Interaction between Monomeric Vanadium Oxide Clusters Supported on Titania and Its Influence on Their Reactivity

Abstract

Density functional theory is used to study monomeric vanadium oxide clusters formed by exposing to oxygen a rutile TiO 2 (110) surface on which vanadium atoms have been preadsorbed. Past calculations examined such systems by using one V atom per supercell. This forces all VO x clusters to be identical and precludes interactions between them. To examine the consequences of this choice, we examine a VO x and a VO y cluster in the same supercell. We find that the most stable configuration is obtained when the supercell contains one VO 2 and one VO 3 cluster. The stabilization occurs because of a Lewis acid−base interaction: VO 2 donates electron charge to VO 3 and this lowers the energy substantially. No such interactions were found when the supercell contained VO x clusters with the same number of oxygen atoms. These finding demonstrate that in some cases it is essential to have two clusters in the supercell and that results obtained with one cluster in the supercell can be misleading. We have also studied the reactivity of the clusters with hydrogen, which is considered a descriptor of the ability of the cluster to catalyze the breaking of a C−H bond in an alkane.

Chemisorption of NH3 on Monomeric Vanadium Oxide Supported on Anatase TiO2: A Combined DRIFT and DFT Study

“…28 Recent studies by Kristoffersen et al about the interaction between supported monomeric vanadium oxide species and titania provide great insights into the origin of the stability of vanadia clusters. 29,30 However, there are relatively few papers investigating the acidic properties of VO x /TiO 2 catalyst with the computational method, and there have also been few attempts to correlate the experimental results with the calculated ones precisely.…”

Stability of V2O5 Supported on Titania in the Presence of Water, Bulk Oxygen Vacancies, and Adsorbed Oxygen Atoms

“…Reciprocal space is sampled by 2 × 2 k-points, and all calculations are spin polarized. This computational setup is the same as in our previous work, 7 and the calculations are directly comparable. The formal charges of the V atoms are helpful for understanding the stability and reactivity of the different vanadium oxide clusters.…”

“…Monomeric vanadium oxide clusters have been predicted to exist as pairs of spatially separated VO 3 − and VO 2 + clusters, if TiO 2 (110) is stoichiometric and the system is in thermal equilibrium with a pure oxygen atmosphere, at temperatures above 200 K. 7 Figure 1b shows the structure of the TiO 2 surface having adsorbed a pair (VO 3 − , VO 2 + ) of monomers. We use the notation (VO 3 − , VO 2 + )/TiO 2 because the VO 2 cluster donates an electron to the VO 3 cluster.…”

“…[2][3][4] DFT calculations of total energies suggested that monomeric vanadium oxide clusters, supported on stoichiometric, rutile, TiO2(110), will bind to each other to form larger clusters. [5][6][7] The calculations presented here study how the stability of monomeric versus polymeric vanadium oxide clusters depends on the presence of other species on the surface. The species consider are adsorbed H2O, bulk oxygen vacancies and an adsorbed oxygen atom.…”

“…Vanadium oxide supported on other oxides is an interesting partial oxidation catalyst. − It is widely accepted that this system is most active if vanadium oxide is a submonolayer, but there is an ongoing debate regarding the nature of the active species. At submonolayer coverage, the vanadium oxide can exist as monomeric or polymeric clusters, depending on the method of preparation and on the reaction condition. − DFT calculations of total energies suggested that monomeric vanadium oxide clusters, supported on stoichiometric, rutile, TiO 2 (110), will bind to each other to form larger clusters. − The calculations presented here study how the stability of monomeric versus polymeric vanadium oxide clusters depends on the presence of other species on the surface. The species considered are adsorbed H 2 O, bulk oxygen vacancies, and an adsorbed oxygen atom.…”