The solid-gas interface 143
AES can be used for studying specific crystal faces. For example,
nitrogen chemisorption on iron has been shown to be much stronger
on the more open Fe(lll) crystal faces than on the more compact
Fe(lOO) and Fe(llO) faces (see Figure 5.16),Low energy electron diffraction (LEED)We now turn from the study of surface composition to that of surface
structure. Low energy electron diffraction is the basic technique for
studying the arrangement of atoms at solid surfaces, just as X-ray
diffraction is the basic technique for studying the three-dimensional
arrangements of atoms in crystalline solids. The electron energy
range exploited in LEED is roughly 20-300 eV. In this range, two
basic, but conflicting, requirements are satisfied.- The electron energy is high enough for the de Broglie wavelength
(A = hip — h/(2m e)^) to be of the same order of magnitude as
interatomic spacings (e.g. c = 150 eV corresponds to A = 10~^10 m). - The energy is low enough to ensure that most of the electrons
(incident at right-angles to the surface) do not penetrate much
beyond the outer atomic layer of the solid, thus giving a technique
appropriate for surface studies.
Crystalline solids consist of periodically repeating arrays of atoms,
ions or molecules. Many catalytic metals adopt cubic close-packed
(also called face-centred cubic) (Co, Ni, Cu, Pd, Ag, Pt) or hexagonal
close-packed (Ti, Co, Zn) structures. Others (e.g. Fe, W) adopt the
slightly less efficiently packed body-centred cubic structure. The
different crystal faces which are possible are conveniently described
in terms of their Miller indices. It is customary to describe the
geometry of a crystal in terms of its unit cell. This is a parallelepiped
of characteristic shape which generates the crystal lattice when many
of them are packed together.
In two dimensions, the equivalents of unit cell and lattice are unit
mesh and net, respectively. Crystallography in two dimensions is,
obviously, simpler than that in three dimensions, and there are only
five types of net (illustrated in Figure 5.15). The choice of unit mesh
is arbitrary. The primitive unit mesh (illustrated at the bottom left
hand corner of each net) is the smallest possible repeating quadrilateral
with lattice points only at the corners. However, it may be appropriate