Catalyzed diffusion medium

As we discussed above, there are two major types of CL fabrication techniques. One is to apply the catalyst ink onto the gas diffusion layer to form a catalyzed diffusion medium (CDM), and the other is to apply the catalyst ink onto the PEM to form a CCM. Normally, applying the ink to the gas diffusion medium has the advantage of preserving the membrane from chemical attacks by the solvents in the catalyst ink. However, it seems that the CL does not come into close contact with the membrane and therefore the electrode is prone to delamination. Regarding CCM, there are two ways of applying the catalyst ink to the membrane, namely the decal transferring process and the direct coating process. In the former, the CL is cast onto a PTFE blank... 

Many problems involving competitive reaction kinetics may be treated by invoking the steady-state assumption within the digital simulation this has been done in at least two instances [29-34]. The first of these involves the development of a model for enzyme catalysis in the amperometric enzyme electrode [29-31]. In this model, the enzyme E is considered to be immobilized in a diffusion medium covering an electrode that is operated at a fixed potential such that the product (P) of enzyme catalysis is electroactive under diffusion-controlled conditions. (This model has also served as the basis for the simulation of the voltammetric response of the enzyme electrode [35].) The substrate (S) diffuses through the medium that contains the immobilized enzyme and is catalyzed to form P by straightforward enzyme kinetics ... 

Zhabotinsky et al. [32] studied refraction and reflection of waves in the ferroin-catalyzed BZ reaction-diffusion medium using the oxygen inhibition of excitability in the BZ reaction [33] to create a sharp boundary between two regions with different wave velocities. Figure 1 shows refraction of a chemical wave at the boundary between two regions of different wave speed. Measurements of the angles and speeds have shown that refraction of chemical waves obeys Snell s Law within experimental accuracy. 

Figure 3. Mitochondrial fatty acid oxidation. Long-chain fatty acids are converted to their CoA-esters as described in the text, and their fatty-acyl-groups transferred to CoA in the matrix by the concerted action of CPT 1, the acylcarnitine/carnitine exchange carrier and CPT (A) as described in the text. Medium-chain and short-chain fatty acids (Cg or less) diffuse directly into the matrix where they are converted to their acyl-CoA esters by a acyl-CoA synthase. The mechanism of p-oxidation is shown below (B). Each cycle of P-oxidation removes -CH2-CH2- as an acetyl unit until the fatty acids are completely converted to acetyl-CoA. The enzymes catalyzing each stage of P-oxidation have different but overlapping specificities. In muscle mitochondria, most acetyl-CoA is oxidized to CO2 and H2O by the citrate cycle (Figure 4) some is converted to acylcamitine by carnitine acetyltransferase (associated with the inner face of the inner membrane) and exported from the matrix. Some acetyl-CoA (if in excess) is hydrolyzed to acetate and CoASH by acetyl-CoA hydrolase in the matrix. Enzymes ...

The reaction was studied for all coinage metal nanoparticles. In the case of GMEs the rate follows zero-order kinetics with IT for all the coinage metal cases. The observed IT for the Cu catalyzed reaction was maximum but its rate of reduction was found to be minimum. Just the reverse was the case for Au and an intermediate value was obtained for the Ag catalyzed reaction (Figure 7). The adsorption of substrates is driven by chemical interaction between the particle surface and the substrates. Here phe-nolate ions get adsorbed onto the particle surface when present in the aqueous medium. This caused a blue shift of the plasmon band. A strong nucleophile such as NaBH4, because of its diffusive nature and high electron injection capability, transfers electrons to the substrate via metal particles. This helps to overcome the kinetic barrier of the reaction. 

R NCIO system also makes the medium acidic, however the strength of the EGA is much weaker than that of a CH Clj—LiClO system due to neutralization of EGA with trialkyl amine produced by cathodic reduction of the ammonium cation Therefore, an EGA-catalyzed reaction can be performed even in a CH Cl — —R NCIO or MeCN—R NCIO system when a divided cell is employed. Presumably, amines generated in the cathode compartment diffuse slowly into the anode compartment where the EGA reaction occurs. 

In purple photosynthetic bacteria, electrons return to P870+ from the quinones QA and QB via a cyclic pathway. When QB is reduced with two electrons, it picks up protons from the cytosol and diffuses to the cytochrome bct complex. Here it transfers one electron to an iron-sulfur protein and the other to a 6-type cytochrome and releases protons to the extracellular medium. The electron-transfer steps catalyzed by the cytochrome 6c, complex probably include a Q cycle similar to that catalyzed by complex III of the mitochondrial respiratory chain (see fig. 14.11). The c-type cytochrome that is reduced by the iron-sulfur protein in the cytochrome be, complex diffuses to the reaction center, where it either reduces P870+ directly or provides an electron to a bound cytochrome that reacts with P870+. In the Q cycle, four protons probably are pumped out of the cell for every two electrons that return to P870. This proton translocation creates an electrochemical potential gradient across the membrane. Protons move back into the cell through an ATP-synthase, driving the formation of ATP. 

Fig. 5.18 Schematic representation of the mechanism of haloperoxidases. In the presence of Cl", HOCI is formed that (a) diffuses from the active site and oxidizes substrates in the medium, although in some cases, (b) oxidation may occur within the active site. In the absence of Cl", thiol-ligated haloperoxidases can (c) catalyze oxygen transfer to their substrates in a cytochrome P450-like reaction...

Let catalytic X-sites exist on a plane, and catalytic F-sites on another plane located parallel to the first, at a distance x = L in space. For the consecutively catalyzed reaction scheme VIII, intermediate B molecules must now diffuse from a = 0 to a = L through a medium having diffusiv-ity D. 

Viscosity of the medium can also play a role in the kinetics due to the importance of diffusion in the observed rate constants. In the bulk radical polymerization of 2-phenoxyethyl methacrylate, thiol chain-transfer reagents operate at rates close to those observed for MMA while the rate of CCT catalyzed by 9a is an order of magnitude slower (2 x 103 at 60 °C) than that of MMA.5 The thiol reactions involve a chemically controlled hydrogen transfer event, whereas the reaction of methacrylate radicals with cobalt are diffusion controlled. The higher bulk viscosity of the 2-phenoxyethyl methacrylate has a significant influence on the transfer rate. 

Catalytic supercritical water oxidation is an important class of solid-catalyzed reaction that utilizes advantageous solution properties of supercritical water (dielectric constant, electrolytic conductance, dissociation constant, hydrogen bonding) as well as the superior transport properties of the supercritical medium (viscosity, heat capacity, diffusion coefficient, and density). The most commonly encountered oxidation reaction carried out in supercritical water is the oxidation of alcohols, acetic acid, ammonia, benzene, benzoic acid, butanol, chlorophenol, dichlorobenzene, phenol, 2-propanol (catalyzed by metal oxide catalysts such as CuO/ZnO, Ti02, MnOz, KMn04, V2O5, and Cr203), 2,4-dichlorophenol, methyl ethyl ketone, and pyridine (catalyzed by supported noble metal catalysts such as supported platinum). ... 

Because of their tunable properties, supercritical solvents provide a useful medium for enzyme-catalyzed reactions.f The mechanism of enzyme-catalyzed reactions is similar to the mechanism described for solid-catalyzed reactions. External as well as internal transport effects may limit the reaction rate. Utilizing supercritical fluids enhances external transport rate due to increase in the diffusivity and therefore mass transfer coefficient. Internal transport rate depends on the fluid medium as well as the morphology of the enzyme. Supercritical fluids can alter both. 

P. B, Weisz (Socony Mobil Oil Co.) In connection with the question of how the two types of activity centers collaborate, Dr. Mills has already stated that physical mixtures of considerable particle size will successfully catalyze the reaction. We have recently shown that successful cooperation of the two types of centers can be had through the medium of ordinary gaseous diffusion as a transport mechanism for the molecules of intermediates (1). For example, we find that a typical, useful reaction rate can be supported through an intermediate species existing at a vapor pressure of 10 atm. if the two catalytic functions are as far as 100 A. apart, or through an intermediate at a partial pressure of 10 atm. for distances as large as about 50y. The latter partial pressure represents, in fact, the magnitude of the thermodynamically attainable concentration of some of the olefins in hydroisomerization reactions. 

Some of the first considerations of the problem of diffusion and reaction in porous catalysts were reported independently by Thiele [E.W. Thiele, Ind. Eng. Chem., 31, 916 (1939)] Damkohler [G. Damkohler, Der Chemie-Ingenieur, 3, 430 (1937)] and Zeldovich [Ya.B. Zeldovich, Acta Phys.-Chim. USSR, 10, 583 (1939)] although the first solution to the mathematical problem was given by Jiittner in 1909 [F. Jiittner, Z. Phys. Chem., 65, 595 (1909)]. Consider the porous catalyst in the form of a flat slab of semi-infinite dimension on the surface, and of half-thickness W as shown in Figure 7.3. The first-order, irreversible reaction A B is catalyzed within the porous matrix with an intrinsic rate (—r). We assume that the mass-transport process is in one direction though the porous structure and may be represented by a normal diffusion-type expression, that there is no net eonveetive transport eontribution, and that the medium is isotropic. For this case, a steady-state mass balance over the differential volume element dz (for unit surface area) (Figure 7.3), yields... 

In order to eliminate the possibility for in situ carbene formation Raubenheimer et al. synthesized l-alkyl-2,3-dimethylimidazolium triflate ionic liquids and applied these as solvents in the rhodium catalyzed hydroformylation of l-hejEne and 1-dodecene [178]. Both, the classical Wilkinson type complex [RhCl(TPP)3] and the chiral, stereochemically pure complex (—)-(j7 -cycloocta-l,5-diene)-(2-menthyl-4,7-dimethylindenyl)rhodium(i) were applied. The Wilkinson catalyst showed low selectivity towards n-aldehydes whereas the chiral catalyst formed branched aldehydes predominantly. Hydrogenation was significant with up to 44% alkanes being formed and also a significant activity for olefin isomerization was observed. Additionally, hydroformylation was found to be slower in the ionic liquid than in toluene. Some of the findings were attributed by the authors to the lower gas solubility in the ionic liquid and the slower diffusion of the reactive gases H2 and CO into the ionic medium. 

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