Diffusion, bulk surface

In general, homogeneous nucleation of nanoparticles occurs when a solute (C) diffuses to surface of a cluster from a bulk solution, and then incorporates into the cluster through surface reaction until a nucleus (C ) is obtained ... 

Erosion is typically characterized by either occurring on the surface or in the bulk. Surface erosion is controlled by the chemical reaction and/or dissolution kinetics, while bulk erosion is controlled by diffusion and transport processes such as polymer swelling, diffusion of water through the polymer matrix, and the diffusion of degradation products from the swollen polymer matrix. The processes of surface and bulk erosion are compared schematically in Fig. 1. These two processes are idealized descriptions. In real systems, the tendency towards surface versus bulk erosion behavior is a function of the particular chemistry and device geometry (Tamada and Langer, 1993). Surface erosion may permit the... 

If a catalytic cycle should be maintained, oxygen diffusion out to the surface must be complemented by an inward diffusions of surface-activated oxygen resulting from accumulation of reduced metal centers required to activate gas-phase oxygen. Not all studies mentioned here ensured in their experiments that the conditions of lattice oxygen catalysis were such as to fulfill the conditions of cyclic reversibility [34, 51, 82,131,132] as opposed to stoichiometric and irreversible reduction [133] caused by a structural phase transition. As long as complex MMO oxides are being used and the extent of reduction is kept to levels where no bulk transformation can be detected this condition can be verified [20,99,118,121,134,135], The kinetics of re-oxidation of partly reduced oxide catalysts was found to be rapid [77, 78, 80, 82] and always faster than its reduction. 

The effect of reaction temperature on the ammoniation of silica is summarized in figure 12.4. It is noticeable that for all regions an overlap of effects can be observed at almost every reaction temperature, a mixture of several surface species is formed. Again, it is also important to note that at higher reaction temperatures, the ammoniation of silica is not longer restricted to the surface, but will also involve reaction in the bulk skeleton siloxanes, combined with a diffusion of surface species into the bulk structure. 

Step 3 Gas phase diffusion and surface adsorption of CO and from the bulk of sink gas phase (S) to e working electrode surface at location (III). 

The calculations have shown that in SixHvPJ clusters H atoms initially located in near-surface layers diffuse to surface forming covalent bonds with some surface Si atoms and stabilizing them. Surface atoms relax to a cluster bulk. The H atoms which are in deeper layers are located in interstitial positions. Initial amorphization of silicon layers under action of H atoms is observed. 

The necessity of a larger ensemble for the dissolution of carbon into nickel than for activating methane corresponds to observations in surface physics. Adsorbed carbon atoms result in a distortion of the metal atom geometry whereas the bonding of adsorbed methane may require only 3-4 free nickel atoms. In simple terms, the two-dimensional surface sulfide prevents a distortion of the surface being necessary for the diffusion of surface carbon atoms into the bulk nickel phase. 

The gas is applied as a mixture to the retentate (high pressure) side of the membrane, the components of the mixture diffuse with different rates through the membrane under the action of a total pressure gradient and are removed at the permeate side by a sweep gas or by vacuum suction. Because the only segregative mechanisms in mesopores are Knudsen diffusion and surface diffusion/capillary condensation (see Table 9.1), viscous flow and continuum (bulk gas) diffusion should be absent in the separation layer. Only the transition state between Knudsen diffusion and continuum diffusion is allowed to some extent, but is not preferred because the selectivity is decreased. Nevertheless, continuum diffusion and viscous flow usually occur in the macroscopic pores of the support of the separation layer in asymmetric systems (see Fig. 9.2) and this can affect the separation factor. Furthermore the experimental set-up as shown in Fig. 9.11 can be used vmder isobaric conditions (only partial pressure differences are present) for the measurement of diffusivities in gas mixtures in so-called Wicke-Callenbach types of measurement. 

Our present task is to build on the foundations laid in chap. 7, but now with special reference to the diffusive processes that take place at extended defects. The basic argument will be that by virtue of the more open atomic-level environments near extended defects, the activation energy both for point defect formation and migration will often be reduced relative to bulk values. We will build our case around a fundamental case study through the consideration of diffusion at surfaces. The surface diffusion example will illustrate not only how diffusive processes are amended at extended defects, but will also illustrate the shortcomings of the transition state formalism when the detailed atomic-level mechanisms are not known a priori. 

More generally, adsorption is controlled by a combination of transport mechanisms in macropores or micropores, depending on the pore size distribution, the sorbate concentration, the isotherm, and other conditions. The combining bulk diffusion with surface diffusion gives the effective macropore diffusivity ... 

Overview In many industrial reactions, the overall rate of reaction is limited by the rate of mass transfer of reactants between the bulk fluid and the catalytic surface. By mas,s transfer, we mean any proces.s in which diffusion plays a role. In the rate laws and catalytic reaction steps described in Chapter 10 (diffusion, adsorption, surface reaction, desorption, and diffusion), we neglected the diffusion steps by saying we were operating under conditions where these steps are fast when compared to the other steps and thus could be neglected. We now examine the assumption that diffusion can be neglected. In this chapter we consider the external resistance to diffusion, and in the next chapter we consider internal resistance to diffusion. 

In technological applications as well as in scientific experiments specific boundary conditions are often given, such as definite changes of the interfacial area. A schematic representation is given in Fig. 4.2. which shows various bulk and interfacial transport processes of surface active molecules diffusion in the bulk, interfacial diffusion, bulk flow of different origin, interfacial compression and dilation. 

The deposition precipitation (DP) method has been successfiilly applied in order to functionalize the mesoporous silica SBA-15 with platinum. [Pt(NH3)4](OH)2 was used as the platinum precursor. The applicability of the SBA-15 for the DP is discussed in terms of sufficient hydrothermal stability, and the SBA-15, prepared at 100 °C was found to be a suitable support. The impact of the impregnation was monitored by means of Na adsorption-desorption and X-ray diffraction measurements. Platinum surface species coexisting with platinum nanoparticles on impregnated solids after a mild calcination at 300 °C was detected by UV-Vis diffuse reflectance. Reduction of the bulk surface specie occurs at 60 °C whilst a dispersive phase, which interacts stronger with the support, is reduced at around 120 °C as shown by T emperature-Programmed-Reduction (TPR). 

XRD and TEM analyses provided the bulk information on the M-CZ and CZ catalysts. However, as another information, the surface distribution of Zr and Ce species is very important for the evaluation of OSC because the reduction-oxidation mainly occurs on the surface not in the bulk. Surface Zr/Ce ratios by XPS spectra were calculated to be 0.49 for the CZ catalyst and 0.84 for the M-CZ catalyst, respectively. Probably, the Ce rich surface on the CZ catalyst is formed during the diffusion process in the solid-solid reaction when the as-prepared was reduced at 1473 K and the surface Ce species might be present as CeOa form observed by XRD(Fig. 1). Consequently, it is found that the segregation on the surface is preventable by the presence of MgO. 

We have confined ourselves to a description of the dynamics of surface roughness and the influence of the interaction forces on these dynamics. In reality, however, there are many more dynamic processes in the film and especially in the adsorbed monolayers that should be considered to describe in full detail the film dynamics. Apart from dynamics of the film surfaces parallel to the normal of the interfaces, motions of the adsorbed surface molecules in the interface must be considered. According to Lucas-sen-Reynders and Lucassen, the actual stresses in an interface are described by four rheological coefficients, reflecting the viscoelastic properties of the interface. Two of these, the surface dilatational elasticity and the surface dilatational viscosity, measure the surface s resistance against changes in area. The dilatational module e, considered before, expresses the dilatational elasticity. In our description of the film system, we neglected the viscous behavior of the interface, which implies that no diffusion of surface active molecules between bulk and interface was considered. If, however, surface-to-bulk diffusion is taken into account, the expression... 

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