Catalysts kinetic method

Plot of equation 13.18 showing limits for which a chemical kinetic method of analysis can be used to determine the concentration of a catalyst or a substrate. 

Noncatalytic Reactions Chemical kinetic methods are not as common for the quantitative analysis of analytes in noncatalytic reactions. Because they lack the enhancement of reaction rate obtained when using a catalyst, noncatalytic methods generally are not used for the determination of analytes at low concentrations. Noncatalytic methods for analyzing inorganic analytes are usually based on a com-plexation reaction. One example was outlined in Example 13.4, in which the concentration of aluminum in serum was determined by the initial rate of formation of its complex with 2-hydroxy-1-naphthaldehyde p-methoxybenzoyl-hydrazone. ° The greatest number of noncatalytic methods, however, are for the quantitative analysis of organic analytes. For example, the insecticide methyl parathion has been determined by measuring its rate of hydrolysis in alkaline solutions. 

Kinetic methods. These methods of quantitative analysis are based upon the fact that the speed of a given chemical reaction may frequently be increased by the addition of a small amount of a catalyst, and within limits, the rate of the catalysed reaction will be governed by the amount of catalyst present. If a calibration curve is prepared showing variation of reaction rate with amount of catalyst used, then measurement of reaction rate will make it possible to determine how much catalyst has been added in a certain instance. This provides a sensitive method for determining sub-microgram amounts of appropriate substances. 

In the results presented in Table 13.5, the addition of tin affects the kinetic selectivity r differently, depending on the catalyst preparation method. When compared to the monometallic PdO catalyst, r slightly decreases for the coimpregnated PdSn catalyst, but it sharply increases for the PdOSn catalyst prepared via the colloidal oxide synthesis. As the intrinsic kinetic constant rates k do not show significant discrepancies between the different catalysts, the main contribution of the variation of the kinetic selectivity is ascribed to the adsorption constant ratio fBo/ Butenes- In the case of the PdOSn catalyst, formation of but-l-ene is favored compared to its consumption because the X Bo/ Butenes ratio increases, indicating that olefin adsorption is much more destabilized than diene adsorption. Thus, the olefin easily desorbs before being hydrogenated into butane. 

The apparatus s step change from ambient to desired reaction conditions eliminates transport effects between catalyst surface and gas phase reactants. Using catalytic reactors that are already used in industry enables easy transfer from the shock tube to a ffow reactor for practical performance evaluation and scale up. Moreover, it has capability to conduct temperature- and pressure-jump relaxation experiments, making this technique useful in studying reactions that operate near equilibrium. Currently there is no known experimental, gas-solid chemical kinetic method that can achieve this. 

Catalymetry — A kinetic method for chemical analysis in which the -> analyte is the - catalyst whose concentration is determined from its effect upon the - reaction rate data of the - indicator. Thus, conditions are provided in such a way that the concentration of the... 

The same sequence is applicable to ketones, but in this case TiCU is superior to ZnBr2 as the catalyst. The method can be used with both cyclic and acyclic ketones and is applicable to both kinetic and thermodynamic silyl enol ethers for control of regiospecificity. ... 

Attempts to determine the number of active centers in Ti complex/MgCl2 catalysts have been made using the chemical or radiochemical and kinetic methods commonly employed in Ziegler-Natta catalysis. 

The kinetic method consists of determining the number of polymer chains N as a function of polymerization time or catalyst yield Y. Extrapolation of N to time or yield zero provides the number of active centers according to the following types of equations 87,136) ... 

From the above it becomes evident that the kinetic methods only make it possible to determine the number of initial propagation centers which, in the case of supported catalyst is often coincident with their maximum values. On the other hand, since the chemical methods can be applied at any moment during the reaction, they make it possible to follow the evolution of the number of the active centers with time. 

Many kinetic methods of analysis involve reactions whose rates depend on catalysts in solution (Section 15-7) most of these involve redox systems. A catalyst may be defined broadly as an agent that alters the rate of a reaction without shifting the position of equilibrium. The catalyst itself imdergoes no permanent change, although it may enter the reaction mechanism in a cyclic manner. The mechanisms and activation of catalytic reactions have been reviewed. ... 

An assessment of the methods for determining active centre concentrations has been made by Schnecko and Kern [213], and they conclude that radioactive assay is more reliable than indirect kinetic methods. Propene and butene-1 polymerized by TiClj/AlEtjCl on termination with iodine or BuO H gave comparable extrapolated values of ca. 0.5% for catalyst efficiency. This is lower than earlier estimates but higher than the very low values (0.1%) obtained by Coover and Guillet [121], although the shapes of the curves obtained by the two groups were very similar. 

Kinetic methods greatly extend the number of chemical reactions that can be used for analytical purposes because they permit the use of reactions that are too slow or too incomplete for thermodynamic-based procedures. Kinetic methods can be based on complexation reactions, acid-base reactions, redox reactions, and others. Many kinetic methods are based on catalyzed reactions. In one type of catalytic method, the analyte is the catalyst and is determined from its effect on an... 

Many inorganic cations and anions catalyze indicator reactions—that is, reactions whose rates are readily measured by instmmental methods, such as absorption spectrophotometry, fluorescence spectrometry, or electrochemistry. Conditions are then employed such that the rate is proportional to the concentration of catalyst, and, from the rate data, the concentration of catalyst is determined. Such catalytic methods often allow extremely sensitive detection of the catalyst concentration. Kinetic methods based on catalysis by inorganic analytes are widely applicable. For example, the literature in this area lists more than 40 cations and 15 anions that have been determined by a variety of indicator reactions. Table 29-3 gives catalytic methods for several inorganic species along with the indicator reactions used, the method of detection, and the detection limit. 

Kinetic methods are necessary to determine enzyme activities since the enzyme is a catalyst and affects only the reaction rate. 

As noted earlier, kinetic methods based on uncatalyzed reactions are not nearly as widely used as those in which a catalyst is involved. We have already described two of these methods (pages 896 and 898). 

In this chapter we shall consider first the general characteristics of heterogeneous catalysis and adsdilptiQnlCphy and chemic and then physical properties of solid catalysts and methods of preparation. Kinetics... 

The N2O decomposition resulted in N2 and O2 with 2 1 molar ratio. The reaction is practically independent both on the catalyst preparation method and on the pretreatment process. The measured kinetic curves can be seen on Figures 7 and 8. These results are characteristic for the oxidized catalysts, and practically no differences could be observed using reduced catalysts. 

To apply the equations of the multiplet theory one should know the bond energies with the catalysts, Qak The bond energies may be found experimentally by thermochemical, comparative, and kinetic methods. 

The Kinetic Method of Determining Bond Energies with a Catalyst... 

The kinetic method of determining the bond energies of reacting atoms with the surface of a heterogeneous catalyst was suggested by the author... 

Another kinetic method has also been developed (395-400). This variant is also based on the application of Eqs. (II.9) and (11.12) of the multiplet theory and it is best applied to metallic catalysts. Here the matrix of the coefficients is not square but triangular. The procedure consists in determination of, first of all, Q h k for the reaction of para-ortho conversion of H2 whose index contains only one type of atom. 

---