Solid state redox processes

The electrochemical processes involving Prussian blue and organic dyes studied above can be taken as examples of solid state redox processes involving transformation of a one solid compound into another one. This kind of electrochemical reactions are able to be used for discerning between closely related organic dyes. As previously described, the electrochemistry of solids that are in contact with aqueous electrolytes involves proton exchange between the solid and the electrolyte, so that the electrochemical reaction must in principle be confined to a narrow layer in the external surface of the solid particles. Eventually, however, partial oxidative or reductive dissolution processes can produce other species in solution able to react with the dye. 

Fig. 10 Schematic representation of a three-electrode cell for probing solid-state redox processes. (Reproduced with the permission of the American Chemical Society from P.). Kulesza, L R. Faulkner, J. Chen,...

Over the past twenty-five years the development of efficient catalysts for selective oxidation resulted in a new generation of commercial processes which utilize inexpensive olefinic and paraffinic feeds, replacing more reactive and costly raw materials. The catalysts are complex solid metal oxide systems which selectively activate hydrocarbons. Olefins, in particular, are activated via an allylie intermediate formation. The catalysts contain facile solid state redox couples which allow for efficient electron and lattice oxygen transport between reactant, adsorption and surface active site, and the surface reoxidation site which is then reconstituted by gaseous oxygen. 

The electrochemical reversibility of the employed redox material in a pseudocapacitor normally means that the redox process follows Nerstian behavior [2]. These redox materials include (1) electrochemically active materials that can be adsorbed strongly on an electrically conductive substrate surface such as a carbon particle and (2) solid-state redox materials that can combine with or intercalate into an electrode substrate to form a hybrid electrode layer. For example, adsorption on an electrode substrate surface is commonly observed as underpotential deposition of protons on the surface of a crystalline metal electrode (Ft, Rh, Pd, Ir, or Ru). In the case of Ru, the protons can pass through the surface into the metal lattice by an absorption process, similar to the transitional behavior seen in lithium battery intercalation electrodes. 

Redox Processes and Solid State Transformations In Bismuth Molybdates... 

Here we mention as an example that in the coordination-chemistry field optical MMCT transitions between weakly coupled species are usually evaluated using the Hush theory [10,11]. The energy of the MMCT transition is given by = AE + x- Here AE is the difference between the potentials of both redox couples involved in the CT process. The reorganizational energy x is the sum of inner-sphere and outer-sphere contributions. The former depends on structural changes after the MMCT excitation transition, the latter depends on solvent polarity and the distance between the redox centres. However, similar approaches are also known in the solid state field since long [12]. 

The nuclear decay of radioactive atoms embedded in a host is known to lead to various chemical and physical after effects such as redox processes, bond rupture, and the formation of metastable states [46], A very successful way of investigating such after effects in solid material exploits the Mossbauer effect and has been termed Mossbauer Emission Spectroscopy (MES) or Mossbauer source experiments [47, 48]. For instance, the electron capture (EC) decay of Co to Fe, denoted Co(EC) Fe, in cobalt- or iron-containing compormds has been widely explored. In such MES experiments, the compormd tmder study is usually labeled with Co and then used as the Mossbauer source versus a single-line absorber material such as K4[Fe(CN)6]. The recorded spectrum yields information on the chemical state of the nucleogenic Fe at ca. 10 s, which is approximately the lifetime of the 14.4 keV metastable nuclear state of Fe after nuclear decay. 

Unlike solid state -stacks, however, double helical DNA is a molecular structure. Here CT processes are considered in terms of electron or hole transfer and transport, rather than in terms of material conductivity. Moreover, the 7r-stack of DNA is constructed of four distinct bases and is therefore heterogeneous and generally non-periodic. This establishes differences in redox energetics and electronic coupling along the w-stack. The intimate association of DNA with the water and counterions of its environment further defines its structure and contributes to inhomogeneity along the mole-... 

Such pentacarbonyl species can be further decarbonylated when the sample is heated to 373 K under an inert gas stream and under reduced pressure. This slow decarbonylation process provides the surface Mo(CO)3 species depicted in Figure 9.4, which is stable up to 473 K [14]. In contrast with the relevant chemical behavior in solution (9.1 and 9.2), in the solid state, where the species are somewhat diluted and present low mobility, no dimeric species have been identified as resulting from penta- or tricarbonyl species. Heating to 673 K gives rise to the evolution of H2, CO, CO2 and CH4, due to redox reactions between the metal center and the OH surface groups. The resulting oxidation states, as determined by XPS measurements, are mainly II and IV, besides some Mo(0) species ]20]. It is worth underHn-... 

Non-stoichiometry is a very important property of actinide dioxides. Small departures from stoichiometric compositions, are due to point-defects in anion sublattice (vacancies for AnOa-x and interstitials for An02+x )- A lattice defect is a point perturbation of the periodicity of the perfect solid and, in an ionic picture, it constitutes a point charge with respect to the lattice, since it is a point of accumulation of electrons or electron holes. This point charge must be compensated, in order to preserve electroneutrality of the total lattice. Actinide ions having usually two or more oxidation states within a narrow range of stability, the neutralization of the point charges is achieved through a Redox process, i.e. oxidation or reduction of the cation. This is in fact the main reason for the existence of non-stoichiometry. In this respect, actinide compounds are similar to transition metals oxides and to some lanthanide dioxides. 

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