Reactions on Surfaces

Several reactions of industrial interest occur at inconveniently slow rates in the absence of a catalyst, but can be accelerated considerably by the solid surface of a metal. These include the Haber process for the synthesis of ammonia, the cracking of petroleum, the Fischer-Tropsch synthesis and others. Originally, it was thought that the main effect of the solid catalyst was presenting the reactants at considerably higher local concentrations than in the gas phase. However, this is not generally true, since in some cases, different surfaces cause a substance to react in different ways. We will analyse some of these. 

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SECM Scanning electrochemical microscopy [40] An STM serves as microelectrode to reduce electroactive species Electrochemical reactions on surfaces... 

Photochemical reactions (chapter A3.13) and heterogeneous reactions on surfaces (chapter A3.10) are discussed in separate chapters. 

Langmuir i 1922 Chemicai reactions on surfaces Trans. Faraday See. 17 607... 

A3.10 Reactions on surfaces corrosion, growth, etching and catalysis... 

The characteristic times on which catalytic events occur vary more or less in parallel with the different length scales discussed above. The activation and breaking of a chemical bond inside a molecule occurs in the picosecond regime, completion of an entire reaction cycle from complexation between catalyst and reactants through separation from the product may take anywhere between microseconds for the fastest enzymatic reactions to minutes for complicated reactions on surfaces. On the mesoscopic level, diffusion in and outside pores, and through shaped catalyst particles may take between seconds and minutes, and the residence times of molecules inside entire reactors may be from seconds to, effectively, infinity if the reactants end up in unwanted byproducts such as coke, which stay on the catalyst. 

Here we use the convenient notation for the transition state A 2d- As we have already have seen, it is customary to introduce the concept of surface coverage when dealing with reactions on surfaces. The coverage of A is given by... 

Gas-surface interactions and reactions on surfaces play a crucial role in many technologically important areas such as corrosion, adhesion, synthesis of new materials, electrochemistry and heterogeneous catalysis. This chapter aims to describe the interaction of gases with metal surfaces in terms of chemical bonding. Molecular orbital and band structure theory are the basic tools for this. We limit ourselves to metals. 

Figure 7.12 neatly illustrates how dissociation reactions on surface are greatly affected by interactions with neighboring adsorbate species. This is because dissociations often require ensembles of sites in a specific arrangement. From our discus-... 

Finally, some details of the dynamics of NO conversion reactions on surfaces have been recently probed by using angle-resolved TPD. For instance, in a study of the decomposition of N20 on Rh(110), Matsushima and co-workers have identified four N2 peaks between 60 and 150 K originating from direct N20-dissociation, and a fifth feature at 160 K attributed to the desorption of N2 molecules adsorbed on the surface after previous decomposition (Figure 3.2) [20]. The appearance of each of these peaks... 

Drawing and Naming Surface Species in Organic Reactions on Surfaces... 

In any field, certain definitions and language must be understood, and the field of catalysis is no exception. Thus we start with some definitions before describing organic reactions on surfaces. 

DRAWING AND NAMING SURFACE SPECIES IN ORGANIC REACTIONS ON SURFACES... 

Another approach is that of including dynamics in the calculations. A dynamical formalism of DFT was first developed by Car and Parrinello [31], and has been employed in a wide range of areas, e.g. solvation problems, reactions on surfaces, solid-state interactions, and a variety of biochemical applications. In CP-MD one normally uses a plane wave basis to reduce the computational requirements and enable easy implementation of periodic boundary conditions. Nonetheless, CP-MD simulations are rather costly, and are normally not applied to systems larger than, say, 1-200 atoms, and over relatively short time frames. 

The simplest theories of reactions on surfaces also predict surface rate laws in which the rate is proportional to the amount of each adsorbed reactant raised to the power of its stoichiometric coefficient, just like elementary gas-phase reactions. For example, the rate of reaction of adsorbed carbon monoxide and hydrogen atoms on a metal surface to produce a formyl species and an open site,... 

M. W. Roberts reviews the contribution of photoelectron spectroscopy to provide chemical information at the molecular level to the catalytic reactions on surfaces. The use of organic probes to study the rate-determining steps and mechanisms of catalytic reactions is reviewed by R. W. Maatman and M. Kraus, respectively. 

Specific research subjects have emerged with respect to improved descriptions of specific phenomena. Some time ago, it was speculated that gas-solid interactions and turbulence effects on reaction kinetics would be important areas of advance in the modeling art. Gas-solid interactions include both chemical formation of aerosols and reactions on surfaces of pre-existing suspended particulate matter. Because of differing effects of a material in the gas phase and in some condensed phase, it will be important to characterize transformation processes. The achex (Aerosol Characterization hYperiment) program recently carried out under the direction of Hidy will provide an extensive data base with which to test new ways of treating the gas-solid interaction problem. 

Wall termination reactions immediately introduce a complexity to all chain reactions, namely, that the overall reaction rate can be a strong function of the size of the reactor. In a small reactor where the surface-to-volume ratio is large, termination reactions on surfaces can keep the radical intermediate pool small and thus strongly inhibit chain reactions (nothing appears to happen), while in a large reactor the surface-to-volume ratio is smaller so that the termination rate is smaller and the effective rate increases by a large factor (and the process takes oft). 

Interestingly, heterogeneous processes appear to be involved in HONO formation, certainly in smog chambers and indoor air environments and most likely on a variety of surfaces outdoors. It is produced from gaseous N02 and adsorbed water in a heterogeneous reaction on surfaces (see Chapter 7) ... 

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