Surface catalysis, mechanism

The physical chemist is very interested in kinetics—in the mechanisms of chemical reactions, the rates of adsorption, dissolution or evaporation, and generally, in time as a variable. As may be imagined, there is a wide spectrum of rate phenomena and in the sophistication achieved in dealing wifli them. In some cases changes in area or in amounts of phases are involved, as in rates of evaporation, condensation, dissolution, precipitation, flocculation, and adsorption and desorption. In other cases surface composition is changing as with reaction in monolayers. The field of catalysis is focused largely on the study of surface reaction mechanisms. Thus, throughout this book, the kinetic aspects of interfacial phenomena are discussed in concert with the associated thermodynamic properties. 

Surface catalysis. As the name implies, surface catalysis takes place on the surface atoms of an extended solid. This often involves different properties for the surface atoms and hence different types of sites (unlike molecular catalysis, in which all the sites are equivalent). Because the catalyst is a solid, surface catalysis is by nature heterogeneous (see (6) below). The extended nature of the surface enables reaction mechanisms different from those with molecular catalysts. 

In surface catalysis, where X is an adsorption complex and Y and W are non-existent, the mechanism may be represented as follows ... 

Conformational effects, on reactivity of cycloamyloses, 23 242, 245-249 Constant Ci, values for, 33 273, 274 Contact catalysis, mechanism of, 2 251 Contact catalysts, surface area measurements for studying, 1 65... 

Organic Substituent Effects as Probes for the Mechanism of Surface Catalysis... 

Surface catalysis affects the kinetics of the process as well. Saltzman et al. (1974) note that in the case of Ca -kaolinite, parathion decomposition proceeds in two stages with different first-order rates (Fig. 16.14). In the first stage, parathion molecules specifically adsorbed on the saturating cation are quickly hydrolyzed by contact with the dissociated hydration water molecules. In the second stage, parathion molecules that might have been initially bound to the clay surface by different mechanisms are very slowly hydrolyzed, as they reach active sites with a proper orientation. 

Site Density and Entropy Criteria in Identifying Rate-Determining Steps in Solid-Catalyzed Reactions Russell W. Maatman Organic Substituent Effects as Probes for the Mechanism of Surface Catalysis M. Kraus... 

Although the division of surface reaction mechanisms into LH or ER dates to the early days of catalysis, ER/HA surface reactions have only been demonstrated recently and only for strongly reactive atomic gas phase species, e.g., H, O. There are many differences between the ER/HA mechanism and the LH mechanism that can be used to separate them experimentally. For example, ER/HA reactions of reactive incident atoms are very exothermic relative to the equivalent LH reaction, typically by several eV. Much of this released energy should end up in the gas-phase product molecule. ER/HA are direct non-activated reactions whose final state properties depend on the initial conditions of the incoming atom and not Ts. This is of course the exact opposite of LH properties. 

In the traditional surface science approach the surface chemistry and physics are examined in a UHV chamber at reactant pressures (and sometimes surface temperatures) that are normally far from the actual conditions of the process being investigated (catalysis, CVD, etching, etc.). This so-called pressure gap has been the subject of much discussion and debate for surface science studies of heterogeneous catalysis, and most of the critical issues are also relevant to the study of microelectronic systems. By going to lower pressures and temperatures, it is sometimes possible to isolate reaction intermediates and perform a stepwise study of a surface chemical mechanism. Reaction kinetics (particularly unimolecular kinetics) measured at low pressures often extrapolate very well to real-world conditions. 

Keywords Silane ester silanol hydrolysis condensation surfaces kinetics mechanisms catalysis steric polar functionality Taft Brensted. 

Similar reaction sequences have been identified in other chemically reacting systems, specifically catalytic combustion (52, 53), solid-fuel combustion (54), transport and reaction in high-temperature incandescent lamps (55), and heterogeneous catalysis (56 and references within). The elementary reactions in hydrocarbon combustion are better understood than most CVD gas-phase reactions are. Similarly, the surface reaction mechanisms underlying hydrocarbon catalysis are better known than CVD surface reactions. 

Concepts of Modem Catalysis and Kinetics, I. Chokendorff and J. W. Niemantsverdriet, Wiley-VCH 2003, 452 pp., ISBN 3-527-30574-2. This specialized book deals only with classic gas/solid heterogeneous catalysis. It contains excellent technical explanations and has a strong mathematical and physical approach, which makes for rather heavy reading. It covers many surface reaction mechanisms and catalyst characterization techniques. 

This section reports a series of examples of application of the cluster model approach to problems in chemisorption and catalysis. The first examples concern rather simple surface science systems such as the interaction of CO on metallic and bimetallic surfaces. The mechanism of H2 dissociation on bimetallic PdCu catalysts is discussed to illustrate the cluster model approach to a simple catalytic system. Next, we show how the cluster model can be used to gain insight into the understanding of promotion in catalysis using the activation of CO2 promoted by alkali metals as a key example. The oxidation of methanol to formaldehyde and the catalytic coupling of prop)me to benzene on copper surfaces constitute examples of more complex catalytic reactions. 

The rate equation with strongly acidic catalysts is also second order in silanol and first order in catalyst (75). The mechanism is proposed to proceed via protonation of silanol, followed by an electrophilic attack of the conjugate acid on nonprotonated silanol. The condensation processes outlined in reactions 16a and 16b for sulfonic acids is also an applicable mechanism for the acid catalysis. The condensation polymerization in emulsion catalyzed by dodecylbenzenesulfonic acid is second order in silanol, but the rate has a complex dependence on sulfonic acid concentration (69). This process was most likely a surface catalysis of the oil-water interface and was complicated by self-associations of the catalyst-surfactant. 

Studies of the dissolution rates of alum inum oxide and beryllium oxide (Furrer and Stumm, 1986) showed that the rates of these reactions are facilitated by increased concentrations of protons and various aliphatic and aromatic ligands at the oxide surface. The key link for determining the mechanism of reaction catalysis is establishing the relation between the concentration of these species in solution and the concentration at the solid surface. This relation is determined by potentiometric titration of the solution-solid mixture, just as described earlier for the surface catalysis of Mn oxidation. The relations between the dissolution rate of aluminum oxide and the hydrogen concentration both in solution and on the solid surface are presented along with the proposed mechanism in Fig. 9.9. The solution dependence of the dissolution rate on pH (Fig. 9.9A) has a fractional order of 0.4, which is similar to that of other oxide dissolution reactions (Table 9.4). But when the same relation is plotted as... 

According to the lUPAC, the Langmuir-Hinshelwood mechanism is defined as a mechanism for surface catalysis in which the reaction occurs between species that are adsorbed on the surface. This mechanism is expected to exhibit a second order kinetics with respect to the surface coverage of the two reactants. 

The discussion of a number of topics in electrocatalysis, including adsorption phenomena, surface reaction mechanisms and investigation techniques, electrocatalytic activity and selectivity concepts, and reaction engineering factors, may seem at first too diverse. We believe, however, that fundamental principles cannot be divorced from their natural counterpart, praxis. Here, we attempt to establish ties between basic and applied electrocatalysis and with their conventional similes, catalysis, surface physics (and spectroscopy) and reaction engineering. By taking a vitae parallelae perspective, we hope that a synthetic analysis of the present state of the art emerges. 

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