Vanadyl groups

Figure 2.15 Proposed setup to effect two-qubit gates onto Mo12V2, a mixed-valence POM that combines two localized electrons on the lateral vanadyl groups with a variable number of delocalized electrons in the Keggin core.

Virtually all amphoteric oxides are converted to monomeric anions in sufficiently strong basic media. Rieger and co-workers studied strongly basic oxovanadium(IV) solutions by ESR, optical and Raman spectroscopy.472 Raman absorption at 987 cm-1 confirms the presence of the vanadyl group in solution, and from ESR spectra and titration, the authors concluded that vanadium(IV) exists as a monomer [VO(OH)3] for pHs 12. 

The nature of the 0(s) species for ethane activation probably varies depending on the catalyst. On reduced Mo/Si02, the surface O species generated by the decomposition of N20 has been shown to react readily with ethane (15), and O has also been suggested as the active species for the Li-Mg oxide catalyst (17). Oxygen vacancies or surface vanadyl groups may also be active sites. 

For KVOF4 very similar results have been obtained [59], In the crystal structure of this solid there are isolated VOF4 groups which contain a vanadyl group. Ab-initio calculations on complexes with a short V-O bond can account for the low oscillator strength of the lowest absorption transition, and therefore also of the emission transition [60, 61]. 

If the OH group is not blocked at all but left free, and the epoxidation reagent is the vanadium complex VO(acac)2 combined with f-BuOOH, the syn epoxide is formed instead. The vanadyl group chelates reagent and alcohol and delivers the reactive oxygen atom to the same face of the alkene. 

Figure 5 Crystal stmctures of (a) /3 - VOPO4 and (b) (V0)2P207. ( 0) shows a vanadyl group (V=0)...

A range of coordination complexes of has been prepared in which the vanadyl group is absent, a number of these compounds being synthesized as part of an effort to understand the nature of biological vanadium sites, such as amavadin," which contains an octadentate, hydroxyiminodipropionate ligand bound to vanadium A number of model complexes of this type have been prepared and discussion of this area can be found in the article Vanadium in Biology Further discussion in this section follows the main ligand donor atom to vanadium. 

It now seems clear that after vanadium is deposited onto the catalyst it may be mobile in an atmosphere of steam and oxygen (from air). Mobile vanadia species migrate into the particles but may be enriched at the surface segment of low surface area particles such as our inert clay microspheres or old sintered equihbrium catalyst. Surface vanadyl groups are transferred to other FCC particles and migrate through them as well. Migration ceases when a reaction occurs with catalyst moieties, such as A1 or rare earth, to form stable vanadates (17). 

The nature of the oxidic support plays an important role in modulating the nature of vanadium species and the catalytic behavior of the materials. In fact, changes in the electron density of the vanadyl double bond can be produced by the inductive effect through the bonds anchoring the tripodal vanadyl groups to the oxidic support [111]. These electronic effects can be clearly demonstrated using... 

Scheme 2.1 Schematic representation of the CT processes on the surface vanadyl groups of the tetrahedrally coordinated...

Figure 2.23 Emission and excitation spectra recorded at 300 K of vanadyl group anchored on metal oxide, (a) hydrated 0.8V/ Si02 (b) 3V/Ti02 (c) 2V/AI2O3 (d) 20V/MgO. Reprinted from ref [111], with permission from Springer.

PL spectra collected at low temperature (77 K) allow the observation of the vibrational profile of the vanadyl groups. This particular application of PL is extremely useful for distinguishing different kinds of tetracoordinated vanadium sites, as reported by Dzwigaj and coworkers for species incorporated in a P-zeoUte framework [93, 113]. The emission spectra collected at 77 K with = 250 nm of three V-P samples (VSiP as-synthesized, C-VSip calcined and C-Hyd-VSip calcined and then hydrated) are reported in Figure 2.25. All the solids exhibit emission... 

Figure 6.2 V 2p and O Is region ofV203 (0001) on W(llO) in comparison with V2O5 according to Dupuis and coworkers [52]. The spectra indicate that the V2O3 (0001) surface is terminated with vanadyl groups.

As shown in Fig. 8, the vibrational bands are clearly resolved in the photoluminescence spectrum of vanadium oxide supported on Vycor glass 33, 34, 36-38). The vibrational separation of about 1040 cm is in good agreement with the energy of the V=0 stretching vibration of the ground state of the vanadyl group of the oxide as measured by IR or Raman spectroscopies. 

Figure 19 shows the typical photoluminescencc spectrum of the anchored vanadium oxide catalyst prepared by photo-CVD methods (a), its corresponding excitation spectrum (b), and the UV absorption spectrum of the catalyst (c) (56,115,116). These absorption and photoluminescence spectra (phosphorescence) are attributed to the following charge-transfer processes on the surface vanadyl group (V=0) of the tetrahedrally coordinated VO4 species involving an electron transfer from to V and a reverse radia-... 

As shown in Fig. 20, a vibrational fine structure of the phosphorescence due to the V=0 double bond of the vanadyl group is clearly observed at 77 K. However, the fine structure is not observed at 298 K because of the significant contribution of an efficient radiationless deactivation arising from various types of vibrational interaction on the surfaces. From an analysis of the vibrational fine structure, the energy gap between the (0 -> 0) and (0 -> 1) vibrational transitions is determined to be about 1035 cm in good agreement with the vibrational energy of the surface V=0 bond obtained by IR and Raman measurements (727, 722). 

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