Pore mouths

Fig. 10. Catalyst macropores showing D noble metal sites and (a) narrowed micropores after exposure to high temperatures where H represents thermally damaged noble metal sites and (b) pore mouth plugging from poisons where A, if aUowed, diffuses in to be converted to B.

The accumulation of matter on the surface of the catalyst restricts gas passage into the catalyst by a mechanism known as pore mouth plugging (see Fig. 10b). It takes only a smaU amount of material on the surface of the catalyst to restrict the free passage of gases into and out of the active porous... 

Pore mouth or shell) poisoning occurs when the poisoning of a pore surface begins at the mouth and moves gradually inward. In this case the reactant must diffuse through the dead zone before it starts to react. P is the fraction of the pore that is deac tivated, Ci is the concentration at the end of the inac tive region, and x = — P)L is the coordinate there. 

Some studies of potential commercial significance have been made. For instance, deposition of catalyst some distance away from the pore mouth extends the catalyst s hfe when pore mouth deactivation occui s. Oxidation of CO in automobile exhausts is sensitive to the catalyst profile. For oxidation of propane the activity is eggshell > uniform > egg white. Nonuniform distributions have been found superior for hydrodemetaUation of petroleum and hydrodesulfuriza-tion with molybdenum and cobalt sulfides. Whether any commercial processes with programmed pore distribution of catalysts are actually in use is not mentioned in the recent extensive review of GavriUidis et al. (in Becker and Pereira, eds., Computer-Aided Design of Catalysts, Dekker, 1993, pp. 137-198), with the exception of monohthic automobile exhaust cleanup where the catalyst may be deposited some distance from the mouth of the pore and where perhaps a 25-percent longer life thereby may be attained. 

The concentration of gas over the active catalyst surface at location / in a pore is ai [). The pore diffusion model of Section 10.4.1 linked concentrations within the pore to the concentration at the pore mouth, a. The film resistance between the external surface of the catalyst (i.e., at the mouths of the pore) and the concentration in the bulk gas phase is frequently small. Thus, a, and the effectiveness factor depends only on diffusion within the particle. However, situations exist where the film resistance also makes a contribution to rj so that Steps 2 and 8 must be considered. This contribution can be determined using the principle of equal rates i.e., the overall reaction rate equals the rate of mass transfer across the stagnant film at the external surface of the particle. Assume A is consumed by a first-order reaction. The results of the previous section give the overall reaction rate as a function of the concentration at the external surface, a. ... 

Because the pore dimensions in narrow pore zeolites such as ZSM-22 are of molecular order, hydrocarbon conversion on such zeolites is affected by the geometry of the pores and the hydrocarbons. Acid sites can be situated at different locations in the zeolite framework, each with their specific shape-selective effects. On ZSM-22 bridge, pore mouth and micropore acid sites occur (see Fig. 2). The shape-selective effects observed on ZSM-22 are mainly caused by conversion at the pore mouth sites. These effects are accounted for in the hydrocracking kinetics in the physisorption, protonation and transition state formation [12]. 

Alkane physisorption on ZSM-22 can be described using an additivity method accounting for the number of carbon atoms inside and outside the ZSM-22 micropores [22]. Linear alkanes can fully enter the micropores while branched alkanes can only enter the pore mouths. Multiple physisorption modes exist at the pore mouths where branched alkanes can enter the pore mouth with each of their straight ends . 

Alkene protonation at pore mouths can exclusively lead to secondary carbenium ions. In addition, the alkene standard protonation enthalpies increase with the number of carbon atoms inside the micropore because charge dispersive effects are supposed to be more effective on carbon atoms inside the micropores. 

Transition state formation is sterically hindered at ZSM-22 pore mouths if the elementary reaction requires the ionic centre to move too far away from the deprotonated acid... 

Fig. 2. Isometric view of ZSM-22 crystallite location of pore mouth, bridge acid and micropiore acid sites.

GL 18] [R 6a] [P 17] CFD calculations were performed to give the Pd concentration profile in a nanopore of the oxide catalyst carrier layer [17]. For wet-chemical deposition most of the catalyst was deposited in the pore mouth, in the first 4 pm of the pore. Hence most of the hydrogenation reaction is expected to occur in this location. For electrochemical deposition, large fractions of the catalyst are located in both the pore mouth and base. Since the pore base is not expected to contribute to large extent to hydrogenation, a worse performance was predicted for this case. 

In the first step the solution enters the pores. The driving forces for the flow are capillary forces. During the flow, adsorption by ion exchange occurs. Due to the high rate of adsorption an uneven distribution of Pt ions results. Subsequently, a situation exists in which the diffusion through the pore mouth becomes rate determining. The active phase is pre.sent as a shell, which moves towards the interior of the particle as shown in Fig. 3.30. 

Diffusion Poisons. This phenomenon is closely akin to catalyst fouling. Blockage of pore mouths prevents full use of the interior surface area of the pellets Entrained dust par ticles or materials that can react on the catalyst to yield a solid residue give rise to this type of poisoning. 

The numerator of the right side of this equation is equal to the chemical reaction rate that would prevail if there were no diffusional limitations on the reaction rate. In this situation, the reactant concentration is uniform throughout the pore and equal to its value at the pore mouth. The denominator may be regarded as the product of a hypothetical diffusive flux and a cross-sectional area for flow. The hypothetical flux corresponds to the case where there is a linear concentration gradient over the pore length equal to C0/L. The Thiele modulus is thus characteristic of the ratio of an intrinsic reaction rate in the absence of mass transfer limitations to the rate of diffusion into the pore under specified conditions. 

From equation 12.3.13, the concentration gradient at the pore mouth is found to be... 

To evaluate the effectiveness factor we require only the derivative of the concentration at the pore mouth. This parameter may be obtained by multiplying both sides of equation 12.3.31 by d(C/C0)/d(x/L) ... 

This situation is termed pore-mouth poisoning. As poisoning proceeds the inactive shell thickens and, under extreme conditions, the rate of the catalytic reaction may become limited by the rate of diffusion past the poisoned pore mouths. The apparent activation energy of the reaction under these extreme conditions will be typical of the temperature dependence of diffusion coefficients. If the catalyst and reaction conditions in question are characterized by a low effectiveness factor, one may find that poisoning only a small fraction of the surface gives rise to a disproportionate drop in activity. In a sense one observes a form of selective poisoning. 

The two limiting cases for the distribution of deactivated catalyst sites are representative of some of the situations that can be encountered in industrial practice. The formation of coke deposits on some relatively inactive cracking catalysts would be expected to occur uniformly throughout the catalyst pore structure. In other situations the coke may deposit as a peripheral shell that thickens with time on-stream. Poisoning by trace constituents of the feed stream often falls in the pore-mouth category. 

Poisoning curves for porous catalysts. Curve A is for a porous catalyst with hT very small and poison distributed homogeneously. Curve B is for large hT with the poison distributed homogeneously. Curves C and D correspond to preferential adsorption of poison near the pore mouths. For curve C, hT = 5, and for curve D, hT = 20. 

If a fraction a of the total catalyst surface has been deactivated by poison, the pore-mouth poisoning model assumes that a cylindrical region of length (aL) nearest the pore mouth will have... 

In order to demonstrate the selective effect of pore-mouth poisoning, it is instructive to consider the two limiting cases of reaction conditions corresponding to large and small values of the Thiele modulus for the poisoned reaction. For the case of active catalysts with small pores, the arguments of all the hyperbolic tangent terms in equation 12.3.124 will become unity and... 

Now consider the other extreme condition where diffusion is rapid relative to chemical reaction [i.e., hT( 1 — a) is small]. In this situation the effectiveness factor will approach unity for both the poisoned and unpoisoned reactions, and we must retain the hyperbolic tangent terms in equation 12.3.124 to properly evaluate Curve C in Figure 12.11 is calculated for a value of hT = 5. It is apparent that in this instance the activity decline is not nearly as sharp at low values of a as it was at the other extreme, but it is obviously more than a linear effect. The reason for this result is that the regions of the catalyst pore exposed to the highest reactant concentrations do not contribute proportionately to the overall reaction rate because they have suffered a disproportionate loss of activity when pore-mouth poisoning takes place. 

What is your interpretation of these data Do the data indicate whether homogeneous or pore-mouth poisoning takes place ... 

X distance from pore mouth AG Gibbs free energy change... 

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