Oxygen vacancy formation on clean and hydroxylated low-index ${\text{V}}_2{\text{O}}_5$ surfaces: A density functional investigation

We present ab initio density functional studies of the stability and the formation of oxygen vacancies on three low-index surfaces of ${\text{V}}_2{\text{O}}_5$. In agreement with the experimental results on the morphology of ${\text{V}}_2{\text{O}}_5$ crystallites we find that the surface energies of the (100) and (001) surfaces are considerably higher than that of the (010) surface such that in equilibrium 85% of the surface area is occupied by (010) surfaces. However, we also find that the energies required for the formation of oxygen vacancies are considerably lower on the energetically less favorable surfaces. As reported in earlier theoretical studies we find that on the (010) surface, the elimination of an oxygen atom from the vanadyl group [denoted O(1)] requires considerably less energy ($E_f=4.7\text{eV}$ relative to atomic oxygen) than the formation of a vacancy by desorption of twofold or threefold coordinated O(2) and O(3) oxygen atoms $\left(E_f=6.5\text{eV}\right)$. In addition, the thermodynamic analysis shows that under conditions required for vacancy creation (very low value of the oxygen partial pressure) the ${\text{V}}_2{\text{O}}_5$ phase is unstable and may transform into ${\text{VO}}_2$ 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 requires 1.0–1.5 eV less energy than on the (010) surface. A thermodynamic analysis demonstrates that on the (010) surfaces only O(1) vacancies are marginally stable against reoxidation under strongly reducing conditions, while on the (100) and (001) surfaces all types of vacancies are 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 is an exothermic process. Adsorption energies on vanadyl O(1) atoms are larger by about 1.1 and 1.6 eV on the (001) and (100) surfaces than on the (010) surface and adsorption energies on O(2) sites are larger by 0.8–1.2 eV. Vacancy formation by elimination of a hydroxyl group requires less energy than abstraction of an oxygen atom. On the (010) surface the vacancy formation energies are reduced by 1.6–1.7 eV; on the (001) and (100) surfaces the reduction varies between 0.1 and 0.9 eV. However, the lowest vacancy formation energies are still 3.1 eV on the (010) and 2.9 and 2.3 eV on the (001) and (100) surfaces, respectively. The lower vacancy formation energies mean that although in equilibrium only about 15.5% of the surface area of a crystallite consists of (001) and (100) facets, these surfaces will make a considerable contribution to the activity of ${\text{V}}_2{\text{O}}_5$ as an oxidation catalyst.

Figure 1

Perspective view of the crystal structure of ${\text{V}}_2{\text{O}}_5$ and top view of the three low-index surfaces: (010), (001), and (100). Large spheres represent V; small spheres O atoms. The black frame shows the bulk unit cell, the shaded areas the large (L), medium (M), and small (S) surface cells. Oxygen atoms with one, two, or three V neighbors are marked as O(1)–O(3). For the (001) surface ${\text{O}}^{\text{t}}\left(1\right)$ and ${\text{O}}^{\text{t}}\left(2\right)$ mark oxygen atoms located on the plateaus and ${\text{O}}^{\text{e}}\left(2\right)$ oxygen atoms on the edge of the plateau (cf. text).

Figure 2

Relaxed geometries around O(1), O(2), and O(3) vacancies on the (010) surface (for the L supercell). Structures are presented in top view, except for the O(1) vacancy, where a side view depicts the formation of a new bond between surface and subsurface s layers. For clarity, ${\text{V}}^{\ast }$—the vanadium atoms which were bonded to the removed O atom are depicted in solid black.

Figure 3

Oxygen vacancies at the O(1), O(2), and ${\text{O}}^{\text{e}}\left(2\right)$ sites on the (001) surface (for M supercell). Atoms from deeper subsurface layers are all depicted as white circles to emphasize surface reconstruction. For clarity, ${\text{V}}^{\ast }$—the vanadium atoms which were bonded to the removed O atom are depicted in solid black.

Figure 4

Relaxed surface structure around oxygen vacancies on the (100) surface (for L supercell). Top views of the surface after removing one O(1) or O(2) oxygen atom per supercell are shown. Atoms from deeper subsurface layers are all shown as white circles. Vanadium and oxygen atoms are represented by large and small gray circles, respectively. For clarity, ${\text{V}}^{\ast }$—the vanadium atoms which were bonded to the removed O atom are depicted in solid black.

Figure 5

Relaxed geometries after adsorption of individual hydrogen atom at oxygen active sites O(1), O(2), and O(3) on the (010) surface (for the L supercell). Vanadium atoms are represented by large gray circles and oxygen atoms by smaller gray circles. For clarity, atoms of the OH group are depicted in solid black.

Figure 6

Relaxed geometries after adsorption of individual hydrogen atom at the O(1), O(2), and ${\text{O}}^{\text{e}}\left(2\right)$ sites on the (001) surface (for the M supercell). Atoms from deeper subsurface layers are all shown as white circles. Vanadium atoms are represented by large gray circles and oxygen atoms by smaller gray circles. For clarity, atoms of the OH group are depicted in solid black.

Figure 7

Relaxed geometries for hydrogen adsorption on the (100) surface (for the L supercell). Top views of the surface after adsorption of individual hydrogen atom at the O(1) and O(2) oxygen sites are shown. Atoms from deeper subsurface layers are all shown as white circles. Vanadium and oxygen atoms are represented by large and small circles, respectively. For clarity, atoms of the OH group are depicted in solid black.