Catalytic kinetic measurements

The development of methods for the kinetic measurement of heterogeneous catalytic reactions has enabled workers to obtain rate data of a great number of reactions [for a review, see (1, )]. The use of a statistical treatment of kinetic data and of computers [cf. (3-7) ] renders it possible to estimate objectively the suitability of kinetic models as well as to determine relatively accurate values of the constants of rate equations. Nevertheless, even these improvements allow the interpretation of kinetic results from the point of view of reaction mechanisms only within certain limits ... 

Various investigators have tried to obtain information concerning the reaction mechanism from kinetic studies. However, as is often the case in catalytic studies, the reproducibility of the kinetic measurements proved to be poor. A poor reproducibility can be caused by many factors, including sensitivity of the catalyst to traces of poisons in the reactants and dependence of the catalytic activity on storage conditions, activation procedures, and previous experimental use. Moreover, the activity of the catalyst may not be constant in time because of an induction period or of catalyst decay. Hence, it is often impossible to obtain a catalyst with a constant, reproducible activity and, therefore, kinetic data must be evaluated carefully. 

Gamer and Jennings [431] studied nucleation during the dehydration of potassium and ammonium chromium alums. Detailed kinetic measurements were made for the relatively enhanced rate of nucleation which followed admission of water vapour to the solid after a period of vacuum nucleation. This catalytic effect of water vapour is ascribed to its participation in the reorganization of the lattice which had collapsed during previous treatment in vacuum. 

A typical electrochemical promotion experiment includes kinetic measurements under open and closed circuit conditions as well as study of the effect of catalyst potential or work function on catalytic rate and selectivity under steady state and transient conditions. In kinetic measurements one should change the partial pressure of each reactant while... 

Much of the pioneering work which led to the discovery of efficient catalysts for modern Industrial catalytic processes was performed at a time when advanced analytical Instrumentation was not available. Insights Into catalytic phenomena were achieved through gas adsorption, molecular reaction probes, and macroscopic kinetic measurements. Although Sabatier postulated the existence of unstable reaction Intermediates at the turn of this century. It was not until the 1950 s that such species were actually observed on solid surfaces by Elschens and co-workers (2.) using Infrared spectroscopy. Today, scientists have the luxury of using a multitude of sophisticated surface analytical techniques to study catalytic phenomena on a molecular level. Nevertheless, kinetic measurements using chemically specific probe molecules are still the... 

Kinetic measurements are essential for the elucidation of any catalytic mechanism since catalysis, by definition and significance, is purely a kinetic phenomenon [6],... 

Most of the techniques discussed above are typically used ex situ for catalyst characterization before and after reaction. This is normally the easiest way to carry out the experiments, and is often sufficient to acquire the required information. However, it is known that the reaction environment plays an important role in determining the structure and properties of working catalysts. Consequently, it is desirable to also try to perform catalytic studies under realistic conditions, either in situ [113,114,157, 191-193] or in the so-called operando mode, with simultaneous kinetics measurements [194-196], In addition, advances in high-throughput (also known as combinatorial) catalysis call for the fast and simultaneous analysis of a large number of catalytic samples [197,198], This represents a new direction for further research. 

Inhibition of a catalytic reaction by impurities present may take place and sometimes this may have a temporary character. If it is permanent one cannot be mistaken in the kinetic measurements. Impurities that are more reactive than the substrates to be studied may block the catalyst if they react according to a scheme like that of Figure 3.7. Only after all inhibitor has been converted the conversion of the desired substrate can start. Another type of deactivation that may occur is the formation of dormant states, which is very similar to inhibition. Either the regular substrate or an impurity may lead to the formation of a stable intermediate metal complex that does not react further. There are examples where such intermediates can be rescued from this dormant state for instance by the addition of another reagent such as dihydrogen (Chapter 10, dormant states in propene polymerisation). 

Catalyst decomposition ( die-out ) during the catalytic reaction is a common phenomenon also distorting the kinetic measurements. If the decomposition reaction obeys a rate equation in a well-behaved manner, one can include the decomposition reaction in the kinetics, but usually one will prefer the use of a stable catalyst. Catalyst decomposition is an import issue in applied catalysis although it has received relatively little attention in literature as far as homogeneous catalysis is concerned [5],... 

Which has been studied on supported Ni catalysts and on Ni films . Studies such as those described here show that methane can be catalytically synthesized over Ni by an active (carbidic) carbon formed via the Boudouard reaction and its subsequent hydrogenation to methane. However, to demonstrate that this surface carbon route is the major reaction pathway, kinetic measurements of both carbon formation from CO and its removal by H2 were carried out . 

In studies of reaction mechanisms, the necessary experimental information is, in general, obtained from kinetic measurements and from nonkinetic exploration of the reaction course, intermediates, and products. With non-catalytic homogeneous reactions, the kinetic evidence usually plays an important role, and the nonkinetic results often serve only for support, independent confirmation, and elucidation of finer points. However, the... 

For the rational design of transition metal catalyzed reactions, as well as for fine-tuning, it is vital to know about the catalytic mechanism in as much detail as possible. Apart from kinetic measurements, the only way to learn about mechanistic details is direct spectroscopic observation of reactive intermediates. In this chapter, we have demonstrated that NMR spectroscopy is an invaluable tool in this respect. In combination with other physicochemical effects (such as parahydrogen induced nuclear polarization) even reactive intermediates, which are present at only very low concentrations, can be observed and fully characterized. Therefore, it might be worthwhile not only to apply standard experiments, but to go and exploit some of the more exotic techniques that are now available and ready to use. The successful story of homogeneous hydrogenation with rhodium catalysts demonstrates impressively that this really might be worth the effort. 

Figure 12.10 Examples of structured catalytic reactors for kinetic measurements (a) annular reactor [47, 61] (b) plate cell reactor [75].

The experimental strategy in studying catalytic kinetics usually involves measuring the extent of conversion of gas passing in steady flow through a batch of solids. Any flow pattern can be used, as long as the pattern selected is known if it is not known then the kinetics cannot be found. A batch reactor can also be used. In turn we discuss the following experimental devices ... 

As noted by Carberry in 1987, only phenomenological values can be measured in the laboratory since it is not possible to a priori distinguish between A (the catalytic area) and A (exposed measurable area), per volume of catalyst agent. This yields a structure-sensitive reaction that is dependent on crystallite size. While it is clear that a mechanism cannot be determined from purely kinetic measurements, a proposed mechanism is only accepted after it can predict the observed kinetic measurements. The dominant issue of the observed measurements is whether A or A is being measured. This correct measurement will yield the proper intrinsic kinetics, but will not reveal much insight into the mechanism. Thus, it is imperative to identify and obtain as much information as possible on the nature of intermediate chemical species. 

A similar catalytic effect of PbS on the decomposition of thiourea had been suggested previously by Norr [47]. Kinetic measurements by Rieke and Bentjen suggested that CdS likewise catalyzed thiourea decomposition [37], Ortega-Borges and Lincot also deduced such a mechanism based on kinetic measurements of the CdS deposition using a quartz crystal microbalance [48], In this case, the measurements were found to fit best with a complex-decomposition model. Both they and Rieke and Bentjen found optimum deposition to occur under conditions where Cd(OH)2 was formed as a surface species on the substrate but not in the bulk of the solution. Kinetic measurements also led Dona and Herrero to a similar conclusion of a complex-decomposition mechanism, but with the main difference that the initial adsorbed species is not Cd(OH)2 itself but an ammine-hydroxide [49] ... 

The kinetic measurements reported in the following sections are concerned with the polymerization of propylene the results obtained with this monomer can, however, be extended to other olefins (e.g., normal butene-1, pentene-1, or branched). For this reason, although we limit ourselves to recording measurements made with one monomer only and with two types of catalytic system, we have given the most general title to this paper. 

Both these difficulties depend on changes in the catalytic surface of the electrode, and such changes take time. The idea of making electrode kinetic measurements at short times ( transients ) had been introduced by Bowden and Rideal in 1928, but their aim was less to overcome undesired surface changes and more to make use of... 

In terms of catalytic kinetics, the implications of the dynamic changes in catalyst morphology during methanol synthesis are dramatic. Figure 16a shows the agreement between the predictions of a static microkinetic model and the measured rates of methanol synthesis catalyzed by Cu/ZnO/A1203... 

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