Structure of V2O5•nH2O Xerogels

Vanadium oxide is a layered compound that forms V2O5 center dot nH(2)O xerogel when intercalated by water. The xerogel consists of V2O5 bilayers with water between them. The structure of each V2O5 layer in the bilayer is close to the structure of a single layer in bulk V2O5. However, the distance between the two layers in the bilayer is much smaller than the t distance between single layers in the bulk. The xerogel is a Bronsted acid that has been used as an acid catalyst and whose protons are mobile and can be exchanged with other cations. Here, we use density functional theory to examine five possible models for the structure of the xerogel. In the model that has the properties established by experiments, the vanadyl groups in the two layers point toward the outside of the bilayer, while in the bulk V2O5 they point toward the space between layers. This change in the vanadyl positions allows the two layers to get unusually close to each other. This structure is unstable in the absence of water. Water stabilizes it by reacting with bilayers to form two H3O+ ions and one oxygen atom that bridges two vanadium atoms. It is this reaction that confers acidity to the gel.