Oligomerization of Supported Vanadia: Structural Insight Using

Article pubs.acs.org/JPCC

Oligomerization of Supported Vanadia: Structural Insight Using Surface-Science Models with Chemical Complexity Philipp S. Waleska and Christian Hess* Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt, Alarich-Weiss-Strasse 8, 64287 Darmstadt, Germany S Supporting Information *

ABSTRACT: In light of the ongoing debate on the structure of supported vanadia, we report on a spectral marker enabling the direct identification of oligomeric surface structures. A series of VOx/SiO2/Si(100) planar samples with chemical complexity was synthesized by spin-coating and investigated in detail by UV resonance Raman spectroscopy at 256.7 nm excitation as well as by X-ray photoelectron spectroscopy. The enhanced Raman sensitivity allows vibrational spectra to be recorded as a function of vanadium loading (0 ≤ LV ≤ 20.2 V nm−2) despite the small surface area of the planar model samples. At low loadings (LV < 7.3 V nm−2) the spectra are dominated by dispersed vanadia species, whereas at higher loadings the presence of crystalline V2O5 is also observed. We identify new spectral features at 492, 562, and 676 cm−1, which are attributed to V−O−V-related modes of oligomeric vanadia surface species. The vanadia surface species show a linear increase with vanadium loading, saturating at a loading of LV = 7.3 V nm−2, at which V2O5 crystallite formation is observed to increase significantly. Our spectroscopic results are consistent with a growth model that includes oligomerization of vanadia surface species to increase the packing density, thereby reducing the number of V−O−Si linkages to the support.

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INTRODUCTION Silica-supported vanadia systems constitute an important class of heterogeneous catalysts. They are applicable for a number of selective oxidation and reduction reactions, such as the oxidative dehydrogenation (ODH) of propane to propene, the partial oxidation of ethanol to acetaldehyde, and the selective catalytic reduction (SCR) of nitrogen oxides.1−12 Profound knowledge about the structure of the surface vanadia species is essential to understanding the catalytic operation in detail. In light of the ongoing debate on the structure of supported vanadia species, our work has been motivated by the search for a distinctive spectral feature allowing differentiation between monomeric and oligomeric surface structures. For example, a marker for oligomeric vanadia surface species would be the direct spectroscopic detection of V−O−V vibrational modes. Such markers could enable the identification of specific reaction pathways. For example, two different reaction mechanisms for the selective oxidation of ethanol were postulated by Kilos et al. and Beck et al.13,14 The first mechanism, which was discussed in the context of aluminasupported vanadia catalysts, involves the breaking of a V−O−V bond to enable the adsorption of ethanol on vanadia dimers. In contrast, the second mechanism, which was postulated for vanadia on various support materials (Al2O3, CeO2, TiO2, ZrO2), includes the breaking of the interphase (V−O−M, M = © XXXX American Chemical Society

Al, Ce, Ti, Zr) bond as a crucial step for ethanol adsorption. Thus, the direct spectroscopic identification of vibrational modes related to V−O−V bonding would open up the possibility of investigating both mechanisms. As an important first step toward this goal, VOx/SiO2/ Si(100) model catalysts were analyzed in the work reported here regarding their vanadia surface structure using deep UV Raman spectroscopy. The characteristics of such planar model systems were highlighted previously by Thüne and Niemantsverdriet.15 Chemical complexity is inherent to these systems owing to their preparation by controlled spin-coating impregnation, mimicking pore-volume impregnation typically used for powder catalysts. On the other hand, while the properties resemble those of their real-life counterparts, they possess a lower structural inhomogeneity, as the vanadia is deposited onto a flat substrate rather than a porous, spongelike support with high surface area. Such an approach is beneficial when studying specific structural aspects, as in this work. While we are not aware of any Raman studies on such planar vanadia systems, Mojet et al. investigated similar model systems for dispersed molybdena using UV Raman spectroscopy.16 Received: February 18, 2016 Revised: July 28, 2016

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DOI: 10.1021/acs.jpcc.6b01672 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Preparation of VOx/SiO2/Si(100) surface-science models by spin-coating impregnation.

Table 1. Characteristics of the Prepared Samplesa SiO2/Si(100) 0.5VOx/SiO2/Si(100) 1.2VOx/SiO2/Si(100) 2.9VOx/SiO2/Si(100) 4.9VOx/SiO2/Si(100) 7.3aVOx/SiO2/Si(100) 12VOx/SiO2/Si(100) 20VOx/SiO2/Si(100) 7.3bVOx/SiO2/Si(100)c a b

precursor concn [mmol·L−1]

vanadia loading LV [V nm−2]

V:Si

± ± ± ± ± ± ± ±

0 0.47 ± 0.02 1.16 ± 0.05 2.9 ± 0.1 4.9 ± 0.2 7.3 ± 0.3 12.1 ± 0.5 20.2 ± 0.8 7.3 ± 0.4

0 0.01 ± 0.02 0.04b 0.07b 0.1b 0.126 ± 0.001 0.14b 0.13b 0.09 ± 0.02

0.17 0.43 1.07 1.78 2.66 4.44 7.40 2.66

0.004 0.004 0.01 0.02 0.02 0.03 0.05 0.05

Vanadia loadings were calculated according to the model of van Hardeveld et al.25 Atomic ratios were determined on the basis of XPS results. Uncertainty n.a. cSample 7.3bVOx/SiO2/Si(100) was resynthesized to examine the reproducibility and does not belong to the dilution series.

Briefly, we employed 9 × 9 mm sized Si(100) wafer pieces, which were initially cut and cleaned. Subsequently, calcination was carried out at 800 °C for 12 h to form a 120 nm thick SiO2 layer. The layer thickness was quantified by ellipsometry. After the samples had been rehydroxylated in boiling water for 1 h, they were spin-coated using the approach of van Hardeveld et al.25 To this end, 40 μL of a precursor solution of 2-propanol (anhydrous, Sigma-Aldrich) and vanadium isopropoxide (Sigma-Aldrich) was injected onto a wafer piece followed by sample rotation at 2800 rpm in a nitrogen atmosphere for 3.2 s. The vanadia loadings of the model catalysts were adjusted by diluting the concentration of vanadium isopropoxide in the solution. Finally, the samples were calcined at 550 °C for 2 h. During this process, the overall concentration of the precursor solution is low and mainly dispersed vanadia surface species are formed by reaction with the hydroxyl groups of the silica surface. A layer growth in the sense of a PVD, CVD (physical, chemical vapor deposition), or ALD (atomic layer deposition) process is therefore not expected. As a reference compound, crystalline V2O5 (99.5%, Sigma-Aldrich) was employed. The characteristics of the prepared samples are summarized in Table 1. UV Raman Spectroscopy. For Raman experiments we employed a wavelength variable in situ Raman spectroscopic setup including a tunable Ti:sapphire solid state laser (Indigo Coherent) as excitation source. The laser wavelength is converted into the UV range by frequency tripling with anisotropic BBO (BaB2O4) and LBO (LiB3O5) crystals. For dispersion and detection of the Raman scattered light, a triplestage spectrometer (Princeton Instruments, TriVista 555) with an attached CCD (charge-coupled device) camera (Princeton Instruments, Spec10:2kBUV) is employed. The spectral resolution of the spectrometer is ∼1 cm−1. A more detailed description of the Raman setup can be found in ref 26. To improve the sensitivity of the UV Raman setup, a new mirror system was designed, which is based on 90° off-axis parabolic mirrors (Thorlabs) as shown in Figure 2. A spherical mirror (Edmund Optics) focuses the laser radiation through a

There are only a few techniques that provide sufficient sensitivity to obtain information on vibrational modes of surface species on planar model systems but at the same time allow in situ experiments at elevated temperatures in air. Regarding Raman spectroscopic techniques, these include resonance Raman (RR) spectroscopy, surface-enhanced Raman spectroscopy (SERS), and tip-enhanced Raman spectroscopy (TERS). SERS provides large enhancement (∼106) in Raman scattering for species located at roughened metal surfaces or neighbored by nanometer-scale metal particles.17,18 A variation of SERS is TERS, where enhancement arises from a metallic scanning probe microscopy tip rather than from the substrate. Whereas SERS is not applicable to this study as all samples are metal-free, TERS requires specific instrumentation. Besides these methods, sum frequency generation vibrational spectroscopy (SFG-VS) and infrared reflection−absorption spectroscopy (IRAS) are alternative techniques for the investigation of model catalysts. IRAS is only suitable on reflecting metal surfaces.17,19 SFG-VS is a surface-specific technique based on a second-order nonlinear process forbidden in centrosymmetric media. Comparing the detection limits, resonance Raman spectroscopy has a slightly higher sensitivity in the low-frequency region than SFG-VS.17 Planar model systems such as the VOx/SiO2/Si(100) samples are highly challenging for Raman spectroscopic investigations owing to their small surface area. Therefore, we chose an excitation wavelength within the range 200−260 nm to stimulate the oxygen → vanadium charge transfer band of the vanadia surface species.20−22 This allowed us to exploit the selective resonance enhancement of the vanadia Raman bands and ensured sufficient sensitivity to resolve changes in the vibrational signature during the measurements.17,23

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EXPERIMENTAL SECTION Sample Preparation. A series of VOx/SiO2/Si(100) samples was prepared according to the synthesis procedure of Hess and Schlögl shown schematically in Figure 1.24 B


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Raman spectra were analyzed using least-squares fitting (Origin 8) based on Lorentzian functions. Prior to the fitting analysis, background correction and smoothing (SMA, simple moving average) were performed using Matlab R2010a. In the right panel of Figure 5, Raman peaks owing to crystalline V2O5 were removed on the basis of fitted peaks. The extent of subtraction applied corresponds to the subtraction necessary to fully remove the intensity of the V2O5-related band at 990 cm−1. X-ray Photoelectron Spectroscopy (XPS). To determine the surface composition of the samples, XP spectra were taken on an SSX 100 ESCA spectrometer (Surface Science Laboratories Inc.) under ultrahigh vacuum conditions. The spectrometer is equipped with a monochromatic Al Kα X-ray source. The radiation is focused onto the sample surface to yield a spot size of 0.25 × 1.0 mm2. The use of flat substrates strongly reduces charging as compared to powder samples. To further minimize charging effects, an electron flood gun was employed at 0.5 eV. All XP spectra were recorded at room temperature with a step size of 0.5 eV. Binding energies were corrected by shifting the main component of the C 1s peak to 285 eV. Data analysis included subtraction of a Shirley background.

Figure 2. Scheme of the UV Raman experiment representing the optical beam path of the newly designed mirror system.

hole in parabolic mirror B onto the sample. Using this design it was possible to realize a confocal setup with only three optical components. A UV Raman setup based on one 90° off-axis elliptical mirror was reported previously by the group of Peter Stair.17 The samples were measured in a reaction cell (Harrick Scientific). For the reported spectra the cell was not sealed to avoid disrupting signals from the sapphire window. To establish the dehydrated state of vanadia, all samples, including the V2O5 reference, were heated to 220 °C and kept at this temperature during the Raman analysis. All essential spectroscopic parameters are listed in Table 2. The samples were moved continuously during the measurements using an xyz-stage to avoid laser-induced heating.

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RESULTS AND DISCUSSION X-ray Photoelectron Spectra. Based on the XPS spectra, the surface composition of the VOx/SiO2/Si(100) samples was quantified as a function of vanadium loading. The corresponding V:Si ratios are summarized in Table 1. Figure 3 depicts the V:Si ratio as a function of vanadium loading. At low loadings, the ratio increases linearly, as indicated by the black line. Starting at vanadium loadings of about LV = 2.9 V nm−2, an increasing deviation from linearity is observed. This is illustrated by the red line, which was calculated with a logarithmic regression function. A very similar behavior was observed previously and explained by the formation of vanadia (micro)crystallites starting at vanadium loadings of about 2−3 V nm−2, causing a decrease in the V 2p3/2 emission.24 The XP spectra also provide information about the silica layer of the wafer series. All Si 2p signals are located at binding energies of about 103 eV, characteristic for Si atoms in an

Table 2. Summary of Essential Raman Spectroscopic Parameters Used for the Experiments laser wavelength [nm] acquisition time [min] laser power [mW] spot size [mm2] sample temperature [°C]

256.7 255 9 0.6 220

Figure 3. Dependence of XPS V:Si ratio on vanadium loading. Lines are included to guide the eye. The black line indicates the linear progression of the V:Si ratio, while the red line illustrates the deviation from the linear behavior at higher loadings. C


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Figure 4. Raman spectra (256.7 nm) of VOx/SiO2/Si(100) wafer samples taken at 220 °C. Spectra are offset and normalized to the Raman feature at 512 cm−1. The peak at 1359 cm−1 is due to boron nitride used for the adjustment of the spectrometer.39,40 Marked peaks are spectroscopic artifacts from cosmic radiation.41 Inset (a) illustrates the fitting analysis for the sample with LV = 7.3 V nm−2. Inset (b) gives an enlarged view of the overtone region.

Figure 5. (a) Raman spectra (256.7 nm) of the two highest-loaded samples (12, 20 V nm−2) in comparison to V2O5. The spectra of the loaded samples were normalized to the Raman feature at 512 cm−1, while the intensity of the V2O5 spectrum was adapted based on the peak at 990 cm−1. Spectra are offset for clarity. The inset gives an enlarged view of the overtone region. (b) Difference spectra of the low-frequency region of the spectra after elimination of the V2O5 contribution. The inset illustrates the fitting analysis for the sample with LV = 7.3 V nm−2.

amorphous SiO2 network.27 In particular, no signal for crystalline silicon at 99.3 eV could be identified. Thus, we conclude that the SiO2 layer completely covers the sample surface within the accuracy of the analysis (spot size: 0.25 × 1.0 mm2). UV Raman Spectra. Figure 4 shows UV Raman spectra of the VOx/SiO2/Si(100) wafer samples at 256.7 nm excitation. All Raman spectra were offset and normalized to the peak at 512 cm−1. Non-vanadia-related features are observed at 512, 940, 1557, and 2334 cm−1. The two sharp features at 1557 and 2334 cm−1 originate from gas-phase molecular oxygen and

nitrogen, respectively.28,29 For the weakly loaded samples, the oxygen feature also exhibits some rotational fine structure.29 Besides, substrate features due to crystalline silicon are detected at 512 and ∼940 cm−1.16,30,31 In particular, the peak at 512 cm−1, which is slightly red-shifted due to the sample temperature, corresponds to a Si−Si stretching mode, while the broad band at ∼940 cm−1 is assigned to the Si overtone region.32 Within the region 400−900 cm−1, the three lowest-loaded samples exhibit a very similar spectral behavior characterized by a shoulder at 478 cm−1 and two peaks at 606 and 810 cm−1. All D


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Table 3. Literature Overview of Reported Positions (cm−1) for V−O−V Stretching Vibrations and Deformation Vibrations of the Vanadyl Group for Crystalline Vanadia Compounds ν(V−O−V)

structure

2-fold-coord oxygen

not specified α-V2O5

orthorhombic, D2h

(s)

β-V2O5 NH4VO3

monoclinic, C2h metavanadate

(s) (s)

KVO3

metavanadate

(s)

α-NaVO3

metavanadate

(s)

β-NaVO3

metavanadate

(s)

RbVO3 CsVO3 LiVO3 Ca3V10O28·17H2O

metavanadate metavanadate metavanadate pascoite (decavanadate)

(s) (s) (s) (s)

Ca3V10O28·15H2O

pascoite (decavanadate)

(s)

(VO3)nn− (VO3)nn−

metavanadate metavanadate (tetrameric, trimeric) pyrovanadate pyrovanadate decavanadate decavanadate

(aq) (aq)

(V2O7)4− (HV2O7)3− (V10O28)6− (HV10O28)5−

(aq) (aq) (aq) (aq)

350 404

954 482a 686, 574, 476 646, 496 643, 497 643.5, 530, 496.5 648, 498 634.2, 547.2, 507.9

701, 701, 530, 502 701, 574 736, 574

406, 355 − − 384, 341, 322, 309 375, 360, 330 379, 360, 339, 328 383, 362, 322 376.4, 358.4, 344.8, 316.6 380, 361, 345 − − 375, 357, 330 375, 355, 332 388.5, 352.4, 326 362, 334, 317

58 57 58 55 55 59 60

368, 336

61

360 360

62 63

351 351 357, 320 320

62 62 61 61

837, 815, 619, 584, 546 880, 820, 594, 560, 499 630, 490 630,d 540d, 490d 503 500 550 550

ref

701, 701 702, 702, 528,a 507

636, 551,c 509 732.8, 557.6, 431.2 737, 557,c 509 648, 485 652, 475 648.9, 559.9, 524.6

810, 805, 840, 850,

δ(VO)

3-fold-coord oxygen

848,b 526 778.2b

42 45, 47, 48 46 44 44 54 55 56 55 57

a

Note that the symmetric V−O−V stretching vibration is not a genuine stretching mode, because it is coupled with a V−O−V bending mode.44 Vibration could not be observed and is only based on calculation. cThis band was assigned to combinations by the authors. dThis band was assigned to a ring breathing mode by the author. b

and 700 cm−1 can be identified at higher loadings (12, 20 V nm−2) by comparison to a spectrum of a V2O5 reference (see Figure 5a). Based on the literature, crystalline V2O5 is expected to be already present on silica supports at a loading of LV = 2.9 V nm−2.20,21 Therefore, we decided to eliminate V2O5-related signals from the spectra in Figure 5a, starting at loadings of LV = 2.9 V nm−2. The corresponding difference spectra are shown in Figure 5b. Figure 5b reveals a decrease of the Si−Si phonon signal with increasing vanadium loading. This behavior is consistent with the observations by Holtz et al. and Song et al., who recognized a decrease in the silicon signal when the thickness of the top layer was increased.49,50 Since higher loadings cause an increased absorption of laser radiation, the information depth of the Raman analysis will decrease, affecting mainly the signal originating from the deepest layer, i.e., the silicon bulk. Please note in this regard that almost no intensity decrease of the silica signal at 450 cm−1 was observed. On the other hand, the D1 defect signal of silica at 492 cm−1 increases in intensity with increasing vanadium loading. Besides, fitting analysis reveals a broadening of this feature. This implies a superposition with another Raman feature corresponding to dispersed vanadia. In addition, all difference spectra show broad bands at 347, 562, and 676 cm−1. The presence of these features is strongly supported by comparison with a high surface area sample (see Figure S1 in the Supporting Information). Prior to our

three signals represent characteristic vibrations of silica.33,34 Detailed analysis reveals that the shoulder at 478 cm−1 is a superposition of a strong and broad SiO2 band around 450 cm−1 and the D1 defect mode of silica at 488 cm−1 (see Figure 5b). The band at 450 cm−1 corresponds to a pure “bending” motion of one oxygen atom without any motion of the Si atoms.35 The peaks at 606 and 810 cm−1 result from the D2 defect mode and a Si−O−Si stretching vibration, respectively.21,33,34 It should be mentioned that the D2 defect mode of silica was not included in the fitting analysis because of its low peak area and therefore negligible influence. In addition, all loaded samples show Raman bands at 1025 and 1062 cm−1, which are attributed to a vanadyl (VO) stretching vibration and an out-of-phase V−O−Si stretching vibration.20,22,36−38 In agreement with the literature, both bands are assigned to dispersed tetrahedrally coordinated surface vanadia species. Detailed analysis of the Si overtone region at ∼940 cm−1 reveals an intensity increase and peak broadening with increasing vanadium loading. This behavior strongly suggests that the Si overtone band additionally has the in-phase stretching vibration of the V−O−Si interphase bond superimposed on it.20,22,36−38 250−750 cm−1. Starting from a loading of LV = 7.3 V nm−2, the vanadyl stretching mode of crystalline V2O5 at 990 cm−1 is observed.20,21,42−48 This vibration will be referred to as νk. Additional features of crystalline V2O5 at 294, 408, 474, 522, E


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Figure 6. Enlarged view of the spectrum of crystalline V2O5 together with the results of least-squares fitting. (a) Spectral region of the vanadyl (V O) stretching mode νk. (b) Enlarged view of the corresponding overtone region.

We now turn to the discussion of the assignment of the remaining three bands at 492, 562, and 676 cm−1. In principle, based on Table 3, all bands can be attributed to V−O−V stretching vibrations. The assignment of these three features to dispersed vanadia structures is supported by isotopic exchange experiments using H218O for sample 7.3bVOx/SiO2/Si(100) as well as powder samples (see Figures S2 and S3). Regarding sample 7.3bVOx/SiO2/Si(100), besides a new feature at 983 cm−1 assigned to V18O stretching of dispersed vanadia structures, the disappearance of intensity around 550 cm−1 is observed, indicating the presence of V−O−V bands subject to the exchange process. In more detail, there are various possible ways that the oxygen atom may be bound within the bridging V−O−V bonds. Starting with a structure resembling that of V2O5, the oxygen atom could be coordinated with three vanadium atoms. In this scenario, the bands at 676 and 562 cm−1 may be V−O stretching vibrations arising from 3-foldcoordinated oxygen atoms, while the band at 492 cm−1 is a V− O deformation mode of a 2-fold-coordinated oxygen atom. Here we follow the interpretation by Clauws et al.45 However, please note that the assignment of V2O5 bands is not fully consistent in the literature (see Table 3). Good agreement with the above band positions can also be found based on the structure of metavanadate. In this case, the bands at 492 and 676 cm−1 are allocated to a symmetric and an asymmetric stretching vibration of a V−O−V bond in a (VO3)n chain arrangement, respectively, whereas an assignment of the 562 cm−1 band is difficult. In the literature, Raman bands are found at this position for some of the solid metavanadates. However, the intensity ratio of the three unknown bands in our spectra is different from that found for bands of solid metavandate. A plausible explanation is provided by Griffith, who predicted a ring configuration of VO3 units.63 The characteristic Raman band of the ring configuration (breathing mode) is found at 540 cm−1 for aqueous solutions (see Table 3). Following this interpretation, VOx oligomers preferably form tetrameric or trimeric rings. On the other hand, the presence of configurations resembling pyrovanadate or decavanadate seems less likely, because no bands in the region 750−880 cm−1 were found in our spectra. Assignment of the band at 562 cm−1 to V−O−V stretching also allows the assignment of another as yet unidentified band at 1124 cm−1 (see Figure 4). We propose this band to represent

proposed assignment of these bands, we provide an overview of the available literature. Comparison of the UV Raman results from this work with literature Raman data for visible excitation illustrates the effect of different conditions for resonance enhancement on the observed spectra (see Table S1 in the Supporting Information). Accordingly, UV excitation enhances the intensity of dispersed vanadia species, while visible Raman spectra are dominated by the signature of crystalline V2O5. Wu et al. observed Raman bands at 570 and 690 cm−1 on silica-based powder samples using 244 nm excitation.38 They ascribed this observation to a dispersed VOx structure, which becomes detectable as a result of the resonance effect. Also Gao et al. observed strong bands within the range 400−600 cm−1, which they could not explain by a simple combination of SiO2- and V2O5-related bands.20 As a result, they proposed the presence of an additional yet unknown vanadia species. In addition, Moisii et al. detected a broad Raman band at 686 cm−1 for silica-supported vanadia using 488 and 514 nm excitation.51 They assigned this band to a symmetric stretching mode of V−O−V arising from oligomerization. Applying a resonance enhancement of Raman features related to the vanadia surface species (257 nm), Moisii et al. also found two additional broad bands at 568 and 496 cm−1.52 Both were assigned to stretching vibrations of the Si−O−V interphase with the aid of normal-mode analysis. Using density functional theory (DFT) calculations, Döbler et al. and Magg et al. proposed the band positions of the V−O−V mode for dimeric, trimeric, and tetrameric silica-supported VOn species to be located in the range 760−860 cm−1.48,53 Table 3 gives a summary of reported wavenumber positions of V−O−V stretching and vanadyl (VO) deformation vibrations for crystalline vanadia compounds. According to Table 3, the position of V−O−V vibrational modes can be limited to the region 430−850 cm−1. In the literature, the location of symmetric V−O−V stretching modes is often assigned to the region 430−560 cm−1, while the asymmetric stretching modes are found at higher wavenumber values. Features in the range 300−400 cm−1 are assigned to a deformation mode of the vanadyl group. In particular, based on the data in Table 3, all crystalline vanadia compounds have Raman bands around 350 cm−1. This strongly suggests that the band observed at 346 cm−1 in our spectra originates from a V O deformation mode. F


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Figure 7. (a) Dependence of integrated UV Raman signals of dispersed vanadia species at 562, 1025, 1062, and 1124 cm−1 on vanadium loading LV. The different slopes represent different Raman cross sections. (b) Linear correlation between the overtone pair of the V−O−V bands at 562 and 1124 cm−1.

the first overtone of the fundamental band at 562 cm−1. A corresponding overtone feature related to the V−O−V band at 492 cm−1 would be expected in the range of the Si band at 940 cm−1. This V−O−V overtone may contribute to the broadening and increasing intensity of the Si band, but a detailed analysis in this range was hampered by the strong overlap of bands. 1800−2200 cm−1. Inset b of Figure 4 shows that samples with loadings LV ≥ 2.9 V nm−−2 exhibit a first overtone of the vanadyl stretching mode νd at 2045 cm−1.22,26,38 For loadings 2.9 ≤ LV ≤ 7.3 V nm−2 the overtone possesses a symmetrical shape, whereas an additional shoulder at around 1978 cm−1 is observed for the two highest loadings. To clarify the origin of the shoulder, the spectra of the two highest-loaded samples were compared with the spectrum of V2O5 (see Figure 5a). In fact, the crystalline reference exhibits a band in this region that to our knowledge has not yet been discussed in the literature. The maximum of this V2O5 overtone band is located at 1978 cm−1, but clearly an additional shoulder is observed, which is blue-shifted to the maximum. As shown in Figure 6b, a more detailed analysis of this region reveals three components, located at 1847 ± 6, 1978 ± 1, and 2020 ± 2 cm−1. We assign the contribution at 1978 cm−1 to the first overtone of the vanadyl stretching mode νk at 990 cm−1. The origin of the two other components, at 1847 and 2020 cm−1, is less obvious. A closer look at vibrational modes that may represent the fundamental to these overtones (see Figure 6a) provides four additional weak bands at 926 ± 1, 1020 ± 2, 1095 ± 2, and 1182 ± 6 cm−1. The bands at 926 and 1020 cm−1 have already been described in the literature.42,45,46,64 In this context, Gilson et al. assigned a band at 954 cm−1 (T = 293 K) to a V− O−V stretching mode (B1g).46 On the other hand, Clauws et al. attributed Raman features at 963 cm−1 (B2g, T = 77 K) and 1024 cm−1 (Ag, B2g, T = 77 K) to combination bands,45 while Abello et al. proposed the asymmetrical vanadyl stretching vibration to be located at 976 cm−1 (B1g, T = 100 K).42 Finally, Anderson and Verble reported the presence of a sharp band at 1010 cm−1 and assigned this band to an internal mode of a polyhedral vanadia unit.64 To our knowledge, the bands at 1095 and 1182 cm−1 have not yet been mentioned in the literature. Thus, we tentatively attribute the two bands at 1095 and 1182 cm−1 to combination or overtone modes. In addition, we propose the bands at 926 and at around 1020 cm−1 to

correspond to the overtone features at 1847 and 2020 cm−1, respectively. The effect of water on the Raman spectra was examined for a sample with a loading of LV ≥ 7.3 V nm−−2 as a representative (see Figure S4). Upon dehydration, changes in the vanadyl region are observed. As indicated by the intensity increase of the vanadyl feature at 1026 cm−1, the amount of tetrahedrally coordinated dispersed vanadia species increases in agreement with previous studies.22 Vanadia Growth Model. Figure 7a depicts the loadingdependent behavior of the integrated intensities of the four strongest Raman features of the vanadia surface species at 562, 1025, 1062, and 1124 cm−1. It may serve as a basis for a discussion of the growth characteristics of the vanadia layer. Based on Figure 7a, the amount of vanadia surface species increases linearly (R2 ≥ 0.9) up to a loading of LV = 7.3 V nm−2, where it saturates. In light of the literature, this is a surprising result, because Gao et al. reported the maximum coverage of vanadia surface species for powder silica supports to be reached at a loading of LV = 2.6 V nm−2.20 They explained this value by the onset of characteristic V2O5 peaks in the Raman spectrum (λ = 514.5 nm). According to ref 65, this is called the titration point, at which all surface hydroxyl groups have been consumed and additional deposition of vanadia leads only to the formation of crystallites.65 Nevertheless, for powder samples it is known that V2O5 crystallites may transform into dispersed vanadia species if thermal energy is provided.66 Therefore, we conclude that the higher temperature in our experiments (T = 220 °C) causes a shift of the titration point to higher loadings, enabling coverages of dispersed vanadia species comparable to those of other support materials (e.g., TiO2, Al2O3).65,66 This hypothesis was tested by examining Raman spectra recorded at 26 and 220 °C, both under ambient conditions (see Figure S4). In particular, a V2O5-related vanadyl feature at 990 cm−1 appears when the temperature is lowered from 220 to 26 °C. This indicates that at higher temperatures a higher amount of oligomerized, noncrystalline vanadia is present as compared to room temperature. In this context, there is the question of how the extra vanadia species are accommodated on the surface. A first explanation may be that not all hydroxyl groups are consumed when a loading of LV = 7.3 V nm−2 is achieved. Since the detection of the weak Si−OH Raman band at 976 cm−120−22,26,38 was not possible owing to G


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Figure 8. Proposed vanadia growth model based on our spectroscopic data. Yellow dots and rounded rectangles represent monomeric and oligomeric vanadia surface species, respectively, while orange rectangles represent V2O5 crystallites.

ring configuration of tetrameric or trimeric oligomers combined with species arranged in (VO3)n chains resembling the structure of metavandates, and/or oligomeric structures with 3-foldcoordinated oxygen atoms comparable to the structure of V2O5. The increase of Raman signals related to the dispersed vanadia species saturates at a loading of LV = 7.3 V nm−2. This high value of vanadia density on silica is explained by an increased degree of oligomerization of surface species at elevated temperatures.

the small surface area of the wafer samples, this aspect could not be pursued experimentally in this work. However, considering the literature value for the typical hydroxyl concentration of amorphous silica of 4.6 OH nm −2 , consumption of all silanol groups is expected at even lower loadings.67 A second explanation may be based on structural changes of the dispersed species. Thereby, a higher density of vanadia surface structures is achieved by the formation of networking bonds, i.e., an increasing degree of oligomerization. Thus, the rearrangement from a situation at lower loadings, where e.g. each vanadium is anchored by at least one OH, to higher loadings may proceed by increasing the number of V− O−V bonds.68 As a result, fewer V−O−Si linkages are needed to anchor the vanadia to the support. Such a behavior may ultimately lead to vanadium(V) centers containing one vanadyl group and three V−O−V linkages to other vanadium centers, thus vanadium(V) centers without linkage to the support. Such a scenario seems reasonable considering the increasing intensity of the V−O−V bands at 562 and 1124 cm−1. To this end, Figure 7b shows the linear regression between these two bands, revealing that the overtone at 1124 cm−1 reaches about 26% of the intensity of the fundamental band at 562 cm−1 and may thus be used as an indicator for V−O−V in future studies. Based on our findings we propose the following model for the vanadia growth (see Figure 8): At low loadings (LV < 2.9 V nm−2), dispersed vanadia species dominate. These may include monomeric species but also smaller aggregates of oligomeric species as discussed previously.22 At higher loadings (2.9 V nm−2 < LV < 7.3 V nm−2), besides dispersed species the presence of crystalline V2O5 is observed. In this loading range, a significant increase in the density of surface vanadia species occurs by an increase in the degree of oligomerization, as observed directly via the V−O−V Raman signals. At even higher loadings (LV > 7.3 V nm−2), owing to saturation of the surface with vanadia species, further growth is only possible in the third dimension, leading to an increased formation of crystalline V2O5.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b01672. Results from Raman spectroscopic experiments (Figures S1−S4, Table S1) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 0049-61511621975. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Karl Kopp for performing the XPS experiments and Nicolas Sobel for performing the ellipsometry measurements. Also, the authors gratefully acknowledge Jürgen Schildmann and Oliver Rohm (both S&I Spectroscopy and Imaging GmbH) for technical support regarding the UV Raman spectrometer.

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CONCLUSIONS In this work, the identification of V−O−V as a spectral marker enabling the direct identification of oligomeric surface structures is reported. We employ VOx/SiO2/Si(100) model samples with chemical complexity. By resonantly enhancing Raman features related to vanadia surface species, the sensitivity is significantly increased, allowing vibrational spectra of vanadia surface species despite the small surface area of the planar samples. Besides the well-known bands of dispersed and crystalline vanadia, new Raman features arising from V−O−V vibrations are observed at 492, 562, 676, and 1124 cm−1. Detailed analysis reveals that they may originate either from a

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Oligomerization of Supported Vanadia: Structural Insight Using

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