Theoretical Study of the Effect of (001) TiO₂Anatase Support on V₂O₅

Abstract

The effect of (001) TiO 2 anatase support on the electronic and catalytic properties of a V 2 O 5 monolayer is analyzed using density functional theory (DFT). The catalyst is represented by both clusters and periodic slabs. Using two experimentally relevant models of monolayer V 2 O 5 /TiO 2 (anatase) catalyst, both weak and strong interactions between a V 2 O 5 monolayer and the TiO 2 support have been investigated. In the first model, where a crystallographic (001) V 2 O 5 layer is placed on top of the (001) TiO 2 support, the weak interaction between vanadia and titania does not result in a major reconstruction of the active phase. Nevertheless, the changes in the electronic properties of the system are evident. The deposition of the vanadia monolayer on the titania substrate results in charge redistribution, enhancing the Lewis acidity of vanadium and the chemical hardness above the vanadyl oxygen, and in a shift of the Fermi level to lower binding energies accompanied by a reduction in the band gap. In the second model, where the (001) titania anatase structure is extended with a VO 2 film terminated by half a monolayer of vanadyl oxygen, apart from a similar electronic effect, the strong interaction of the vanadia phase with the titania support resulting from a high order of epitaxy has an important effect on the structure of the active phase. Atomic hydrogen adsorption is most favorable on the vanadyl oxygen of all the investigated surfaces, while the adsorption energy on this site increases by ∼10 kJ/mol due to the weak interaction between vanadia and titania and is further increased by ∼50 kJ/mol as a stronger interaction between the two phases is achieved, all in agreement with the increase in the negative electrostatic potential above the vanadyl site. The observed trends in the reactivity of the oxygen sites in H adsorption for the different catalyst models are successfully explained in terms of a frontier orbital analysis. IntroductionTransition metal oxides are widely used as catalysts for the oxidation of hydrocarbons. Especially vanadium oxide based catalysts are among the most active for the oxidation of both aliphatic and aromatic hydrocarbons. An important example of such a catalyst is V 2 O 5 /TiO 2 (anatase), which is used in industry as an effective catalyst for the production of phthalic anhydride from o-xylene.1,2 For the anatase support, although the (101) surface is thermodynamically more stable, the (100) and (001) faces are found in the industrial TiO 2 powders.3 Earlier experimental studies indicate that this catalytic system is of high performance when it consists of a monolayer of V 2 O 5 upon a TiO 2 (anatase) substrate, showing activity and selectivity not observed in the unsupported V 2 O 5 or TiO 2 anatase.2,4 The enhanced catalytic performance can be attributed to a synergetic effect between the active phase and the support. In order to unravel the role of the TiO 2 support on a V 2 O 5 monolayer catalyst, ab initio methods can help to provide a better understanding of the rel...

DFT Study of Hg Oxidation across Vanadia-Titania SCR Catalyst under Flue Gas Conditions

“…These results from DFT may aid in the design of improved catalysts for mercury oxidation while simultaneously maintaining optimal performance for NO x reduction. The TiO 2 (001) surface has been studied previously, 16,25,30,72 and is shown in Figure 9. It is composed of five-coordinated Ti atoms bonded to two O atoms along the (001) direction and to three O atoms in the (010) direction.…”

Vanadium Oxide Oligomers and Ordered Monolayers Supported on CeO2(111): Structure and Stability Studied by Density Functional Theory

“…Two approaches to construct models of supported vanadium oxide ML structures are reported in the literature: (i) substitution of metal atoms of the support with vanadium atoms, for example in anatase TiO2(001), 37 α-Al2O3(0001), 38 κ-Al2O3(001) 39 and Al2O3/NiAl(110), 40 or (ii) deposition of vanadium oxide layers cut from bulk phases on the support, e.g. V2O3 on…”

“…Al2O3/NiAl(110) 40 and V2O5 on anatase TiO2(001). 37,[41][42] Thus, the monolayers are typically structurally similar either to the support or the corresponding bulk vanadium oxide. This is different with monolayers on CeO2(111).…”

Chapter 2. Theoretical studies of selective propane oxidation and ammoxidation over vanadium-based multi-metal oxides

“…25,44,45 Accordingly, theoretical studies of surface VO x species have focused to date on understanding the nature of the VO x -support interactions, the molecular structure-reactivity relationships for propane ODH, and especially the mechanism of the initial C-H bond activation step. 47 Extensive experimental and theoretical evidence has been gathered in support of the basic tetrahedral structure of an isolated VO x species where a V 51 cation forms a short double bond to terminal oxygen and 3 bonds to oxide support via bridging oxygens. [48][49][50][51] Cheng et al 52 has studied conversion of propane to propanol by the anatase TiO 2 -supported VO x monomers and dimers using B3LYP.…”

“…Therefore, the high coverage of vanadia on the support surface is beneficial for the selectivity of supported vanadia to propene. Alexopoulos et al 37,47 have performed extensive DFT calculations of propane oxidation over anatase TiO 2 -suppoted V 2 O 5 (001) surface employing cluster and periodic slab models. They found that the anatase support enhances the Lewis acidity of the vanadia monolayer and the catalytic activity of the vanadyl oxygen leading to lower H binding energy on the vanadyl site.…”