Solid redox mechanism

From Fig.2 (a), A solid phase transformation fiom hematite, Fc203 to magnetite, Fe304, is observed, indicating that the active sites of the catalj are related to Fc304. Suzuki et. al also found that Fe304 plays an important role in the formation of active centers by a redox mechanism [6]. It is also observed that the hematite itself relates to the formation of benzene at the initial periods, but no obvious iron carbide peaks are found on the tested Li-Fe/CNF, formation of which is considered as one of the itsisons for catalyst deactivation [3,6]. 

The effect of ionizing radiation on molecular or ionic solids is to eject electrons, which often subsequently react at sites in the material well removed from the residual electron-loss centre. These electron-loss and electron-gain centres, or breakdown products thereof, are paramagnetic and have been extensively studied by e.s.r. spectroscopy. Results for a wide range of organo metals both as pure compounds and as dilute solid solutions are used to illustrate this action. Aspects of the electronic structures of these centres are derived from the spectra and aspects of redox mechanisms are discussed. 

The active site on the surface of selective propylene anmioxidation catalyst contains three critical functionalities associated with the specific metal components of the catalyst (37—39) an CC-H abstraction component such as Bi3+, Sb3+, or Te4+ an olefin chemisorption and oxygen or nitrogen insertion component such as Mo6+ or Sb5+ and a redox couple such as Fe2+/Fe3+ or Ce3+/ Ce4+ to enhance transfer of lattice oxygen between the bulk and surface of the catalyst. The surface and solid-state mechanisms of propylene ammoxidation catalysis have been determined using Raman spectroscopy (40,41), neutron diffraction (42—44), x-ray absorption spectroscopy (45,46), x-ray diffraction (47—49), pulse kinetic studies (36), and probe molecule investigations (50). 

Normal-pulse polarography may be applied to solid-electrode voltammetry, which often is beset by electrode fouling (there is no periodic renewal of the electrode surface, as there is at the dme). This can be demonstrated for the electrolytic deposition of Ag and Au at solid electrodes and for the study of redox mechanisms of metal complexes ... 

All selective oxidation and ammoxidation catalysts possess redox properties. They must be capable not only of reduction during the formation of acrolein or acrylonitrile, but also subsequent catalyst reoxidation in which gaseous oxygen becomes incorporated into the lattice as to replenish catalyst vacancies (Scheme 2). As mentioned earlier, the incorporation of such redox properties into solid state metal oxides was one of the salient working hypotheses on which the development of the Sohio ammoxidation process was based (2). Later, Keulks (70) confirmed the involvement of lattice oxygen in propylene oxidation by using as a vapor phase oxidant. The results showed that the incorporation of O into the acrolein (and CO2) increases with time (Fig. 11), which is consistent with the above redox mechanism. 

The aikoxy group then ioses a second hydrogen and desorbs as an aldehyde or ketone. This series of transformations forms the first part of the Mars-van Krevelen redox mechanism, which is then followed by adsorption of oxygen from the gas phase, transfer of electrons from the solid to adsorbed oxygen molecule and incorporation of oxygen ions into the lattice of the oxide, which completes the redox cycle. The cycle involves two adsorbed redox couples ... 

In this chapter I will propose a kinetic estimate for the thermodynamics of reactions like Eq (lb). The solid phases listed in Table 1 may act as a reductant or an oxidant. One of the prominent geochemical electron donors is pyrite. From an estimate of global pyrite weathering of 36 Tgy"1 (Garrels et al., 1973) we may deduce an average electron flux on the land surface in the order of 0.02 mol m 2 y1. At redox boundaries in salt marshes and in lake sediments microbial sulfate reduction will intensify this electron cycling. Luther (Chapter 6, this volume) discusses the details of sulfide redox mechanisms. 

Benzo-15-crown-5 has been acetylated efficiently using KIO clays ion-exchanged by Cu, Fe or Zr. The best results were obtained with Fe -KlO. The catalytic properties are not related to the acidity of the solid and it can therefore be admitted that the C-Cl bond is activated by a redox mechanism, as proposed earlier for the alkylation of aromatic compounds by benzyl chloride. 

When Pb(OH)2 solid is not present, control over Pb activity may be exerted by redox mechanisms. For example, if the system contains Mn304 undergoing disproportionation, there could be a diversion of electron transfers from the manganese ions to the lead ions, which may be represented by the equation... 

This group of catalysts comprises solids exhibiting electrical conductivity, that is, having mobile electrons (metals and semiconductors). Many reactions proceed by the redox mechanism, for example ... 

The presence of mobile electrons in metals and other solid substances (to which some liquids may be added, like mercury" ), introduces in the modelling of the liquid interface new features not present in the previously-quoted interfaces nor in the bulk liquids. For many phenomena occurring at such interfaces there is the need for taking into explicit account electrons flowing from one phase to the other, and of related electron transfers occurring via redox mechanisms. The interaction potentials have to be modified and extended accordingly." ... 

The catalytic mechanism of the total methanol oxidation may be a redox mechanism in a manner first proposed of Mars and van Krevelen [28]. According to this mechanism the consumed lattice-oxygen is continuously restored by oxygen from gas phase. It remains open what the exact nature of lattice oxygen means. It may well be that this term does not designate a physically discernible oxygen species but rather means a rate constant of a solid state reaction (the formation of the active oxide) much different from the rate constants of the catalytic processes. 

The superior mechanical properties of elastomeric electrolytes, combined with the simplicity of fabrication of thin separator films from these materials, has led to very active pursuit of solid state cells based on solid polymeric materials. However, the development of a novel class of solid state or nic positive electrodes which not only operate on an entirely new principle for energy storage, but have demonstrated performances in a number of instances exceeding those for intercalation electrodes in solid state hthium and sodium batteries. These new electrodes have been called solid redox polymerization electrodes (SRPE) and arc based on polysulfide polymers [-S(R) 0 (R) S-] where R is an organic radical such as CH3, QH, CF3, etc. The redox mechanism for the positive electrode is in essence a redox dimeriza-tion/scission reaction which occurs in two steps as... 

A method to circumvent the problem of chalcogen excess in the solid is to employ low oxidation state precursors in solution, so that the above collateral reactions will not be in favor thermodynamically. Complexation strategies have been used for this purpose [1, 2]. The most established procedure utilizes thiosulfate or selenosulfate ions in aqueous alkaline solutions, as sulfur and selenium precursors, respectively (there is no analogue telluro-complex). The mechanism of deposition in such solutions has been demonstrated primarily from the viewpoint of chemical rather than electrochemical processes (see Sect. 3.3.1). Facts about the (electro)chemistry of thiosulfate will be addressed in following sections for sulfide compounds (mainly CdS). Well documented is the specific redox and solution chemistry involved in the formulation of selenosulfate plating baths and related deposition results [11, 12]. It is convenient to consider some elements of this chemistry in the present section. 

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