Reoxidation rate

Reoxidation occurs when the metallic iron in hot DRI reacts with oxygen in the air to form either Ee O or Ee202. The reaction continues as long as the DRI remains hot and sufficient oxygen is avadable. Because reoxidation reactions are exothermic and DRI is a good insulator, it is possible that once reoxidation begins inside a pde, the DRI temperature increases and accelerates the reoxidation rate. Although the inner core of the pde may reach temperatures up to the fusion point of iron, the maximum temperature of the outer parts of the pde will be much lower because of heat dissipation. 

The transients shown in Figure 6 (see (9)) suggest that in the H2O/N2 phase, t O reacts with adsorbed CO to produce C0 and H2 ana that the Hj wavefront concentration is two-fold of the CO2 wavefront concentration. The overproduction of H on the wavefront seems to have been caused by the reoxidation of the catalyst by H 0. The reoxidation rate is therefore on the wavefront equal to the rate of the shift reaction, it is namely the limiting step in the global relaxation process. Furthermore, the fact that I CC is 2 1 on the wavefront suggests that presumably the shift con-... 

Equations 2.26 and 2.27 carmot be solved analytically except for a series of limiting cases considered by Bartlett and Pratt [147,192]. Since fine control of film thickness and organization can be achieved with LbL self-assembled enzyme polyelectrolyte multilayers, these different cases of the kinetic case-diagram for amperometric enzyme electrodes could be tested [147]. For the enzyme multilayer with entrapped mediator in the mediator-limited kinetics (enzyme-mediator reaction rate-determining step), two kinetic cases deserve consideration in this system in both cases I and II, there is no substrate dependence since the kinetics are mediator limited and the current is potential dependent, since the mediator concentration is potential dependent. Since diffusion is fast as compared to enzyme kinetics, mediator and substrate are both approximately at their bulk concentrations throughout the film in case I. The current is first order in both mediator and enzyme concentration and k, the enzyme reoxidation rate. It increases linearly with film thickness since there is no... 

The reduction of laccase by O J obtained by pulse radiolysis in the presence of Oj was followed by the decrease of the absorbance at 614 nm of type-1 Cu. Only a very partial reduction was observed (up to 7%). The binding of F anions to type-2 Cu lowered the reduction and the reoxidation rates... 

Reduction and reoxidation rates were also measured by Brueckman et al. [61], who used a static circulation reactor at 220—460°C. The reduction with hydrogen or propene at 460°C proceeds to Mo02 and Bi°. The very fast reoxidation was studied at much lower temperatures. Bi° is reoxidized first. The reduction process is rather complicated for the molybdenum-rich phases (Bi/Mo = 1/1 and 2/3), which appear initially to form a mixture of Mo02 and the 7-phase. Kinetic equations are presented by the authors, but do not seem relevant for catalysis in view of the too severe... 

Although this model cannot correctly reflect a redox mechanism, it indicates that the reduction and reoxidation rates have the same order of magnitude, and hence both influence the kinetics. 

Fe, Fe/V 0.08 Increase of catalytic activity Fe replaces in (VO)2P207. The reoxidation rate is increased (219,220)... 

Enzyme Oxidant Signal of intermediate Putative intermediate Rate of reoxidation Rate of decay of intermediate Reference... 

Fig. 3 strikingly illustrates the fact that exactly the same reaction mechanism is responsible for activation (transforming M0O3 to M018O52 or M08O23) and deactivation (M018O52 or M08O23 to M04O11 represented in Fig. 3c). Generally speaking, avoiding deactivation implies that the reduction and reoxidation rates of Fig. 1 are perfectly balanced.

Although the reoxidation rate is usually not rate determining in the overall redox cycle, the ability of the catalyst to rapidly replenish its reservoir of lattice oxygen is clearly necessary in order to sustain the surface catalyzed reaction. The reconstitution of catalyst surface after reduction occurs by means of shearing processes (Fig. 13) (77, 16), which are essential for effective oxidation catalysts. While reoxidation at higher temperatures is generally rapid for all selective oxidation catalyst systems, it becomes more... 

Figure 5. Dependence of the j eduction and reoxidation rate on V2.O5 content in the rhombic phase of M0O3.

In absence of acrolein the reoxidation rate of the reduced catalyst also increases markedly as the water vapor concentration is raised, figure 7. This effect may partly explain the... 

Other functions of cytochrome b PSI cyclic electron transport. The rate of reoxidation of cyt after a flash is increased by a factor of 4-5 if heme b is reduced prior to the flash (57). This suggests a mechanism for PSI cyclic phosphorylation that incorporates the oxidant-induced reduction of heme bp concomitant with reduction of heme b by PSI and ferredoxin, possibly through a quinone site (62). The oxidation of the two hemes by plastoquinone would be cooperative, as implied by the effect of b reduction on the reoxidation rate. The proposal of a quinone niche near the center of the bilayer that could oxidize both hemes resembles the semiquinone cycle model (63). One confusing aspect is the action of antimycin A which inhibits PSI cyclic phosphorylation (64). Because this compound is a classic n-side inhibitor of the mitochondrial cyt 6, it would be expected to act on cyt b y but there is no clear spectrophotometric effect. The slow reduction of cyt b mediated by ferredoxin in the dark (65) may be a problem for this model, although this would be explained if the reduction of cyt b as well as its oxidation is cooperative. 

In most cases, the reoxidation rate does not limit the overall rate of the process. The high and similar activation energies of these reactions, proportionality of their rates to the methane partial pressure, and manifestations of the kinetic isotope effect with fccH4/ fccD4 > 1 confirm the validity of the assumption that the rate-limiting step in these oxidation processes is due to the cleavage of the C—H bond on the catalyst surface. 

The reoxidation rate of the reduced metal is important in an efficient catalyst system. The metal-centered catalyst may have the proper redox potential for O2 oxidation, but if the rate of catalyst decomposition is competitive, then loss of the active catalyst results. To overcome issues of low turnover number (TON) and turnover frequency (TOE), added reagents are often used to couple multiple catalytic cycles in a biomimetic fashion [18]. Transition metals known to activate O2 based on biological precedence are commonly used as coupling reagents, including Cu(II), Co(II), Ee(II) and Mn(II) [2]. Many other organic molecules, such as quinones and catechols, can also act as redox couples. 

---