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 (O_t), promote the oxidation of CO adsorbed on IrO₂(110). On the O_t-covered surface, CO oxidation by O_t atoms produces a CO₂ TPRS peak at ∼370 K, and all of the initially adsorbed CO oxidizes to CO₂ when the initial O_t 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 O_t 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.