Catalysts, general velocity constants

It has been mentioned above that with nonuniform catalyst surfaces, as they mostly occur in practice, the above equations give merely an upper limit of the velocity although in such catalysts the true surface would be expected to be greater than the geometrical one. In nearly all the cases where a reaction has been measured on different catalysts, it has been found that fco is by no means a universal constant as would have to be expected from the simple theory. There is a regular connection between fco and the activation energy, of the general form ... 

Deactivation parameters obtained by plotting ln[(l — a) a)] versus time are listed in Table XIX for a number of nickel and nickel bimetallic catalysts. The fact that these plots were generally linear confirms that these data are fitted well by this deactivation model. These data, which include initial site densities for sulfur adsorption, deactivation rate constants, and breakthrough times for poisoning by 1-ppm H2S at a space velocity of 3000 hr-1 provide meaningful comparisons of sulfur resistance and catalyst life for both unsupported and supported catalysts. Table XIX shows that the... 

In general, higher gas velocities, higher values of catalyst loading, lower particle diameters, larger reactor diameters, and higher reaction-rate constants make the system dynamics faster. At any point in the reactor, the liquid-phase concentration responds faster than the catalyst-surface concentration. 

The conversion of chloromethane over ZSM-5 to gasoline-range hydrocarbons occurred under conditions comparable to those for the conversion of methanol. The reaction was typically conducted at constant temperature, whereas conversions and product distributions were determined as functions of space velocity or catalyst time-on-stream. The mass-selective detector allowed identification of most of the components in the liquid samples. Generally, the products contain ten carbons or less, and a large fraction of the products are aromatic. The ZSM-5 catalyst was stable under extended exposure to chloromethanes. Figure 2 illustrates the catalyst activity after nearly 700 hours of exposure to chloromethane, during which time the catalyst had been oxidatively regenerated to remove coke that had been deposited on the catalyst. 

The reactions catalysed by enzymes are equilibrium reactions. When the ratio of substrate to product has reached a constant value, the velocities of the forward reaction and the back reaction are equal. It does not mean tliat the substrate and product are present in the same concentration. Usually one or other predominates by a factor of several hundred. An enzyme may induce a reaction that cannot be appreciably detected in its absence, but it is unable to affect the equilibrium position. All that the enzyme does is to increase the rate at which the reaction proceeds to equilibrium. It is generally considered that enzymes combine with their substrates at three or more points. There is probably a simultaneous attack on the substrate by two groups of the enzyme, one withdrawing an electron from one position, whilst the other is donating an electron to a different atom of the substrate. This would explain why enzymes are much more effective then mono-functional catalysts like acids or bases. 

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