Pores of a catalyst

Reactants must diffuse through the network of pores of a catalyst particle to reach the internal area, and the products must diffuse back. The optimum porosity of a catalyst particle is deterrnined by tradeoffs making the pores smaller increases the surface area and thereby increases the activity of the catalyst, but this gain is offset by the increased resistance to transport in the smaller pores increasing the pore volume to create larger pores for faster transport is compensated by a loss of physical strength. A simple quantitative development (46—48) follows for a first-order, isothermal, irreversible catalytic reaction in a spherical, porous catalyst particle. 

In the discussion so far, the fluid has been considered to be a continuum, and distances on the molecular scale have, in effect, been regarded as small compared with the dimensions of the containing vessel, and thus only a small proportion of the molecules collides directly with the walls. As the pressure of a gas is reduced, however, the mean free path may increase to such an extent that it becomes comparable with the dimensions of the vessel, and a significant proportion of the molecules may then collide direcdy with the walls rather than with other molecules. Similarly, if the linear dimensions of the system are reduced, as for instance when diffusion is occurring in the small pores of a catalyst particle (Section 10.7), the effects of collision with the walls of the pores may be important even at moderate pressures. Where the main resistance to diffusion arises from collisions of molecules with the walls, the process is referred to Knudsen diffusion, with a Knudsen diffusivily which is proportional to the product where I is a linear dimension of the containing vessel. 

If the pores of a catalyst pellet are randomly oriented, geometric considerations require that if one takes an arbitrary cross section of the porous mass, the fraction of the area occupied by the solid material will be a constant that will... 

If we consider a reaction with intrinsic kinetics of simple nth order form that takes place within the pores of a catalyst pellet, the observed rate of reaction per unit mass of catalyst may be written as... 

The discussion of diffusion in pores of a catalyst requires taking into account the specific features of diffusion in narrow capillaries. 

Accumulated protons generated within the pores of a catalyst cause the formation of a proton gradient. The extent of such a gradient is predominantly a matter of the proton formation rate, which is dependent on the immobilized enzyme s activity and the mass transfer driven transport of protons to the outside of the catalyst particles. At steady state a mass balance occurs. 

The main use of the pore volume is in eq. (2) below. If the pores of a catalyst were assumed to be a single continuous cylinder of uniform size with smooth walls then the average pore radius would be given by ... 

What is the value of the apparent activation energy of a reaction the rate of which is measured in a regime controlled by diffusion through the pores of a catalyst pellet, if the true activation energy of the 6rst-order reaction is equal to E ... 

I 4 Chemical Reaction Engineering Table 4.5.2 Classification of pores of a catalyst. 

Impregnation is a common technique for distributing active metals within the pores of a catalyst support. Calcined supports are especially porous. Like sponges, they use capillary action to suck up aqueous solutions containing... 

Fig. 16.2-1. Four types of diffusion-controlled reactions. In (a), a reagent slowly diffuses to a solid surface and quickly reacts there this case occurs frequently in electrochemistry. In (b), a reagent slowly diffuses into the pores of a catalyst pellet, quickly reacting all along the way this reaction is modeled as if it were homogeneous. In (c), a circular solute quickly reacts with a mobile carrier, thus faciUtating the solute s diffusion across the membrane.

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