Enhanced Selectivity for C2H4 Production from C2H6 on Partially Chlorinated IrO2(110) Surfaces

Abstract

Modifying metal oxide surfaces to limit their oxidizing activity can provide a means of improving catalytic selectivity toward the partial oxidation of light alkanes. In this study, we investigated the oxidation of C₂H₆ on Cl-modified IrO₂(110) surfaces using temperature-programmed reaction spectroscopy (TPRS) and first-principles microkinetic modeling. We find that substituting Cl for O in the IrO₂(110) surface enhances the selectivity for C₂H₆ conversion to C₂H₄ during TPRS by suppressing extensive oxidation to CO _x products, while also either enhancing C₂H₄ production or altering it to a lesser extent, depending on the initial C₂H₆ coverage. The C₂H₄ selectivity increased with increasing C₂H₆ and Cl coverage, but reached a limiting value below 50%. The Cl coverage changed negligibly during C₂H₆ oxidation, and the surface reactivity decreased only marginally for Cl coverages up to 0.5 ML (monolayer). TPRS simulations using a microkinetic model predict C₂H₄ and CO _x product yields as a function of the Cl coverage that agree closely with the experimental results. According to the simulations, C₂H₆ conversion to C₂H₄ occurs on Cl-IrO₂(110) by the hydrogenation of C₂H₃* species adsorbed in blocked states, in which neighboring sites are occupied only by unreactive HO and Cl species. The microkinetic modeling shows that H-hopping away from surface HO groups provides a relatively efficient route for C₂H₃* to escape blocked configurations and dehydrogenate, and that this process can limit the C₂H₄ selectivity on Cl-IrO₂(110) under the conditions studied. Overall, our results demonstrate that Cl-substitution into IrO₂(110) enhances the selectivity for C₂H₄ production from C₂H₆ and provides insights into the reaction mechanism that can guide strategies to further improve the C₂H₄ selectivity.