Surveying the Homogeneity of a Molecular Electrocatalyst Embedded in a Metal-Organic Framework Using Operando Characterization.

Homogeneous catalysis generally yields low catalytic current densities due to the small number of catalytic centers at the electrode surface. Incorporating molecular catalysts in metal-organic frameworks (MOFs) has been proposed as a viable approach to immobilize them on electrodes, increasing current densities. In addition, molecular catalysts do not always remain in their homogeneous state, sometimes partially taking on a more heterogeneous character, which challenges the clear identification of the active species. Despite the risk of homogeneity loss, most studies on molecular catalysts embedded in MOFs have so far overlooked the possibility of heterogeneous deposit formation during electrocatalysis. In this work, a more comprehensive study on the changes of homogeneity exhibited by an MOF-embedded molecular catalyst is presented. The Cu species formed in the NU1000|Cu-tmpaCOOH MOF before, during, and after the oxygen reduction reaction using operando X-ray absorption spectroscopy are investigated. The initial Cu²⁺ catalyst forms Cu⁰ clusters of diameter <2 nm upon application of a reductive potential. This work demonstrates that for Cu-based molecular catalysts embedded in MOFs, it is essential to account for the possible changes in a molecular catalyst's homogeneity, regardless of the catalytic benefits its supporting structure might grant.