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.

Carbene iridium complexes for efficient water oxidation: scope and mechanistic insights

“…from experiments using a quartz microbalance as a probe and from catalyst immobilization. [35][36][37] Direct comparison of results across the broad range of mechanistic investigations reported in literature is difficult due to varying reaction conditions including the use or absence of organic co-solvents and buffered or unbuffered aqueous conditions. Following our initial discovery of triazolylidene iridium complexes as potent water oxidation catalysts, we engaged in careful ligand modification in an attempt to improve catalyst performance and to expand our understanding of oxidation mechanisms with application to a broad variety of fuel producing photosystems.…”

Ligand Exchange and Redox Processes in Iridium Triazolylidene Complexes Relevant to Catalytic Water Oxidation

“…19 Moreover, kinetic and mechanistic studies have provide increasingly compelling evidence that some complexes are precursors for homogeneous rather than heterogeneous [20][21][22] water oxidation catalysts and that the oxidation therefore occurs at an iridium center that is in a well-defined environment. 19,[23][24][25][26][27][28] This environment has remained elusive up to now despite various efforts to trap and isolate catalytically competent species. [29][30][31][32] In particular iridium cyclopentadienyl complexes [Ir(Cp*)(L,L)X] + containing a chelating N,N-, C,N-, or C,Cbidentate ligand motive afforded high catalytic activity (Cp* = = C 5 Me 5 -).…”

Heterogenized Iridium Water-Oxidation Catalyst from a Silatrane Precursor

“…Pentamethylcyclopentadienyl iridium dichloride dimer, (IrCp*Cl 2 ) 2 , 3 3-aminophenylsilatrane, 4 and Cp*Ir(2-(2'-pyridyl)-2-propanolate)Cl (Cp* = pentamethylcyclopentadienyl), 5 were synthesized and characterized by previously reported methods.…”