Site densities reactants adsorption

Zeolites as cracking catalysts are characterized hy higher activity and better selectivity toward middle distillates than amorphous silica-alumina catalysts. This is attrihuted to a greater acid sites density and a higher adsorption power for the reactants on the catalyst surface. 

JSnchen et al. [64] have reported that the heats of adsorption of acetonitrile on mesoporous (MCM-41) and microporous (FAU and MFI) molecular sieves are mainly influenced by a specific interaction with the acidic sites, while the adsorption heats of a non-polar molecule like w-hexane are determined by the pore size or density of those materials. However, a pore-size effect, affecting the heats of acetonitrile adsorption on acidic molecular sieves, has to be taken into account when employing those heats as a measurement of acidic strength. The contribution of the pore-size governed dispersion interaction in mesoporous MCM-41 is about 15 kJ mof less than that in the narrow channels of MFI. The adsorption of molecules of different sizes (toluene, xylenes, etc.), and the consecutive adsorption of these same molecules, studied by adsorption microcalorimetry together with reaction tests, can provide useful indications of the pore geometry and reactant accessibility of new zeolitic materials such as MCM-22 [65] or ZSM-11, SSZ-24, ZSM-12, H-M and CIT-1 [66]. 

Convection refers to fhe fransport of the reactant or product species by bulk fluid motion driven by natural or applied mechanical forces. The natural convection limitations are due to convective transport caused by differences in densities as a result of temperature or concentration. The species transport to the interface can also be limited by fhe fuel cell flow sfrucfures and fheir conditions. For example, in PEMFC, blockage of flow channels or pore structures in diffusion or elecfrode-cafalysf layers owing to the liquid phase can restrict the supply of fhe reactant to the interface. Accumulation of inert gases that do not participate in chemical reaction will limit the partial pressure of the reactant at the interface. This results to decreased reactions at the interface. The accumulation of chemical impurities at the reaction sites will prevent adsorption of desired reactant species. For example, in PEMFC, the presence of carbon monoxide degrades the platinum catalyst because the platinum preferentially adsorbs carbon monoxide, leaving few reaction sites for hydrogen adsorption and oxidation. This leads to high anodic overpotential. 

In these expressions is the rate of adsorption of species j, which for A may be written as A + S AS, where A is the gas-phase molecule. S is an empty site on the surface, and AS is the adsorbed molecule. We can consider adsorption as a bimolecular chemical reaction that is proportional to the densities of the two reactants A and S to give... 

From the above consideration it is clear that the exchange current density is the main factor affecting the activation overpotential, then the optimization of a PEM fuel cell performance requires the maximization of io- This can be obviously accomplished by increasing the catalyst activity, that means to raise the surface area, cell temperature, and reactant pressure (this last effect should also favor gas adsorption on catalyst sites). 

Reaction 88 represents the conversion of A- to B . The formation of B is 1 directly proportional to [ >Me-A], the adsorbed reactant. Estimates of reaction rate based solely on [AbUlk] are unreliable, since mass transfer, adsorption j reactions, and the availability of free sites all influence the surface density of the reacting species > Me-A. 

E vs. Aa seems to be most sensitive to product concentrations near the external surface of the catalyst and adsorption/desorption equilibrium constants. I c.surf. I d, surf, and 6>, directly affect the vacant-site fraction on the interior catalytic surface and the rate of reactant consumption. In the previous simulations, product molar densities near the external surface of the catalyst were varied by a factor of 50 (i.e., from 0.1 to 5), and 0, was varied by a factor of 20 (i.e., from 0.05 to 1). The effectiveness factor increases significantly when either 4 c,surf, I d. surf or 6i is larger. E vs. Aa is marginally sensitive to a stoichiometric imbalance between reactants A2 and B, but I B.sur ce was only varied by a factor of 4 (i.e., from 0.5 to 2). A four-fold decrease in the molecular weight of reactant B, which produces two-fold changes in 30b, effective and 5b, does not affect E. 

At relatively low pressures, what dimensionless differential equations must be solved to generate basic information for the effectiveness factor vs. the intrapellet Damkohler number when an isothermal irreversible chemical reaction occurs within the internal pores of flat slab catalysts. Single-site adsorption is reasonable for each component, and dual-site reaction on the catalytic surface is the rate-limiting step for A -h B C -h D. Use the molar density of reactant A near the external surface of the catalytic particles as a characteristic quantity to make all of the molar densities dimensionless. Be sure to define the intrapellet Damkohler number. Include all the boundary conditions required to obtain a unique solution to these ordinary differential equations. 

Figure 26. Cluster modeling results for the oxidation of galena by Fe suggesting the importance of proximity effects to surface reactivity on galena, modified from Becker et al. (2001). The adsorption of an Fe ion to one side of the cluster results in a coupled exchange of electron density from comer sulfur sites to the ferric ion and spin density (indicated by vertical arrows) from the ferric ion to the comer sulfur sites (a). This causes the equilibrium position of a hydroxyl on the opposite side of the cluster to migrate towards tlK nearest spin polarized comer site (b). The calculations demonstrate that delocalized orbitals typical of semiconducting minerals can couple reactants spatially separated by several bond lengths in the substrate.

To include an adsorbed species, one or more surface water molecules are removed and the adsorbate added. Figure 3.7(c) illustrates the adsorption structure of sulfate at a fully solvated electrode surface with one water molecule displaced. The choice of cell size, adsorbate site (which also sets position within the water layer), water density, and water structure can all affect the computed results. As these choices can only be loosely based on matching an actual system, and testing of the impact of all of them is often intractable, choices should be made consistently between reactants and products to take advantage of cancellation of variability. For the anion adsorption reaction [eqn (3.7)], adsorption occurs with displacement of some number of water molecules ... 

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