Methyl viologen colloids

I - Zn - tetraphenyl porphyrin p- sulphonic acid, methyl viologen, colloidal platinum and AA as aii electron donor. 

Although Ru(bipy)2+ alone will not split water into hydrogen and oxygen, it has been accomplished with Ru(bipy)2+ using various catalysts or radical carriers. Perhaps the most studied system for the photoreduction of water involves using methyl viologen as the quencher, EDTA as an electron donor (decomposed in the reaction) and colloidal platinum as a redox catalyst (Figure 1.19). 

Methyl viologen (l,T-dimethyl-4,4 -bipyridylium dichloride, MV " ) promotes photoanodic dissolution in aerated CdS solution Figure 8 shows how the rate of dissolution depends on the concentration. The colloid has a weak fluorescence at 620 nm which is quenched by. The curves for fluorescence and dissolution in Fig. 8 are symmetric, which indicates that the two processes have a common intermediate that reacts with M. These effects are explained by the following mechanism ... 

Fig. 8. Intensity of fluorescence and rate of photoanodic dissolution of colloidal CdS as function of the concentration of added methyl viologen...

Photoanodic dissolution in the presence of air and its promotion by methyl viologen was also observed for alkaline solutions of colloidal cadmium phosphide, CdjPj and cadmium arsenide, CdjAsj Bismuth sulfide, Sb Sj, photo-dissolves in the presence of air mainly according to... 

The metal formed is deposited on the colloidal particles and causes the absorption of the solution to increase at all wavelengtlK. The depositel cadmium is very reactive. It is reoxidized when air is admitted to the illuminated solution. Addition of methyl viologen to the illuminated solution under the exclusion of air yields the blue colour of the semi-ceduced methyl viologen, MV". As the absorption coefficient of MV" is known, the concentration of reduced cadmium can be readily calculated. reoxidises cadmium atoms ... 

Nosaka and Fox determined the quantum yield for the reduction of methyl viologen adsorbed on colloidal CdS particles as a function of incident light intensity. Electron transfer from CdS to MV " competes with electron-hole recombination. They derived a bimolecular rate constant of 9 10 cm s for the latter process. 

In the pulse radiolysis studies on the reaction of MV with TiOj, the sol contained propanol-2 or formate and methyl viologen, MV Ionizing radiation produces reducing organic radicals, i.e. (CH3)2COH or C02 , respectively, and these radicals react rapidly with MV to form MV. The reaction of MV with the colloidal particles was then followed by recording the 600 nm absorption of MV . The rate of reaction was found to be slower than predicted for a diffusion controlled reaction. 

With respect to using methyl viologen as electron relay, it might be of interest to note tlmt MV " can be oxidized by positive holes produced in illuminated colloidal semiconductors such as Ti02 Two oxidation products of MV are 1, 2 -di-hydro-l,r-dimethyl-2 -oxo-4,4 -bipyridylium chloride and 3,4-dihydro-l,r-dime-thyl-3-oxo-4,4 -bipyridylium chloride, which can readily be detected by their strong fluorescences at 516 nm and 528 nm, respectively. These products are also produced in the direct photolysis of MV " solutions and in the reaction of MV "" with OH radicals in homogeneous solution... 

Instead of postulating Zn," as intermediate, as it has a highly negative potential and is possibly unstable in ZnO, one may write the above mechanism with Zn e pairs. The blue-shift in the absorption upon illumination was explained by the decrease in particle size. The Hauffe mechanism was abandoned after it was recognized that an excess electron on a colloidal particle causes a blue-shift of the absorption threshold (see Fig. 19). In fact, in a more recent study it was shown that the blue-shift is also produced in the electron transfer from CH2OH radicals to colloidal ZnO particles When deaerated propanol-2 solutions of colloidal ZnO were irradiated for longer times, a black precipitate of Zn metal was formed. In the presence of 10 M methyl viologen in the alcohol solution, MV was produced with a quantum yield of 80 %... 

Metal oxide, e.g. RUO2 [35,36] and WO3 [151], loaded Cd8 colloidal suspensions with and without Ft functionalization were investigated for water decomposition under visible light. WO3-Pt/Cd8, in an aqueous solution of methyl viologen (MV ) which serves as an electron relay, has been found most efficient to date in terms of water splitting, see reactions (7.4.1) to (7.4.8) [151] ... 

Other acceptors which have reduction potentials below the conduction band edge of the illuminated semiconductor can also function in this role. The single electron reduction product derived from methyl viologen, for example, can be detected spectroscopically when a colloidal suspension of titanium dioxide is flashed in the presence of this electron acceptor, Eq. (2)... 

In the particular case of the carboxylates of the eosin family, a careful examination of electron transfer paths was made by K. Tokumaru and cowor-kers [158, 159, 160]. They found that eosin Y (EY ) was able to sensitize hydrogen evolution from water under visible light irradiation in a system made of an amine donor, methyl viologen (MV2+) mediator and redox catalyst such as colloidal platinum. In a water-ethanol solution of EY, in the presence of MV2+ and triethanolamine (TEOA), a maximum quantum yield of 0.3 was measured for the MV + radical cation formation. Kinetic measurements of the EY triplet quenching constant by MV2+ (kq = 3.109 M 1 s 1) and TEOA... 

The photophysics and photochemistry of gaseous PuFe have been ex-amined. Studies involving zinc porphyrins have been reported and include photo-oxidations in aqueous solution, photoreductions of Zn-TPP with hydrazines, and the role of Zn-TPPSa/ethyl viologen in photoredox processes. The mechanism of the photo-oxidation of water to oxygen with silver chloride has been discussed, and the synthesis of bis(chlorosilyl)-mercury compounds described. Colloidal CdSe has been shown to sensitize the photoreduction of O2 and of methyl viologen by cysteine. ... 

Bard and co-workers have reported on the attainment of equilibrium between the nanosized particles and an electrode in the presence of a redox mediator [25a]. The study refers to the production of a mediator (methyl viologen radical cation) that reduces water in the presence of colloidal gold and platinum metal catalyst. An electrochemical model based on the assumption that the kinetic properties are controlled by the half-cell reactions is proposed to understand the catalytic properties of the colloidal metals. The same authors have used 15 nm electrodes to detect single molecules using scanning electrochemical microscopy (SECM) [25b]. A Pt-Ir tip of nm size diameter is used along with a ferrocene derivative in a positive feedback mode of SECM. The response has been found to be stochastic and Ear-adaic currents of the order of pA are observed. 

---