Catalyst Characterization
朱信
Hsin Chu
Professor
Dept. of Environmental Eng.
National Cheng Kung University
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1. Introduction
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Physical properties: pore size, surface area, and
morphology of the carrier; and the geometry and
strength of the support
Chemical properties: composition, structure, and
nature of the carrier and the active catalytic
components
Changes during the catalysis process:
deactivation
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2. Physical Properties of Catalysts
2.1 Surface Area and Pore Size of the Carrier
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Surface area
Pore size
Pore size distribution
Pore structure
Pore volume
The size and number of pores determine the internal
surface area. It is usually advantageous to have high
surface area (large number of small pores) to maximize
the dispersion of catalytic components.
However, if the pore size is too small, diffusional
resistance becomes a problem.
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2.2 surface Area and Pore Size Measurements
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A standardized procedure for determining the internal surface
area of a porous material with surface area greater than 1 or 2
m2/g is based on the adsorption of N2 at liquid N2 temperature
onto the internal surfaces of the carrier.
Each adsorbed N2 molecule occupies an area of the surface
comparable to its cross-sectional area (16.2Å2)
By measuring the number of N2 molecules adsorbed at
monolayer coverage, one can calculate the internal surface
area.
Next slide (Fig. 3.1a)
BET gas adsorptin plot
Flatten part: monolayer
Second rise: multiple layers
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The Brunaure, Emmett, and Teller (BET) equation describes the
relationship between N2 volume adsorbed at a given partial
pressure and the volume adsorbed at monolayer coverage:
P
1
(C  1) P


V ( P0  P ) VmC VmCP0
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where P = partial pressure of N2
P0 = saturation pressure at the experimental temperature
V = volume adsorbed at P
Vm= volume adsorbed at monolayer coverage
C = a constant
Next slide (Fig. 3.1b)
Linear form of the BET equation
The most reliable results are obtained at relative pressure (P/P0)
between 0.05 and 0.3.
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The same equipment can be used to determine the pore
size distribution of porous materials with diameters less
than 100Å, except that high relative pressures are used
for condensing N2 in the catalyst pores.
Next slide (Fig. 3.1c)
Pore size distribution measurement
The procedure involves measuring the volume adsorbed
in either the ascending or descending branch of the BET
plot at relative pressures close to 1.
This plot shows the carrier having a significant volume
of mesopores (diameters between 20 and 500Å)
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Capillary condensation occurs in the pores in accordance with the
Kelvin equation:
ln(
P
2 V cos 
)
P0
rRT
σ = surface tension of liquid nitrogen
θ = contact angle
V = molar volume of liquid nitrogen
r = radius of the pore
R = gas constant
T = absolute temperature
P = measured pressure
P0 = saturation pressure
The form of the Kelvin equation describes the desorption isotherm,
and it is the preferred one for calculation of pore size distribution.
(desorption requires a lower pressure compared to adsorption)
where
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2.3 Pore Size by Mercury Intrusion
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For materials with pore diameters greater than about 30Å, the
mecury intrusion method is preferred.
The penetration of mercury into the pores of a material is a
function of applied pressure.
The Washburn equation: 4 cos
d
P
where d = pore diameter, nm
p = applied pressure, atm
θ= wetting or contact angle, between the mercury and the
solid is usually 130o
γ= the surface tension of the mercury, 0.48 N/m
Next slide (Fig. 3.2)
The mercury intrusion method: 30Å-106Å
the BET method: < 100Å
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2.4 Particle Size Distribution of the Carrier
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Sieves of various mesh sizes
Reliable only for particles larger than about 40μm
(#400=38 μm)
Light scattering, image analysis, sedimentation,
centrifugation, and volume exclusion (coulter counter)
Reliable for finer particles
Laser techniques (more recently)
Next slide (Fig. 3.3)
Laser-generated particle size distribution for an Al2O3
carrier
F: fraction
U: cumulation
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2.5 Washcoat Thickness
Optical microscopy is the most commonly used method of obtaining
washcoat thickness.
2.6 Mechanical Strength of a Monolith
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Monoliths, particularly when used in a stacked mode (e.g., in
stationary pollution abatement) must resist crushing axially.
For automobile and truck application, and for ozone abatement in
aircraft, resistance to vibration and radial strength is important.
2.7 Adhesion
Erosion
Thermal shocks (start up and shutdown)
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2.8 Location and Analysis of Species within the Catalyst
 Scanning electron microscope (SEM) is equipped with an
energy dispersive analyzer (EDX) or wavelength
dispersive analyzer (WDS).
The bombardment of a sample with electrons generates X
rays characteristic of the elements present.
 Next slides (Fig. 3.4 a ~ Fig. 3.4 d)
WDS analysis for catalysts deposited on double-layered
washcoats
(a)(b): topcoat Pt dispersed on bottom coat SiO2
(c)(d): topcoat Pd dispersed on bottom coat Al2O3
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Next slide (Fig. 3.5)
A electron microscope line profile of a poisoned
automobile exhaust catalyst
P and Zn, originating from the lubricating oil, are
concentrated on the edge of the washcoat
indicating they physically deposited as aerosols.
Gaseous sulfur compounds such as SO2/SO3,
penetrate deeply and more uniformly into the
washcoat.
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3. Chemical Properties
3.1 Elemental Analysis
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Small amounts of promoter oxides intentionally added
(often < 0.1 %) can influence catalyst activity, selectivity,
and life.
Carriers are derived from raw materials, which contain
various impurities such as alkali and alkaline-earth
compounds, if used in excess, causing sintering or loss of
surface area in Al2O3. When added in the proper amount,
the same impurities can enhance stability against
sinterting or, in some cases, improve selectivity.
Therefore, the quantitative procedures used to analyze the
composition of catalysts are important.
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3.2 Thermal Gravimetric Analysis (TGA)
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TGA: a useful technique to follow microscopic weight
changes
A few milligrams of contaminated catalyst can be loaded into
a quartz pan suspended in the microbalance. A controlled gas
flow and temperature ramp is initiated and a profile of weight
change versus temperature is recorded.
Frequently, TGA units are equipped with a mass spectrometer
so that the offgases from the catalyst can be measured as a
function of temperature.
The weight-temperature profile is helpful in establishing
procedures for regenerating the catalyst and other processes in
the process reactor.
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3.3 Analysis by X-Ray Diffraction (XRD)
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Provided a material is sufficiently crystalline to diffract
X rays and is present in an amount greater than 1%, XRD
can be used for qualitative and quantitative analysis.
The angles of diffraction differ for the various planes
within the crystal. Thus, every compound or element has
its own somewhat unique diffraction pattern.
3.4 Structural Analysis: structure of Al2O3 carriers
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Next slide (Fig. 3.6)
XRD patterns of amorphous γ-Al2O3 and α-Al2O3
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γ-Al2O3: the high-surface-area, lower temperature structure
α-Al2O3: produced at high temperature and has
low surface area
Amorphous: Materials with crystallites smaller
than 50Å.
A well-defined X-ray pattern will not be
obtained.
Need to be characterized by other techniques
listed below.
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3.5 Dispersion or Crystallite Size of Catalytic Species
3.5.1 Chemisorption
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When a structure has a definite XRD pattern, it usually has less
than optimum activity. This is because most catalytic reactions
are favored by either amorphous materials or extremely small
crystallites.
Frequently, the purpose of the preparation technique is to
disperse the catalytic components to maximize their
availability to reactants:
no. of catalytic sites on the surface  100%
% dispersion =
theoretical no. of sites present
When this is done effectively, only small crystals are present
and the diffraction of X rays is minimized.
Selective chemisorption can be used to measure the accessible
catalytic component on the surface by noting the amount of gas
adsorbed per unit weight of catalyst.
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One assumes that the catalytic surface area is proportional to
the number of active sites. A gas that will selectively
chemisorb only onto the metal and not the support is used
under predetermined conditions.
Hydrogen and CO are most commonly used as selective
adsorbates for many supported metals.
The measurements are usually carried out in a static vacuum
system similar to that used for BET surface area
measurements.
Next slide (Fig. 3.7a)
Chemisorption isotherm (monolayer coverage, 1 H per metal
site)
One can determine the catalytic surface area by multiplying
molecules adsorbed by cross-sectional area of the site and
dividing by the weight of catalyst, e.g., the cross-sectional
area of Pt is 8.9Å2 and Ni, 6.5Å2.
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The static vacuum technique is time-consuming.
Alternatively, a dynamic pulse technique has been used
in which a pulse of adsorbate such as H2 or CO is
injected into a stream of inert gas and passed through a
bed of catalyst.
Next slide (Fig. 3.7b)
Pulse chemisporption profiles
The static method measures only species that are strongly
adsorbed.
The dynamic method, performed under equilibrium
conditions, measures strong and weakly chemisorbed
species.
Thus, static techniques usually give better dispersion
results.
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3.5.2 Transmission Electronmicroscopy (TEM)
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A thin sample is subjected to a beam of electrons. The
dark spots on the positive of the detecting film
correspond to dense areas in the sample that inhibit
electron transmission.
These dark spots form the outline of metal particles or
crystallites and, hence, their sizes can be determined.
Next slide (Fig. 3.8)
A TEM of sintered Pt, dispersed on TiO2 (500Å)
Following slide (Fig. 3.9)
Crystallites of Pt about 100Å in size, dispersed on CeO2
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3.5.3 X-Ray Diffraction
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The larger the crystals of a given component, the sharper the peaks on the
XRD pattern for each crystal plane.
The scherrer equation:
B
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k
L cos 
Where B = the breadth at half-peak height of an XRD line
L = the size of the crystallites
λ = X-ray wavelength
θ = diffraction angle
k = a constant usually equal to 1
As the crystallite size increases, the line breadth B decreases.
Next slide (Fig. 3.10)
XRD profile for different crystallite sizes of CeO2
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3.6 Surface Composition of Catalysts
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XRD and TEM measure the structure and/or chemical
composition of catalysts extending below the catalytic
surface.
The composition of the surface is usually different from
that of the bulk.
It is on these surfaces that the active sites exist and where
chemisorption, chemical reaction, and desorption take
place.
The tools available for surface composition
characterization are X-ray photoelectron spectroscopy
(XPS), Auger electron spectroscopy (AES), ion
scattering spectroscopy (ISS), and secondary-ion mass
spectroscopy (SIMS).
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XPS is used more widely than the others for studing the
surface composition and oxidation states of industrial catalysts.
XPS refers to the technique of bombarding the surface with Xray photons to produce the emission of characteristic electrons.
These are measured as a function of electron energy.
Because of the low energy of the characteristic electrons, the
depth to which the analysis is made is only ~40Å .
The composition of this thin layer as a function of depth can
be determined by removing or sputtering away top layers and
analyzing the underlying surfaces.
This technique can provide properties including oxidation
state of the active species, interaction of a metal with an oxide
carrier, and the nature of chemisorbed poisons and other
impurities.
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