Internal hindered pore diffusion

There are several resistances which may hinder the movement of a molecule of adsorbate from the bulk fluid outside a pellet to an adsorption site on its internal surface, as shown in Figure 17.15. Some of these are sequential and have to be traversed in series, whilst others derive from possible parallel paths. In broad terms, a molecule, under the influence of concentration gradients, diffuses from the turbulent bulk fluid through a laminar boundary layer around a solid pellet to its external surface. It then diffuses, by various possible mechanisms, through the pores or the lattice vacancies in the pellet until it is held by an adsorption site. During desorption the process is reversed. 

A catalyst possesses a considerable inner surface the access to which is difficult for gases, due to windings and small diameters of pores. Oxidation proceeds mainly in the internal diffusion region and as the temperature is raised to 400°, it passes into the external diffusion region, with sharp heating up of the catalyst to 100-110° (at a reaction temperature 420°). The phthalic anhydride yielded by naphthalene inside the pores oxidizes to the end-products, C20 and H20, due to hindered diffusion and the increased contact time (Fig. 10) this results in lower selectivity. Boreskov (142) found that the contribution of the inner surface is different for various industrial catalysts. Inside the catalyst grains the reaction is several times slower than at the external surface. 

They are of great interest in catalysis because of their uniform and large pores, which allow sterically hindered molecules (i.e., unable to diffuse effectively through the smaller microporous chaimels of zeolites) to diffuse easily to internal active sites. These properties could also be interesting for application in gas sorption processes. 

Easier diffusion of B produces features such as a smoother dissolution front, internal vacancy clusters (polyatomic voids), and islands of A-type atoms hindered from dissolution. Qualitatively similar conclusions are drawn on 3D lattices except for the specific generation of pores with easier diffusion of B atoms as predicted [201 ] by a nonstochastic approach. This tendency to generate a tunneling attack at the cost of only surface diffusion could be considered as a likely explanation of pit nucleation at the atomic level, with no need for the concept of passive film breakdown. 

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