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.本研究以氧饱和锯齿型边（zigzag edge）与扶手椅型边（armchair edge）的类石墨烯碳团簇为模型体系，开展碳催化剂表面烃分子氧化脱氢（oxidative dehydrogenation, ODH）过程的密度泛函理论（density-functional theory）研究。研究发现，反应第一步基元步骤的产物为烃自由基（hydrocarbon radical）或表面醚（surface ether），且该产物严格取决于催化剂的自旋多重度（spin multiplicity）；尽管反应初始态的能量呈现自旋简并（spin-degenerate）特性。轻质烃（light hydrocarbons，即甲烷methane、乙烷ethane与丙烷propane）在锯齿型边碳团簇上发生ODH的第一步能垒介于59–104 kJ/mol，高于乙苯（ethylbenzene）对应的18–58 kJ/mol能垒；而第二步氢提取（H abstraction）步骤的能垒通常为限速步骤（rate-limiting），能垒范围为82–106 kJ/mol。扶手椅型边对烃类反应呈催化惰性，但可与烃自由基几乎无能垒地发生反应。研究发现，氧气（O2）介导的再氧化（reoxidation）能垒随碳团簇氢原子饱和程度的提升，从161 kJ/mol降至69 kJ/mol。