Cálculo de la Demanda Total Actual

1.2.6.1

In situ UV-vis spectroscopy

Ultraviolet visible spectroscopy (UV-vis) is an excellent spectroscopic tool to characterize the individual oxidation state of a transition metal oxide catalysts and its coordination environment. As mentioned in section 1.2.1, the structure of supported VOx catalyst is highly dependent on reaction conditions. Therefore, experiments aimed at elucidating relevant chemical structure of these materials need to be performed in situ. UV-vis spectroscopy can thus be applied to investigate the structures of pentavalent vanadium (V+5)-containing materials due to the ligand-to-metal charge transfer (LMCT) transitions of V in the UV-vis (208 - 500 nm, 20000 - 48000 cm-1 ) range. In the case where VOx catalyst are supported on SiO2 or Al2O3, the electronic transitions from the supports are negligible as compared to the strong absorption of V+5 47–49. However, in the case of the VOx/TiO2 catalysts, the LMCT from TiO2 overlaps with that from V cations. Scientists tried to overcome this difficulty by mathematically subtracting the TiO2

absorption band from the DRS spectra of the VOx/TiO2 catalysts based on the expectation that only adsorption by V+5 was displayed50–52.

Gao and Wachs extensively used UV-vis spectroscopy to provide the structural characteristic of supported VOx species on various metal oxide supports under different environmental conditions53_{. They observed that the band gap (E}

g) of VOx/TiO2 catalysts with different vanadia loadings (1% and 5%) showed at 2.77 and 2.65 eV, respectively. The authors pointed out that the slight change in band gap between the two catalysts resulted likely from the strong TiO2 support absorption in the higher energy region (200 – 300 nm) that overlaps the weak absorption from a small amount of V+5 in the same region51,53. Because the Eg values for reducible metal oxide supports such as TiO2, and CeO2 are so close to that of VOx, they, thus, suggested that the investigation of structures of VOx/TiO2 catalysts may not be reliable, especially for catalysts with low VOx loadings.

Nonetheless, correlations between the edge energy (Eg) and the number of covalent V-O- V bonds (CVB) has been empirically well-established, as shown in Fig.1.7. This correlation was based on the edge energy (Eg) values obtained for several crystalline pentavalent vanadium (V+5)-containing compounds and oxides (V2O5, MgV2O6, NH4VO3, Mg2V2O7, and Mg3V2O8). The obtained Eg values are inversely proportional to the CVB number. The equation of the correlation line can be expressed by CVB = 14.03 - 3.95·Eg (eV)53. It is well known that the Eg of nanosized particles shifted to higher energies as the particle size decreases. This is due to the valence and conduction bands in nanosized particles comprising discrete electronic levels as opposed to continuous energy bands present in large particles. Moreover, Weber previously proposed that the energy band gap values of molecularly sized clusters associates with the degree of spatial delocalization of the molecular orbitals involved in the electronic transitions of these materials54_.

Figure 1.7. Edge energies of V5+_{-containing reference oxides/compounds as a}

function of number of covalently bonded V-O-V bonds in the coordination sphere of central V+5 _{cation. Reproduced with permission from [53].} 53

UV-vis spectroscopy is also a powerful technique to characterize the reduced species present in the catalyst. This can be accomplished by defining “the extent of reduction” of the supported VOx catalysts as the number of transferred electrons to vanadia per vanadium atom (∆e−/ 𝑉 𝑎𝑡𝑜𝑚). Argyle and coworkers extensively analyzed various optical spectral features of oxidized and reduced supported VOx species on alumina, in particular, the pre-edge spectral features caused by the reduction47,48_{. The values that} represent the extent of reduction were obtained by the authors from a series of experiments: first, the catalysts (VOx/Al2O3) were reduced in flowing H2. Then, the catalyst was exposed to oxygen and the amount of O2 consumed by the catalyst was measured while the UV-Vis absorption spectra were obtained, this continued until the catalysts returned back to its fully oxidized state (V+5). The extent of reduction (∆e−/

𝑉 𝑎𝑡𝑜𝑚) was calculated by assuming that each O atom accepts two electrons. The results showed a linear correlation between the calculated extent of reduction and the pre-edge absorption intensities measured by UV-vis spectroscopy. This analysis was applied during propane oxidative dehydrogenation reaction carried over alumina supported catalyst with different vanadia loadings. The results of such analysis indicate that the extent of reduction per surface vanadium (∆e−_{/ 𝑉 𝑎𝑡𝑜𝑚) atom increase with increasing}

propane to O2 ratio in the influent stream and also with increasing VOx loading in the catalysts (Fig.1.8). A low extent of reduction (i.e. 0.11 ∆e−/ 𝑉 𝑎𝑡𝑜𝑚 at global maximum in Fig.1.8) was observed regardless of VOx loading, and turnover rate values increased with vanadium loading. The significant differences in the extent of reduction that resulted from the use of catalyst with different VOx loadings and the low values of the extent of reduction observed for all catalysts tested indicated that the absolute number of reduced vanadium atoms present during catalytic turnovers is unrelated to catalytic activity.

Figure 1.8. Dependence of the extent of catalytically relevant reduction per surface V-atom on the C3H8/O2 ratio for VOx/Al2O3 catalysts during propane oxidative

dehydrogenation [filled symbols: C3H8 dependence (1.0-16 kPa C3H8, 4.0 kPa O2,

balance Ar, 603 K); open symbols: O2 dependence (4.0 kPa C3H8, 1.0-16 kPa O2,

balance Ar, 603 K); diamonds: 3.5 wt % V2O5/Al2O3 (2.3 V/nm2); squares: 10 wt %

V2O5/Al2O3 (8.0 V/nm2); triangles: 30 wt % V2O5/Al2O3 (34 V/nm2); filled circles are

C3H8 dependence (8.0 kPa O2, 1.0-12 kPa O2, balance Ar, 603 K) for