Vanadyl, monomeric

Surface vanadium appears to be most stable (to reduction) at low (<1%) V concentration when present as monomeric vanadyl units. Its stability decreases with increasing V levels. It is least stable (to reduction) at high (5%) V levels when present as a supported Vanadia phase. This difference in reactivity with V concentrations is believed responsible for the rapid decline in cracking activity observed in dual function cracking catalysts containing alumina when V start to exceed the 1.0-1.25 wt.% level (4). Further details of the mechanism of catalyst deactivation by V age the subject of continuing investigations 1n our laboratories by 31V solid state NMR, XPS, and Raman spectroscopy. 

Fig. 5.18. Monomeric vanadyl (left) and polymeric vanadates (right). Adapted ft-om [65].

It has been shown that titania-supported vanadia materials comprise a distribution of monomeric vanadyl, polymeric vanadates and crystalline vanadia, the amount of which is dependent on the vanadia content. 

Three types of vanadium-containing species are present at the surface of the vanadia on titania catalysts. Monomeric vanadyl, polymeric vanadates, and crystalline vanadia depending on the vanadia loading (see Fig. 5.19). Moreover, Bronsted acid sites and Lewis sites are present at the surface of vanadia on titania catalysts. All species are needed to explain the mechanism of the SCR reaction over this type of catalyst. The active sites are related to previously mentioned vanadia/vanadium species. A variety of active sites were proposed such as two adjacent V =0 groups or co-ordinated vanadyl centres [81,84-86] V =0 and... 

Went et al. [70] used TPD and laser Raman spectroscopy to determine the structure of the catalyst and of the adsorbed species. It was found that the specific activity of polymeric vanadate species was 10 times greater than that of monomeric vanadyls at 500 K. Monomeric species produce N2 both in the presence and in the absence of oxygen whereas polymeric species produce both N2 and N2O. The selectivity towards N2O increases with increasing O2 concentration in the feed. 

During TPR water is formed and desorbs from the surface [35]. It is observed that the hydrogen consumption starts at 350 K, whereas water is formed at temperatures above 425 K. The monomeric vanadyls are restored after one cycle of reduction and oxidation. For polymeric vanadates terminal vanadyl oxygen atoms are removed leaving the structure unchanged. Upon oxidation the polymeric vanadate splits into a dimeric form and a monomeric vanadyl (Fig. 2). Oszkan et al. [36] concluded from their TPD studies that two types of ammonia species were present at the surface. One type was ammonia coordinatively bonded to surface vanadyls (Lewis sites) and a second type of ammonia species formed by the reaction of NH3 and surface hydroxyl groups [36]. 

Lietti and Forzatti [41] have shown by means of transient techniques such as TPD, TPSR, TPR and SSR (steady state reaction experiments) that isolated vanadyls and polymeric metavanadate species are present on the surface of vanadia on titania catalysts with V2O5 loadings of up to 3.56 wt%. Polyvanadate species are more reactive than isolated vanadyls due to the presence of more weakly bonded oxygen atoms. It has been shown that titania-supported vanadia materials comprise of a distribution of monomeric vanadyl, polymeric vanadates, and crystalline vanadia, the amount of which is dependent on the vanadia content. 

The oxidation state, local structure, and redox properties of vanadium are usually studied by DRS UV-vis, XPS, NMR, ESR, and EXAFS/XANES techniques. Most studies show a presence of well-dispersed, immobile monomeric vanadyl(IV) species attached to the framework, which are still active in oxidation reaction (190-207) (Table 6). The scheme for the oxidation and reduction of... 

The [V0(C03)20H] species is apparently the form of vanadyl sulfate present under these conditions and the [V(0)2(C03)2] species is formed from the monomeric V(IV) species by oxidation. Similar processes result in the formation of the V2O3 core when a V(V) source is reduced in the same buffer [101]. 

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. 

Other complexes with N,0 donor ligands include the dimeric compound (144) and monomeric (145).841 The ESR spectra of the vanadyl dimer (144) in frozen DMSO and DMF exhibited only zero-field splitting and the V—V distance estimated from this was 8.66 A. 

The boron trihalide complexes with a number of phosphine sulphides and selenides can be isolated for the chloride, bromide, or iodide but, consistent with its reduced Lewis acidity, the trifluoride does not react.600 Alkyldithio-phosphonic acids give tin, lead, and mercury derivatives such as Me3SnSP-(S)FEt, Pb[SP(S)FMe]2, and MeHgSP(S)FMe, which are monomeric in solution, and there is n.m.r. evidence for a bidentate phosphonate group in the tin compound.601 Vanadyl chelates of alkoxy-ethyl and alkoxy-phenyldi-thiophosphonates (87) have been synthesized and e.s.r. measurements... 

Namely, the spectral features of the low loaded VPS 2 catalyst (Fig. 4a), consisting of a main absorption band at ca. 40,000 cm associated with the CT process in the terminal vanadyl V=0 bonds monitors in such system the presence of a monomeric tetrahedral coordinated (TJ) vanadia species (7v) [10], while the component at lower wavenumbers ( 34,000 cm ) arises from the CT in di/oligomeric structures formed by condensation of the monomeric ones,... 

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