Vanadyl species

In a. p. and s.o. ZV(a) and ZV(i) samples, no ESR signals were detected. In a.p. ZV(acac), a weak ESR signal of vanadyl species was detected (5% of total V), absent after the s.o. treatment. The spectra of samples reduced with CO at 400 to 623 K consisted of a signal showing a resolved hyperfine structure (Vh), overlapping a broad (AHnp = 300 Gauss) and nearly-isotropic band (Vb, giso = 1 -97) (Fig. 3). When recorded at 77 K, both Vh and Vb maintained the same shape as at RT, and their Intensity as a function of temperature followed the Curie law. 

Analysis of structure-activity relationships shows that various species characterized by different reactivities exist on the surface of vanadium oxide-based catalysts.339 The redox cycle between V5+ and V4+ is generally accepted to play a key role in the reaction mechanism, although opposite relationships between activity and selectivity, and reducibility were established. More recent studies with zirconia-supported vanadium oxide catalysts showed that vanadium is present in the form of isolated vanadyl species or oligomeric vanadates depending on the loading.345,346 The maximum catalytic activity was observed for catalysts with vanadia content of 3-5 mol% for which highly dispersed polyvanadate species are dominant. 

Using electron spin echo modulation spectroscopy, Narayana et al.33 showed that in 0.25% V205 on Si02 a vanadyl species with two co-ordinated water molecules, giving octahedral co-ordination, is present. This species loses the two water molecules even at 100°C leaving a tetrahedral species. 

It was found that on the surface of zirconia-supported vanadia catalysts vanadium was presented in the form of isolated vanadyl species or oligomeric vanadates, or as V205 nanocrystals, and that V5+ and V4+ ions coexisted in octahedral and tetrahedral coordination. Within the bulk of zirconia matrix, V4+ ions were stabilized in a VxZri x02 solid solution [39],... 

This, having a d2 configuration, is particularly suitable for octahedral mononitrido complexes such as [OsNC15]2-, to which the Ballhausen-Gray bonding model for vanadyl species can be applied.312 Taking the nitrido ligand to lie on the z axis the Os=N triple bond is formed from a a... 

Figure 4.6 UV Raman spectra (X citation = 220nm, 287nm) of dehydrated 0.16V sample and 9-alumina. Two distinct Raman bands at 997cm and 1015cm for the 0.16V sample are attributed to V=0 stretching vibrations from two distinct vanadyl species. A broad band at 897cm can be assigned to V—O—Al stretching vibrations.

The results obtained from such dynamic photoluminescence studies shown in Fig. 41, together with the results obtained for O2 and CO. permit calculation of the absolute rate constants of quenching for the various molecules, as follows 9.34 x 10 for O2, 3.52 x 10 for C2H4, 2.24 x 10 for rram -2-butene, and 1.51 x 10 for N2O, all in units of (g/mol s), respectively (33,34,56,69,115-117). Consequently, the reactivities of these molecules toward the charge-transfer excited state of the vanadyl species decrease in the order O2 > CO > C2H4 > CsHg > trans-l-C Wg > N2O (120, 208-210). 

These absolute quenching rate constants are larger than the values for the excited triplet state of the tetrahedral oxo-vanadyl species of the supported vanadium oxide but decrease in the same order 2.56 x 1(T, 4.35 x 10, ... 

Ramis et al. [44] studied the effect of dopants and additives on the state of surface vanadyl species of vanadia on titania catalysts by means of FTIR spectroscopy. Additives such as alkali and alkali-earth metal cations (typically Cs, K, Na, Li and Mg), oxoanions (such as sulphates and arsenates), and other species (such as AP+, MoO +, and WO ), influence the position of V=0 stretching frequencies. The position of vy=o for a 3 wt% V2O5 on titania was observed at 1035 Two percent W or Mo did not show any shift of the stretching frequency of V=0, whereas Cs lowered the band position by 45 cm . This was explained in terms of the formation of strong basic sites and the exchange of Ti in 0=V-0-Ti by 0=V-0-Cs. The elements Al, S, and As shift the position of vv=o to higher frequencies. Oxoanions are coordinatively bond to vanadyl centers [44],... 

FT-IR spectra of the four samples are reported in Figure 1. The catalysts prepared by coprecipitation (1,2,3) have the same FTIR spectra. It is possible to see the stretching absorbance of the V=0 bond at around 988 cm this shifting to lower frequencies with respect to the absorbance of the crystalline V2O5 (1022 cm l) may be attributed to two effects, firstly to the interactions between the vanadyl species and the rutile type Sn02 matrix and... 

The ET-IR spectra of the sample calcined in air are quite similar, it is possible to see the typical absorbance bands of the tin oxide (around 629 cm ) and of vanadyl species (around 988 cm ) described in the previous section. After treatment at higher temperatures, with an increase in crystallinity and the progressive evolution of volatile compounds, the band, typical of Sn-0 bond in the Sn/Sb system, becomes more well defined. Moreover, some initial modifications of spectra due to Sb-O-Sb bond of antimony oxides are observed in the sample calcined at 800°C... 

The sample calcined in nitrogen at 700°C is less active with respect to one calcined in air at the same temperature and the selectivity in acrylonitrile is very low, owing to combustion. The absence of vanadyl species, evidenced by FTIR spectra, seems to influence the catalytic performance of the mixed oxide in a dramatically negative way. 

The chemical form of sulphur is important in determining the availability of metal ions in marine environments. In area I of Fig. 5.34 sulphur is generally in the form of S042 and nickel is available for bonding (as Ni2+), but vanadium is unavailable. Oils from source rocks deposited under these conditions, such as those from Mesozoic and Tertiary reservoirs in the Uinta Basin (USA), have V/(V + Ni) values <0.1 and S content <1%. In area II sulphate is again the major form of sulphur, while Ni2+ and V02+ (vanadyl) are available for bonding, but the vanadyl species may... 

It is apparent from the chemical shifts (g-values), the hyperfine coupling constants (A-values), and the linewidths that the free radicals and vanadyl species are in very similar environments in both samples. It was not possible to obtain meaningful values for the absolute numbers of spins per gram for either species, but estimates of the relative concentrations obtained by measuring peak heights indicate that the vanadyl and free-radical concentrations do not differ significantly between the two asphaltenes. It thus appears that heat treatment of Cold Lake asphaltenes to 320°C does not alter the nature or abundance of paramagnetic centers. 

The EPR spectra of catalytic materials with vanadium contents between those of Vo.1Mgo.9Ox and V0.gMg0.2Ox (3.3 < V content / wt.-% < 15) show also a superposition of a hyperfine splitting multiplet (hfe) of isolated VO " species and a more or less broad isotropic singlet of interacting VO species. With increasing vanadium concentration the fraction of interacting vanadyl species increases. 

---