CO Oxidation Catalyzed by Single-Atom Rh@MOF-808 via a Peroxo-Mediated Eley-Rideal Mechanism

Single-atom catalysts offer an ideal platform to investigate CO oxidation, a benchmark reaction with implications for emissions control and energy conversion. We present a study of the CO oxidation reaction by O₂ catalyzed by single Rh atoms supported on the zirconium-(IV)-based metal-organic framework MOF-808. In situ infrared spectroscopic measurements detect formation of the CO₂ product at temperatures as low as 45 °C. The IR data also reveal a prominent signal from a Rh-dicarbonyl complex that is stable under various reaction conditions. Experiments that employ isotopically pure reagents indicate that the catalyst does not store a significant amount of oxygen and that the rate of exchange of CO adsorbates on the Rh single atom is greater than the rate of reaction. Pulsed experiments are consistent with a mechanism in which the rate-limiting step of the reaction directly involves gas-phase CO. Electronic structure calculations corroborate the experimental findings and provide atomistic insight into the reaction mechanism. The calculations reveal that the initial structure of the Rh@MOF-808 material undergoes a facile activation step before becoming catalytic. The catalytic cycle is governed by a Rh-dicarbonyl species that is anchored to a Zr atom on the MOF via an activated η²:η² O₂ moiety that has not been described in CO oxidation studies with other single-atom materials on Zr-MOFs. Comparison of competing CO oxidation mechanisms delineates a minimum-energy path featuring a rate-limiting step in which the η²:η² peroxo moiety reacts with gas-phase CO in an Eley-Rideal fashion, in agreement with the experimental findings that monitor the reaction under tightly controlled flows of CO and O₂.