Vanadia Gel Synthesis via Peroxovanadate Precursors. 2. Characterization of the Gels

Abstract

Vanadium oxide gels derived from the reaction of H2O2 and V2O5 have been investigated using 51V MAS NMR, TGA, XRD, SEM, and laser Raman spectroscopy. Based primarily on the 51V MAS NMR and TGA results, the coordinations of five distinct vanadia sites have been detailed, which possibly include a previously unreported dimer. The relative concentration of these sites changed as dehydration progressed, and a model of this process has been proposed based on the numerical analysis of the NMR MAS spectra. In addition, the coordination of the most tightly bound water has been postulated. Depending on sample treatment, it was possible to synthesize both layered and nonlayered materials. The laser Raman spectra revealed differences between layered and nonlayered materials. These differences have been attributed to the interaction of coordinated water molecules, which were trapped between layers and held firmly in place, thus restricting or altering certain Raman-active vibrations.

Practical aspects of 51V and 93Nb solid-state NMR spectroscopy and applications to oxide materials

“…VO 3 (OH), (VO 3 ) n , VO 2 (OH) 2 , V 10 O 26 (OH) 2 , VO(OH) 3 , V 10 O 27 (OH), (V 2 O 7 ) 4À , or V 10 O 28 6À , depending on the net surface pH, as was demonstrated using Raman spectroscopy by Deo and Wachs [253]. This agrees with recent 51 V NMR data, which have also shown that surface complexes remain attached to the surface, though not as strongly as in the dehydrated state [148,254,255]. Surface vanadia species in hydrated VO x /SiO 2 are similar to those found in V 2 O 5 AEnH 2 O gels.…”

“…In a similar fashion, the structure of the tailor-made macroporous vanadium oxide foams has been examined by 51 V NMR in [244] (Table 5). It has been revealed that the local structure of vanadium sites in these foams is similar to that reported earlier for V 2 O 5 xerogels [148].…”

“…and even several V-containing biological systems, have also recently been studied [93,94,[144][145][146][147][148][149]. It is noteworthy, that almost Table 4 Different types of vanadium sites all types of vanadium sites shown in Table 4 can be found in these individual compounds.…”

“…When V-O l2 -V bonding is absent or very weak, the pyramids are said to be isolated. Vanadium occupying these sites has an axially symmetric chemical shielding tensor with a large anisotropy (400 < d r < 500 ppm), the quadrupolar coupling constant can vary from 1 to 4 MHz, while the CSA asymmetry parameter g r changes from 0 to 0.2, and d^$ À400 to À600 ppm [144][145][146][147][148][149]177].…”

Modification of the Properties of Vanadium Oxide Thin Films by Plasma-Immersion Ion Implantation

“…51 V MAS NMR spectra of both initial and hydrogenated films (Figure 11(a)) contain the only signal with isotropic shift around −575 ppm, which corresponds to vanadium in the vanadium pentoxide phase of V 2 O 5 -xerogel [24]. However, the intensity of the signal in hydrogenated film is lower by 2.7 times than in initial one.…”