Page 124 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: ZBU 2nd Revised Pages
Encyclopedia of Physical Science and Technology EN002G-87 May 19, 2001 20:3

522 Catalyst Characterization

of the metal surface, the nature of the metal, interac- thermal wave that acts as a piston on an inert gas layer
tion of the metal with the support material, and other fea- at the surface of the sample and produces a sound wave
tures of heterogeneity, such as variations in crystallite size that is detected by a microphone. Although the applica-
and surface coverage. A completely satisfactory explana- tion is relatively new to catalysis, it has been used suc-
tion of CO chemisorption on metals from IR data has yet cessfully in the study of acid sites on silica–alumina cat-
to be realized. Changing ratios of linear to bridged species alysts and similar materials. The photothermal deflection
signals caution in interpretating the dispersion of metal on technique is an outgrowth of the photoacoustic method.
a support from CO chemisorption measurements. Disper- In this technique the IR radiation is focused on the sur-
sion calculations from chemisorption data must assume a face of the catalyst sample, producing thermal gradients in
surface stoichiometry for the chemisorbed molecule, and the gas just above the sample. A laser beam probe passes
IR results have demonstrated that an assumed model can through the gas just above the sample and is deflected
easily be off by 20 or 30% depending on the ratio of linear by the changes in the gas density. The deflected beam is
to bridged species. detected with a position sensitive detector, and the an-
The reflection technique has not been used as exten- gle is measured between the deflected and undeflected
sively as transmission. Its slow development may be at- beams.Thephotothermaltechniquehasthedistinctadvan-
tributed to several factors. It is used primarily on highly tage of being adaptable to in situ measurements of catalyst
polished metal surfaces including those involved in fun- behavior. The application of both techniques to catalyst
damental studies of single crystals. The theoretical frame- studies is new and relatively unexplored.
work for reflection IR spectra has been developed only
recently. Ultrahigh-vacuum techniques are required and 2. Raman Spectroscopy
modifications are needed for standard IR spectrometers.
Since the reflection technique can be used with single crys- Application of Raman spectroscopy to a study of cat-
tals of metals, it is a bridge between the more sophisticated alyst surfaces is increasing. Until recently, this tech-
surface techniques used in surface science and the IR stud- nique had been limited to observing distortions in ad-
ies of the more practical catalysts. sorbed organic molecules by the appearance of forbidden
Recent studies using the reflection technique have re- Raman bands and giant Raman effects of silver surfaces
vealed a flaw in interpretation of the data from transmis- with chemisorbed species. However, the development of
sion studies of CO chemisorption on metals. Studies of laser Raman instrumentation and modern computerization
CO chemisorption on polycrystalline copper and several techniques for control and data reduction have expanded
different faces of single crystals of copper have shown these applications to studies of acid sites and oxide struc-
marked variations in the frequency shift of the IR spec- tures. For example: The oxidation–reduction cycle oc-
tra. Similar results have been obtained on palladium films. curring in bismuth molybdate catalysts for oxidation of
The polycrystalline and high-index faces show an absorp- ammonia and propylene to acrylonitrile has been studied
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tion band at ∼2100 cm , whereas low-index faces show in situ by this technique. And new and valuable informa-
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a corresponding band at ∼2070 cm . This suggests that tion on the interaction of oxides, such as tungsten oxide
the support may influence the development of high-index and cerium oxide, with the surface of an alumina support,
planes and steps on the surfaces of small crystallite de- has been obtained.
posits. On palladium films deposited at low temperatures Future applications of the Raman technique to catalyst
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a similar CO absorption band is at 2112 cm . Films an- studies appear promising.
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nealed at elevated temperatures, 150 C, have absorption
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bands in a region between 1930 and 1990 cm . These
3. Magnetic Resonance
results suggest that the original assignment of linear and
bridged CO species on supported palladium may, in fact, Magnetic resonance spectra are obtained from species
be associated with low- and high-index faces on the crys- with net magnetic moments. Two species are recognized:
tallites.Theconceptofthesupportstronglyinfluencingthe a rotating electron that produces electron paramagnetic
type of crystallite faces formed on impregnation and de- resonance and a spinning nucleus with a net magnetic
position of metal on oxide surfaces is reinforced by these moment that produces nuclear magnetic resonance.
observations. Resonance spectra are obtained by placing a sample in
Some recent developments in IR techniques have in- a magnetic field. The magnetic field removes the degen-
cluded IR photoacoustic and photothermal beam de- eracy of magnetic states of the system. The addition of
flection spectroscopy. In photoacoustic spectroscopy the a small oscillating field perpendicular to the direction
IR wave incident to the solid surface of the catalyst is of the magnetic field adds an additional small increment
absorbed by the sample. The radiation is converted to a of energy to the system. When the frequency of the