Surface Chemistry
No one knew very much about the surfaces of
the respirator charcoals that those in the PCL at the time had striven so hard
to improve. The activation by steam, whereby some 30% of the carbon is burnt
away, develops activity, and the resulting structure was believed to consist of
porous aggregates of graphite crystallites, with exposed surfaces which are
heterogeneous, with active sites and with some surface oxide. Why one
briquetted coal should give a more active charcoal than another was not
understood. Douglas Everett retained an interest in these problems and attacked
the question of the thermodynamics of the adsorption process and of hysteresis.
Following a conversation with Victor Goldsmidt, in which it was suggested that
dielectric measurements might throw light on the interactions between water and
clays, Leslie Sutton carried out experiments first on brucite and later on
kaolinite. There seemed to be a dipole rotation that was hindered to varying
degrees as concentration and temperature were varied, but no simple model
sufficed and interpretation was inevitably made difficult by sample
heterogeneity. It was not until some years later that the chemistry of surfaces
was taken up again in the PCL, this time by Robert Gasser, who used ultra-high
vacua to study the adsorption of gases on the surfaces of pure metals.
The most important breakthrough in the study
of gases adsorbed on solids was the development of ultra-high vacuum technique
because it made it possible to study clean, and therefore better defined,
surfaces. This had become available when Gasser started his surface work in the
early sixties. He used a combination of three techniques to study the fate of
species adsorbed on clean surfaces, flash desorption, analysis of desorbed
species by mass spectrometry, and isotopic labelling. In flash desorption the
surface is heated rapidly after material has been adsorbed on it and different
surface species are released over narrow temperature, and therefore narrow
time, ranges. By combining this with mass spectrometry the desorbing species
could usually be identified with some certainty. A precise picture could
therefore be built up of the different species formed on the surface and of
some of the kinetic processes involved. Isotopic labelling adds further power
to the analysis in that different isotopic species are readily distinguished in
a mass spectrometer and therefore such processes as the dissociation of N2 could be followed by 15N/14N exchange. It is interesting that the combination of
flash desorption and mass spectrometry used by Gasser over a period of some 15
years is now being used again in the department by John Foord to study chemical
precursors at the gas/solid interface. In 1985, some time after he had moved
out of research, Robert Gasser wrote a book, An Introduction to
Chemisorption and Catalysis by Metals (OUP, 1985) which is widely read by
chemistry undergraduates.
One of the most exciting developments in the
sixties was the application of neutron scattering methods to the study of
condensed matter, including surfaces. John White was one of the pioneers in
this field and he showed how neutron scattering could be applied to understand
processes in a wide variety of systems. The early experiments were done at
Harwell, only twenty miles from Oxford. Although these facilities have recently
closed, the fruit of the research in the sixties, to which White made a
substantial contribution, led to the participation of the U.K. in the joint
research reactor, the Institut Laue-Langevin at Grenoble, and to the later
development of the pulsed neutron source ISIS at Chilton, near Oxford. The two
facilities are the best of their type in the world. John White spent from
1975-80 as the British director at the Institut Laue-Langevin.
The power of neutron scattering is that it
can be used to determine structure at a molecular level and to study dynamic
processes ranging from slow diffusive motions on surfaces to the high
frequencies of intramolecular vibrations. John White's neutron scattering
experiments covered a wide range of systems. He studied vibrational excitations
in polymers, as well as diffusion in liquid crystals and in liquids. Two
noteworthy experiments on diffusion were the motion of water in clays,
especially as influenced by the surface of the clay, and a study of the
contribution of proton transfer to the overall diffusion of protons in acid
solutions. John White also made some of the first applications of neutron
scattering to the study of surface species, using neutrons to study the
structure and motion of small molecules physisorbed on graphite and in
zeolites. One of the most interesting ideas developed by him was to apply small
angle neutron scattering to the study of colloidal systems, an area which has
blossomed in both fundamental and applied colloid science.
Bob Thomas, after an apprenticeship with
John White, has further developed the use of neutron scattering as a surface
technique. Neutrons interact only weakly with matter and therefore are not an
obvious choice for investigating surfaces where the premium is on very high
sensitivity. However, for gases on solids this can be overcome by making use of
the widely different neutron cross sections of different atoms and isotopes.
Thomas has studied the way small molecules pack together in layers on the
surfaces of weakly adsorbing materials. More recently, he has pioneered the use
of grazing incidence reflection techniques, which use both x-rays in the
laboratory and neutrons. This technique allows the structure of wet surfaces to
be probed for the first time and is now being widely used. A further
application of neutron scattering, developing from earlier work by John White,
has been the study of the swelling of clays and its relation to the theory of
colloid stability.
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The X-ray
diffractometer in Bob Thomas' laboratory |
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John Foord joined the PCL from the Inorganic
Chemistry Laboratory in 1990, although his group will not move into the
Laboratory until the autumn of 1992. He is concerned with the problems of
chemical reactions at solid surfaces, and areas of current interest are: the
design and fabrication of new solid state materials, with particular emphasis
on the role of gas phase chemical precursors at the growing gas-solid
interface; photochemistry of adsorbed layers; heterogeneous catalysis - metal
multilayer structures which are chemically etched give materials like real
catalysts, but which are sufficiently well characterised to be used in studies
of the link betwen structure and catalytic activity; and, reaction dynamics and
surface scattering.
The study of surface chemistry at the PCL
will be strengthened by the arrival, in 1991, of Colin Bain from Cambridge. His
current interests centre on the use of high power, tunable, pulsed infrared
lasers to obtain vibrational spectra of organic molecules at interfaces -
particularly `wet' interfaces that are difficult to study by other means. The
principal technique exploited in this work is sum-frequency generation, a
non-linear optical effect that allows, in principle, the determination of the
nature, concentration and orientation of molecules adsorbed at a surface.
Photoacoustic and photothermal techniques are also being developed.
Laser induced desorption experiments link
the themes of energy transfer and of surface chemistry, and such experiments
are being pursued by Stephen Simpson and his colleagues.
