Catalytic properties kinetic parameters

Table 6.18.7 gives material properties, kinetic parameters, and typical reaction conditions of catalytic NO reduction with ammonia on a monolithic Ti-V-W catalyst. 

The steady state experiments showed that the two separate phases and the mixture are not very different in activity, give approximately the same product distributions, and have similar kinetic parameters. The reaction is about. 5 order in methanol, nearly zero order in oxygen, and has an apparent activation energy of 18-20 kcal/mol. These kinetic parameters are similar to those previously reported (9,10), but often ferric molybdate was regcirded to be the major catalytically active phase, with the excess molybdenum trioxide serving for mechanical properties and increased surface area (10,11,12). 

Determination of the thermodynamic and kinetic parameters of interest requires monitoring of the surface concentration of the binding molecule. With large biomolecules, the surface concentrations are small, and simple redox labeling will not allow sufficient sensitivity. Labeling of the target biomolecule with a redox enzyme obviates this difficulty, thanks to the catalytic properties of the enzyme. 

The input parameters for the model are the thermodynamics of the gas phase, chemisorption energy and spectroscopic properties for the intermediates, the kinetic parameters for the rate limiting step and the number of active sites on the catalyst. No reference to experimental data for catalytic reaction rates are made in the determination of the input parameters. 

Before an immobilized enzyme can be used for an industrial process, it is essential to characterize it in terms of its catalytic and kinetic properties. A quantitative assay must be developed to measure the activity, kinetic parameters, and stability of the enzyme. In a coupling reaction, H202 rapidly reacts with phenol and 4-aminoantipyrine (electron donor) in the presence of peroxidase to produce a quinoneimine chromogen (Equation E12.2, Figure El 1.2), which is intensely colored with a maximum absorbance at 510 nm. (This is the same as the product formed in the analysis of cholesterol in Experiment 11.)... 

For linear mechanisms we have obtained structurized forms of steady-state kinetic equations (Chap. 4). These forms make possible a rapid derivation of steady-state kinetic equations on the basis of a reaction scheme without laborious intermediate calculations. The advantage of these forms is, however, not so much in the simplicity of derivation as in the fact that, on their basis, various physico-chemical conclusions can be drawn, in particular those concerning the relation between the characteristics of detailed mechanisms and the observable kinetic parameters. An interesting and important property of the structurized forms is that they vividly show in what way a complex chemical reaction is assembled from simple ones. Thus, for a single-route linear mechanism, the numerator of a steady-state kinetic equation always corresponds to the kinetic law of the overall reaction as if it were simple and obeyed the law of mass action. This type of numerator is absolutely independent of the number of steps (a thousand, a million) involved in a single-route mechanism. The denominator, however, characterizes the "non-elementary character accounting for the retardation of the complex catalytic reaction by the initial substances and products. 

In summary, it can be seen for the three-step reaction scheme of this example that the net rate of the overall reaction is controlled by three kinetic parameters, KTSi, that depend only on the properties of the transition states for the elementary steps relative to the reactants (and possibly the products) of the overall reaction. The reaction scheme is represented by six individual rate constants /c, and /c the product of which must give the equilibrium constant for the overall reaction. However, it is not necessary to determine values for five linearly independent rate constants to determine the rate of the overall reaction. We conclude that the maximum number of kinetic parameters needed to determine the net rate of overall reaction is equal to the number of transition states in the reaction scheme (equal to three in the current case) since each kinetic parameter is related to a quasi-equilibrium constant for the formation of each transition state from the reactants and/or products of the overall reaction. To calculate rates of heterogeneous catalytic reactions, an addition kinetic parameter is required for each surface species that is abundant on the catalyst surface. Specifically, the net rate of the overall reaction is determined by the intrinsic kinetic parameters Kf s as well as by the fraction of the surface sites, 0, available for formation of the transition states furthermore, the value of o. is determined by the extent of site blocking by abundant surface species. 

Much has been written about solid metal electrodes, which have now largely displaced liquid mercury. Those most often used as redox ( inert ) electrodes for studying electron transfer kinetics and mechanism, and determining thermodynamic parameters are platinum, gold, and silver. However, it should be remembered that their inertness is relative at certain values of applied potential bonds are formed between the metal and oxygen or hydrogen in aqueous and some non-aqueous solutions. Platinum also exhibits catalytic properties. 

If one were to describe the essence of electrode kinetics in one short phrase, it would be the transition from electronic to ionic conduction, and the phenomena associated with and controlling this process. Conduction in the solution is ionic, whereas in the electrodes and the connecting wires it is electronic. The transition from one mode of conduction to the other requires charge transfer across the interfaces. This is a kinetic process. Its rate is controlled by the catalytic properties of the surface, the chemisorption of species, the concentration and the nature of the reacting species and all other parameters that control the rate of heterogeneous chemical reactions. 

Table 3.1 Catalytic properties and kinetic parameters of selective hydrogenation of acetylene alcohols with micellar catalysts based on PS-b-P4VPl l... 

As opposite to parameter K (or Keq) and kcat, V is not a fundamental property of the enzyme since it depends on its concentration as indicated by Eq. 3.5. This has to be taken into consideration when determining the kinetic parameters. The catalytic rate constant (kcat) is a fundamental property of the enzyme that can be expressed in different ways and in different units, according to how e is expressed (moles gL UL ). If e is expressed in moles L , kcat has dimension of T (known as turnover number). This requires the knowledge of the molecular weight and the specific activity and number of active centers of the enzyme. Sometimes this information is not available so that kcat is expressed in dimensions of M T (mass of substrate converted per unit time and unit of enzyme activity). If U is expressed in international units (lU), then kcat reduces to a dimensionless value of 1, which is to say that it is equivalent to V. 

Because of the numerous influencing parameters, prediction of the catalytic properties is not possible. They can only be determnined by measurement of the reaction kinetics. This makes it clear why catalyst production is based on special company know-how and that not all details are publicized. 

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