Metal oxides are well known oxidation catalysts. Oxidation of hydrocarbons on metal oxides is believed to occur via Mars–van Krevelen type mechanism, where hydrocarbons are oxidized using the lattice oxygen. However, the nature and role of different surface oxygen species and the underlying bond activation mechanism is not yet fully understood. Similarly, in the case of molecules with oxygen containing functional groups, exchange of surface lattice oxygen with the reactant is reported, with little insight into the role of lattice oxygen in catalyzing the oxidation reaction. Hence, in this study, we performed Density functional theory (DFT) calculations to evaluate the catalytic activity of surface lattice oxygen of metal oxides, with CuO as an example, in two cases: methane C-H activation in natural gas utilization and the formyl group C-H activation in biomass reactions.
In the first case, we investigated the catalytic mechanism of the oxidation of glucose to gluconic acid on CuO catalyst. The first-principles calculations reveal that the surface lattice oxygen of copper oxide (CuO) activates the formyl C–H bond in glucose with an activation energy barrier of 87 kJ/mol and it incorporates itself into the glucose molecule to oxidize it to gluconic acid. The complete reaction mechanism involving the insertion of the surface lattice oxygen into the sugar molecule, in perfect agreement with the experimental findings, is revealed. The formyl C–H bond activation by surface lattice oxygen is compared with that of chemisorbed oxygen on the surface and the crucial role of surface lattice oxygen in the oxidation of glucose to gluconic acid, with minimum C–C cleavage, is explained.
In the second case, the synergistic role of unsaturated metal (Cu) and surface lattice oxygen (O) site of CuO in activating the C-H bond of methane is investigated. C-H activation barrier was computed as 169 kJ/mol on Cu, but reduced to 132 kJ/mol when facilitated by chemisorbed oxygen on Cu and significantly lowered to 77 kJ/mol on CuO surface. The comprehensive evaluation of methane C-H bond activation on CuO and on chemisorbed oxygen on Cu surface reveal that both Cu and O sites are needed to facilitate the C-H bond activation by enhanced stabilization of the transition state.
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