Reduction by water

Su YO, Kuwana T, Chen SM. 1990. Electrocatalysis of oxygen reduction by water-soluble iron porphyrins. Thermodynamic and kinetic advantage studies. J Electroanal Chem 288 177. 

Since it is known that the tetranuclear mixed-valent platinum-blue complexes such as 19 and 57 undergo disproportionation and reduction by water as Eqs. (1)—(3) and (7)—(9) show (106, 113), all the species appearing in Eqs. (1)—(3) and (7)—(9) are present in the solution. However, only one or several of the four species in the solution may in fact be resposible for the catalytic olefin oxidation. To clarify this point, the effect of the Pt oxidation state in the platinum complexes was compared. The results are summarized in Table VII, which... 

Table 1.12 Water reduction by water-reducing agents as a function of workability...

The crystal held model can also be used to account For the stability of particular oxidation states. In aqueous solution Co(lll) is unstable with respect to reduction by water to form Co(Il)- Although there are several energy terms involved, this may be viewed as a reflection of the high third ionization energy of cobalt. If various moderate-to-strong field ligands are present in the solution, however. theCo(IIl)ion is perfectly stable. In fact, m some cases it is difficult or impossible to prevent the oxidation of Co(ll) to Coflll). 

It has been known for some time that Ag2+ ions could be stabilized by the use of strongly addic conditions.531 Studies in perchloric, sulfuric and phosphoric adds have emphasized the importance of silver(II)-solvent complexes and the role of kinetic factors in determining the lifetime of Agu with regard to its reduction by water.532,533... 

Stability Ce(TV) solutions in perchloric acid are sensitive to photochemical reduction by water. Weiss and Porret postulated that a hydroxyl radical is formed from activated Ce(TV). The radical can either reoxidize Ce(III) or react to release oxygen ... 

Finally, spray drift caused by other factors, e.g. temperature inversions and droplet size reduction by water evaporation, cannot be addressed by the above measures (Elliott and Wilson, 1983). It is not clear whether the consequences of temperature inversion can be addressed by formulation design. However, the loss of water from spray droplets might be usefully minimised by the use of humectants. 

Neatu. S. Macia-Agullo, J. A. Concepcion, P. Garcia, H. Gold-Copper Nanoalloys Supported on Ti02 as Photocatalysts for C02 Reduction by Water. J. Am. Chem. Soc., 2014,136,15969—15976. 

The viscosity reduction by water addition is not due to the presence of the surfactant (HAB). For the sand-in-bitumen suspension, the viscosity variation is shown in Figure 31. It can be observed from Figure 31 that the sand-in-bitumen suspensions are slightly shear thinning type. A low shear limit viscosity is observed, although a yield stress may be assumed if Casson s model is used (194). The shear thinning behavior is more severe when the solid volume fraction is increased. 

Radiation reduction by water injection. (From Leary, K., Knott, D., and Thompson, R., Oil Gas Journal, 100, no. 18, 76-83, 2002. With permission.)... 

Figure 17, UV-visible spectra showing the decay of Ag(II), in the form of AgN03+, due to reduction by water. Note the strong absorbance at approximately 390 nm. The electrolyte was 0.05 M AgN03 and 3.25 M HNO3 at ambient temperature.

Electrocatalytic nitric oxide reductions were noted in the previously described studies by Meyer s group on nitrite reduction by water-soluble porphyrins. As NO is a proposed intermediate, it was used independently as a substrate to verify the assigned potential for the ferrous nitfosyl to nitroxyl transformation. Bettelheim et al. also investigated NO reduction by Fe(TMPyP) immobilized onto a GCE with a Nafion coating that repelled anions like nitrite from the electrode surface. Differential pulse voltammetry of this system obtained three distinct reduction waves at potentials ca. 0.5, -0.6, and —0.8 V, assigned using the mechanistic scheme of Meyer " to sequential reductions of Ee (NO" ), Ee (NO), and Ee (NO ). 

Cheng, S.-H. and Y.O. Sn (1994). Electrocatalysis of nitric oxide reduction by water-soluble cobalt porphyrin. Spectral and electrochemical studies. Inorg. Chem. 33, 5847-5854. 

The experimental data that exists confirms that the range of LnlV species is very limited. Given that the potential for the oxidation of water is -1.23 V, it would be expected that all of the tetravalent lanthanide ions should be unstable to reduction by water. 

The hterature, incidentally, often does not clarify the meaning of the most stable , but it normally means either the most thermodynamically or the most kineti-cally resistant to thermal decomposition, to dismutation, to oxidation by air and to hydrolysis, oxidation or reduction by water (liquid or vapor) at ambient T. 

Signiflcantly, the average oxidation number of americium remains iv when Am(OH)4 is dissolved in either perchloric, nitric or sulfuric acids [157], indicating no significant reduction by water, in contrast to the reduction of Cm(iv) [358]. 

Scheme 1.3 Schematic representation of an ideal photocatalyst performing CO2 reduction by water...

The reasons why CO2 reduction by water is so inefficient are both thermodynamic and kinetic. From the thermodynamic point of view, CO2 reduction is highly endothermic and consequently requires more than one photon to occur. It should be noted that in natural photosynthesis the transfer of one electron requires the absorption of two photons, one in each of the two photosynthetic centers (Scheme 1.5) [18], Also from the kinetic point of view, the process of CO2 reduction is very complex since, depending on the products formed, several steps of reduction and protonatiOTi have to take place. For instance, conversion of CO2 into methane is a process that requires eight electrons and eight protons, and most probably this transformation has to occur stepwise, each step consisting in the transfer of one electron followed more or less synchronously by protonation. For the reaction of Eq. 1.1, formation of methanol requires the reduction of six electrons and six protons. 

As commented earlier, one of the most efficient photocatalysts for CO2 reduction by water to form methane consists of N-doped Ti02 anatase nanotubes modified by Pt and Cu as co-catalysts (Scheme 1.7). In this case, N atoms, introduced during the anodization process of Ti02 nanotubes, come from the NHa electrolyte or even atmospheric N2, and its presence increases the photocatalytic activity by allowing visible light absorption by the photocatalyst. 

Neaju , Macia-Agullo JA, Concepcion P, Garcia H (2014) Gold-copper nanoalloys supported on Ti02 as photocatalysts for C02 reduction by water. J Am Chem Soc 136 15969-15976... 

---