Elementary Processes of Gas-Surface Interaction

A certain fraction of the incident gas atoms are trapped in the attractive potential well, and once trapped they can move along the surface by diffusion. The adsorbed species may desorb from the surface if sufficient energy is imparted to it at a given surface site to overcome the attractive surface forces. The types of interactions that take place between a gas atom or molecule and the surface depend on the energy of the gaseous species (kinetic or translational energy, internal energy, rotation, vibration, or electronic excitation when appropriate), the temperature, and the atomic structure of the solid surface. 

During the collision of the adsorbed gas atom with the surface, it exchanges kinetic or translational energy T with the vibrational modes of the surface atoms. The type of energy transfer that takes place in this circumstance is often called the T F, energy exchange. During the collision of gas molecules with the surface. 

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In order to understand the dynamics of gas-surface interaction, it is necessary to determine how much energy is exchanged between the gas and surface atoms through the various energy-transfer channels. In addition the kinetic parameters (rate constants, activation energies, and preexponential factors) for each elementary surface step of adsorption, diffusion, and desorption are required in order to obtain a complete description of the gas-surface energy transfer process. 

In this chapter we shall review the motion of atoms and molecules at surfaces. First we discuss how atoms vibrate about their equilibrium surface sites. Then, the elementary surface processes during the collisions of gas atoms and molecules with surfaces are described. We then discuss several elementary gas-surface interactions adsorption, surface diffusion, and desorption. 

In the oil flotation process emulsion drops to which ore particles adhere are used instead of gas bubbles. In the case of film flotation the crushed ore is dumped on a continuously moving water surface. The easily wetted particles sink while the poorly wettable ones remain at the surface and are then collected with a special device. The theory of particle extraction by flotation have been further developed due to the detailed study of the elementary stages of particle adhesion and attachment to bubbles, accounting for the interaction forces of wetting films on solid substrate [e.g. 15-17]. 

The elementary adsorption process takes place at the interaction of gas molecules with the surface of the SE. Therefore, the concentration of adsorbed molecules is proportional to the number of gas interactions with the surface. 

The most complete mathematical model of a nonuniform adsorbed layer is the distributed model, which takes into account interactions of adsorbed species, their mobility, and a possibility of phase transitions under the action of adsorbed species. The layer of adsorbed species corresponds to the two-dimensional model of the lattice gas, which is a characteristic model of statistical mechanics. Currently, it is widely used in the modeling of elementary processes on the catalyst surface. The energies of the lateral interaction between species localized in different lattice cells are the main parameters of the model. In the case of the chemisorption of simple species, each species occupies one unit cell. The catalytic process consists of a set of elementary steps of adsorption, desorption, and diffusion and an elementary act of reaction, which occurs on some set of cells (nodes) of the lattice. 

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