Page 114 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN002G-87 May 19, 2001 20:3

512 Catalyst Characterization

fluid–solid interface, it is likely to be lost in an abrasive itive of the detecting film correspond to dense areas in the
environment. sample, which inhibit the transmission of electrons. These
The optimum catalyst location must be determined by dark spots form the outline of metal particles or crystal-
a delicate balance of the kinetics, which will control the lites, and hence their sizes can be determined. Electron
overall reaction rate, and anticipated deposits or abrasive diffraction can also be conducted, allowing the determi-
components contained in the feed. nation of the particle structure.
Transmission and scanning electron microscopes and The electron microprobe is similar to the scanning elec-
electron microprobes are electron optical instruments with tron microscope; however, its primary function is to de-
the same basic features, including an electron gun with tect characteristic X-rays produced by the electron beam
a variable high-voltage supply, magnetic lenses to focus interaction with the specimen. The X-ray emissions can
the electron beam, and detectors to record the images or be used to determine the elemental composition of the
characteristic X-ray emissions from the sample. All these specimen quantitatively and the location of a particular
features are packaged in a high-vacuum system capable element within the morphology or topological structure
of reaching at least 10 −5 torr. Transmission and scan- of the specimen.
ning electron microscopes and the electron microprobe
each exhibit capabilities that complement each other. The 2. Electron Microscopy: X-Ray Analysis
essential operating features of the STEM (an instrument
Both the electron microprobe and scanning electron mi-
combining scanning and transmission capabilities) will be
croscope have proved to be very useful in determining the
described since it embodies the important characteristics
location of metal in catalyst particles. The morphology
of each of these instruments.
and metal location of a typical carbon-supported metal
The basic electron optical system consists of an elec-
catalyst are shown in Fig. 11. The carbon particles are
tron source, an array of magnetic lenses to collimate the
potted in epoxy resin and then cut, ground, and polished
electron beam, objective lenses, and a projector lens that
to provide a cross-sectional surface to be examined. From
focuses the images at the focal plane. The collimated elec-
the morphology shown in Fig. 11, the particles are a wood-
tron beam passes through the specimen, is focused by the
basedcarbonwithporestypicalofthistype.Thepalladium
condenser lens on thebackfocal plane,andis magnified by
X-ray maps (Fig. 11B) clearly outline the edge coating of
intermediate lenses and finally the projector lens to form
palladium on the carbon particle. The coating penetrates
the final image. With the addition of deflector coils to the
no more than 15 µm into the interior of the carbon particle,
magnetic lens system, the electron beam can be rastered
which is ∼50 µm in diameter.
across a small area of the specimen. Defocusing of the
The location of the metal in a particle can also be deter-
objective lens increases the depth of field of the image
mined by running a line scan to detect the X rays charac-
in the transmission mode. Compared with that of opti-
teristic of palladium. As the electron beam moves across
cal microscopes the depth of field is much greater before
the particle, the detector signal is integrated and displayed
any loss of resolution is observed. When the electrons are
on a cathode ray tube as intensity versus electron beam
brought into focus in the back focal plane by the objec-
position. The resulting signal is shown in Fig. 11C.
tive lens, an electron diffraction pattern is obtained. The
Metal location is but one of a number of applications for
diffraction pattern provides structural information of crys-
scanning electron microscope studies in catalysis. Other
talline materials equivalent to X-ray diffraction patterns.
applications are the study of the morphology of platinum–
The crystal structure and consequently identification of
rhodium gauzes used in the oxidation of ammonia and
very small crystallites can be obtained by this technique,
the poisoning of catalysts, in which the scanning electron
an important measurement in catalytic research.
microscope results show the location of poisons such as
In the scanning mode the electron beam focused on the
compounds containing sulfur, phosphorus, heavy metals,
sample is scanned by a set of deflection coils. Backscat-
or coke relative to the location of the catalytic components.
tered electrons or secondary electrons emitted from the
sample are detected. As the electron beam passes over the
surface of the sample, variations in composition and topol- III. CHEMICAL PROPERTIES
ogy produce variations in the intensity of the secondary
electrons. The raster of the electron beam is synchronized A. Chemical Composition
with that of a cathode ray tube, and the detected signal
1. Elemental Analysis
then produces an image on the tube.
In the transmission mode a thin sample, usually pre- The precise nature of active sites is still the subject of con-
pared by a microtome, is subjected to a beam of electrons, siderable research; however, there is a huge body of data
and those transmitted are noted. The dark spots on the pos- relating various metals, metal oxides, and compounds that,