Assessment of competing mechanisms of the abstraction of hydrogen from CH4 on Li/MgO(001)

The electronic structure of alkali doped alkaline earth metal oxides: Li doping of MgO studied with DFT-GGA and GGA+U

“…This 5 layer slab thickness is consistent with the similarly converged results of Ref. [19], in which a subsurface doped structure with 4-6 layers was studied and the differences between the different slab thicknesses are small. Testing the plane wave energy cut-off and k-point sampling gives a 500 eV plane wave energy cut-off and a 2 · 2 · 1 k-point sampling mesh, with the third vector along the surface normal, since the inter-slab interaction in the vacuum direction is negligible.…”

“…This delocalisation of the hole state was confirmed by analysis of the charge density and is in disagreement with the UHF results of [14][15][16] and experimental data [1][2][3][4]. The energetics for the reaction of Li-doped MgO with methane and with hydrogen were presented for the structures found with GGA [19]. However, it is not clear what effect the delocalisation found with the GGA computations will have on the energetics.…”

“…It was concluded that LSDA incorrectly delocalises the unpaired electron. Despite this, Yang et al [17] have used the generalised gradient approximation (GGA) of DFT with the Perdew-Wang 1991 (PW91) [18] exchange-correlation functional, in the framework of the full potential linear augmented plane wave (FPLAPW) approach, to study the Li-doped MgO (1 0 0) surface and Dash and Gillan [19] have applied the same exchange-correlation functional within the pseudopotential approach. The latter study is the only study to consider both surface and subsurface dopant sites.…”

“…In the DFT studies [17,19] the electronic density of states (EDOS) calculated for Li-doped MgO displays no states in the energy gap and the valence band crosses the Fermi level. This is concluded to be due to the presence of holes [17].…”

“…Experimental data indicates that the activation energy for conversion of methane to methyl radicals over Li/MgO is 2.29 eV (220 kJ mol À1 ) [9], while from DFT, the energy for hydrogen abstraction from methane has been found to be 0.25 eV (24 kJ mol À1 ) [19]. Periodic Hartree-Fock computations result in a hydrogen abstraction energy of 1.18 eV (113 kJ mol À1 ) [15], while a correlation correction to the UHF approach results in a reaction energy of 0.79 eV (76 kJ mol À1 ) [20].…”