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J. Phys. Chem. C 2010, 114, 412–422
Multiwavelength Raman Spectroscopic Study of Silica-Supported Vanadium Oxide Catalysts Zili Wu,* Sheng Dai, and Steven H. Overbury Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 ReceiVed: September 2, 2009; ReVised Manuscript ReceiVed: October 14, 2009
The molecular structure of silica-supported vanadium oxide (VOx) catalysts over wide range of surface VOx density (0.0002-8 V/nm2) has been investigated in detail under dehydrated conditions by in situ multiwavelength Raman spectroscopy (laser excitations at 244, 325, 442, 532, and 633 nm) and in situ UV-vis diffuse reflectance spectroscopy. Resonance Raman scattering is clearly observed using 244 and 325 nm excitations, whereas normal Raman scattering occurs using excitation at the three visible wavelengths. The observation of strong fundamentals, overtones, and combinational bands due to selective resonance enhancement effect helps clarify assignments of some of the VOx Raman bands (920, 1032, and 1060 cm-1) whose assignments have been controversial. The resonance Raman spectra of dehydrated VOx/SiO2 show a VdO band at a smaller Raman shift than that in visible Raman spectra, an indication of the presence of two different surface VOx species on dehydrated SiO2 even at submonolayer VOx loading. Quantitative estimation shows that the two different monomeric VOx species coexist on silica surface from very low VOx loadings and transform to crystalline V2O5 at VOx loadings above the monolayer. It is postulated that one of the two monomeric VOx species has pyramidal structure and the other is in the partially hydroxylated pyramidal mode. The two VOx species show similar reduction-oxidation behavior and may both participate in redox reactions catalyzed by VOx/SiO2 catalysts. This study demonstrates the advantages of multiwavelength Raman spectroscopy over conventional single-wavelength Raman spectroscopy in structural characterization of supported metal-oxide catalysts. 1. Introduction Silica-supported vanadium oxide (VOx/SiO2) catalysts represent an important class of heterogeneous catalysts widely used in a variety of oxidation reactions such as selective oxidation of methane to formaldehyde,1–3 oxidative dehydrogenation of alkanes,4,5 and selective photooxidation of alcohols6 and hydrocarbons.7 A primary step to understanding the structurecatalysis relationship is to obtain knowledge of the molecular structure of surface vanadium oxide species anchored on the silica surface. To this end, structural characterization of VOx/ SiO2 has been the subject of many spectroscopic studies including Raman, IR, UV-vis-NIR, EXAFS, NMR, and EPR,3,8–20 among which Raman spectroscopy is the most commonly used tool because it is powerful in providing molecular bonding information. It is commonly agreed that vanadium oxide is present on the dehydrated silica surface in two different forms, that is, as monomeric VO4 species at low loadings (typical 2 VOx nm2).18 On the basis of these techniques (mainly Raman spectroscopy), it has been long accepted that a pyramidal model (OdV-(O-Si)3) is the only structure for dehydrated VOx/SiO2 at low VOx loading (573 K) dehydrated VOx /SiO2, whereas the bigrafted and umbrella models are likely to be present at less dehydrated surfaces. Until now, both experimental and theoretical work have concluded that the pyramidal and the umbrella structures can coexist at a support surface, their relative ratio depending on the hydration degree, temperature, and preparation conditions of the catalyst material.25,33,34 These studies confirm the traditionally accepted pyramidal model for stable VOx
10.1021/jp9084876 2010 American Chemical Society Published on Web 11/10/2009
Multiwavelength Raman of VOx/SiO2 species under highly dehydrated conditions. However, even the exclusive presence of monomeric vanadia species on the silica surface was questioned recently by Schlogl and co-workers40,41 via comparing the theoretical and experimental NEXAFS spectra of a silica SBA-15-supported vanadia catalyst. They suggested that under in situ conditions different molecular vanadia species, in particular nonmonomeric VOx, may exist at the catalyst surface. Clearly, there is still controversy on the exact molecular structure of VOx species present on a dehydrated silica surface despite the above-mentioned extensive experimental and computation studies of the VOx /SiO2 system. In this work, we investigate the molecular structure of the VOx/SiO2 catalytic system using multiwavelength (MW) Raman spectroscopy. The advantages of MW Raman spectroscopy over conventional single wavelength Raman spectroscopy have been highlighted in recent studies of supported VOx catalysts.22,26,29–31,42 On VOx/Al2O3, excitations by both UV and visible lasers lead to the selective distinction between monovanadate and polyvanadate species on the surface of alumina, providing more complete structural information of supported vanadia.31,42 Furthermore, three types of monovanadate structures have been identified on the alumina surface as three different vanadyl stretching bands were observed on a low loaded VOx/Al2O3 sample when excited by different laser wavelengths.29 The reason lies in the strong electronic absorptions throughout the UV and visible wavelength regions exhibited by vanadium oxide, which makes it possible to measure resonance-enhanced Raman spectra. Under circumstances where supported VOx species are present in a distribution of cluster sizes or coordination geometries, it is likely that these species also possess a corresponding distribution of electronic absorption wavelengths. Even within one type of VOx species, the different bonds (VdO, V-O-V, V-O-support, etc.) may have different electronic absorption through charge transfer from oxygen to vanadium.30 Excitation of Raman spectra within the absorption region will produce resonance-enhanced spectra from the subset of VOx species and/or local V-O bonding with absorptions at/near the excitation wavelength. Through measurement of the Raman spectra at several wavelengths, more detailed structural information can be obtained not only on the various VOx species in the distribution but also on the local V-O bondings in a specific VOx species. With these advantages of MW Raman over traditional single-excitation Raman, it is desirable to revisit the VOx/SiO2 system for the purpose of getting a clearer picture of how vanadia is anchored on the surface of SiO2. In this work, cabosil silica-supported vanadia (loading ranging from 0.0002 to 8 V/nm2) has been investigated with a MW Raman system (laser excitations at 244, 325, 442, 532, and 632.8 nm). It is shown for the first time that there are two types of monomeric VOx species coexisting on the dehydrated surface thanks to the selective resonance enhancement effect. Also, resonance Raman effect enables the observation of the overtones of VdO up to the fourth order and combination bands of VdO and V-O-Si modes, which are of help in interpreting Raman spectra in the fundamental region. 2. Experimental Section 2.1. Sample Preparation. The VOx/SiO2 samples were prepared via incipient-wetness impregnation of SiO2 (Cabot Cabosil EH-5, SBET ) 319 m2/g) with 2-propanol solutions of vanadium isopropoxide (VO(O-Pri)3, Alfa-Aesar 95-99% purity) in a Schlenk line. Typically, 1 g of precalcined SiO2 (823 K for 6 h in air) powder was loaded into a fritted reactor, which was sealed with a rubber septum at one end and connected
J. Phys. Chem. C, Vol. 114, No. 1, 2010 413 to a Schlenk line at the other end. The initially loaded sample was evacuated at 523 K for 1 h and cooled down to room temperature before switching to nitrogen atmosphere. Through a syringe, 10 mL of anhydrous 2-propanol (Aldrich, 99.8%) and a certain amount of vanadium isopropoxide were transferred into the reactor. The solution was stirred overnight at room temperature and dried via vacuum filtration. The impregnated powder was switched to nitrogen flow and further dried in the reactor at 523 K for 1 h before cooling down and exposed to air. The sample was transferred to an oven and calcined in air at 573 K for 1 h and then at 773 K for 4 h. Surface VOx density (V/nm2) instead of surface VOx loading (wt %) is used to describe the supported VOx samples. A VOx/SiO2 sample with surface VOx density of Y V/nm2 will be denoted as YV in the following text. 2.2. Raman Spectroscopy. The VOx/SiO2 samples were dehydrated in situ in a Raman catalytic reactor (Linkam CCR1000) before Raman spectral collection at room temperature. The dehydration was done by heating the sample (ca. 20 mg) in flowing 10% O2/He (60 mL/min) from room temperature to 773 K (ramping rate 10 K/min) and holding there for additional 2 h. The sample was then cooled down to room temperature in O2/He and ready for Raman measurement. The isotopic 18O exchange was accomplished with H218O (Isotec, Water-18O, normalized min 97 atom % 18O) at elevated temperatures in the in situ Raman reactor. Lee et al.28 have shown that use of H218O vapor gives similar but much more efficient isotopic exchange effect than by 18O2 and an exchange temperature of 723 K is appropriate for silica-supported vanadia samples. The room temperature H218O was bubbled into the Raman reactor with flowing He (30 mL/min) after the sample was dehydrated at 773 K and cooled down in He to 723 K for isotopic exchange. A complete exchange for surface VOx species can be obtained after 1 h treatment at 723 K, whereas the silica support does not show obvious exchange. The Raman measurements were performed on a newly built multiwavelength Raman system at Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences. Raman scattering was collected via fiber optics connected directly to the spectrograph stage of a triple Raman spectrometer (Princeton Instruments Acton Trivista 555). Edge filters (Semrock) were used in front of the UV-vis fiber optic bundle (Princeton Instruments) to block the laser irradiation. The 244 nm laser excitation (∼2 mW at sample) is obtained via second-harmonic generation (SHG) from an Ar ion laser (Coherent, MotorFred I300C) with fundamental at 488 nm. The 325 (5 mW at sample) and 442 nm (10 mW at sample) excitations are from a HeCd laser (Melles Griot). The 532 nm excitation (20 mW at sample) is emitted from a solid-state laser (Princeton Scientific, MSL 532-50). The 632.8 nm (20 mW at sample) is obtained from a high-power HeNe laser (Melles Griot). The laser spot size is ca. 50-150 um depending on the laser wavelength used. A UVenhanced liquid N2-cooled CCD detector (Princeton Instrument) was employed for signal detection. The Raman reactor sits on an XY stage (Prior Scientific, OptiScan XY system) and translates in raster mode while collecting the spectrum. The fast translation has shown been able to eliminate/minimize any laser damage of the samples. Cyclohexane solution is used as a standard for the calibration of the Raman shifts. 2.3. UV-vis Diffuse Reflectance Spectroscopy (UV-vis DRS). The UV-vis DRS measurements were taken on a Varian Cary 5E UV-vis-NIR spectrophotometer with an integration sphere diffuse reflectance attachment (Harrick Praying Mantis Attachment, DRA-2). The precalcined VOx/SiO2 samples were
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Figure 1. In situ multiwavelength Raman spectra (A) and UV-vis DRS spectra (B) of dehydrated 0.25 V sample.
loaded as loose powder into an in situ cell (Harrick, HVC-DR2) and dehydrated at 773 K in flowing Ar (∼60 mL/min) for 2 h. The samples were cooled down to room temperature and the UV-vis DRS spectra were collected from 200 to 800 nm. The reflectance of the SiO2 support was used as the standard baseline. 3. Results 3.1. Raman Spectra as a Function of Laser Excitations. For multiwavelength Raman study of the VOx/SiO2 system, a low-loaded sample, 0.25 V, is selected for the comparison of the different laser excitations. Part A of Figure 1 shows the Raman spectra of a dehydrated 0.25 V sample excited at 244, 325, 442, 532, and 632.8 nm. Generally, the spectra excited by visible lasers (442, 532, and 632.8 nm), the visible Raman spectra, are similar to each other while quite different from those excited by UV lasers (244 and 325 nm), the UV Raman spectra, in terms of spectral contour and bands position. This is due to the fact that the 0.25 V sample barely absorbs in the visible region (part B of Figure 1 for the UV-vis DRS spectrum), whereas it has strong electronic absorption in the UV region (below 400 nm), which may offer a resonance enhancement effect in the UV Raman spectra. The visible excited spectra show a sharp band at 1041 cm-1 that is related to the surface VOx species and usually assigned to vanadyl (VdO) stretching, although this assignment has been questioned recently.26 We
Wu et al. will still use this traditional assignment to VdO in the context before coming to a detailed discussion on the bands assignment. In the range below 1000 cm-1, Raman bands are observed at 976, ∼800, 605, 484 cm-1, all due to the SiO2 support, have been assigned to Si-OH stretching, Si-O-Si stretching, D2 defect mode, and D1 defect mode, respectively.16,28,43 For UV excited spectra, no apparent features due to SiO2 can be observed, consistent with the strong absorbance of the surface VOx species in the UV region (part B of Figure 1). Thus the Raman spectra are dominated by features from surface VOx species. The characteristic VdO stretching band is observed at 1032 cm-1 in the UV Raman spectra for both 325 and 244 nm excitation. This band is about 9 cm-1 lower than that observed in the visible Raman spectra and is probably responsible for the lower Raman shift tailing of the 1041 cm-1 band in the visible Raman spectra. The observation of two different VdO stretching bands suggests the presence of two types of VOx species on the silica surface. The 325 nm excited spectrum exhibits an additional sharp band at 461 cm-1 and a weak one at 920 cm-1. The band at 920 cm-1 is assigned to interface V-O-Si mode. The band at 461 cm-1 is usually ascribed to the bending mode of V-O because of its low frequency. However, its sharpness is in general conflict with spectral characteristics of bending modes. It is thus likely a stretching mode and has recently been assigned to primarily V-O-Si stretching.26 The 244 nm excited spectrum shows a shoulder at 1060 cm-1 on the intense band at 1032 cm-1. The shoulder band at 1060 cm-1 was previously assigned to silica network TO mode and Si(-O-)2 and Si-O- functionalities12,16 but recent theoretical work suggests that it is due to the V-O-Si stretching mode.22,26,36,37 Weak bands are also observed at 920, 690, and 570 cm-1 in the 244 nm-excited spectrum, assignable to V-O-Si stretching and V-O bending modes, respectively. The observation of these new bands below 1000 cm-1 in the UV Raman spectra is a strong indication of resonance enhancement of certain modes of surface VOx species. Detection of different VdO stretching bands is evidence for the presence of at least two different VOx species. The edge energy obtained from the UV-vis-DRS spectrum in part B of Figure 1 is about 3.4 eV, very close to that of monomeric vanadia.11,44,45 This implies that the two VOx species are monomeric in nature. 3.2. Raman Spectra as a Function of VOx Loading. VOx/ SiO2 samples with wide VOx loadings (0.0002-8 V/nm2) were prepared and measured by MW Raman spectroscopy. Because the visible-laser excited spectra (442, 532, and 632.8 nm) are similar as shown in the case of 0.25 V sample, only 532 nm excited spectra are shown together with the 244 and 325 nm excited ones in Figure 2. 532 nm Excited Spectra (Part A of Figure 2). A weak band at 1041 cm-1 starts to appear on the 0.01 V sample and increases in intensity with surface VOx density. The asymmetry of this band indicates the existence of another VdO stretching at a lower Raman shift, which is resonance enhanced in the UV excited spectra. Meanwhile, the bands at 976, ∼800, 605, and 484 cm-1 due to the SiO2 support a decrease in intensity when vanadia species are gradually anchored on the SiO2 surface at increasing density. At a surface density of 2.2 V/nm2 and higher, Raman bands characteristic of crystalline V2O5 are observed (995, 703, 528, 483, and 405 cm-1) instead of the silica support bands. This is consistent with the observation of a large extension of adsorption into visible region in the UV-vis DRS spectra (Figure E-1 in the Supporting Information) due to the presence of V2O5 in 2.2 and 8 V samples. It is clear that the presence of crystalline V2O5 can be more easily detected by
Multiwavelength Raman of VOx/SiO2
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Figure 2. Raman spectra excited by 532 nm (A), 325 nm (B), and 244 nm (C) for dehydrated VOx/SiO2 samples with different surface VOx density.
visible Raman than UV Raman (below), similar to a previous observation on VOx/Al2O3 system.31 There are no bands due to dispersed vanadia observed below 1000 cm-1. The consistent position of the 1041 cm-1 band indicates that the structure of the surface-dispersed vanadia species is unchanged throughout the wide vanadia loading range. 325 nm Excited Spectra (Part B of Figure 2). At surface vanadia loading lower than 0.05 V/nm2, the spectra are dominated by bands from the silica support except for the development of a sharp band at 1032 cm-1 due to VdO. For vanadia density of 0.05 V/nm2 and above, another strong band at 462 cm-1 overtakes the strong features of SiO2 at the lower spectral region due to resonance enhancement effect. A weak band at 920 cm-1 is observed at all vanadia loadings and assigned to the V-O-Si stretching mode. Different from visible Raman spectra, bands from V2O5 are barely observable at VOx density up to 8 V/nm2, apparently due to the strong electronic absorption of dispersed vanadia species in the UV region. The first overtone progress of the VdO is also shown in the spectra at 2052 cm-1. The constant Raman shifts for the fundamental and overtone of VdO stretching together with the unchanged intensity ratio of the fundamental to the first overtone band are
indications that the 325 nm excitation is resonantly detecting the same type of surface VOx species. 244 nm Excited Spectra (Part C of Figure 2). Similar to the 325 nm excited spectra, the 244 nm excited spectra show features due to SiO2 support at surface VOx loading lower than 0.05 V/nm2 and the spectra are taken over by Raman bands due to VOx species at higher loadings. This is again due to the strong absorption of UV radiation (decreases signal from SiO2) by the surface VOx species and resonance enhancement effect (increases signal from VOx species). Resonance enhancement is clearly indicated by the extremely low detection limit of surface VOx by UV Raman, because some features due to VOx species are already observable at density as low as 0.0002 V/nm2. An example is the broadband centered at around 1060 cm-1 visible above the SiO2 background. To our knowledge, this is the lowest detection limit ever reported for surface VOx species by vibrational spectroscopy. As the VOx density increases to 0.001 V/nm2, a shoulder develops at 1032 cm-1 together with the band at 1060 cm-1, assigned to VdO and V-O-Si stretching modes, respectively. The V-O-Si stretching mode is preferably resonance enhanced with 244 nm excitation than other wavelengths used in this work. With the
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Figure 3. (A) Raman spectra (λex. ) 244 nm) of the 0.25 V sample collected at 723 K during 18O isotopic exchange with different durations. (B) Raman spectra (λex. ) 244 nm) of 0.001 V, 0.01 V, and 0.25 V collected at room temperature before and after 18O isotopic exchange. (C) Raman spectra (λex. ) 325 nm) of 0.25 V collected at room temperature before and after 18O isotopic exchange. (D) Raman spectra (λex. ) 532 nm) of 0.25 V collected at room temperature before and after 18O isotopic exchange.
increase of VOx density, the intensity ratio between the band at 1032 and 1060 cm-1 reverses, that is, the band at 1032 cm-1 gradually evolves into the major band, whereas the one at 1060 cm-1 becomes a shoulder. This intensity ratio change versus VOx loading is not likely due to the structural change of surface VOx species because the VdO stretching stays constantly at 1032 cm-1 and ratio of its intensity to that of the first overtone band (at 2056 cm-1) is constant for all samples. Instead, it is probably due to the change of anchoring sites of VOx species on the support surface. As Wachs and co-workers pointed out,11 VOx species are bonded onto SiO2 surface via both the available terminal surface hydroxyl groups and also vicinal hydroxyl groups that are formed by breaking the 3-member rings on the silica surface during sample calcination process. The two sites may pose different stress on the V-O-Si interface bond so that its Raman scattering property changes when the VOx density increases. In the region below 1000 cm-1, weak bands at 920, 690, and 570 cm-1 are observed over silica background on samples with surface VOx density higher than 0.05 V/nm2 due
to resonance enhancement. Raman bands due to V2O5 are observed at the highest VOx density as in the 325 nm excited case. 3.3. Oxygen Isotopic Exchange. 18O isotopic labeling in Raman spectroscopy provides useful information for band assignment and structural identification of metal-oxide catalysts.28,46 In this study, the isotopic exchange was performed at 723 K by interacting H18O2 with the VOx/SiO2 samples. Isotopic exchange does not occur for the SiO2 support under such condition. Part A of Figure 3 shows the time-resolved Raman spectra (excited by 244 nm) collected at 723 K during oxygen isotopic exchange on 0.25 V sample. Before isotopic exchange, the VdO band is observed at 1028 cm-1, whereas the shoulder at 1060 cm-1 and the band at 920 cm-1 are less evident compared to room temperature spectrum. The red shift of the VdO band from 1032 to 1028 cm-1 is due to thermal expansion and changes in the population of the vibrational energy levels of the V-O bonding with increasing temperature.12,47 Upon exposure to H18O2, a new band at 986 cm-1 due to Vd18O appears and gradually takes over as the band at 1028 cm-1
Multiwavelength Raman of VOx/SiO2 (Vd16O) decreases. There is no further spectra change after 45 min exchange. The spectrum is now dominated by the band at 986 cm-1 (Vd18O) with a shoulder at 1016 cm-1 probably due to V-18O-Si stretching. The ability to observe the band due to V-18O-Si is attributed to the fact that the two bands associated to VdO and V-O-Si separate better when oxygen is isotopically exchanged. Part B of Figure 3 compares the room temperature Raman spectra before and after oxygen isotope exchange. After 18O exchange, the VdO band shifts from 1032 to 989 cm-1, and the bridging V-O-Si stretchings at 1060 and 920 cm-1 shift to 1018 and 885 cm-1, respectively. These shifts match well with the theoretical isotopic shifts for the replacement of 16O by 18O in the VdO and V-O-Si bondings. There also appear shifts of the bending mode bands in the range 400-700 cm-1 but the broadness of these bands prevents it from accurately calculating the isotopic shifts. The shoulder band at 1018 cm-1 due to V-18O-Si is more evident on samples with lower VOx density (0.001 and 0.01 V/nm2) as also shown in part B of Figure 3, becoming a resolved band on 18O isotopically exchanged 0.001 V and 0.01 V samples. The Vd18O band at 989 cm-1 on these two samples is more intense than the V-18O-Si band at 1018 cm-1, as opposed to the unexchanged spectra. This is due to the overlapping of the Vd18O band and the surface Si-OH band (976 cm-1), which also makes the Vd18O band appear at a lower Raman shift on the 0.001 V sample. The silica support does not undergo 18O isotopic exchange under current experimental conditions as evidenced by the consistent position of the silica bands before and after exchange. Figure 3C and 3D give the spectra of 0.25 V before and after 18 O exchange excited at 325 and 532 nm, respectively. For 325 nm excited spectra, 18O exchange causes the VdO band to shift from 1032 to 989 cm-1 and the V-O-Si band from 920 to 885 cm-1, similar to the 244 nm excited spectra. The strong band at 462 cm-1 due to V-O-Si stretching also shifts to 451 cm-1 after 18O exchange. For 532 nm excited spectra, the VdO band at 1041 cm-1 shifts to 995 cm-1 and both bands are asymmetrically shaped before and after 18O exchange. A weak band at 1032 cm-1 is evident after 18O exchanged, likely due to the remaining unexchanged Vd16O, which is a shoulder of the main band at 1041 cm-1 before 18O exchange. Its presence after 18O exchange probably indicates that it is due to one of the two types of VOx species and is more difficult for oxygen isotopic exchange. 3.4. Overtones and Combination Bands. One of the characteristics of resonance Raman is the high possibility of observing overtones and combination bands, which provide insight for the symmetry and electronic properties of the fundamental vibrations. We observed overtones and combination bands for most of the VOx/SiO2 samples (loading >0.001 V/nm2) when excited only by the UV lasers and the results on 0.25 V are shown here. The full-range spectra before and after 18O exchange are shown Figure 4. For unexchanged sample, fundamental bands are observed at 462 (V-16O-Si, denoted as υa), 920 (V-16O-Si, υb), and 1032 (Vd16O, υc) when excited by 325 nm as exhibited in part A of Figure 4. Overtones are clearly observed at 2052, 3059, 4065, and 5064 cm-1, corresponding to 2υc, 3υc, 4υc, and 5υc, respectively. To our knowledge, this is the first resonance Raman spectrum showing overtones of vanadyl stretching up to the fourth order. Bands at 1362, 1493, and ∼1831 cm-1 are also observed beyond the fundamental vibrations and can be assigned to combination bands of (υa + υb), (υa + υc), and an overtone band of 2υb, respectively. Upon complete oxygen isotopic exchange, all of
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Figure 4. Full-range Raman spectra excited by 325 nm (A) and 244 nm (B) for 0.25 V collected at room temperature before and after 18O isotopic exchange.
the fundamental bands, overtones, and combination bands are shifted to lower Raman shifts as shown in part A of Figure 4 and collected in Table 1. Because a control experiment on the pure SiO2 support does not show obvious isotopic exchange under current experimental conditions, the shifts upon oxygen isotopic exchange observed on 0.25 V confirms that the bands observed above the fundamental region are indeed due to overtones and combination bands of surface vanadia species. For 244 nm excited spectra (part B of Figure 4), overtones up to the fourth order of vanadyl stretching are also observed at similar Raman shifts as in 325 nm excited spectra (Table 1), suggesting that the 1032 cm-1 band observed in both 244 and 325 nm excited spectra are due to a vanadyl vibration in the same type of VOx species. The difference between the two excited spectra is that the overtones of VdO in the 244 nm excited spectrum are weaker, indicating less resonance enhancement of VdO mode compared to 325 nm excitation. Bands at ∼1831 and 1362 cm-1 are observed above the fundamental region and are assigned to the overtone of the 920 cm-1 band (2υb) and combination band of (υa + υc), respectively. Although the υa mode (462 cm-1) is not clearly resolved from the silica background, the strong υc band makes their combination band
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TABLE 1: Fundamental and Overtones Observed for 1032 cm-1 Band in 244 and 325 nm Excited Resonance Raman Spectra of 0.25 V Sample and the Calculated Anharmonicity, Harmonic Frequency, Dissociation Energy, and Force Constant 244 nm 325 nm
O-16 O-18 O-16 O-18
νVdO (cm-1)
2νVdO (cm-1)
3νVdO (cm-1)
4νVdO (cm-1)
5νVdO (cm-1)
xmm
νm (cm-1)
D0 (kJ/mol)
fe (mdyn/Å)
1032 989 1032 989
2056 1969 2052 1968
3064
4072
5069
3059 2941
4065 3898
5064 4846
4.6 4.5 4.7 4.7
1041 998 1040 998
698 656 682 628
7.99 8.03 7.98 8.03
observable. The combination band of (υa + υb) is not observed because both modes are very weak in the fundamental region. Upon 18O exchange, all of the Raman bands shift downward, again confirming that the overtones and combination bands are due to surface VOx species. The observation of overtones of VdO (νc) up to the fourth order allows the calculation of both the harmonic wavenumber, ωe, and the anharmonic constant ωexe () xmm). ωe () ωm) is the vibrational frequency corrected for anharmonicity and represents the frequency associated with infinitely small vibrations about the equilibrium separation in the actual potential field (anharmonic oscillator).30,48 The observed wavenumbers, νm(n), for fundamental (n ) 1), first to fourth overtones (n ) 2-5) from a polyatomic anharmonic oscillator are given by the expression νm(n) ) nωm - n(n + 1)xmm, where m is the normal mode identifier, for example, ω1 and x11 for the ν1 mode and ω2 and x22 for the ν2 mode. A plot of νm(n)/n versus n should be a straight line of slope xmm and an intercept, which gives ωm - xmm and thus ωm. Plots of the overtone progressions of νVdO (νc) in 0.25 V before and after 18O exchange are shown in Figure 5. The values of ωm and xmm determined by the plots are included in Table 1. The availability of the harmonic frequency, ωe, and anharmonic constant, xmm, allows the determination of anharmonicitycorrected force constant, fe, and dissociation energy, D0. The anharmonic force constant can be obtained using the diatomic oscillator approximation from the equation ωe ) (2π)-1(fe µ)0.5, where µ is the reduced mass. The calculated fe for VdO (1032 cm-1) and V-O-Si (920 cm-1) are 7.99 and 4.57 mdyn/Å respectively, matching well with the general trend f(VdO) > f(V-O). The value of fe for VdO is in agreement with the VdO force constant from VOx/Al2O3 (7.37-7.58 mdyn/Å)30 and vanadium bulk compounds (7.10-7.70 mdyn/Å).49 The value of fe for V-O-Si is less than that for V-O-Al in VOx/Al2O3
Figure 5. Plots of νVdO(n)/n versus n for the overtone progressions of VdO stretching observed in 244 and 325 nm excited Raman spectra of 0.25 V sample before and after 18O isotopic exchange.
(∼6.40 mdyn/Å),30 consistent with the picture that vanadia species bond weaker on SiO2 than on Al2O3. The dissociation energy along a normal coordinate, D0, is the sum of the vibrational levels of the anharmonic oscillator and represents the depth of the vibrational potential well below the dissociation limit. It can be calculated using the standard equation for an anharmonic oscillator by neglecting cubic and higher-order terms: D0 (in cm-1) ) (0.25 ωm2 /xmm) - (ωm/2) + (xmm/4). The D0 (kJ/mol) value for VdO (νc) mode determined from the resonance Raman spectra excited by 325 and 244 nm (before and after 18O exchange) are included in Table 1 as well as the calculated anharmonic force constant. The D0 of VdO ranges from 628 to 698 kJ/mol. The variation is likely due to the inaccuracy of determining the Raman shift of the broad overtone bands. The D0 of V-O-Si (band at 920 cm-1) can be roughly estimated from its first overtone band in 244 and 325 nm excited spectra and it ranges from 517 to 574 kJ/mol. The D0 of V-O-Si is less accurate than that of VdO because both the fundamental band (920 cm-1) and the overtone progression (∼1830 cm-1) are weak and broad. The value for both D0 indicates that the bonds responsible for the two modes have bond order of about 2, consistent with the VdO and V-O-Si designation. 4. Discussion 4.1. Raman Bands Assignment. Silica-supported vanadia catalysts have been the subject of numerous Raman spectroscopy studies.1–4,8–16,18–28,50 It has been generally agreed that Raman bands in the range 800-1200 cm-1 are due to stretching modes and the bands below 800 cm-1 are due to bending/stretching modes of V-O.16,28 However, there is still controversy on the assignment of major Raman bands above 800 cm-1,16,22,24–28,33 namely, the three bands at 920, 1032(1041), and 1060 cm-1 observed in this study. We will concentrate our discussion on the assignment of these three bands based on our spectral data. It may not be possible to assign each of the Raman bands to a single specific mode because of the strong coupling between vanadia and silica vibrations.22 So the assignments are made to the modes that contribute most to each of the bands. Assignment of Bands at 1032 (1041) and 1060 cm-1. A sharp band at ca. 1040 cm-1 is always observed in the visible Raman spectrum of dehydrated VOx/SiO2 for VOx loading less than a monolayer and has been traditionally assigned exclusively to VdO stretching in a pyramid VO4 structure.16,28 A weak shoulder at ca. 1065 cm-1 is also often observed and assigned to perturbed silica vibrations that are indicative of the formation of V-O-Si bonds.16,28,51 A recent infrared and UV Raman study22 on both model and powder VOx/silica samples showed that there is a pair of sharp bands observed at 1035 and 1065 cm-1 for the VOx/silica sample. Aided with DFT calculation, it was suggested there is a strong coupling between the vanadia and the silica substrate so that it is not possible to attribute the sharp Raman bands above 1000 cm-1 exclusively to VdO or V-O-Si. The assignment of the sharp band at ∼1040 cm-1 to VdO was further challenged by a recent resonance Raman study of VOx/SiO2.26 The Raman results were combined with a
Multiwavelength Raman of VOx/SiO2 normal-mode analysis of the primary stretching modes of the vanadium oxo group using a central force approximation. It was concluded that the bands at 1032 and 1064 cm-1 are dominated by V-O-Si stretches while the weak band at 923 cm-1 is dominated by VdO stretching.26 On the basis of our results, we assign the 1032 (1041) cm-1 band to VdO stretching and 1060 cm-1 to V-O-Si stretching. Our evidence derives from analysis of the overtones and of the temperature dependence as summarized below. (1) Overtone bands. Strong overtone progressions of the 1032 cm-1 band (υc) are observed in both 244 and 325 nm excited Raman spectra. The intensity of the overtone bands becomes weaker with progression order, whereas the bandwidth becomes broader with progression order. These observations are generally consistent with resonance enhancement by Albrecht A term,52–55 which indicates that υc is a totally symmetric mode with a large displacement of the vanadium oxo bond length in the excited state. The calculated dissociation energy of this mode from the overtone progression is in the range 628-698 kJ/mol, in nice agreement with the reported D0 for diatomic VdO (600-640 kJ/mol).56,57 The calculated force constant also matches well with the VdO force constant for both supported-vanadia30 and unsupported-vanadium compounds.49 The symmetry, dissociation energy, and force constant associated with the υc mode strongly suggest it can only be due to the VdO stretching. Strong overtone progressions are also observed in resonance Raman spectra for MdO (M stands for transition metal) stretching of supported MOx where the surface metal-oxide species is not vibrationally coupled with the support such as VOx/Al2O330 and MoO3/Al2O3.58 These observations support the assignment of the 1032 cm-1 band to VdO stretching. No overtone progression is observed for the 1060 cm-1 band, even though its fundamental band is resonance enhanced in 244 nm excitation. According to the Albrecht A term,55 this band is associated with a nontotally symmetric mode without displacement of the potential curve in the excited state so that it has no overtone progression. Therefore, the 1060 cm-1 band can be assigned to an antisymmetric V-O-Si mode, in agreement with the resonance Raman study of VOx/silica by Moisii et al.26 Upon 18 O2 isotope exchange, this band shifts downward to 1018 cm-1, indicating it is associated with V-O vibration but not silica vibrations because no obvious 18O isotope exchange is found on the silica support. Very recently, Sauer and co-workers37 investigated the VOx/silica system with hybrid DFT/MM calculation methods and concluded that in the 1000-1080 cm-1 region, the V-O-Si interface band should be at higher wavenumbers than the characteristic vanadyl band. This conclusion is consistent with our experimental results and Raman assignments. (2) Temperature effect. As shown in part A of Figure 3, the shoulder band at 1060 cm-1 is hardly observable, whereas the main band at 1032 cm-1 shifts downward to 1028 cm-1 when the 0.25 V sample is heated up to 723 K. The disappearance of the shoulder band at 1060 cm-1 at high temperature is an indication that the V-O bond associated with this band is weaker than the one associated with band at 1032 cm-1 based upon Xie et al.,47 which showed that more severe temperature effects on the intensity of Raman band will be found on metal oxide with a weaker metal-oxygen bond. This temperature effect on the bands’ intensity can be more easily observed on samples with a lower VOx density where the 1060 cm-1 band is more prominent. On a 0.001 V sample (Figure E-2 in the Supporting Information), the spectra collected from room temperature to 773 K clearly show that the relative intensity of
J. Phys. Chem. C, Vol. 114, No. 1, 2010 419 the band at 1060 cm-1 decreases faster with increasing temperature than the band at 1032 cm-1. These confirm that the band at 1032 cm-1 is associated with VdO, whereas the one at 1060 cm-1 is associated with the V-O-Si moiety. Xie et al.47 also indicated that the closer the absorption edge of oxide species to the laser excitation, the more severe will be the effects of temperature on the intensity of Raman spectrum. Thus, the higher sensitivity to temperature effect of the band at 1060 cm-1 relative to that of the band at 1032 cm-1 may imply that the charge transfer band of V-O-Si is closer to 244 nm than that of VdO. Assignment of Band at 920 cm-1. Several different assignments have been proposed previously for the Raman band at around920cm-1,includingassignmenttoV-O-Vvibration,20,38,59,60 O-O vibration,23,24,39 V-O-support mode,22,25,31–37 Si-O- and Si(-O-)2 modes as a result of anchoring the vanadium oxide,11,51 and VdO vibration.26 The traditional assignment to V-O-V mode was excluded by recent EXAFS and Raman results22,25,31,39 that no V-O-V connection is found in EXAFS measurements for low-loaded vanadium oxide, whereas the Raman band at around 920 cm-1 is readily observable on sample with VOx density as low as 0.01 V/nm2. Our current study also shows the presence of this band at surface VOx density ranging from 0.01 to 8 V/nm2 in both 244 and 325 nm excited Raman spectra. The assignment to the O-O mode was initially proposed by Gijzeman et al.23 and later elaborated by a DFT calculation24 where the two oxygen atoms were described as a chemisorbed oxygen molecule to the V center. However, the broadband at around 920 cm-1 is clearly observed for our hightemperature (above 673 K) dehydrated VOx/SiO2 samples and so is not likely due to O-O vibration because solid-phase vanadium peroxo compounds are thermally unstable and dissociate between 473 and 573 K.61 So this assignment can be also discarded for a highly dehydrated VOx system. The assignment to Si-O- and Si(-O-)2 related silica vibration modes is not likely because the band shifts to lower frequency (to 885 cm-1) after 18O isotope exchange, whereas silica is found not to undergo isotope exchange under current experimental conditions. Finally, the VdO origin26 of the weak and broad 920 cm-1 band is not likely because the vanadyl stretching usually gives a very intensive and narrow Raman band in both bulk and supported vanadium oxide systems.16,62,63 Instead, our data supports the assignment of 920 cm-1 band to V-O-Si vibration and has been suggested previously by both Raman experiments and DFT calculations.22,25,37 Our resonance Raman spectra show an overtone progression for the 920 cm-1 band, implying the fundamental vibration is a symmetric mode. The value of the calculated force constant (4.58 mdyn/Å) indicates the vanadium oxo is not doubly bonded as in VdO whose force constant is much higher (>7 mdyn/Å).30,49 This is consistent with the assignment of the 920 cm-1 band to V-O-Si interface mode. The observation of this band across the wide VOx loading range (0.01-8 V/nm2) is also in line with this assignment. The direct observation of the V-O-Si bridging modes at 920 and 1060 cm-1 in the UV Raman spectroscopy is of significance to the understanding of redox reactions catalyzed by silica-supported vanadia catalysts because the interface bond has been considered as a key active site for these reactions. The ability to observe these modes is a crucial advantage of selective resonance enhancement Raman for identifying active and selective sites in supported metal-oxide catalysts. 4.2. Structure of Surface Dispersed VOx Species. Although it has been generally accepted that the dehydrated vanadia species exist exclusively as monomers on the silica surface when
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Figure 6. Distribution of surface monomeric species and crystalline V2O5 as a function of surface VOx density.
the VOx loading is less than monolayer, the exact structure of the monomer VOx is still in debate because of the different spectral assignments as illustrated above. The proposed structure includes the long-time accepted pyramidal model (trigrafted), bigrafted model, and umbrella model (monografted).25 With both experimental and theoretical efforts,22,25,33–37 it is now accepted that the pyramid structure is the most stable model for highly dehydrated supported VOx system, whereas the bigrafted and umbrella models are likely to be present at less dehydrated surfaces. Under current experimental conditions, we consider the VOx/SiO2 samples are highly dehydrated after 773 K calcinations. So the surface VOx species is supposed to have a pyramidal structure at low surface VOx density and to evolve into crystalline V2O5 at higher density. The interesting observation is that two different VdO stretching bands at 1032 and 1041 cm-1 are found for VOx dispersed on silica, suggesting the existence of two different VOx species that are monomeric in nature according to the edge energy measured from UV-vis DRS spectra. The VdO stretching at 1032 cm-1 dominates the 325 and 244 nm excited spectra due resonance enhancement effect. Under normal Raman condition (nonresonance enhancement, 442, 532 and 633 nm excitations), the 1032 cm-1 band is weak and buried in the strong VdO stretching band at 1041 cm-1. This implies that the monomeric VOx species giving VdO stretching at 1041 cm-1 (type A) dominates on the silica surface, whereas the other monomeric species (type B) exhibiting VdO stretching at 1032 cm-1 is the minor one. Assuming that the Raman cross section of the VdO in the two species are the same under 532 nm excitation (no resonance enhancement) and the Raman cross section of V2O5 is known 10 times larger than dispersed vanadia species,12 a quantitative determination of the distribution of different surface VOx species can be obtained by peak fitting the 532 nm excited spectra. As shown in Figure 6, the percentage of species B (with VdO stretching at 1032 cm-1 detected by UV Raman) continues to drop, whereas that of species A (with VdO stretching at 1041 cm-1 detected by visible Raman) increases at low loadings and then decreases at higher loadings. This indicates that VOx species maybe anchored on the silica surface initially as species B, which transforms partially into species A at higher VOx loadings. This is consistent with the detection of species B by UV excitations at VOx density as low as 0.0002 V/nm2. Both VOx species transform to
Wu et al. crystalline V2O5 at higher surface density as evidenced by the increasing percentage of V2O5. Because the pyramidal structure has been considered the most stable form for dispersed VOx on highly dehydrated SiO2,22,33–37 it is highly likely that the major VOx species A observed in our nonresonant visible Raman spectra has such a structure with three V-O-Si bonds and one VdO bond. The exact molecular structure of species B observed by UV resonance Raman is not clear but some candidates can be proposed. Because species B is anchored on silica surface prior to species A, the silica surface is more hydroxyl group-rich when species B is formed. This surface is an analogue of a partially hydrated one when compared to the surface where both species A and B are anchored. The VdO vibration of species B appears at a lower Raman shift (1032 cm-1) than that of species A (1041 cm-1), in line with the fact that the VdO stretching frequency of a partially hydrated VOx species is generally lower than that of a dehydrated species.11,12 Therefore, species B may have a partially hydroxylated pyramidal structure where a hydroxyl group is either hydrogen-bonded to the bridging oxygen atom in the V-O-Si bonds (designated as hydrogen-bonded pyramidal structure) or bonded directly to the V center (similar to bigrafted structure). The hydrogen-bonded pyramidal structure is more likely than the bigrafted structure because all recent theoretical calculations suggest that only the trigrafted VOx structure is stable under highly dehydrated condition (>573 K). Further theoretical modeling is warranted to bring a clear picture of structure of the VOx species B. The redox property of the two VOx species was tested with a preliminary hydrogen reduction experiment followed by in situ UV and visible Raman spectroscopy. Parts A and B of Figure 7 give the 244 (detecting species B) and 442 nm (detecting mainly species A) excited Raman spectra collected at room temperature after different reduction temperatures for 0.25 V sample. For the 244 nm excited spectra, the intensity ratio of the bands at 1032 to 1060 cm-1 decreases as reduction temperature increases. This intensity ratio change is in the reverse direction of the case when VOx density increases on silica surface (part C of Figure 2), indicating the decrease of species A coverage due to hydrogen reduction. By comparing the intensity ratio of the two bands at 1032/1060 cm-1 to that in part C of Figure 2, it is estimated that about 90% of species B has been reduced at temperature higher than 973 K. The observation of rather strong Raman features due to unreduced VOx even after 1073 K reduction is attributed not only to the resonance enhancement effect but also to the decreased selfabsorption64 because supported vanadia oxide shows less absorption in the UV region when reduced.65,66 While in the 442 nm excited spectra, a continuous decrease in intensity of the band at 1041 cm-1 is observed with increasing reduction temperature. This band disappears at reduction temperatures of 973 K and above as a result of reduction of species A (and species B). However, the disappearance of the VdO band does not necessarily indicate a complete reduction of VOx species because the Raman intensity decrease can also be partially caused by the increased self-absorption effect64 of the reduced sample in the visible Raman region.66 Nevertheless, most of species A and B seems to have been reduced at high temperatures. Reoxidation in oxygen is able to restore the reduced vanadia oxide species back to oxidized ones because the Raman spectra of freshly calcined and reoxidized samples resemble with each other. Therefore, it appears that VOx species A and B have no significant difference in reduction and oxidation behaviors and thus may both participate in redox reactions that VOx/SiO2
Multiwavelength Raman of VOx/SiO2
J. Phys. Chem. C, Vol. 114, No. 1, 2010 421 as a function of surface VOx density. The majority species of the dispersed VOx species has a pyramidal structure, whereas the minor species is assumed to possess a partially hydroxylated pyramidal structure. There is no significant difference in redox behavior of the two species and thus they may both participate in the redox reactions catalyzed by VOx/silica. Further in situ MW Raman spectroscopy of supported metal-oxide catalysts under reaction conditions offer the possibility of differentiating active metal-oxide species (monomeric species vs polymeric species) and active sites (e.g., MdO vs. M-O-S, M stands for metal, S for support) from spectators. Acknowledgment. This research was supported by the Center for Nanophase Materials Sciences (CNMS), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. We thank Dr. Ilia Ivanov at CNMS for help with the in situ UV-vis-DRS measurements. We are grateful for the fruitful discussion with Prof. Peter C. Stair at Northwestern University. Supporting Information Available: Figure of in situ UVVis DRS spectra of dehydrated VOx/SiO2 samples, and a figure of Raman spectra of dehydrated 0.001 V sample collected during heating in O2/He from 298 to 773 K. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 7. Raman spectra excited by 244 nm (A) and 442 nm (B) for 0.25 V sample collected at room temperature after hydrogen reduction at different temperatures (673 - 1073 K).
catalyzes. Further in situ Raman study under redox reaction conditions such as alkane oxidative dehydrogenation will provide insights into the catalytic function of the two different VOx species. 5. Conclusions In situ multiwavelength Raman spectroscopy and UV-visDRS have been used to investigate the VOx/SiO2 catalytic systems under dehydrated conditions. Multiwavelength excitation permits selective resonance enhancement that can be used to differentiate surface metal-oxide species and clarify spectral assignments, underscoring the advantage of this technique in characterizing the structure of supported metal-oxide catalysts. The three major bands at 920, 1032 (1041), and 1060 cm-1 observed on dehydrated VOx/SiO2 samples are demonstrated to be mainly associated with symmetric V-O-Si, VdO, and antisymmetric V-O-Si modes, respectively. This well supports the recent DFT-calculated Raman results on VOx/silica system.37 At least two different monomeric VOx species have been identified coexisting on highly dehydrated silica surface in extremely wide VOx loading range. Quantification of the two dispersed VOx species as well as crystalline V2O5 is achieved
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