Water Oxidation with Molecularly Defined Iridium Complexes: Insights into Homogeneous versus Heterogeneous Catalysis

Abstract

Molecularly defined Ir complexes and different samples of supported IrO(2) nanoparticles have been tested and compared in the catalytic water oxidation with cerium ammonium nitrate (CAN) as the oxidant. By comparing the activity of nano-scaled supported IrO(2) particles to the one of organometallic complexes it is shown that the overall activity of the homogeneous Ir precursors is defined by both the formation of the homogeneous active species and its conversion to Ir(IV)-oxo nanoparticles. In the first phase of the reaction the activity is dominated by the homogeneous active species. With increasing reaction time, the influence of nano-sized Ir-oxo particles becomes more evident. Notably, the different conversion rates of the homogeneous precursor into the active species as well as the conversion into Ir-oxo nanoparticles and the different particle sizes have a significant influence on the overall activity. In addition to the homogeneous systems, IrO(2)@MCM-41 has also been synthesized, which contains stabilized nanoparticles of between 1 and 3 nm in size. This latter system shows a similar activity to IrCl(3)⋅xH(2)O and complexes 4 and 5. Mechanistic insights were obtained by in situ X-ray absorption spectroscopy and scanning transmission electron microscopy.

Heterogeneous Catalysis for Water Oxidation by an Iridium Complex Immobilized on Bipyridine‐Periodic Mesoporous Organosilica

“…Junge et al reported that a significant amount of the Ir IV species (28.5-82 % IrO 2 of Ir species) was formed for homogeneous iridium catalysts after water oxidation reaction at the high CAN concentration of 170 mm. [6] Our results suggest that no iridium oxide nanoparticles form within the first seven reactions and that iridium complexes strongly coordinate with the pore surface of BPy-PMO. The slight differences in EXAFS results are due to the decomposition of the Cp* ligand and the change in the coordination environment of iridium.…”

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 -).…”