Thermogravimetry and X-Ray Absorption Spectroscopy Study of Heated V[sub 2]O[sub 5]⋅nH[sub 2]O Aerogels and Ambigels

We have used thermogravimetry and X-ray absorption spectroscopy to examine the water content, oxidation state, and local structure of V for V 2 O 5 ⋅ n H 2 O aerogels and ambigels in the pristine state and after heating under vacuum (100 μTorr) at 114 and 220°C for 25 h. On the basis of thermogravimetric analysis, the composition of the aerogels and ambigels consists of about 2 and 1 mol of H 2 O per mole of V 2 O 5 in the pristine state and after heating at 114°C, respectively. Thermogravimetric analysis reveals the absence of loosely bound H 2 O in the network of the aerogels and ambigels heated at 220°C. Analyses of X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra reveal that the oxidation state of V for aerogels and ambigels in the pristine state ( V 2 O 5 ⋅ 2.0 H 2 O ) as well as those heated at 114°C ( V 2 O 5 ⋅ 1.0 H 2 O ) is similar to that of V in V 2 O 5 ; that is to say + 5 (pentavalent V). Heat-treating the aerogels and ambigels at 114°C does not lead to a measurable change in the oxidation state of V. The local structure of V under these conditions is similar to that of V in orthorhombic V 2 O 5 except for a higher degree of local symmetry within the structure of the VO 5 square pyramid. Analyses of XANES and EXAFS spectra of aerogels and ambigels heated at 220°C, on the other hand, reveal the presence of both tetravalent and pentavalent V. The fractions of tetravalent V are estimated to be 0.29 ± 0.08 and 0.26 ± 0.08 for the aerogels and ambigels, respectively. The mixed valency can be in the form of a single phase with a solid solution of tetravalent and pentavalent V sites or a two-phase system with one phase containing tetravalent V and the other phase containing pentavalent V.