Ab Initio Assignments of FIR, MIR, and Raman Bands of Bulk Ba

12286

J. Phys. Chem. C 2009, 113, 12286–12292

Ab Initio Assignments of FIR, MIR, and Raman Bands of Bulk Ba Species Relevant in NOx Storage-Reduction Holger Hesske, Atsushi Urakawa, and Alfons Baiker* Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Ho¨nggerberg, HCI, 8093 Zurich, Switzerland ReceiVed: December 23, 2008; ReVised Manuscript ReceiVed: May 12, 2009

Vibrational characteristics of bulk Ba species relevant in NOx storage and reduction, namely, BaO, BaO2, Ba(NO3)2, and BaCO3, have been investigated by first-principle calculations and compared with experimental Far-IR, Mid-IR, and Raman spectra. IR and Raman intensities were calculated and compared to experimental data reported in literature. The study provides an unambiguous and firm assignment of experimentally observed bands for these important barium compounds and a detailed description of the corresponding vibrational modes. Introduction Barium and its compounds have found important applications ranging from medical1 to various industrial uses.1,2 Ba is one of the alkaline-earth metals found in nature as carbonate and sulfate. Among various other applications barium compounds serve in the oil and gas industries to make drilling mud and are used in the manufacturing of paint, bricks, tiles, glass, and rubber. More recently BaO, BaO2, BaCO3, Ba(NO3)2, and Ba(OH)2 have gained interests in environmental catalysis due to their role in NOx storage-reduction (NSR) catalysis.3-6 NSR utilizes unsteady-state periodic operation by switching the atmosphere over a catalyst between fuel-lean (oxygen rich) and fuel-rich conditions. NOx species react with the storage material, typically Ba or other alkali/alkali earth metal compounds, and form their nitrates during fuel-lean periods, while in the following fuelrich periods they are reduced by the assistance of precious metal such as Pt, restoring the catalyst for subsequent NOx storage in the proceeding fuel-lean period. Among in situ spectroscopic techniques, vibrational spectroscopy (in particular IR and Raman) is the common and valuable technique applied to identify participating species during NSR. IR or Raman spectral features are usually taken as characteristic fingerprints for certain bulk and surface species, and based on their existence or absence it is decided whether a chemical species participates in the investigated process. Although there are numerous studies aimed at elucidating the NSR reaction mechanism, comparison of the reported results is rather difficult.7-10 Apparent reasons are the different reaction parameters used, such as catalyst preparation, temperature, pressure, NOx gas, and reducing agents. The wide variety of these parameters renders drawing unambiguous conclusions difficult.9,11-18 Another complicating factor for spectral interpretation is that the applied sampling configuration in IR spectroscopy has a strong influence on the spectral features, for example, band appearance and positions, depending on its local sensitivity toward bulk or surface.19 Therefore, unambiguous assignments of IR and Raman bands, especially of involved Ba species, are an absolute prerequisite to gain a firm fundamental * Corresponding author. E-mail: [email protected]. Fax: +41 44 632 11 63.

understanding of the mechanism of this important catalytic reaction system. Most studies agree that surface nitrates and bulk Ba(NO3)2 are the key barium species formed during the NOx storage process, while BaCO3 and Ba(OH)2 are known and suggested to play an important role. Less clear is the existence and possible role of BaO and BaO2 under real exhaust gas conditions, where water and carbon dioxide are present, and their existence and role is still debated.6 With this background in mind, this study is aimed to provide unambiguous IR and Raman band assignments of the bulk Ba species relevant in NSR. Far-IR (FIR), Mid-IR (MIR), and Raman spectra were measured and compared with the spectral features obtained by first-principle calculations. The obtained experimental and ab initio spectra were carefully compared with clear identifications of vibrational modes. Although Ba(OH)2 and its hydrates likely play an important role during NSR, their investigation has been excluded due to an uncertainty of the crystal structure for first-principle calculations and the great difficulty in preparing well-defined Ba(OH)2 · xH2O without BaCO3 formation on the surface. Theoretical and Experimental Methods Computational Methods. All calculations were based on the density functional theory (DFT) and its implementation in the Quantum-ESPRESSO package (QE).20 Interactions between atomic cores and valence electrons were described by normconserving pseudopotentials, which are included in QE or obtained and tested with the atomic code inside QE. The Kohn-Sham orbitals were expanded in plane-waves up to an energy cutoff of 120 Ry. They were sampled in the reciprocal space using a Monkhorst-Pack grid of k-points, dense enough to ensure total energy convergence. The exchange-correlation contribution was approximated by the functional of Perdew, Burke, and Ernzerhof (PBE),21 which has been demonstrated to be able to deal with these systems,8,22-24 although this functional is known to underestimate binding energies and vibrational frequencies by about 3-4% in comparison to experimental values. To overcome this known issue, the obtained frequencies were scaled according to the calculated overestimation of the respective cell length. In the investigated systems the scaling factors are therefore 1.026, 1.015, 1.031, and 1.034 for BaO, BaO2, Ba(NO3)2, and BaCO3, respectively.

10.1021/jp811366f CCC: $40.75  2009 American Chemical Society Published on Web 06/17/2009

Ba Species Relevant in NOx Storage Reduction

J. Phys. Chem. C, Vol. 113, No. 28, 2009 12287

TABLE 1: Calculated and Measured Frequencies and Relative Intensities of the Vibrational Modes of Ba(NO3)2a external modes wavenumber [cm-1] 28 54 59 87 91 95 95 119 125 127 134 144 163 164 173 191 197 200 201 202 210 238 254 266 a

calc. intensity IR

R

internal modes expt. data IR

R

0.00 0.01

80 83b

0.10 0.01

95 0.07

114c

0.04

133b

0.17 1.00

152 0.00

0.00 0.11

171 0.04 0.00 205c

0.00 0.01 0.00 0.04 0.06

factor group E_u A_g F_u F_g A_u F_u A_u F_g F_u E_u F_g F_u F_g F_u F_u E_g F_g E_u A_u F_u A_g F_g F_u F_g

wavenumber [cm-1] 716 720 724 734 735 735 790 793 796 803 1033 1042 1048 1052 1339 1348 1366 1376 1378 1402

calc. intensity IR

R

expt. data IR

0.03 0.03 0.00 0.01

730.5 732b

b

729 0.01

733.5b

0.00 0.00

821b

1.00 0.00

815 0.03 1.00

6.00

1048b 1336 1357b 1388b 1405b

0.01 0.00 0.03 1.00

factor group

Rb

1413

F_g E_g F_u F_u F_g E_u A_u A_g F_g F_u F_u F_g A_g A_u F_u E_g F_g F_g E_u F_u

assignment in-plane bend (ν4)

out-of-plane bend (ν2)

sym. stretch (ν1)

asym. stretch (ν3)

The vibrational modes were assigned according to calculated eigenmodes. b Data from ref 43. c Data from ref 46.

The properties of the bulk systems were calculated and verified using different sizes of super cell (1 × 1 × 1, 1 × 1 × 2, 2 × 2 × 1, and 2 × 2 × 2 unit cells) to avoid artifacts from periodic images. The initial configurations were taken from crystal structures25-28 and ionic relaxations were performed until interatomic forces were less than 1 × 10-4 Ry/Å and the change in the electronic energy below 1 × 10-8 Ry. After convergence, cell relaxation calculations with a pressure threshold of 0.1 kbar were performed, which were finally used for the calculation of vibrational modes and intensities according to Lazzeri and Mauri.29 Experimental Methods. BaCO3 (Fluka, >99%), Ba(NO3)2 (Fluka, >99%), and BaO2 (Aldrich, >95%) were used as received. All compounds show clearly visible crystallites and were ground for the measurements. Additionally X-ray diffraction patterns were obtained and show an average crystallite size between 100 and 300 nm (see Supporting Information for the XRD patterns). This means that the measured spectra should resemble the single crystal behavior quiet well. Furthermore, the experimental spectra should be comparable to the calculated spectra for well-defined bulk solids. Due to its instability in air, BaO was prepared prior to the measurements (see Supporting Information for a detailed description). It could be shown that the used preparation methods yield carbonate contaminated BaO and the obtained spectra are hardly comparable to calculated data for pure BaO. The FIR spectra were measured in transmission mode on a BRUKER VERTEX 70 spectrometer equipped with a DLATGS (CsI window) detector. The powder sample was deposited on a stretched plastic sheet (Parafilm, Pechiney Plastic Packaging, Inc.), which was taken as the background. An attenuated total reflection (ATR) configurations (Harrick, MVP (TM)) using the same spectrometer was employed to obtain MIR spectra of powder samples pressed onto a ZnSe crystal used as internal reflection element (IRE). The spectra were measured with a liquid nitrogen cooled MCT detector with 45° angle of incidence

at 4 cm-1 resolution. All results are shown without wavelengthdependent penetration-depth correction. Raman spectra were recorded with an Ocean Optics QE6500 spectrometer equipped with a fiber optics probe (InPhotonics, RIP-RPB-785) and a 785 nm excitation laser. Results and Discussion Ba(NO3)2. Ba(NO3)2 is generally accepted as the main resulting component after NOx storage and there are numerous studies describing the band positions and vibrational modes for bulk1,15,30-35 and surface11,17,19,22,33,36-40 barium nitrates. In this section, detailed MIR, FIR, and Raman band assignments of Ba(NO3)2 crystal based on ab initio calculations are given taking a proper crystal symmetry into account. Ba(NO3)2 is accepted to crystallize in a cubic unit cell (a ) 8.1148 Å) and belongs to the Th6-Pa3 space group,27 although there had been intensive discussions on its crystal structure due to apparent discrepancies observed in spectroscopic and diffraction measurements.30,31,41,42 Our calculation yielded an optimized cell length of a ) 8.370 Å, which exceeded the experimental value by about 3.1%. Considering the known features of the PBE functional, this result is within the expected error range and therefore reasonable. The calculated vibrational modes, their assignments including factor group and the experimentally observed internal (intramolecular) and external (lattice) fundamental vibrational frequencies of Ba(NO3)2 are presented in Table 1. Detailed discussions about the history of Ba(NO3)2 crystal studies, the symmetry-based band assignments, and the factor group analysis are given in the Supporting Information. Figure 1 shows the experimental and calculated MIR spectra. The most prominent band in the experimental spectrum is located at 1335 cm-1, which is clearly assigned to the asymmetric stretching mode (ν3) of the nitrate ions (Figure 1b). The broadness of this band is remarkable and characterizes the nature

12288

J. Phys. Chem. C, Vol. 113, No. 28, 2009

Hesske et al.

Figure 2. Experimental (black) and calculated (gray) FIR spectra of Ba(NO3)2 and the corresponding eigenvector representations. The unit cell is displayed in the direction of the space diagonal to emphasize the symmetry of the modes. For the sake of clarity only two inverse nitrate ions are colored, while all other atoms are displayed in gray (colored according to Figure 1). Calculated bands are broadened with Gaussian function of 5 cm-1 fwhm.

Figure 1. Experimental (black) and calculated (gray) MIR spectra of Ba(NO3)2. Selected vibrational modes are visualized by their eigenvectors along the C3 axis (a-d). For the sake of clarity only two inverse nitrate ions are colored (N in blue, O in red, Ba in green), while all other atoms are displayed in gray. Calculated bands are broadened with Gaussian function of 5 cm-1 fwhm and 25 cm-1 fwhm for the asymmetric stretching band.

of the mode. Depending on the applied method, the peak maxima are shifted within 1330-1349 cm-1. For the single crystal,30 the peak maximum was obtained at 1349 cm-1, while studies on polycrystalline samples19,43 reported the maxima at 1345 or 1336 cm-1. A possible explanation for the broadness of this band was given by Brooker,31 who discussed a significant population of excited state nitrate ions above 200 K, leading to delocalization of the vibrational modes. Due to the crystal symmetry, the band for the asymmetric stretching of the nitrate ions splits up and shows an additional band at 1413/1402 cm-1 in the experiment/calculation, respectively (Figure 1a). The observed sharpness of this band compared to that at 1335 cm-1 contradicts the previous explanation and a further investigation is necessary to explain the remarkable broadening in detail. Within the framework of harmonic approximation used in the vibrational analysis, the calculation of overtone and combination bands is not possible without rough assumptions. For this reason overtone/combination band frequencies are estimated and discussed based on the simple summation of the fundamental frequencies, neglecting a possible anharmonicity. Previously, the band at 1413 cm-1 was assigned to an overtone of the in-plane bending mode (2ν4).19,44 Our ab initio calculation and the factor group analysis (Supporting Information) unambiguously show that the assignment is most likely incorrect and it appears due to the splitting of the asymmetric stretching of nitrate ions as discussed above. Instead, the band at 1462 cm-1, which is close to the doubled frequency of the in-plane bending

mode ν4 (729 cm-1), can be assigned to the previously described overtone band. Our calculation yields the frequency of 1468 cm-1 by the simple estimation and a good agreement with the experimental band position. Another experimentally prominent combination/overtone band is located at 1774 cm-1 and is widely assigned to the combination of symmetric stretching and inplane bending of the nitrate ion (ν1 + ν4). Taking the IR-forbidden symmetric stretching and the most IR intense inplane bending mode into consideration, the estimated frequency from the calculation is 1782 cm-1, which agrees well with the experiment. Interestingly, the crystal symmetry causes the symmetric stretching (ν1) of the nitrate ion to be IR allowed. However, this band was not observed experimentally, probably due to its low intensity as well as the possible band broadening. The largest frequency differences between the calculated and the experimental results are observed for the out-of-plane bending mode ν2 (Figure 1c), obtained at 803 and 815 cm-1 for calculation and experiment, respectively. A better agreement of calculated and experimental frequencies is obtained for the in-plane bending mode ν4 (Figure 1d). The first experimental FIR spectrum of Ba(NO3)2 was reported by Bloor45 in 1965. This work described lattice modes very carefully, while the assignment to experimental bands appeared rather empirical. The FIR spectra obtained experimentally and by our calculation for polycrystalline Ba(NO3)2 are displayed in Figure 2. The most prominent band was observed at 152 cm-1, which is assigned to a translation of nitrate ions (Figure 2a). Another band position, which was well reproduced by the calculation, is located at 171 cm-1 and could be assigned to a nitrate libration. Larger deviations were obtained for the intensities of the modes below 100 cm-1, which is likely caused by the difficulties to reach convergence for these modes. Several more FIR bands for Ba(NO3)2 were reported by Bon et al.,30 although the assignments are based on a different space group from that considered to be correct currently.

Ba Species Relevant in NOx Storage Reduction

Figure 3. Experimental (black) and calculated (gray) Raman spectra of Ba(NO3)2. Selected eigenvectors of Raman active modes are displayed (a-d). In the case of b and d, the unit cell is presented in the direction of the space diagonal (shown in a) to emphasize the mode symmetry. For the sake of clarity, only two inverse nitrate ions are colored, while all other atoms are displayed in gray (colored according to Figure 1). Calculated bands are broadened with Gaussian function of 5 cm-1 fwhm.

Raman spectra obtained from calculation and experiment are displayed in Figure 3. The used Raman setup limited the observable frequency to the region >200 cm-1; therefore the experimental Raman data reported by Brooker et al.43 and Couture46 are given in Table 1 for the bands below 200 cm-1. The most prominent band was the symmetric stretching of the nitrate ions (ν1). The peak position of this band was calculated and measured at 1048 cm-1 (Figure 3b). Due to the crystal symmetry (Table 1 and Supporting Information), the symmetric vibration split up and an additional band emerges at slightly lower frequency. This band was expected at 1042 cm-1 by theory, while it could be not observed experimentally as a distinct band. One reason for this apparent absence might be the very high intensity of the observed mode at 1048 cm-1, which could cover the comparable weak second band. Furthermore, the significantly larger splitting of the asymmetric stretching mode ν3 (experimentally obtained at 1357, 1388, and 1405 cm-1) could be well reproduced by the calculation (1348, 1366, and 1376 cm-1, respectively). BaCO3. BaCO3 is also assumed to play a crucial role in the NSR process due to its relatively high stability and the presence of CO2, which can readily react with BaO to form BaCO3 under reaction conditions when using hydrocarbons as reducing agents. In the bulk phase it is known that barium carbonate is more stable than nitrate and exists almost always as an impurity in Ba compounds when stored in air. The existence of different modifications of BaCO3 is known, which can lead to different spectra in IR and Raman, especially for the lattice vibrational modes. For the theoretical model the witherite structure is

J. Phys. Chem. C, Vol. 113, No. 28, 2009 12289

Figure 4. Experimental (black) and calculated (gray) MIR spectra of BaCO3 and selected eigenvector representations for IR-active modes. Barium ions are displayed in green, carbon atoms in cyan, and oxygen in red. Two carbonate ions are shown in gray for clarity. Calculated bands are broadened with Gaussian function of 5 cm-1 fwhm.

chosen, because it is the thermodynamically most stable structure at room temperature. It belongs to orthorhombic crystal symmetry (Pmcn space group) with a ) 5.313 Å, b ) 8.904 Å, and c ) 6.430 Å and four formula units per unit cell.26,47 On average the calculated cell dimensions are 3% larger, giving 5.435, 9.149, and 6.645 Å, respectively. The calculated frequencies and intensities in comparison to the experimental data are given in Table 2. A more detailed discussion about the factor group analysis of BaCO3 and its isostructural compound is given in the Supporting Information along with a description of the observed mode splitting. Although there have been numerous spectroscopic studies on the BaCO3 structure since 1947,46 the band assignments for BaCO3 are not discussed in detail.32,48-50 The experimental and calculated MIR spectra are shown in Figure 4. Similar to the previously discussed Ba(NO3)2, the most prominent band is assigned to the asymmetric stretching (ν3). In the MIR spectrum the band maximum is located at 1412 cm-1, while the band shape shows a shoulder at slightly higher frequency. This shoulder may be caused by the two different but very close bands for this vibrational mode (1404 and 1407 cm-1), which appear as one band with the applied broadening with the Gaussian function line shape (Figure 4). Another explanation for the presence of the shoulder is the occurrence of the first overtone of the in-plane bending (2ν4) as observed for Ba(NO3)2. Neglecting anharmonic contributions, the proposed position of the overtone band from calculation would yield a band at 1394-1404 cm-1, which is exactly in the range of the asymmetric stretching band. The combination band of symmetric stretching and in-plane bending (ν1 + ν4), previously discussed for Ba(NO3)2, is experimentally obtained at 1750

12290

J. Phys. Chem. C, Vol. 113, No. 28, 2009

Hesske et al.

TABLE 2: Calculated and Measured Frequencies and Relative Intensities of the Vibrational Modes of BaCO3a external modes wavenumber [cm-1] 72 77 77 78 84 95 97 138 141 150 156 162 163 168 168 171 172 173 174 195 196 204 205 208 212 217 222 226 230 231 233 249 250

calc. intensity IR

R

internal modes

expt. data IR

R

0.00 0.00 0.02

75 0.00 0.00 0.00 0.00

0.00 0.33 0.12 0.00 0.00 0.62 0.09 0.00 1.00

170 0.01

0.45 0.03 0.00 0.54 0.00 0.63 0.00 0.02 0.05 0.02 0.00

220 232

0.02

factor group A_g B_2g B_2u A_u B_1g B_3g A_g B_3g B_1u B_3u B_2g A_g A_u B_2u B_1u B_3g B_1g B_1u A_g B_3u B_2g B_1g B_3u A_u B_3g B_2u A_u B_2g B_1g B_3g B_2u B_1u A_g

assignment

wavenumber [cm-1]

Lib.Y Lib.Y

Lib.X

Lib. Z Lib. Z Lib. Z

696 697 698 698 699 702 703 707 851 852 871 872 1055 1055 1057 1058 1366 1384 1398 1404 1407 1407 1419 1499

calc. intensity IR

R

0.06

expt. data IR

R

693 0.02 0.05 0.00

687

0.08 0.00 0.00 1.00

856 0.00 0.00

0.00 1.00 0.00 0.03 0.00

1056 1059

0.03 0.01 24.7 24.8

1412 0.03 0.11 0.12

1416 1504

factor group A_u B_3u B_1g A_g B_2g B_2u B_3g B_1u B_1u A_g B_3g B_2u A_g B_3g B_1u B_2u A_u B_1g B_1u B_3u B_2u A_g B_2g B_3g

assignment in-plane bend (ν4)

out-of-plane bend (ν2)

sym. stretch (ν1)

asym. stretch (ν3)

Lib.X Lib.Y Lib. Z Lib.Y Lib.X Lib.X

a The vibrational modes were assigned according to calculated eigenmodes. Librational modes are assigned with the direction of the rotational axis.

cm-1, while the calculation would propose 1754-1759 cm-1, depending on the chosen consisting fundamental modes. A very good agreement between experiment and calculation is obtained for the symmetric stretching mode ν1 (Figure 4b). The band positions are 1059 and 1057 cm-1 for experiment and calculation, respectively. Slightly larger deviations between calculation and experiment are obtained for the out-of-plane bending mode (ν2, Figure 4c) and the in-plane bending mode (ν4, Figure 4d). The first experimental FIR spectrum for BaCO3 were published by Morandat,48 but the study focused mainly on other minerals and the assignments for the observed bands were not given. A comparison of calculated and experimentally obtained FIR spectra is shown in Figure 5. Obviously the experimental bands for external modes are much broader than the internal ones, which makes it more complicated to assign all modes properly. The most IR-intense lattice mode is assigned to the antiparallel motion of barium and carbonate ions and is observed at 173 cm-1 (Figure 5c). Several librational modes are calculated in the range of 200-250 cm-1, which contributes to the experimentally observed bands in this region. Interestingly the lowest frequency mode at 75 cm-1 is very well reproduced by the calculation and could be assigned to a cell-breathing mode (Figure 5d). Raman studies on bulk BaCO3 are focused mainly on the internal region and to the best of our knowledge no data are available for external modes. In Figure 6 the calculated and

experimentally obtained spectra for BaCO3 are displayed. The symmetry based splitting of the asymmetric stretching mode (1504 and 1416 cm-1) is very well reproduced by the calculation (1499 and 1419 cm-1, respectively). The largest deviation from experimental data is observed for the calculated in-plane bending mode ν4, which is consistent with the results of the IR calculation, where the frequency of this mode is also slightly overestimated. A much better agreement of calculation and experiment is obtained for the symmetric stretching mode ν1, which is the most Raman intense one (Figure 6b). A detailed discussion of the calculated Raman modes for the external region is difficult, due to the lack of experimental data. The calculation show several Raman-allowed modes, while the obtained intensities are rather low. The most intense modes could be assigned to librations of the carbonate ions (171, 231 cm-1). BaO. Barium oxide is often discussed as the storage material, although it is well-known for its very high chemical affinity toward water and CO2. In fact, the commercial BaO (Aldrich, 99.99%) consists of BaCO3, Ba(OH) · 8H2O, and BaO1.3 after several weeks under air in a closed container.19 In this study, BaO was prepared by two routes, namely, by the decomposition of BaO2 or BaCO3 (the details are described in the Supporting Information). The decomposition reactions leading to BaO is subject to a very fast subsequent transformation into a mixture of BaCO3 and Ba(OH)2 in air, which makes the measurement of pure BaO very difficult. This transformation behavior could

Ba Species Relevant in NOx Storage Reduction

Figure 5. Experimental (black) and calculated (gray) FIR spectra of BaCO3 and selected eigenvector representations of FIR-active modes. Barium ions are displayed in green, carbon atoms in cyan, and oxygen in red. Two carbonate ions are shown in gray for clarity. Calculated bands are broadened with Gaussian function of 5 cm-1 fwhm.

be clearly observed by FIR measurements recorded immediately after decomposition. From a theoretical point of view, vibrational modes of BaO are first-order Raman-forbidden due to its NaCl structure (a ) 5.539 Å25). It should further show only one IR active mode, which is directly assigned to the antiparallel lattice vibration of Ba and oxygen ions. The frequency of this mode is calculated to be 237 cm-1 with a unit cell length of a ) 5.683 Å (+2.6%). An experimental FIR spectrum was obtained from a freshly prepared sample. Although the air exposure of this sample was less than 30 s, the calculated and experimental spectra are not in a good agreement. Although a shoulder could be found in the experimental spectra at 240 cm-1, the most intense peak is located at 170 cm-1. As discussed above, this band should be assigned to BaCO3, which is present due to its incomplete thermal decomposition and to the extremely high reactivity of BaO with CO2 upon exposure to air. Based on the strength of the calculated transition dipole moment, it can be further concluded that the lattice vibration of BaCO3 has a much higher absorption coefficient in comparison to that of BaO. This fact leads to the apparent abundance of the observed BaCO3 in the BaO sample. The observed bands between 400 and 650 cm-1 varies considerably depending on the chosen decomposition route and the sample pretreatment. These bands are most probably assigned to the bending modes of surface adsorbed CO2 and CO32+ and to the vibrational modes of hydroxy-species, hydroxy carbonates, and adsorbed H2O. The BaO band at 878-868 cm-1, discussed previously by Pandey51 et al. and Roedel,19 et al. could not be observed for the freshly prepared BaO in this study, but it was observed for commercially

J. Phys. Chem. C, Vol. 113, No. 28, 2009 12291

Figure 6. Experimental (black) and calculated (gray) Raman spectra of BaCO3 and selected eigenvector representations for Raman-active modes. Barium ions are displayed in green, carbon atoms in cyan and oxygen in red. Two carbonate ions are shown in gray for clarity. Calculated bands are broadened with Gaussian function of 5 cm-1 fwhm.

available BaO, which is better described by BaO1.3. In agreement with Mestl et al.,14,52 these features are assigned to peroxide anions, which are most likely present in this material. BaO2. BaO2 has not been observed as crystalline species during the NSR process.33,53 However, an evidence that peroxide anions take part in the catalytic reaction is given by spectroscopic data reported by Mestl et al.52 Crystalline BaO2 shows a tetragonal symmetry with the I4/mmm space group and two formula units. The experimentally obtained cell lengths are a ) 3.8016 Å and c ) 6.7786 Å,28 while the calculation overestimates the cell lengths by about 1.5% (a ) 3.866 Å, c ) 6.878 Å). The calculated O-O distance is 1.538 Å and exceeds the experimental value (1.482 Å) by about 3.8%. The factor group analysis yields three IR-active modes and three Raman-active modes. In both cases, two modes are degenerate, hence, there will be two IR-active and two Raman-active bands. A very detailed investigation of pure BaO2 by Raman spectroscopy is given by Haller et al.54 where they clearly show that the most Raman-active mode is located at 848 cm-1. This mode is assigned to the stretching vibration of the peroxide anions, which is obtained at 852 cm-1 by calculation and at 838 cm-1 by the experiment of this study (spectrum not shown). The calculated doubly degenerate Raman-active mode at 196 cm-1 is assigned to a rotation of the peroxide ion within the crystal, showing two percent of the band intensity of the peroxide anion stretching mode. The IR-active lattice vibrations are calculated at 205 cm-1 and 231 cm-1 (doubly degenerated). The experimental spectrum

12292

J. Phys. Chem. C, Vol. 113, No. 28, 2009

(not shown) supported these results, but it also showed very broad bands and features characteristic for BaCO3 and BaO, indicating the presence of the latter in the BaO2 sample and high reactivity of BaO2 surface with the air components. Conclusions Vibrational characteristics (IR- and Raman-active modes and their intensities) of bulk barium species relevant in the NSR processes were evaluated by DFT calculations and compared with corresponding experimental FIR, MIR, and Raman spectra. With the help of the ab initio assignments the spectral features could be clearly interpreted and all modes were unambiguously ascribed to their fundamental vibrations. From the analysis it emerges that some band assignments reported in the literature are not precise and may require careful reevaluation. The study should serve as a basis for the band assignments of surface species and particle size effects. Acknowledgment. The authors thank Dr. Nobutaka Maeda and Dr. Eva Roedel for providing some spectroscopic data, Dr. Marek Maciejewski and Niels van Vegten for assistance in the thermogravimetric experiments and fruitful discussions, Wouter van Beek and SNBL for XRD-measurements at the SNBL, ESRF in Grenoble, and the Foundation Claude and Giuliana for financial support. The Swiss Center of Scientific Computing (CSCS) in Manno, Switzerland, and ETH Zurich are acknowledged for providing computational resources. Supporting Information Available: Additional details about Ba species and XRD patterns. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kresse, R.; Baudis, U.; Jaeger, P.; Riechers, H. H.; Wagner, H.; Winkler, J.; Wolf, H. U. Barium and Barium Compounds. Ullmann’s Encyclopedia of Industrial Chemistry; 7th ed.; Wiley-VCH: Weinheim, 2008; p 301. (2) Kirk-Othmer Encyclopedia of Chemical Technology; John Wiley & Sons, Inc.: New York, 2000; Vol. 3; p 342. (3) Epling, W. S.; Campbell, L. E.; Yezerets, A.; Currier, N. W.; Parks, J. E. Catal. ReV. Sci. Eng. 2004, 46, 163. (4) Liu, Z.; Woo, S. I. Catal. ReV. Sci. Eng. 2006, 48, 43. (5) Takeuchi, M.; Matsumoto, S. Top. Catal. 2004, 28, 151. (6) Roy, S.; Baiker, A. Chem. ReV. 2009, DOI: 10.1021/cr800496f. (7) Desikusumastuti, A.; Staudt, T.; Qin, Z. H.; Happel, M.; Laurin, M.; Lykhach, Y.; Shaikhutdinov, S.; Rohr, F.; Libuda, J. ChemPhysChem 2008, 9, 2191. (8) Kwak, J. H.; Kim, D. H.; Szailer, T.; Peden, C. H. F.; Szanyi, J. Catal. Lett. 2006, 111, 119. (9) Lietti, L.; Nova, I.; Forzatti, P. J. Catal. 2008, 257, 270. (10) Yamazaki, K.; Suzuki, T.; Takahashi, N.; Yokota, K.; Sugiura, M. Appl. Catal., B 2001, 30, 459. (11) Fridell, E.; Skoglundh, M.; Westerberg, B.; Johansson, S.; Smedler, G. J. Catal. 1999, 183, 196. (12) Hodjati, S.; Bernhardt, P.; Petit, C.; Pitchon, V.; Kiennemann, A. Appl. Catal., B 1998, 19, 209. (13) Lietti, L.; Forzatti, P.; Nova, I.; Tronconi, E. J. Catal. 2001, 204, 175. (14) Mestl, G.; Rosynek, M. P.; Lunsford, J. H. J. Phys. Chem. B 1998, 102, 154.

Hesske et al. (15) Prinetto, F.; Ghiotti, G.; Nova, I.; Lietti, L.; Tronconi, E.; Forzatti, P. J. Phys. Chem. B 2001, 105, 12732. (16) Symalla, M. O.; Drochner, A.; Vogel, H.; Philipp, S.; Gobel, U.; Mu¨ller, W. Top. Catal. 2007, 42-43, 199. (17) Urakawa, A.; Maeda, N.; Baiker, A. Angew. Chem., Int. Ed. 2008, 120, 9256. (18) Yi, C.-W.; Szanyi, J. J. Phys. Chem. C 2008, 113, 2134. (19) Roedel, E.; Urakawa, A.; Kureti, S.; Baiker, A. Phys. Chem. Chem. Phys. 2008, 10, 6190. (20) Giannozzi, P.; http://www.quantum-espresso.org. (21) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (22) Broqvist, P.; Gro¨nbeck, H.; Fridell, E.; Panas, I. Catal. Today 2004, 96, 71. (23) Lu, N.-X.; Fu, G.; Xu, X.; Wan, H.-L. J. Chem. Phys. 2008, 128, 034702. (24) Mei, D. H.; Ge, Q. F.; Kwak, J. H.; Kim, D. H.; Szanyi, J.; Peden, C. H. F. J. Phys. Chem. C 2008, 112, 18050. (25) Chang, Z. P.; Graham, E. K. J. Phys. Chem. Solids 1977, 38, 1355. (26) Holl, C. M.; Smyth, J. R.; Laustsen, H. M. S.; Jacobsen, S. D.; Downs, R. T. Phys. Chem. Miner. 2000, 27, 467. (27) Nowotny, H.; Heger, G. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1983, 39, 952. (28) VerNooy, P. D. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1993, 49, 433. (29) Lazzeri, M.; Mauri, F. Phys. ReV. Lett. 2003, 90, 036401. (30) Bon, A. M.; Benoit, C.; Giordano, J. Phys. Status Solidi B 1976, 78, 453. (31) Brooker, M. H. J. Solid State Chem. 1979, 28, 29. (32) Jayasooriya, U. A.; Kettle, S. F. A. Phys. ReV. B 1984, 29, 2227. (33) Mestl, G.; Rosynek, M. P.; Lunsford, J. H. J. Phys. Chem. B 1997, 101, 9321. (34) Taha, S.; Tosson, M. Thermochim. Acta 1994, 236, 217. (35) Zverev, P. G.; Jia, W.; Liu, H.; Basiev, T. T. Opt. Lett. 1995, 20, 2378. (36) Desikusumastuti, A.; Happel, M.; Dumbuya, K.; Staudt, T.; Laurin, M.; Gottfried, J. M.; Steinruck, H. P.; Libuda, J. J. Phys. Chem. C 2008, 112, 6477. (37) Desikusumastuti, A.; Staudt, T.; Gro¨nbeck, H.; Libuda, J. J. Catal. 2008, 255, 127. (38) Epling, W. S.; Parks, J. E.; Campbell, G. C.; Yezerets, A.; Currier, N. W.; Campbell, L. E. Catal. Today 2004, 96, 21. (39) Hodjati, S.; Bernhardt, P.; Petit, C.; Pitchon, V.; Kiennemann, A. Appl. Catal., B 1998, 19, 221. (40) Nova, I.; Castoldi, L.; Lietti, L.; Tronconi, E.; Forzatti, P.; Prinetto, F.; Ghiotti, G. J. Catal. 2004, 222, 377. (41) Birnstock, R. Zeit. Krist. 1967, 124, 310. (42) Brooker, M. H.; Bates, J. B. Spectrochim. Acta, Part A 1973, A29, 439. (43) Brooker, M. H.; Irish, D. E.; Boyd, G. E. J. Chem. Phys. 1970, 53, 1083. (44) Pandey, S. K.; Steiner, A.; Roesky, H.; Stalke, D. Angew. Chem., Int. Ed. 1993, 32, 596. (45) Bloor, D. Spectrochim. Acta 1965, 21, 133. (46) Couture, L. Ann. Phys. (Paris, Fr.) 1947, 2, 5. (47) DeVilliers, J. P. R. Am. Mineral. 1971, 56, 758. (48) Morandat, J.; Lorenzel, V.; Lecomte, J. J. Phys. (Paris) 1967, 28, 152. (49) White, W. B. Carbonate Minerals. In Infrared Spectra of Minerals; Farmer, V. C., Ed.; Mineralogical Society: London, 1974; Vol. 4, p 227. (50) Martens, W. N.; Rintoul, L.; Kloprogge, J. T.; Frost, R. L. Am. Mineral. 2004, 89, 352. (51) Pandey, A. N.; Upadhyaya, K. S. Phys. Status Solidi B 1976, 78, 399. (52) Mestl, G.; Rosynek, M. P.; Lunsford, J. H. J. Phys. Chem. B 1997, 101, 9329. (53) Xie, S. B.; Mestl, G.; Rosynek, M. P.; Lunsford, J. H. J. Am. Chem. Soc. 1997, 119, 10186. (54) Haller, K.; Lunsford, J. H.; Laane, J. J. Phys. Chem. 1996, 100, 551.

JP811366F