Size and Site Dependence of the Catalytic Activity of Iridium Clusters toward Ethane Dehydrogenation.

Abstract

This research focuses on optimizing transition metal nanocatalyst immobilization and activity to enhance ethane dehydrogenation. Ethane dehydrogenation, catalyzed by thermally stable Irn (n = 8, 12, 18) atomic clusters that exhibit a cuboid structure, was studied using the B3LYP method with triple-ζ basis sets. Relativistic effects and dispersion corrections were included in the calculations. In the dehydrogenation reaction Irn + C2H6 → H-Irn-C2H5 → (H)2-Irn-C2H4, the first H-elimination is the rate-limiting step, primarily because the reaction releases sufficient heat to facilitate the second H-elimination. The catalytic activity of the Ir clusters strongly depends on the Ir cluster size and the specific catalytic site. Cubic Ir8 is the least reactive toward H-elimination in ethane: Ir8 + C2H6 → H-Ir8-C2H5 has a large (65 kJ/mol) energy barrier, whereas Ir12 (3 × 2 × 2 cuboid) and Ir18 (3 × 3 × 2 cuboid) lower this energy barrier to 22 and 3 kJ/mol, respectively. The site dependence is as prominent as the size effect. For example, the energy barrier for the Ir18 + C2H6 → H-Ir18-C2H5 reaction is 3, 48, and 71 kJ/mol at the corner, edge, or face-center sites of the Ir18 cuboid, respectively. Energy release due to Ir cluster insertion into an ethane C-H bond facilitates hydrogen migration on the Ir cluster surface, and the second H-elimination of ethane. In an oxygen-rich environment, oxygen molecules may be absorbed on the Ir cluster surface. The oxygen atoms bonded to the Ir cluster surface may slightly increase the energy barrier for H-elimination in ethane. However, the adsorption of oxygen and its reaction with H atoms on the Ir cluster releases sufficient heat to yield an overall thermodynamically favored reaction: Irn + C2H6 + 1/2O2 → Irn + C2H4 + H2O. These results will be useful toward reducing the energy cost of ethane dehydrogenation in industry.