Catalytic sites diffusion

Because the reaction takes place in the Hquid, the amount of Hquid held in the contacting vessel is important, as are the Hquid physical properties such as viscosity, density, and surface tension. These properties affect gas bubble size and therefore phase boundary area and diffusion properties for rate considerations. Chemically, the oxidation rate is also dependent on the concentration of the anthrahydroquinone, the actual oxygen concentration in the Hquid, and the system temperature (64). The oxidation reaction is also exothermic, releasing the remaining 45% of the heat of formation from the elements. Temperature can be controUed by the various options described under hydrogenation. Added heat release can result from decomposition of hydrogen peroxide or direct reaction of H2O2 and hydroquinone (HQ) at a catalytic site (eq. 19). 

Fig. 13. Single-sheet diffusion transfer plate (a) stmcture (b) upon exposure to light (c) development and (d) washing off and finish. In (a) the plate is first coated with a receiver layer of small (<5 nm) catalytic sites. The photographic layer is a spectrally sensitized silver haUde emulsion. In (c) the exposed areas develop as silver metal. Unexposed areas diffuse down to the receiver layer and form the printing image. In (d) the emulsion is washed off, revealing...

The mechanism of poisoning automobile exhaust catalysts has been identified (71). Upon combustion in the cylinder tetraethyllead (TEL) produces lead oxide which would accumulate in the combustion chamber except that ethylene dibromide [106-93-4] or other similar haUde compounds were added to the gasoline along with TEL to form volatile lead haUde compounds. Thus lead deposits in the cylinder and on the spark plugs are minimized. Volatile lead hahdes (bromides or chlorides) would then exit the combustion chamber, and such volatile compounds would diffuse to catalyst surfaces by the same mechanisms as do carbon monoxide compounds. When adsorbed on the precious metal catalyst site, lead haUde renders the catalytic site inactive. 

Mass transport may constitute another problem. Since many catalysts are porous systems, diffusion of gases in and out of the pores may not be fast enough in comparison to the rate of reaction on the catalytic site. In such cases diffusion limits the rate of the overall process. 

The rate of a catalytic reaction depends on the rate of diffusion of both substrates and products to and from the catalytic sites. Therefore it is of outmost importance that the catalytically active sites are freely accessible for reactions. Only dendrimers of low generation number can possibly be expected to be suitable carriers for catalytically active sites, especially when these are located in the interior. In high-generation dendrimers with crowded surfaces catalytic activity of an internal site would be prevented. On the other hand, a crowded surface will not only hinder access to an interior ligand site but will also cause steric hindrance between groups attached to it and thus prevent high reactivity of sites at the periphery. 

In a bulk silica matrix that differs from the silica nanomatrix regarding only the matrix size but has a similar network structure of silica, several kinetic parameters have been studied and the results demonstrated a diffusion controlled mechanism for penetration of other species into the silica matrix [89-93]. When the silica is used as a catalyst matrix in the liquid phase, slow diffusion of reactants to the catalytic sites within the silica rendered the reaction diffusion controlled [90]. It was also reported that the reduction rate of encapsulated ferricytochrome by sodium dithionite decreased in a bulk silica matrix by an order of magnitude compared to its original reaction rate in a homogeneous solution [89], In gas-phase reactions in the silica matrix, diffusion limitations were observed occasionally [93],... 

Smith and coworkers recently proposed a specific and novel mineral-based solution to the problem of dilution and diffusion of prebiotic reactants. They have suggested [132-134] the uptake of organics within the micron-sized three-dimensional cross-linked network of pores found to exist within the top 50 xm, or so, of alumina-depleted, silica-rich weathered feldspar surfaces. These surfaces incorporate cavities typically about 0.5 pm in diameter along with cross inter-connections of about 0.2 pm. The nominal area of the weathered feldspar surface is apparently multiplied by a factor of about 130 arising from this network. The similarity of these pores to the catalytic sites in zeolite-type materials is pointedly mentioned. 

The necessity of forming zeolite powders into larger particles or other structures stems from a combination of pressure drop, reactor/adsorber design and mass transfer considerahons. For an adsorption or catalytic process to be productive, the molecules of interest need to diffuse to adsorption/catalytic sites as quickly as possible, while some trade-off may be necessary in cases of shape- or size-selective reactions. A schematic diagram of the principal resistances to mass transfer in a packed-bed zeolite adsorbent or catalyst system is shown in Figure 3.1 [69]. 

The speed of provision of the feed molecules to the adsorption/catalytic sites must be balanced with engineering issues such as pressure drop in a reactor/ adsorber, so the parhcle size and pore structure of engineered forms must be optimized for each appHcation. A hierarchy of diffusion mechanisms interplays in processes using formed zeoHtes. Micropore, molecular, Knudsen and surface diffusion mechanisms are all more or less operative, and the rate Hmifing diffusion mechanism in each case is directly affected by synthesis and post-synthesis manufacturing processes. Additional details are provided in Chapter 9. 

Assuming that Ti(IV) is distributed statistically in all tetrahedral positions, it can be easily seen that even for crystallite sizes of 0,2 m the great majority of T1(IV) is located inside the pore structure. Assuming that every Ti(IV) is a catalytic centre with equal activity, diffusion limitations for molecules of different sizes should be observed. This is in fact the case. It has been shown [27] that the rate of oxidation of primary alcohols decreases regularly as the chain length increases, while for iso-butyl alcohol a sudden drop in the rate is observed. Also the reactivity order of olefins on TS-1 is different from the order observed with homogeneous electrophilic catalysts, while as already indicated very bulky molecules are unreactive when TS-1 is used as the catalyst. All these facts can only be interpreted as due to diffusion limitations of the larger molecules, which means that the catalytic sites are located inside the pore structure of the solid. 

The efficiency of the inhibition by a mechanism-based inhibitor depends on the selective addition of the electrophilic entity on a nucleophile of the catalytic site, and not on the other numerous nucleophilic sites of the enzyme. Factors leading to a lack of selectivity are the insufficient reactivity of the electrophilic entity (a long half-life is often accompanied by the diffusion of the species outside the active site) and a large distance between the electrophile and the nucleophile of the active site of the enzyme. 

As already mentioned, supported rhodium(I) complexes have virtually the same selectivities as their homogeneous counterparts. In addition, their activities are also very similar to their homogeneous counterparts, except at low temperatures (i.e., close to room temperature) 45, 91, 95) the latter suggests that their activity may under such circumstances be limited by diffusion to and from the catalytic site. 

The computer-reconstructed catalyst is represented by a discrete volume phase function in the form of 3D matrix containing information about the phase in each volume element. Another 3D matrix defines the distribution of active catalytic sites. Macroporosity, sizes of supporting articles and the correlation function describing the macropore size distribution are evaluated from the SEM images of porous catalyst (Koci et al., 2006 Kosek et al., 2005). Spatially 3D reaction-diffusion system with low concentrations of reactants and products can be described by mass balances in the form of the following partial differential equations (Koci et al., 2006, 2007a). For gaseous components ... 

Shape-selective reactions occur by differentiating reactants, products, and/or reaction intermediates according to their shape and size in sterically restricted environments of the pore structures of microporous crystals16. If all of the catalytic sites are located inside a pore that is small enough to accommodate both the reactants and products, the fate of the reactant and the probability of forming the product are determined by molecular size and configuration of the pore as well as by the characteristics of its catalytic center, i.e., only a reactant molecule whose dimension is less than a critical size can enter into the pore and react at the catalytic site. Furthermore, only product molecule that can diffuse out through the pore will appear in the product. 

Product selectivity occurs when some of the products formed in the catalyst pore are too bulky to diffuse out, being converted to less bulky molecules (e.g., by equilibration or cracking). The large product molecules, which cannot diffuse out, may eventually deactivate the catalytic sites by blocking the pores. 

With ion exchangers as catalysts for olefin hydration, special attention was paid to transport problems within the resin particles and to their effects on the reaction kinetics. In all cases, the rate was found to be of the first order with respect to the olefin. The role of water is more complicated but it is supposed that it is absorbed by the resin maintaining it in a swollen state the olefin must diffuse through the water or gel phase to a catalytic site where it may react. The quantitative interpretation depends on whether the reaction is carried out in a vapour system, liquid-vapour system or two-phase liquid system. In the vapour system [284, 285], the amount of water sorbed by the resin depends on the H20 partial pressure it was found at 125—170°C and 1.1—5.1 bar that 2-methyl-propene hydration rate is directly proportional to the amount of sorbed water... 

If monolayers are involved then the substrate probably reacts with catalytic sites directly on the electrode surface. However, when multilayers are involved the substrate is denied access to the surface and movement of electrons is by the previously described process of electrochemical charge transport the substrate also diffuses through the multilayers. Under certain circumstances the presence of multilayers and hence of an increased number of catalytic sites may increase the rate of reaction. 

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