Unraveling the Surface State Evolution of IrO2 in Ethane Chemical Looping Oxidative Dehydrogenation

Abstract

Based on the advantages of favorable thermodynamics and coking resistance of ethane oxidative dehydrogenation and the challenge of low ethylene selectivity, chemical looping oxidative dehydrogenation (CL-ODH) over the IrO2 catalyst was examined, including the dehydrogenation and regeneration processes. The stoichiometric S-IrO2 and reduced R-IrO2 catalysts as two extreme states of the IrO2 surface structure with dynamic changes were considered. Density functional theory (DFT) calculations and kinetic Monte Carlo simulations showed that the mechanisms of ethane dehydrogenation over S-IrO2 and R-IrO2 catalysts were quite different. Over the S-IrO2 catalyst, ethane oxidative dehydrogenation to C2H4(g) with H2O(g), CO(g), and CO2(g) taking away surface lattice oxygen, followed by lattice oxygen migration from the bulk to the surface, leads to the reduction of the S-IrO2 catalyst. Over the R-IrO2 catalyst, ethane directly dehydrogenates to C2H4(g) and H2(g). Furthermore, the oxidation degree in the regeneration process is greater than the Ov concentration in the dehydrogenation process, which can easily achieve oxygen replenishment in the regeneration process. More importantly, the IrO2 catalyst can be neither completely reduced in the dehydrogenation process nor completely oxidized in the regeneration process, both S-IrO2 and R-IrO2 simultaneously exist for the IrO2 catalyst, and both 750 K and 0.8 bar C2H6(g) pressure were obtained to be the optimal reaction conditions; thus, for ethane CL-ODH over the IrO2 catalyst, the proposed mechanism starts from the oxidative dehydrogenation process; with the consumption of surface lattice oxygen and the oxygen migration from the bulk to the surface, the oxidative and nonoxidative dehydrogenations occur simultaneously until the regeneration. The present study broadens the understanding of ethane CL-ODH over metal oxide catalysts and provides valuable information for the optimization of the CL-ODH process and the development of other high-performance metal oxide catalysts in other alkane CL-ODH processes.