Multiwavelength Raman Spectroscopic Study of Silica-Supported

May 11, 2011 - of his criticisms is our Raman band (associated with VdO and. VАOАSi modes) assignment of the VOx/SiO2 system, where. Stiegman made t...
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Reply to Comment on “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, United States n his comment,1 Stiegman criticized certain points of our paper2 titled “Multiwavelength Raman Spectroscopic Study of Silica-Supported Vanadium Oxide Catalysts”. The essential point of his criticisms is our Raman band (associated with VdO and V O Si modes) assignment of the VOx/SiO2 system, where Stiegman made the assignment based on an 8-atom model study while our assignment was supported by a DFT study on cluster models.3 As shown below, we believe Stiegman’s model does not sufficiently consider the profound effect of the surface structure and environment of silica on the vibrational spectra and normal modes of surface vanadia species. We will start by showing how our Raman results are well supported by the DFT cluster models compared with Stiegman’s model. We will also address his other comments.

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1. NORMAL MODE ANALYSIS OF THE VOX/SiO2 SYSTEM First of all, in agreement with Stiegman’s comments, we emphasized in our paper2 in both the Introduction and at the beginning of the Discussion that there is strong vibrational coupling between the vanadia species and the silica support. As such, it may not be possible to assign each of the Raman bands to a single specific mode, and the assignments are made to the modes that contribute most to each of the bands. So, the VdO and V O Si modes are still used throughout the paper. This is in line with the normal modes theory. One of the objectives of our paper was to make assignments of the Raman bands at 920, 1032, and 1060 cm 1 to the VdO or V O Si mode. Since both the VdO and V O Si vibrations couple with those of the silica support,3,4 the VdO mode refers to a mode with dominating contribution from VdO stretching over V O Si stretching. The same is true for the V O Si mode where V O Si stretching dominates over VdO. Determination of the contribution from VdO and V O Si stretching to specific Raman bands is not straightforward in the complex VOx/SiO2 system and needs theoretical modeling. Although we did not perform theoretical calculation in our paper, we were guided by a recent DFT study3 of the effect of local surface structure of silica on the vibrations of supported vanadia to support our assignments (ref 37 in our paper). Stiegman assigned Raman bands of silica-supported vanadia with help from a normal coordinate analysis of the primary stretching modes of the vanadium oxo group in VO(Si O)3 using a central force approximation.1,5 Valuable information was obtained from this analysis; e.g., all the vibrational bands have significant contribution from all of the coupled oscillators, and especially V O Si stretches are a very important component to all the modes. Actually, a similar conclusion was already drawn in our previous combined experimental and DFT study of r 2011 American Chemical Society

a silica-supported VOx system with the Sauer group.4 However, the Raman bands were assigned differently in the two studies; e.g., Stiegman assigned the bands at 920 and 1033 cm 1 to the VdO and V O Si modes, respectively, while Sauer assigned them to the V O Si mode and VdO or V O Si mode, respectively. The difference is most likely because of the different calculation models used. In quantum mechanical calculation of vibrational normal modes and vibrational spectra, the crucial step is the choice of a suitable model system to represent the surface species. Since the real catalyst system is often very complex, it is sensible to start with finite size models cut out of the surface structure, so-called cluster models that model the direct environment of the reactive center. Extended models for the bulk support can be considered in a second step to see the effect of interactions with the bulk on the vibrational normal modes and frequencies. In a recent hybrid DFT/MM calculation of silica-supported vanadia,3 Sauer and co-workers built several different cluster models that model the different environment of the vanadia center on the silica surface. Their results show that the contribution of VdO and V O Si stretches to the normal modes varies significantly depending on the cluster model used, emphasizing both the essential role of silica surface structure and environment on the vibrational spectra of vanadia and the hazards of a simplified model. The 8-atom model, VO(Si O)3, used by Stiegman necessarily ignores the surface and bulk structure and environment of silica and thus cannot well represent the complex silica-supported VOx system. So the result from his approximation may not reasonably explain and assign the Raman spectra measured from a complex VOx/ SiO2 sample. In the following section, we will show how our results are fully supported by the cluster model study by Sauer and co-workers. It is necessary to briefly summarize some of the calculation results from Sauer’s recent paper3 to support our assignment of the Raman bands at 920, 1032, and 1060 cm 1. Amorphous silica exhibits a variety of different local surface structures and is difficult to model by DFT as it requires huge pseudocells, and the generation of a realistic structure model is not an easy task. Sauer and co-workers3 adopted a strategy by using three differently ordered silica phases (Edingtonite, hexagonal prism slab, and Cristobalite) to generate different local environments of the vanadyl site. These span the range of local structures of silica that may be found in real powder catalysts. Both Edingtonite (Figure 1 in ref 3) and Cristobalite (Figure 4 in ref 3) models represent the hydroxylated silica surface where monomeric vanadia species are Received: July 5, 2010 Revised: January 18, 2011 Published: May 11, 2011 10925

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The Journal of Physical Chemistry C surrounded by silanol groups. One difference between the two structures is that the surface OH groups are denser (closer in distance) on Cristobalite. Hexagonal prisms (Figure 3 in ref 3) are used to model monomeric vanadia without neighboring silanol groups, namely, a dehydrated silica surface. The vibrational frequencies as well as the contribution of VdO and V O( Si) bonds to a specific band were calculated. Similarly, normal modes with dominating contributions of VdO and V O( Si) bonds were referred to as “VdO” (vanadyl) and “V O Si” (in phase/out-of-phase) modes. For details about the cluster models and DFT methods, readers are referred to the original paper.3 Since our Raman study was concentrated on monomeric vanadia species on the dehydrated silica surface, the system can be closely represented by models with no or few surface silanol groups. So, among the three models, results from modeling vanadia on a hexagonal prism can best represent our dehydrated VOx/SiO2 system. The results from vanadia on the Edingonite model may also be relevant because surface silanol groups are still present on a real VOx/SiO2 system even after dehydration at 773 K. The highly hydroxylated Cristobalite model represents more closely the hydrated VOx/SiO2 system but is not relevant for our studies. The embedded cluster results on both VOx/ hexagonal prism and VOx/Edingonite models showed that the vanadyl mode in monomeric vanadia species, ranging from 1064 to 1012 cm 1, has a dominating contribution of ∼90% from VdO stretching (Tables 3 and 4 in ref 3) and thus is almost separated from the bulk mode. This provides strong support that the VdO mode in our VOx/SiO2 system can be treated as a pseudodiatomic VdO stretch. We acknowledge that had we briefly discussed the “pure” nature of the VdO mode in the VOx/SiO2 system in our original paper, and it would be more solid to carry out the diatomic approximation analysis associated with overtones of the VdO mode, harmonic wavenumber, force constant, and dissociation energy. Nevertheless, we maintain that the analysis is still valid and can be used to rationalize the assignment of the 1032 cm 1 band to VdO mode in a dehydrated VOx/SiO2 system. The force constants are given in Table 1 (last column) in our paper,2 although we have found a small error in calculating the published values as pointed out by Stiegman. The correct value for the four constants from top to bottom should be 7.76, 7.79, 7.75, and 7.79, respectively. The correct equation for calculating the anharmonic force constant (ωe = (2πc) 1(fe/μ)0.5) was used but was published with an error. None of the discussions and conclusions are affected by these corrections. The calculated vibrational frequency3 for the in-phase and outof-phase V O Si mode of monomeric vanadia species also supports our band assignment: 920 cm 1 to the out-of-phase V O Si mode and 1060 cm 1 to the in-phase V O Si mode. The cluster model results (Tables 3 and 4 in ref 4) showed that there is much stronger coupling of bonds in V O Si to the Si O Si bonds of silica than in the VdO mode. In the VOx/ hexagonal prism model which is very close to our dehydrated VOx/SiO2, the out-of-phase V O Si mode is less coupled to the slab. In all cases, the V O contribution to the V O Si modes is much larger than the VdO contribution. Realizing the strong coupling, the force constant of V O( Si) was calculated in our paper2 using the effective reduced mass of V, O, and Si rather than the diatomic reduced mass of V and O. The correctly calculated force constant for the out-of-phase V O( Si) stretch should be 4.32 mdyn/Å instead of the erroneously calculated

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4.57 mdyn/Å in the paper. We acknowledge that this force constant is associated with the average of all the contributing bonds, not to a single V O( Si) bond. Since the V O Si contribution is much larger than VdO to the V O Si mode,3 the calculated force constant could be used very roughly to represent the triatomic V O Si/Si O Si bond. Because V O Si and Si O Si have similar bond length and force constant (this is the reason for the strong coupling between the two types of bonds vibrations) but different from those of VdO,3,4 the calculated force constant can still be used approximately to distinguish V O( Si) from VdO bonds. Thus, we maintain that the analysis of the force constant and dissociation energy of the V O Si in our paper is still meaningful. It is worthwhile to reiterate the DFT results from Sauer’s paper3 that show how important it is to consider the surface structure and environment of silica in the modeling. Although the computed frequencies of VdO and V O Si stretches are found similar in the VOx/Cristobalite model in comparison to other two models (VOx/hexagonal prism and VOx/Edingonite), the VdO stretch contribution to the VdO mode is much less in VOx/Crystobalite (highest at 35%). The drastic change of the VdO stretch contribution to the VdO mode on different cluster models highlights the significant role of support surface structure and environment on the vibrations of vanadia species. This conclusion is supported experimentally in a recent Raman and DFT study6 of VOx supported on dealuminated BEA zeolite, where both the VdO and V O Si modes are found to be affected greatly by the presence of a hydroxyl group on the V site or in the vicinity. These DFT studies manifest the importance of constructing a close-to-real model in theoretical calculation of vibrational normal modes and vibrational spectra of a catalyst system. This observed dependence cannot be reproduced by an 8-atom cluster used in Stiegman’s analysis, and thus his results are not expected to represent the real complex VOx/SiO2 system. We acknowledge that normal modes should be considered in vibrational spectra analysis of supported metal oxide systems as suggested by Stiegman. This is particularly necessary for systems where the metal oxide vibrations are close in energy with the support oxide vibrations such as VOx/SiO2. When the metal oxide vibrations are not strongly coupled with the support vibrations, it is still meaningful to use the traditional MdO and M O S (M and S stand for metal and support, respectively) designations, and any conclusion drawn from the spectroscopic analysis should be valid. For example, our previous paper of combined experiments and DFT calculation4 showed that for V/alumina the vanadyl vibrations are separated from the interface mode (V O Al) and can be considered as an independent oscillator because the alumina vibrations occur at much lower frequency than the VdO stretch. Namely, the VdO mode in VOx/Al2O3 is “pure” and can be considered as a VdO diatomic stretch mode.

2. 18O LABELING STUDIES Stiegman1 argued that the spectral intensity change in the 18O isotopic exchange (Figure 3A in our paper2) does not support the assignment of the 1032 cm 1 band to the isolated diatomic VdO stretch. We persist in the assignment that the 1032 cm 1 band is mainly associated with the VdO stretch and provide here two additional pieces of evidence, shown in Figures 1a and 1b. Figure 1a gives the time-resolved Raman spectra (excited by 244 nm) collected at 723 K during oxygen isotopic exchange on 10926

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The Journal of Physical Chemistry C

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and overtone regions clearly support that the 1032 cm 1 band is associated with the VdO fundamental mode with close-todiatomic nature.

Figure 1. (a) Raman spectra (λex = 244 nm) of the 0.25 V sample in the fundamental and overtone regions collected at 723 K during 18O isotopic exchange with different durations. Spectra normalized to 400 600 cm 1 region bands where no appreciate isotopic exchange occurred. (b) Plots of the intensity of Raman bands at 1028 and 986 cm 1 vs oxygen isotopic exchange time on 0.25 V sample from Figure 1a.

0.25 V/SiO2. Different from Figure 3A in our original paper, the first overtone region is also included in Figure 1a. Because of the interference from the in-phase V O Si mode (1060 cm 1) in the fundamental region of the VdO mode, the exchange process can be more clearly observed in the overtone region where the inphase V O Si mode does not show an overtone band. The band at 2049 cm 1 apparently disappears with concomitant increase of a band at 1956 cm 1 with increasing degree of isotopic exchange. Clearly, no other bands can be seen between the two bands. Figure 1b shows the intensity change of the two bands at 1028 and 986 cm 1 as a function of oxygen isotopic exchange time where the decrease of the 1028 cm 1 band is accompanied by the increase of the 986 cm 1 band. Moreover, the total intensity of the two bands is approximately constant during the exchange process. The results from both fundamental

3. TEMPERATURE EFFECT ON RAMAN BAND INTENSITY Stiegman1 provided a different explanation of the temperature effect on the intensity of the Raman bands at 1060 and 1032 cm 1. In our original paper, the explanation of the stronger temperature effect on the intensity of the 1060 band than on the 1032 cm 1 band was that the V O associated with the 1060 cm 1 band is weaker than the one associated with the band at 1032 cm 1 based on Xie et al.,7 who showed that more severe temperature effects on the intensity of Raman bands will be found on metal oxide with a weaker metal oxygen bond. The conclusion by Xie et al. was drawn from experimental observation of the temperature effect on the Raman band intensity of several metal oxides including Nb2O5, WO3, MoO3, and V2O5. Stiegman instead related the temperature effect in Raman bands merely to the change of population of the ground-state phonons as given by Boltzmann distribution. It is true that temperature would affect the Raman band intensity by affecting the population of the ground state and has different effects on bands at different Raman shifts, with lower frequency modes decreasing more rapidly than higher frequency. Accordingly, the intensity of the Raman band at 1032 cm 1 is expected to decrease more rapidly than the one at 1060 cm 1 when temperature increases if the change in the ground state density plays the main role. However, this is contrary to our Raman observation (Figure 3A and E-2 in Supporting Information in our paper2) where the 1060 cm 1 band suffers more severely from a temperature increase than the 1032 cm 1 band. Evidently, the population of ground state phonons is not the primary factor controlling the observed temperature dependence, and other factors must be at work. We proposed in our paper that two factors may contribute to this observation: one is that the bond associated with 1060 cm 1 is weaker than the one with 1032 cm 1; the other is that the charge transfer band in the mode (V O Si) associated with the 1060 cm 1 band is closer to 244 nm excitation than that of VdO, which would cause higher sensitivity to temperature as indicated in the study by Xie et al.7 4. DISCRIMINATION OF DIFFERENT VANADIA SPECIES VIA MULTIWAVELENGTH EXCITATION Stiegman1 challenged the spectroscopic principle we have employed to explain the detection of different surface metal oxide species using different laser excitations.2,8 10 He raised two arguments. First is that “it will always be uncertain whether one is observing a new species or just the resonant enhancement of a new mode of a single species”. We agree that resonance enhancement of different modes of the same species sometimes can happen by changing laser excitation wavelength, such as the 1060 cm 1 mode in our study which was selectively enhanced when excited by 244 nm excitation. However, the simultaneous observation of the two bands at 1041 and 1032 cm 1 in the nonresonance visible Raman spectra (Figures 2A and 6 in the original paper) and the change of their relative intensity as a function of vanadia loading strongly suggest that these bands are the same mode but of different vanadia species. Furthermore, DFT calculations3,4,8,11 14 of supported vanadia systems 10927

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The Journal of Physical Chemistry C consistently suggested that Raman bands in the spectral range 1000 1050 cm 1 are due to vanadyl stretch. So, we believe that two bands at 1041 and 1032 cm 1 are due to the same mode, i.e., VdO. The variation of vanadyl stretch reflects the change of the structure of vanadia species and thus new species. The second argument by Stiegman is that “the electronic absorption spectrum of different species derived from the same compounds (i.e., different vanadium oxo species) will have electronic absorption bands in the same energy regime (e.g., all oxo-to-vanadium charge transfer) and, in fact, are likely to have very similar spectral features”. This is not consistent with previous literature. The structures of the vanadia species, e.g., monovanadate vs polyvanadate, are quite different in terms of geometry of the species and coordination number of the vanadium center. The electronic properties of the species are thus greatly affected, and absorption spectra could be very different. This has been confirmed in the UV vis spectra of VOx/Al2O3 samples that show very different absorption bands when different vanadia species are present on the alumina surface.10,15 17 It was shown15 that tetrahedrally coordinated monovanadate gives UV vis absorption bands at around 240 and 290 nm, while bands at 270, 340, and 412 nm have been ascribed to polyvanadate in either tetrahedral or pentahedral coordination. Crystalline V2O5 shows several bands in the range 220 650 nm.15 Clearly, these different vanadium oxo species have electronic absorption bands at very different energy regimes. In fact, the edge energy measured from UV vis DRS spectra has been successfully employed to distinguish between monovanadate and polyvanadate and quantify them.17 Therefore, excitation of Raman spectra within the absorption region of one species will produce resonance-enhanced spectra from that species with absorptions at the excitation wavelength. Through measurement of the Raman spectra at different wavelengths, different vanadia species can be selectively resonance enhanced. Stiegman also contested that the 9 cm 1 difference between 1041 and 1032 cm 1 is unremarkable and arises from the fact that when enhanced there is spectral congestion of nearby bands (in this case, the E modes at 1060 cm 1) whose overlap will distort the apparent peak position. We disagree with this. The 1032 cm 1 band is observed not only in 244 nm excitation but also in 325 nm excitation where no congestion by 1060 cm 1 is observed (see Figure 1 in the original paper2). Furthermore, in the VOx/Al2O3 system10 where there is no spectral congestion above the 1000 cm 1 region, different VdO stretches were observed when excited by UV and visible wavelengths. The 9 cm 1 shift is also not an experimental error because: (1) the spectra were carefully calibrated using a quadratic fit of the observed to the actual wavenumbers of the cyclohexane standard and (2) the bending mode of the silanol groups is at the same frequency (around 976 cm 1) in the thus-calibrated UV Raman spectra as in the visible Raman spectra (see Figure 2 in our paper2). For the VOx/Al2O3 system,10 well-calibrated UV and visible Raman spectra also consistently show VdO stretches at different Raman shifts. Altogether, the small difference in Raman shift of the VdO mode from UV and visible excitations is real and can be reasonably used as evidence for the presence of different vanadia species. To conclude, we have addressed the comments from Stiegman, and we stand by the results and conclusions of our paper.2 We agree with Steigman that there is strong coupling between vanadia and silica vibrations (as stated in our paper), and so caution should be taken in assigning Raman bands of the VOx/SiO2

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system; however, further we think that the vibration properties are strongly affected by the silica structure and environment. Theoretical calculation of the vibrational normal modes and vibrational spectra of supported vanadia species should be based upon highly realistic models that take into consideration the extended surface structure and surface environment of the supporting materials.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ 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. ’ REFERENCES (1) Stiegman, A. E. J. Phys. Chem. C 201010.1021/jp1045468. (2) Wu, Z. L.; Dai, S.; Overbury, S. H. J. Phys. Chem. C 2010, 114, 412. (3) Do^bler, J.; Pritzsche, M.; Sauer, J. J. Phys. Chem. C 2009, 113, 12454. (4) Magg, N.; Immaraporn, B.; Giorgi, J. B.; Schroeder, T.; Baumer, M.; Dobler, J.; Wu, Z. L.; Kondratenko, E.; Cherian, M.; Baerns, M.; Stair, P. C.; Sauer, J.; Freund, H. J. J. Catal. 2004, 226, 88. (5) Moisii, C.; van de Burgt, L. J.; Stiegman, A. E. Chem. Mater. 2008, 20, 3927. (6) Lewandowska, A. E.; Banares, M. A.; Tielens, F.; Che, M.; Dzwigaj, S. J. Phys. Chem. C 2010, 114, 19771. (7) Xie, S. B.; Iglesia, E.; Bell, A. T. J. Phys. Chem. B 2001, 105, 5144. (8) Kim, H. S.; Zygmunt, S. A.; Stair, P. C.; Zapol, P.; Curtiss, L. A. J. Phys. Chem. C 2009, 113, 8836. (9) Kim, H. S.; Stair, P. C. J. Phys. Chem. A 2009, 113, 4346. (10) Wu, Z. L.; Kim, H. S.; Stair, P. C.; Rugmini, S.; Jackson, S. D. J. Phys. Chem. B 2005, 109, 2793. (11) Todorova, T. K.; Dobler, J.; Sierka, M.; Sauer, J. J. Phys. Chem. C 2009, 113, 8336. (12) Islam, M. M.; Costa, D.; Calatayud, M.; Tielens, F. J. Phys. Chem. C 2009, 113, 10740. (13) van Lingen, J. N. J.; Gijzeman, O. L. J.; Havenith, R. W. A.; van Lenthe, J. H. J. Phys. Chem. C 2007, 111, 7071. (14) Ohde, C.; Brandt, M.; Limberg, C.; Doebler, J.; Ziemer, B.; Sauer, J. Dalton Trans. 2008, 326. (15) Gao, X. T.; Wachs, I. E. J. Phys. Chem. B 2000, 104, 1261. (16) Olthof, B.; Khodakov, A.; Bell, A. T.; Iglesia, E. J. Phys. Chem. B 2000, 104, 1516. (17) Tian, H. J.; Ross, E. I.; Wachs, I. E. J. Phys. Chem. B 2006, 110, 9593.

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