Reactions at Surfaces

Growth reactions at surfaces will certainly continue to be tlie focus of much research. In particular, the synthesis of novel materials is an exciting field that holds much promise for the nanoscale engineering of materials. Undoubtedly, the advent of STM as a means of investigating growth reactions on the atomic scale will influence the llitiire of nanoscale teclmology. 

On the atomic level, etching is composed of several steps diflfiision of the etch molecules to the surface, adsorption to the surface, subsequent reaction with the surface and, finally, removal of the reaction products. The third step, that of reaction between the etchant and the surface, is of considerable interest to the understanding of surface reactions on an atomic scale. In recent years, STM has given considerable insight into the nature of etching reactions at surfaces. The following discussion will focus on the etching of silicon surfaces [28]. 

Reactions with Radicals and Electron-deficient Species, Reactions at Surfaces... 

Other reactions at surfaces (heterogeneous catalysis and reduction reactions)... 

Reaction with radicals and electron-deficient species reaction at surfaces... 

Chemical reactions of surfeces. Diffraction can be used qualitatively to identify different surface phases resulting from adsorption and chemical reaction at surfaces. Reaction rates can be investigated by following the evolution of diffracted beam intensities. 

C. R. Brundle. In Molecular Spectroscopy. K. R. West, Ed.) Heyden, London, 1976. This review discusses both the use of XPS and UPS in studying adsorption and reactions at surfaces. 

In a recent paper [11] this approach has been generalized to deal with reactions at surfaces, notably dissociation of molecules. A lattice gas model is employed for homonuclear molecules with both atoms and molecules present on the surface, also accounting for lateral interactions between all species. In a series of model calculations equilibrium properties, such as heats of adsorption, are discussed, and the role of dissociation disequilibrium on the time evolution of an adsorbate during temperature-programmed desorption is examined. This approach is adaptable to more complicated systems, provided the individual species remain in local equilibrium, allowing of course for dissociation and reaction disequilibria. 

Models of chemical reactions of trace pollutants in groundwater must be based on experimental analysis of the kinetics of possible pollutant interactions with earth materials, much the same as smog chamber studies considered atmospheric photochemistry. Fundamental research could determine the surface chemistry of soil components and processes such as adsorption and desorption, pore diffusion, and biodegradation of contaminants. Hydrodynamic pollutant transport models should be upgraded to take into account chemical reactions at surfaces. 

In the near future probably computer modelling, allowing the analysis of adsorption and elementary reactions at surfaces, will become increasingly helpful in catalyst selection. On the experimental side the field is changing drastically. Parallel testing equipment is now the state of the art. This field is often referred to as Combinatorial Chemistry . It is expected to have a large impact already in the near future. In fact, at present already companies have been formed in this field. 

An opportunity exists to apply to the study of reducing agent reactions at surfaces some of the analytical techniques successfully used to study the intermediates and poisons in fuel cell reactions, e.g. methanol. 

Reactions with Radicals and Electron-Deficient Species Reactions at Surfaces 494... 

No new reaction at surfaces, neither hydrogenation nor electrochemical reaction, was found for the systems falling within the scope of this chapter. 

Electrostatic vs. Chemical Interactions in Surface Phenomena. There are three phenomena to which these surface equilibrium models are applied regularly (i) adsorption reactions, (ii) electrokinetic phenomena (e.g., colloid stability, electrophoretic mobility), and (iii) chemical reactions at surfaces (precipitation, dissolution, heterogeneous catalysis). 

Finally we draw attention to simulations of the growth of adsorbate islands for models of chemical reactions at surfaces, such as A(a) + B(a) - AB(g) where two reactants (A, B) are adsorbed at the surface (a) while the reaction product AB is rapidly desorbing to the gas phase (g) . A well-known system exhibiting such behavior is 0(a) -I- C0(a)- C02(g) on metal surfaces . ... 

AU reactions occur by collisions between molecules or by collisions of molecules with surfaces. We will consider reactions at surfaces later, but here we consider the theory of homogeneous reactions. We wiU not attempt a quantitative or thorough description of reaction mechanisms but will only describe them in enough detail to be able to see how the engineer can control them. These collisions occur as sketched in Figure 4-12. 

Another convention in catalytic reactions at surfaces is to use the partial pressure Pj rather than the concentration Cj in describing densities in the bulk phase above catalyst surfaces, although there are of course many situations where the reactants and products are in the liquid phase. These are simply related for ideal gases by the expression Cj = Pj/RT. 

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