Sacrificial electron donor

Ru(bipy)3 formed in this reaction is reduced by the sacrificial electron donor sodium ethylenediaminetetra-acetic acid, EDTA. Cat is the colloidal catalyst. With platinum, the quantum yield of hydrogenation was 9.9 x 10 . The yield for C H hydrogenation was much lower. However, it could substantially be improv l by using a Pt colloid which was covered by palladium This example demonstrates that complex colloidal metal catalysts may have specific actions. Bimetalic alloys of high specific area often can prepared by radiolytic reduction of metal ions 3.44) Reactions of oxidizing radicals with colloidal metals have been investigated less thoroughly. OH radicals react with colloidal platinum to form a thin oxide layer which increases the optical absorbance in the UV and protects the colloid from further radical attack. Complexed halide atoms, such as Cl , Br, and I, also react... 

Following the first observations by Darwent and Porter numerous studies have been carried out into the generation of hydrogen by illuminating CdS suspensions or CdS sols with visible light A sacrificial electron donor such as SH ,... 

Hydrogen is readily formed in the near UV illumination of sols or suspensions of platinized TiO containing a sacrificial electron donor Figure 27 shows the... 

In fact, the formation of was observed, and it was assumed that the stoichiometric amount of must have been generated too. However, the oxygen was not measured and the question then arose whether water was indeed decomposed into -I- Oj, the O2 escaping analysis, or whether an unknown sacrificial electron donor was present which interfered with reaction 55. 

Later, an improved system for C02 photofixation was reported by the same authors.164 The new system consisted of 6.5 x 1(T5 M tris(2,2 -bipyridine)ruthenium(II), Ru(bpy)3, as the photosensitive electron donor, methyl viologen (MV2+, 20 mM) as the electron acceptor, and triethanolamine (TEOA, 0.6 M) as a sacrificial electron donor in a C02-saturated aqueous solution (Fig. 18). Under irradiation with a 300-W high-pressure Hg lamp with a CuS04 chemical filter (A > 320 nm), formic acid, which was detected by isotachophoresis, was produced in quantum yields of ca. 0.01%. Recently, however, Kase et al.165 have repeated this experiment using a 13C02 tracer and have claimed that the formic acid obtained was produced not by C02 reduction but rather by oxidative cleavage of TEOA. 

Lehn and Ziessel166 have also developed systems for the photochemical reduction of C02. These systems are similar to those represented by Fig. 18. Visible-light irradiation of C02-saturated aqueous acetonitrile solutions containing Ru(bpy)2+ as a photosensitizer, cobalt(II) chloride as an electron acceptor, and triethyl-amine as a sacrificial electron donor gave carbon monoxide and... 

Fig. 9. Postulated oxidation state changes occurring at the Mo and Fe centers of SO during the catalytic oxidation of sulfite, and concomitant reduction of cyt c. The one-electron reduction shown with a dashed arrow connecting MoVIFem and MoVIFen can be initiated with a laser pulse, in a solution containing 5-deazariboflavin (dRF) and a sacrificial electron donor (AH2). Reprinted with permission from Ref. (20). Copyright 1999, Society of Biological Inorganic Chemistry.

Figure 17. Formation of viologen radicals on visible light irradiation in the presence of EDTA as a sacrificial electron donor.

In 0.1 N NaOH electrolyte solutions, hydrogen evolution is seen exclusively at the Ti02 photoanode in the presence of a sacrificial electron donor, CH3OH, by the following reaction ... 

AOT-isooctane-H20 reversed micelles 50-A-diameter CdS particles generated in situ in reversed micelles from CdCl2 or Cd(N03)2 by H2S Reversed-micelle-entrapped CdS was fluorescence quenched by methylviologen band-gap excitation in the presence of Rh as catalyst and PhSH as sacrificial electron donor resulted in water photoreduction 611... 

Band-gap irradiation of CdS, led to sacrificial 565 photoreduction of Ag+ at the outer surfaces of SUVs via electron exchange between CI6MVi + and CiSMV+i, located at the inner and at the outer surfaces of the vesicles, at the expense of benzyl alcohol as a sacrificial electron donor... 

Visible-light irradiation of DHP-vesicle-incorporated, 626, 627 Rh-coated CdS in the presence of PhSH as a sacrificial electron donor resulted in water photoreduction... 

Rh-coated CdS resulted in sacrificial water photoreduction a thiol-functionalized surfactant acted as the sacrificial electron donor, system could be recycled... 

Some Ni(II) macrocyclic complexes mediate electron transfer and produce CO during the photochemical reduction of C02. However, photochemical reduction of C02 using a photosensitizer, a sacrificial electron donor, and [Ni(cyclam)]2+ as the catalyst has been only moderately successful compared to the electrochemical reduction of C02 using [Ni(cyclam)]2+. 

Sensitization of Ti02 nanosized particle films soaked in water was tried by dissolving a sensitizer and a sacrificial electron donor (EDTA) in the water phase (Fig. 19.7). Photocurrent was strongly dependent on the concentration of Ru(bpy)32+, reaching saturation at higher concentrations beyond 2 mM. By analysis of the photocurrent-vs.-concentration curve, a Langmuir-type adsorption of the dye was suggested. 

Sensitization of Ti02/Pt powders in water by dissolved sensitizer was carried out under visible light irradiation in the presence of sacrificial electron donor EDTA to produce H2. It was found that tris(bipyrimidine)Ru(II) (5) is an efficient sensitizer for visible light H2 formation. 

The majority of studies have led to the photochemical production of hydrogen or oxygen at the expense of a sacrificial electron donor or acceptor, but some systems for cyclic water cleavage have been reported. 

For example, hydrogen can be produced from a sacrificial electron donor, a chromophore, an electron transfer catalyst and a redox catalyst. The role of the sacrificial electron donor is to... 

Replacing CTAC micelles with a cationic polysoap protecting colloidal Pt also leads to successful charge separation and in this case H2 production is observed in the absence of a sacrificial electron donor.138 The use of a positively charged polysoap is highly important since it allows reaction of C14M V+ with the Pt catalyst but protects [Ru(bipy)3]3+ from the reduced Pt particle by electrostatic repulsion.138 It has been claimed that, in the presence of Ru02, H2 and 02 are produced but details have not been published.138... 

Much interest has also been expressed in tetra-azamacrocyclic compounds, due to their role in the natural reduction of C02 to CH4 by a nickel tetrapyrrole coenzyme found in methane-producing bacteria. Tinnemans et al. used Co(II) tetra-azamacrocyclic complexes with [Ru(bpy)3]2+ as the photosensitizer and ascorbic acid as the sacrificial electron donor in aqueous C02-saturated solutions at acidic pH [33]. Whilst the TON for the total observed products of CO and H2 exceeded 500, they were formed in a ratio of 0.27 1, respectively. 

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