92168.pdf

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

order of somewhere between about 108 and 1014!

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