Article pubs.acs.org/JPCC
Reconstruction of Low-Index α‑V2O5 Surfaces Henrik H. Kristoffersen and Horia Metiu* Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106-9510, United States S Supporting Information *
ABSTRACT: Density functional theory (DFT) calculations with a PBE+U functional and a larger supercell than used previously find that that the (100) and (001) surfaces reconstruct. These reconstructions involve fairly extensive rearrangements of the surface atoms and lead to changes in the density of states and the energies of oxygen-vacancy formation.
positions; they occur through an extensive “reorganization” of the surface atoms. To assess how the reconstructions change surface chemistry, we calculated the energies of oxygen-vacancy formation on the reconstructed faces. This is of interest because catalytic oxidation by V2O5 often follows the Mars−van Krevelen mechanism28 in which the surface oxygen atoms act as oxidants and the resulting oxygen vacancies are healed by gas-phase oxygen. Therefore, oxygen-vacancy formation energies give a simple estimate for the relative oxidation strength of different facets and their importance in catalysis. We have found one low-energy reconstruction for the (001) surface and two distinct low-energy reconstructions for the (100) surface, which are very close in energy, hence equally likely to be observed. The reconstructions reduce the (100) and (001) surface energies to around 60% of the values for the unreconstructed (“bulk-terminated”) surfaces. The vacancy formation energies are found to be 1.26 eV for the (010) surface, 1.34 and 0.68 eV for the two reconstructed (100) surfaces, and 0.95 eV for the reconstructed (001) surface. These values are quite different from those obtained for the unreconstructed surfaces.
1. INTRODUCTION Vanadium pentoxide (V2O5) is used industrially for the preparation of the sulfuric acid catalyst and as a catalyst for selective reduction of NOx with ammonia.1 Numerous academic studies have shown that supported V2O5 facilitates oxidative dehydrogenation of alkanes2−4 and selective oxidation of methanol.5−7 In addition, silica-supported V2O5 has been tested as a catalyst for the oxidation of methane.8,9 V2O5 is also used as a cathode for lithium batteries.10,11 α-V2O5 has a layered structure in the [010] direction. The layers are kept together by van der Waals interactions,12 and αV2O5 can therefore be cleaved13 or exfoliated14−16 along the (010) facet. This means that the face formed by cleavage has the smallest surface energy and the largest surface area in a V2O5 particle. For this reason, the (010) surface has received the most attention in both surface science experiments17−19 and modeling.20−22 However, it is often found that easy-tocleave facets are not as reactive as other facets: examples include graphite, mica, and molybdenum sulfide. Therefore, other facets of α-V2O5, such as (100) and (001), might be important for catalysis in spite of being a smaller fraction of the total area of a nanoparticle. In this article we use density functional theory (DFT) calculations with the PBE+U functional to show that the αV2O5(100) and α-V2O5(001) surfaces reconstruct and that the reconstructions affect surface chemistry. These surfaces have been studied with DFT before, and the surface energy,23 the electronic structure,24 oxygen-vacancy formation energy,25 and the adsorption energy of hydrogen,25 water,26 and ammonia27 have been calculated. Those calculations were based on a surface structure with bulk periodicity, and as a result the surface geometry was very similar to that of the bulk. In this article we called these geometries bulk-terminated surfaces. The reconstructions reported here are not just small displacements of the surface atoms as compared to the bulk © 2015 American Chemical Society
2. COMPUTATIONAL METHODS The DFT calculations were performed with the VASP program.29−32 We used the PBE functional,33 together with a U = 3 eV correction applied to vanadium d-states34 and the DFT-D2 correction to account for van der Waals interactions.35 Two surface energies were calculated without U to assess the effect of the +U correction. The surface energy of the reconstructed (100)R2 surface was unchanged (0.027 eV/Å2 compared to 0.026 eV/Å2 with +U), while the bulk-terminated Received: March 11, 2015 Revised: April 21, 2015 Published: May 4, 2015 10500
DOI: 10.1021/acs.jpcc.5b02383 J. Phys. Chem. C 2015, 119, 10500−10506
Article
The Journal of Physical Chemistry C (100) surface was found to be slightly more stable (0.040 eV/ Å2 compared to 0.047 eV/Å2). Still, the reconstructed surface is significantly more stable in both methods. In the calculations, we treated explicitly, within the PAW method, 11 electrons for vanadium and 6 for oxygen. The remaining electrons were treated by the frozen core approximation. A plane wave basis set with a 450 eV cutoff was employed. Surfaces were modeled by surface slabs separated from their periodic image by more than 12 Å of vacuum. The stability of different surfaces is compared on the basis of surface energies (eq 1) γ=
Eslab − n·E V2O5(bulk) Figure 1. (a) Bulk α-V2O5 unit cell. (b) The square-pyramidal structure of the oxygen atoms around a vanadium atom in the bulk. (c) 1 × 2 × 3 repetition, where the layered structure is visible. In all pictures of atomic configurations, bonds are drawn between vanadium atoms (light gray) and oxygen atoms (colored) that are closer than 2.2 Å. The oxygen atoms in the vanadyl are colored brown, and the bridging oxygen atoms are red. Vanadium−oxygen atoms closer than 1.7 Å are considered to be vanadyl groups.
(1)
2A
The surface energy γ is calculated as the energy difference between a slab exposing the surface in question (Eslab) and the equivalent number of bulk V2O5 units (n·EV2O5(bulk)). The energy difference is divided by the total surface area of the slab (2A). Spin-paired electronic configurations were obtained for all stoichiometric, low-energy V2O5 surfaces. The energy of oxygen-vacancy formation (Evac) was calculated as the energy of the slab with one oxygen-vacancy (Evac+slab) plus the energy of half the O2 gaseous molecule ((1/ 2)EO2(g)) minus the energy of the stoichiometric slab (Eslab) (eq 2). In this formulation, positive defect formation energy means an endoergic vacancy formation process. Evac = Evac + slab +
1 EO (g) − Eslab 2 2
a vanadyl oxygen atom. One oxygen atom is 2-fold coordinated (O(2) in Figure 1c) and bridges two vanadium atoms. The remaining three oxygen atoms are equivalent (by symmetry), and each is coordinated to three vanadium atoms (O(3) in Figure 1c). V2O5 is a layered compound with the layers stacked in the [010] direction (Figure 1c). The bonds between layers are weak (essentially van der Waals), and the oxide can be easily cleaved13 or exfoliated.14−16 In the literature, the [010] and [001] directions are sometimes interchanged,13 and the cleavage surface is sometimes labeled (010) and sometimes (001). We label the cleavage surface (010) as indicated in Figure 1, which is consistent with the notation used in the work of K. Hermann.20,24
(2)
The formation of an oxygen vacancy results in two unpaired electrons in the system. These excess electrons are expected to be localized on vanadium atoms.36,37 This is similar to other reducible transition metal oxides such as rutile TiO238−40 and CeO2.41,42 By increasing the distance between the cation on which the electron is to be localized and the neighboring oxygen atoms, one can control the location of the unpaired electrons. This causes an unpaired electron to move to the cation. After this has happened, one relaxes the oxygen atoms and optimizes the structure.38,43 By using this method we have explored different locations of the unpaired electrons formed when an oxygen atom is removed from the surface to create a vacancy. We have performed spin-polarized calculations and examined both the singlet (all electron spins paired) and triplet (two spin-unpaired electrons); we found that the lowest energy is obtained if the two electrons, left behind when the oxygen is removed, form a triplet. However, the energy difference between the triplet and singlet spin state is very small (
