THEORY 1 Catalytic processes

Recently there has been an increasing interest in self-oscillatory phenomena and also in formation of spatio-temporal structure, accompanied by the rapid development of theory concerning dynamics of such systems under nonlinear, nonequilibrium conditions. The discovery of model chemical reactions to produce self-oscillations and spatio-temporal structures has accelerated the studies on nonlinear dynamics in chemistry. The Belousov-Zhabotinskii(B-Z) reaction is the most famous among such types of oscillatory chemical reactions, and has been studied most frequently during the past couple of decades [1,2]. The B-Z reaction has attracted much interest from scientists with various discipline, because in this reaction, the rhythmic change between oxidation and reduction states can be easily observed in a test tube. As the reproducibility of the amplitude, period and some other experimental measures is rather high under a found condition, the mechanism of the B-Z reaction has been almost fully understood until now. The most important step in the induction of oscillations is the existence of auto-catalytic process in the reaction network. 

Firstly and primarily, it seeks to disclose the elementary (microscopic) mechanism of the catalytic act. Every heterogeneous catalytic process, like any chemical process, is based in the final reckoning on an electronic mechanism. It is the aim of the theory to elucidate this mechanism. This is necessary if the theory of catalysis is to rise above vulgar empiricism and to show how to control the activity and selectivity of catalysts, i.e., how to vary them to the required degree and in the required direction. 

Every heterogeneous catalytic process begins with the act of adsorption. Therefore, the theory of heterogeneous catalysis should proceed from the... 

Here the role of the geometrical factors in chemisorption is especially vividly expressed. These factors have been analyzed in detail by A. A. Balandin and co-workers in their papers (see, for example, ref. 18) on the multiplet theory of catalysis, in which they show their prime importance in a number of cases of the catalytic process. The electronic mechanism of chemisorption does not at all exclude these factors, but just stresses their role it retains the geometrical schemes of the multiplet theory but gives them physical content. 

Investigations on the adsorption of gases, on the rates of catalytic processes (see Rideal and Taylor, Catalysis in Theory and Practice) and on the dissociation of crystalline solids such as hydrates or carbonates have revealed another interesting phenomenon. 

The description of bonding at transition metal surfaces presented here has been based on a combination of detailed experiments and quantitative theoretical treatments. Adsorption of simple molecules on transition metal surfaces has been extremely well characterized experimentally both in terms of geometrical structure, vibrational properties, electronic structure, kinetics, and thermo-chemistry [1-3]. The wealth of high-quality experimental data forms a unique basis for the testing of theoretical methods, and it has become clear that density functional theory calculations, using a semi-local description of exchange and correlation effects, can provide a semi-quantitative description of surface adsorption phenomena [4-6]. Given that the DFT calculations describe reality semi-quantitatively, we can use them as a basis for the analysis of catalytic processes at surfaces. 

We would like to conclude this introductory Chapter by the following general comment. Most of the papers dealing with the fluctuation-controlled reactions, focus their attention on the simplest bimolecular A + B —> B and A + B —> 0 reactions. To our mind, main results in this field are already obtained and the situation is quite clear. In the nearest future the most prospective direction of kinetic theory seems to be many-stage catalytic processes the first results are discussed in Chapters 8 and 9. Their study (stimulated also by the technological importance) should be continued using in parallel both refined mathematical formalisms of the fluctuation-controlled kinetics and full-scale computer simulations. 

The electronic structure of a solid metal or semiconductor is described by the band theory that considers the possible energy states of delocalized electrons in the crystal lattice. An apparent difficulty for the application of band theory to solid state catalysis is that the theory describes the situation in an infinitely extended lattice whereas the catalytic process is located on an external crystal surface where the lattice ends. In attempting to develop a correlation between catalytic surface processes and the bulk electronic properties of catalysts as described by the band theory, the approach taken in the following pages will be to assume a correlation between bulk and surface electronic properties. For example, it is assumed that lack of electrons in the bulk results in empty orbitals in the surface conversely, excess electrons in the bulk should result in occupied orbitals in the surface (7). This principle gains strong support from the consistency of the description thus achieved. In the following, the principle will be applied to supported catalysts. 

Intra- and intermolecular hydrogen transfer processes are important in a wide variety of chemical processes, ranging from free radical reactions (which make up the foundation of radiation chemistry) and tautomeriza-tion in the ground and excited states (a fundamental photochemical process) to bulk and surface diffusion (critical for heterogeneous catalytic processes). The exchange reaction H2 + H has always been the preeminent model for testing basic concepts of chemical dynamics theory because it is amenable to carrying out exact three-dimensional fully quantum mechanical calculations. This reaction is now studied in low-temperature solids as well. 

Though homogeneous-catalytic processes can be explained by one of these two theories [28], in practice, the mechanism is more complex. 

Characterization of the Surfaces of Catalysts Measurements of the Density of Surface Faces for High Surface Area Supports. - It has always been a tenet of theories of catalysis that certain reactions will proceed at different rates on different surface planes of the same crystal. Experiments with metal single crystals have vindicated this view by showing that the rate of hydrogenolysis of ethane on a nickel surface will vary from one plane to another. In contrast the rate of methanation remains constant for the same planes.4 Because of this structure sensitivity of catalytic processes there is a requirement for methods of determining the number of each of the different planes which a catalyst and its support may expose at their surfaces. Electron microscopy studies of 5nm Pt particles supported upon graphite show them to be cubo-octahedra with surfaces bound by (111) and (100) planes.5 Similar studies of Pd and Pt prepared by evaporation reveal square pyramids of size 60-200 A bounded by incomplete (111) faces.6... 

The so-called renaissance of electrochemistry has come about through a combination of modem electronic instrumentation and the development of a more pragmatic theory implemented with the data-piocessing and computational power of computers. Within the area of physical chemistry, numerous thermodynamic studies of unstable reaction intermediates have made use of modem electrochemistry. In addition, extensive studies of the kinetics of electron-transfer processes in aqueous and nonaqueous media have been accomplished. The electrochemical characterization of adsorption phenomena has been of immense benefit to the understanding of catalytic processes. 

Within the framework of the transition state theory [112,113], the observed activation energy, Eobs, for a monomolecular catalytic process in the heterogeneous case is Eobs = E0 + A//ads [act. complex], where E0 is the energy of the reaction without a catalyst and A//.lds act. complex] is the adsorption enthalpy of the activated complex [114], In the monomolecular cracking of n-alkanes catalyzed by... 

In view of the proven multistep catalytic process, transport of the reacting species from one type of site to another before desorption as an alkane, cycloalkane, or benzene seems necessary. Since desorbed olefin plays a significant role in exchange of cycloalkanes with deuterium on Pd films even at ambient temperatures, olefins and even dienes could be responsible in the transport steps. It is useful to recall that this is, in fact, the basis of the classic theory of dualfunctional catalysis. 

The catalytic properties of simple metals with respect to the system H2-02 have been investigated experimentally for the long time in detail. Pt and Pd show the best catalytic activity (CA), as it is well known. The general theory of catalytic processes is developed rather poorly, as it is well known too. However, numerous experimental data point to a strong dependence of material CA on the processes of reagents adsorption and products desorption (Fig. 1) [1],... 

Industrial catalysis is an old practice. Catalysts have always been used in the production of wine and beer. Among the first industrial catalytic processes are a few inorganic oxidation processes, viz. the Deacon process (oxidation of HC1 into CI2) and the production of sulphuric acid. These processes were developed before a scientific basis of chemical reactivity was established. Only after the formulation of the theory of chemical equilibria by van t Hoff did a framework for catalyst development become available. This had a major impact on the development of a process for the synthesis of ammonia at the beginning of the twentieth century, allowing a systematic, scientifically based search for a good catalyst to be performed. It also initiated the development of chemical process engineering as we know it today. 

Many catalytic processes are accompanied by side reactions (for example, by coke formation) that decrease the catalyst activity. The deactivation of pore catalysts by coke formation is a complex phenomenon that includes coverage of active sites, simultaneously with coke growth and pore blockage (64 -67). The latter phenomenon can be described employing percolation theory (44-46). 

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