Vanadium oxide spectroscopy

In this paper selectivity in partial oxidation reactions is related to the manner in which hydrocarbon intermediates (R) are bound to surface metal centers on oxides. When the bonding is through oxygen atoms (M-O-R) selective oxidation products are favored, and when the bonding is directly between metal and hydrocarbon (M-R), total oxidation is preferred. Results are presented for two redox systems ethane oxidation on supported vanadium oxide and propylene oxidation on supported molybdenum oxide. The catalysts and adsorbates are stuped by laser Raman spectroscopy, reaction kinetics, and temperature-programmed reaction. Thermochemical calculations confirm that the M-R intermediates are more stable than the M-O-R intermediates. The longer surface residence time of the M-R complexes, coupled to their lack of ready decomposition pathways, is responsible for their total oxidation. 

Cortez, G.G. Banares, M.A. A Raman Spectroscopy Study of Alumina-Supported Vanadium Oxide Catalyst During Propane Oxidative Dehydrogenation with Online Activity Measurement /. Catal. 2002, 209, 197-201. 

This book does not follow a chronological sequence but rather builds up in a hierarchy of complexity. Some basic principles of 51V NMR spectroscopy are discussed this is followed by a description of the self-condensation reactions of vanadate itself. The reactions with simple monodentate ligands are then described, and this proceeds to more complicated systems such as diols, -hydroxy acids, amino acids, peptides, and so on. Aspects of this sequence are later revisited but with interest now directed toward the influence of ligand electronic properties on coordination and reactivity. The influences of ligands, particularly those of hydrogen peroxide and hydroxyl amine, on heteroligand reactivity are compared and contrasted. There is a brief discussion of the vanadium-dependent haloperoxidases and model systems. There is also some discussion of vanadium in the environment and of some technological applications. Because vanadium pollution is inextricably linked to vanadium(V) chemistry, some discussion of vanadium as a pollutant is provided. This book provides only a very brief discussion of vanadium oxidation states other than V(V) and also does not discuss vanadium redox activity, except in a peripheral manner where required. It does, however, briefly cover the catalytic reactions of peroxovanadates and haloperoxidases model compounds. 

A criterion for the suitability of a spectroscopy cell for investigations of working catalysts can be formulated as follows the activity or selectivity data and activation energy values have to be in agreement with the catalytic performance data measured with a conventional fixed-bed reactor. Table 1 is a comparison of the conversion and selectivity values characterizing an alumina-supported molybdenum-vanadium oxide catalyst during propane ODH obtained with a conventional fixed-bed reactor and with a spectroscopic cell that fulfills this requirement (Banares and Khatib, 2004). Similar considerations have also been reported earlier for other methods, such as X-ray diffraction (Clausen et al., 1991). 

TABLE 1 Catalytic Activity of a Monolayer Catalyst of Molybdenum and Vanadium Oxides on Alumina with a Mo V Atomic Ratio of 1 1 for Propane ODH (Banares and Khatib, 2004), Measured in a Conventional Fixed-Bed Reactor and a Fixed-Bed Reactor Cell for Raman spectroscopy. 

These results led to questioning of the presumed similar performance of polymeric and isolated surface species. Raman spectroscopy and UV-vis DRS investigations of zirconia- and alumina-supported vanadia showed that the oxidation state of the surface vanadium oxide species was determined by the propane-to-C>2 ratio in the feed (Garcra-Cortez and Banares, 2002). The combination of two spectroscopic techniques provided more detail about the structural state of the supported species during moderate reduction under reaction conditions (Gao et al., 2002). 

Raman Spectroscopy of Vanadium Oxide Supported on Alumina... 

The molecular state ot vanadium oxide supported on ditterent alumina phases (7, S-0, and a) was investigated with Raman spectroscopy. The supported vanadium oxide was "found to "form a molecularly dispersed overlayer on the di-f-ferent alumina phases. The molecular state oT the surTace vanadium oxide phase, however, was dependent on the nature oT the alumina support. This variation was primarily due to the presence oT surTace impurities, in particular sodium oxide. The surface sodium oxide content was found to increase with the calcination temperature required to form the different transitional alumina phases (a, 6-0, 7). The... 

Recent studies of supported vanadium oxide catalysts have revealed that the vanadium oxide component is present as a two-dimensional metal oxide overlayer on oxide supports (1). These surface vanadium oxide species are more selective than bulk, crystalline V2O5 for the partial oxidation of hydrocarbons (2). The molecular structures of the surface vanadium oxide species, however, have not been resolved (1,3,4). A characterization technique that has provided important information and insight into the molecular structures of surface metal oxide species is Raman spectroscopy (2,5). The molecular structures of metal oxides can be determined from Raman spectroscopy through the use of group theory, polarization data, and comparison of the... 

In the present investigation, the interaction ot vanadium oxide with dif-ferent alumina phases (7, 6-6, and a) is examined with Raman spectroscopy. Comparison ot the Raman spectra ot the supported vanadium oxide catalysts with those obtained -from vanadium oxide reference compounds allows for the structural assignment of these supported species. The present Raman data demonstrate that the molecular structures of the surface vanadium oxide phases are significantly influenced by the presence of surface impurities on the alumina supports and this overshadows the influence, if any, of the alumina substrate phase. 

Resonance Raman Spectroscopy - 0-Al2O3-Supported Vanadium Oxide Catalysts as an Illustrative Example... 

I 4 Resonance Raman Spectroscopy - 6-AlfiySupported Vanadium Oxide Catalysts 4.2... 

Chemistry, spectroscopy and the role of supported vanadium oxides in heterogeneous catalysis. Catalysis Today, 78 (1-4), 25 6. 

Went, G.T., Oyama, S.T. and Bell, A.T. (1990) Laser Raman spectroscopy of supported vanadium oxide catalysts. Journal of Physical Chemistry, 94 (10), 4240-6. 

Sun, Q., Jehng, J.-M., Hu, H. Herman, R.G., Wachs, l.E. and Klier, K. (1997) In situ Raman spectroscopy during the partial oxidation of methane to formaldehyde over supported vanadium oxide catalysts. Journal of Catalysis, 165 (1), 91-101. 

Gao, X. and Wachs, I.E. (2000) Investigation of surface structures of supported vanadium oxide catalysts by UV-vis-NIR diffuse reflectance spectroscopy. Journal of Physical Chemistry B, 104 (6), 1261-8. 

S. and Jackson, S.D. (2005) On the structure of vanadium oxide supported on aluminas UV and visible Raman spectroscopy, UV-visible diffuse reflectance spectroscopy, and temperature-programmed reduction studies. Journal of Physical Chemistry, 109, 2793-800. 

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