The first principles studies on the reaction pathway of the oxidative dehydrogenation of ethane on the undoped and doped carbon catalyst

Abstract

The oxidation dehydrogenation (ODH) of ethane on a carbon catalyst is studied by first principles calculations. From the examination of the ethane chemisorption at the various oxygen functional groups, the ketonic-like groups, diketone and quinone are identified as the active sites. Two different pathways are explored after the first C–H bond breaking in the ethane molecule, which has a barrier of 1.48 eV: (1) the ethyl radical becomes the adsorbed ethoxide and the further forwarding reaction encounters a large barrier, which is not kinetically favorable. (2) The ethyl radical is reoriented and directly activated at the diketone site to form ethylene. Nitrogen and boron doping can effectively alter the reaction pathway. It is found that the barrier of the first C–H bond breaking at the armchair termination is decreased after doping. Also, nitrogen doping makes the intermediate ethoxide less stable, which facilitates a further reaction to ethylene formation. The mechanism of the regeneration of the active sites is different from the one for the metal oxide catalyst. The combination of two neighboring hydroxyls into a water molecule is not feasible. Instead, two routes for active site regeneration on the carbon catalyst are proposed using either the oxygen molecule or the dissociated oxygen atom as the oxidant. The current work gives a detailed description of the whole process of the ODH of ethane on a carbon catalyst. The results from the calculations not only deepen the understanding of the reaction mechanism of ODH of ethane on a carbon catalyst, but also provide valuable insights for the optimization of the carbon catalyst performance in general.