Photocatalytic Reduction of Water

Among the numerous catalysts of dihydrogen evolution known at present (see e.g. the reviews [253-257]) the most promising for application in the vesicle systems 

For example Kurihara and Fendler [258] succeeded in forming colloid platinum particles, Ptin, inside the vesicle cavities. An analogous catalyst was proposed also by Maier and Shafirovich [164, 259-261]. The latter catalyst was prepared via sonification of the lipid in the solution of a platinum complex. During the formation of the vesicles platinum was reduced and the tiny particles of metal platinum were adsorbed onto the membranes. Electron microscopy has shown a size of 10-20 A for these particles. With the Ptin-catalyst the most suitable reductant proved to be a Rh(bpy)3+ complex generated photochemically in the inner cavity of the vesicle (see Fig. 8a). With this reductant the quantum yield for H2 evolution of 3% was achieved. Addition of the oxidant Fe(CN), in the bulk solution outside vesicles has practically no effect on the rate of dihydrogen evolution in the system. Note that the redox potential of the bulk solution remains positive during the H2 evolution in the vesicle inner cavities, i.e. the inner redox reaction does not depend on the redox potential of the environment. Thus redox processes in the inner cavities of the vesicles can proceed independently of the redox potential in the bulk solution. 

Now let us discuss how one can conjugate the process of H2 evolution on the catalyst anchored to the membrane or located in a water phase with the vectorial PET across this membrane. 

Dihydrogen can be rather easily evolved with Systems 10-15 of Table 1, where, with PET across the membrane, the water-soluble radical cation MV+ is produced outside the vesicle. This radical cation can evolve dihydrogen from water in the presence of various catalysts. This was demonstrated by Tsvetkov et al. [262] for System 12 of Table 1. As a catalyst, the c water soluble hydrogenase from Thiocapsa roseopersicina was used or a heterogeneous rhodium-polymer catalyst [263]. The quantum yield of H2 production was comparable with the quantum yield of MV+ generation. 

Thus the data presented in this Section prove that vectorial PET across membranes in vesicle systems indeed can be conjugated with the catalytic process of water reduction to dihydrogen. 

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A sensor for nitric oxide was constructed utilizing multiwaUed carbon nanotubes that were previously modified with ethylenediamine and sonicated in a solution containing Co -TSPP. Drop cast electrodes were prepared and characterized by field-emission transmission electron microscopy, X-ray photoelectron spectroscopy and by electrochemical techniques [220]. Cyclic voltammetry, electrochemical impedance, and chronoamperometry were utilized to evaluate the electrocatalytic activity of the hybrid sensor that exhibited linear response in the 6.6 x 10 to 1.3 X 10 mol L range, with a detection limit of 6.6 x 10 mol L. Electrodes composed of porphyrins, platinum nanowires, and Nafion were proposed for photocatalytic reduction of water [221]. After an extensive search for optimization of each component and under the best conditions (—350 mV and visible light irradiation) a detectable amount of hydrogen was produced, but Nafion can diminish the diffusion of ions to the porphyrin active sites. 

Fig. 5.14 Inoue et al. carried out a systematic study of the photocatalytic reduction of CO2 by different semiconductor powders in aqueous suspensions. Shown here is the energy correlation between semiconductor catalysts and redox couples in water, as presented in their paper. In principle, the solution species with more positive redox potential with respect to the conduction band level of the semiconductor is preferably reduced at the electrode. Photoexcited electrons in the more negative conduction band certainly have greater ability to reduce CO2 in the solution. (Reproduced from [240])...

Sayama, K., Arakawa, H. 1993. Photocatalytic decomposition of water and photocatalytic reduction of carbon-dioxide over ZrOj catalyst. J Phys Chem 97 531-533. 

In water, two H02 can be recombined if their concentration allow them to react significantly yielding H2O2 and O2. It follows photocatalytic reduction of H2O2 by scavenging an electron from the conduction band where OH radicals are generated [95] ... 

Photocatalytic reduction of C02 can be accomplished by suspending photosensitive semiconductor powders in aqueous solutions under irradiation, usually using UV light.129156 Photoreduction of C02, however, is in competition with H2 formation due to water decomposition, and leads to mixtures of reduced carbon products. Selectivity, therefore, is one of major problems of these processes. 

Another approach to wards photocatalysis is to use dy as a sensitizer instead of a semiconductor as in photosynthesis. It is not the aim of this book to cover all the aspects of the sensitized photochemical conversion system, but typical sensitized systems for photocatalytic reactions of water are described in Chapter 18 The concept of a photochemical conversion system using a sensitizer and water oxidation/reduction catalysts is mentioned in Chapter 19, accompanied by a discussion on the sensitization of semiconductors. 

Besides, in an interesting study Sing Tan et al. [63] reported the possibilities to use the photocatalytic reduction of carbon dioxide with water to produce hydrogen and methane. 

Reduction of C02 photocatalyzed by semiconducting materials may lead to formation of formaldehyde, formic acid, methanol, methane, and oxalate among other products [7], The earliest report on photocatalytic reduction of C02 with water at Ti02 was published by Inoue et al. [25], Almost at the same time the photoreduction of C02 to methane at SrTi03 was reported by Hemminger et al. [26], The same process can also be performed on AgCl/zeolites [27],... 

Photochemical splitting of water achieved by combining two photocatalytic reactions on suspended Ti02 particles namely, the reduction of water to H2 using bromide ions and the oxidation of water using Fe(III) species. High efficiency also observed for the photoassisted OER on TiCte in the presence of Fe(III) ions. 326, 327... 

The principles of photochemical water splitting can be extended to the design of systems using photocatalytic semiconductors in the form of particles or powders suspended in aqueous solutions (Bard, 1979, 1980). In this system, each photocatalyst particle functions as a microphotoelectrode performing both oxidation and reduction of water on its surface (Figure 4). 

The ability of the surface-bonded peroxo species to undergo reduction, via conduction-band electrons, in the region of potentials close to that of reversible hydrogen electrode in the same solution, gives rise to very effective hole-electron recombination, decreasing the yield of photocatalytic dissociation of water. 

Ti/Si binary oxides having different Ti contents were prepared by the sol-gel method from mixtures of tetraethylorthosilicate and titaniumisopropoxide. Ti/Si gels were obtained by keeping the mixture at room temperature for several days, washed with sufficient amounts of boiled water and then calcined in dry air at 725 K for 5 h. The Ti/Si binary oxides were crushed and sieved to 0.25-mm-size particles. Prior to spectroscopic measurements and photocatalytic reactions, the catalysts were treated with O2 at 725 K for 2 h and then evacuated for 2 h at 475 K. The photocatalytic reduction of CO2 with H2O was carried out with the catalysts (150 mg) in a quartz cell connected to a conventional vacuum system. UV irradiation of the catalysts in the presence of CO2 (24 pmol) and gaseous H2O (120 pmol) was carried out using a high-pressure Hg lamp (X, > 280 nm) at 328 K. The reaction products collected in the gas phase were analyzed by gas chromatography. 

The influence of aliovalent cation doping of the support (Ti02) on the catalytic properties of supported Pt and Rh crystallites was also investigated under other reaction conditions, among which the photocatalytic cleavage of water [116] and the reduction of NO by propylene [117] in the presence or absence of oxygen. In the case... 

These processes occur in Ti02 suspensions in water. Zr02 and other oxides were also found to be active in the photocatalytic reduction of carbon dioxide. 

Jeyalakshmi, V. Mahalakshmy, R. Krishnamurthy, K. R. Viswa-nathan, B. Photocatalytic Reduction of Carbon Dioxide by Water A Step towards Sustainable Fuels and Chemicals. Mater. Sci. Forum, 2012, 734, 1-62. 

Other recent reports inclnde photocatalytic disinfection [203], photocatalytic H2 evolntion from water [198], and photocatalytic reduction of CO2 with H2O [196], A list of representative examples of carbon-Ti02 composite photocatalysts and their applications to photodegradation reactions is given in Table 13.1. 

Because one possible rate determining step is the water oxidation process, the photocatalytic reduction of CO2 has been tried in the presence of sacrificial electron donors, such as alcohols [94, 118, 121, 133, 139], It has been reported that photocatalytic CO2 reduction by Ti02 in the presence of 2-propanol selectively gives CH4 [121]. Acetone was also detected as an oxidation product of 2-propanol. However, only a slight increase in the reaction efficiency was observed following the addition of 2-propanol. The reported TN and TF of CH4 production are TN = 1.3 pmol (g cat.) and TF = 0.43 pmol (g cat.) h [121]. 

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