CO oxidation on IrO2(110) surfaces

Abstract

We investigated the oxidation of CO on stoichiometric and O-rich IrO2(110) surfaces using temperature programmed reaction spectroscopy (TPRS), density functional theory (DFT) calculations and microkinetic simulations. Adsorbed CO on the s-IrO2(110) surface generates CO and CO2 peaks near 545 K during TPRS, and only about 38 % of the CO oxidized to CO2 when the initial CO layer was saturated. Pre-adsorbed O-atoms, so-called on-top oxygen (Ot), promote the oxidation of CO adsorbed on IrO2(110). On the Ot-covered surface, CO oxidation by Ot atoms produces a CO2 TPRS peak at ∼370 K, and all of the initially adsorbed CO oxidizes to CO2 when the initial Ot coverage is greater than the CO coverage. In agreement with the TPRS results, DFT calculations predict that the barrier is about 100 kJ/mol lower for CO oxidation by an Ot atom than a lattice O-atom of IrO2(110). A microkinetic model, parameterized with energy barriers computed using DFT, accurately reproduces the CO and CO2 TPRS traces only after CO binding energies are lowered to values determined using a hybrid exchange-correlation functional and the barrier for CO molecules to fill bridging O-vacancies is lowered. The simulations predict that O-vacancies play an important role in mediating the CO oxidation kinetics on s-IrO2(110), and thereby demonstrate the importance of future spectroscopic studies aimed at characterizing the nature of the surface CO and O species involved in reaction. This study provides new insights for understanding CO oxidation on IrO2(110), and provides evidence that several elementary steps can be involved in governing this chemistry.