The design of single-atom based catalysts has been an ultimate goal for many years for the catalyst design. In addition to the desire to maximize the cost-efficiency of using precious metals, single-atom catalysts are highly interesting also from a fundamental point of view because of the well-defined nature of the active site compared to dispersed metal nanoparticles. Identification of the active site is a precondition for a comprehensive understanding of the mechanistic of catalytic reactions, and thus for a systematic control of their activity and selectivity. With their high porosity and exceptionally large surface area, the structurally well-defined metal–organic frameworks (MOFs) represent a robust and very selective platform for the atomic dispersion of metals, stabilizing individual metal atoms. Recently, we reported a single-atom catalyst, Cu/UiO-66, prepared by a covalent attachment of Cu atoms to the defect sites at the zirconium oxide clusters of the MOF UiO-66. Catalytic performance tests showed that these catalysts are highly active and stable during CO oxidation under technically relevant gas mixtures (O2/CO = 20 and temperature up to 350°C). Here, we expanded on our studies to understand the reaction mechanism on these catalysts, by combining insights from operando spectroscopies (X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR) and diffuse reflectance FTIR), and activity / kinetic isotope effect (KIE) studies during CO oxidation together with DFT computations of different reaction pathways. These studies revealed that the reaction cycle starts by the creation of an O-vacancy defect site at the interface between Cu single atoms and Zr oxide clusters, forming an F-center. Then O2 is activated on this O-vacancy, followed by CO adsorption on Cu1+, and subsequent reaction with the atomic oxygen. The reaction cycle starts over again by recreation of the F-center (i.e., vacancy creation) by reaction with CO. KIE measurements, in good agreement with DFT calculations, revealed that CO removal by reaction with active oxygen is the rate limiting step in this reaction.

Reaction mechanism and rate limiting step during CO oxidation on single Cu-atom catalysts supported by metal-organic-frameworks

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.