Single-step water splitting

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 ... 

Figure 10.5 Schematic of photocatalytic water splitting by one-step excitation in a single photocatalyst (0) light irradiation and absorption, (1) photo-excitation of electron-hole pairs, (2) migration of...

Thimsen E, Rastgar N, Biswas P (2008) Nanostructured Ti02 films with controlled morphology synthesized in a single step process performance of dye-sensitized solar cells and photo water splitting. J Phys Chem C 112 4134-4140... 

One possible alternative to the complex modular scheme has been proposed by Meyer and co-workers, and is based upon the integration of excitation, electron, and proton transfer in one single concerted step. Instead of conventional PCET, where light excitation precedes a proton and electron transfer event, the authors propose a mechanism based on photo-EPT. This as a concept has been tested for intramolecular charge transfer in hydrogen bonded dyes, but further development is needed before it can be applied to light driven water splitting devices. 

An ab initio study on the structure and splitting of the uracil dimer anion radical (see Scheme 3-66 and keep R = H) gives preference to the one-step mechanism (Voityuk Roesch 1997). Anion radical anions of the pyrimidine dimers cleave with rate constants in excess of 106 sec 1 (Yeh Falvey 1991). However, the cyclobutyl dimer of a quinone, dithymoquinone, also cleaves upon single-electron reduction but much more slowly than the pyrimidine dimers (Robbins Falvey 1993). It is truly an unresolved issue as to why the anion radical cleavage depicted in Scheme 3-66 is so facile. Water participation can probably decrease the barrier of the cycloreversion on physiological conditions (Saettel Wiest 2001). 

A characteristic common to the monomers of all condensation, or step-growth, polymers is that they possess two (or more than two) more-or-less reactive functional groups. The functional groups of a single monomer may be different, in which case the polymer may be produced by reaction of the single monomer with itself (Table 20.1, first two examples) or the functional groups required for condensation reactions may be on different monomers, in which case two monomers are required (Table 20.1, last three examples). Examples 1, 2, and 4 from this table are also typical of polycondensations in which a small molecule is split out, water in the first two cases and alcohol in... 

In Figure 1 a simplified process scheme of the antisolvent crystallization of sodium chloride is displayed. The process is divided into three steps the crystallization, the solid-liquid separation and the antisolvent recovery or liquid-liquid separation. In the first step sodium chloride is crystallized by mixing the feed brine with an antisolvent. The crystallization is carried out at temperatures below the liquid-liquid equilibrium line in the single liquid phase area (see Figure 2). In the second step the crystals are separated from their mother liquor, e.g. by filtration or in a centrifuge. In the third and final step the antisolvent is separated from the water phase at a temperature above the liquid-liquid equilibrium line in the two liquid phase area, in which the ternary amine-water-salt system splits up into an amine and an aqueous phase. The recovered antisolvent is recycled within the process and most ideally the water phase is reused for the dissolution of crude sodium chloride. In this paper the crystallization and the liquid-liquid separation steps will be treated. 

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