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As previously discussed, vibrational spectroscopy is a sensitive probe of the atomic structure and chemical bonding and thus of electronic structure. Most frequently used vibrational techniques are Infrared, and Raman spectroscopy. Raman spectroscopy, based on inelastic scattering effects, has become a powerful tool for studying the structure of catalysts and identifying catalytic active sites21,51,55–59_{. For the specific case of vanadia,} this technique is considered the gold standard for the identification of the specific type surface vanadium-oxygen bonds present in the catalyst57.

Here, bulk vanadium oxide (V2O5) typically exhibits Raman peaks at 990, 700, and 483 cm-1 15,51. The strong peak at 990 cm-1 is typically assigned to the characteristic symmetric stretching of the terminal vanadyl bond (V=O) and is characteristic of large vanadium oxide clusters. On the contrary, supported well-dispersed VOx structures do not exhibit this characteristic sharp Raman band found in bulk V2O5. In the case of supported VOx, instead, other characteristic peaks appear depending on the degree of agglomeration of the vanadium oxide clusters (referred as polymerization). Under fully oxidizing dehydrated conditions the Raman bands of oxidized supported vanadium oxide appear at ~1030 and ~920-950 cm-1, as depicted in Fig.1.9. The strong and intense peak at ~1030 cm-1 is assigned to the characteristic symmetric stretching of a mono-oxo terminal vanadyl species and the broad Raman band at ~920-950 cm-1, assigned to bridging V-O- V moities. In contrast to these V=O and V-O-V bonds, which are easily identified through Raman signals it is difficult to detect Raman bands from V-O-M (M describing the metal atom of the support) moieties, since these bonds have very weak Raman scattering cross-section, resulting in the inability to direct detect these type of vibrations60.

Figure 1.9. In situ Raman spectroscopy (532 nm) of dehydrated (500 °C) VOx/TiO2

catalysts as a function of surface vanadium density (V atoms per nm2_{) prepared by}

impregnation of ammonium metavanadate precursor in aqueous oxalic acid. Reproduced by permission from ref [61]. 61

Jehng et al. reported that sample temperature and presence of water vapor on the surface of vanadia supported over TiO2 sometimes influences the position of Raman bands of the vanadium oxygen bonds, specifically the frequency of the symmetric stretching mode of the terminal V=O Raman band62. In their study they observed that for the case of samples with a low vanadia loading (such as 1wt.% as V2O5), the terminal Raman band shifts from 1024 cm-1 at 450 oC to 1006 cm-1 at 120 oC and becomes broad in the

presence of water vapor. They reported as well that, for high loading VOx/TiO2, samples a red shift of terminal V=O Raman band is observed from 1029 to 1018 cm-1 when the temperature is decreased from 450 oC to 120 oC. Similarly, the characteristic band appearing at ~920-950 cm-1 commonly assigned to bridging V-O-V moieties was affected by the introduction of water vapor. On the other hand, the Raman bands of terminal V=O bonds at 990 cm-1 in large crystalline VOx species were not affected by the presence of water62_.

Raman studies in combination with 18_O/16_{O isotope oxygen exchange measurements have} also provided critical insights into the behavior of supported vanadia during redox catalysis. For instance, an isothermal isotopic exchange of 18O2 with 16O2 taking place over VOx/ZrO2 at relatively high temperatures (450oC) was conducted by Weckhuysen and coworkers63. After successive reduction-reoxidation cycles, a single new Raman band (18O=V), red-shifted by ~50 cm-1 from the original Raman band previously observed for the equivalent bond (16O=V), was generated at the expense of the original V=O band (16O=V). Therefore, it could be concluded that in this system, the terminal oxygens (16O) can be completely exchanged to 18O to form V=18O bonds63.

Moreover, isotopic Raman studies can also provide critical information regarding the elementary steps of catalyst reoxidation that takes place during redox catalysts. Though, to the best of our knowledge this has not been accomplished yet on the VOx/TiO2 system. However a study of this type was carried over a similar redox catalytic system (MoOx/TiO2)59. The results reported are applicable to vanadia since in the molybdena/titania system molybdenum is present in a mono-oxo tetrahedral configuration O=Mo(-O-Ti)3, analogous to vanadia in the VOx/TiO2 system. Mo can also shift between several redox states, in a similar way as vanadium. The authors carried isotopic oxygen exchange at the same time that Raman spectra was acquired. They reported that a gradual red-shift in the terminal (16_O=Mo(-16_O-Ti)

3) band at 994 cm-1 to 988 cm-1 (16O=Mo(-18O-Ti)3) and to (18O=Mo (-18O-Ti)3) 944 cm-1after repetitive reduction-reoxidation cycles was observed. It was then suggested that this Raman shift in terminal oxygen band is due to a sequential process where 18O/16O exchange takes place first at oxygen sites in the titania support followed by exchange at the terminal oxygen of

theO=Mo functionality. On the basis of these observation, they also suggested that the most favorable sites for exchange could be the bridging Mo-O-Ti species or the Mo-O- Mo sites to a less extent. Summarizing these results, the 18O/16O isotopic exchange sequentially occurs as follow:

𝑂 16 _{= Mo(− 𝑂}16 _{− Ti)} 3 𝑂 18 𝑂 16 → 16𝑂 = Mo(− 𝑂18 − Ti) 3 𝑂 18 𝑂 16 → 18𝑂 = Mo(− 𝑂18 − Ti) 3

Such an isotopic substitution then takes place selectively before the substitution of the terminal 16_{O of the terminal Mo=O site. They also pointed that unsaturated Ti-O-Ti sites} are easier to be substituted during isotopic exchange process. Thus, 18_{O substitution take} place on the unsaturated surface Ti-O-Ti sites followed by a surface diffusion/spillover of dissociated 18O toward Mo-O-Ti sites. Although direct extrapolation of these results to the VOx/TiO2 system needs to be carefully considered, a viewpoint where the V-O-M (M: metal support atom) moieties in the catalyst play a similar role could be proposed for the case of reducible metal oxides supports such as TiO2, and CeO2.