Page 126 - 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

524 Catalyst Characterization

The measurement of small-angle scattering of neutrons backscatter line has been used to measure pore sizes in
provides information similar to that furnished by other porous materials. The advantage of the technique is that it
small-angle techniques, such as small-angle X-ray diffrac- measures smaller pore sizes than can be measured by gas
tion. Small-angle scattering results from the differences in adsorption techniques, and it is capable of measuring pore
neutron scattering length density of pores and particles in size distributions as a function of depth within the pellet
the solid matrix of the catalyst. The method does not dis- by varying the energy of the incoming ion beam.
tinguish between open and closed pores. A combination
of the two techniques provides more complete characteri-
C. Radioisotope Techniques
zation of a supported heavy-metal catalyst. A combination
of neutron and X-ray scattering and gas adsorption on a This section deals with spectroscopic methods that depend
supported metal catalyst measures pore size, metal parti- on nuclear events. Radiotracer techniques that are very
cle size, and whether metal has been encapsulated in small valuable in studying the mechanisms of catalytic reactions
pores by the conditions of its usage. have been omitted.
Inelastic scattering of neutrons yields neutron scattering
spectra that measure the vibrational energy levels of the
1. M¨ossbauer Spectroscopy
material under study. For example, the chemisorption of
water on Raney nickel was shown by inelastic scattering M¨ossbauer spectroscopy depends on low-level nuclear
both to produce hydroxyl groups and to chemisorb water transitions that emit or absorb low-energy γ rays. Cou-
molecules on the surface at less than monolayer coverages. pling of the nucleus with its electronic surroundings re-
sults in changes in the spectrum of the nuclear energy
levels. These changes can be interpreted in terms of the
2. Ion Diffraction
valence state of the M¨ossbauer atom and the electronic and
Ion scattering spectrometry and secondary ion mass spec- magnetic surroundings of its environment. The three basic
trometry are the best-known types of ion-beam-induced parameters observed in M¨ossbauer spectra are the isomer
analyses applicable to catalysis. Other ion-beam tech- shift δ, quadrupole splitting E q , and magnetic splitting.
niques have not enjoyed wide use, probably because ac- The isomer shift is a result of changes in the electron
celerators are required to produce sufficiently energetic density at the nucleus, which produce small changes in the
ion beams. nuclear energy levels. The result is shifts in the centroid
The ISS and SIMS techniques are sensitive to less than of the M¨ossbauer spectrum relative to a standard material.
monolayer coverages and detect many of the lighter el- Asymmetry in the electric field surrounding the nucleus
ements, including hydrogen. Their application to prac- produces splitting of the energy levels. Additional split-
tical catalysts is somewhat limited. The sampling area ting results from a permanent magnetic field surrounding
2
is ∼1mm , and the results obtained from examining the the nucleus. The benefits of the detailed information on
surface of a typical catalyst pellet can be very ambiguous. oxidation state at the atomic level is offset by the limited
Other ion-beam techniques applicable to catalysts number of elements with useful M¨ossbauer isotopes. The
are proton-induced X-ray emission (PIXE), Rutherford application of the 57 Fe isotope has been widespread not
backscattering, and resonance ion-beam backscattering. only in the characterization of iron catalysts, but also as an
The proton-induced X-ray emission technique is very sim- atomic probe in studies of other supported metal catalysts.
ilar to electron microprobe analysis. Characteristic X-rays The spectra in Fig. 23 show the effect of the chemisorp-
are generated by bombardment of the sample with a pro- tion of NH 3 on the 57 Fe resonance of a highly dispersed
ton beam. The advantages of PIXE are high sensitivity, Fe/SiO 2 catalyst. The iron is in the ferrous state following
particularly for light elements, and a high signal-to-noise thehydrogenreductionofmicrocrystallineferricoxidede-
ratio, which allows analysis even of trace elements in a posits. The addition of NH 3 to the reduced and outgassed
few minutes. sample has a marked effect on peak 2 of the spectra. Peak 2
In Rutherford backscattering the sample is bombarded decreases in relative area as small doses of NH 3 are added.
with light ions such as helium ions, and the energy of the Since peak 2 has been assigned as half of a doublet pro-
light ions that have been backscattered from the sample duced by surface ferrous ions, the relative area of this
is measured. When the ion is reflected back from the sur- peak is used as a measure of surface sites available for
face layer the energy lost is inversely proportional to the chemisorption. There is no noticeable change in the spec-
atomic number of the scattering species. By measuring trum with the addition of the first small amounts of ad-
the energy loss the element can be identified and a sim- sorbate. However, the relative area under peak 2 begins to
ple spectrum obtained. The broadening of the resonant decrease with further addition of NH 3 , starting at ∼2.0 ×