Catalytic CO Oxidation by Cu Single Atoms on the UiO-66 Metal–Organic Framework: The Role of the Oxidation State

Recent experiments have demonstrated catalysis by single
transition-metal
atoms supported on metal–organic frameworks (MOF) but have
yet to decipher the role of the transition-metal oxidation state and
the reaction mechanism. In this work, we present a study of the aerobic
CO oxidation reaction on single Cu atoms supported on the UiO-66 MOF.
Characterization of the Cu@UiO-66 catalyst after reduction via infrared
spectroscopy of a CO probe molecule reveals the presence of both Cu­(I)
and Cu­(II) oxidation states. Operando infrared spectroscopic experiments
further show both oxidation states are also present during reaction,
and subsequent mechanistic studies using quantum mechanical calculations
have therefore examined the reaction with both Cu­(I) and Cu­(II). The
calculations reveal a variety of different Langmuir–Hinshelwood
CO oxidation mechanisms, including those involving coordination of
only O2 to Cu at the rate-limiting transition state or
both CO and O2. Overall, CO oxidation on Cu­(II)@UiO-66
exhibits a lower maximum energetic span than on Cu­(I) and is therefore
predicted to be faster under operational conditions for most methods
examined. The reaction mechanisms for MOF-supported Cu single atoms
differ from those involving single-Cu atoms on metal-oxide supports,
which opens new avenues for increasingly efficient catalysis.