Catalytic activity distribution

Stepwise catalytic activity distributions has also been simulated assuming the same average activity whatever the distribution. Since the converter operates under transfer control at steady state, steady state conversions are nearly independent of the distribution. However, high upstream activity is responsible for slightly earlier light-off. 

Aluminum distribution in zeolites is also important to the catalytic activity. An inbalance in charge between the silicon atoms in the zeolite framework creates active sites, which determine the predominant reactivity and selectivity of FCC catalyst. Selectivity and octane performance are correlated with unit cell size, which in turn can be correlated with the number of aluminum atoms in the zeolite framework. ... 

Tosi, C Sequence Distribution in Copolymers Numerical Tables. Vol. 5, pp. 451 to 462. Tsuchida, E. and Nishide, H. Polymer-Metal Complexes and Their Catalytic Activity. 

Diffusion effects can be expected in reactions that are very rapid. A great deal of effort has been made to shorten the diffusion path, which increases the efficiency of the catalysts. Pellets are made with all the active ingredients concentrated on a thin peripheral shell and monoliths are made with very thin washcoats containing the noble metals. In order to convert 90% of the CO from the inlet stream at a residence time of no more than 0.01 sec, one needs a first-order kinetic rate constant of about 230 sec-1. When the catalytic activity is distributed uniformly through a porous pellet of 0.15 cm radius with a diffusion coefficient of 0.01 cm2/sec, one obtains a Thiele modulus y> = 22.7. This would yield an effectiveness factor of 0.132 for a spherical geometry, and an apparent kinetic rate constant of 30.3 sec-1 (106). 

Influence of Metal Particle Size in Nickel-on-Aerosil Catalysts on Surface Site Distribution, Catalytic Activity, and Selectivity... 

A porphinatoaluminum alkoxide is reported to be a superior initiator of c-caprolactone polymerization (44,45). A living polymer with a narrow molecular weight distribution (M /Mjj = 1.08) is ob-tmned under conditions of high conversion, in part because steric hindrance at the catalyst site reduces intra- and intermolecular transesterification. Treatment with alcohols does not quench the catalytic activity although methanol serves as a coinitiator in the presence of the aluminum species. The immortal nature of the system has been demonstrated by preparation of an AB block copolymer with ethylene oxide. The order of reactivity is e-lactone > p-lactone. 

The advantages of microreactors, for example, well-defined control of the gas-liquid distributions, also hold for photocatalytic conversions. Furthermore, the distance between the light source and the catalyst is small, with the catalyst immobilized on the walls of the microchannels. It was demonstrated for the photodegradation of 4-chlorophenol in a microreactor that the reaction was truly kinetically controlled, and performed with high efficiency [32]. The latter was explained by the illuminated area, which exceeds conventional reactor types by a factor of 4-400, depending on the reactor type. Even further reduction of the distance between the light source and the catalytically active site might be possible by the use of electroluminescent materials [19]. The benefits of this concept have still to be proven. 

For many solubilized enzymes the greatest catalytic activity and/or changes in conformation are found at R < 12, namely, when the competition for the water in the system between surfactant head groups and biopolymers is strong. This emphasizes the importance of the hydration water surrounding the biopolymer on its reactivity and conformation [13], It has been reported that enzymes incorporated in the aqueous polar core of the reversed micelles are protected against denaturation and that the distribution of some proteins, such as chymotrypsine, ribonuclease, and cytochrome c, is well described by a Poisson distribution. The protein state and reactivity were found markedly different from those observed in bulk aqueous solution [178,179],... 

The catalytic pyrolysis of R22 over metal fluoride catalysts was studied at 923K. The catalytic activities over the prepared catalysts were compared with those of a non-catalytic reaction and the changes of product distribution with time-on-stream (TOS) were investigated. The physical mixture catalysts showed the highest selectivity and yield for TFE. It was found that the specific patterns of selectivity with TOS are probably due to the modification of catalyst surface. Product profiles suggest that the secondary reaction of intermediate CF2 with HF leads to the formation of R23. 

Schiith et al. reported that Pt particles on MCM-41 show high catalytic activity in CO oxidation [13]. The Pt particles have bimodal size distribution of 2 and 20 nm. 

Support materials are commonly u.sed in heterogeneous catalysis. Their major function is to maximize the dispersion of the active phase by providing a large surface area over which the active phase can be distributed. In this way the cataly.st material is shaped into a form suitable for use in technical reactors. Supports are not always chemically inert they can also show certain catalytic activity and often they act as a stabilizer for the actual active phase. A number of materials are u.sed as catalyst supports. Table 3.2 gives an overview. 

At this stage, it should be pointed out that modihcation of a Pt-Sn catalyst by Ru atoms increases cell performance (and hence catalytic activity with regard to ethanol electro-oxidation), but has no effect on the OCV or on product distribution [Rousseau et al., 2006]. It seems, then, that the oxidation mechanism is the same on Pt-Sn and Pt-Sn-Ru, which supports the proposition that Ru allows OH species to be produced when the anode potential is increased and noncatalytically active tin oxides are formed. 

Ir catalysts supported on binary oxides of Ti/Si and Nb/Si were prepared and essayed for the hydrogenation of a,P-unsaturated aldehydes reactions. The results of characterization revealed that monolayers of Ti/Si and Nb/Si allow a high metal distribution with a small size crystallite of Ir. The activity test indicates that the catalytic activity of these solids is dependent on the dispersion obtained and acidity of the solids. For molecules with a ring plane such as furfural and ciimamaldehyde, the adsorption mode can iirfluence the obtained products. SMSI effect (evidenced for H2 chemisorption) favors the formation of unsaturated alcohol. 

Improved Filtration Rate Filterability is an important powder catalyst physical property. Sometimes, it can become more important than the catalyst activity depending on the chemical process. When a simple reaction requires less reaction time, a slow filtration operation can slow down the whole process. From a practical point of view, an ideal catalyst not only should have good activity, but also it should have good filtration. From catalyst development point of view, one should consider the relationship between catalyst particle size and its distribution with its catalytic activity and filterability. Smaller catalyst particle size will have better activity but will generally result in slower filtration rate. A narrower particle size distribution with proper particle size will provide a better filtration rate and maintain good activity. 

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