Density functional theory study on propane dehydrogenation over VOx based catalysts

Abstract

This thesis describes a density functional theory (DFT) study for the mechanism of propane dehydrogenation on the surface of vanadium-based oxide catalysts. Through DFT calculations, a range of catalysts were used in the reaction simulations to demonstrate the catalytic activity of different vanadium oxide catalysts. These include oxygen-vacant V2O5 catalysts, platinum-doped VOx catalysts and γ-Al2O3 supported VOx catalysts.

On the clean surface of V2O5, it is found from DFT calculations that in the environment with propane, the oxygen atoms in the one coordination state readily combine with the hydrogen atoms in the propane to form water and leave the surface, thus creating an oxygen vacancy at the top site. There are four top-site oxygen atoms on the surface of a 1×2 V2O5 supercell, all of which can be reduced off the surface in the same way. In the two-layer V2O5 structure, the vanadium atom in the oxygen vacancy can bind with the top oxygen in the second layer. This makes the two-layer structure with the oxygen vacancy more stable than the single layer. Where the oxygen-vacant vanadium entoxide structure is involved in the propane dehydrogenation reaction, we find that the breakage of the carbon-hydrogen bonds is relatively simple. Both the di- and tricoordinated oxygen atoms are catalytically active for carbon-hydrogen bond cleavage and the di-coordinated oxygen is more active. However, in the process of hydrogen binding, the hydrogen-oxygen bond formed on the oxygen site is difficult to break, while the weak hydrogen atom adsorption of the vanadium site to the hydrogen atom can facilitate the bonding of the hydrogen atom. The problem, however, is that the transfer of hydrogen atoms to metallic vanadium sites is difficult on clean vanadium-oxygen surfaces.

To solve this problem, we have introduced platinum atoms to modify the clean vanadium-oxygen surface. By comparing the stability of the surface when the platinum atoms are in different sites, it is illustrated that the replacement of the vanadium atoms with platinum atoms is the most stable. The transfer of the hydrogen atoms to the platinum atoms is much easier on the platinum-doped surface, which is very favourable for the binding of the hydrogen atoms. In addition, the catalytic activity of the Pt-doped surface is also higher than that of a clean V2O5 surface for the breakage of hydrogencarbon bonds.

In addition to the introduction of platinum atoms for surface modification, turning catalyst supports may be also an approach to increase the activity. The porous γ-Al2O3 supports can form a wide range of unpredictable and complex structures with VOx catalysts. Molecular dynamics methods were applied to help search for γ-Al2O3 supported vanadium-oxygen catalysts. Different stoichiometry ratios of vanadium and oxygen exhibit different reduced states of vanadium; it is found that the lower the vanadium alence, the more unstable the catalyst structure is. However, when they are used as catalysts for the dehydrogenation of propane, the lower-valency vanadium atoms have a higher reactivity. Furthermore, it is surprising that on the VxOy supported on alumina surface in a reduced state, the lattice oxygen of the alumina can act as an active site and that the hydrogen atoms adsorbed at this site are more favourable to the H2 formation than those in the vanadium-oxygen structures. This solves the biggest challenge of propane dehydrogenation reactions in the VOx systems to date.

Thesis is embargoed until 31 July 2025.

Date of Award	Jul 2023
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Jillian Thompson (Supervisor) & Peijun Hu (Supervisor)