Kinetics surface-catalytic

Adsorption of reactants on the surface of the catalyst is the first step in every reaction of heterogeneous catalysis. Flere we focus on gases reacting on solid catalysts. Although we will deal with the adsorption of gases in a separate chapter, we need to discuss the relationship between the coverage of a particular gas and its partial pressure above the surface. Such relations are called isotherms, and they form the basis of the kinetics of catalytic reactions. 

Similar disproportionation is likely to occur during catalytic hydrocarbon oxidation since the Bl2Mo20g catalyst is subjected to continuous redox cycling under such conditions. Therefore, any kinetic or catalytic information about Bi2Mo20n is suspect unless the absence of surface restructuring can be confirmed. 

Many physical-chemical processes on surfaces of solids involve free atoms and radicals as intermediate particles. The latter diffuse along the adsorbent-catalyst surface and govern not only kinetics of catalytic, photocatalytic, or some heterogeneous radiative processes, but also creation of certain substances as a result of the reaction. 

Recently there has been a growing emphasis on the use of transient methods to study the mechanism and kinetics of catalytic reactions (16, 17, 18). These transient studies gained new impetus with the introduction of computer-controlled catalytic converters for automobile emission control (19) in this large-scale catalytic process the composition of the feedstream is oscillated as a result of a feedback control scheme, and the frequency response characteristics of the catalyst appear to play an important role (20). Preliminary studies (e.g., 15) indicate that the transient response of these catalysts is dominated by the relaxation of surface events, and thus it is necessary to use fast-response, surface-sensitive techniques in order to understand the catalyst s behavior under transient conditions. 

These assumptions are the basis of the simplest rational explanation of surface catalytic kinetics and models for it. The preeminent of these, formulated by Langmuir and Hinshelwood, makes the further assumption that for an overall (gas-phase) reaction, for example, A(g) +...- product(s), the rate-determining step is a surface reaction involving adsorbed species, such as A s. Despite the fact that reality is known to be more complex, the resulting rate expressions find wide use in the chemical industry, because they exhibit many of the commonly observed features of surface-catalyzed reactions. 

The effect of metal structure and phase formation on the kinetics of catalytic oxidation reactions was treated in detail by Savchenko et al. (see, for example, refs. 83, 84, 117 and 118). In metal surface layers both reconstruction of the metal proper (faceting) and processes associated with the formation of surface oxides can take place. In this case the first to form can be chemisorption structures (without breaking the metal-metal bond) and then the formation of two-dimensional surface oxides is observed. Finally, three-dimensional subsurface oxides are produced. An important role is played by the temperature of disordering the adsorbed layer. 

The kinetic of catalytic oxygen reduction is closely connected with chemical condition of surface nanodispersed diamond. 

As the above examples illustrate, the kinetics of surface-catalytic events depend on complex structural and electronic considerations that, thus far, have not been understood at the level of detail that would permit predictive mathematical modeling and therefore rational design. For this reason, the molecular-level engineering of catalytic surfaces harbors perhaps the greatest future potential for chemical reaction engineering, at least from the standpoint of the design of catalysts. 

The kinetics of catalytic reactions on nonuniform surfaces have been discussed by Roginskii (330,331) certain general features of his discussion will be presented here. The rate of a complex multistage heterogeneous catalytic reaction is controlled by the rate of the slowest step. The slowest step may be the adsorption of the reactants, the chemical reactions on the surface, desorption of the products or diffusion of reactants or products through the gaseous phase near the surface of the catalyst. 

The efficiency of fuel cells is largely limited by the kinetic barriers of the surface catalytic electrode reactions. In particular, the electroreduction of molecular oxygen at a PEMFC cathode severely limits high reaction rates and hence currents near the equilibrium cell voltage. 

Oct. 14, 1922, Kromeriz, then Czechoslovakia - Aug. 10, 2005, Berlin, Germany) Koutecky was a theoretical electrochemist, quantum chemist, solid state physicist (surfaces and chemisorption), and expert in the theory of clusters. He received his PhD in theoretical physics, was later a co-worker of -> Brdicka in Prague, and since 1967 professor of physical chemistry at Charles University, Prague. Since 1973 he was professor of physical chemistry at Freie Universitat, Berlin, Germany. Koutecky solved differential equations relevant to spherical -> diffusion, slow electrode reaction, - kinetic currents, -> catalytic currents, to currents controlled by nonlinear chemical reactions, and to combinations of these [i-v]. For a comprehensive review of his work on spherical diffusion and kinetic currents see [vi]. See also Koutecky-Levich plot. 

The discussion to this point has emphasized kinetics of catalytic reactions on a uniform surface where only one type of active site participates in the reaction. Bifunctional catalysts operate by utilizing two different types of catalytic sites on the same solid. For example, hydrocarbon reforming reactions that are used to upgrade motor fuels are catalyzed by platinum particles supported on acidified alumina. Extensive research revealed that the metallic function of Pt/Al203 catalyzes hydrogenation/dehydrogenation of hydrocarbons, whereas the acidic function of the support facilitates skeletal isomerization of alkenes. The isomerization of n-pentane (N) to isopentane (I) is used to illustrate the kinetic sequence associated with a bifunctional Pt/Al203 catalyst ... 

The theoretical results presented in this review show that, due to purely kinetic factors, the kinetics of catalytic reactions occurring on nm-sized metal particles, exposing different crystalline facets, may be unique compared to those observed on poly- or single-crystal surfaces. Experimental studies focused on such factors are still rare but certainly will attract more attention in the near future. 

This novel effect has been termed non-Faradaic electrochemical modification of catalytic activity (NEMCA effect [5-15]) or electrochemical promotion [16] or in situ controlled promotion [20]. Its importance in catalysis and electrochemistry has been discussed by Haber [18], Pritchard [16] and Bockris [17], respectively. In addition to the group which first reported this new phenomenon [5-7], the groups of Lambert [12], Haller [10], Sobyanin [8], Comninellis [13], Pacchioni [21] and Stoukides [11] have also made important contributions in this area, which has been reviewed recently [14,15]. In this review the main phenomenological features of NEMCA for oxidation reactions are briefly surveyed and the origin of the effect is discussed in the light of recent kinetic, surface spectroscopic and quantum mechanical investigations. 

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