First-principles density functional studies were made of the stability and formation

of oxygen vacancies on three low-index surfaces of V2O5. In agreement with

experimental results on the morphology of V2O5 crystallites, it was found that the

surface energies of the (100) and (001) surfaces were considerably higher than that

of the (010) surface; such that - at equilibrium - 85% of the surface area was

occupied by (010) surfaces. However, it was also found that the energies required

for the formation of oxygen vacancies were considerably lower on the energetically

less favorable surfaces. As found in previous theoretical studies, it was noted that -

on the (010) surface - the elimination of an oxygen atom from the vanadyl group,

O(1), required considerably less energy (Ef = 4.7eV relative to atomic oxygen) than

did the formation of a vacancy by the desorption of twofold or threefold

coordinated O(2) and O(3) oxygen atoms (Ef = 6.5eV). In addition, thermodynamic

analysis showed that, under the conditions required for vacancy creation (very low

value of the oxygen partial pressure), the V2O5 phase was unstable and could

transform into VO2 by releasing molecular oxygen. Due to extensive framework

relaxation, the formation of vacancies at the O(1) and O(2) sites of the (001) and

(100) surfaces required 1.0 to 1.5eV less energy than on the (010) surface. A

thermodynamic analysis demonstrated that, on the (010) surfaces, only O(1)

vacancies were marginally stable against re-oxidation under strongly reducing

conditions while, on the (100) and (001) surfaces, all types of vacancy were stable even under much higher partial pressures of oxygen. In addition, the adsorption of

atomic hydrogen and the formation of hydroxyl groups had been studied on all

three surfaces. Hydrogen adsorption was an exothermic process. The adsorption

energies on vanadyl O(1) atoms were larger, by about 1.1 and 1.6eV, on the (001)

and (100) surfaces than on the (010) surface and the adsorption energies on O(2)

sites were larger by 0.8 to 1.2eV. Vacancy formation by elimination of a hydroxyl

group required less energy than did abstraction of an oxygen atom. On the (010)

surface, the vacancy formation energies were reduced by 1.6 to 1.7eV. On the

(001) and (100) surfaces, the reduction varied between 0.1 and 0.9eV. However,

the lowest vacancy formation energies were still 3.1eV on the (010) and 2.9 and

2.3eV on the (001) and (100) surfaces, respectively. The lower vacancy formation

energies meant that, although in equilibrium only about 15.5% of the surface area

of a crystallite consisted of (001) and (100) facets, these surfaces could make a

considerable contribution to the activity of V2O5 as an oxidation catalyst.

Oxygen Vacancy Formation on Clean and Hydroxylated Low-Index V2O5 Surfaces:

a Density Functional Investigation. J.Goclon, R. Grybos, M.Witko, J.Hafner:

Physical Review B, 2009, 79[7], 075439