Decomposition, water

The electrolyte in lead-acid batteries is dilute sulfuric acid that contains the component water . Its stability is an important factor since it can be decomposed into hydrogen and oxygen, and the two broken lines in Fig. 1 represent the borderhnes of this stability. They show the equilibrium potentials of hydrogen and oxygen evolution and their dependence on the pH value. 

The Hj/H line in Fig. 1 represents the equilibrium potential E° of the hydrogen evolution according to Eq. (5) 

This is the curve H2/H in Fig. 1 for = latm. The standard value of this equation (proton activity =1 mol L - latm) 

In Fig. 1, H2O/O2 and H,/H curves are straight lines which have the same slope of - 0.059V per pH unit. 

---

Direct, One-Step Thermal Water Splitting. The water decomposition reaction has a very positive free energy change, and therefore the equihbrium for the reaction is highly unfavorable for hydrogen production. 

Main cathode reaction Anodic water decomposition Two principal cathode by-products... 

Although the rate of these reactions is slow, according to its thermodynamic situation the lead dioxide electrode is not stable. Since a similar situation applies to the negative electrode, the lead-acid battery system as a whole is unstable and a certain rate of water decomposition cannot be avoided. 

Navol Tank. Torpedo. A tank forming part of a torpedo assemblage, provided for the storage of solution of hydrogen peroxide in water. Decomposition of the hydrogen peroxide furnishes the oxygen required to effect combustion of the fuel, alcohol... 

Photopolymerizable coatings relief-image-forming systems, 6,125 Photoreactivity environmental effects, 1, 394 Photoredox properties bipyridyl metal complexes, 2, 90 Photoresist systems, 6,125 Photosensitive materials, 6, 113 Photosynthesis anoxygenic, 6, 589 magnesium and manganese, 6, 588 water decomposition models, 6, 498... 

To provide an appropriate background for a discussion of the title reactions note that 10 9 secs after irradiation the water decomposition (products) can be expressed as ... 

Recently, we have shown that the combination of barium tetratitanate, BaTi40g and sodium hexatitanate, NagTigOis, with ruthenium oxides leads to active photocatalysts for water decomposition[1,2]. The unique feature of these photocatalysts is that no reduction of the titanates is required to be activated this is intrinsically different from conventional photocatalysts using TIO2 which are often heat-treated in a reducing atmosphere. Such different photocatalytic characteristics suggest that efficiency for the separation of photoexcited charges (a pair of electrons and holes) which is the most important step in photocatalysis is... 

Margulis MA, Mal tsev (1968) Estimation of energetic yield of chemical reactions initiated by ultrasonic waves. II. Initial chemical acoustic yields of products of water decomposition by ultrasound. Zh Fiz Khim 42 2660-2663, Russ J Phys Chem (42) 1447-1451... 

Figure 22 The energy level diagram (Z-scheme) for photocatalytic water decomposition by a tandem cell.

The reaction H + OH— eh is undoubtedly responsible for the increase of G(eh) at high pH. Similarly, the reaction eh + H+—H must be responsible for the reduction of the hydrated electron yield in acid solution. The increase of total reducing yield and water decomposition yield at pH = 1.3 is not clearly understood, but it may also be due to secondary reactions. 

Sato, S. and White, J.M., Photocatalytic water decomposition and water-gas shift reactions over NaOH-coated platinized TiOz, J. Catal., 69,128,1981. 

Inoue, Y., Niiyama, T., and Sato, K., Photocatalysts using hexa- and octa-titanates with different tunnel space for water decomposition, Top. Catal., 1,137,1994. 

Ogura, S., Kohno, M., Sato, K., and Inoue, Y., Effects of RuOz on activity for water decomposition of a Ru02/Na2Ti307 photocatalyst with zigzag layer structure,. Mater. Chem., 8,2335,1998. 

H. Kato, A. Kudo, New tantalate photocatalysts for water decomposition into H2 and O2, Chem. Phys. Lett. 295 (1998) 487-492. 

Y. Inoue, New photocatalyst group for water decomposition of Ru02-loaded p-block metal (In, Sn, and Sb) oxides with d10 configuration, J. Phys. Chem. B 105 (2001) 6061-6063. 

N. Saito, H. Nishiyama, Y. Inoue, Photocatalytic activity for water decomposition of Ru02-dispersed Zn2Ge04 with d10 configuration,... 

Y. Inoue, Photocatalytic activity of the Ru02-dispersed composite p-block metal oxide LiInGe04 with d10-d10 configuration for water decomposition,... 

Fig. 6-2. Chemical cell and electrolytic cell (a) hydrogen-oxygen fuel cell (chemical cell), (b) water decomposition cell (electrolytic cell).

On the other hand, a typical electrolytic cell is the cell of water decomposition and its cell reaction is given in Eqn. 6-2 ... 

The potential required to split water into and O, i.e., (E - E is equal to 1.229 V. Though the theoretical potential is 1.23 V for water electrolysis, in practice the actual water decomposition will occur only above 1.7 V. The extra potential, which is essential for the water decomposition, is called overpotential. Overvoltages are composed of activation or charge transfer overvoltage, concentration or diffusion or mass transfer overvoltage and resistance overvoltage. Overvoltage is evaluated mainly as a function of current and temperature (Viswanathan, 2006). 

High temperature (often exceeding 1000 K) drives the endothermic chemical reactions. Multistep cycles for water splitting are used because very high temperatures are required before an appreciable amount of water decomposes in single-step cycles. Thus, in one or more subsequent chemical reactions, the intermediary compounds can be recovered to the original substance, which is used repeatedly. The thermochemical water decomposition steps involve the following five principal reactions ... 

Kato, H., Kudo, A. 1998. New tantalite photocatalysts for water decomposition into Hj and Oj. Chem Phys Lett 295 487 92. 

---