Thermochemical Redox Reactions

Keywords Solar Energy, Chemical Equilibrium Thermodynamics, Tin Oxide, Alternative Fuels, Thermochemical Redox Reactions, Second Law Analysis... 

Steinfeld A (2002) Solar hydrogen production via a two-step water-splitting thermochemical cycle based on Zn/ZnO redox reactions. Int J Hydrogen Energy 27 611-619... 

Photocalorimetry offers a convenient alternative to other methods of AH determination and, in some instances, may be the only practical method. The ligand substitution reactions of robust Werner-type complexes are a case in point. Conventional thermochemical measurements are complicated by the slowness of the substitution process and/or by competing reactions. Some of these same complexes, however, undergo clean photosubstitutions with high quantum yields and thus are excellent candidates for photocalorimetry. Examples include [Cr(NH3)6]3+, [Cr(CN)6]3-and [Co(CN)6]3-.192 Photocalorimetric measurements of AH have also been obtained for isomerization and redox reactions of coordination compounds.193194... 

A reaction is said to be enthalpy-driven if it involves a large, negative AH" with a smaller and usually unfavourable TAS at all accessible temperatures. In a thermochemical analysis of such a reaction, and in comparing several such related reactions, only the enthalpy terms need normally be considered. Most redox reactions and acid-base reactions come into this category. The latter term can be interpreted liberally to include many instances of complex formation, e.g. ... 

Since rates of arsenic redox reactions are slow at room temperature (5), it is assumed that the oxidation state data represent adjustment of arsenic species to the electron activity of the solution at 300°C. A quantitative assessment of the Eh of the basalt-water system at 300°C requires high-temperature thermochemical data for aqueous arsenic species. Such data are not available and, therefore, approximations were used to calculate Eh at 300°C. 

Steinfeld, A., Solar Hydrogen Production via Two-Step Water Splitting Thermochemical Cycle Based on Zn/ZnO Redox Reaction, Int. J. 

Various tropolonato complexes of protoactinium have been synthezised and characteriz-ed 2 . However, no thermochemical data (including emf and hence AG) are seemingly available, even though there are simple redox reactions interrelating these species. 

As we saw earlier in the chapter, the redox reactions between compounds of S in different oxidation states are often slow, and values of E° for half-reactions are invariably obtained from thermochemical information or estimated on the basis of observed chemistry. The data in Figure 15.19 illustrate the relative redox properties of some S-, Se- and Te-containing species. Points to note are ... 

Thermodynamic Analysis of Solar Fuel Production via Thermochemical H2O and/or CO2 Splitting Using Tin Oxide Based Redox Reactions... 

Due to the continuous increase in the population of world and drastic depletion of the fossil fuel reservoirs, it is highly essential to invest towards renewable energy technologies such as solar energy (storage, conversion and utilization). A two-step solar thermochemical H2O and/or CO2 splitting process which utilizes metal oxide (MO) based redox reactions is one of the... 

As the experiments showed, iodide and iodate were formed in a ratio of about 5 1 upon contact of I2 with the basic aerosol materials. Apparently, it is easier for I2 to disproportionate on the surface than it is for it to undergo a redox reaction with ions in the crystal to form Csl alone. Thermochemical data show that the formation of iodide and iodate would result in a lower free energy of reaction than formation of iodide alone. Formation of iodate alone would give a lower iodine potential than formation of iodide and iodate however, iodate formation seems to be limited by reaction kinetics. The extent of I2 reaction with anhydrous CS2O and CS2CO3 is probably limited only by the surface concentration of iodide and iodate which prevents or delays further interaction between I2 and the host crystal. In tests in which saturated aqueous solutions of these compounds were present, no such limitation was observed, nor had it been expected. 

The ceria-based thermochemical redox cycle offers a promising pathway to split water while storing intermittent sunhght in the form of hydrogen. Similar to other metal oxide thermochemical cycles, the cerium—cerium oxide cycle consists of two steps. The first reaction is the reduction of ceria (Ce02) to form cerium (III) oxide ... 

Another cycle for hydrogen production of special interest is the solar thermochemical cycle based on Zn—ZnO redox reactions. This cycle splits water into its constitoents via two steps. In the first step, ZnO is dissociated thermally into Zn gas and O2 during an endothermic reaction at temperatures of above 2000°C. Since this temperature is difficult to achieve, concentrated solar energy is used as the heat source. This reaction is given by the following expression ... 

Amend and McCollom this volume) echo and amplify this view by calculating thermochemical parameters of redox reactions that must have constituted the earliest metabolisms at geochemically relevant temperatures, pressures, and chemical compositions. They thus demonstrate that the molecular components that make up a prokaryotic cell can be produced through aqueous synthesis from inorganic precursors. 

In this paper selectivity in partial oxidation reactions is related to the manner in which hydrocarbon intermediates (R) are bound to surface metal centers on oxides. When the bonding is through oxygen atoms (M-O-R) selective oxidation products are favored, and when the bonding is directly between metal and hydrocarbon (M-R), total oxidation is preferred. Results are presented for two redox systems ethane oxidation on supported vanadium oxide and propylene oxidation on supported molybdenum oxide. The catalysts and adsorbates are stuped by laser Raman spectroscopy, reaction kinetics, and temperature-programmed reaction. Thermochemical calculations confirm that the M-R intermediates are more stable than the M-O-R intermediates. The longer surface residence time of the M-R complexes, coupled to their lack of ready decomposition pathways, is responsible for their total oxidation. 

The usefulness of the As(m)-As(V) couple as a redox indicator in hydrothermal experiments is limited by the availability of thermochemical data and by the range of Eh dictated by analytical and reaction rate constraints. For low temperature (<150°C) short-term experiments, selenium oxidation state analysis may be more appropriate than arsenic, since the Se(VI)-Se(IV) reduction occurs at relatively high Eh (36). 

In a pulse radiolysis study of CS2 Roebke and co-workers identified several derivative species (258). SC(OH)S, its conjugate base, and the pKa (4.4) that relates them were determined from the reaction of CS2 with OH. SCSH and its pKa (1.6) were determined by the reactions of eaq and H with CS2. The redox characteristics of these species remain unexplored. A subsequent pulse radiolysis study has not contributed any additional thermochemical information (56a). 

The redox-process is based on a thermochemical cycle making use of the following set of chemical reactions (for simplicity the reaction is formulated with pure iron oxides as the solid reactants) ... 

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