Page 125 - 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

Catalyst Characterization 523

oscillating field matches the energy difference between spectrum of a reference bulk material with known near-
magnetic states, resonance occurs and energy is absorbed est neighbor distances. The precision of the measurements
from the oscillator. of distances to neighboring atoms is high, and these dis-
Electron paramagnetic resonance (EPR) has been used tances can be assigned to bond type, such as metal–metal
extensively in studies of the mechanism of catalytic reac- or metal–oxygen. The number of nearest and next nearest
tions. It has been used to identify free radicals and ion- neighboring atoms is obtained from the intensity of the
radicals formed by chemisorbed species on catalytically absorption peaks.
active sites and to study the structure and distribution of Most of the published EXAFS studies have been con-
paramagnetic catalytic sites such as those produced by cerned with the structure of very small metal crystallites
transition metals or metal ions on a catalyst surface. EPR supported on high surface area materials such as silica gel,
remainsprimarilyaresearchtoolforstudyingmechanisms alumina,orzeolites.Typically,crystallitesaresmallerthan
˚
of catalytic reactions. 100 A in diameter.
The principles of NMR are similar to those of EPR. As an example of the application of EXAFS to cata-
In this case the nucleus is the probe. The chemical in- lyst systems, consider studies of platinum crystallites sup-
formation is contained in shifts in the nuclear magnetic ported on silica gel, alumina, and Y zeolite. In each case
energy levels as a result of coupling with the external the coordinate number for the nearest neighbor atoms was
electronic system of the atom. Until recently very few smaller than in the bulk metal. It was 8 for platinum on
examples of applications of NMR to catalysts appeared silica gel and 7.2 for platinum on alumina. The nearest
in the literature. Before the recent discovery of the phe- neighbor distances were similar to bulk distances, namely,
˚
nomenon of magic angle spinning, solids exhibited broad, 2.775 A for platinum on silica gel and Y zeolites. How-
featureless absorption bands in the NMR region. Magic ever, the distance was much smaller for platinum on alu-
angle spinning removes the dipolar broadening found in mina. The results suggest that small crystallites, similar
the solid state, and sharp absorption lines result. The ap- to bulk metal, are formed on silica and Y zeolite sup-
plication of magic angle spinning to catalyst character- ports, but strong interactions exist between platinum and
ization has not been fully developed and this technique alumina to produce the small interatom distances and low
could become an important characterization tool in the coordination number. The absorption near edge fine struc-
future. ture results from electron transitions to bound states in
the adsorbing species. The fine structure is related to the
electronic structure of the adsorbing atom and provides
4. EXAFS and XANES
information on its valence state and interactions with the
Extended X-ray absorption fine structure (EXAFS) and support or with chemisorbed species. ZANES’ data on
X-ray absorption near edge fine structure (XANES) have the platinum catalyst mentioned above demonstrated that
been used with great success to characterize highly dis- in the reduced state the supported platinum was electron
persed supported metals. EXAFS can provide informa- deficient compared to the bulk metal, indicating that the
tion on the local structure of highly dispersed metal on a platinum 5d electrons form bonds with the support.
support, such as alumina, and XANES, measured at the Unfortunately, EXAFS cannot be routinely applied to
same time, provides information on the valence state of the characterization of catalysts. Practical applications re-
the metal. quire high-intensity X-ray sources obtainable only at syn-
With EXAFS oscillations are observed in the photon chrotron installations.
energy spectrum just above the X-ray absorption edge of
an element. Theoretical studies of these oscillations have
B. Particle Diffraction and Scattering
demonstrated that they contain structural information on
the immediate surroundings of the atom undergoing X-ray The scattering of neutrons, ions, or electrons results in
absorption. When an X-ray photon is absorbed by an atom, changes in both the energy and momentum of these
a photoelectron is emitted. The outgoing photoelectron particles. Structural information is obtained from the mea-
wave may be reflected from the neighboring atoms. The surement of these changes.
constructive and destructive interference between the out-
going and reflected photoelectron waves produces the os-
1. Neutrons
cillations observed just above the X-ray absorption edge.
The structural information is extracted by a Fourier trans- Structural information important for catalyst characteri-
form of the oscillations. The resulting absorption spectrum zation can be obtained from neutron diffraction, inelastic
shows peak intensity as a function of nearest-neighbor scattering, and small-angle scattering. Each experimental
distances. The abscissa is calibrated from the oscillation technique yields a different type of structural information.