Photocatalytic reduction

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])...

In general, the process of photocatalytic reduction of C02 requires a milder conditions and lower energy consumption than chemical reduction [22]. 

Hirano, K., Inoue, K., and Yatsu, T. (1992) Photocatalysed reduction of C02 in aqueous Ti02 suspension mixed with copper powder. Journal of Photochemistry and Photobiology A Chemistry, 64 (2), 255-258. Adachi, K., Ohta, K., and Mizuno, T. (1994) Photocatalytic reduction of carbon dioxide to hydrocarbon using copper-loaded titanium dioxide. Solar Energy,... 

Koci, K., Obalova, L.,and Lacny, Z. (2008) Photocatalytic reduction of C02 over Ti02 based catalysts. Chemical Papers, 62 (6), 1-9. 

Anpo, M., Yamashita, H., Ichihashi, Y., and Ehara, S. (1995) Photocatalytic reduction of C02 with H20 on various titanium oxide catalysts. Journal of Electroanalytical Chemistry, 396 (1-2), 21-26. 

Brezova, V., Blazkova, A., Surina, I., and Havlmova, B. (1997) Solvent effect on the photocatalytic reduction of 4-nitrophenol in titanium dioxide suspensions. Journal of Photochemistry and Photobiology A Chemistry, 107 (1-3), 233-237. 

Ferry, J.L. and Glaze, W.H. (1998) Photocatalytic reduction of nitro organics over illuminated titanium dioxide role ofthe Ti02 surface. Langmuir, 14 (13), 3551-3555. 

Wang, H., Yan, J., Chang, W., and Zhang, Z. (2009) Practical synthesis of aromatic amines by photocatalytic reduction of aromatic nitro compounds on nanoparticles N-doped Ti02. Catalysis Communications, 10 (6), 989-994. 

Ikeue, K., Nozaki, S., Ogawa, M., and Anpo, M. (2002) Characterization of self standing Ti-containing porous silica thin films and their reactivity for the photocatalytic reduction of C02 with H20. Catalysis Today, 74 (3-4), 241—248. 

Fig. 44. Yields of CH4 and CH3OH in the photocatalytic reduction of C02 with H20 on Ti02 powder (a) TS-1 (b) Ti-MCM-41 (c) Ti-MCM-48 and (d) zeolite catalysts [from Anpo etal. (263)].

Nanosized selenium particles were deposited onto Ti02 by the photocatalytic reduction of selenate (Se(VI) and selenite (Se(IV) ions) [317], The deposition of Se particles on TiOz was only observed in... 

Photocatalytic Reductive Dissolution of Hematite in the Presence of Oxalate... 

In the absence of oxygen the photocatalytic reductive dissolution of hematite in the presence of oxalate occurs according to the following overall stoichiometry (Siffert and Sulzberger, 1991) ... 

H. Kato, A. Kudo, Photocatalytic reduction of nitrate ions over tantalate photocatalysts, Phys. Chem. Chem. Phys. 4 (2002) 2833-2838. 

G. Williams, B. Seger, P. V Kamat, Ti02-graphene nanocomposites. UV-assisted photocatalytic reduction of graphene oxide, ACS Nano, 2 (2008) 1487-1491. 

Liang, Y. T. Vijayan, B. K. Gray, K. A. Hersam, M. C., Minimizing Graphene Defects Enhances Titania Nanocomposite-Based Photocatalytic Reduction of C02 for Improved Solar Fuel Production. Nano Lett. 2011,11 2865-2870. 

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] ... 

Fig. 3 Results for the photocatalytic reduction of nitrate in the presence of 0.04 M HCOOH as hole scavenger over titania and Ag/Ti02 catalysts prepared by impregnation (IMP) or photodeposition (Photo) and comparing use of Degussa P25 (49 m g ) with Hombikat UV 100 (250 m g ) as supports. ...

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. 

On the other hand, the production of desired compounds through reduction of starting material requires the electron donors to be oxidized (reductant). Alcohols are often used not only as a solvent but as the donor to produce useful compounds, e.g., anilines from nitrobenzenes,22) alcohols from aldehydes,23) and secondary amines from the corresponding Schiff bases.24) From the organic synthetic point of view, however, the separation of undesired products, aldehydes or ketones, from the alcohols is necessary unless subsequent reaction processes consume them25,26) or they are easily removed by distillation or other procedures. A recent report has shown that water acts as the electron donor and is converted into 02 in the photocatalytic regio-selective reduction of terpenes mixed with aqueous suspension of Ti02.27,28) It is notable that isolation of the desired product from the reaction mixture is simple in this type of photocatalytic reduction. 

Photocatalytic reductions are not observed as frequently as photocatalytic oxidations. This is due to the fact that the reducing power of a conduction band electron is significantly lower than the oxidizing power of a valence-band hole and because most reducible substrates do not compete kinetically with oxygen in trapping photogenerated conduction band electrons. Most photocatalytic reductions require a co-catalyst such as platinum. 

The use of high pressure photocatalysis resulted in greater turnover number for photocatalytic reduction of CO2 to CO with Re(bpy)(CO)3Cl and [Re(bpy)(CO)3(POiPr)3]+, in addition to greater catalyst stability [97,98]. For example, for sacrificial quenching in DMF solution the former exhibited 5.1 times the turnover number at normal pressure (0.1 Pa) using a pressure of 2.45 Pa of CO2. The latter showed a 3.8 times increase at 3.8 Pa. 

Chen and Ray [48] in a study on the photocatalytic reduction of environmentally relevant metal ions on Ti02 suspensions, observed that the presence of dissolved oxygen, acting as electron acceptor, inhibits the reduction of metal ions with low reduction potentials, while the presence of organic reductants promotes photocatalytic reduction. 

Moreover, the photocatalytic reduction has been used to convert some species in others of interest, such as nitrite to ammonia [53], or p-chloronitrobenzene to p-chloroaniline [54]. 

Liu et al. [21] investigated the simultaneous photocatalytic reduction of Cr(VI) and oxidation ofbisphenol A (BPA) in an aqueous solution in the presence of Fe(III)-OH complexes as catalysts, achieving a synergy effect of the simultaneous photocatalytic oxidation and reduction of both pollutants. Papadam et al. [55], instead, coupled the reduction of Cr( VI) to the oxidative degradation of an azodye, while in another study it was reported the simultaneous photocatalytic reduction of Fe(VI) and oxidation of ammonia [50]. 

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. 

Scheme 56 Illustration of the photocatalytic reductions conducted by Matsushita et al. (2006) in a quartz microreactor.

In an analogous manner, Matsushita et al. (2006) investigated the use of a quartz microreactor [channel dimensions = 500 pm (wide) x 100 pm (deep) x 0.40 cm (long)], in which the bottom and sides of the microchan-nel were coated with Ti02 (anatase), for the photocatalytic reduction of 4-nitrotoluene 193 (Scheme 56a) and benzaldehyde 116 (Scheme 56b) using a UV-LED (A = 365nm). As the photoreduction of an analyte requires a corresponding oxidation step to occur, a series of alcohols were evaluated as potential reaction solvents. 

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