Insight into Fundamental, Overtone, and Combination IR Bands of Surface and BulkBa(NO3)2 by Ab Initio Molecular Dynamics

Holger Hesske,† Atsushi Urakawa,*,†,‡ Joost VandeVondele,§ and Alfons Baiker*,†

Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich,Honggerberg, HCI, CH-8093 Zurich, Switzerland, Institute of Chemical Research of Catalonia, ICIQ, AV.Paısos Catalans 16, E-43007 Tarragona, Spain, and Institute of Physical Chemistry, UniVersity of Zurich,Winterthurerstrasse 190, CH-8057 Zurich, Switzerland

ReceiVed: June 14, 2010; ReVised Manuscript ReceiVed: August 10, 2010

Vibrational characteristics of Ba(NO3)2, one of the key components in an important automotive catalytictechnology, NOx storage and reduction (NSR), were investigated by ab initio molecular dynamics. In particular,the fundamental, overtone, and combination bands of surface and bulk Ba(NO3)2 were calculated and comparedwith experimental infrared (IR) spectra measured by internal and diffuse reflection sampling configurations.Using the densities of characteristic internal vibrational modes, the origins of the experimental IR bands inthe regions of fundamental as well as overtone and combination vibrations were clarified. Furthermore, themolecular dynamics based vibrational analysis showed that the bands in the overtone and combination bandregion (1600-3000 cm-1), typically neglected in NSR studies, contain chemically rich information and canassist in the firm identification of surface nitrates and their adsorption configurations.

Introduction

In situ vibrational spectroscopic methods, commonly in situIR and Raman spectroscopy, are valuable tools to gain specificinformation about molecular species involved in a chemicalreaction. Based on observed bands, particular functional groups,chemical species, or intermediates present during the reactionscan be identified. In reality, band assignments are occasionallynot straightforward because of the strong dependence of bandcharacteristics, such as position (vibrational frequency), intensity,and width on the chemical environment (e.g., in vacuum,solvated, or coordinating to a surface). As a particular example,the surrounding environment of chemical species is often diverseand ill-defined when they reside at or close to a surface. In sucha case, band assignments are rather empirical, consequentlyleading to unjustified band assignments and thus ambiguousinterpretations of underlying chemistry.

Ba(NO3)2 has been subject to recent extensive studies becauseof its important function in the NOx storage and reduction (NSR)catalysis, where Ba(NO3)2 is formed after the storage of gaseousNOx under oxygen-rich conditions over Ba species existing asBaO, BaO2, Ba(OH)2, and BaCO3.1-3 A number of vibrationalspectroscopic studies have been reported to elucidate NOx

storage mechanisms by clarifying the structure of barium nitratesand surface nitrate species formed on metal oxide supportmaterials during the storage process.4-12 In spite of considerableinsight gained by recent in situ/operando vibrational spectro-scopic studies, the reported range of vibrational frequencies forbarium nitrates, particularly those formed during NSR, is broad,scattered, and often contradicting, which is partially explainedby the different local sensitivities of the applied samplingconfigurations, for example, transmission or diffuse reflectionIR method, giving rise to distinct spectral features.13 Clearly

firm band assignments of bulk and surface Ba(NO3)2 are requiredto identify the nature of bands observed during NSR.

Over the past 15 years, the positions of IR-active bands ofBa(NO3)2 have been discussed controversially in several studies.In the first IR study on Ba(NO3)2 reported by Brooker et al.14

the observed bands up to 1800 cm-1 were carefully assignedand discussed. In 1994 Taha and Toson15 assigned observedovertones and combination bands at different temperatures. Theusefulness of this overtone assignment for the identification ofNOx surface species was discussed by Hadjiivanov.16 Vibrationalfrequencies and spectra of bulk Ba(NO3)2 have been reportedusing different sampling configurations in the mid-IR range,transmission far-IR, Raman, and theoretical approaches.5,6,9-13,17-24

Most of the experimental studies employed materials containingcomponents (e.g., Ba/Al2O3) typically contained in the formula-tion of NSR catalysts (e.g., Pt-Ba/Al2O3), investigated NO/O2

or NO2 adsorption on these materials, and proposed variouspossible configurations of surface adsorbed NOx. With the helpof density functional theory (DFT) calculations, assignmentsof observed bands to different adsorbed surface species havebeen attempted.5,8,24-29

To the best of our knowledge, calculated IR band positionsand intensities originating from nitrates residing at the surfaceof Ba(NO3)2 crystal have not been reported so far, although afirm assignment of these bands is a necessary prerequisite forinterpreting nitrate bands of more complex systems. Therefore,this study aims at identifying and differentiating vibrationalcharacteristics arising from the bulk and the surface of Ba(NO3)2

by state-of-the-art ab initio molecular dynamics (MD) simula-tions. Moreover, strong focus is given to the assignment ofovertone and combination bands of surface and bulk nitratesusing ab initio MD in order to explore the potential of thesebands to gain further chemical information of surface nitratespecies and determine configurations allowing more unambigu-ous interpretation of experimental vibrational bands.

* To whom correspondence should be addressed. Fax: +41 44 632 1163. E-mail: aurakawa@iciq.es; baiker@chem.ethz.ch.

† ETH Zurich.‡ Institute of Chemical Research of Catalonia.§ University of Zurich.

J. Phys. Chem. C 2010, 114, 15042–1504815042

10.1021/jp105435h 2010 American Chemical SocietyPublished on Web 08/19/2010

Methods

Computational Details. A unit cell of Ba(NO3)2 wasconstructed in such a way that the xy-plane represents the {111}-facet, because the surface of natural and synthetic Ba(NO3)2

contains almost exclusively {111}-facets.30 The smallest unitcell that fulfills this requirement is of hexagonal symmetry (a) b ) 11.756 Å, c ) 14.332 Å, R ) � ) 90°, γ ) 60°) andcontains 108 atoms. In Figure 1, the slab system used for ourMD-based IR spectra calculation is shown composed of 1 × 1× 4 unit cells and additional 17 Å of vacuum to separate theperiodic images from each other. The surface nitrate ions werecategorized into three types (layers 1-3) to investigate differentvibrational contributions from the respective surface layers.

Recently, we have reported the fundamental vibrationalfrequencies and modes of bulk Ba(NO3)2 obtained by DFTcalculation using plane wave basis sets and vibrational analysisbased on the finite difference method (PW-FD).24 The studyshowed an excellent agreement with experiments and clarifiedsome controversy in the band assignments. However, a directuse of this approach to surface vibrational spectrum calculationis not suited because of a considerable increase of computationalcosts by the use of a slab system containing surface and avacuum space to ensure the absence of interaction betweensurfaces of periodic images. Therefore, in this study a compu-tationally efficient Gaussian and plane wave (GPW) basis setwas used to describe the Kohn-Sham orbitals as implementedin the CP2K code.31-33 In this approach, the interaction of atomiccores and valence electrons is described by pseudopotentials.The barium pseudopotential, containing 10 valence electrons,was created and tested for bulk BaO and BaO2 structures,showing a very good agreement with experimental latticeparameters (<3.5%), bond length (<1.6%), and binding energies(<1.0%). The corresponding barium basis set of double-� pluspolarization (DZVP) quality has been optimized as describedby VandeVondele and Hutter34 using six exponents. The samequality pseudopotentials and basis sets have been employed fornitrogen and oxygen. All basis sets and pseudopotentials havebeen made available with the CP2K package.35 The plane wavescutoff used for the density was 320 Ry and the exchange-correlation contributions were approximated by the functionalof Perdew, Becke, and Ernzerhof (PBE)36 and the electronicstructure was refined in a self-consistent field approach37 (wavefunction convergence threshold: 1.0 × 10-6). Ionic relaxationwas performed until the maximum gradient became less than4.5 × 10-4 hartree/bohr and the trace of the analytical stresstensor was less than 100 bar.

After geometry optimization or equilibration, vibrationalspectra were calculated by two methods, namely, finite differ-ence method within the harmonic approximation (GPW-FD) andab initio MD simulation (GPW-MD). For the MD-basedvibrational analysis, the complete slab system (Figure 1) wascalculated and the vibrations originating from the two unit cellslocated in the middle were considered as vibrations of bulkBa(NO3)2, while the top three nitrate layers in the topmost unitcell were taken into account to calculate surface vibrationalspectra. The IR spectrum of bulk Ba(NO3)2 based on the finitedifference method was calculated using a single unit cell withoutvacuum.

In this work, we aimed to identify the spectral featuresoriginating from nitrates residing at or near surface andparticularly from different configurations of the surface nitrates.MD test calculations at 600 K (close to the operating temperatureof NSR) showed spontaneous releases and relaxations of somesurface nitrates, interchanging their configurations in differentsurface layers. To identify the spectral contributions fromdifferent layers, we simulated at lower temperature (300 K)where the surface nitrates remained in the same configurationduring the course of the simulation. After an initial equilibrationof 2 ps at 300 K in a canonical ensemble (NVT, velocityrescaling) with a time step of 1 fs, the sampling was changedto a microcanonical ensemble (NVE), resulting in an averagetemperature of 315 ( 70 K. For the calculation of the vibrationalspectra, simulations of 16-20 ps have been employed.

Vibrational Analysis and Assignments. A detailed descrip-tion of the method to obtain vibrational spectra and IR intensitiesfrom ab initio MD is given by Aida and Dupuis.38 Here weintroduce some internal coordinates, which were used in thisstudy to identify the contributions from the fundamental,overtone, and combination vibrational modes. In general,autocorrelation functions (ACFs) of any dynamical variable aregiven by

Its spectral density I(ω) can be calculated by the Fouriertransform of the ACF:

Using the atomic velocity ACF (VACF) of a system thevibrational density of states (VDOS) is obtained. For theidentification and assignment of vibrational modes, internalcoordinates such as particular bond length, bond angle, anddihedral angle, characteristic to a certain vibrational mode (e.g.,bond stretching and bending in in-plane or out-of-plane) canbe used to obtain partial VDOS and identify vibrational modesappearing in the spectrum of VACF or Dipole ACF (DipACF),which is the IR spectrum. Here, we focus on the identificationof nitrate ion (NO3-, trigonal planar arrangement) vibrationalmodes, because contributions from the vibrations involvingbarium atoms are not significant in the mid-IR range we focuson in this study.

The following internal coordinates are used to identify partialvibrational contributions of the nitrate ions in the vibrationalspectrum: TACF, torsion angle of OOON; AACF, anglebetween three oxygen atoms of a nitrate ion; BACF, NO bondlengths, r(N-On) {n ) 1, 2, 3}; and SymACF, averaged NOdistance, 1/3Σrn.

Figure 1. Investigated slab system and the respective surface unit cellof Ba(NO3)2. Surface nitrates showing the same configuration areaccounted into layers and accordingly colored (layer 1, cyan, chelatingbidentate; layer 2, orange, bridging monodentate; layer 3, yellow, flat).Barium atoms are shown in black, while bulk nitrates are depicted insilver/gray.

C(t) ) ⟨x(0) · x(t)⟩

I(ω) ) 12π ∫-∞

+∞C(t) exp(-iωt)dt

IR Study of Ba(NO3)2 by Ab Initio MD J. Phys. Chem. C, Vol. 114, No. 35, 2010 15043

The torsion angle ACF (TACF) represents distortion of theplanar structure of the ion and hence contributions from thenitrate out-of-plane bending vibration. The angle (AACF)formed by the three oxygen atoms of the nitrate ion signifiesthe contributions from in-plane bending vibrations as well asfrom the asymmetric stretching mode of the ion. For a betteridentification, the latter mode is additionally described by theNO bond length ACF (BACF), where each NO bond isseparately considered. The symmetric stretching, formally IR-inactive, mode is identified by the ACF of the average bondlengths (SymACF). The Mulliken charges were stored for allatoms at every time step to rebuild local dipoles, which wereused further to calculate IR spectra from its ACF (DipACF)and Fourier transform. The calculation of maximally localizedWannier centers would be more precise for the local dipolereconstruction; however, we have taken the former methodbecause of the higher computational costs of the latter method.Our method possibly leads to an erroneous estimation of theIR intensities if the center of atomic charges differs from theactual atomic position. Furthermore, the intensities can be alsooverestimated when the short-range local dipole variationscancel out the long-range contributions as it will be shown forthe case of out-of-plane bending mode where a nitrate ionvibrates in harmony with other nitrate ions. The IR spectra (i.e.,DipACF) were calculated with the specden plug-in in the VMDsoftware.39

Results and Discussion

Fundamental Modes of Bulk Ba(NO3)2. Figure 2 shows theIR spectra obtained by the GPW approach using the finitedifference (GPW-FD) and MD methods (GPW-MD). In com-parison, the previously reported IR spectra obtained experi-mentally as well as by the PW approach (PW-FD) are alsopresented.24 At first glance, despite some notable differencesbetween the spectra, all the theoretical methods successfullyreproduced representative vibrational modes of the nitrate ionin Ba(NO3)2. The differences between GPW-FD and GPW-MD(at finite temperature) results are expected due to anharmoniceffects taken into account by the MD approach, while lackingin standard FD calculations. Summarized in Table 1, the

calculated frequencies showed a reasonable degree of deviationsfrom the experimental values. The vibrational mode with thelargest frequency differences between experiment and calcula-tions was the out-of-plane bending mode (ν2) with maximum40 cm-1 underestimation for the calculated frequency. The samemode showed also a remarkable overestimation of the intensityby GPW-MD. The former may be due to the way the localdipole has been calculated and the latter to the neglect of dipolecancellation as mentioned in the experimental section. Obviouslythere are other factors influencing the vibrational frequenciesand intensities as evidenced from the comparison between thespectra of the PW-FD and GPW-FD methods, which can becaused by several differences in the methodologies and aredifficult to be narrowed down. Also using the same type of basisset (GPW), the FD- and MD-based calculations showed notice-able shifts and differences in the vibrational frequencies andintensities.

The vibrational characteristics of the IR spectrum of bulkBa(NO3)2 obtained by GPW-MD were investigated and areshown in Figure 3 by analyzing the vibrational frequencies ofthe internal coordinates, as described in the Methods, namely,the VACF for all vibrations arising from the nitrate ions, theTACF for out-of-plane bending (ν2), the AACF for in-planebending (ν4), the SymACF for symmetric stretching (ν1), andthe BACF for asymmetric stretching (ν3). For the bendingmodes, the TACF represents the out-of-plane bending mode (ν2)exclusively, while the AACF shows major contribution to thein-plane bending (ν4) as well as a minor one to the asymmetricstretching mode (ν3). For the stretching modes, the averagedNO bond length (SymACF) represents exclusively the contribu-tion for the symmetric stretching vibration (ν1), while the singleNO bond length, BACF, shows the asymmetric stretching (ν3)as the major component and some minor contributions to the

TABLE 1: Experimental and Calculated VibrationalFrequencies [cm-1] of the Fundamental Modes of BulkBa(NO3)2

fundamentalmode exp.13 PW-FD24 GPW-FD GPW-MD (bulk)

ν4 729 734 721 700ν2 815 803 790 775ν1 1033 1039/1056 1031ν3a 1335 1339 1355 1347ν3b 1413 1402 1404 1401

Figure 3. Calculated IR spectrum (DipACF), the total VDOS (VACF),and the densities of states of the internal vibrational modes of nitrateions in Ba(NO3)2.

Figure 2. Experimental and calculated IR spectra in the region offundamental vibrations of bulk Ba(NO3)2. Internal reflection IRspectrum was taken from Roedel et al.13 and PW-FD spectrum wastaken from Hesske et al.24 The finite difference based calculations areperformed on a single unit cell and the bands are broadened by Gaussianfunctions.24

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symmetric vibration (ν1) and the in-plane bending mode (ν4).The IR-silent symmetric stretching mode ν1 could be determinedby the VACF and SymACF to exist around 1031 cm-1, whichagrees well with the calculated frequency and the experimentalRaman spectra.24 The results from finite difference vibrationalanalysis with the GPW approach showed two different frequen-cies for this, formally degenerated mode24 (Table 1). This isbecause of the neglect of crystal symmetry in our GPWcalculations, leading to a slight numerical differences andresulting in shifted frequencies and an (erroneous) IR-intensityfor the Raman-active mode (1056 cm-1). The IR-active com-ponent of this mode (1039 cm-1) showed the same magnitudeof intensity as the other, Raman-active mode. For the sake ofcompleteness and to avoid misinterpretations both frequenciesare given.

Overall, the band assignments based on the selected internalcoordinates of the nitrate ion in the bulk Ba(NO3)2 weresatisfactory and will be used to identify the nature of surface,overtone, and combination vibrational modes.

Fundamental Modes of Surface Ba(NO3)2. The spectralfeatures of surface nitrates can shed light on detailed mecha-nisms of adsorption processes and the structures, given that theirmodes are assigned unambiguously. A detailed review byHadjiivanov40 describes several nitrato, nitrito, and othernitrogen containing species on different surfaces and theirexpected or experimentally proven spectral contributions. Thestudy concluded that a firm assignment of the surface modes toa single configuration or species is difficult because of smallband shifts and a large portion of IR light absorption by thebulk, that is, minor contribution of surface species in spectra.In NSR, numerous studies have attempted this challengingassignment and reported various kinds of adsorbed NOx speciesboth experimentally and theoretically, discussing in detail theirconfigurations and vibrational characteristics.5-11,20,21,23,26-28,41

In general, surface nitrates are categorized to exist in fourdifferent configurations, namely, monodentate, bridging mono-dentate, chelating bidentate, or bridging bidentate.40 On the usedBa(NO3)2 {111}-surface, two of these configurations are present.In the topmost layer (layer 1) the nitrate ions show a chelatingbidentate binding, while layer 2 contains nitrates bound in abridging monodentate configuration (Figure 1). The nitrate ionsin the third layer (layer 3) are incorporated in plane, with the{111}-layer formed by the barium atoms and do not belong tothe above-described surface configurations. It is thereforeexpected that the spectral features of this layer are remarkablydifferent from those of layers 1 and 2 and similar to those ofthe bulk Ba(NO3)2.

The results of the MD-based vibrational analysis (GPW-MD)for the surface nitrates are displayed in Figure 4. Below thecalculated IR spectra (DipACF), the VACFs together with thecontributions from different internal vibrational modes (TACF,AACF, BACF, and SymACF) are shown for different sets ofnitrates. The two top panels represent the spectra obtained fromthe surface layers 1 (Figure 4a) and 2 (Figure 4b), while thethird panel (Figure 4c) displays the combined spectra of all threesurface layers. For comparison, the bottom panel (Figure 4d)shows the vibrational spectra obtained from the whole surfaceunit cell. Three significant differences between surface andbulklike nitrates were revealed.

The most obvious one arises from the ν4 mode in the topmostnitrate layer (layer 1), showing a significant red-shift (about 150cm-1) of the ν4 band. A similar trend of the band shiftaccompanying a broadening was observed for layer 2, but theband shift with respect to the position of the bulk nitrate (Figure

3) was minor (30 cm-1). This observed ν4 mode red-shift is inagreement with calculated band positions of adsorbed nitrateson a BaO surface reported by Broqvist et al.26 Generally,experimental observations of the shifted ν4 band have not beenreported in literature because of different equipment restrictionsof in situ IR measurements (detector and optical components)in this frequency range. Still, the Raman activity of the ν4 modesuggests that the surface nitrates and their configurations maybe identified by Raman spectroscopy.

The second difference concerns the asymmetric stretching mode(ν3), exhibiting different splitting patterns and red-shifts for thesurface layers (Figure 4a,b) compared to the bulklike spectra ofthe whole unit cell (Figure 4d) or the bulk spectra (Figure 3). Theshift and splitting of the ν3 bands and their assignment have beendiscussed in detail by several authors.6,10,12,22,24,26 In general, allstudies including the data presented here reported the lower-frequency ν3 band between 1240-1300 cm-1, while the positionof the higher-frequency ν3 band is reported in a broader rangeof 1480-1650 cm-1 and one study reported even at 1448cm-1.4,6,26 It is interesting to note that the band splitting wasnot observed for the nitrate in layer 1 (chelating bidentate) butobserved in layer 2 (bridging monodentate). When the threesurface layers are considered altogether, the splitting becomesmore evident (Figure 4c), showing two broad bands at 1200-1300and 1300-1450 cm-1. The vibrational frequency of the latterband was underestimated compared to literature.4,26 This maybe due to the different underlying substrate used here (bariumnitrates) compared to literature.6,26 For example, Kwak et al.,6

who studied vibrational features of nitrates formed on BaO/Al2O3 and assigned the split mode to features between 1234-1320and 1574-1620 cm-1, reported considerably higher frequencies.

The third difference is the IR activity of the surface nitratesymmetric stretching mode at 1030 cm-1, which is IR-silent inthe bulk. Although the calculated intensity of this band is verysmall, the presence was confirmed in the surface spectra (Figure4a,b). This formally IR-silent band has been observed experi-mentally at this position.6,26 A firm identification of the bandby IR spectroscopy in the actual NSR system is highlydemanding because of the possible presence of surface adsorbedNO2 and nitrito ions, which also shows bands in this region.Raman spectroscopy is more suited for the identification of thevibration because of its high Raman activity; however, no

Figure 4. Calculated IR spectra (DipACF), VDOSs of total vibrations(VACF) and internal vibrations of nitrate ions at the different layers:(a) layer 1, (b) layer 2, (c) all nitrates in the surface layers (layers 1-3),and (d) all nitrates in the simulated cell (Figure 1).

IR Study of Ba(NO3)2 by Ab Initio MD J. Phys. Chem. C, Vol. 114, No. 35, 2010 15045

considerable gain about its surface adsorption state is expectedfrom the subtle differences between the nitrate layers (Figure4).

The MD approach of this study clarified several importantvibrational characteristics of surface nitrates such as bandpositions and splitting depending on the adsorption configura-tions. It is, however, challenging to use solely this informationfor unambiguous identification of adsorption geometry of nitratesbecause of overlaps between the bands of nitrates in variousconfigurations as well as other nitrogen oxides species onsurfaces in actual systems.

Overtones and Combination Bands of Bulk Ba(NO3)2. Oneway to enhance the information about the types of surface nitratespecies and their structural configurations is to utilize overtoneand combination bands which appear above 1600 cm-1 for NOx

species. The usefulness of such an approach has been discussedalready by Hadjiivanov for the adsorption of NOx species onZrO2.16

Typically the bands in this region are not discussed in detailbecause of uncertainties and the difficulties in verifyingcontributing fundamental modes. Theoretical approaches toestimate the position of overtones and combination bands aswell as their intensities based on harmonic approximation areerror-prone and not widely applied. On the other hand, ab initioMD simulations intrinsically contain all necessary informationto calculate band positions from the accurate description of theanharmonicity of vibrations.38 Furthermore, the intensities ofthese bands can be calculated similar to the fundamentalvibrations by applying the DipACF, and it is straightforwardto identify the nature of vibrations by analyzing internalvibrational modes in the same way as shown in Figure 3.

Experimental IR spectra measured in diffuse reflection (moresurface sensitive) and internal reflection (more bulk sensitive)configurations13 are shown in Figure 5 in comparison to thecalculated IR spectrum of bulk Ba(NO3)2. It should be notedthat the diffuse reflection sampling configuration enhances therelative band intensities in this frequency region (in other words,using the internal reflection configuration the bands in this regionare minor compared to the fundamental bands13) and, therefore,all spectra shown in Figure 5 are roughly scaled using theintensity of the band at about 1770 cm-1.

The calculated spectrum reproduced all four major bandsobserved experimentally at 1750-1770, 2000-2150, 2350-2500,

and 2600-2900 cm-1, which are typically assigned to ν1 + ν4,2ν1, ν1 + ν3, and 2ν3 combination/overtone vibrational modes,respectively.15 A very good agreement was found between thecalculated spectral features and the spectrum obtained by internalreflection configuration, while significant differences wereobserved in comparison to diffuse reflection configurationspectra. This is reasonable considering the local sensitivity ofthe two experimental sampling configurations as mentionedabove; the former configuration is more bulk sensitive than thelatter and the calculated spectrum was for bulk Ba(NO3)2.

Similar to the assignment of the fundamental vibrationalmodes using the population density of internal vibrations (Figure3), the overtone and combination bands of bulk Ba(NO3)2 above1600 cm-1 can be firmly assigned (Figure 6). The band at1750-1770 cm-1 shows contributions from SymACF andAACF, characteristic for symmetric stretching (ν1) and in-planebending (ν4) vibrations, respectively. BACF, characteristic forthe asymmetric stretching (ν3) vibration appears at the sameposition; however, this is due to the imperfectness of the internalvibrational mode (BACF) to describe exclusively the ν3 vibra-tion. BACF represents simple changes in the N-O bond lengths,and, consequently, there is a contribution of symmetric vibrationin BACF. The assignments of ν1 + ν4 without a ν3 contributionwas confirmed from the sum of consisting vibrational frequen-cies (i.e., ν3 plus ν1 and ν4 exceeds the observed frequency).

The bands at 2000-2150, 2350-2500, and 2600-2900 cm-1

can be firmly assigned to 2ν1 (SymACF), ν1 + ν3 (SymACF +BACF), and 2ν3 (BACF), respectively, in accordance with thecommon band assignation.15 Minor apparent vibrational statesfrom other modes are present, but their actual contributions canbe discarded based on the simple criteria by taking the sum ofconsisting vibrations like in the case of ν1 + ν4 to discard ν3.Notably, there is a contribution of TACF close to 2ν1, whichcan be assigned to the 3ν2 overtone vibration; however, it isclear from Figure 6 that there is no contribution of the 3ν2

overtone vibration to the IR spectrum (DipACF).Overtones and Combination Bands of Surface Ba(NO3)2.

The assignments of the overtone and combination IR bands ofbulk Ba(NO3)2 generally used in literature were confirmed bythe analysis of internal vibrational modes and their populationdensities obtained by ab initio MD. Interestingly, some remark-able differences in this frequency region have been reported byinternal and diffuse reflection sampling configurations,13 hintingthat additional useful information to distinguish the configura-

Figure 5. Experimental IR spectra of Ba(NO3)2 powder measured indiffuse and internal reflection configurations13 for the overtone andcombination band region and calculated IR spectrum (DipACF) of bulkBa(NO3)2.

Figure 6. IR spectrum (DipACF) and VDOS (VACF) of the overtoneand combination vibrational modes of bulk Ba(NO3)2. The contributionsfrom the selected internal vibrations are shown by colored bars.

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tions of nitrate ions at the surface of Ba(NO3)2 may be gainedfrom the spectral features in this frequency region. Here weinvestigate in detail the calculated spectral features and bandassignments of the surface Ba(NO3)2.

Figure 7 shows the calculated vibrational spectra from GPW-MD for depth-dependent surface nitrate layers in the same wayas for the region of fundamental vibrations (Figure 4). Severalstriking differences were revealed between different nitratelayers and in comparison with bulk nitrates (Figure 6). Inparticular, the absence (or a very minor presence) of ν1 + ν4

and 2ν1 bands for layer 1 nitrates at about 1750 and 2100 cm-1

and the presence of ν1 + ν3 and 2ν3 bands (2300 and 2550cm-1) are remarkable compared to the bands of bulk nitrates(Figure 6). It should be noted that for both ν1 + ν3 and 2ν3

modes considerable contributions from ν1 and ν3 are present,and the assignments were given based on the sum of composingfundamental frequencies (Figure 4). The unique spectral featureof the bridging bidentate nitrates in layer 1 is understood fromthe spectral characteristics of the fundamental modes of layer1, showing the large shift in ν4 and consequently leading to agreat red shift in the ν1 + ν4 combination band.

On the other hand, the surface nitrates in bridging monoden-tate configuration in layer 2 show the major four bands asobserved for bulk nitrates (Figure 5) but with considerable red-shifts of about 50-100 cm-1, at about 1684 (ν1 + ν4), 1950(2ν1), 2320 (ν1 + ν3), and 2665 (2ν3) cm-1 (Figure 7).Furthermore, the vibrational characteristic of the nitrates in layer3 can be concluded to resemble those of bulk nitrates as clearlyconfirmed by the spectral features of bulk Ba(NO3)2 (Figure 6or Figure 7d because the most nitrates in the cell can beconsidered as bulk nitrates) present in the averaged spectrumof layers 1-3 in Figure 7.

The large differences calculated between layer 1, layer 2, andbulk strongly suggest a potential to use the spectral informationof the overtone/combination bands for identification of adsorp-tion modes of surface nitrates. In general, no attention to theovertone/combination band region has been paid in NSR studieswhere the roles of surface nitrate species have been intensivelydiscussed. Therefore, here we reevaluate the IR spectrumobtained during NSR in this frequency region, which wasobtained by the more surface-sensitive diffuse reflection sam-pling configuration and previously reported for fundamentalmodes up to 1800 cm-1.7

Figure 8 shows the in situ IR spectrum measured at the endof the lean phase (NO + O2 atmosphere, the moment whennitrates were observed to the maximum extent during the NSRcycle) over a Pt-Ba/Al2O3 catalyst at 623 K.7 The experimentalspectrum showed remarkable differences to the IR spectrum ofbulk Ba(NO3)2 (Figure 5). First, the ν1 + ν4 band is split andtwo clear bands were confirmed at about 1750 cm-1. This featureis well-reproduced by the surface spectrum obtained from layers1-3 (Figure 8), which clearly shows both contributions fromthe surface (layers 1 and 2) and the bulklike (layer 3) nitrates.The lower frequency band at 1684 cm-1 originates from thebridging monodentate nitrates in layer 2 (Figure 7), while thehigher one at 1747 cm-1 from the bulk nitrates (Figure 5).Second, the 2ν1 band is red-shifted and observed at 2026 cm-1.This band position is also reproduced from the calculatedspectrum of layer 2 showing the band at about 1950 cm-1. Theband at 2311 cm-1 likely arises from the nitrates in layers 1and 2, and the band at 2374 cm-1 from the bulk ones as indicatedin the calculated spectra shown in Figure 7. There is a clearband observed at 2490 cm-1, which fits well to the position ofthe red-shifted 2ν3 band from layer 1. The shoulder band at2690 cm-1 and the band at 2770 cm-1 are 2ν3 bands from thenitrates in layer 2 and the bulk, respectively, according to Figure7.

The agreement between the experimental diffuse reflectionIR spectrum and the calculated surface is striking. Thisagreement as well as the large differences between the diffusereflection IR spectra obtained for bulk Ba(NO3)2 (Figure 5) andBa(NO3)2 formed during NSR (Figure 8) can be explained bythe difference in the particle sizes; the crystal size of the formerbeing >50 µm, while that of the latter was 65 nm.7,13 The surface

Figure 7. Calculated IR spectrua (DipACF) and the VDOS (VACF)for overtone and combination modes obtained for (a) layer 1 nitrates,(b) layer 2 nitrates, (c) all nitrates in the surface layers (layers 1-3),and (d) all nitrates in the simulated cell (Figure 1). The contributionsfrom internal variables are shown by colored bars.

Figure 8. Experimental diffuse reflection IR spectrum recorded at theend of the lean phase (NO + O2 in He) at 623 K during NSR7 andcalculated IR spectrum (DipACF) from the nitrate in layers 1-3. Thelast spectrum of the rich phase (H2 in He) was taken as the backgroundof the experimental spectrum.

IR Study of Ba(NO3)2 by Ab Initio MD J. Phys. Chem. C, Vol. 114, No. 35, 2010 15047

area of Ba(NO3)2 increases with decreasing particle size andthe surface contribution to the IR spectrum becomes moredominant.

In conclusion, we can state that the region of the overtoneand combination vibrations of nitrates contains rich and practicalinformation for the identification of surface nitrate configurationsbecause of clear separation from the bands originating from bulkBa(NO3)2. The presence of bands at 2026/2490 and 1731/2690cm-1 is a strong indication of bridging bidentate and bridgingmonodentate nitrates on the surface, respectively, while thebands at 1764/2374/2770 cm-1 are indicative for bulk Ba(NO3)2.In combination with the information gained for fundamentalvibrational modes, these can serve as criteria for the bandassignments of surface and bulk nitrates with a great awarenessand care for higher complexity of actual surfaces because ofthe presence of various surface species and defect structures.

Conclusions

Fundamental, overtone, and combination vibrational modesand their IR intensities of nitrate ions in the bulk and surfaceof Ba(NO3)2 were investigated by means of ab initio MD. Firmband assignments could be given by the partial vibrationaldensities of states for selected internal vibrational modes,confirming widely used assignments of IR bands for funda-mental, overtone, and combination vibrations of bulk Ba(NO3)2.New insight was gained for vibrations of nitrates on a surface,specifically for those adsorbed as bridging monodentate andbridging bidentate configurations. The calculated and experi-mental IR spectra strongly suggest that the information containedin the region of overtone and combination bands, typicallyneglected in the vibrational analysis, can provide clearer hintsfor proper identification of surface nitrate bands and configura-tions compared to the information extracted from fundamentalvibrations. This study shows the potential of extended vibrationalanalysis in the overtone/combination band region for morerigorous assignments of involved surface nitrate species duringNSR.

Acknowledgment. The Swiss National SupercomputingCentre (CSCS) in Manno, Switzerland, and the ETH Zurichare acknowledged for computational resources. Financial supportby the Foundation Claude and Giuliana is kindly acknowledged.We thank Dr. Nobutaka Maeda for providing us spectroscopicdata and Dr. Florian Schiffmann for fruitful discussions.

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