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.