Carrier transport catalyst

Because of its cyclic nature, this process presents analogies with molecular catalysis it may be considered as physical catalysis operating a change in location, a translocation, on the substrate, like chemical catalysis operates a transformation into products. The carrier is the transport catalyst which strongly increases the rate of passage of the substrate with respect to free diffusion and shows enzyme-like features (saturation kinetics, competition and inhibition phenomena, etc.). The active species is the carrier-substrate supermolecule. The transport of substrate Sj may be coupled to the flow of a second species S2 in the same (symport) or opposite antiport) direction. 

Let us first consider the catalyst/polyolefin particle in the early stage of its evolution. The particle consists of the solid catalyst carrier with catalyst sites immobilized on its surface, polymer phase, and pores. The first-principle-based meso-scopic model of particle evolution has to be capable of describing the formation of polymer at catalyst sites, transport of monomer(s) and other re-actants/diluents through the polymer and pore space, and deformation of the polymer and catalyst carrier (including its fragmentation). Similar discrete element modeling techniques have been applied previously to different problems (Heyes et al., 2004 Mikami et al., 1998 Tsuji et al., 1993). 

Figures 12 and 13 show the effects of CO2 feed partial pressures, pc02 on Rc02 and a for dry and water-containing membranes, respectively. In both cases, as the CO2 feed pressure increased, Rco2 decreased while Rn2 was nearly constant. The decrease in Rc02 observed for both dry and water-containing membranes suggests that CO2 permeates by the carrier transport mechanism in both conditions. However, the mechanism may be different for the two cases. In the dry membranes, the facilitated transport of CO2 is expected to be attributable to the weak acid-base interaction between CO2 and amine moiety, as suggested by Yoshikawa et al. (27). Therefore, the dry membrane is a fixed carrier membrane. On the other hand, tertiaiy amine groups in the wet membrane are considered to act as catalyst for the hydration of CO2 as in the case of triethanolamine in a supported liquid membrane (28). The mechanism is schematically represented as follows ...

The platinum concentrations in the platinized carbon blacks are reported to be between 10 and 40% (by mass), sometimes even higher. At low concentrations the specific surface area of the platinum on carbon is as high as lOOm /g, whereas unsupported disperse platinum has surface areas not higher than 10 to 15m /g. However, at low platinum concentrations, thicker catalyst layers must be applied, which makes reactant transport to reaction sites more difficult. The degree of dispersion and catalytic activity of the platinum depend not only on its concentration on the carrier but also on the chemical or electrochemical method used to deposit it. 

ORR catalysis by Fe or Co porphyrins in Nation [Shi and Anson, 1990 Anson et al., 1985 Buttry and Anson, 1984], polyp5rrolidone [Wan et al., 1984], a surfactant [Shi et al., 1995] or lipid films [CoUman and Boulatov, 2002] on electrode surfaces has been studied. The major advantages of diluting a metalloporphyrin in an inert film include the abUity to study the catalytic properties of isolated molecules and the potentially higher surface loading of the catalyst without mass transport Umit-ations. StabUity of catalysts may also improve upon incorporating them into a polymer. However, this setup requires that the catalyst have a reasonable mobUity in the matrix, and/or that a mobile electron carrier be incorporated in the film [Andrieux and Saveant, 1992]. The latter limits the accessible electrochemical potentials to that of the electron carrier. 

Electron transport in electrode coatings containing redox centers is a necessary ingredient of their functioning as a catalytic device. They indeed serve as an electron shuttle between the electrode and the catalyst present inside the film. As discussed in the next section, the same molecule may play the role of catalyst and of electron carrier, since as shown earlier, redox catalysis is possible in these multilayered coatings. They may also be different, as exemplified is Section 4.3.6. 

The catalyst activity depends not only on the chemical composition but also on the diffusion properties of the catalyst material and on the size and shape of the catalyst pellets because transport limitations through the gas boundary layer around the pellets and through the porous material reduce the overall reaction rate. The influence of gas film restrictions, which depends on the pellet size and gas velocity, is usually low in sulphuric acid converters. The effective diffusivity in the catalyst depends on the porosity, the pore size distribution, and the tortuosity of the pore system. It may be improved in the design of the carrier by e.g. increasing the porosity or the pore size, but usually such improvements will also lead to a reduction of mechanical strength. The effect of transport restrictions is normally expressed as an effectiveness factor q defined as the ratio between observed reaction rate for a catalyst pellet and the intrinsic reaction rate, i.e. the hypothetical reaction rate if bulk or surface conditions (temperature, pressure, concentrations) prevailed throughout the pellet [11], For particles with the same intrinsic reaction rate and the same pore system, the surface effectiveness factor only depends on an equivalent particle diameter given by... 

Transporters, particularly those carrying nonlipophilic species across biomembranes or model membranes, can be regarded as vectorial catalysts (and are also called carriers, translocators, permeases, pumps, and ports [e.g., symports and antiports]). Many specialized approaches and techniques have been developed to characterize such systems. This is reflected by the fact that there are currently twenty-three volumes in the Methods in Enzymology series (vols. 21,22,52-56,81,88,96-98,125-127,156-157, 171-174, and 191-192) devoted to biomembranes and their constituent proteins. Chapters in each of these volumes will be of interest to those investigating transport kinetics. Other volumes are devoted to ion channels (207), membrane fusion techniques (220 and 221), lipids (14, 35, 71, and 72), plant cell membranes (148), and a volume on the reconstitution of intracellular transport (219). See Ion Pumps... 

If the activity of the immobilised catalyst is sufficiently high, the reaction which it mediates occurs essentially at the interface between the catalyst and the substrate solution. In the case of the surface immobilised enzyme or a thin microbial film this will, of course, occur irrespective of the level of activity. Under these conditions the limiting process for transporting substrate from the bulk of the solution to the immobilised enzyme is molecular or convective diffusion through the layer of solution immediate to the carrier. Under steady-state conditions, the rate of reaction at the active sites is equal to the rate at which substrate arrives at the site. This... 

Varying the side groups X in 27b affects both the stability and selectivity of the complexes (lateral discrimination), and allows the receptor-substrate interactions in biological systems to be modelled, for instance, the interaction between nicotinamide and tryptophan [2.109b]. One may attach to 27b amino acid residues (leading to parallel peptides [2.109] as in 27c), nucleic acid bases or nucleosides, saccharides, etc. The structural features of 27 and its remarkable binding properties make it an attractive unit for the construction of macropolycyclic multisite receptors, molecular catalysts, and carriers for membrane transport. Such extensions require sepa-... 

One may imagine extending this type of methodology to reactivity, catalysis and transport by generating suitable libraries for the discovery of novel synthetic reagents, reactions, catalysts [9.176c] and carriers as well as for the exploration of product preparation through supramolecular assistance to synthesis (see Section 9.6). 

Most drugs that are fully charged or otherwise too polar for passive diffusion cross membranes with assistance from carrier or transport proteins. Carrier proteins span the membrane and can shuttle small molecules from one side to the other. These proteins are technically catalysts because they accelerate a process (membrane crossing) without being consumed. 

Studies were carried out In a one-liter, mechanically-stirred autoclave operated In a semi-continuous fashion In that the catalyst and liquid carrier (normal-octacosane) remain In the reactor whereas synthesis gas is sparged to the reactor and volatile products removed overhead. The phases are well mixed, which simplifies Interpretation of experimental results. Moreover, the degree of mass transport can be controlled by varying the degree... 

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