Quantum-Chemical Investigation of Hydrocarbon Oxidative Dehydrogenation over Spin-Active Carbon Catalyst Clusters

Graphene-like carbon clusters with oxygen-saturated zigzag and armchair edges were used as models for density-functional theory investigations of the oxidative dehydrogenation (ODH) of hydrocarbon molecules over carbon catalysts. The product of the first elementary step of the reaction, which is either a hydrocarbon radical or a surface ether, is found to be strictly dependent on the spin multiplicity of the catalyst, although energies of the initial state are spin-degenerate. The barriers of the first step of the ODH of light hydrocarbons (methane, ethane, and propane) over zigzag-edge carbon clusters are higher (59–104 kJ/mol) than those for ethylbenzene (18–58 kJ/mol), and the barrier of the second H abstraction is generally rate-limiting (82–106 kJ/mol). The armchair edge is passive toward reaction with hydrocarbons, but it reacts almost without a barrier with hydrocarbon radicals. The barrier of reoxidation by O₂ was found to decrease from 161 to 69 kJ/mol with an increasing level of saturation with H atoms.