Resolving the Mechanism Complexity of Oxidative Dehydrogenation of Hydrocarbons on Nanocarbon by Microkinetic Modeling
Zan Lian, Chaowei Si, Faheem Jan, Min Yang, Bo Li
Abstract
As metal-free catalysts with great potential for widespread application, nanostructured carbon materials have excellent performance comparable to traditional metal oxide catalysts in the oxidative dehydrogenation (ODH) of alkanes. Unlike the mature narrations of the mechanism on the metal catalyst, the mechanism of ODH on the nanocarbon catalyst is still disputable and inconclusive. In this work, density functional theory (DFT) calculation and microkinetic modeling (MKM) are combined to provide unique insights into the mechanism. Four possible mechanisms were studied, which are Eley–Rideal (ER), Langmuir–Hinshelwood (LH), and Mars–van Krevelen (MvK), together with the reactions between weakly adsorbed species and radicals. The simulation clearly identified the mechanism of the reaction that the ER pathway is more likely than both LH and MvK pathways under various conditions. Carbonyl groups are the main active sites for the first hydrogen abstraction of ethylbenzene. On the other hand, the hydrogen abstraction from intermediate hydrocarbons (such as C6H5CHCH3·) by O2* and HO2· yields a significant contribution to the total reaction rate under certain conditions. The reaction is first-order in ethylbenzene and zero-order in O2 when the ratio of reactants nears stoichiometry. Moreover, the defective site does not affect the intrinsic catalytic performance. The defective site is subsequently transformed into the carbonyl group at the steady state and thus increases the number of active sites. This study provides a decisive description of the mechanism of ODH on nanocarbon catalysts under various reaction conditions and sheds light on design of the catalyst and optimization of reaction conditions.