Vanadium pentoxide I. Structures and properties

Abstract

Structure and properties of V 2O 5 are described and the role of three types of oxygen atoms present in the lattice: vanadyl oxygen atoms O(1) coordinated only to one vanadium atom, and bridging oxygen atoms O(2) and O(3) coordinated to two and three vanadium atoms, respectively, is discussed. Equilibration of gas phase oxygen with vanadium oxides results in the formation of the intrinsic defect structure of V 2O 5 consisting of oxygen vacancies. Discussion of the properties of vacancies as deduced from measurements of electrical conductivity, EPR and IR spectra and the stability of different oxygen vacant sites as obtained from quantum-chemical calculations is given. Mechanism of the reduction of V 2O 5 through crystallographic shear, resulting in the formation of V 6O 13 or V 4O 9, is illustrated. It is shown that because of a pronounced anisotropy of V 2O 5 crystal structure sensitivity of catalytic reactions appears. Two types of crystal planes are exposed. The (0 1 0) basal plane has all chemical bonds almost fully saturated. The non-bonding d-orbitals of V ions have the LUMO character and act as Lewis acid sites, whereas the lone electron pairs of bridging oxygen atoms have the HOMO character and behave as Lewis basic sites. On the (1 0 0) and (0 0 1) planes cleavage leaves coordinatively unsaturated vanadium and oxygen ions, which develop Brönsted acid-base interactions with reacting molecules, causing the heterolytic chemisorption. Oxygen vacancies in the lattice are replenished through oxidation by gas phase oxygen, which sometimes is considered as oxygen chemisorption.