption of H2 at an O vacancysite is lower than the barrier for homolytic dissociation,and the barrier for transforming the hydridic defects intothe isoelectronic counterparts (which is accompanied bya reduction of ZnO) is quite high. Therefore, the hydridicdefects F++/H2 and F

0/H2 might have a long enough life-time that they may act as potential hydrogenation sourcefor the methanol synthesis.

3

Figure 1. Schematic overview of previously proposed reaction mechanisms, as discussed in the text, for methanol synthesis from CO andH2 over ZnO (adopted from Ref. 3).

Only a few theoretical studies have aimed at explor-ing the different proposed reaction mechanisms for themethanol formation from CO and H2 over ZnO catalysts.Jones et al.37 reported the energetics of the intermediatesand one transition state for a reaction pathway similar toMechanism A (formyl, hydroxymethylene, formaldehyde,methoxide, methanol) based on DFT calculations andsmall cluster models which mimic the ideal, defect-freepolar Zn-terminated ZnO(0001) surface. Recently, thiswork has been revisited and extended by Chuasiripat-tana et al.38 Fink et al.13 calculated the stability of theintermediates according to Mechanism C using a highlyaccurate approximate coupled cluster method and an em-bedded cluster model for an F++ center on the hydrogen-stabilized polar ZnO(0001) surface. This study was re-cently vastly extended by Rossmuller et al.39,40 to includefurther possible intermediates, other charge states of theO vacancy and the most important activation barriers.Rossmuller et al.39,40 also employed an embedded clus-ter model, but density functional theory was used.

In part III of our study “Methanol synthesis onZnO(0001).III. Free energy landscapes, reaction path-ways, and mechanistic insights” (see Ref. 41) we demon-strated the usefulness of accelerated sampling ab initiomolecular dynamics (AIMD) techniques42 by generatingthe whole reaction network and automatically disclos-ing all molecular species of methanol synthesis from COand H2 over ZnO(0001), see Ref. 43 for a review of thisapproach to computational heterogeneous catalysis. Al-lowing direct access to the free energy surfaces of the

chemical processes at this catalyst surface, the acceler-ated AIMD approach was able to further refine individualreaction channels. Moreover, we were able to reveal reac-tion pathways being exclusively open at certain processconditions, i.e. reducing and oxidizing conditions as rele-vant to the industrial processes of methanol synthesis andin the methanol decomposition reaction, respectively.43

The aim of the present paper is to provide an exhaus-tive characterization of the possible reaction intermedi-ates in methanol synthesis from CO and H2 at O va-cancies on the partially hydroxylated polar ZnO(0001)surface. We have calculated the structure and energeticsof various reaction intermediates proposed in the Mech-anisms A to C. Furthermore, the list of possible inter-mediates has been extended by formic acid, dioxome-thylene and hydroxymethoxide. All these species havebeen observed in our recent AIMD sampling.41 Differ-ent alternative configurations of the intermediates in-side or at the edge of the vacancies are taken into ac-count. For the O vacancy we considered the charge statesF++/H2, F

0, F0/H2, and F−−, which we have identi-fied in part I of our study as the most relevant vacancyconfigurations under reaction conditions.34 We have re-stricted our study to configurations with an even num-ber of electrons since it reduces the computational costby a factor of two. For all defects considered we alsotested the influence of the specific arrangement of theH atoms around the vacancy. Moreover, we included theF++ centers in the present investigation, although theywill not play an important role at the temperatures and

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pressures in methanol synthesis.34 However, they mightbecome abundant under UHV conditions which will allowus to connect our results to single crystal UHV experi-ments and to previous theoretical studies where F++ wasconsidered as the active site.13 Finally, for a selected setof the most stable configurations the vibrational frequen-cies are calculated and compared to existing experimentaldata, whereas these important properties are predictedfor yet undetected species that have been disclosed bythe previous AIMD simulations.

II. COMPUTATIONAL DETAILS

All calculations were carried out using the CPMD soft-ware package.44,45 Throughout this study we employedthe identical setup and the same computational param-eters as in our preceding work.34,41,43 Briefly, the DFTcalculations are based on the Perdew, Burke and Ernz-erhof gradient-corrected exchange correlation functionalPBE46, Vanderbilt ultrasoft pseudopotentials47 and aplane wave basis set. Well-converged results for atomicstructures and energies were obtained with a plane wavecut-off energy of 25Ry. In the computations of vi-brational frequencies the cut-off energy was increase to30Ry.The ZnO(0001) surface structures were modeled by

periodically repeated slabs with a lateral extension of(4×4) surface unit cells. The slabs consisted of fourZn-O double layers and were separated by a vacuumregion with a thickness of 15 A. Altogether, the hexag-onal supercell contained 64 zinc and 64 oxygen atoms.The O–terminated top side of the slab was covered with1/2 monolayer of hydrogen,36 and the broken surfacebonds at the bottom of the slab were saturated withpseudohydrogen atoms carrying a nuclear charge of +3/2.Due to the large size of the supercells the Brillouin zonesampling could be restricted to the Γ–point.34

Oxygen vacancies on the ZnO(0001) surface were cre-ated by removing a single O atom from the top surfacelayer. The different charge states of the vacancy wereobtained by adjusting the number of surface H atomsso that throughout all calculations the slabs remainedcharge neutral.34 In the geometry optimizations only thepositions of the atoms in the upper half of the slab to-gether with the adsorbates were relaxed, whereas theatoms in the bottom half of the slab were kept fixed. Theconvergence criterion for the residual atomic forces was0.01 eV/A. When vibrational frequencies were computed,the convergence criterion was tightened to 0.001 eV/A.For more details on the computational setup, see Ref. 34.The vibrational frequencies were calculated in har-

monic approximation by a finite difference scheme. Theatoms of the adsorbate molecule together with the firsttwo coordination shells of surface atoms around the ad-sorption site were displaced by 0.01 A in all three Carte-sian directions, and the dynamical matrix was deter-mined from the resulting atomic forces. To adjust our

calculated frequencies for the DFT error and for devia-tions due to anharmonicities we scaled all frequencies byvibrational mode dependent scaling factors. The scal-ing factors were determined by taking the ratio betweenthe experimental and the computed frequency of a smallset of isolated gas phase molecules (formic acid, formal-dehyde and methanol, see Supporting Material49). Thevibrational frequencies of the gas phase molecules werecomputed by using a large cubic box with a size of 24 bohrand otherwise exactly the same setup as in the slab cal-culations.We describe the energetics and relative stability of the

different intermediate species in terms of a global forma-tion/reaction energy Espec

f which we define as the relativeenergy change along the reaction pathway with respectto the starting configuration, i.e., the slab with the initialO vacancy and one CO and two H2 molecules in the gasphase:

Especf = Espec

slab (NH2)− (

Evslab + ECO

mol +NH2EH2

mol

). (1)

NH2is the number of subsequently adsorbed H2

molecules along the reaction pathway and Especslab , E

vslab,

ECOmol, E

H2

mol are the total energies of the slab with the ad-sorbed intermediate species, the adsorbate-free slab withthe initial O vacancy and the isolated gas phase CO andH2 molecules, respectively. For reaction intermediateswhich are stable gas phase molecules it is sometimes moreinstructive to look at the adsorption energy Espec

ad of thisspecies:

Especad = Espec

slab − Evslab − Espec

mol (2)

with the total energy Especmol of the isolated adsorbate

molecule in the gas phase. The difference between Especad

and Especf is given by the gas phase formation energy of

the adsorbate molecule from CO and H2.All formation and binding energies will be reported

without correction for the zero-point vibration energy(ZPE). To test the accuracy of our calculational setup(PBE functional, ultrasoft pseudopotentials) we have cal-culated the gas phase formation energies of methanol,formaldehyde and formic acid from CO, CO2 and H2

(see Table I). They deviate by about 0.2 eV from the ex-perimental formation enthalpies at 298K. This gives usa good estimate of the overall accuracy which we canexpect in our study for the formation and adsorption en-ergies of the intermediates along the reaction pathway.Altogether we have calculated the structure and en-

ergy of more than 170 configurations. As a general rule itturned out that for a given charge state of the O vacancythe specific arrangement of the H atoms around the de-fect typically changes the intermediate formation energyonly by 0.1 to 0.2 eV. In many cases the same holds trueif the intermediate could be bound at similar (but notstrictly symmetry-equivalent) positions or orientations inor next to the O vacancy. From our set of more than 170configurations we will therefore focus in the paper onlyon a selected set of low energy structures. All other con-figurations are listed in the Supporting Material.49

5

Table I. Experimental reaction enthalpies Hexpf (at 298K) and calculated zero-temperature formation energies EPBE

f for some selected

gas phase reactions as obtained from the same calculational setup that is used throughout this study of the ZnO catalyst surfaces. TheZPE correction was determined from the experimental vibration frequencies.48 All energies are given in eV. ΔE is the difference betweenthe calculated formation energy including ZPE and the experimental enthalpy.

Reaction Hexpf EPBE

f w/o ZPE ZPE EPBEf +ZPE ΔE

CO + 2H2 −→ H3COH −0.94 −1.84 +0.70 −1.14 −0.20CO2 + 3H2 −→ H3COH + H2O −0.49 −1.12 +0.82 −0.30 +0.19CO + H2 −→ H2CO −0.06 −0.56 +0.31 −0.25 −0.19CO2 + H2 −→ HCOOH +0.15 −0.15 +0.32 +0.17 +0.02

(a) ECOf (F++) = −0.30 eV (b) ECO

f (F0) = −0.05 eV

Figure 2. Top view of CO molecules adsorbed at O vacanciesin different charge states on the polar ZnO(0001) surface. For asummary of all optimized structures see Fig. S1 in the SupportingMaterial.49 The Zn O, C and H atoms are depicted as silver-gray,red, cyan and blue spheres, respectively. The O atoms in the adsor-bates are represented by smaller spheres than the surface O ions.The same color coding is used in all other figures.

III. RESULTS AND DISCUSSION

A. Carbon monoxide (CO)

In all three proposed reaction mechanisms itis assumed that methanol synthesis starts from atightly bound CO molecule at the surface. Recentexperimental28,50,51 and theoretical50–53 studies haveshown that CO interacts only weakly with defect-freeZnO surfaces. The most stable CO adsorption con-figuration is on-top of Zn ions with the carbon sidedown.54 Depending on the experimental and computa-tional method CO adsorption energies between −0.28and −0.36 eV have been reported for the low coveragelimit. CO can also bind to surface OH groups (C-downorientation)50,55 with a somewhat lower adsorption en-ergy of about −0.20 eV. In addition to CO adsorbed ondefect-free surface parts of ZnO powders and single crys-tals also more strongly bound CO species are seen incalorimetric and thermal desorption spectroscopy (TDS)

measurements.19,28,56 Such CO species, which give rise toTDS desorption peaks above 200K, have been assignedin part to CO stemming from the decomposition of othersurface species (mainly formate and carbonate), and inpart to CO bound to defect sites, for example, O vacan-cies on the ZnO surfaces.

We therefore begin our investigation with scanning allpossible adsorption configurations of CO molecules atsingle O vacancies on the ZnO(0001) surface. For allcharge states of the O vacancy we performed full struc-ture optimizations starting from three different initialorientations of the CO molecule: perpendicular to thesurface with either the C or the O atom in the defect (C-down and O-down orientation) and parallel to the surfacewith the CO molecule flat inside the vacancy.

The strongest interaction of CO with O vacancies onZnO(0001) is found for the F++ charge state. The ad-sorption energy of about −0.30 eV is in the same range asfor CO molecules coordinated to Zn sites on defect-freesurfaces (note that in the case of adsorbed CO intermedi-ates the adsorption and formation energies are the sameaccording to our definitions). The CO molecule sits up-right above the center of the underlying Zn triangle withan C-down orientation [see Fig. 2(a)]. For the O-downorientation of the CO molecule the adsorption energyis reduced to only −0.05 eV. These results are in excel-lent agreement with two recent DFT/embedded clusterstudies.39,57 Herein, Fink reported adsorption energies of−0.30 eV and −0.01 eV for the C-down and O-down ad-sorption of CO in F++ centers. As discussed in detailin Ref. 57, since DFT overestimates the CO dipole mo-ment by about 30%, the binding of CO with the C enddown is usually slightly overestimated and the interactionstrength with the O end is underestimated. Accordingly,in an approximate coupled-cluster calculation a slightlylarger CO adsorption energy of −0.15 eV for the O-downorientation of CO in F++ centers was found.13

On oxide surfaces typically a blue shift of the CO vi-bration frequency ν(CO) is observed.58,59 This is alsothe case for ZnO. In IR60 and high resolution electronenergy loss spectroscopy (HREELS)60,61 a CO vibrationfrequency of about 2193 cm−1 has been reported (corre-sponding to a blue shift of 50 cm−1 compared to the COgas phase frequency of 2143 cm−1), which was attributedto CO bound to Zn sites on defect-free parts of the sur-faces. For CO bound to the charge deficient F++ center

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[see Fig. 2(a)], however, we obtain a slight red shift ofthe ν(CO) mode to 2099 cm−1 (since for the gas phasemolecule our calculated value of 2145 cm−1 for the COstretch is almost identical to the experimentally observedfrequency62 of 2143 cm−1, no scaling was applied).For the charge states F0 and F−− we find a very pe-

culiar behavior for the interaction with CO. When a COmolecule approaches the O vacancy, first a shallow van-der-Waals-like minimum at a distance of 3.8 A above theZn triangle in the vacancy is observed. The most stableorientation of the CO molecule is slightly tilted but al-most parallel to the surface with an adsorption energy of−0.05 eV [see Fig. 2(b)]. Such shallow van-der-Waals-likeminima are typical for the PBE functional, but of course,they don’t represent accurate and reliable descriptions ofthe van der Waals interaction. When the CO molecule isshifted closer to the surface a significant activation bar-rier has to be overcome before a new minimum in thepotential energy surface evolves. The energy of the mini-mum is higher than the energy for the CO molecule in thegas phase, giving rise to a positive adsorption energy ofabout +0.83 eV and +0.41 eV for the F0 and F−− center,respectively. The CO molecules has rotated to an uprightorientation and the C atom sits 2.12 A (F0) and 2.31 A(F−−) above the center of the Zn triangle in the vacancy.A significant charge transfer from the defect to the COmolecule has occurred, which is the source of the appear-ance of the activation barrier. Due to this charge transfer,the CO bond is weakened and elongated to 1.22 A at theF−− defect, which is apparent from the strong redshiftof the CO stretch frequency. The same behavior for theCO interaction with F0 and F−− was found by Fink57 inher DFT/embedded cluster study. She reported adsorp-tion energies of +0.93 eV and −0.23 eV for the two chargetransfer minima. The negative CO adsorption energy forthe F−− defect is most likely an artefact of the embeddedcluster approach where the excess charge of the F−− de-fect is confined to the small spacial region of those atomsthat are treated quantum mechanically. However, as wehave shown with periodic slab calculations, part of thischarge will be delocalized over the whole surface area.34

For all other orientations of the CO molecule and theother charge states F++/H2 and F

0/H2 the CO moleculeeither relaxed into a shallow van-der-Waals-like minimumwith a parallel orientation to the surface as describedabove, or it drifted towards a surface OH group and aweakly physisorbed state is formed as it is known fromdefect-free surfaces.50,55

B. Formate (HCOO−)

In experiments on methanol and formic acid decom-position as well as the water–gas shift reaction on ZnOpowders and single crystals formate intermediates couldbe identified by IR and XPS spectroscopy.19–27,63–67 Ac-cordingly, in reaction mechanism B (see Fig. 1) it wasproposed that surface-bound formate species are formed

upon interaction of CO with surface OH groups. This ispossible via two different reaction mechanisms, either adirect insertion of CO into a surface OH group41

CO+OH−surf −⇀↽− HCOO− (3)

or proton abstraction from an OH group and formateformation on a neighboring O 2 –

surf surface ion39

CO+O 2−surf +

+H(−Osurf

) −⇀↽− HCOO− . (4)

In our calculations we considered 30 different configu-rations of formate species on the ZnO(0001) surface (seeFig. S2 in the Supporting Material49). After structuralrelaxation we observed two different adsorption modes:(i) a bidentate mode with one O atom in a bridging po-sition between two Zn ions of the vacancy and the otherO atom from the surface next to the defect [see Fig. 3(a)],and (ii) a monodentate mode with the carbonyl oxygenatom oriented out of the vacancy where it may developan H bond to a neighboring OH group of the surface [seeFig. 3(b)].The bidentate mode is exclusively found at the charge

deficient F++ center, exhibiting formation energies ofabout −0.69 eV [note that the formate formation ener-gies reflect the reaction energies of the insertion reactionsof Eq. (3) and Eq. (4), which are competing processes toCO adsorption at Zn sites; thus, the thermodynamicallypreferred process can be identified by comparing formateformation and CO adsorption energies]. The bidentatebinding mode is favored, because the negatively chargedadsorbate partially compensates the charge deficit of theF++ center. The formate molecule is almost symmetri-cally bound to the surface via its two O ends and the twoC−O bonds have a similar length of about 1.28 A. Alter-natively to CO being inserted into a surface OH group,this configuration can be regarded as formate being ad-sorbed in an O di-vacancy, since the O atom from theinitial CO molecule and the surface O atom are almostindistinguishable [see Fig. 3(a)].At electron-rich vacancies formate species bind with

a monodentate mode. The reason is that the nega-tively charged formate is repelled by the localized elec-trons within the defect.34 In the monodentate configura-tion H bonds between the carbonyl oxygen and surfaceOH groups are now possible, which can lead to slightlylower formation energies of about 0.1 eV to 0.2 eV com-pared to the F++ center, i.e.−0.76 eV at F0 and−0.82 eVat F−−. The two C−O bonds are different in length, i.e.1.23 A for the carbonly bond and 1.34 A for the C−Obond to the surface oxygen, both are very similar tothe values of formic acid. Thus, the negative charge inthe format ion is not evenly distributed over both C−Obonds, but rather directed to the surface O atom. Assuch, these formate species at the electron-rich vacan-cies might be alternatively addressed as adsorbed formylspecies bound to a surface O ion.Altogether, we find that the thermodynamic stability

of adsorbed formate species is basically independent of

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(a) EHCOOf (F++) = −0.68 eV (b) EHCOO

f (F0) = −0.72 eV (c) EHCOOHf (F0)=+0.82 eV

Figure 3. Top view of (a)+(b) formate (HCOO – ) and (c) formic acid (HCOOH) species adsorbed at O vacancies in different chargestates on the polar ZnO(0001) surface. The most stable binding mode of formate is bidentate at F++ centers and monodentate with theformate O atom oriented toward a surface OH group at F0. For a summary of all optimized structures see Fig. S2+S3 in the SupportingMaterial.49

Table II. Vibrational frequencies (in cm−1) of selected formate species adsorbed on defective ZnO(0001). Δν(CO) is the differencebetween the two C-O stretch vibrational frequencies. The binding mode of formate is bidentate at F++ and monodentate for all othercharge states of the O vacancy.

defect δ(OCO) νs(OCO) νas(OCO) ν(C=O) π(CH) δ(CH) ν(CH) Δν(CO)F++ Fig. S2a 772 1344 1510 1004 1309 2981 166F++ Fig. S2c 776 1351 1534 1008 1288 2986 183F++/H2 Fig. S2e 753 1157 1675 1018 1355 2911 518F0 Fig. S2n 783 1202 1644 1019 1360 2903 442F0/H2 Fig. S2z 752 1176 1652 1032 1360 2913 476F−− Fig. S2D 762 1180 1641 1032 1362 2900 461exp. Ref. 27 no data 1367 1585 no data 1380 2872 218exp. Ref. 68 ∼750 1371 1605 1080 no data 2928 234

the charge state of the O vacancy. In addition, the for-mate species are significantly more stable by 0.4 eV to0.7 eV than pre-adsorbed CO molecules, independentlywhether the CO molecules are adsorbed at regular Znsurface sites or within the respective defect. Comparingthe thermodynamic stability of formate with respect totransformation to formyl and formaldehyde as well as thedesorption of formic acid, we find that in all cases for-mate is energetically far more stable for all charge statesof the vacancy, with one notable exception: at stronglyreduced vacancies, e.g. F0/H2 centers, the reduction offormate to formaldehyde is thermodynamically preferredby about 0.4 eV.

To gain further insights into the distinct binding statesof formate species on the defective ZnO(0001) surface wehave computed the vibrational frequencies for six repre-sentative formate configurations (see Table II). The cal-culated frequencies are significantly different for biden-tate binding at F++ centers and the monodentate ad-sorption mode at the other charge states of the vacancies.A very characteristic measure is the difference Δν(CO)

between the frequency of the asymmetric stretching bandνas(OCO) and the symmetric one νs(OCO), which is of-ten used in the analysis of experimental data to identifybidentate and monodentate binding of formate species.Unfortunately, the other vibrational modes are not veryspecific. In particular, they do not allow to distinguishbetween charge states of the defect other than F++ or thepresence of co-adsorbed hydride anions in the O vacancy.

Our calculated values for bidentate formate at F++

centers show a very good agreement with HREELS datafrom single crystals experiments obtained under UHVconditions68–70 and frequencies from DRIFTS measure-ments while studying methanol decomposition over ZnOpowders.71–75 These experiments have in common thatthey were carried out not in a reducing atmosphere butunder rather oxidizing conditions. From the agreementof the splitting Δν(CO) and the absolute values of thevibrational frequencies we can thus conclude that F++

centers were dominating in those experiments, which arenot the representative defects under the H-rich reducingenvironment of the industrial process of methanol synthe-

8

sis. At the electron-rich F centers the frequency splittingΔν(CO) is largely increased and the symmetric stretchmode νs(OCO) is red-shifted by about 140 cm−1 to190 cm−1. Both changes are inherent to the change of theadsorption mode from bidentate to monodentate. Suchstructure-induced shifts of νs(OCO) of adsorbed formatespecies are known from experiment, but with smallermagnitude than predicted from our calculations.68,76 Fora final clarification to what extend formate intermediatesare also present in methanol synthesis (and not methanoldecomposition), in-situ experiments under the reducingconditions of the industrial process are needed.

C. Formic acid (HCOOH)

Adsorbed formic acid molecules can be generated frommonodentate formate configurations via transfer of a pro-ton from a surface OH group

HCOO− + +H(−Osurf

) −⇀↽− HCOOH . (5)

This reaction is clearly part of the methanol decompo-sition process in which formate via formic acid eventu-ally decomposes into CO2, CO, H2 and H2O.

77 Similarly,our recent exhaustive study employing advanced AIMDtechniques showed that Eq. (5) is only a side reaction inmethanol synthesis from CO and H2 when carried outunder highly reducing conditions, but becomes a preva-lent reaction path if the reaction conditions are changedto oxidizing.43

Accordingly, in our calculations we found only formicacid species with formation energies of at least 1.5 eVhigher compared to formate at the reduced ZnO surface(see Fig. S3 in the Supporting Material49). These ther-modynamically highly unfavorable formic acid species ex-clusively bind in a monodentate mode to the F0 centers[see Fig. 3(c)]. From the high reaction energy for the pro-cess of Eq. (5) we can conclude that formic acid will beirrelevant as an intermediate in the methanol synthesisreaction. Instead, formic acid will dissociatively adsorbon defective ZnO by forming a formate group bound tothe defect site and donating a proton to a surface oxygen.Of course, formic acid may also decompose directly uponcontact with the surface to give CO2, CO, H2 and H2O,but these processes were not studied in the present inves-tigation. However, since also CO2 and H2O are formed,formic acid decomposition might be a relevant processfor creating new oxygen vacancies on the surface.

D. Formyl (HCO−)

In-situ IR spectroscopy in high pressure CO and H2

co-adsorption experiments on ZnO powders at room tem-perature and below suggested the presence and forma-tion of formyl species on the ZnO surfaces.78–80 In addi-tion, at temperatures close to that of the catalytic pro-cess chemical trapping allowed for an indirect detection

Table III. Vibrational frequencies (in cm−1) of selected formylspecies adsorbed on defective ZnO(0001). In the first two configu-rations for each charge state formyl is bended and in the third itsits upright in the O vacancy.

defect ν(CO) π(CH) δ(CH) ν(CH)F++ Fig. S4b 1603 752 1374 2720F++ Fig. S4d 1571 730 1321 2760F++ Fig. S4f 1369 490 1262 2651F0 Fig. S4h 1511 734 1369 2733F0 Fig. S4j 1570 733 1366 2692F0 Fig. S4l 1391 428 1290 2619exp. Ref. 78 1520 no data 1370 2661/2770

of this species,81 and also XPS experiments have indi-cated formyl on ZnO(0001).37 Based on these results thebinding state of formyl species could be narrowed downto either monodentate or bidentate with only the C atomor both C and O atom coordinating to a metal center,respectively.In mechanism A and C of Fig. 1 it was proposed that

formyl species are formed from activated CO adsorbedat low-coordinated Zn sites or Zn3 clusters. In a previ-ous theoretical study formation of a linear upright formylfrom CO bound with the O end to the three Zn in anO vacancy was suggested.13 However, since in all thesecases the initial CO is only weakly bound to the surface,it is more likely that CO stemming from the gas phaseis directly reduced to give formyl. This can occur via re-action with hydride anions bound to O vacancies at thesurface

CO + H−vac −⇀↽− HCO− , (6)

or, alternatively, CO is reduced by the two electrons ofan O defect and a proton from a surface OH group istransferred34,41

CO+ 2 e−vac ++H

(−Osurf

) −⇀↽− HCO− . (7)

In the present study we investigated the possible bind-ing modes of formyl at O vacancies of F++ and F0

charge state after reaction Eq. (6) or Eq. (7) has oc-curred. Depending on the initial configuration of formylat these O defects we find two major adsorption modesafter structural relaxation (see Fig. S4 in the Support-ing Material49): (i) a bend mode in which formyl ar-ranges parallel within the layer of surface atoms andbinds with the C and O atom to Zn ions in the vacancy[see Fig. 4(a)], and (ii) an upright mode with only thecarbonyl oxygen being bound to the underlying Zn [seeFig. 4(b)].Irrespective of the charge state of the O vacancy we

find that the bend structure of formyl is by about 1 eVmore stable than the upright configuration. The mainreason for this energetic stabilization of adsorption mode(i) is that the valencies of both C and O atom are satu-rated by their coordination to the Zn ions, while in theupright mode this is only the case for the O atom. A

9

(a) EHCOf (F0) = −0.22 eV (b) EHCO

f (F0) = +0.74 eV (c) EHCOHf (F0) = +0.19 eV (d) EHCOH

f (F0) = −0.04 eV

Figure 4. Top view of (a)+(b) formyl (HCO – ) and (c)+(d) hydroxymethylene (HCOH) species adsorbed at O vacancies on ZnO(0001).Bended formyl configurations in which both C and O atom are coordinated to Zn ions within the vacancy are significantly more stablethan linear formyl species sitting upright in the vacancy. In contrast, for hydroxymethylene the CH group prefers to insert into a surfaceZn-O bond and the OH group sticks out from the surface and receives an H bond from a surface hydroxyl. For a summary of all optimizedstructures see Fig. S4–S6 in the Supporting Material.49

similar binding mode with comparable stability is knownfor formyl species on the Pt(111) surface.83

Taking gas phase CO and H2 as energy reference, thelowest formation energy of formyl is realized in the bendmode at the F++ center with a value of −1.24 eV [seeFig. S4(b) in the Supporting Material49], while at themore reduced F0 defect the formation energy is reducedto −0.22 eV. The reason for this strong destabilization offormyl at F0 is the electrostatic interaction between thenegatively charged adsorbate (CHO – ) and the localizedelectronic charge in the F0 center.34 Similar as formate,also formyl is energetically more stable than pre-adsorbedCO molecules. Compared to a stepwise procedure, inwhich CO is first adsorbed and then reduced and pro-tonated in a second step, the direct mechanisms to giveadsorbed formate and formyl species by starting fromgas phase CO molecules are thermodynamically more fa-vorable. Formyl species, however, provide the main ad-vantage that the first reduction step has been alreadyrealized, which is not the case for the formate species.In all formyl configurations the C−O bond distance isslightly elongated with respect to typical C−−O doublebonds (∼1.23 A), with 1.26 A in the bend and 1.31 A inthe upright binding mode.

In Table III we summarize the calculated vibrationalfrequencies for a characteristic set of formyl configura-tions in both adsorption modes and for both charge statesof the O vacancy. While the vibrational frequencies donot allow to discriminate between the reduction stateof the vacancy, upright and bend adsorption mode areclearly distinguishable. The most pronounced differencecan be seen in the ν(CO) stretch, but also the π(CH)and δ(CH) modes show a characteristic red shift for up-right molecules compared to the corresponding values

for the bend configuration. Furthermore, the values ofthe ν(CO) stretch frequency for the upright and bendconfigurations lie in-between the typical values for C−−Odouble and C−O single bonds and thus indicate thatformyl species adsorbed at these O defects are alreadyactivated beyond the first reduction step. Overall, thecalculated frequencies of the bend configurations at theF0 center show the best agreement with the experimen-tally recorded values.78–81 Nevertheless, our bend config-uration is a bidentate adsorption, i.e. both C and O atomcoordinate to different Zn ions of the defect, and not ofdihapto-character as suggested earlier.81

E. Hydroxymethylene (HCOH)

Molecular hydroxymethylene is a metastable species.In reaction mechanism A of Fig. 1 these species were pro-posed as intermediates which are formed by protonationof the O atom of C-bound formyl

HCO− + +H(−Osurf

) −⇀↽− HCOH . (8)

Furthermore, based on experimental indications andDFT calculations for the Zn-terminated ZnO(0001) sur-face, a trans-hydroxymethylene was proposed as possibleintermediate to give formaldehyde species.37 Such trans-hydroxymethylene species have been successfully isolatedin an argon matrix only very recently.82 No further directexperimental evidence exists up to now of hydroxymethy-lene adsorbed on ZnO surfaces.Starting from protonated formyl at its O end within

its most stable bend configuration we obtained two differ-ent final configurations for hydroxymethylene after struc-tural relaxation: (i) a Zn-bound species with the C atom

10

and the OH group coordinated to the Zn ions of the de-fect [see Fig. 4(c)], or alternatively with the OH groupsticking out of surface and forming an H bond to a sur-face O ion (see Fig. S5 in the Supporting Material49), and(ii) an adsorption mode where hydroxymethylene is in-serted into a Zn−O bond and the O atom receives anH bond from a surface OH group [see Fig. 4(d)].The calculated formation energies show that the Zn-

bound adsorption mode (i) is thermodynamically not sta-ble with respect to gas phase CO and H2. At the oxi-dized surface with F++ centers the formation energies areabout +0.35 eV. The values are slightly lower at the re-duced F0 center with about +0.09 eV. In contrast, hydr-oxymethylene adsorbed via insertion mode (ii) is stablefor all charge states of the O vacancy. The formationenergies are −0.64 eV, −0.25 eV, and −0.24 eV for theoxidized F++, the reduced F++/H2, and the strongly re-duced F0/H2 defect, respectively (see Fig. S6 in the Sup-porting Material49). The transformation from adsorptionmode (i) to (ii) seems to be barrierless in some cases, sincewe have obtained mode (ii) during geometry optimizationwhen starting from configurations of mode (i). However,altogether the transformation of formyl to hydroxyme-thylene (even when adsorbed in its most stable form byinsertion into a Zn−O bond) is thermodynamically unfa-vorable by about 0.7 eV at F++ and 0.2 eV at F0 centers.From the structural point of view hydroxymethylene

species exhibit C−O bond lengths which correspond toprototypical single bonds and thus are elongated com-pared to the formyl species. This single-bond characterand the fully four-fold coordination of the C atom suggesta carbon oxidation state corresponding to that present inmethanol. However, in both adsorption modes of hydr-oxymethylene further protonation of the C center towardmethanol is sterically hindered. Alternatively, the hydr-oxymethylene in the inserted adsorption mode can beinterpreted as formic acid species adsorbed in an O di-vacancy and therefore might represent an intermediatebelonging to a side reaction producing formic acid.

F. Formaldehyde (H2CO)

Two reduction steps are required to form methanolfrom CO and H2. Thus, formaldehyde represents thecentral reaction intermediate in methanol synthesis afterthe first reduction. In mechanism A of Fig. 1 it has beenproposed that formaldehyde may form from Zn-boundformyl species by receiving a proton from an adjacentOH group of the surface

HCO− + +H(−Osurf

) −⇀↽− H2CO . (9)

Alternatively, it has been suggested that formaldehydespecies are obtained from hydroxymethylene via an in-termolecular hydrogen transfer.37,82

In our calculations we performed structural relaxationsfor many different configurations of formaldehyde on the

defective ZnO(0001) surface (see Fig. S7 in the Support-ing Material49). From the results two general adsorp-tion modes can be distinguished, both are closely relatedto those of formyl species: (i) an upright mode wherethe O atom binds to all three Zn ions of the defect [seeFig. 5(a)], and (ii) a bend mode with the C atom bindingto one and the O atom to the remaining two Zn ions ofthe defect [see Fig. 5(b)].The upright configuration represents the most stable

adsorption mode under oxidizing conditions when F++

centers are present. The formation energy is about−1.15 eV [see Fig. 7(c) in the Supporting Material49], andthe C−−O bond distance of about 1.23 A is close to thevalue for formaldehyde molecules in the gas phase. At theelectron-deficient F++ defects formaldehyde can be onlystabilized by electrostatic interactions of the non-bondingp-orbitals of the carbonyl O atom with the Zn ions of thevacancy.When going to reduced surfaces and F0 centers, the up-

right adsorption mode becomes metastable. Instead, wefind that formaldehyde in a bend configuration is morestable by about 0.8 eV than upright adsorption. However,the absolute formation energy of −1.02 eV in the bendconfiguration at F0 is slightly less favorable comparedto upright CH2O at F++ centers. The two defect elec-trons of the F0 defect are well-localized within the oxy-gen vacancy.34 However, if molecules are adsorbed withinsuch electron-rich defects, it becomes arbitrary whetherto assign the excess electrons still to the vacancy or tothe adsorbate, which would then have to be described asbeing in a higher reduction state. In the present case,the assignment of the defect electrons to formaldehydewould lead to H2CO

2 – di-anions

H2CO+ 2 e−vac −⇀↽− H2CO2− (10)

which are in the same oxidation state as methanol.41 Thispoint of view can be justified by looking at the bindingconfiguration of formaldehyde at F0 centers. In the bendadsorption configuration an optimum of electronic sta-bilization is realized by a four-fold coordination of theC atom in an almost tetrahedral environment, and theC−O distance of about 1.46 A corresponds to the lengthof a single bond. In contrast, within the thermodynami-cally metastable upright mode at the same F0 center theC−O bond is shorter with 1.35 A, which is in-betweentypical double and single bond lengths. The destabi-lization of the upright configuration can be explained bythe additional electronic charge stemming from the twoelectrons of the O vacancy, which in a planar geome-try of formaldehyde can only occupy the anti-bondingπ∗C−O molecular orbital and thus enforce the C−O bondto elongate.Formaldehyde species are thermodynamically most

stable at highly reduced F−− centers. We obtained ex-clusively the bend adsorption mode and the formationenergy is as low as −1.40 eV [see Fig. 7(l) in the Sup-porting Material49]. Although formaldehyde is more sta-ble by 0.4 eV at F−− compared to F0centers, the atomic

11

(a) EH2COf (F++) = −1.07 eV (b) EH2CO

f (F0) = −1.02 eV (c) EH2COOf (F++) = −1.90 eV (d) EH2COO

f (F0) = −1.03 eV

Figure 5. Top view of (a)+(b) formaldehyde (H2CO) and (c)+(d) dioxomethylene (H2COO 2 – ) species adsorbed at O vacancies in

different charge states on the polar ZnO(0001) surface. At F++ centers formaldehyde prefers to sit upright in the vacancy, whereas forhigher charge states of the vacancy the molecule is stabilized by bending into the defect and formation of a C-Zn bond. For dioxomethylenethe most stable binding mode is bidentate at F++ and monodentate with an O atom oriented toward a surface OH group at F0. For asummary of all optimized structures see Fig. S7+S8 in the Supporting Material.49

Table IV. Vibrational frequencies (in cm−1) of selected formaldehyde species adsorbed on defective ZnO(0001). Formaldehyde sits uprightin the O vacancy at F++, and it is bended for all other charge states.

defect ν(CO) ω(HCH) ρ(HCH) δ(HCH) νs(HCH) νas(HCH)F++ Fig. S7a 1718 1192 1253 1490 2861 2974F++ Fig. S7c 1698 1184 1255 1488 2871 2986F0 Fig. S7d 948 1203 1222 1453 2878 2929F0 Fig. S7h 956 1196 1221 1451 2878 2932F−− Fig. S7l 971 1210 1214 1451 2839 2881F−− Fig. S7n 968 1210 1227 1447 2836 2907exp. Ref. 89a 1710 1250 1490 2850exp. Ref. 90b 1426, 1393 no data 1484 2916 2984

a formaldehyde on AgO(110)b polymer (H2CO)x on ZnO/SiO2

structure is very similar at both defects. Formaldehydein its bend configuration does not only fit structurallyand electronically very well into the surface defect andtherefore stabilizes the charge density which is localizedin reduced F centers, but it can further host parts of theexcess charge of the highly reduced F−− centers whichis delocalized in a surface band of ZnO(0001).34 Thus,bend formaldehyde is able to re-establish and compen-sate most of the perfect structural and electronic satu-ration of the ideal hydroxylated ZnO(0001) surface. Theadditional electronic stabilization of formaldehyde at theF−− center is furthermore reflected when comparing theadsorption instead of the formation energies. The ad-sorption energy of formaldehyde at F−− of −0.84 eV is30% lower than the corresponding values of −0.46 eV atthe less-reduced F0 defect and −0.58 eV at F++.

While the difference between the adsorption energies istoo small to allow to discriminate between the differentadsorption modes of formaldehyde and the charge state

of the O vacancy in experiment, the change between up-right and bend configuration should be reflected in thevibrational frequencies. In Table IV we have summarizedthe calculated vibrational frequencies for a set of the moststable formaldehyde species at different F centers. Themost pronounced difference between upright and bendconfiguration can be seen in the C−O stretch frequencyν(CO). In the upright adsorption mode this frequency isvery similar to the value of gas phase formaldehyde (seeTable S-II in the Supporting Material49), with a slightred shift of about 30 cm−1 to 50 cm−1. It is blue-shiftedcompared to the experimental result for paraformalde-hyde on ZnO/SiO2,

90 but very close to measured valuesfor formaldehyde on AgO(110).89 In the bend configu-ration, the ν(CO) frequency is significantly lower, indi-cating that the C−−O double-bond has become a singlebond. Indeed, the calculated values are very characteris-tic for C−O single bonds and are even slightly red-shiftedcompared to the C−O stretch frequency of methanol in

12

the gas phase. Besides the ν(CO) frequency, the valuesof the other vibrational modes are not very sensitive tothe configuration of formaldehyde or charge state of thedefect (see Table IV).

G. Dioxomethylene (H2COO2 – )

Dioxomethylene species adsorbed on ZnO surfaces maybe regarded as formaldehyde which is bound to a surfaceO atom. When bound in the vicinity of an O vacancy onthe polar ZnO(0001) surface, in principle the followingconfiguration change between dioxomethylene and for-maldehyde is possible41

H2CO+O 2−surf

−⇀↽− H2COO2− . (11)

Alternatively, dioxomethylene can be obtained by reduc-tion of formate, either by a hydride anion or by two defectelectrons and a proton transfer

HCOO− + 2 e−vac ++H

(−Osurf

) −⇀↽− H2COO2− . (12)

For the ternary Cu/ZnO/Al2O3 low-pressure catalyst ithas been speculated that the hydrogenation of formateto dioxomethylene is the rate determining step.84–87 Inmethanol synthesis over ZnO, however, dioxomethyleneis not much discussed. It is only vaguely mentioned inmechanism B of Fig. 1. The reason is, that here CO isthe main carbon source and not CO2 as for the ternarycatalyst. The lack of experimental evidence of a dioxome-thylene species on ZnO surfaces may be due to its shortlifetime at the high pressure and temperature conditionsof the catalytic process and the low intensity of the char-acteristic H2COO

2 – bands in the vibrational spectra.88

We have investigated more than 30 configurations ofdioxomethylene adsorbed in or at the edge of O vacanciesin different charge states (see Fig. S8 in the SupportingMaterial49). Two different adsorption modes are found,both of them are very similar to the configurations whichwe have obtained for formate species (i) a bidentate modewith one O atom in a bridging position between a twoZn ions of the vacancy and the other O atom from thesurface next to the defect [see Fig. 5(c)], and (ii) a mon-odentate mode involving a surface O atom and the otherO develops an H bond to a neighboring surface OH group[see Fig. 5(d)].At F++ centers we find that dioxomethylene adsorbs

exclusively within the bidentate mode with formationenergies as low as −1.90 eV. This is about 0.8 eV lowerin energy than formaldehyde bound to the same defectand suggests that under oxidizing conditions dioxome-thylene rather than formaldehyde is present at the sur-face. In contrast, at the reduced F0 and the stronglyreduced F0/H2 centers the dioxomethylene formation en-ergies are higher with values of −1.03 eV and −1.13 eV,respectively. In addition, monodentate adsorption is nowmore favorable than the bidentate configuration. At F0

dioxomethylene and formaldehyde have almost the same

formation energy, whereas at strongly reduced defects di-oxomethylene has become less stable than formaldehydeby about 0.3 eV.For the dependence of the formation energy on the va-

cancy charge state we have seen opposite trends for for-maldehyde and dioxomethylene. Although both speciesare in the same reduction state, their slightly differentatomic structure provides alternative ways of electronicstabilization depending on the charge of the vacancy.Within the bidentate configuration of dioxomethylene atF++ centers the carbon is fully four-fold saturated withC−O bond distances of about 1.43 A to 1.45 A. The twoelectrons of the di-anion are localized at the O atomsand donate a maximum of stabilizing charge density tothe O defect, which is not possible in the upright for-maldehyde adsorption mode with the electrons occupyingthe anti-bonding π∗C−O molecular orbitals. In contrast,the excess electrons of the reduced and strongly reducedoxygen defects do not provide further stabilization for di-oxomethylene, but for the bend formaldehyde species asdescribed in the previous section. Only by changing froma bidentate to a monodentate binding mode, H bond for-mation is now possible for dioxomethylene.In summary, the formation energies of dioxomethylene

at the reduced and strongly reduced F centers are ratherclose to each other and similar to the corresponding val-ues of formaldehyde, which eventually does not allow foran easy and straight forward discrimination of the twospecies and an identification of the reduction state of theZnO surface in experiment. Instead, vibrational frequen-cies might provide some insights. Calculated frequenciesfor a representative set of vacancy charge states and di-oxomethylene adsorption modes are summarized in Ta-bleV. Similar as for formaldehyde, the obtained frequen-cies allow only for a discrimination between monodentateand bidentate binding mode of dioxomethylene species.In particular, the asymmetric stretching mode νas(OCO)is red-shifted by about 100 cm−1 when dioxomethylenebinds bidentately. The absolute values of νas(OCO) areclose to the values characteristic for C−O single bondsand thus can be distinguished from the correspondingν(CO) frequencies which are at least 100 cm−1 higher(see Table IV). Otherwise, the vibrational modes are notvery sensitive and similar for the F++/H2, F

0 and F0/H2

charge states. Since no experimental data for ZnO exists,we can only refer to vibrational frequencies of dioxome-thylene species adsorbed on AgO(110), which show qual-itative agreement to our calculated values (see TableV),but does not hold for a quantitative analysis.

H. Hydroxymethyl (H2COH– )

The consideration of hydroxymethyl species as reactionintermediate in mechanism A of Fig. 1 was substantiallymotivated by the theoretical work on methanol decompo-sition over the non-polar ZnO surface.3,91 Recently, othertheoretical work on CO hydrogenation over Pt(111) pro-

13

Table V. Vibrational frequencies (in cm−1) of selected dioxomethylene species adsorbed on defective ZnO(0001). The binding mode ofdioxomethylene is bidentate at F++ and in the first configuration of F0 and monodentate in all other cases.

defect δ(OCO) νs(OCO) νas(OCO) ω(HCH) τ(HCH) ρ(HCH) δ(HCH) νs(HCH) νas(HCH)F++ Fig. S8f 640 879 1016 1122 1209 1360 1489 2879 2968F++ Fig. S8m 615 863 1023 1125 1236 1372 1481 2912 2973F++/H2 Fig. S8r 666 797/868 1120 1126 1204 1362 1503 2822 2876F0 Fig. S8x 626 896 1030 1116 1188 1341 1517 2881 2914F0 Fig. S8B 641 794 1133 1111 1185 1353 1511 2807 2831F0/H2 Fig. S8E 688 865 1106 1122 1184 1346 1516 2788 2814exp. Ref. 89a 620 960 1100 obscured obscured obscured 1430 2960

a η2-methylenedioxy on AgO(110)

(a) EH2COHf (F0) = −0.96 eV (b) EH2COH

f (F0) = −1.35 eV (c) EH2COOHf (F0) = −1.23 eV (d) EH3CO

f (F0) = −3.04 eV

Figure 6. Top view of (a)+(b) hydroxymethyl (H2COH – ), (c) hydroxymethoxide (H2COOH – ) and (d) methoxide (H3CO – ) species

adsorbed at O vacancies on ZnO(0001). At F0 centers the preferred binding mode of hydroxymethyl and hydroxymethoxide is monodentate,with the OH group forming an H bond to a surface O anion. Methoxide anions are strongly bound in O vacancies, irrespective of thecharge state. For a summary of all optimized structures see Fig. S9–S11 in the Supporting Material.49

posed hydroxymethyl as active species toward methanolproduction,83,92 which might compete with an alterna-tive reaction pathway via methoxide species.93,94 How-ever, no experimental evidence exists whether hydroxy-methyl species may be actively involved in the methanolproduction over ZnO or is even present on these surfaces.

Examining the structure and relative stability ofvarious hydroxymethyl configurations on the defec-tive ZnO(0001) surface (see Fig. S9 in the SupportingMaterial49), we find two different adsorption modes: (i) atridentate Zn-bound hydroxymethyl [see Fig. 6(a)], whichis similar to the tridentate formaldehyde structure ofFig. 5(b), and (ii) a monodentate configuration with theC atom bound to one Zn ion of the defect, but comparedto (i) the OH group is rotated out of the vacancy andforms and receives an H bond with a surface O atom andOH group, respectively [see Fig. 6(b)].

Hydroxymethyl species bound via adsorption mode (i)can be obtained directly from bend and tridentate for-maldehyde species initially adsorbed at reduced F0 andF−− centers [see Fig. 5(b)] via transfer of a proton from

a surface OH group [see Fig. 6(a)]

H2CO2− + +H

(−Osurf

) −⇀↽− H2COH− , (13)

which, together with the electron transfer of Eq. (10),completes the second reduction step. At F++ and F0 de-fects both adsorption modes of hydroxymethyl are sta-ble with rather similar formation energies. For the moststable configuration we obtain −2.34 eV and −1.35 eVat F++ and F0, respectively. However, if we comparehydroxymethyl to formaldehyde by considering the reac-tion energy for the hydrogen transfer plus the reductionprocesses according to Eq. (13), we see that this processis endothermic by approximately +0.5 eV [note that thereaction energies are given by the difference between theformation energies of hydroxymethyl at F++ (F0) andformaldehyde at F0 (F−−), adjusted by the difference ofthe F++ and F0 (F0 and F−−) vacancy formation en-ergy]. The reason for the reduced stability of hydroxy-methyl compared to formaldehyde is the chemical satu-ration of the oxygen end by hydrogen, leaving only onenon-saturated, directional bond of the sp3 carbon atomto interact with the Zn ions of the defect.

14

The overall binding of hydroxymethyl is strongest atthe F++ center, because the negatively charged adsorbatemolecule compensates for the electron deficit introducedby the O vacancy and mostly restores the electronic sat-uration of the half-hydroxylated ZnO(0001) surface.34 Incontrast, at the reduced F0 center additional electrons areavailable to stabilize the polar surface, which results in aless strong binding of the hydroxymethyl anion comparedto the F++ center. At F0 there is an interesting relationbetween the hydroxymethyl binding mode and the local-ization/delocalization of the two defect electrons, whicheventually results in very similar formation energies forthe tridentate and monodentate configuration. In thetridentate adsorption mode (i) the hydroxymethyl an-ion binds deep into the vacancy and saturates all threeZn ions inside the defect, while the defect electrons oc-cupy the conduction band of the ZnO surface. The elec-tronic structure of this configuration is very similar tothe ideal half-hydroxylated ZnO(0001) surface after hav-ing adsorbed a H2 molecule (see Fig. 2 of Ref. 34). Al-ternatively, in the monodentate adsorption mode (ii) thedefect electrons localize within the defect and stabilizetwo of the non-saturated Zn ions in the vacancy whilethe hydroxymethyl binds to the third Zn. The localizeddefect electrons and the hydroxymethyl anion repel eachother, which leads to the monodentate adsorption mode.The charge density distribution in this configuration re-sembles very much a F++/H2 defect, as it is shown inFig. 8 of Ref. 34.

I. Hydroxymethoxide (H2COOH– )

In mechanism B of Fig. 1) hydroxymethoxide was pro-posed to play an active role in the catalytic cycle ofmethanol formation. However, similar as for hydroxy-methyl, no experimental evidence exists from methanolsynthesis studies for the presence of hydroxymethoxideon the ZnO catalyst surfaces.Hydroxymethoxide [see Fig. 6(c)] may be obtained

from monodentate dioxomethylene [see Fig. 5(d)] viatransfer of one proton from a surface OH group

H2COO2− + +H

(−Osurf

) −⇀↽− H2COOH− . (14)

This is the same chemical process by which Zn-boundand reduced formaldehyde di-anions are transformed intohydroxymethyl (see Eq. (13)), albeit now realized for O-bound species. The main difference between the twoprotonation reactions is, however, that formaldehyde di-anions and hydroxymethyl are already in the same re-duction state as methanol, whereas the transformationof hydroxymethoxide to methanol requires a final reduc-tion step.In our calculations we obtained very similar formation

energies for hydroxymethoxide bound to different O de-fects, with values of −1.31 eV at F++/H2, −1.23 eV at F0,and −1.26 eV at F0/H2 for the most stable configuration(see Fig. S10 in the Supporting Material49). Evaluation

of the reaction energy for Eq. (14) shows that hydrox-ymethoxides bound at F++/H2, F

0 or F0/H2 are slightlymore stable by about 0.2 eV than the corresponding di-oxomethylene species. This additional stabilization orig-inates from the H bonds between the hydroxymethoxideOH groups and O ions of the ZnO(0001) surface.Based on the atomic configuration of hydroxymetho-

xide on the ZnO surface [see Fig. 6(c)] we can speculatethat the next hydrogenation step toward methanol willbe difficult. A hydride anion would need to attack thealready tetrahedrally four-fold coordinated C atom andsimultaneously one of the two C−O bonds has to bebroken. It is therefore more likely that hydroxymetho-xide reacts back to dioxomethylene (left side of Eq. (14)).However, since hydroxymethoxide is slightly more stablethan dioxomethylene, this side reaction may constitutea severe trap that prevents methanol formation from di-oxomethylene.

J. Methoxide (H3CO– )

In basically all proposed reaction mechanisms formethanol formation it is assumed that the final reac-tion step proceeds via the methoxide species (see Fig. 1).Long-living methoxide species on ZnO surfaces are well-known from methanol decomposition studies and havebeen characterized by IR, TPRS and other experimen-tal techniques.19–27 Applying the principle of microscopicreversibility it was therefore suggested that methoxideis the final species in the catalytic cycle of methanolsynthesis.3 However, direct in-situ evidence for the for-mation and presence of methoxide on ZnO under the re-action conditions of the catalytic process is still missing.Methoxide may be either formed by a reduction of for-

maldehyde

H2CO+ 2 e−vac ++H

(−Osurf

) −⇀↽− H3CO− , (15)

or by a reduction of dioxomethylene

H2COO2− +H−vac −⇀↽− H3CO

− +O 2−surf . (16)

Of course, also the reduction of formaldehyde by an hy-dride anion and of dioxomethylene by two defect elec-trons together with a proton transfer are possible.Methoxide can adsorb on defective ZnO(0001) only in

one configuration. The molecule binds upright into theO vacancy with the oxygen end coordinating to the threeZn ions of the defect [see Fig. 6(d)]. This key–lock-typestructure is very stable, and independent of the chargestate of the vacancy methoxide is strongly bound (seeFig. S11 in the Supporting Material49). The calculatedformation energies of −4.06 eV at F++ and −3.04 eV atF0 indicate deep local minima on the potential energysurface. The methoxide O atom is four-fold coordinated,electronically saturated and sterically shielded againstprotonic attack. In both oxidation states of the vacancythe oxygen of methoxide sits slightly above the position of

15

Table VI. Vibrational frequencies (in cm−1) of selected methoxide species adsorbed on defective ZnO(0001). For all charge statesmethoxide sits upright in the O vacancy.

defect ν(CO) ρ(CH3) δs(CH3) δas(CH3) νs(CH3) νas(CH3)F++ Fig. S11a 1008 1157 / 1166 1442 1475 / 1484 2901 2970 / 3003F++ Fig. S11b 1004 1164 / 1165 1442 1469 / 1489 2912 2986 / 3003F0 Fig. S11c 1004 1146 / 1154 1430 1473 / 1485 2900 2968 / 2992F0 Fig. S11d 1001 1151 / 1159 1431 1476 / 1489 2901 2975 / 2996exp. Ref. 27 ∼1067 no data 1468 2817 2932exp. Ref. 20 no data no data 1470 2830 2930

the original surface O ion, i.e. 0.10 A at F++ and 0.05 A atF0. Methoxide at F++ centers restores to a large extendthe ideal electronic saturation of the half-hydroxylatedZnO(0001) surface. The reason for the destabilizationof methoxide at F0 by about 1.0 eV is that the defectelectrons of F0, which were previously localized insidethe O vacancy, need to be shifted into the ZnO conduc-tion band upon methoxide adsorption. Furthermore wefind that the number of OH groups in direct vicinity ofthe methoxide species has a moderate influence of up to0.3 eV on the formation energy. The stability of methox-ide decreases with higher number of adjacent OH groups.

The high stability of methoxide at the defectiveZnO(0001) surfaces also becomes apparent when we lookat the dissociative adsorption energies of methanol whenmethanol decomposes at oxygen vacancies, which corre-sponds to the back reaction of Eq. (17). The methanoladsorption energies are −2.20 eV at F++ and −1.09 eV atF0, indicating a high energy barrier for methanol recom-bination and desorption. In particular at the oxidizedsurface with F++ defects methoxide will be a deep trapand will have a long lifetime. This is why methoxide isso easy to detect in methanol decomposition experiments(representing oxidizing conditions). However, if methox-ide at F++ were present in the catalytic cycle of methanolsynthesis, the O vacancies would be blocked most of thetime by methoxide species and the number of possible ac-tive sites would be strongly reduced. On the other hand,at F0 the activation energy for methanol desorption isreduced to 1.09 eV, a value that can be overcome at thetemperature and pressure conditions of methanol synthe-sis. So obviously, for methanol synthesis to work, ZnOhas to be in a reduced state. This result indicates thatthere is a fundamental difference whether experimentsare done for methanol decomposition (oxidizing condi-tions, mostly F++ defects present) or methanol synthe-sis (severe reducing conditions, defects in higher chargestates dominating).

The calculated vibrational frequencies for four methox-ide configurations with either F++ or F0 defects are sum-marized in Table VI. The vibrational modes of the fourconfigurations are very similar and do not depend signifi-cantly on the charge state of the O vacancy or the distinctarrangement of OH groups next to the methoxide anion.All calculated frequencies agree nicely with the availableexperimental data from methanol decomposition experi-

(a) EH3COHf (F++) = −3.82 eV (b) EH3COH

f (F0) = −2.40 eV

Figure 7. Top view of methanol (H3COH) molecules adsorbed

at O vacancies in different charge states on the polar ZnO(0001)surface. H bonds are indicated by blue dashed lines.

ments.

K. Methanol (H3COH)

The final step for methanol formation from methoxideis a proton transfer from a surface OH group

H3CO− + +H

(−Osurf

) −⇀↽− H3COH . (17)

Alternatively, methanol can be obtained by protonationof hydroxymethyl as suggested in mechanism A of Fig. 1

H2COH− + +H

(−Osurf

) −⇀↽− H3COH , (18)

or by reduction of hydroxymethoxide as already discussedin Sec. III I.In the final set of calculations we were able to locate

two stable configurations of molecular methanol adsorbedat O vacancies on ZnO(0001): a key–lock-type arrange-ment with the methanol O atom coordinated to a Zn ioninside the vacancy at F++ defects [see Fig. 7(a)], and anH-bridge bound configuration above the F0 vacancy [seeFig. 7(b)].

16

The structure of molecular adsorbed methanol at F++

is quite similar to the configuration of methoxide species(see previous section). The O atom of methanol is located0.62 A above the O position of the vacancy, which is only0.5 A higher than the O atom of methoxide. In addition,the formation energy of −3.82 eV and the adsorption en-ergy of −1.97 eV of methanol at the F++ defect are onlyslightly higher than the corresponding values for methox-ide. Therefore we assume that the molecular methanoladsorption is only a shallow local energy minimum andthat only a small barrier has to be overcome to dissociatemethanol at F++ to form methoxide on the surface (seediscussion in the previous section).In contrast, the additional charge of the reduced F0

center repels the methanol adsorbate from a methoxide-like adsorption. In the relaxed configuration of Fig. 7(b)the O atom of methanol sits 2.25 A above the vacantO position of the defect and the C atom is located 3.79 Aabove the Zn ions of the vacancy. The correspondingformation energy of −2.40 eV and the adsorption energyof −0.55 eV suggest a rather weak binding via hydrogenbonds. Indeed, a value of −0.55 eV is quite typical for thestrength of two H bonds. Such weakly-bound methanolwill easily desorb from the surface under the elevatedtemperatures of the methanol synthesis process.

IV. SUMMARY

We have reported on the structures and energetics ofa set of ten molecular species, from reactants to inter-mediates to products, which are relevant in the hetero-geneous catalytic process of methanol synthesis from COand H2 over the defective O–terminated ZnO(0001) sur-face. In particular, we demonstrate that the thermody-namic stability and the properties of distinct adsorptionmodes of CO, formate, formic acid, formyl, hydroxyme-thylene, formaldehyde, dioxomethylene, hydroxymethyl,hydroxymethoxide, methoxide and methanol species atoxygen vacancies are intimately coupled to the thermody-namic properties of the gas phase that is in contact withthe ZnO surface. This complex interplay of physical andchemical processes opens up a vast configurational spaceof putative reaction intermediates which we studied us-ing DFT calculations carried out on thermodynamicallyoptimized surface slab models34.The comprehensive energy level diagram of Fig. 8 sur-

veys the most stable adsorption modes of molecularspecies being adsorbed at oxygen vacancies of electronrich and electron deficient nature, which are relevant un-der the reducing and oxidizing conditions of methanolsynthesis and methanol decomposition, respectively. Fur-ther characterization of the individual adsorption modesof key species is provided by the calculation of vibra-tional frequencies, which are in good agreement with ex-perimental data for the known intermediates, but alsoprovide reference data for future experimental work onnot yet characterized species.

Starting the discussion of the energy level diagram,Fig. 8, with the adsorption of the CO species at F++

centers, we could not identify a configuration with a suf-ficiently large binding energy that could favor a reac-tion mechanism proposed many years ago.13 Accordingto our present investigation the chemisorption of CO ina oxygen defect site representing the first reaction stepis rather unlikely. Instead we propose a direct chemicalreaction between CO and the surface, i.e. either hydro-genation or protonation of the CO.39 This can be realizedeither (i) through a successful collision with with a hy-dride in the vacancy or (ii) via a direct insertion into asurface hydroxyl group.41 These two possible reactionsgive rise to the formation of (i) weakly bound formyl or(ii) strongly bound formate species, respectively. More-over, significant stability is gained with respect to ad-sorbate structures investigated earlier13 when changingthe formyl species from upright to bend configurationswhich now bind with both carbon and oxygen atoms tothe Zn atoms of the O vacancy due to the tilt. In ad-dition, moving formaldehyde slightly to the edge of theO vacancy and forming dioxomethylene will provide ad-ditional stabilization of almost 1 eV, while the alterna-tive process of transferring a proton from a neighboringsurface OH group to the formaldehyde to give a hydroxy-methylene species is thermodynamically even more favor-able. Having obtained several species being significantlymore stable than those considered in early work13 does,however, not change the fundamental problems of anyreaction mechanism that is based on the fully oxidizedF++ center, i.e. the extremely low energetic minimum ofthe methoxide intermediate which blocks the active site,as well as the final desorption of methanol once this prod-uct has been formed. We rather propose here the reverseprocess to be preferred, i.e. methanol decomposition totake place under such oxidizing conditions thus involvingF++ centers.

This picture, being most unfavorable for methanol pro-duction, changes completely when moving towards reduc-ing conditions such as those used in the actual industrialprocess of methanol synthesis. In the energy level dia-gram of Fig. 8 blue and red color mark the most stableconfigurations of species being bound to F0 and F++/H2

as well as to F−− and F0/H2 centers, while arrows markthe corresponding process of H2 adsorption being a dy-namical process which can take place easily during thecatalytic cycle.34 Obviously, the energy levels are lowerfor all intermediates, since the formation energy of thepure F0 defect is well below that of F++. Clearly, at theelectron rich F centers Zn-bound species will stabilize farmore in bent adsorption modes, which subsequently re-sults in a reduction of the molecules. Within such modes,formyl species and especially formaldehyde will receivesubstantial activation by the electronic charge density,i.e. leading even to a di-anionic formaldehyde specieswhich is already in the same reduction state as methanol.In stark contrast, the same charge density destabilizesand partially even repels bidentate O-bound species from

17

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Figure 8. Energy level diagram of the most stable configurations of various molecular intermediates in the reaction of CO and H2 to

methanol on defective ZnO(0001) depending on the charge state of the O vacancy: oxidized F++ (black color), reduced F++/H2 or F0 (bluecolor) and strongly reduced F0/H2 or F−− (red color). The energy axes represent formation energies relative to an initial F++ center (left)and alternatively relative to an initial F0 defect (right). The formation energy of methanol in the gas phase, EPBE

f,H3COH= 1.84 eV, is given as

further reference. Explicit values of the formation energy are given for adsorbed species at F++ defect sites. Arrows represent the relativeenergy change by homolytic H2 adsorption, i.e. conversion ofF++ to F0 (blue color) and conversion of F0 to F−− (red color). Metastableconfigurations with alternative binding modes are indicated by dashed lines. Adsorption modes are classified as monodentate (mono),bidentate (bi), upright and inserted as indicated in the scheme. Additionally, the specific location of the species inside the O vacancy,(Zn-bound), insertion into a ZnO dimer or at the rim of the defect (O-bound) is given.

the F centers, thus favoring corresponding monodentateadsorption modes. Nevertheless, at the reduced defectsZn- and O-bound species, despite their distincly differ-ent binding modes, often show a very similar stability.In particular, for formyl and hydroxymethylene, formal-dehyde and dioxomethylene, as well as hydroxymethyland hydroxymethoxide the formation energies are withina small window of about 0.1–0.2 eV. Moreover, the finalhydrogen transfer in the second reduction step is largelybiased by the steric hindrance due to the carbon atomof the most stable adsorption modes of formaldehyde,hydroxymethyl, dioxomethylene, hydroxymethoxide andmethoxide species. The latter species, however, is signif-icantly destabilized at these O vacancies hosted by suchreduced ZnO(0001) surfaces, which makes the desorptionof the final product methanol far less endothermic whencompared to the F++ center. According to our energydiagram (see Fig. 8) the desorption of methanol from aF0 center may become even exothermic when the chargestate is changed to F−− as a result of H2 adsorption.

Having obtained species which are energetically moreor less equivalent at oxygen defects present under re-ducing conditions, a large space of possibilities in bothchemistry and configuration space opens up that mustbe considered. Typically, this is done by locating vari-ous transition states and calculating all reaction barriersthat inter-connect these species in addition to consider-ing re-reduction of the surface by H2 adsorption. More-over, after applying thermodynamic corrections eventu-

ally one should be able to disclose low energy pathways ofmethanol synthesis from CO and H2 over ZnO(0001).

39

In our approach to computational heterogeneous catal-ysis, however, we do not proceed along these lines. In-stead, we have already demonstrated that acceleratedab initio molecular dynamics is indeed a versatile toolfor computational heterogeneous catalysis which by purecomputational means successfully solved this type of puz-zle of dynamical chemical processes for methanol syn-thesis from CO and H2 over the defective ZnO(0001)surface.41,43 While keeping any a priory input on prod-ucts, intermediates, reaction channels, or mechanismto a minimum this automated molecular dynamics ap-proach generated the whole reaction network and thusthe species from Fig. 8, which have now been analyzedand characterized in detail in the present study. Futurework will transfer our approach to heterogenous catalysisto the ternary system, i.e. to the Cu/ZnO/Al2O3 catalystof the ICI process, for which the basic ab initio thermo-dynamics as well as the strong metal-support interactionhave already been analyzed.12

ACKNOWLEDGMENTS

We thank K. Fink, G. Roßmuller, J. Koßmann,Ch. Hattig, Y. Wang, Ch. Woll and M. Muhler for fruit-ful discussions. This work was supported by the GermanResearch Foundation (DFG) via Collaborative Research

18

Center SFB 558 “Metal-Substrate Interactions in Hetero-geneous Catalysis”. Computational resources were pro-vided by BOVILAB@RUB (Bochum), LRZ (Munchen),and RV–NRW.

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