Bulk reductant

The reason for the ultramicrochemical test was to establish whether the bismuth phosphate would carry the plutonium at the concentrations that would exist at the Hanford extraction plant. This test was necessary because it did not seem logical that tripositive bismuth should be so efficient in carrying tetrapositive plutonium. In subsequent months there was much skepticism on this point and the ultramicrochemists were forced to make repeated tests to prove this point. Thompson soon showed that Pu(Vl) was not carried by bismuth phosphate, thus establishing that an oxidation-reduction cycle would be feasible. All the various parts of the bismuth-phosphate oxidation-reduction procedure, bulk reduction via cross-over to a rare earth fluoride oxidation-reduction step and final isolation by precipitation of plutonium (IV) peroxide were tested at the Hanford concentrations of... 

Anaerobic bio-reduction of azo dye is a nonspecific and presumably extracellular process and comprises of three different mechanisms by researchers (Fig. 1), including the direct enzymatic reduction, indirect/mediated reduction, and chemical reduction. A direct enzymatic reaction or a mediated/indirect reaction is catalyzed by biologically regenerated enzyme cofactors or other electron carriers. Moreover, azo dye chemical reduction can result from purely chemical reactions with biogenic bulk reductants like sulfide. These azo dye reduction mechanisms have been shown to be greatly accelerated by the addition of many redox-mediating compounds, such as anthraquinone-sulfonate (AQS) and anthraquinone-disulfonate (AQDS) [13-15],... 

A bulk reduction of Cjq in solution is also possible with less electropositive metals. This was demonstrated by the reduction of Cjq with mercury, leading to Cjq or 55 [141]. The addition of a drop of mercury to a 1 mM solution of tetra-... 

Figure 14.4 Selection of environmentally relevant redox couples including organic pollutants such as nitroaromatic and halogenated compounds, as well as examples of electron transfer mediators and important bulk reductants. The values given represent reduction potentials at pH 7 at equal (except otherwise indicated) concentrations of the redox partners but at environmental con-centrations of the major anions involved ...

A correlation between the catalytic qualities and the reducibility of this type of catalyst is suggested by Massoth and Scarpiello [ 205]. They performed reduction experiments both with hydrogen and with butene. Reduction may destroy the lattice, and the best catalysts appear to be those that are only superficially reduced. The effect of the introduction of Cr in the ferrites, mentioned above, is shown to be essentially due to an increase of the stability against bulk reduction. 

An additional consideration in formulating redox reactions is the possibility of catalysis by substances that mediate the transfer of electrons between the bulk reductant (or oxidant) and the substrate being transformed. Such considerations arise frequently in many areas of chemistry, especially electrochemistry and biochemistry (e.g., 97). In environmental applications, the most common model for mediated electron transfer involves a rapid and reversible redox couple that shuttles electrons from a bulk electron donor to a contaminant that is transformed by reduction. 

Reduction of nitro aromatic compounds often appears to be a two-step process, in which a mediator is required for facile transfer of electrons from a bulk reductant to the contaminant. A well documented example is the coupling of organic matter oxidation by iron reducing bacteria to "abiotic" nitro reduction by biogenic Fe(II) that is adsorbed to mineral surfaces in a column containing aquifer material (36, 39, 76). 

Although it has long been known that the adsorbed Fe(II) can be an effective reductant, its potential role as a mediator of reductive transformations of contaminants only recently has gained widespread recognition. Of particular interest are its possible roles in "natural attenuation" (65) and remediation technologies where the bulk reductant is dithionite (79) or Fe° (66). 

Like the various forms of iron, NOM apparently serves as both bulk reductant and mediator of reduction as well as bulk reductant (recall section 2.2.2). NOM also can act as an electron acceptor for microbial respiration by iron reducing bacteria (26), thereby facilitating the catabolism of aromatic hydrocarbons under anaerobic conditions (103). In general, it appears that NOM can mediate electron transfer between a wide range of donors and acceptors in environmental systems (104,105). In this way, NOM probably facilitates many redox reactions that are favorable in a thermodynamic sense but do not occur by direct interaction between donor and acceptor due to unfavorable kinetics. 

As previously mentioned, the promoters render the catalysts more difficult to reduce and their effect seems to be a double one, being related to the bulk reduction as well as to the final clean up of the surface. The promoters are assumed to create and stabilize surface sites of high excess of free energy. 

This brings the discussion of the changes in the solid full circle. Spiltover hydrogen can exchange with the surface. It may react with and replace methoxyls with hydroxyls. It may be incorporated into the bulk with a change in the bulk crystal structure. Bulk reduction may occur. The species spilling over may react only with the surface, with coke, or with other sorbed species. In addition, spillover may promote or inhibit reaction on the surface. 

The influence of spillover species on an acceptor phase can be in the extreme either subtle or profound. Many of the phenomena associated with hydrogen spillover are as subtle as the influences of type-2 hydrogen on the activity of ZnO (189) or as significant as bulk reduction, bronze formation, or catalytic activation. The effects may be similar to the exposure of a surface to a hydrogen plasma. 

Balducci, G., Kaspar, J., Fornasiero, R, Graziani, M., Islam, M.S., and Gale, J.D., Computer simulation studies of bulk reduction and oxygen migration in Ce02-Zr02 solid solutions, Journal of Physical Chemistry B, 1997, 101, 1750-1753. 

If the TPR profiles for the NM/Ce02 catalysts and the bare support, also included in Figure 4.3, are compared, a common high temperature feature centred at 1090 K may be noted. This peak is generally interpreted as due to the bulk reduction of ceria (61, and references there in). In agreement with several earlier studies (73,110,283), the position of this peak does not seem to be modified by the presence of any supported metal. This observation is typically interpreted in terms of a kinetic model (205) which assumes that the high temperature reduction process is controlled by the slow bulk difhision of the oxygen vacancies created at the surface of the oxide. 

These results evidence that OSC is larger for Ir-based catalysts than for Rh catalysts in the whole temperature range from 200 to SOO C. Additionally, OSC for ceria-supported catalysts varies slightly with temperature and reaches a maximum after full reduction of the surface. In this case> OSC appears to be limited by surface diffusion. On the opposite, for ceiia-zirconia supported catalysts, OSC is multiplied by a factor of 3 to 4 - depending on the metal - between 200 and 500°C. Bulk reduction is then responsible for such a large increase of the OSC. In that case, oxygen storage would be limited by bulk diffusion. 

As mentioned in the introduction, the ability of cerium to switch between the +4 and -h3 oxidation states determines many important applications of ceria-based materials. For this reason, the energy change associated with the bulk reduction in... 

Typically, stabilization of the catalyst consists of forming an oxide layer on the outer surface of the metal, while keeping the inner surface reduced. This is the result of controlled oxidation of the catalyst at low temperature. The effect is that 5-20% of the metal is oxidized and shields the inner reduced material from air. The outer oxide layer readily reduces in situ with hydrogen or with the reactant gases at a temperature often lower than the bulk reduction temperature when the plant is started. 

Extensive bulk reduction with epitactic formation of an external metal deposit... 

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