Dynamics of gas-surface interactions and reactions

The number of surface atoms per unit area is of the order of lO cm. But as soon as we generate a pristine surface, say by cleaving a solid, a surface tension is generated, therefore molecules from the gas phase get absorbed on the bare surface and, as we discuss below, are reluctant to leave. It is worthwhile to note how quickly a clean surface gets covered. The flux, /, of molecules coming from the gas phase and striking the surface is / = n (v) /4 where n is the number density and (v) is the mean velocity of molecules at thermal equilibrium. Problem B shows where the factor of 1 /4 comes from. In practical units and relating number density to pressure, 

For N2 at room temperature and at the quite low pressure of 10 torr, the surface will be covered in a few seconds, assuming that every molecule sticks. For this reason, ulfrahigh vacuum ( 10 torr) conditions are necessary in experiments that study reactions on clean surfaces. Even then the surface will be contaminated in about an hour. Otherwise, a surface is always covered and under the higher-pressure conditions (required in industrial catalytic processes to achieve a high throughput of material) there can be several absorbed layers, one on top of the other.  

only the first layer is strongly bound to the surface and the additional layers can easily be evaporated. Water is an example to the contrary because of the hydrogen bonding (Henderson, 2002). See Problem C for an example of water-saturated air in equilibrium with Ru. 

Clusters of atoms serve as a useful model for this and other surface processes. Small clusters exhibit non-monotonic variations in reactivity with size reflecting this optimization process that leads to different packings for different numbers of atoms. 

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