Computational Study of Structure and Reactivity of Oligomeric Vanadia Clusters Supported on Anatase and Rutile TiO₂ Surfaces

Abstract

We use density functional theory to examine structure− activity relationships of small vanadia clusters supported on anatase TiO 2 (001) and rutile TiO 2 (110) surfaces. A thermodynamic analysis indicates that the vanadia monomer cluster can be stabilized on the anatase TiO 2 (001) surface in a catalytically relevant oxygen environment. On the other hand, vanadia clusters tend to aggregate into dimers on the rutile TiO 2 (110) surface because this surface binds the monomer less strongly as compared to anatase. Hydrogen adsorption is found to be exothermic on the vanadia monomer adsorbed on both supports, enhanced by a charge transfer between the adsorbate and the substrate. There is no such charge transfer on vanadia dimers and tetramers, where the hydrogen adsorption energies are similar to that on the single crystal V 2 O 5 (001) surface. The improved catalytic performance of the anatase support can be attributed to the ability of this surface to stabilize the catalytically active vanadia monomer clusters. ■ INTRODUCTIONThe growing demand of olefins and aldehydes as raw materials in industry calls for catalysts that can produce them in an efficient and economical way. One promising way is the catalytic oxidative dehydrogenation (ODH) of light alkanes and alcohols to the corresponding alkenes and aldehydes. The advantages of ODH over conventional nonoxidative routes rely on the fact that the ODH reaction is (i) exothermic and can operate at low temperature and (ii) minimizes coke deposition which eventually deactivates the catalysts. Oxide-supported vanadium oxide catalysts (VO x ) have been demonstrated to exhibit high activity for the ODH reaction. 1−3 However, the yield of desirable products is still limited by undesirable total combustion reactions. An increase in the activity and selectivity of this reaction would be aided by a fundamental understanding of the structure−activity relationship at the molecular level.The structure−activity relationship of VO x has long been pursued in experiments, 4−8 where different structures of VO x are prepared by varying the vanadium loading on a given support. With increasing vanadium loading, VO x catalysts are proposed to change in structure from monomers, oligomers, monolayers, to 3D crystalline V 2 O 5 nanoparticles. The structure of VO x clusters at different coverages and their corresponding performance are characterized and measured to establish structure−activity relationships. For the TiO 2 support, which is of interest in this study, it is believed that the turnover frequency (TOF) at the VO x /TiO 2 catalyst is relatively insensitive to coverage, whereas V 2 O 5 nanoclusters with a size of 1−2 nm exhibit high ODH activity. At higher coverages, the activity drops when extensive crystalline V 2 O 5 forms.Although these studies provide valuable insight, a structure− activity relationship with molecular resolution should be pursued since the currently employed characterization methods do not provide unambiguous structural information at the atomic scale. Density f...

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

“…6,26 Calculations show that V 2 O 5 is more stable than (VO 3 − , VO 2 + ). Therefore, experiments with isolated clusters need to be conducted under conditions that ensure that their mobility along the surface is low.…”

The reaction mechanism for the SCR process on monomer V5+sites and the effect of modified Brønsted acidity

“…16,28,29 A ball and stick model of the initial state is depicted in Fig. 16,28,29 A ball and stick model of the initial state is depicted in Fig.…”

Cation synergies affect ammonia adsorption over VOX and (V,W)OX dispersed on α-Al2O3 (0001) and α-Fe2O3 (0001)

“…The catalytic properties of surface species such as VO X are heavily influenced by the oxide support [17][18][19][20][21]. V cations are more reducible in the presence of other reducible metal cationic species, such as Ti 4+ ; however, V reducibility decreases as its density increases, since crystalline V 2 O 5 does not interact as strongly with the support material [15].…”