Catalytic water oxidation

Although catalytic water oxidation (dark reaction) is the first and important reaction of the electron flow in the photosynthesis represented by Fig. 19.1 whereby water is used as the source of electrons provided to the whole system, its catalyst and reaction mechanism are not yet established.10-13) In the photosynthesis Mn-protein complex works as a catalyst for the difficult four-electron oxidation of two molecules of water to liberate one 02 molecule (Eq. (19.2)). It is inferred that at least four Mn ions are involved in the active center, but its structure is not yet completely elucidated. 

Nafion film coated on an ITO electrode to understand the structural transformations of the dimer in the Nafion coating during the catalytic water oxidation process . The absorption spectral changes observed during the oxidation scan from 0.4 to 1.4 V (s. SCE) (Fig. lOA) showed a decrease in the absorbance at 655 nm with simultaneous increase in the absorbance at around 450 nm with clear isosbestic points at 430 and 545 nm. In the reductive scan from 1.4 to 0.4 V (vs. SCE) (Fig. lOB), the absorbance at 655 nm increased and a simultaneous decrease in the absorbance at 450 nm was observed with an isosbestic point at 555 nm. The absorbance at 655 nm was almost recovered back. Initially the oxidation of the dimer complex H20-Ru" -Ru "-0H2 leads to the formation of H20-Ru "-Ru -0H2 with an absorption maximum at around 450 nm and further oxidation at higher positive potentials must lead to Ru -Ru formation in a successive oxidation process. The Ru -Ru would be rapidly reduced by water molecules to produce H20-Ru -Ru -0H2 at pH 1. The same in situ spectrocyclic voltammetry experiments at pH 9.3 showed an absorption maximum at around 500 nm with the formation of H20-Ru" -Ru -OH in the Nafion film. In relevant to the absorbances at 450... 

The reactions observed for the dimer complex adsorbed in a Nafion film coated on an ITO electrode at different pH by in situ absorption spectral measurements are summarized as shown in Fig. 11. At higher positive potentials and at potentiostatic conditions, a band at around 450 nm was observed indicating the formation of H20-Ru "-Ru -OH2 at acidic conditions and formation of H20-Ru" "-Ru -OH at basic conditions in addition to the absorbance at 655 nm. This shows that during the catalytic water oxidation process, the diaquo dimer complex exists as an intermediate. In a Nafion polymer membrane, the metal complex is isolated and experiences a micro-heterogeneous environment imposed by hydrophobic fluorocarbon moiety and... 

Nation film during the catalytic water oxidation. The absorption spectra did not show any further changes in repeated cyclic scans. When an applied potential of 1.2 V (vs. SCE) was applied, Ru-brown underwent oxidation to produce Ru -Ru -Ru complex and this Ru -Ru -Ru complex is reduced by water molecules to produce Ru-brown (Ru -Ru" -Ru ) by four-electron process. Independent of pH condition, the cyclic reaction between Ru -Ru" -Ru and Ru Ru -Ru showed only the spectrum of Ru-brown The reactions involved at Nafion membrane is summarized in Fig. 14. This report clearly shows that in the cyclic catalysed water oxidation reaction the Ru-red and Ru-brown complexes combined in a Nafion membranes act as four-electron catalyst, independently of pH, and the complex is stabilized against decomposition in the membrane. 

The most intensively studied molecular systems mimicking the action of the WOC are based on the / -oxo-bridged ruthenium dimer [m-(bpy)2Ru(0H2)]20" +, where each ruthenium is associated with two bipyridyl ligands and one water molecule. The catalytic water oxidation cycle involves abstraction of two electrons from each of the Ru(II) centers forming two dioxoruthenium(IV) moieties which convert back to the starting state under oxygen release and subsequent re-aquation of the ruthenium metal centers [119]. 

Scheme 2 Redox active ligand participation in catalytic water oxidation by a binuclear Ru catalyst containing ortho-quinone/semiquinone ligands...

The counterpart of catalytic water oxidation to O, catalytic evolution via water reduction, is of equal importance in the context of water splitting (artificial photosynthesis) [15]. Innature, hydrogenase enzymes show excellent catalytic rates and efficiencies and can catalyze both proton reduction and oxidation. Consequently, several biomi-metic complexes have been developed which show excellent catalytic activity towards electrochemical production from water. 

Shafirovich VY, Khannanov NK, Strelets VV (1980) Chemical and light-induced catalytic water oxidation. Nouv J Chim 4 81-84... 

Cao R, Ma H, Geletii YV, Hardcastle KI, Hill CL (2009) Stmcturally characterized Iridium (in)-containing polytungstate and catalytic water oxidation activity. Inorg Chem 48 5596-5598... 

Experiment 13-1 Catalytic Water Oxidation by a Trinuclear Ru-ammine Complex (Ru-red) Incorporated into a Nafion Membrane (Section 13.2.3.1) [19]... 

Catalytic water oxidation This membrane with adsorbed Ru-red was placed in distilled water (5 mL) in a cell equipped with a rubber septum, Ar gas was bubbled into the water for 1 h, and then cerium(IV) diammonium nitrate powder (6T0 mol) was added quickly. The Ce salt dissolves in water rapidly, and dioxygen bubbles were seen on the surface of the Nafion membrane. The evolved O2 was analyzed on a gas chromatograph equipped with a 5-A molecular sieve column using Ar carrier gas (flow rate 40 mL min" ) at 50 C. 

Electrode-assisted Catalytic Water Oxidation and Related Electrochemical Reactions... 

Catalytic Water Oxidation by Single-Site Ruthenium Catalysts."... 

The first nonheme ferryl complex in pure water was observed with the pentadentate bispidine at neutral to acidic pH at 0°C, and this had some implications for catalytic water oxidation with ferryl compounds (Section 6.6.5) [ 10]. Interestingly, the kinetic analysis of the formation of [(L )Fe =0] in water indicates that this is a clean monophasic reaction (see Figure 6.2) that is, there... 

Fig. 11 Oxygen Yields as a Function of pH for Catalytic and Non-Catalytic Water Oxidation hy One Electron Oxidants...

The mechanism of catalytic water oxidation by the blue dimer was studied in detail by Baik at the DFT level. Unrestricted B3LYP calculations... 

Duan L, Fischer A, Xu Y, Sun L. Isolated seven-coordinate Ru(IV) dimer complex with [HOHOH]—bridging ligand as an intermediate for catalytic water oxidation. JAm Chem Soc. 2009 131 10397-10399. 

ConcepcionJJ.JurssJW, Templeton JL, MeyerTJ. One site is enough. Catalytic water oxidation by [Ru(tpy)(bpm)(OH2)]. Jz4m Chem Soc. 2008 130 16462. 

HuUJF, Balcells D, Blakemore JD, et al. Highly active and robust Cp iridium complexes for catalytic water oxidation, y Am Chem Soc. 2009 131 8730-8731. 

The failure of the mononuclear [Ru(bpy)2(py)OH2] to promote oxygen evolution followed by success of the blue dimer and the fact that the OEC itself contains a multi-metal centre, lead researchers to believe that multiple sites were required for catalytic water oxidation. However, the mechanism proposed for the activation of the blue dimer showed that in reality, only one of the metal centres is involved in the oxygen-oxygen bond formation step. This suggestion was later confirmed when Meyer and co-workers showed that single site ruthenium-aqua complexes were capable of oxidizing water in the presence of Ce(iv). One of these catalysts, [Ru(tpy)(bpm)OH2] " (tpy = terpyridine, bpm = 2,2 -bipyrimidine), is shown in Scheme 5.2. 

Acknowledgements We thank the U.S. Department of Defense (the Army Research Office and the Defense Threat Reduction Agency (DTRA)) for funding complex catalysts for air-based oxidations and the Department of Energy, Office of Basic Energy Sciences for funding our research on catalytic water oxidation. 

There are numerous catalytic applications of NHC complexes (hydrogenation, hydrosilation, metathesis, coupling chemistry, etc.) in which they show advantages over phosphines. Rates can be faster, and the catalysts usually do not need protection from air during catalysis. Imidazoles are also more readily synthesized in a variety of structural modifications, although subsequent formation of the M-C bond can be somewhat more difficult than in the case of PR3. Ruthenium NHCs can even be stable under intensely oxidative and acidic conditions in catalytic water oxidation driven by Ce(IV). Since free NHCs would be easily oxidized, this emphasizes the kinetic inertness of M-NHC bonds and contrasts with the ease of oxidation of many M-PR3 to give 0=PR3. 

Scheme 7 Proposed mechanism for the light-induced catalytic water oxidation using the [Ru (bpylsl as a photosensitizer and [S208] as a sacrificial electron acceptor [169]...

Complexes 35 and 36 were studied for catalytic water oxidation using (NH4)2[Ce(N03)g] (CAN) as sacrificial oxidant (Scheme 3.7). When using a 900 1 Ce /Ir ratio, turnover numbers (TONs) were limited by the availability of the sacrificial oxidant and essentially quantitative conversions were reached in less than two hours. Excellent long-term catalytic activity was observed and, within five days, complex 35 accomplished up to 10000 turnovers with complete consumption of the sacrificial Ce oxidant, suggesting that even higher TONs might be accessible if more oxidant is available. 

A series of ruthenium complexes bearing triazolylidene ligands was syn-thethized and evaluated for catalytic water oxidation with CAN as a sacrificial... 

Solar energy conversion into chemical fuels is one of the holy grails of the 21st century. Significant research efforts are currently underway toward understanding natural photosynthesis and artificial biomimetic systems. Photocatalytic cells absorb solar energy and use it to drive catalytic water oxidation at photoanodes ... 

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