One major difference between classical homogeneous catalysts and heterogeneous catalysts is the use and availability of ligands as part of the active catalytic site. Ligands are often used to increase the activity and selectivity of homogeneous catalysts due to their ability to modify structural or electronic properties of the active metal site. In a similar way, supported atomically dispersed catalysts have been shown to exhibit similar effects. This starts with viewing the support as a ligand and goes up to molecular complexes that are not part of the support being used as ligands.<br/>Here we present the unusual case of a heterogeneous catalyst that uses one of the reactants as a fixed ligand as part of the active site. Specifically, we show that for the Ir1/TiO2 single atom catalyst, the active catalytic site contains an inert CO that is ever present throughout the reaction cycle of CO oxidation. Using a combination of isotopically labeled 13CO, infrared and X-ray absorption spectroscopy experiments, activity measurements, and DFT modeling, we show that the ligand CO remains part of the active complex through 100s of turnovers of the Eley-Rideal/Mars-van Krevelen reaction pathway we propose. This is a rare instance of CO acting as a ligand for CO oxidation in a single atom catalyst. It also further develops the potential improvements that can be gained through control of the site in heterogeneous catalysis by using the principles common to homogeneous catalysts.

CO as a ligand for CO oxidation on supported Ir single atom catalysts

ACS Fall 2021

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