title: Distinguishing homogeneous from heterogeneous catalysis in electrode-driven water oxidation with molecular iridium complexes.
abstract: Molecular water-oxidation catalysts can deactivate by side reactions or decompose to secondary materials over time due to the harsh, oxidizing conditions required to drive oxygen evolution. Distinguishing electrode surface-bound heterogeneous catalysts (such as iridium oxide) from homogeneous molecular catalysts is often difficult. Using an electrochemical quartz crystal nanobalance (EQCN), we report a method for probing electrodeposition of metal oxide materials from molecular precursors. Using the previously reported [Cp*Ir(H(2)O)(3)](2+) complex, we monitor deposition of a heterogeneous water oxidation catalyst by measuring the electrode mass in real time with piezoelectric gravimetry. Conversely, we do not observe deposition for homogeneous catalysts, such as the water-soluble complex Cp*Ir(pyr-CMe(2)O)X reported in this work. Rotating ring-disk electrode electrochemistry and Clark-type electrode studies show that this complex is a catalyst for water oxidation with oxygen produced as the product. For the heterogeneous, surface-attached material generated from [Cp*Ir(H(2)O)(3)](2+), we can estimate the percentage of electroactive metal centers in the surface layer. We monitor electrode composition dynamically during catalytic turnover, providing new information on catalytic performance. Together, these data suggest that EQCN can directly probe the homogeneity of molecular water-oxidation catalysts over short times.
authors.affiliation: Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520-8107, USA.
authors.forename: Nathan D
authors.initials: ND
authors.lastname: Schley
authors.forename: James D
authors.initials: JD
authors.lastname: Blakemore
authors.forename: Navaneetha K
authors.initials: NK
authors.lastname: Subbaiyan
authors.forename: Christopher D
authors.initials: CD
authors.lastname: Incarvito
authors.forename: Francis
authors.initials: F
authors.lastname: D'Souza
authors.forename: Robert H
authors.initials: RH
authors.lastname: Crabtree
authors.forename: Gary W
authors.initials: GW
authors.lastname: Brudvig