Diffusion-limited oxidation regime

Because of the appearance of equation 65, B is called the parabolic rate constant. This limiting case is the diffusion-controlled oxidation regime that occurs when oxidant availability at the Si-Si02 interface is limited by transport through the oxide (thick-oxide case). 

Figure 2.3 Left, reduction models. In the shrinking core or contracting sphere model the rate of reduction is initially fast and decreases progressively due to diffusion limitations. The nucleation model applies when the initial reaction of the oxide with molecular hydrogen is difficult. Once metal nuclei are available for the dissociation of hydrogen, reduction proceeds at a higher rate until the system comes into the shrinking core regime. Right the reduction rate depends on the concentration of unreduced sample (1-a) as f(a) see Expressions (2-5) and (2-6).

Ammonia is adsorbed on the surface of an SCR catalyst in a diffusion limited laminar flow regime. The ammonia combines with vanadium pentoxide V2O5, a catalytic metal impregnated on the surface of the catalyst, to form a Bronsted acid site. NOx reduction takes place on this acid site to form nitrogen and water. The spent -OH site is restored to -OH via oxidation to repeat the catalytic cycle. Once the vanadium site can no longer revert back into the -1-5 oxidative state, then that site is no longer active for NO reduction. Figure 17.7 shows the catalytic cycle for the SCR reactions. 

A summary picture is presented in Fig. 5.43. The catalytic process can be carried out in the kinetic and in the diffusion-limited regime. In the figure the former case is represented by the smooth Pt data. The catalyst potential stabilizes at a rather high value ( 1 V) — at lower values the alcohol oxidation would not be able to keep up with the 02-reduction rate. At this potential the Pt surface is covered to an appreciable extent with Oad species (that are not involved in the... 

The TS-SSR is a temperature scanning reactor intended for kinetic studies of interactions between fluids and solids. It is therefore appropriate fin- studies of adsorption/desorption, ore roasting, solid oxidations/reductions and so on. Because a TSR reports rates of reaction regardless of the mechanism, a TS-SSR will report reaction rates in all diffusion regimes. Mechanistic studies seeking to determine the rates of interaction between the solid surface and the fluid free of diffusion limitations will have to be proceeded by careful delineation of conditions that allow diffusion-free measurements to be made. 

Laminar flame speed is one of the fundamental properties characterizing the global combustion rate of a fuel/ oxidizer mixture. Therefore, it frequently serves as the reference quantity in the study of the phenomena involving premixed flames, such as flammability limits, flame stabilization, blowoff, blowout, extinction, and turbulent combustion. Furthermore, it contains the information on the reaction mechanism in the high-temperature regime, in the presence of diffusive transport. Hence, at the global level, laminar flame-speed data have been widely used to validate a proposed chemical reaction mechanism. 

Fig. 8 Long range charge transport between dppz complexes of Ru(III) and an artificial base, methyl indole, in DNA. The methyl indole is paired opposite cytosine and separated from the intercalating oxidant by distances up to 37 A. In all assemblies, the rate constant for methyl indole formation was found to be coincident with the diffusion-controlled generation of Ru(III) (> 107 s )> indicating that charge transport is not rate limiting over this distance regime...

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