CO Oxidation over HKUST-1 Catalysts: The Role of Defective Sites

Abstract

CO oxidation over metal–organic framework (MOF) HKUST-1 catalysts that contain coordinatively unsaturated sites has been investigated using density functional theory (DFT) calculations. Three model catalysts, that is, the pristine HKUST-1, the defective HKUST-1 with one linker displaced by a hydroxyl group (HKUST-DE1), and the defective HKUST-1 with one linker completely removed (HKUST-DE2) are used to understand how the defective site affects CO oxidation reaction mechanisms and kinetics with high-spin and low-spin states. Theoretical DFT calculations indicate that the CO oxidation activity follows the order of HKUST-DE2 > HKUST-DE1 > HKUST-1. On the pristine HKUST-1, CO oxidation only occurs via the Eley–Rideal (ER) mechanism. With the introduction of the defective site to the HKUST-1 framework, CO oxidation may occur via both Langmuir–Hinshelwood (LH) and ER mechanisms, while the LH mechanism is more favored. In particular, the presence of dangling OH groups over HKUST-DE1 provides a new reaction pathway of forming COOH intermediate that further accelerates the CO oxidation reaction. Furthermore, CO oxidation is pronouncedly promoted by both steric and electronic effects derived from the missing linker over HKUST-DE2, which is largely due to the formation of a Cu–C–O–O–Cu five-membered ring intermediate on the reduced Cuδ+/Cuδ+ (δ < 2) dimer. The present study demonstrates that catalytic properties of MOF catalysts can be rationally controlled by the defect-engineering strategy.