Direct Observation of a Hydroperoxide Surface Intermediate upon Visible Light-Driven Water Oxidation at an Ir Oxide Nanocluster Catalyst by Rapid-Scan FT-IR Spectroscopy

Electrodeposition of iridium oxide layers from soluble precursors provides a route to active thin-layer electrocatalysts for use on water-oxidizing anodes. Certain organometallic half-sandwich aqua complexes of iridium form stable and highly active oxide films upon electrochemical oxidation in aqueous solution. The catalyst films appear as blue layers on the anode when sufficiently thick, and most closely resemble hydrous iridium(III,IV) oxide by voltammetry. The deposition rate and cyclic voltammetric response of the electrodeposited material depend on whether the precursor complex contains a pentamethylcyclopentadieneyl (Cp*) or cyclopentadienyl ligand (Cp), and do not match, in either case, iridium oxide anodes prepared from non-organometallic precursors. Here, we survey our organometallic precursors, iridium hydroxide, and pre-formed iridium oxide nanoparticles. From electrochemical quartz crystal nanobalance (EQCN) studies, we find differences in the rate of electrodeposition of catalyst layers from the two half-sandwich precursors; however, the resulting layers operate as water-oxidizing anodes with indistinguishable overpotentials and H/D isotope effects. Furthermore, using the mass data collected by EQCN and not otherwise available, we show that the electrodeposited materials are excellent catalysts for the water-oxidation reaction, showing maximum turnover frequencies greater than 0.5 mol O(2) (mol iridium)(-1) s(-1) and quantitative conversion of current to product dioxygen. Importantly, these anodes maintain their high activity and robustness at very low iridium loadings. Our organometallic precursors contrast with pre-formed iridium oxide nanoparticles, which form an unstable electrodeposited material that is not stably adherent to the anode surface at even moderately oxidizing potentials.

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“…This result agrees well with recent reports of the observation of a hydroperoxide intermediate in WO with [Ru(bpy) 3 ] 3+ as the oxidant. 55 …”

Section: ■ Resultsmentioning

confidence: 99%

Comparison of Amorphous Iridium Water-Oxidation Electrocatalysts Prepared from Soluble Precursors

et al. 2012

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“…(Fig. 12b) [63,69,70] and confirms that the intermediate is a kinetically relevant species of the catalytic water oxidation cycle [68]. The observed frequency of the OO mode does not allow us to identify the metal oxidation state of the IrOOH group.…”

Section: Ir Oxide Catalyst: Combining Bond Specificity Of Infrared Spmentioning

confidence: 87%

Towards a Molecular Level Understanding of the Multi-Electron Catalysis of Water Oxidation on Metal Oxide Surfaces

Zhang

Frei

2014

Catal Lett

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Earth abundant metal oxides play a central role as catalysts in the essential chemical transformations of sunlight to fuel conversion, which are the oxidation of water and the reduction of carbon dioxide. The rapidly growing interest in renewable fuel generation by using the energy of the sun has recently led to substantial breakthroughs in the use of first row transition metal oxides as catalysts for oxygen evolution from water. Substantive improvements of rates and lowering of overpotentials have been achieved by exploiting materials properties on the nanoscale, or taking advantage of the synergy of multiple metals. Moreover, knowledge derived from mechanistic investigations with structure specific spectroscopy is accelerating efficiency improvements. Monitoring by timeresolved FT-infrared spectroscopy reveals the molecular nature of active sites, while in situ X-ray and optical spectroscopy under reaction conditions provides insights into the electronic structure of the surface metal centers participating in the catalysis. By combining the bond specificity of vibrational spectroscopy with the metal electronic structure specificity of optical, X-ray absorption or photoelectron spectroscopy, a complete understanding of active surface sites on metal oxides begins to emerge. Charge flow driving the chemical transformations probed by optical spectroscopy across time scales from ultrafast to very slow reveals the processes that control the productive use of charges delivered to the catalyst. Coupling of the water oxidation catalysis at a metal oxide catalyst with carbon dioxide reduction at a heterobinuclear chromophore, which is the goal of the artificial photosystem approach, is demonstrated by a well-defined all-inorganic polynuclear unit.

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“…Hydroperoxy species have been detected as intermediates in the electrochemical evolution of oxygen on gold using surface-enhanced Raman spectroscopy (SERS) [22] and in the photocatalytic oxidation of water on IrO2 nanoclusters [23]. Hydroperoxy species can desorb from the transition metal active sites to the solution, which may result in a number of reactions.…”

Section: Introductionmentioning

confidence: 99%

Selectivity of Nanocrystalline IrO2-Based Catalysts in Parallel Chlorine and Oxygen Evolution

et al. 2014

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This paper examines the detection and quantification of the potentially corrosive byproducts of the oxygen evolution reaction on iridium and iridium-ruthenium mixed oxides. A conventional but stationary ring-disc electrode was employed in a flow cell configuration for the detection of volatile reaction product other than oxygen, and the formation of at least two active species was detected.Potential-modulated UV-VIS reflectance spectroscopy helped to identify one of these volatile reaction byproducts as hydrogen peroxide, and the other is tentatively suggested to be ozone. It was found that these species are formed under potentiodynamic conditions due to the chemical recombination of the absorbed reaction intermediates. The influence of the electrode material composition on the production yield of these byproducts was studied, and we found that the generation of these corrosive reaction byproducts is suppressed the more active the catalyst is for the OER. The formation of by-products can therefore be addressed by a rational choice of the electrode materials based on the adsorption energy of the reaction intermediates as is done for the OER itself. Steady-state electrolysis minimizes hydrogen peroxide formation as byproduct of the oxygen evolution reaction.

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Direct Observation of a Hydroperoxide Surface Intermediate upon Visible Light-Driven Water Oxidation at an Ir Oxide Nanocluster Catalyst by Rapid-Scan FT-IR Spectroscopy

Cited by 122 publications

References 37 publications

Comparison of Amorphous Iridium Water-Oxidation Electrocatalysts Prepared from Soluble Precursors

Comparison of Amorphous Iridium Water-Oxidation Electrocatalysts Prepared from Soluble Precursors

Towards a Molecular Level Understanding of the Multi-Electron Catalysis of Water Oxidation on Metal Oxide Surfaces

Selectivity of Nanocrystalline IrO2-Based Catalysts in Parallel Chlorine and Oxygen Evolution

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