DFT modelling of methane oxidation with H2O2 over heterogeneous catalysts

• Main Author: Thetford, Adam
• Published: Cardiff University 2012
• Subjects: 541; QD Chemistry
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.567488

DFT methods were used to study the mechanism of methane oxidation using H2O2 over a TiO2 supported Au and Fe-ZSM-5 catalyst to produce CH3OH. DFT+U was employed to improve the model of the oxide surface, framework and framework metal centre in CHA and MFI structures. The catalysts were modelled in VASP. Au10 clusters were used to produce a preliminary mechanism, which was tested on the different design catalysts. The Au10 clusters were supported on TiO2 to model small clusters and extended Au, Pd and PdO surfaces were used to model larger clusters. The mechanism is tested on [Fe2O2]2+ which is in the zeolite extra-framework in both the CHA structure and MFI structure with two Al ions in the framework as counter ions. H2O2 is found to spontaneously break the HO – OH bond in the presence of Au10 clusters, Au(111) surface and Pd(111) surface. CH3OOH is produced via OOH formed from H2O2 and a radical methyl on the Au(111) and Pd(111) surfaces. Fe2O2 as an extra-framework species is modified with water to produce an active site. The active site was then further modified with H2O2 to produce a Fe4+ ═ O which is used to break a C – H bond in methane. The mechanism on the metal/oxide was shown to differ from the mechanism in the zeolite. The formation of the C – O bond in CH3OOH is the most important step on the metal/oxide whereas this is low energy or spontaneous in the zeolite. CH4 directly producing CH3OOH has a barrier of 50 kJ mol-1 on the modified Fe2O2 in the Al-MFI structure.