Page 148 - Introduction to Colloid and Surface Chemistry

138 The solid-gas interface

solids can be cleaved to expose specific crystal faces. A study of clean
solid surfaces, either as such or following exposure to known
adsorbate, is then possible.
The most obvious application of these studies is towards an
understanding of heterogeneous catalysis. Despite its immense
importance, heterogeneous catalysis has tended to be an empirical
subject, with the mode(s) of action far from understood. With the
advent of modern surface chemical techniques, this situation is
changing, though not without certain difficulties. The information
provided can be of controversial value for a number of reasons.
Usually, only stable species are detected and these may be of limited
significance when considering the overall reaction mechanism. The
surface may be unrepresentative by virtue of its mode of preparation
and/or the perturbation caused by the experiment itself. The
experiments involve low pressures, whereas industrial heterogeneous
catalytic processes tend to be at atmospheric, or higher, pressure.
The consequent 'pressure gap', therefore, involves a factor of the
8 14
order of somewhere between about 10 and 10 !
The two fundamental questions concerning heterogeneous catalysis
relate to the surface chemistry and to the kinetics of the process, and
are, respectively,

1. what is the nature of the catalyst surface?
2. what is the reaction mechanism?

These questions, of course, are strongly related. The following
surface chemical techniques are described and exemplified very much
with the first of these questions in mind, including subsidiary
questions such as to (a) the nature of an active site and (b) the role of
catalyst promoters. For an account of heterogeneous catalytic
reaction mechanisms, the reader is referred to modern texts on
reaction kinetics/catalysis.

Electron spectroscopy

Electron spectroscopy is the study of electrons emitted when matter
is irradiated with photons or bombarded with particles. In photoelectron
spectroscopy (PES), a monochromatic source of irradiation is used
and the kinetic energies of directly ejected (primary) electrons are
analysed by means of an electron spectrometer, i.e. an intensity