Surface atoms, adsorption, catalysis

A catalytic reaction is in general composed of a series of elementary processes, and the surface atoms are necessarily involved in making intermediates and/or activated complexes, as well as in the adsorption of reactant and product molecules. For this reason, the participation of surface atoms is indispensable in all of the elementary processes accomplishing the catalysis, but the kinetics are mainly influenced by the manner of the participation of surface atoms in the rate-determining steps. Accordingly, in order to make clear the roles of the surface atoms in catalysis, it should be clarified how the surface atoms participate in the rate-determining steps as well as in the other elementary processes. 

Hydrogenation of olefins is a good example for demonstrating the roles of the surface atoms in catalysis. The orbital symmetry rule in chemical reactions suggests that the highest occupied molecular orbital (HOMO) of one reaction partner and the lowest unoccupied molecular orbital (LUMO) of the other should meet the symmetry requirements. In this respect, a concerted addition of an H2 molecule to the double bond of an olefin, that is, a molecular addition reaction, is a forbidden process. Adsorption of olefin on transition metal surfaces undoubtedly changes the population of electrons in the HOMO (7tu) and the LUMO (re ) as shown schematically in Fig. 1. In spite of such perturbation of the electron densities of the HOMO and the... 

There has been a general updating of the material in all the chapters the treatment of films at the liquid-air and liquid-solid interfaces has been expanded, particularly in the area of contemporary techniques and that of macromolecular films. The scanning microscopies (tunneling and atomic force) now contribute more prominently. The topic of heterogeneous catalysis has been expanded to include the well-studied case of oxidation of carbon monoxide on metals, and there is now more emphasis on the flexible surface, that is, the restructuring of surfaces when adsorption occurs. New calculational methods are discussed. 

The mechanism of heterogeneous catalysis is often complex and not well understood. Important steps, however, involve (1) attachment of reactants to the surface of the catalyst, a process called adsorption, (2) conversion of reactants to products on the surface, and (3) desorption of products from the surface. The adsorption step is thought to involve chemical bonding of reactants to the highly reactive metal atoms on the surface with accompanying breaking, or at least weakening, of bonds in the reactants. 

Another rewarding field of applications is given by cluster simulations of the role of SOC in surface catalysis, for instance oxidation on the surface. Dissociative adsorption of O2 on metal surfaces leads to inclusion of atomic oxygen in the oxidation reaction. Ground state 0(3P) atom insertion in the C=C bond is spin forbidden, so the epoxidation of olefins on metal surfaces must find a way to overcome this prohibition. Other types of surface reactions can also illustrate the importance of SOC effects in spin catalysis [211]. 

It is generally accepted that heterogeneous catalysis represents a sequence of elementary reactions such as the adsorption of the reactant on the catalyst surface, atomic rearrangements of the adsorbed particles, and desorption of the products, the overall reaction rate being governed by the slowest step of these elementary reactions. The rate of the slowest... 

The present finding shows the critical importance of the atomic-scale design of the surface of a material. The arrangements of Ti4 + ions and O2- ions create new catalytic functions for desired chemical processes. The active sites can be produced in situ under the catalytic reaction conditions, even if there are no active sites at the surface before the catalysis. The surface is also modified by adsorption of a reactant to form a new surface with a different add-base character from the intrinsic property. The dynamic acid-base aspect at a catalyst surface is the key issue to regulate the acid-base catalysis, which may provide a new strategy for creation of acid-base catalysts. 

The term surface reactions is used to include the interaction of one or more types of foreign atoms with or at the surface atoms of the solid substrate. This includes epitaxy, adsorption, place exchange, oxidation, and catalysis. 

Desorption (sect. 4). Desorption is discussed in less detail than adsorption, simply because it is generally the reverse of the adsorption process. Nevertheless, desorption is used widely in surface science and catalysis as a diagnostic of the adsorbed state of atoms and molecules, and these aspects will be dealt with in this section. 

Chemical heterogeneity of a surface is an important property affecting adhesion, adsorption, wettability, biocompatibility, printability and lubrication behavior of a surface. It seriously affects gas and liquid adsorption capacity of a substrate and also the extent of a catalysis reaction. As an example, the partial oxidation of carbon black surfaces has an important, influence on their adsorptive behavior. In a chemically heterogeneous catalyst, the composition and the chemical (valence) state of the surface atoms or molecules are very important, and such a catalyst may only have the power to catalyze a specific chemical reaction if the heterogeneity of its surface structure can be controlled and reproduced during the synthesis. Thus in many instances, it is necessary to determine the chemical... 

It is probable that catalysis for certain reactions requires a site made up of a particular ensemble of surface atoms. The number of adjacent atoms required may increase from 2 for the dissociative adsorption of oxygen or hydrogen to something like 12 for ethane hydrogenolysis (105). The B5 sites seem to have particular importance in catalysis, and it is claimed that they are necessary for the appearance of infrared-active adsorption of nitrogen (106). 

Such effects suggest an influence arising from the chemical properties of the catalyst. Specific catalytic effects depend on properties of the surface atoms, and these properties, in turn, are a function of the chemical nature of the catalytic substance. This is shown by the fact that the same type of active surface is not developed on copper as on platinum, and both differ from the active surface that can be formed on carbon. Properties of surface atoms that are important in catalysis appear to be linked to specific adsorptive powers, thus hydrogenating and dehydrogenating catalysts adsorb hydrogen, whereas oxidizing catalysts adsorb oxygen. 

Low energy electron diffraction is the dominant diffraction method for studying adsorption structures. It gives information in many ways complementary to that obtained from the field ion microscope (i). Brief comparison of LEED with field ion microscopy (FIM) is instructive because these two high resolution methods of finding surface atom positions differ greatly in their actual and potential applications for study of catalysis. 

Most of the chapters discuss local properties of surface atoms and molecules atomic structure, chemical bonding, adsorption, catalysis, and mechanical properties. Transport properties, electron transport, surface magnetism, and optical properties are important subjects of surface science but are not treated here. 

Earlier we described the catalytic reaction as a series of consecutive steps at the surface, in which adsorbate and adsorbate-surface bonds are formed and/or broken on the reaction path towards the product molecule. The forces between surface atoms and adsorbate atoms responsible for rearrangement of the chemical bond are similar to those responsible for strong adsorption (E > 10 kcal/nx)l). The adsorption process dominated by such interaction is called chemisorption. Even on a single crystal metal surface, several adsorption modes are conceivable and for dissociation of a diatomic molecule many different reaction paths can be envisioned. However, usually only one particular surface atom configuration is preferred to lead to the idea of catalytic active site. If catalysis of a molecule is studied that has several reaction possibilities, some desirable and others not, a selective reaction usually requires a particular surface atom composition and rearrangement. 

As early as 1929 Balandin introduced the multiplet theory, which is based on purely structural and geometric considerations, into the field of catalysis. If we assume that the molecule to be adsorbed is large and therefore is not adsorbed at a single active center (single-point adsorption), but at two or more centers (multipoint adsorption), then it becomes clear that the steric conditions and topology of the surface are of crucial importance for the activation of the reactants. Balandin referred to the principle of geometric correspondence between the reactant molecules and the surface atoms of the catalyst. 

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