Sacrificial electron acceptor

Loaded with 10-20 nm nanoparticles of rhodium-chromium mixed oxide (Rh2-yCry03) as a cocatalyst on the surface of [(Gai xZnx)(Ni-xOx)], the quantum efficiency of overall water splitting is about 2.5% at 420-440 nm [55,180-182]. Use of AgN03 as a sacrificial electron acceptor leads to an increase in oxygen evolution. The mixed oxide exhibited high and stable photocatalytic activity in aqueous H2SO4 solution at pH 4.5 [182] both lower and higher pH... 

The catalytic photochemical oxidation of water can also be achieved272,279 by similar methods, for example photolysis of [Ru(bipy)3]2+ using [Co(NH3)sCl]2+ as a sacrificial electron acceptor. The quantum yield for 02 production is ca. 0.025272 in the absence of added redox catalyst, although this result has been questioned.280,281... 

Of the oxidants described above, only [Ru(bipy)3]3+ can be efficiently prepared in a photochemical reaction and hence has potential for use in a cyclic water cleavage system. Many studies have, thus, been carried out on the Ru02 catalysis of 02 production from [Ru(bipy)3]3+ generated photochemically from [Ru(bipy)3]2+ and a sacrificial electron acceptor such as [Co(NH3)5C1]2+ or [S208]2-. 

Somewhat similar results to those obtained with Ru02 have been obtained using Mn02296 in the photochemical system employing [Co(NH3)5Cl]2+ as sacrificial electron acceptor. Thus, the... 

Fig. 16a, b. Examination of reduction and oxidation processes of artificial photosynthetic devices by means of sacrificial components a) application of sacrificial electron donor in a photosensitized reduction process b) application of a sacrificial electron acceptor in a photosensitized oxidation process... 

The fast decomposition of the sacrificial electron acceptor prevents back electron transfer. The formation of molecular oxygen results from... 

TasNs.476 This material evolves H2 and O2 under visible irradiation (X < 600 nm) in the presence of sacrificial electron acceptors such as Ag+ and a co catalyst for HER such as Pt.476 Control of solution pH was found to be critical for suppressing the photoanodic corrosion of the photocatalyst which is signaled by N2 evolution. The shrinking of the optical band gap was attributed to a conduction band derived from Ta 5d orbitals and a higher lying valence band derived from N 2p orbitals than the counterpart built from 0 2p orbitals.476... 

Under visible light, this Pt-modified photocatalyst evolves O2 from aqueous AgNOs as sacrificial electron acceptor and traces of H2 from aqueous solutions of methanol as sacrificial electron donor. 

The most common sacrificial electron acceptor in the environment is molecular oxygen, whereas the main sacrificial donors are organic compounds. In consequence, the self-cleaning processes consist in oxidation of organic pollutants by molecular oxygen in its triplet ground state the reactions are driven by energy from solar radiation. In nature, many different photoinitiators or photosensitizers are reactive, but the most common environmental photosensitizers include hiunic substances (HS), whereas the best photoinitiators are transition metal complexes. 

The reconstitution of chemically modified heme with apo-Mb has been well established [173-176]. The one-electron reduction potential of ferryl-Mb (Fe -heme) was reported as 0.896 V relative to the NHE [177] which is less positive than the one-electron oxidation potential of [Ru(bpy)3] + (1.25 V). Thus, generation of ferryl-Mb by electron transfer from the met-Mb (ferric state) moiety to the Ru + moiety might be thermodynamically favorable. An appropriate amount of a sacrificial electron acceptor such as [Co(NH)3Cl] + which can quench the Ru + excited state oxidatively was, however, required to produce the Ru + state in competition with the reductive quenching of the Ru + excited state by the met-Mb [178, 179]. No direct electron transfer from the met-Mb (ferric sate) moiety to the Ru + excited state occurs, because this process is thermodynamically disfavored [178, 179]. 

Photoinduced electron-transfer in the opposite direction was demonstrated upon irradiation of the Ru(bpy)3 +-Mb system in the presence of Co +(NH3)5Cl as a sacrificial electron acceptor (Figure 44B) [244]. The photochemical reaction results in the formation of ferryl species (i.e., Fe(IV)-heme), with the intermediate formation of the porphyrin cation radical (as demonstrated using laser flash photolysis [237]). The electron-transfer cascade includes the primary oxidative quenching of the excited chromophore, Ru(bpy)3"+, by Co +(NH3)5Cl to yield Ru(bpy)3 + [E° = +1.01 V vs. SCE). The resulting oxidant efficiently takes an electron from the porphyrin ring (fcet = 8.5 x 10 s ) and the porphyrin cation radical produced further oxidizes the central iron atom, converting it from the Fe(III) state to the Fe(IV) state (/cet = 4.0 x 10 s at pH 7.5). 

The QDH from C. t stosim i was further characterized It oxidizes stereo-specifically the fl ) enantiomer of secondary alcohols. Both, bsat/Km and M increased with the substrate chain length. J ritra, fetricyamde used as sacrificial electron acceptor. i idi O, the excess electrons are most probably transferred to molecular... 

To suppress the electron and electron-hole recombination and continuously supply valence-band electron holes for an oxidative process, a sacrificial electron acceptor must be used to scavenge the electron in the conduction band [15,20], Oxygen is usually used in photocatalytic oxidation because of its ability to capture conduction-band electrons from most semiconductors [8]. The oxygen radical anion formed after the quenching of an electron is an additional oxidant, which is useful for oxidative waste degradation, but may be a source of side reactions in organic synthesis. To reduce such side reactions, the use of other electron acceptors such as methyl viologen and N2O has been reported [74]. 

In water photooxidation by semiconductor photocatalysis, a sacrificial electron acceptor A, such as Fe or Ag" ions, is usually added to the system to prevent accumulation of any photogenerated electrons. Transition metal oxides, such as RUO2 or Ir02, which are recognised O2 evolution catalysts, are often deposited on the surface of the semiconductor catalyst to improve the efficiency of water oxidation. 

The semiconductor sensitised photocatalytic oxidation of water by a sacrificial electron acceptor can be expressed by Eq. (11-6) ... 

Name sacrificial electron acceptors (A) and electron donors (D) which can increase the efficiency of photocatalysts in water splitting. 

The following equations illustrate a possible general scheme for photoinduced water oxidation assisted by a sacrificial electron acceptor. In such equations, P and SA are the photosensitizer and the sacrificial oxidizing agent (primary electron... 

Figure 13-14. A model reaction for photochemical O2 evolution with a sacrificial electron acceptor and colloidal RUO2 catalyst.

PECs). The major challenge is the yield of the photosensitized reaction in the presence of reversible electrons, rather than the classically used triethylamine as sacrificial electron donor or persulfate as sacrificial electron acceptors. 

An example of a system for photocatalytic water oxidation with a molecular catalyst based on abundant metals is shown in Fig. Id. The Cobalt polyoxometalate catalyst is oxidized with Ru —trisbipyridine that is generated by quenching of the photo-excited Ru —trisbipyridine sensitizer with peroxodisulfate as sacrificial electron acceptor. The system operates at pH 8 and exhibits a high (30 %) photon-to-02 yield while the stability of the catalyst allowed for turnover numbers >220 that were limited by depletion of electron acceptor only [7]. This performance of the abundant-metal-based catalyst is superior to that of an analogue ruthenium polyoxometalate water oxidation catalyst. 

For the photosplitting of water to be efficient from a photochemical viewpoint, however, it is necessary to also effectively use the chemical potential of the oxidant Ru(bpy)3 to produce oxygen from water. This reaction has been achieved by the use of a combination of a redox catalyst and a sacrificial electron acceptor. Both... 

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]...

In this Chapter, aspects of the electronic stmctures of complexes formed from n-type electron donors and sacrificial electron acceptors will be examined. An n-type donor donates an electron from essentially a lone-pair atomic orbital on a key atom, and a sacrificial acceptor accepts an electron into an antibonding molecnlar orbital. (MuUiken has also designated n-type donors as increvalent donors). Therefore, for this type of complex, D has a lone-pair of electrons and A has a vacant antibonding orbital in Eqn. (1). We shall assume here that the antibonding orbital of A extends over two adjacent atomic centres, and that the corresponding bonding molecular orbital of A is doubly occupied. 

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