Redox decoupling

Obviously, the manner in which the gas phase is put in contact with the catalyst surface is important as the reactor type influences the catalytic performance through heat and mass transfer (as well as hydrodynamics in special cases). This is true for any catalyst/reaction couple, but alternative designs exploiting the redox mechanism have been proposed in the case of selective oxidation reactions. Redox decoupling... 

Alternatively, the sub-surface or bulk 0 species may diffuse to the surface to refill the vacancy. Note that only reactors operating in a redox decoupling mode, like CFBs, would derive oxygen from the bulk because the oxidation step is carried out in a distinct fluidized-bed reactor. Obviously this diffusion process is slower than the surface re-oxidation by molecular (gaseous) O2, but depending on the cation reducibihty during catalyst equilibration , part of the bulk becomes reduced. At the steady state (rR = rred = Tox) there is a hmited amount of 0 species able to participate in the reaction at each cycle (turn-over). 

Ballarini, N., Cavani, R, Cortelli, C., Giunchi, C., Nobili, R, Trifirb, F., Catani, R., and Cornaro, U. Reactivity of V/Nb mixed oxides in the oxide-hydrogenation of propane under co-feed and under redox-decoupling conditions and references therein. Catal. Today 2003, 78, 353. 

Is the assumed nature of equilibrium appropriate The modeler defines an equilibrium system that forms the core of a geochemical model, using one of the equilibrium concepts already described. The modeler needs to ask whether the reactions considered in an equilibrium system actually approach equilibrium. If not, it may be necessary to decouple redox reactions, suppress miner-... 

The modeler controls which redox reactions should be in equilibrium by interactively coupling or decoupling the redox pairs. For each coupled pair, the model uses the corresponding coupling reaction to eliminate redox species from the reactions in the database. For example, if the pair Fe+++-Fe++ is coupled, the model adds the coupling reaction to the reaction for hematite,... 

Models of natural waters calculated assuming redox disequilibrium generally require more input data than equilibrium models, in which a single variable constrains the system s oxidation state. The modeler can decouple as many redox pairs as can be independently constrained. A completely decoupled model, therefore, would require analytical data for each element in each of its redox states. Unfortunately, analytical data of this completeness are seldom collected. 

By decoupling the ferric-ferrous reaction with the decouple command, we add Fe+++ as a new basis entry in the calculation, setting up a model in which O2 and iron are held in redox disequilibrium. We constrain the new entry using the difference between the total and ferrous iron contents. 

The SPECE8 input script below describes the analysis of a hypothetical ground-water, assuming equilibrium with ferric hydroxide and a soil gas in which fco2 = 10-2. In the script, we decouple a number of redox pairs so that we can constrain the amounts of several elements in two or more redox states. 

A second complication is that we would like to decouple zero-valent sulfur from the element s other redox states, since Reaction 17.28 produces native sulfur, but the database does not include such a coupling reaction. Situations of this nature are not uncommon, occurring when an element in a certain oxidation state is stable as a solid, but no corresponding aqueous species occurs under geochemical conditions. To work the problem, we invent a ficticious zero-valent species S(aq) with an arbitrarily low stability. Setting log K for the reaction... 

To run the simulation, we save the surface complexation model to a dataset FeOH U02.dat , decouple the relevant redox reactions, set the system s initial composition, and define the rate law. The procedure in REACT is... 

Although Se and S are similar chemically, their redox speciation is different enough so that decoupling of Se from S can occur. This is illustrated in the Eh-pH stability diagrams for Se and S given in Figure 1. Under moderately reducing conditions, Se is stable as Se(I V) or Se(0), whereas S(tV) is not stable at all, and S(0) is stable only under a restricted set of conditions. Thus, Se may be separated from S if it is precipitated as Se(0), for example. 

A clear avenue of future research is to explore the S-Fe redox couple in biologic systems. Bacterial sulfate reduction and DIR may be spatially decoupled, dependent upon the distribution of poorly crystalline ferric hydroxides and sulfate (e.g., Canfield et al. 1993 Thamdrup and Canfield 1996), or may be closely associated in low-suUate environments. Production of FIjS from bacterial sulfate reduction may quickly react with Fefll) to form iron sulfides (e.g., Sorensen and Jeorgensen 1987 Thamdrup et al. 1994). In addition to these reactions, Fe(III) reduchon may be coupled to oxidation of reduced S (e.g., Thamdrup and Canfield 1996), where the net result is that S and Fe may be cycled extensively before they find themselves in the inventory of sedimentary rocks (e.g., Canfield et al. 1993). Investigation of both S and Fe isotope fractionations produced during biochemical cycling of these elements will be an important future avenue of research that will bear on our understanding of the isotopic variations of these elements in both modem and ancient environments. 

From a basic standpoint, an increased linkage length, N, can increase the value of A by partially decoupling the motion of the redox center from that of the backbone. In an ideal case, with a well-solvated linkage that does not impair backbone mobility, the value of A should vary with N. In reality, such... 

TAP Hicroreactor and TPROX Studies. In an examination of the role of oxygen in the reaction, it is useful to decouple the redox reactions. In separate TAP experiments we pulsed a 50-50 blend of... 

Zn-bromine flow and vanadium redox flow are special cases of secondary batteries. Here, liquid electrode materials are used on one (Zn-Br flow) or both sides (V redox flow) of the electrochemical cell. In contrast to regular batteries, which are typically completely closed systems, the liquid electrode materials in flow batteries are circulated and replenished from tanks (Figure 3.5.5). Therefore, the flow batteries possess large electrodes, the effective size of which is just limited by the volume of those tanks. This partly decouples energy and power capabilities of the batteries, allowing one to optimize both separately. 

In an effort to systematize differences in the absolute magnitude of benthic phosphate efflux in freshwater versus marine systems, Caraco et al. (1989) argue that more efficient benthic P-release occurs in lake relative to marine sediments as a direct consequence of the presence of higher sulfate in seawater, and that redox conditions exert secondary control. This argument is overly simplistic, however, because redox conditions control production of sulfide from sulfate, and it is the removal of ferrous iron from solution into insoluble ferrous sulfides that decouples the iron and phosphoms cycles (e.g., Golterman, 1995a,b,c Rozen et al., 2002). Thus, the presence of sulfate is a necessary but not sufficient criterion to account for differences in benthic P-cycling in marine versus freshwater systems redox conditions are an equally crucial factor. 

Fig. 3.7. Cubic model of a redox-linlced proton pump. OX and RED denote a redox centre in the oxidised and reduced state. The bar marked M or C next to OX and RED indicates an acidic group, the function of which is linked to the redox centre. M and C mean that the group is connected protonically either with the aqueous matrix or cytoplasmic phases, respectively. When the group is protonated the bar is supplemented with H. Left and right faces of the cube separate states in electronic and protonic contact with the input and output sides of the transducer, respectively. Allowed transitions between these are indicated by thick arrows. Dotted lines denote forbidden transitions. If the latter gain significant probability relative to allowed transitions proton transport becomes decoupled from electron transfer (so-called slipping ). (From Ref. 8.)...

On the other hand, decoupling of redox-active entities with carbonyl functions may result in splitting of the v(CO) bands. 

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