Chemisorption molecular view

Arenes can also bridge tw o metals, as shown in Figure 2.35. In one example shown, the arene bridges and sandwiches two palladium atoms. The arene in the dinuclear nickel complex also bridges two metals. The arene in the osmium complex binds symmetrically to each of the three metal atoms on the triangular face of a cluster. This compound provides a molecular view of the chemisorption of benzene on a metal surface. ... 

The plan of this chapter is as follows. We discuss chemisorption as a distinct topic, first from the molecular and then from the phenomenological points of view. Heterogeneous catalysis is then taken up, but now first from the phenomenological (and technologically important) viewpoint and then in terms of current knowledge about surface structures at the molecular level. Section XVIII-9F takes note of the current interest in photodriven surface processes. 

A major portion of this chapter is concerned with physical adsorption, particularly from a global thermodynamic point of view. This is followed by a molecular-scale examination of crystalline surfaces and a brief discussion of chemisorption and its relevance to heterogeneous catalysis. 

Before 1916, adsorption theories postulated either a condensed liquid film or a compressed gaseous layer which decreases in density as the distance from the surface increases. Langmuir (1916) was of the opinion that, because of the rapidity with which intermolecular forces fall off with distance, adsorbed layers are not likely to be more than one molecular layer in thickness. This view is generally accepted for chemisorption and for physical adsorption at low pressures and moderately high temperatures. 

Figure 3 Schematic diagrams of prototypes of gas-surface interactions as can be probed by molecular beams, presented as side views of the surface atoms or cubes. (A) molecular scattering in which parallel momentum is conserved and die surface is represented by hard cubes. (B) molecular scattering from individual surface atoms. (C) molecular scattering in the presence of a strong chemisorption well. (D) molecular scattering for a partially passivated surface, containing specific sites where chemisorption is possible. Note that in this case the interaction is also strongly orientation dependent. From Ref. [1].

The cluster model approach is based on the premise that it is only necessary to use a limited nuinber of atoms to calculate, with a certain degree of confidence, the local properties that reproduce the experimental data. From the molecular modelling point of view, the cluster model is one of the most widely used toots to study phenomena like chemisorption, physorption and reactions on large atomic aggregates. such as electronic transitions on metal... 

In Uhllg s earlier view, one of the factors determining passivation (and chemisorption) is the ratio of the work function to the enthalpy of sublimation AH. If this ratio is less than unity, conditions are favorafle to passivation because the electron would escape more readily than the atom, favoring the chemisorption of substances like oxygen. A passive film is composed, then, from chemisorbed atomic and molecular oxygen (supplemented perhaps by OH and H 0). The formation of chemical bonds satisfies the surface affinities of the metal without metal atoms leaving their lattice site. 

An important simplification results if we can consider the bonding between atoms to be a local phenomenon. In this event, we would need to consider only the immediate neighbours of the adsorbate or defect atoms, and we arrive at the cluster models circled in Fig. 1. Of course, some properties of the system will depend on its extended nature. Others, including the variation in total energy with small displacements of atoms, should be described satisfactorily by a cluster calculation. In such cases, the problem has been reduced to one of molecular dimensions, so that the methods of molecular physics or theoretical chemistry could be used. For many systems of interest to the solid-state physicist, where a typical problem might be the chemisorption of a carbon monoxide molecule on the surface of a ferromagnetic metal surface such as nickel, the methods discussed in much of the rest of the present volume are inappropriate. It is necessary to seek alternatives, and this chapter is concerned with one of them, the density functional (DF) formalism. While the motivation of the solid-state physicist is perhaps different from that of the chemist, the above discussion shows that some of the goals are very similar. Indeed, it is my view that the density functional formalism, which owes much of its development and most of its applications to solid-state physicists, can make a useful contribution to theoretical chemistry. 

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