Mechanistic insights into collision induced and catalytic activation of methane from first principles
Varghese, Jithin John
Date of Issue2016
School of Chemical and Biomedical Engineering
Methane is the primary component of natural gas, coal bed gases, shale gas and natural gas hydrates, and its commercially recoverable amounts exceed those of petroleum and coal by a few orders of magnitude. The strong C-H bonds of methane make its selective and controlled activation/dissociation difficult and this impedes the widespread utilization of this small alkane as a chemical feedstock to make value added chemicals, fuels and materials. Thus, developing alternate techniques, synthesising new catalytic materials and gaining fundamental mechanistic insights into C-H bond activation on known catalytic surfaces are pivotal to making ‘methane to chemicals and fuels’ a reality. Using state of the art quantum chemical simulations, it is shown for the first time that mechanical impact of methane molecules induces the dissociation of their strong C-H bonds. This novel bimolecular ‘coupling induced dissociation mechanism’ is demonstrated to convert methane molecules undergoing mechanical impact, directly to C2 hydrocarbons like ethane and ethylene. The impact induced vibrational excitation of the molecules allow successive transitions to lower potential energy states, leading to the concerted dissociation of C-H bonds of the molecules and coupling of the carbon atoms. Using ab initio molecular dynamics and Metadynamics simulations, it is shown that the mechanical impact of methane on an inactive transition metal like copper, in the form of nanoclusters, can dissociate it with activation free energy barriers between 60 and 90 kJ mol-1. The impact induced vibrational excitation of the molecule and the formation of a collision complex, which serves as a precursor for the dissociation of methane are key reasons for the low free energy barriers. Additionally, the small 3-dimensional copper nanoclusters have electronic structure and morphological features which favour the impact induced dissociation of methane while subsequent dehydrogenation steps have high barriers. The stabilization of the transition state and the products of dissociation (CH3 and H) by the under-coordinated Cu-O pair on the CuO surfaces, enable the dissociation of methane with an activation energy barrier as low as 76 kJ mol-1 on the most stable CuO(111) surface, compared to the barrier of 170 kJ mol-1 on Cu(111) surface. Thus, oxygen enhances the activity of copper in methane dissociation. The stronger the H chemisorption on the surface lattice oxygen on the different CuO surfaces, the more active is the surface in dissociating methane, making H chemisorption energy a descriptor for the reactivity of the surface. The weaker the copper- lattice oxygen interaction on the surface, the stronger is the H chemisorption on the lattice oxygen. The difference in the intrinsic metal- oxygen interaction on the different CuO surfaces is the root cause for the differences in their reactivity. In contrast to CuO, the 5-coordinated surface lattice oxygen on the most stable NiO surface, NiO(100), makes the surface inactive for methane dissociation with activation barrier of 136 kJ mol-1 compared to 70 kJ mol-1 on Ni(100) surface. NiO surfaces are as active as metallic Ni for methane dissociation when 1) the surface lattice oxygen is 4 coordinated as in the less stable NiO(110) surface; 2) NiO has surface Ni vacancies; and 3) NiO is doped with low valent dopants like Li. On comparing un-doped and doped surfaces, the H chemisorption energy, the descriptor for the reactivity of the lattice oxygen on different NiO surfaces, was found to have a weaker correlation with the intrinsic binding strength of the lattice oxygen on the surfaces. Low valent dopants like Li increase the activity of surface lattice oxygen for methane dissociation by lowering the binding strength of the surface lattice oxygen. In summary, this thesis reveals unusually high reactivity of methane resulting from collisional impact. The findings have implications for understanding of certain planetary phenomena, offer an explanation for the genesis of ‘abiogenic petroleum’ on the earth and also indicate opportunities for the commercial exploitation of methane. Insights into the role of lattice oxygen, vacancies and dopants on metal oxides in methane dissociation, and H chemisorption energy as a descriptor for reactivity can serve as guidelines in screening and designing efficient multifunctional catalysts for methane dissociation and oxidation reactions.