Catalytic Gas-Solid Interactions

For the moment, let us focus our attention on gas-phase reactions catalyzed by solid surfaces. For a catalytic reaction to occur, at least one and frequently all of the reactants must become attached to the surface. This attachment j.s known as adsorptivn and takes place by two ditTerent processes physical adsorption and chemisorption. Physical adsorption is similar to condensation. The process is exothermic, and the heat of adsorption is relatively small, being on the order of I to 15 kcal/mol. The forces of attraction between the gas molecules and the solid surface are weak. These van der Waals forces consist of interaction betwe permanent dipoles, between a permanent dipole and an induced dipole, and/or between neutral atoms and ntolecules. The amount of gas physically adsorbed decreases rapidly with increasing temperature, and above its critic temperature only very small amounts of a substance are i ysically adsorbed. 

The type of adsorption that affects the rate of a chemical reaction is chemisorption. Here, the adsorbed atoms or molecules are held lo the surface by valence forces of the same type as those that occur between bonded atoms in molecules. As a result, the electronic structure of the chemisorbed molecule is penurbed significantly, causing it to be extremely reactive. Interaction with the catalyst causes bonds of the adsorbed reactant to be stretched, making them easier to break. 

One parameter u.sed to quantify the activity of a catalyst is the turnover frequency TOF),f. It is the number of molecules reacting per active site per second at the conditions of the experiment. When a metal catalyst such as platinum is deposited on a support, the metal atoms are considered active sites. The dispersion, D, of the catalyst is the fraction of the metal atoms deposited that are on the surface. 

An example showing how to calculate the turnover number is given on the DVD-ROMAVeb Summary Notes for Chapter 10. 

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