Combined surface exchange/diffusion process

The different elementary steps involved in a combined surface exchange/ diffusion process can be summarized as follows for <7e ffy -... 

At one extreme diffusivity may be so low that chemical reaction takes place only at suface active sites. In that case p is equal to the fraction of active sites on the surface of the catalyst. Such a polymer-supported phase transfer catalyst would have extremely low activity. At the other extreme when diffusion is much faster than chemical reaction p = 1. In that case the observed reaction rate equals the intrinsic reaction rate. Between the extremes a combination of intraparticle diffusion rates and intrinsic rates controls the observed reaction rates as shown in Fig. 2, which profiles the reactant concentration as a function of distance from the center of a spherical catalyst particle located at the right axis, When both diffusion and intrinsic reactivity control overall reaction rates, there is a gradient of reactant concentration from CAS at the surface, to a lower concentration at the center of the particle. The reactant is consumed as it diffuses into the particle. With diffusional limitations the active sites nearest the surface have the highest turnover numbers. The overall process of simultaneous diffusion and chemical reaction in a spherical particle has been described mathematically for the cases of ion exchange catalysis,63 65) and catalysis by enzymes immobilized in gels 66-67). Many experimental parameters influence the balance between intraparticle diffusional and intrinsic reactivity control of reaction rates with polymer-supported phase transfer catalysts, as shown in Fig. 1. 

Besides interacting with suspended particles, a chemical also undergoes direct exchange at the sediment surface by diffusion and advection into the hyporheic zone. Furthermore, resuspension followed by exchange between water and particles also adds to the sediment-water interaction. These processes have been extensively discussed in Chapter 23, especially in Box 23.2. There we concluded that the effect from the different mechanisms can be combined into a flux of the form (see Eq. 23-25) ... 

As an alternative to DET, small, artificial substrate/co-substrate electroactive molecules (mediators) can be used to shuttle electrons between the enzyme and the electrode (Figure 5.3b). This involves a process in which the enzyme takes part in the first redox reaction with the substrate and is re-oxidized or reduced by the mediator which in turn is regenerated, through a combination of physical diffusion and self-exchange, at the electrode surface. The mediator circulates continuously between the enzyme and the electrode, cycled between its oxidized and reduced forms, producing current. This process is known as mediated electron transfer (MET). 

Again, for the purpose of this chapter, this step is deflned as any process which is unaffected by the agitation conditions, e.g. the surface integration step in crystallization and the combined in-pore diffusion and chemical reaction at the surface associated with such particles as ion-exchange resins and catalysts. Figure 17.2 shows diagrammatically this idealized two-step mechanism. 

Let us consider a perovskite membrane covered by a film of metal nanoparticles (e.g., Ni). In such a situation, the different elementary steps involved in a combined H2 splitting over Ni, surface exchange at the feed/perovskite and perme-ate/perovskite membrane sides, and diffusion process within the perovskite membrane can be summarized as follows for Ue ... 

Soil reactions are generally classified according to the nature of the main chemical process involved adsorption, ion exchange, dissolution, etc. However, in order to assess the kinetics one should consider the nature and the rate of the transport processes associated with the chemical reaction flow and diffusion in the soil solution, transport across the solid-liquid interface, diffusion in liquid-filled pores and micropores, and surface diffusion penetration into the solid. An expression for the kinetics of soil reactions can be devised by assigning rate equations to transport and chemical processes and combining these equations. The expression finally obtained has to be validated by comparison to experimental results. 

---