Active sites, nucleation from

COad removal from active sites (nucleation process), and it accounts for finite surface diffusion of CO ad ... 

Future studies have to clarify the role of active sites and active site clustering as well as the mechanism of active site nucleation. In this regard, the model would benefit from electronic structure calculations for systems that mimic heterogeneous nanoparticle surfaces. These calculations could form the basis for model refinements in terms of surface structure representation, reaction pathways, surface mobility, and kinetics of charge transfer steps. Moreover, the role of adsorbate interactions and anion adsorption effects should be evaluated. 

Many high-pressure reactions consist of a diffusion-controlled growth where also the nucleation rate must be taken into account. Assuming a diffusion-controlled growth of the product phase from randomly distributed nuclei within reactant phase A, various mathematical models have been developed and the dependence of the nucleation rate / on time formulated. Usually a first-order kinetic law I =fNoe fi is assumed for the nucleation from an active site, where N t) = is the number of active sites at time t. Different shapes of the... 

The foregoing discussion serves to show that disordered carbon structures are oxidized more readily than well-ordered graphite planes and that dislocations and active sites provide nucleation points for attack of the carbon crystallite. Another factor that must be considered is that dispersed electrocatalysts, such as platinum, on the carbon surface are not benign. The electrocatalysts interact with the carbon causing local oxidation or corrosion, i.e., the platinum catalyzes the corrosion of the carbon itself. In the presence of oxygen, which is the condition under which the electrocatalyst will operate, reduction intermediates from the oxygen (e.g., HOj) can have an accelerated corrosion effect. 

Figure 19. RH values observed for the efflorescence of ammonium sulfate (120 sec residence time) by heterogeneous nucleation as a function of mode diameter of the inclusions for corundum ( ) and hematite ( ). The hnes show F = 0.50 of the optimized fit to the active site model (Eqn. 24). Right axes show saturation ratios, S, of the aqueous phase with respect to crystalline ammonium sulfate and salt mole fractions, x, of the aqueous phase, as calculated from the model of Clegg et al. (1998) when assuming equihbrium between RH and water activity and omitting Kelvin effects. Adapted from Martin et al. (2001). Used by permission of the American Geophysical Unioa...

The second case, corresponding to the first class, results from an evolution of the catalyst state to a biphasic system through nucleation of C.S. planes. Coherent interfaces must exist, near which active sites are in an excited state and in a metastable coordination as compared to the normal sites of the host matrices. 

As the freshly generated steps, associated with nucleation sites, traverse from the tip-crystal domain, as a consequence of dissolution, the activity of the surface decreases and the current falls accordingly. The process outlined above is thus repeated, leading to the observed oscillatory current response shown in Figure 21. This sequence of events is illustrated schematically in Figure 23. 

Let us explain what happens with the formation of a bubble. When the electrocatalytic reaction occurs with the production of a gaseous product, it dissolves in the electrolyte until it reaches saturation. It is transported from the electronic conductor to the ionic conductor only by convective diffusion. When the solution concentration exceeds supersaturation, we are able to activate the nucleation sites to the bubble formation. This condition depends on the morphology of the surface and the kind of electrolyte and its viscosity. The growth of the bubble induces a microconvective flow on the electrolyte, pushing in various radial directions each bubble from an ideal center of the surface ( active site ). When each bubble attains a certain size, the buoyancy exceeds its adhesion and the bubble leaves the surface producing a drag flow. 

Figure 5 shows the potential dependence of the nucleus density obtained from analysis of the current transients according to equation (7). The exponential dependence of the nucleus density on potential suggests thermal activation of nucleation sites, consistent with classical nucleation models [5,8] where N0 °= exp(-eAU/kT). 

Effect of the Heat Transfer Surface. As we saw from the discussion on pages 15.9-15.18, the number of active nucleation sites on the surface at a given wall superheat and heat flux depends on a variety of factors. First, there is the population of potentially active sites, which is a function of the nature and preparation of the surface. Second, there are the wetting characteristics associated with the fluid/surface combination. These characteristics are often expressed in terms of the contact angles (<( , <]> , and < >,). In general, the heat flux (or heat transfer coefficient) in nucleate boiling heat transfer is strongly dependent on the number of active sites. 

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