136 The solid-gas interface
reason why nitrogen is a particularly suitable adsorbate for surface
area determinations is because the value of c is generally high enough
to give a well-defined point B, but not too high to give excessive
localisation of adsorption.
Effective cross-sectional areas of molecules adsorbed on to solid
surfaces are catalogued in reference^72.
Krypton adsorption at 77 K is often used for the determination of
relatively low solid-surface areas. At this temperature the vapour
pressure of krypton (and so the dead-space correction) is small, and a
reasonable precision is attainable.
The specific surface area of a powder can be used to estimate the
average particle size if the particle shape is known.Thermal desorption spectroscopyA clean solid surface at fixed temperature is exposed to gas at low
pressure (c. 10~^8 -10~^7 torr) to allow an appropriate amount of
chemisorption to take place and the remaining adsorptive is then
pumped off. The sample is heated rapidly at a programmed rate.
Pressure surges are detected in characteristic temperature regions at
which the rate of chemidesorption from the various types of
adsorption site becomes appreciable. In addition to allowing different
types of chemisorption site to be distinguished, this technique
provides a means for estimating activation energies of desorption,
which, in turn, set upper limits for the corresponding adsorption
energies.
Composition and structure of solid surfaces
So far in this chapter, gross properties of solid surfaces (specific
surface area, pore size distribution and adsorption energies) have
been considered. In Chapter 3, the use of the electron microscope to
investigate surface morphology was discussed. In this section, some
of the spectroscopic, diffractometric and other techniques which are
available for investigating the atomic detail at solid surfaces will be
considered. Most of these are fairly modern and exploit high
technology to the full (at corresponding cost!). Like some of the
techniques referred to in Chapter 3 (TEM, SEM, PCS, etc.), they are
mostly known by acronyms. They are many in number (Adamson^2