structural analysis of silica-supported molybdena based on ... · typesofoxygen species,o(1)...

Published: July 05, 2011

r 2011 American Chemical Society 15449 dx.doi.org/10.1021/jp2034642 | J. Phys. Chem. C 2011, 115, 15449–15458

ARTICLE

pubs.acs.org/JPCC

Structural Analysis of Silica-Supported Molybdena Based on X-raySpectroscopy: Quantum Theory and ExperimentC. S. Guo,† K. Hermann,*,† M. H€avecker,‡,§ J. P. Thielemann,§ P. Kube,§ L. J. Gregoriades,|| A. Trunschke,§

J. Sauer,|| and R. Schl€ogl§

†Theory Department, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany‡Department of Solar Energy Research, Helmholtz-Zentrum Berlin/BESSY II, Albert-Einstein-Str. 15, 12489 Berlin, Germany§Inorganic Chemistry Department, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany

)Institut f€ur Chemie, Humboldt-Universit€at zu Berlin, Unter den Linden 6, 10099 Berlin, Germany

ABSTRACT:

Oxygen core excitations in different molecular molybdena�silica models are evaluated using density-functional theory (DFT).These results can be compared with in situ X-ray absorption fine structure (NEXAFS) measurements near the O K-edge ofmolybdena model catalysts supported on SBA-15 silica, used for exploratory catalytic activity studies. The comparison allows ananalysis of structural details of the molybdena species. The silica support is found to contribute to the NEXAFS spectrum in anenergy range well above that of the molybdena units, allowing a clear separation between the corresponding contributions. Differenttypes of oxygen species, O(1) in terminal MdO bonds, O(2) in interphase Mo—O—Si bridges and in Mo—O—Mo linkages, aswell as O(2) in terminal Mo—O—H groups can be distinguished in the theoretical spectra of the molybdena species withmolybdenum in tetrahedral (dioxo species), pentahedral (monooxo species), and octahedral coordination. The experimentalNEXAFS spectra exhibit a pronounced double-peak structure in the O 1s to Mo 4d�O 2p excitation range of 529�536 eV.Comparison with the present theoretical data gives clear indications that dioxo molybdena species with tetrahedral MoO4 units canexplain the experimental spectrum. This does not fully exclude species with other Mo coordination, like pentahedral. However, thelatter are believed to exist in the present samples in much smaller amounts. The experimental NEXAFS spectrum for the supportedmolybdena species differs substantially from that for MoO3 bulk material with octahedral MoO6 units where the observedasymmetric peak structure is also reproduced by the calculations.

1. INTRODUCTION

Molybdenum oxide is catalytically active in a wide range ofdifferent organic reactions, including oxidation, hydrogenation,metathesis, and isomerization, that require both redox and acid�base functions. Selective oxidation of propene to acrolein, oxida-tion of acrolein to acrylic acid, methanol oxidation, olefin metath-esis, or hydroprocessing applications are prominent examples ofcommercially implemented molybdenum oxide based catalysts.Oxidation catalysts are often very complex, containing multipleactive sites and different functionalities that may cooperate underoperation conditions. Aiming at an improved fundamental under-standing of oxidation catalysis, supported molybdenum oxidespecies have been studied as model catalysts in partial oxidationof different molecules, e.g., methanol,1 propylene,2 and propane.3

Selectivity in partial oxidation is determined by the relative

abundance of intermediates formed in the complex network ofconsecutive and parallel reactions. The actual reaction mechan-ism depends on the local geometric structure of surface metaloxide species and its interaction with the support, which deter-mines the surface catalyst characteristics as a whole, includingreducibility, acid base properties, and the nature of the activeoxygen species. These parameters in turn are a function of themetaloxide loading, the chemistry of the support, and the preparationprocedure. Therefore, detailed knowledge of the metal oxidesurface structure is essential for the establishment of structure�function relationships, which are required for an improved con-

Received: April 13, 2011Revised: June 24, 2011

15450 dx.doi.org/10.1021/jp2034642 |J. Phys. Chem. C 2011, 115, 15449–15458

The Journal of Physical Chemistry C ARTICLE

trol of selectivity in partial oxidation reactions. Extensive studieshave been devoted to the investigation of relations between theelectronic and geometric structure of surface vanadium oxidespecies and their reactivity in oxidative dehydrogenation andselective oxidation reactions.3�8 These studies reveal analogiesto supported molybdenum oxide catalysts which are generallyless active than vanadia catalysts.3 The origin of the differentperformance of the two catalysts is still unclear.

In this work, we perform density-functional theory (DFT)calculations on oxygen core excitations inmolybdena�silicamodelclusters simulating local sections of supported molybdena catalystsas well as in clusters representing bulk MoO3 substrate. Thetheoretical excitation spectra are then compared with in situ X-rayabsorptionfine structure (NEXAFS)measurements near the oxygenK-edge of molybdena model catalysts supported by SBA-15 silica inorder to identify structural and electronic details of the molybdenaspecies. NEXAFS spectroscopy9 is based on dipole excitations ofhighly localized core electrons by photons. Thus, it can providereliable atom-specific information allowing to distinguish betweenatoms in different local environments. This is used in the presentcalculations to discriminate between differently binding oxygen,in molybdenyl ModO as well as in Mo—O—Mo, Mo—O—Si,Mo—O—H, and Si—O—Si bridges of the silica-supportedmolybdena species. It also offers the possibility to identify specificmolybdena species, such as tetrahedral MoO4, pentahedralMoO5, or octahedral MoO6, among those prepared in this studyexperimentally at the surface of mesoporous silica, SBA-15.10

In section 2, we introduce themodels and discuss details of thecomputational methods used in the spectrum calculations. Section3 describes experimental details connected with sample prepara-tion and NEXAFS measurements, while section 4 presents resultsand discussion. Finally, section 5 summarizes our conclusions.

2. THEORETICAL DETAILS

2.1. Cluster Models. The present calculations are based onmolybdena�silica model clusters whose structures are obtainedfrom chemical reasoning combined with structure optimizationsat the DFT level in this study as well as from earlier work on theoxidation of methane11 and methanol12 on silica-supported mo-lybdena. The clusters represent a variety of arrangements ofmolybdena in different coordinations where the MoOx unitsare bound to silicon of the silica support via Mo�O�Si bridges.

These arrangementswere inspired to someextent by the geometricstructure of bulk MoO3 which forms an orthorhombic crystallattice with a layer structure of weakly binding bilayers parallel to(010) netplanes; see Figure 1a. Each bilayer consists of twointerleaved sheets which can be described by distorted MoO6

octahedra sharing corners while octahedra from adjacent planesshare edges;13,14 see Figure 2a. The asymmetry of the octahedra,with four smaller Mo�O distances (1.67�1.94 Å) and two largerdMo�O values (2.25�2.33 Å),13 see Table 1 and Figure 1b,

Figure 2. Geometric structure of MoO3(010) bilayers illustrating thestacking of (a) octahedral MoO6, (b) tetrahedral MoO4, and (c)pentahedral MoO5 building units; see the text. Molybdenum (oxygen)atoms are shown by large (small) balls.

Table 1. Molybdenum�Oxygen Bond Distances in the ModelClusters Simulating MoO3 Bulk (Figure 3b) and DifferentMolybdena�Silica Clusters (Figure 4)a

cluster dMo�O(1) dMo�O(2) dMo�O(3)

Mo7O30H18 (bulk) 1.67 1.73 (2.25) 1.94 (1.94, 2.33)

cluster d1 d2 d3

(a) MoO3(OH)�Si8O12H7 1.72 (1.91) 1.90

(b) MoO4�Si7O10H8 1.70 1.87

(c) (MoO4)2�Si6O7H6 1.70 1.89

(d) MoO(OH)5�Si8O12H7 1.88�1.97 2.02

(e) MoO2(OH)4�Si7O10H8 1.89�1.96 1.93, 1.96

(f) MoO4(OH)2�Si12O16H12 1.92 1.92�1.93

(g) MoO5�Si4O4H4 1.68 1.94

(h) MoO5�Si12O16H12 1.68 1.91

(i) Mo2O8�Si6O7H6 1.68 1.89 1.85 (2.08)a For the MoO3 bulk cluster, Mo7O30H18, distances dMo�O(1), dMo�O(2),and dMo�O(3) denote those of terminal molybdenyl, ModO, of asym-metric Mo—O—Mo bridges, and of symmetric Mo—O—Mo bridgeswith an additional longer Mo—O distance. For the molybdena�silicaclusters, distances d1 refer to those of terminal molybdenyl ModO (orMo—OH for clusters a, d, e, and f), d2 to those of Mo—O—Si bridges,and d3 to those of Mo—O—Mo bridges. All distances are in angstroms;see also Figure 1b.

Figure 1. (a) Geometric structure of orthorhombic MoO3 with bilayerstacking along the (010) direction. Molybdenum (oxygen) atoms areshown by large (small) balls. Nonequivalent oxygen atoms, O(1, 2, 3),are labeled accordingly. (b) Adjacent octahedral MoO6 building unitsfrom the top and bottom part of the bilayers. The oxygen atoms arelabeled a�f with interatomic Mo�O distances given in angstroms.



suggests an alternative characterization of each bilayer by twoparallel sheets which are composed of ribbons of MoO4 tetra-hedra sharing corners along the (001) direction; see Figure 2b. Athird possible molybdenum coordination may be derived byconsidering the bilayers of bulk MoO3 to consist of two parallelsheets each forming networks of corner linked pyramidal MoO5

units with their tops pointing in opposite directions; seeFigure 2c. These three equivalent descriptions result in octahe-dral, tetrahedral, and pentahedral oxygen environments aroundmolybdenumwhich may be taken as separate molybdena units ofcorresponding coordination.The silica support of the molybdena�silica clusters is modeled

by clusters15 which are derived from polyhedral oligomericsilsesquioxane (POSS), Si8O12H8.

16 This molecule is of cubicstructure with silicon occupying the corners and oxygen near theedge midpoints while hydrogen saturates the dangling bondsat the silicon corners; see Figure 3a. A molybdena species isadded to this molecule or to corresponding fragments to yield themolybdena�silica model clusters shown in Figure 4. Here,Figures 4a�c represent clusters in whichmonomeric and dimericdioxo molybdena species appear with the molybdenum atoms in4-fold tetrahedral coordination, (a) MoO3(OH)�Si8O12H7

with molybdena bound by one Mo�O�Si bridge with the silicasupport, (b) MoO4�Si7O10H8 with two Mo�O�Si bridges,and (c) (MoO4)2�Si6O7H6, yielding two adjacent dioxo mo-lybdena binding as in (b) each.Figures 4d�f show clusters containing octahedrally coordi-

natedmonomericmolybdena species, (d)MoO(OH)5�Si8O12H7

with one Mo�O�Si bridge, (e) MoO2(OH)4�Si7O10H8, and(f) MoO4(OH)2�Si12O16H12 with four Mo�O�Si bridges.Figures 4g�i refer to clusters describing pentahedrally coordi-nated monooxo molybdena, (g) MoO5�Si4O4H4 and (h)MoO5�Si12O16H12 with four Mo�O�Si bridges and (i)Mo2O8�Si6O7H6, yielding two adjacent monooxo molybdenabinding with two Mo�O�Si and Mo�O�Mo bridges each.Note that models d and e are obtained by H2O addition near theMo centers of models a and b, respectively, and cluster f arisesfromH2O addition to model h. As this is an endothermic processwith increasing Gibbs free energy, models d, e, and f representless stable structures than models a, b, and h, respectively.Reference calculations on molybdena environments in bulk

MoO3 are based on a Mo7O30H18 cluster, see Figure 3b,describing a local section of a bilayer parallel to (010) netplanesshown in Figure 1. Here, dangling Mo�O bonds at the clusterperiphery are saturated by hydrogen to account for clusterembedding. Corresponding Mo�O distances of all clusters arelisted in Table 1; see also Figure 1b.

2.2. NEXAFS Spectrum Calculations. Electronic ground andcore excited states of the above clusters are calculated withindensity-functional theory (DFT) using the gradient correctedrevised Perdew�Burke�Ernzerhof (RPBE) exchange-correla-tion functional.17,18 Corresponding Kohn�Sham orbitals arerepresented by linear combinations of atomic orbitals (LCAO)using extended basis sets of contracted Gaussians. For theevaluation of X-ray absorption spectra, silicon and hydrogenare represented by all-electron double-ζ-valence-plus-polariza-tion (DZVP) basis sets while the electronic structure of molyb-denum is described by an effective core potential (ECP) togetherwith a valence basis set for the 14 outermost valence electrons. Atthe core excited oxygen center, extended IGLO-III type basis sets19

are employed to adequately describe inner shell relaxation effectswhile the other oxygen centers in the cluster are accounted for byeffective core potentials (ECPs) for the 1s shell and correspond-ing valence basis sets.20,21 For all calculations, the DFT clustercode StoBe22 is employed.The computation of theoretical O 1s X-ray absorption spectra

of the different clusters considers core to unoccupied orbitalexcitations resulting from dipole transitions. Thus, polarization-resolved spectral intensities I(E, e) are determined by corre-sponding dipole transition matrix elements, vectorsm = (mx,my,mz), together with angle-dependent factors describing the polar-ization vector e = (ex, ey, ez) of the incoming radiation, i.e., by

IðE, eÞ ¼ R 3 E 3 ðmeÞ2, m ¼ Æjf jq 3 rjjcoreæ ð1Þ

Here, R is a global scaling factor, E denotes the transition energy,and transition dipole vectors m involve the initial core orbitaljcore and final excited state orbitals jf. For the present purpose,the complex dependence of the spectral intensities on the photonpolarization direction can be ignored due to the disordered arran-gement of the MoOx particles to be considered. Therefore, the

Figure 3. Geometric structure of the model clusters representing (a)the silica support (silsesquioxane, Si8O12H8) and (b) the MoO3 bulksubstrate (Mo7O30H18); see the text. Atoms are shown by balls ofdifferent radii and labeled accordingly.

Figure 4. Geometric structure of the molybdena�silica model clusters:(a�c) Tetrahedrally coordinated dioxo molybdena species. (d�f)Octahedrally coordinated molybdena species. (g�i) Pentahedrallycoordinated monooxo molybdena species; see the text. Atoms areshown by balls of different radii and labeled accordingly in part g.



angle averaged intensity given by

IðEÞ ¼Z

IðE, eÞ dΩ ¼ 2π=3 3 R 3 E 3 ðmx2 þ my

2 þ mz2Þ

ð2Þ

and summed over all possible final state orbitalsjf will be used inthe analysis. The evaluation of core excited final states andcorresponding matrix elements m is performed within the transi-tion potential approach23 in combination with a double basis settechnique.24This approximation involves the use of a half occupiedO 1s core orbital at the ionization site, thereby accounting forpartial electronic relaxation due to the presence of the excitedelectron. It has been proven earlier that, within this approach, finalstate relaxation effects are taken care of up to second order in theenergy, achieving a balance between the two relaxation mechan-isms, core and valence type.25

The computed orbital energies and corresponding dipoletransition matrix elements in eq 2 are convoluted using Gaussianbroadening of varying width to simulate instrumental, vibrational,and lifetime broadening. A full-width-at-half-maximum (fwhm)value of 1 eV is applied below the ionization threshold while thebroadening is increased linearly to 4 eV up to 20 eV abovethreshold and kept fixed at this value for higher energies. In thetransition potential approach, the electronic core hole relaxation ofthe excited final state is not fully accounted for. This incompleterelaxation can be corrected in an approximate way by shifting allexcitation energies by the difference of the ionization potentialcomputed with the transition potential method and the corre-sponding value from ΔKohn�Sham (ΔSCF) calculations. Thisresults in a global downward shift of about 2 eV. Further, relativisticcorrections are included by applying an additional upward shift ofthe computed spectra by 0.33 eV.26

The present computational approach has been used success-fully to yield highly accurate results for X-ray absorption of lightatoms in gas phase molecules.27�29 It has also been used to studyproperties of hydrogen-bonded systems,27,30,31 of adsorbates onmetal surfaces,32,33 and of differently coordinated oxygen sitesnear vanadium and molybdenum oxide surfaces.34�36 Furtherdetails concerning the methods can be found in the correspond-ing references.

3. EXPERIMENTAL DETAILS

3.1. Synthesis of the Silica-Supported Molybdena andOxidative Dehydrogenation of Propane. The synthesis ofMoOx/SiO2 was adapted from a functionalization/anion exchangeprocedure applied previously in the preparation of silica supportedvanadia catalysts.37,38Themesoporous support, which was synthe-sized based on the original recipe of Yang et al.,39 was function-alized using 3-aminopropyltrimethoxysilane (APTMS) followedby protonation of the resulting amine with HCl. The chlorideions were exchanged by heptamolybdate ions applying ammo-nium heptamolybdate ((NH4)6Mo7O24 3 4H2O) as the Mo pre-cursor. The resulting powder was washed, filtered, and calcined at823 K for 12 h. The samples included in the present studycontained amolybdenum loading of 5.9 and 11.1 wt %. A SBA-15reference sample was prepared applying the same procedure asdescribed above. However, potassium oxalate monohydrate wasintroduced in the material instead of heptamolybdate in the ion-exchange step.

The SBA-15 supportedmolybdenum oxide catalyst containing11.1 wt % molybdenum (internal catalyst ID 6870) was tested inthe oxidative dehydrogenation of propane for temperatures be-tween 723 and 773 K applying contact times between 0.05 and0.6 gs/ml and amolar ratio of propane/oxygen = 2:1 with nitrogenas balance (C3H8/O2/N2 = 10/5/85). The experiments werecarried out in a setup for selective oxidation (Integrated LabSolutions, Berlin, Germany) using eight parallel fixed bed quartzreactors at atmospheric pressure. For the measurements, 100 mgof the catalyst was diluted with 2.9 g of silicon carbide (ratio 1:30)for minimization of temperature gradients. Reactants and pro-ducts were analyzed by an online gas chromatograph (Agilent7890). Separation of the permanent gases CO, CO2, N2, and O2

was achieved with a combination of a Plot-Q and a Plot-molsievecolumn connected to a thermal conductivity detector (TCD).Propane and propylene were separated with a Plot-Q columnconnected to a flame ionization detector (FID).3.2. NEXAFSMeasurements. In situ NEXAFSmeasurements

have been performed at the synchrotron radiation facility BESSYII of the Helmholtz-Zentrum Berlin, Germany (HZB), usingmonochromatic radiation of the ISISS (Innovative Station forIn Situ Spectroscopy) beamline as a tunable X-ray source. Highpressure soft X-ray absorption spectra were measured in thepresence of oxygen at elevated temperature using the high pressurestation designed and constructed at the Fritz-Haber Institute,Berlin. Details of the setup are described elsewhere.40�42 In brief,self-supporting pellets of MoOx/SBA-15 samples are mountedinside a reaction cell onto a sapphire sample holder approxi-mately 1.4 mm in front of the first aperture of a differentiallypumped electrostatic lens system. The home-built electron lensserves as the input system for a modified commercial hemisphe-rical electron analyzer (PHOIBOS 150, Specs-GmbH). Oxygenis introduced into the cell via calibrated mass flow controllers,heating is provided by a NIR laser at the rear of the sample, and thetemperature is monitored by a thermocouple attached directly tothe sample surface. NEXAFS spectra of the dehydrated MOx/SBA-15 samples have been obtained in 0.5 mbar O2 at 350 �C byheating the material in situ in the XAS cell at 5 K/min up to thefinal temperature. Oxygen 1s (K-edge) excitation spectra havebeen recorded in the Auger electron yield mode by operating theelectron spectrometer with a pass energy of 100 eV as an XASdetector to minimize contributions from the gas phase to thespectra.OK-edge spectra of the sample surface have been correctedfor the remaining effects of O2 gas phase absorption. In order toincrease the signal-to-noise ratio, a data reduction by a factor of6 has been applied to the raw spectra (containing about 4000 ptsper scan) by averaging adjacent points. Three scans have beenaveraged, and the X-ray spot position on the sample has beenchanged after each scan to avoid damage of the surface by thebrilliant synchrotron X-ray beam. Absolute energy calibration hasbeen achieved by setting the π* resonance of the O2 gas phasesignal to 530.9 eV, and the spectral resolution was about 150 meV.Further details of themetrology and data treatment are presentedelsewhere.43 The polycrystalline reference bulk sample of R-MoO3 has been prepared by suspending commercial powder(Fluka) in chloroform on a steel plate to create a thin film ofmaterial without charging.

4. RESULTS AND DISCUSSION

Due to the well-defined preparation procedure, the presentsilica supported molybdenum oxide catalyst seems adequately



suited as a model to gain new insight into the nature of the activesites as well as into relations between structural properties andactivity with emphasis on oxidation reactions. The SBA-15 sup-ported molybdenum oxide catalyst containing 11.1 wt % molyb-denum is found to be active in oxidative dehydrogenation ofpropane. Its activity was studied in a temperature range between723 and 773 K (Figure 5). For a contact time of 0.6 gs/ml,propane conversion reaches a maximum of approximately 2%.Under these conditions, only small amounts of total and partialoxidation products CO2 and CO are formed, resulting in a pro-pylene selectivity ofmore than 80%. The Arrhenius plot using thepropane consumption rate yields an apparent activation energyof 162 kJ/mol. This is considerably high compared to valuesreported in the literature for molybdenum oxide species sup-ported on other carrier oxides. The apparent activation energy ofpropane activation has been determined to be 117 kJ/mol on11%MOx/ZrO2 (ref 5) and 97 kJ/mol on 5%MoOx/TiO2 (ref 4)for catalysts with molybdenum oxide loadings corresponding to atheoretical monolayer. This substantiates the strong effect of thesupport in oxidative dehydrogenation of propane.4.1. NEXAFS Spectra for Bulk MoO3. Before examining the

silica-supported MoOx particles, we validate our theoretical meth-ods for bulk MoO3 whose structure is well-known.

13,14 For thiscompound, experimental oxygen K-shell NEXAFS spectra areavailable in the literature36 but have also been re-examined in thisstudy. Figure 6a compares the calculated oxygen 1s core excita-tion spectrum for a model cluster Mo7O30H18 with H termina-tion at its periphery, see Figure 3b, representing a crystal sectionofR-MoO3 bulk (considered earlier in theoretical studies

13) withcorresponding experimental OK-edgeNEXAFS data for samplesof clean R-MoO3 bulk. Both spectra reveal a main peak near530.5 eV and a broader double peak structure between 533.5 and535 eV where the comparison suggests very good agreementconsidering the fact that an absolute energy scale without adjust-ment between theory and experiment is used. This gives con-fidence that the present theoretical methods can also give areliable description of the spectroscopic data for small supportedMoOx particles.The theoretical bulk spectrum of Figure 6a can be character-

ized in detail by analyses of corresponding final state orbitals. The

peak structure between 530 and 535 eV is found to be dominatedby O 1s core excitations to unoccupied orbitals with antibondingO 2p�Mo 4d character. Further, the broad peak at lower energyrefers to final state orbitals where the O 2p admixture pointsperpendicular to the corresponding O�Mo bond, whereas thepeaks at higher energy are due to orbitals with O 2p admixturepointing along the O�Mo bond. Thus, the different amountof antibonding character of the two types of orbitals explains theenergetic peak sequence. Figure 6b shows a decomposition ofthe total theoretical bulk spectrum into individual contributionsfrom the three nonequivalent bulk oxygen species, O(1), O(2),O(3), as defined in Figure 1a. Obviously, the broad low energypeak in the total spectrum originates from three individual peaksshifted by about 1 eV with respect to each other and referr-

Figure 6. (a) Comparison of the theoretical oxygen 1s core excitationspectrum (“Theory”) evaluated for the model cluster Mo7O30H18

(Figure 2b) with an experimental O K-edge NEXAFS spectrum for bulkMoO3 (“Experiment”). (b) Decomposition of the theoretical excitationspectrum into contributions from the three differently coordinated oxygensites, O(1), O(2), O(3), see Figure 1a. The two vertical dashed linesindicate the energy range of the computed ionization potentials of thethree differently coordinated oxygen species.

Figure 5. Propane conversion and product selectivity in oxidativedehydrogenation of propane over 11.1%Mo/SBA-15 for a contact timeof 0.6 gs/ml in a feed mixture containing the reactants in a molar ratio ofC3H8/O2/N2 = 10/5/85.

Figure 7. Experimental O K-edge NEXAFS spectra for dehydratedMoOx/SBA-15 with 5.9 and 11.1 wt % Mo loading; see the text. TheNEXAFS spectra for clean SBA-15 substrate and bulk MoO3 are includedfor comparison.



ing to different oxygen species where the central peak is slightlylarger in intensity than the other two and due to molybdenylO(1) excitations. The peak structure at higher energies in thetotal spectrum is a somewhat more complex superposition ofindividual peaks where also excitations from all oxygen speciescontribute.4.2. Experimental NEXAFS Spectra for SBA-15 Silica and

Supported MoOx Particles. Figure 7 compares experimentaloxygen K-edge NEXAFS spectra of the SBA-15 silica supportwith those of adsorbed MoOx particles of two different molyb-denum loadings, 11.1 and 5.9 wt %, at the SBA-15 surface. Inaddition, the bottom spectrum shows the NEXAFS result forcrystalline MoO3 bulk measured with the same experimentalsetup. The spectrum for the clean SBA-15 support reveals onelarge asymmetric peak near 538 eV with a smaller precursor at535 eV. The origin of these peaks has been analyzed earlier intheoretical cluster studies44 and found to be due to oxygen 1score excitations of Si�O�Si bridges in the SBA-15 support.Deposition of dehydrated molybdena at the support yields forboth loadings two additional peaks, one low energy peak of0.9 eV fwhm at 530.2 eV and a broader peak structure of 1.9 eVfwhm at 532.5 eV where the integral intensities of the two peaksare rather similar. These peaks must originate from core excita-tions of oxygen located at the MoOx particles. It is interesting tocompare theNEXAFS spectra for depositedmolybdena with thatof crystalline MoO3 bulk also included in Figure 7 and discussedin the previous section. Obviously, the two types of samples showrather different spectral behavior in the oxygen K-edge region.This suggests a different geometric arrangement of the oxygen indeposited molybdena compared with that inside the MoO3 bulkin which distorted octahedral MoO6 units exist. On the otherhand, the two-peak structure found in the NEXAFS spectra fordeposited molybdena has been shown to resemble that observedfor crystalline sodium molybdate, Na2MoO4 3 2H2O,

45 contain-ing tetrahedral MoO4 units only.

46 This may indicate structuralsimilarity with correspondingMoOx units at the SBA-15 support.4.3. Theoretical NEXAFS Spectra for MoOx�SiO2 Model

Clusters. 4.3.1. Model Clusters with Tetrahedral MoO4 Units.Figure 8 shows theoretical O 1s core excitation spectra for thethree molybdena�silica model clusters in which the molybdenum

of the molybdena subunits is tetrahedrally coordinated (dioxoMoO4 species), as shown in Figure 4a�c. The left diagram(“Total”) collects total spectra composed of stoichiometricallyweighted superpositions of partial spectra due to excitation at thedifferent types of oxygen, terminal molybdenyl, as well as 2-foldbridging. Obviously, the three clusters yield a two-peak structurein the excitation energy range between 529 and 534 eV with aslightly narrower peak centered near 530.3 eV and a broader peaknear 532.0 eV. The integral intensity of the higher energy peak isonly slightly larger than that of the lower energy peak. Both thepeak positions and their intensities are found to be extremelysimilar for the three clusters. This suggests that the actual geometriccoupling of the dioxo MoO4 species with the silica support, byone Mo�O�Si bridge in cluster a of Figure 4 and by twoMo�O�Si bridges in clusters b and c, influences the spectralfeatures very little. In addition, the close similarity between thespectra for clusters b and c, in which one and two (adjacent)MoO4

species exist, indicates that theO 1s core excitations are highly localprocesses which are affected only little by the electronic interac-tion with nearby MoO4 species.The right diagram of Figure 8 (“Atom res.”) shows a decom-

position of the excitation spectra into contributions from differ-ent oxygen species in clusters a�c, terminal oxygen O(1) ofmolybdenyl ModO, O(2) in interphase Mo—O—Si bridges,and O(20) in Mo—O—H bridges (cluster a only). Contribu-tions from oxygen O(2#) in Si—O—Si bridges of the silicasupport appear only above 535 eV and are not included in thefigure. A comparison of the atom resolved spectra with thecorresponding total excitation spectra evidence that for all threeclusters the two-peak structure in the energy region between 529and 534 eV is determined mainly by core excitations at themolybdenyl oxygen O(1) while contributions from the bridgingoxygen species appear at higher energies. The broad peak at536.5 eV appearing in cluster a and attributed to core excitationsinMo—O—Si bridges is shifted to lower energies, 535.6 eV, andreduced in its width for clusters b and c. This is simply due to thedifferent electronic coupling of the tetrahedrally coordinatedMoO4 species which binds with one Mo—O—Si bridge to thesilica substrate in cluster a but with two bridges in clusters b and c.The type of oxygen core excitations of the double-peak

spectrum in the energy region between 529 and 534 eV can becharacterized in greater detail by examining corresponding finalstate orbitals. As an example, Figure 9 shows the partial spectrumobtained for 1s core excitation at molybdenyl oxygenO(1) in theMoO4�Si7O10H8 cluster (b); see Figure 4b. The figure includes

Figure 8. (left, Total) Theoretical O 1s core excitation spectra for clustersa�c containing tetrahedral MoO4 subunits; see Figure 4. The spectra arecompared with experimental O K-edge NEXAFS data (dashed line) fordehydrated MoxOy/SBA-15 with 11.1 wt % Mo taken from Figure 7.(right, Atom res.) Decomposition of the excitation spectra into contribu-tions from different oxygen species in clusters a�c, terminal molybdenyloxygen O(1), O(2) in Mo�O�Si bridges, and O(20) in Mo�O�Hbridges. The atom resolved spectra of clusters b and c are almost identical.

Figure 9. Theoretical O 1s excitation spectrum calculated for molyb-denyl oxygen O(1) in the MoO4�Si7O10H8 cluster (b); see Figure 4b.The figure includes discrete excitation energies of the two-peak regionshown as vertical lines of lengths determined by the excitation prob-abilities. The iso-surface plots illustrate representative final state orbitals1 and 2 near the centers of the two peaks; see the text.



also discrete excitation energies of the double-peak region givenby vertical lines of lengths determined by the excitation prob-abilities. This shows that the energetically lower peak at 530.3 eVin the spectrum originates from two excitations separated by0.3 eV where the excitation at lower energy contributes about 60%in intensity compared with that at higher energy (main excitation).Further, the energetically higher peak near 532 eV combinesthree excitations separated by 0.5 and 0.8 eV, respectively, wherethe lowest excitation yields about 50% intensity compared witheach of the two higher excitations which are roughly equal inintensity. The final state orbitals referring to these core excita-tions can be analyzed in detail by their atom contributions or bypopulations. All orbitals of the double-peak structure are found torepresent antibondingmixtures of majorMo 4d and smaller O 2pcontributions, the latter at the excitation center O(1). Theamount of antibonding character determines the energetic orderof the excitations. The orbital type is also obvious in correspond-ing three-dimensional iso-surface plots. As an illustration,Figure 9 shows plots of two final state orbitals resulting from(1) the main excitation of the energetically lower peak and (2)the topmost excitation of the higher peak. These orbitals arefocused mainly at the MoO4 unit of the model cluster with onlyvery little orbital admixture from remote oxygen and siliconcenters. (Only theMoO4 part of the cluster is shown in Figure 9.)An inspection of the orbital shapes evidences the 4d type orbitalcharacter near themolybdenum center of theMoO4 unit for bothfinal state orbitals. Further, the orbital contributions at theoxygen center (excitation center, labeled “O” in the right plot)are clearly of 2p type in both cases. In addition, the antibondingcharacter of theMo�Ocoupling is obvious from the iso-surfaces inthe region between the two atom centers which differ in color(shading) for positive and negative orbital values. Obviously, thedirections of the O 2p functions at the excitation centers differ. Forfinal state orbital 1 of the low energy peak, the p function pointsperpendicular to the Mo�O(1) bond axis, yielding less antibond-ing character than found for orbital 2 of the high energy peakwherethe p function points almost along the Mo�O(1) bond axis.Figure 8 includes also the experimental O K-edge NEXAFS

data (dashed line) for dehydrated MoxOy/SBA-15 with 11.1 wt %Mo taken from Figure 7. This spectrum shows, in the energyregion between 529 and 534 eV, a striking similarity with thetheoretical excitation spectra of all three molybdena�silica modelclusters containing tetrahedrally coordinated dioxo MoO4 species.The agreement refers to both the energetic positions of the two-peak structure (the energy scale has not been adjusted betweentheory and experiment) and the relative intensities of the peaks.This may suggest that the dehydrated MoOx prepared in theSBA-15 support can be characterized structurally as tetrahedrallycoordinated dioxo MoO4 species. However, for a conclusivestructural confirmation, other molybdena types need to be con-sidered as will be discussed in the following.4.3.2. Model Clusters with Octahedral MoO6 Units. Figure 10

shows theoretical O 1s core excitation spectra for the threemolybdena�silica model clusters with octahedrally coordinatedmolybdena subunits (MoO6 species), as shown in Figure 4d�f.The left diagram (“Total”) gives total spectra resulting fromstoichiometrically weighted superpositions of partial spectra dueto excitation at the different types of 2-fold coordinated bridgingoxygen. The total spectra of the three clusters yield, in theexcitation energy range between 529 and 534 eV, a main peak at529.8 eV and a much smaller peak at 3.0�3.5 eV above the mainpeak. The peak positions and their relative intensities turn out to

be rather similar for the three clusters. This indicates, analogousto the tetrahedral MoO4 species discussed in section 4.3.1, thatthe actual geometric coupling of the MoO6 species with the silicasupport, by one, two, or four Mo�O�Si bridges in clusters d, e,and f, respectively, affects the spectral features rather little.The right diagram of Figure 10 (“Atom res.”) shows a decom-

position of the excitation spectra into contributions from differentoxygen species in clusters d�f, O(2) and O(2x) of the two typesof Mo�O�Si bridges as well as O(20) and O(200) of the twotypes of Mo�O�H bridges. Contributions from oxygen O(2#)in Si�O�Si bridges of the silica support appear only above535 eV and are not included in the figure. They explain, however,the third peak near 535 eV in the total spectra of the three clusters.A comparison of the atom resolved spectra with the correspondingtotal excitation spectra shows that for all three clusters the mainpeak at 529.8 eV is characterized as a mixture of contributions dueto oxygen core excitations both in Mo�O�Si and Mo�O�Hbridges. Here, the relative intensities of Mo�O�Si comparedwith Mo�O�H bridges increase in going from cluster d to ewhich can be understood as a simple stoichiometric effect. As aresult of the different coupling of theMoO6 species with the silicasupport, the ratio of Mo�O�Si to Mo�O�H bridges is 1:5 forcluster d and 2:4 and 4:2 for clusters e and f, respectively. Thesmaller peak near 533 eV in the total spectrum is explained by theatom decomposition as mainly due to oxygen core excitations inMo�O�H bridges characterizing the dangling OH groups ofthe hydrolyzed MoO6 species.The types of oxygen core excitations appearing in the total

spectra in the energy region between 529 and 534 eV have beenexamined by analyzing corresponding final state orbitals. Overall,the qualitative results agree with those obtained for the tetrahedralMoO4 species discussed in section 4.3.1. All final state orbitals canbe described as antibondingmixtures of majorMo 4d and smallerO 2p contributions where the amount of antibonding characterdetermines the energetic order of the excitations.Figure 10 includes also the experimental O K-edge NEXAFS

data (dashed line) for dehydrated MoxOy/SBA-15 with 11.1 wt %Mo taken from Figure 7. This spectrum with its almost sym-metric double-peak structure between 529 and 534 eV looksrather different compared with the theoretical excitation spectra

Figure 10. (left, Total) Theoretical O 1s core excitation spectra forclusters d�f containing octahedral MoO6 subunits; see Figure 4. Thespectra are compared with experimental O K-edge NEXAFS data(dashed line) for dehydrated MoxOy/SBA-15 with 11.1 wt % Mo takenfrom Figure 7. (right, Atom res.) Decomposition of the excitationspectra into contributions from different oxygen species in clustersd�f, O(2(x)) in Mo�O�Si bridges, and O(20) and O(200) in differentrepresentative Mo�O�H bridges.



of the three molybdena�silica model clusters containing octa-hedrally coordinated MoO6 species. This applies to both therelative intensities of the two peaks found in the theoretical spectraand their energetic separation which is too large in all cases. Thedisagreement suggests that the dehydrated catalytically activeMoOx prepared on the SBA-15 support may not reflect octa-hedrally coordinated MoO6 species. The discrepancy is consis-tent with the unfavorable relative Gibbs free energy of theoctahedral compared with the tetrahedral species mentionedearlier.4.3.3. Model Clusters with Pentahedral MoO5Units. Figure 11

shows theoretical O 1s core excitation spectra for the threemolybdena�silica model clusters in which the molybdenum ofthe molybdena subunits is pentahedrally coordinated (monoxoMoO5 species), as shown in Figure 4g�i. The left diagram(“Total”) gives total spectra resulting from stoichiometricallyweighted superpositions of partial spectra due to excitation atthe different types of oxygen, terminal molybdenyl, and 2-foldbridging. The right diagram (“Atom res.”) shows a decomposi-tion of the excitation spectra into contributions from differentoxygen species in clusters g�i, terminal oxygen O(1) of molyb-denyl ModO,O(2) of mixedMo—O—Si bridges, and O(2*) ofMo—O—Mo bridges (cluster i only).Obviously, the total spectra are quite different between

the three model clusters which can be explained by the ratherdifferent structure of the MoO5 units and their coupling with thesilica support in the clusters. In the smallest model, cluster g,the molybdena unit is anchored at a four-membered silicatering. This constrains the four Mo�O�Si bridges to an angleΘMo�O�Si = 119.5� which reflects an unusually strongMo�O�Si tilt. (Interestingly, cluster g has been used in theore-tical studies of catalytic methane11,47 oxidation.) Here, the totalspectrum is described by a large main peak at 531.3 eV which,from the atom decomposition of the spectrum, is characterizedby core excitations of molybdenyl oxygen O(1). The very smallside peak at lower energy is identified as being due to coreexcitations of oxygen in Mo�O�Si bridges. These excitationscontribute also smaller intensity to the main peak as well as to aminor peak near 535 eV which overlaps energetically with theregion of core excitations of oxygen in Si�O�Si bridges of thesilica support.

Inmodel cluster h, themolybdena�silica binding is also achievedby four Mo�O�Si bridges. However, these involve Si sites of aneight-membered silicate ring which allows a much less pronouncedtilt of the Mo�O�Si bridges, given by an angle ΘMo�O�Si =160.6�. This seems more reasonable compared with the tilt incluster g. (The superiority of model h over model g has also beenpointed out earlier in theoretical reaction studies.12) The totalspectrum of O 1s core excitations of cluster h is composed of twomain peaks, one at 530.9 eV with an additional low energy shoulderat 529.7 eV and one of smaller intensity at 532.5 eV. The atomdecomposition shows that both peaks are due to core excitations ofmolybdenyl oxygen, while the low energy shoulder is attributed toexcitations of oxygen in Mo�O�Si bridges. Analogous to theresults for cluster g, oxygen excitations in Mo�O�Si bridges

Figure 11. (left, Total) Theoretical O 1s core excitation spectra for clusters g�i containing pentahedral MoO4 subunits; see Figure 4. The spectra arecompared with experimental O K-edge NEXAFS data (dashed line) for dehydrated MoxOy/SBA-15 with 11.1 wt % Mo taken from Figure 7. (right,Atom res.) Decomposition of the excitation spectra into contributions from different oxygen species in clusters g�i, terminal molybdenyl oxygen O(1)and O(2) in Mo�O�Si bridges, and O(2*) in Mo�O�Mo bridges.

Figure 12. Comparison of theoretical O 1s core excitation spectra forMoOx/SiO2 clusters reflecting tetrahedral (cluster b of Figure 4),pentahedral (cluster h), and octahedral (cluster e) molybdenum co-ordination. The figure includes for comparison the correspondingtheoretical spectrum for MoO3 bulk (cluster b of Figure 3) as well asexperimental O K-edge NEXAFS data (dashed line) for dehydratedMoxOy/SBA-15 with 11.1 wt % Mo; see Figure 7.



lead also to a peak near 535 eV which overlaps with the energyregion of core excitations of oxygen in Si�O�Si bridges.Model cluster i contains two MoO5 units which couple with

each other via two Mo�O�Mo bridges and bind with the silicasupport by two Mo�O�Si bridges. This binding scheme isdifferent from those of the previous two clusters g and h andaffects the total spectrum of O core excitations greatly. Thespectrum is described by a broad peak structure in the energyrange between 529 and 534 eV. Here, the peak maximum lies at531 eV with a smaller shoulder at 532.3 eV and a broadeningasymmetry near 529 eV. A comparison with the atom resolvedspectra, see right diagram of Figure 11, shows that the peakstructure is dominated by core excitations of both molybdenyloxygen and oxygen in Mo�O�Mo bridges where the latter covera somewhat wider energy range. However, the integral contribu-tions given by the peak areas are about equal for the two oxygenspecies. Core excitations at oxygen in Mo�O�Si bridges leadalso to a peak near 535 eV, as discussed for the previous clusters.The experimental O K-edge NEXAFS data for dehydra-

tedMoxOy/SBA-15 with 11.1 wt %Mo (dashed line in Figure 11,taken from Figure 7) was shown to yield an almost symmetricdouble-peak structure between 529 and 534 eV. A comparisonwith the theoretical excitation spectra of the molybdena�silicamodel clusters containing pentahedrally coordinated MoO5

species evidence disagreement in the peak structures for all threecases. Cluster h, for which the comparison shows the smallestdisagreement, yields an additional low-energy shoulder which isnot observed. Further, the energetic separation between the twomain peaks is too small by 1 eV comparedwith experiment. Finally,the peak intensity ratio is quite different from experiment.Altogether, the agreement of the calculated spectra for thepentahedrally coordinated monooxo MoO5 species with thosemeasured for MoOx on SBA-15 support is much less convincingthan for the tetrahedrally coordinated dioxo MoO4 species.

5. CONCLUSIONS

In this study, we have evaluated theoretical oxygen 1s coreexcitation spectra of silica-supported molybdena model clustersand of clusters for MoO3 bulk using DFT cluster techniques.The results are then compared with spectra of in situ O K-edgeNEXAFSmeasurements for polycrystalline MoO3 bulk as well asfor dehydrated molybdena of different concentration onSBA-15 support. Here, MoOx/SBA-15 has been shown to becatalytically active in the oxidative dehydrogenation of propaneto propylene; see Figure 5. The experiments are typical for acatalyst precursor in the presence of oxygen and in the absence ofany coordinating ligand, such as water, fixing the highest oxida-tion state of Mo.

In the calculations, the partial core excitation spectra ofdifferently binding oxygen species, molybdenyl ModO, as wellas oxygen inMo—O—Mo,Mo—O—Si,Mo—O—H, and Si—O—Si bridges of the silica-supported molybdena species, arefound to differ considerably as to their peak shapes and energydistributions. Therefore, their spectral superposition in differentmolybdena units, tetrahedral dioxo MoO4, octahedral MoO6,and pentahedral monooxo MoO5, distinguish between the unitsin stoichiometrically well-defined material. This can be used toexplain spectral details of in situ NEXAFS measurements on theSBA-15 supported molybdena model catalysts and, thus, to drawconclusions about a possible existence of different molybdenaspecies.

Experimental O K-edge NEXAFS data for samples of clean R-MoO3 bulk exhibit a large main peak near 530.5 eV and a broaderless intense double peak structure between 533.5 and 535 eV; seeFigure 6a. This spectrum is quite nicely reproduced by that of thepresent cluster model calculations. On the other hand, O K-edgeNEXAFS spectra of dehydrated molybdena on SBA-15 supportyield, for energies between 529 and 534 eV, a clear double-peakstructure with a low energy peak at 530.2 eV and a broader peak at532.5 eV where the integral intensities of the two peaks are rathersimilar, as shown in Figure 7. These spectra are obviously quitedifferent from those obtained for MoO3 bulk, hinting at differentlocal geometricMoOx structure. The present theoretical analysis ofoxygen 1s core excitation spectra for different molybdena units invarious molybdena�silica clusters can give a rather clear assign-ment. First, spectral contributions from core excited oxygen in Si—O—Si bridges of the silica support are found to appear only above535 eV, i.e., energetically well separated from the observed double-peak structure. Second, the energy range between 529 and 534 eVis clearly attributed to oxygen core excitations where those inmolybdenyl ModO groups or Mo—O—Si, Mo—O—H, andMo—O—Mo bridges contribute. Further, molybdenyl excita-tions are found to dominate the spectral shape in most cases.

Among the computed spectra for supported tetrahedral MoO4,octahedral MoO6, and pentahedral MoO5, shown in Figures 8, 10,and 11, those for the tetrahedrally coordinated dioxoMoO4 speciesyield the best agreementwith experiment. Those for the pentahedralmonooxo species agree less well (cluster h yields an additional low-energy shoulder which is not observed, and the energetic separationbetween the two main peaks is too small by 1 eV compared withexperiment). Finally, octahedral MoO6 species cannot explain themeasured spectrum at all. This is emphasized in Figure 12 compar-ing theoretical O 1s core excitation spectra for the differentlycoordinatedmolybdenawhere, for each coordination, the computedcluster spectrum fitting best with experiment was chosen. It suggestsstrongly that the dehydrated molybdena prepared in the SBA-15support of the present study does not contain octahedrallycoordinated MoO6 units. It is assumed to contain tetrahedrallycoordinated dioxo MoO4 as the dominant species, while a penta-hedrally coordinated monooxo species seems to exist in onlysmall quantities if at all. For other catalyst preparations, therelative composition of the different molybdena species mayvary from the present result. As an example, we mention Ramanand infrared measurements on MoOx/SiO2 samples48 wherecomputational studies12 suggest the coexistence of tetra- andpentahedrally coordinated molybdena species. Theoretical andexperimental studies of other molybdena reference compounds,such as molybdates whose structures are well-known, may help toget deeper insight into the structure of supported molybdenacatalysts. They may serve as a foundation for in situ reactionstudies where the separation of active phase signatures from thoseof the support as well as the local coordination of theMo species isimportant. Studies along these lines are currently under way.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

’ACKNOWLEDGMENT

This work was partly supported by the Deutsche Forschungs-gemeinschaft (DFG) through its Collaborative Research Centre546, “Transition metal oxide aggregates”. The HZB staff is



acknowledged for continuously supporting the synchrotron basedhigh pressure electron spectroscopy experiments of the Fritz-Haber Institute at BESSY II. Furthermore, the authors thank Prof.Christian Hess, Tech. Univ. Darmstadt, for fruitful discussions.

’REFERENCES

(1) Jehng, J.-M.; Hu, H.; Gao, X.; Wachs, I. E. Catal. Today 1996,28, 335.(2) Desikan, A. N.; Zhang, W. M.; Oyama, S. T. J. Catal. 1995,

157, 740.(3) Chen, K.; Bell, A. T.; Iglesia, E. J. Phys. Chem. B 2000, 104, 1292.(4) Heracleous, E.; Machli, M.; Lemonidou, A.; Vasalos, I. A. J. Mol.

Catal. A: Chem. 2005, 232, 29.(5) Khodakov, A.; Olthof, B.; Bell, A. T.; Iglesia, E. J. Catal. 1999,

181, 205.(6) Wachs, I. E.; Weckhuysen, B. M. Appl. Catal., A 1997, 157, 67.(7) Hess, C. ChemPhysChem 2009, 10, 319.(8) Muylaert, I.; Voort, P. V. D. Phys. Chem. Chem. Phys. 2009,

11, 2826.(9) St€ohr, J.NEXAFS spectroscopy; Springer: Berlin, NewYork, 1992.(10) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed.

1999, 38, 56.(11) Chempath, S.; Bell, A. T. J. Catal. 2007, 247, 119.(12) Gregoriades, L. J.; D€obler, J.; Sauer, J. J. Phys. Chem. C 2010,

114, 2967.(13) Hermann, K.; Witko, M. In Oxide Surfaces; Woodruff, D. P.,

Ed.; The Chemical Physics of Solid Surfaces, Vol. 9; Elsevier: Amsterdam,The Netherlands, 2001; p 136.(14) Kihlborg, L. Ark. Kemi 1963, 21, 357, 443, 461. Acta Chem.

Scand. 1963, 17, 1485.(15) Sauer, J.; Hill, J.-R. Chem. Phys. Lett. 1994, 218, 333.(16) Feher, F. J.; Walzer, J. F. Inorg. Chem. 1991, 30, 1689.(17) Hammer, B.; Hansen, L. B.; Nørskov, J. K. Phys. Rev. B 1999,

59, 7413.(18) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996,

77, 3865.(19) Kutzelnigg, W.; Fleischer, U.; Schindler, M. NMR Basic Princi-

ples and Progress; Springer: Berlin, New York, 1990.(20) Nyberg, M. Ph.D. Thesis, Stockholm University, 2000.(21) Pettersson, L. G. M.; Wahlgren, U.; Gropen, O. J. Chem. Phys.

1987, 86, 2176.(22) Hermann, K.; Pettersson, L. G. M. deMon developers group,

StoBe software V. 3.1, 2011; see http://www.fhi-berlin.mpg.de/KHsoft-ware/StoBe/.(23) Triguero, L.; Pettersson, L. G. M.; Ågren, H. Phys. Rev. B 1998,

58, 8097.(24) Ågren, H.; Carravetta, V; Vahtras, O; Pettersson, L. G. M.

Chem. Phys. Lett. 1994, 222, 75.(25) Leetmaa, M.; Ljungberg, M. P.; Lyubartsev, A.; Nilsson, A.;

Pettersson, L. G. M. J. Electron Spectrosc. Relat. Phenom. 2010, 177, 135.(26) Takahashi, O.; Pettersson, L. G. M. J. Chem. Phys. 2004, 121,

10339.(27) Cavalleri, M.; Odelius, M.; Nordlund, D.; Nilsson, A.; Pettersson,

L. G. M. Phys. Chem. Chem. Phys. 2005, 7, 2854.(28) Kolczewski, C.; P€uttner, R.; Martins, M.; Schlachter, A. S.;

Snell, G.; Sant’Anna, M.; Hermann, K.; Kaindl, G. J. Chem. Phys. 2006,124, 034302.(29) Kolczewski, C.; P€uttner, R.; Plashkevych, O.; Ågren, H.;

Staemmler, V.; Martins, M.; Snell, G.; Schlachter, A. S.; Sant’Anna,M.; Kaindl, G.; Pettersson, L. G. M. J. Chem. Phys. 2001, 115, 6426.(30) Cavalleri, M.; N€aslund, L.Å.; Edwards, D. C.; Wernet, P.;

Ogasawara, H.; Myneni, S.; €Ojam€ae, L.; Odelius, M.; Nilsson, A.;Pettersson, L. G. M. J. Chem. Phys. 2006, 124, 194508.(31) Wernet, P.; Nordlund, D.; Bergmann, U.; Cavalleri, M.;

Odelius, M.; Ogasawara, H.; N€aslund, L.Å.; Hirsch, T. K.; Ojamae,L.; Glatzel, P.; Pettersson, L. G. M.; Nilsson, A. Science 2004, 304,995.

(32) €Ostr€om, H.; Ogasawara, H.; N€aslund, L. Å.; Pettersson,L. G. M.; Nilsson, A. Phys. Rev. Lett. 2006, 96, 146104.

(33) Schiros, T.; Haq, S.; Ogasawara, H.; Takahashi, O.; €Ostr€om, H.;Andersson, K.; Pettersson, L. G. M.; Hodgson, A.; Nilsson, A. Chem.Phys. Lett. 2006, 429, 415.

(34) Kolczewski, C.; Hermann, K. Surf. Sci. 2004, 552, 98.(35) Kolczewski, C.; Hermann, K. J. Chem. Phys. 2003, 118, 7599.(36) Cavalleri, M.; Hermann, K.; Guimond, S.; Romanyshyn, Y.;

Kuhlenbeck, H.; Freund, H. J. Catal. Today 2007, 124, 21.(37) Hess, C.; Hoefelmeyer, J. D.; Tilley, T. D. J. Phys. Chem. B 2004,

108, 9703.(38) Hess, C.; Wild, U.; Schl€ogl, R. Microporous Mesoporous Mater.

2006, 95, 339.(39) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky,

G. D. Nature 1998, 396, 152.(40) Vass, E. M.; H€avecker, M.; Zafeiratos, S.; Teschner, D.;

Knop-Gericke, A.; Schl€ogl, R. J. Phys.: Condens. Matter 2008, 20, 1.(41) Bluhm, H.; H€avecker, M.; Knop-Gericke, A.; Kiskinova, M.;

Schl€ogl, Salmeron, R. M. MRS Bull. 2007, 7, 602.(42) Knop-Gericke, A.; Kleimenov, E.; H€avecker, M.; Blume, R.;

Teschner, D.; Zafeiratos, S.; Schl€ogl, R.; Bukhtiyarov, V. I.; Kaichev,V. V.; Prosvirin, I. P.; Nizovskii, A. I.; Bluhm, H.; Barinov, A.; Dudin, P.;Kiskinova, M. Adv. Catal. 2008, 52, 215.

(43) H€avecker, M.; Cavalleri, M.; Herbert, R.; Follath, R.;Knop-Gericke, A.; Hess, C.; Hermann, K.; Schl€ogl, R. Phys. StatusSolidi B 2009, 246, 1459.

(44) Cavalleri, M.; Hermann, K.; Knop-Gericke, A.; H€avecker, M.;Herbert, R.; Hess, C.; Oestereich, A.; D€obler, J.; Schl€ogl, R. J. Catal.2009, 262, 215.

(45) Thielemann, J. P. PhD thesis, Tech. Univ., Darmstadt, 2011.(46) Bramnik, K. G.; Ehrenberg, H. Z. Anorg. Allg. Chem. 2004, 630,

1336.(47) Chempath, S.; Zhang, Y.; Bell, A. T. J. Phys. Chem. C 2007, 111,

1291.(48) Lee, E. L.; Wachs, I. E. J. Phys. Chem. C 2007, 111, 14410.

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