Methane Activations by Lanthanum Oxide Clusters

Abstract

Density functional theory and coupled cluster theory were employed to study the activations of CH4 by neutral lanthanum oxide clusters (LaO(OH), La2O3, La3O4(OH), La4O6, La6O9) as models for the La2O3 catalysts for the oxidative coupling of methane (OCM) reaction. The physisorption energies (ΔH298 K) of CH4 on the lanthanum oxide clusters were predicted to be −4 to −3 kcal/mol at the CCSD(T) level. CH4 is activated by hydrogen transfer to one of the O sites on the lanthanum oxide clusters, and the energy barriers (ΔE0 K) from the physisorption structures were calculated to be modest at ∼20 kcal/mol for La2O3 and ∼25 kcal/mol for the other clusters. This is accompanied by the formation of a La–CH3 bond, whose bond dissociation energy (ΔE0 K) was calculated to be 53 to 60 kcal/mol. CH4 chemisorption is slightly exothermic on LaO(OH) and La2O3, whereas it becomes increasingly endothermic for the larger lanthanum oxide clusters. The formation of the CH3 radical was predicted to be substantially endothermic, by ∼50 kcal/mol for LaO(OH) and La2O3 and 64 to 76 kcal/mol for La3O4(OH) and La4O6 (ΔH298 K). Calculations on the activation of CH4 by La6O9 with a higher coordination number for both the La and O sites than La4O6 yield an energy barrier slightly higher by <1 kcal/mol, suggesting that the effects of the coordination numbers on the reaction energetics are rather small. The energy barrier for hydrogen abstraction does not correlate well with the negative charge on the O site, and a linear relation between the energy barrier and the chemisorption energy was not found for all the lanthanum oxide clusters, which is attributed to the strong dependency of their correlation on the specific chemical environment of the reactive site. Cluster reaction energies, physisorption and chemisorption energies, energy barriers, and La–CH3 bond energies calculated at the DFT level with the B3LYP and PBE functionals were compared with those calculated at the CCSD(T) level showing that the B3LYP functional yields better cluster reaction energies, chemisorption energies, and energy barriers. Although the PBE functional yields better physisorption energies, the DFT results can deviate substantially from the CCSD(T) values. Although the O2– sites in these cluster models were predicted to be less reactive toward CH4 than the O– sites modeled by the nonstoichiometric La2O3.33(001) surface (Palmer, M. S. et al. J. Am. Chem. Soc. 2002, 124, 8452), they are more reactive than the O22– site modeled on the stoichiometric La2O3(001) surface, which suggests the relevance of the lattice oxygen sites on the La2O3 catalyst surfaces in the OCM reaction.