Tools for the Kinetics of Fast Reactions

Gregory Biausque, Marie Rochoux, David Farrusseng, and Yves Schuurman 

The design of a catalytic reactor requires the knowledge of the reaction rate and product selectivity as a function of the operating conditions. Rate expression based on the intrinsic reaction kinetics allows scaling up of laboratory data to a pilot plant and further to an industrial unit Without reliable kinetics, optimimi reactor design can be difficult or even impossible to achieve. 

This chapter has been divided into two parts. The first part describes techniques suited to study the interaction of oxygen with perovskites. The second part describes tools adapted for the kinetic study of fast reactions under steady-state and transient conditions. 

Perovskites are used extensively for oxidation reactions and oxygen-conducting materials. The detailed interaction of oxygen with these materials is therefore important and has received enormous attention in the literature [1-3]. Two, very closely related, aspects are generally addressed. The first, the thermodynamics of 

Perovskites and Related Mixed Oxides Concepts and Applications, First Edition. 

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To learn that the rotated ring-disc electrode (RRDE) is one of the most powerful analytical tools for following the kinetics of fast homogeneous reactions. 

Ultramicroelectrodes are excellent tools also for the study of the kinetics of fast follow-up reactions, provided that the high sweep rates needed do not bring the electron transfer process into the quasi-reversible region. Thus, a reaction that during a study with con-... 

Pulse radiolysis is the radiation chemical analogue of flash photolysis. It is a fast-kinetics technique that enables transitory processes, initiated by the absorption of ionizing radiation, to be observed in time frames as short as the submicrosecond region. It permits the detection and characterization of short-lived intermediates, the determination of the kinetics of their decay, and a probing of reaction mechanisms. The technique finds use in the study of radiation effects on materials, and as a tool for the examination of mechanistic details. For inorganic systems, pulse radiolysis is used to characterize metal complexes in unusual oxidation states, to examine the kinetics and rates of ligand-labilization reactions and to elucidate the mechanism of electron transfer. 

On-line MS methods enable continuous kinetic profiles to be obtained but they cannot easily accommodate complex sample preparation steps. In the 1980s, enzymatic reactions were monitored by a popular - at that time - ionization technique, namely fast atom bombardment (FAB)-MS [12, 13]. Heidmann etal. [14] used FAB-MS to identify conjugation products of reactive quinones with glutathione by conducting dynamic mass spectral analysis. Soon after the introduction of ESI to MS, its potential in the monitoring of biochemical reactions was recognized, especially in the detection of labile intermediates (cf. [15,16]). Nowadays ESI and MALDI are prime tools for the analysis of biomolecules. Both techniques are also suitable for the investigation of biocatalytic processes with diverse temporal resolutions [17]. 

Thus, complex chemical processes are represented as a number of simple reactions that are very inhomogeneous on a time scale. Generally, it is impossible to separate the fast processes and the slow ones from each other, so that a continuous time monitoring of the total kinetic process is needed to understand the essence of the phenomenon. Mathematical models provide an adequate tool for the scanning of the kinetic curves. Fig. 1(a) shows a typical example of curves where two time scales are present. These time scales differ up to an order of 10 from each other. If one considers the process on the logarithmic scale, then just three different time scales may be identified, see Fig. 1(b). The presence of both fast and slow variables is explained by the occurrence of either large or small factors in the dynamical equations. For example, this is the case for so-called stiff systems of differential equations. 

An important tool for the fast characterization of intermediates and products in solution-phase synthesis are vibrational spectroscopic techniques such as Fourier transform infrared (FTIR) or Raman spectroscopy. These concepts have also been successfully applied to solid-phase organic chemistry. A single bead often suffices to acquire vibrational spectra that allow for qualitative and quantitative analysis of reaction products,3 reaction kinetics,4 or for decoding combinatorial libraries.5... 

SECM is a powerful tool for studying structures and heterogeneous processes on the micrometer and nanometer scale [8], It can probe electron, ion, and molecule transfers, and other reactions at solid-liquid, liquid-liquid, and liquid-air interfaces [9]. This versatility allows for the investigation of a wide variety of processes, from metal corrosion to adsorption to membrane transport, as discussed below. Other physicochemical applications of this method include measurements of fast homogeneous kinetics in solution and electrocatalytic processes, and characterization of redox processes in biological cells. 

Dorfman and collaborators have recently developped a very promising technique for the production of carbenium ions as transient species in halocarbon sdvents based on the dissociative ionisation of suitable precursors induced by pulse radiolysis of the solvent. While the extremely interesting kinetic results vdiich this group is obtaining will be discussed in Sect. II-G4, it is emphasised here that the fast time response of the apparatus used allows the characterisation of carbenium ions hitherto unobservable because of their excessive reactivity. The ultraviolet absorption spectrum and some reactions of the benzylium ion have been studied for the first time wdth this powerful tool. From the point of view of cationic pdymerisation, the information obtained in this type of work is particularly relevant, since it deals vrith the identification and reactivity of carbenium icais formed in very low concentration in the nght kind of medium. Cation radicals had already been prepared by pulse radiolysis involving nondissociative ionization (electron ejection or transfer), as will be discussed in Sect. II-K. 

The kinetics of product evolution in a typical reaction of adamantane hydroxylation showed an initial induction period followed by a fast, apparently zero-order phase with the maximum rate and highest efficiencies (Fig. 2). Deviation from linear behavior took place only after 90% oxygen donor and 80% of the substrate had been consumed. When Ru (TPFPP)(0)2, prepared by reaction of Ru"(TPFPP)(CO) with 3-chloroperbenzoic acid was used as the catalyst, no induction time was detected and zero-order kinetics were observed as well. The well defined and characteristic UV-vls spectra of metalloporphyrins provide an invaluable tool for the mechanistic studies. Thus, monitoring the state of the metalloporphyrin catalysts during the course of both model reactions by UV-vis spectroscopy revealed that the initial form of the catalyst remained the predominant one throughout the oxidation, i.e. in the Ru°(TPFPP)(CO) catalyzed reaction c.a. 80% of the porphyrin catalyst existed as Ru"(TPFPP)(CO) and in Ru (TPFPP)(0)2 catalyzed reaction more than 90% of... 

Microfluidics is promising for developing tools with integrated chenucal processing of proteomics analytes, by virtue of fast reaction kinetics. Sample purification and concentration can be carried out on the microscale by methods such as solid-phase extraction on hydrophobic... 

The rate at which a reaction proceeds is governed by the principles of chemical kinetics, which is one of the major topics of this book. Chenucal kinetics allows us to understand how reaction rates depend on variables such as concentration, temperature, and pressure. Kinetics provides a basis for manipulating these variables to increase the rate of a desired reaction, and minimize the rates of undesired reactions. We will study kinetics first from a rather empirical standpoint, and lat from a more fundamental point of view, one that creates a link with the details of the reaction chenustry. Catalysis is an extremely important tool within the domain of chemical kinetics. For example, catalysts are required to ensure that blood clots form fast enough to fight serious blood loss. Approximately 90% of the chemical processes that are carried out industrially involve the use of some kind of catalyst in order to increase the rate(s) of the desired reaction(s). Unfortunately, the behavior of heterogeneous catalysts can be significantly and negatively influenced by the rates of heat and mass transfer to and from the sites in the catalyst whrae the reaction occurs. We will approach the interactions between catalytic kinetics and heat and mass transport conceptually and qualitatively at first, and then take them head-on later in the book. 

This picture leads to a tool, called the rate-limiting step (RLS) approximation, that is very useful in chemical kinetics. The figure suggests that Reactions 1 (5-H) and 3 (5-J) are essentially in chemical equilibrium because the rates of the forward and reverse reactions are almost equal. The equilibrium expressions for these fast reactions then can be used to solve for the concentrations of the active centers, instead of using the more cumbersome SSA. The rate of the overall reaction can be written in terms of the slow reaction, which is referred to as the rate-limiting step (or rate-determining step or rate-controlling step). 

As these examples have demonstrated, in particular for fast reactions, chemical kinetics can only be appropriately described if one takes into account dynamic effects, though in practice it may prove extremely difficult to separate and identify different phenomena. It seems that more experiments under systematically controlled variation of solvent enviromnent parameters are needed, in conjunction with numerical simulations that as closely as possible mimic the experimental conditions to improve our understanding of condensed-phase reaction kmetics. The theoretical tools that are available to do so are covered in more depth in other chapters of this encyclopedia and also in comprehensive reviews [6, 118. 119],... 

In analyzing the behavior of these types of tetrahedral intermediates, it should be kept in mind that proton-transfer reactions are usually fast relative to other steps. This circumstance permits the possibility that a minor species in equilibrium with the major species may be the major intermediate. Detailed studies of kinetics, solvent isotope effects, and the nature of catalysis are the best tools for investigating the various possibilities. 

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