Supported nanosized gold catalysi the influence of support morphology and reaction mechanism 2

- 28 -
Chapter 2 Experimental Section
2.1 Catalyst Preparation
The method of preparation strongly influences the particle size.
1-4
, which is believed
to be one of the important factors that can influenc catalysts activity.
5-8
For most of
the reactions, only the catalysts with gold particles smaller than 5 nm lead to high
activity; this is especially true for the oxidation of carbon monoxide.
9,10
There are
two kinds of chemical preparation methods that are widely used in the preparation of
gold nano particles supported on metal oxide support.
The first kind is termed as co-precipitation, in which the support and the gold
precursor are formed at the same time. Co-precipitation is the carrying down by a
precipitate of substances normally soluble under the conditions employed.
11
There are
three main mechanisms of co-precipitation: inclusion, occlusion, and adsorption.
12
An
inclusion occurs when the impurity occupies a lattice site in the crystal structure of the
carrier, resulting in a crystallographic defect; this can happen when the ionic radius
and charge of the impurity are similar to those of the carrier. An adsorbate is an
impurity that is weakly bound (adsorbed) to the surface of the precipitate. An
occlusion occurs when an adsorbed impurity gets physically trapped inside the crystal
as it grows. Besides its applications in chemical analysis and in radiochemistry, co-
precipitation is also "potentially important to many environmental issues closely
related to water resources, including acid mine drainage, radionuclide migration in

fouled waste repositories, metal contaminant transport at industrial and defense sites,
metal concentrations in aquatic systems, and wastewater treatment technology".
13
Co-
precipitation is also used as a method of magnetic nanoparticle synthesis.
14
However
- 29 -
the co-precipitation method tends to be difficult to control and to reproduce.
Nucleation and growth can easily occur in the solution rather than on the carrier,
which results in undesirably large metal particles (low dispersion) and an
inhomogeneous metal distribution on the carrier,
The second kind of methods includes impregnation, ion adsorption, deposition-
precipitation and colloid-based methods. In these methods the gold precursor is
applied to the preformed support. Among these methods, deposition-precipitation and
colloid-based method were utilized in the preparation of gold nanoparticles supported
on iron oxide support.
Deposition-precipitation is a modification of the precipitation methods in solution. It
consists of the conversion of a highly soluble metal precursor in another substance of
lower solubility, which specifically precipitates onto a support and not in solution.
The conversion into the low soluble compound, and then into the precipitate, is
usually achieved by raising the pH of the solution. This can also be done by
decreasing the pH, or by changing the valence state of the metal precursor through
electrochemical reactions or by using a reducing agent, or by changing the
concentration of a complexing agent. The key point for a successful deposition-
precipitation is, therefore, the gradual addition of the precipitating agent to avoid local
rise of concentration above the solubility curve, which would cause a rapid nucleation
of the precipitate in solution. Deposition-precipitation is widely used in the
preparation of nanosized gold catalysts, but G.C. Bond mentioned in his book that the
term deposition-precipitation employed here is not so accurate.

15
It might be more
suitable to be described as grafting or ion adsorption, because deposition-precipitation
hints the forming of hydroxide or hydrated oxide for the deposition onto the surface of
- 30 -
the metal oxide support, while precipitation involves a nucleation by the support and
all the active phase attached to the support.
Colloidal-based method has the advantage of small mean particle size and narrow size
distribution under appropriate conditions, and the influence of the support is almost
ignorable. This method was first used in preparing nanosized gold particles by John
Turkevich and his associates in 1950s.
16,17
They used sodium citrate as the reduction
agent for the AuCl
4
-
ion. Many other reducing agents have been used ever since for
the reduction of gold related ions, including phosphorus, sodium thiocyanate, poly
ethylene-imine, tetrakis phosphonium chloride and sodium borohydride. The
advantage of using the colloidal route for preparing supported gold catalysts lies in
the way that condition of preparation can be manipulated to give particles having a
narrow size distribution about the desired mean.
In this thesis Au/iron oxide catalysts were prepared by various methods, including co-
precipitation (CP), deposition-precipitation (DP), and colloids-based methods, using
HAuCl
4
(sigma-aldrich) and Fe(NO
3
)
3

·9H
2
O (sigma-aldrich) as precursors. In the
case of the co-precipitation (CP) method, an aqueous mixture of the HAuCl
4
and
Fe(NO
3
)
3
precursors was poured into an aqueous solution of Na
2
CO
3
(0.25M) which
was maintained at 70
o
C under vigorous stirring (500 rpm). The precipitate was
washed, dried, and calcined in air at 110
o
C for 12 hrs. This co-precipitation sample is
coded AuCP. In the deposition-precipitation method, Au nanoparticles were
deposited on iron oxide support by keeping the pH value of the aqueous solution of
HAuCl
4
at pH = 8 using 0.1M NaOH. The Fe
2
O
3
support was generated, prior to the

DP process, from 1.0 M Fe(NO
3
)
3
solution. Excessive amount of 1.0M NaOH
solution was added to the Fe(NO
3
)
3
solution drop-wisely till all the iron ions in the
- 31 -
solution were deposited. Then the mixed solution was thoroughly washed using DI
water by centrifugation. The slurry after centrifuge was dried in 110
o
C oven for 48
hours. The above prepared sample was then calcined at 500
o
C for 5 hour. The as-
prepared iron oxide was mainly presented in -Fe
2
O
3
phase, with small amount of γ-
Fe
2
O
3
phase detectable by XRD. This self-prepared iron oxide sample was used as the
support for the AuDP catalyst (The deposition-precipitation sample is coded AuDP).
Two other samples, AuCH and AuCM were prepared using colloid-based method

with assistance of the ultrasound irradiation
18
. The support used for AuCH was
commercial Fe
2
O
3
(hematite, Sigma-Aldrich), while that for AuCM was commercial
Fe
3
O
4
(Magnetite, Sigma-Aldrech). In colloid-based method L-lysine was added as a
capping agent, which has better control on gold particle size compared to
conventional DP method used in literature. HAuCl
4
(1mM) was reduced by NaBH
4
(0.1M). During the reduction period, colloid-based method was applied. The nano-Au
particles were deposited on iron oxide supports. The slurry was dried at 70ºC after
centrifuge four times using DI water. As chloride ions is a poison to the catalytic
reaction and may affect the activity of catalyst, the addition of capping agent and
reduction agent and the followed washing procedure are able to remove almost of
chlorine in the solution.
2.2 X-ray Photoelectron Spectroscopy
X-ray Photoelectron Spectroscopy (XPS) is widely used to investigate the chemical
compositions and oxidation state of surfaces. Its surface specificity, applicability to
nearly all elements, and sensitivity to chemical state give XPS great potential for
contributing to the understanding of a wide variety of catalyst problems.
19

The X-rays
penetrate far into the solid (1~10

m) but the mean free path for the escape of a 100-
- 32 -
1500 eV electron without energy loss is only 1-8 nm. Secondary electron emission
and inelastic losses account for much of the background in the spectrum, but the
information carried in the spectral peaks applies to a thin surface layer because of the
relatively short electron mean free path. Thus, XPS is inherently a surface technique.
Surface analysis by XPS is accomplished by irradiating a sample with mono energetic
soft X-rays (usually Mg K

(1253.6 eV) or Al K

(1486.6 eV)) under ultra-high
vacuum (UHV) conditions, causing electrons to be emitted from the surface region by
the photoelectric effect and analyzing the energies of the detected electrons. The
emitted photoelectrons have measured kinetic energies given by.
20
KE h BE
s
  
 
(2.1)
where h

is the energy of the photon, BE is the binding energy of the atomic orbital
from which the electron originates, and

s

is the spectrometer work function. The
binding engery which is theoretically equivalent to the ionization energy of the
electron can also be regarded as the energy difference between the initial and final
states after the photoelectron has left the atom. Thus each element has a unique set of
binding energies. The binding energies of the core-level electrons are sufficiently
affected by differences in the chemical potential and polarizability of the neighboring
compounds. This would cause a detectable photoelectron energy shift, which ranges
from 0.1 up to 10 eV. This kind of shift is termed as chemical shift. Several types of
peaks are observed in XPS spectra. Photoelectron lines, Auger lines and shake-up
lines are the mostly encountered feature in our research. The dominant features in an
XPS spectrum are photoelectron lines. Most intense photoelectron lines are relatively
symmetrical and have typically the narrowest FWHM of 1-2 eV. Less intense
photoelectron lines at higher binding energies are usually 1-4 eV wider than the lines
- 33 -
at lower binding energies. All of the photoelectron lines of insulating solids are of the
order of 0.5 eV wider than photoelectron lines of conductors. Auger lines are
observable in XPS spectra, due to relaxation of the excited ions left after
photoemission. The Auger electron possesses the kinetic energy equal to the
difference between the energies of the initial ion and the doubly charged final ion.
Shake-up losses are final state effects which arise when the photoelectron imparts
energy to another electron of the atom. This electron ends up in a higher unoccupied
state (shake-up). This results in a satellite peak a few eV higher in BE than the main
peak.
During our XPS analysis, charging often occurs because our nano gold on semi-
conductor (CuO or TiO
2
) samples might lack of delocalized electrons to neutralize the
positively charged macroscopic clusters created by the ejection of photoelectrons
and/or Auger electrons. As a result, a positive potential builds up at the surface of the
sample, which retards the outgoing electrons. This retardation appears in the

spectrum as an additional positive shift in binding energies. Referring to the BE of
C1s (284.60.4 eV) contributed from the adventitious carbon is the commonly used
procedure to solve the charging problem. Any shift from this value is taken as a
measure of the steady-state static charging.
Quantification of XPS spectra is quite straightforward. The elemental composition of
the interested sample determined as followed:
%X = (A
x
/S
x
) / Σ
N
i=1
(A
x
/S
x
) (2.2)
where X is the element, A
x
the area under the peak of element X in the spectrum, and
S
x
is the relative atomic sensitivity factor. %X is the fractional atomic concentration.
- 34 -
In our research, the S
x
data are provided by the instrument manufacture VG Scientific
Ltd.
2.3 Secondary ion mass spectrometry (SIMS)

Secondary ion mass spectrometry (SIMS) is a technique used to analyze the
composition of solid surfaces and thin films by sputtering the surface of the specimen
with a focused primary ion beam and collecting and analyzing ejected secondary ions.
It is particularly useful in detecting hydrogen species on surfaces. While only charged
secondary ions emitted from the material surface through the sputtering process are
used to analyze the chemical composition of the material, these represent a small
fraction of the particles emitted from the sample. There are two different modes of
analytical SIMS application: static and dynamic SIMS. In static SIMS,
21,22
the
information on the composition of the uppermost monolayer is generated virtually
without disturbing the composition and structure of the surface. This is achieved by
very low primary ion current densities. For an ion current density of 10
-9
Acm
-2
the
life-time of a monolayer is in the order of some hours. Essentially static SIMS
provides a mass spectrum of the surface. The mix of elemental and cluster ions in the
spectrum can generate a rich store of information regarding the chemistry of the
surface layer. Hence static SIMS is a technique of great potential for understanding
the chemical behavior and structure of surfaces.
In dynamic SIMS, high primary ion-current densities up to some Acm
-2
is applied,
thus resulting in a short lifetime of a monolayer down to the 10
-3
sec range. This fast
surface erosion continuously moves the surface into the bulk material, thus supplying
- 35 -

information on the chemical composition of original subsurface layers of the
bombarded sample.
The secondary ions are measured with a mass spectrometer to determine the elemental,
isotopic, or molecular composition of the surface. SIMS is the most sensitive surface
analysis technique, being able to detect elements present in the parts per billion ranges.
There are three basic analyzers available: sector, quadrupole, and time-of-flight. The
time of flight mass analyzer separates the ions in a field-free drift path according to
their kinetic energy. Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS)
uses a pulsed primary ion beam to desorb and ionize species from a sample surface.
The resulting secondary ions are accelerated into a mass spectrometer, where they are
mass analyzed by measuring their time-of-flight from the sample surface to the
detector. There are three different modes of analysis in TOF-SIMS; 1) mass spectra
are acquired to determine the elemental and molecular species on a surface; 2) images
are acquired to visualize the distribution of individual species on the surface; and 3)
depth profiles are used to determine the distribution of different chemical species as a
function of depth from the surface.
23
It is the only analyzer type able to detect all
generated secondary ions simultaneously and is the standard analyzer for static SIMS
instruments. Static SIMS is the process involved in surface atomic monolayer
analysis. TOF-SIMS provides spectroscopy for characterization of chemical
composition, imaging for determining the distribution of chemical species, and depth
profiling. In the spectroscopy and imaging modes, only the outermost (1-2) atomic
layers of the sample are analyzed. The actual desorption of material from the surface
is caused by a "collision cascade" which is initiated by the primary ion impacting the
sample surface. The emitted secondary ions are extracted into the TOF analyzer by
applying a high voltage potential between the sample surface and the mass analyzer.
- 36 -
TOF-SIMS spectra are generated using a pulsed primary ion source (very short pulses
of <1 ns). Secondary ions travel through the TOF analyzer with different velocities,

depending on their mass to charge ratio (ke=½mv
2
).
2.4 X-ray Powder Diffraction (XRD)
X-ray diffraction is accomplished by beaming X-ray photons onto a sample and
recording the diffracted X-ray beam. The diffraction would take place only at those
angles which fulfill the Bragg condition.
24
n d
 
 2 sin
n =1, 2, (2.3)
where

is the wavelength of the X-ray, d inter-planar spacing of crystal lattice planes,

the glancing angle of incidence of the primary beam relative to the atomic plane,
and n an integer. The X-ray detector moves at an angular velocity twice that of the
rotating sample during a scan. This will ensure that the X-ray glancing angle of
incidence is equal to the diffracted beam glancing angle. Whenever the Bragg
condition is fulfilled, X-ray is reflected to the detector. The diffracted beam intensity
is recorded and plotted against 2 by a computer.
Information about the structure, composition as well as state of polycrystalline
materials can get from XRD analysis. A few typical applications include identification
of unknown structures based on the crystalline peaks, variable temperature studies,
precise measurement of lattice constants and residual strains as well as refinement of
atomic coordinates.
Bruker XRD D8 advance with Cu K
α
radiation was used for examination of the

crystalline structure. (see Figure 2.1) The intensity data were collected over a 2 θ
range of 20˚-70˚ with a scan speed of 5˚/min and a scan step of 0.02˚.
- 37 -
Figure 2.1 Bruker XRD D8 advance
2.5 Transmission Electron Microscopy (TEM)
The transmission electron microscope was first developed in the 1930’s. In a
transmission electron microscope, high energy electron beam is collimated by
magnetic lenses and allowed to pass through the specimen under high vaccum.
25
This
results in the diffraction pattern that contained transmitted beam and a number of
diffracted beams, which will be then imaged on a fluorescent screen below the
specimen. In this case, one can obtain the lattice spacings and symmetry information
for the structure under consideration.
25
The transmitted beam or one of the diffracted beams can also form magnified image
of the sample on the viewing screen. One can obtain the high-resolution image that
contains information on the atomic structural of the materials if the transmitted beam
and one or more diffracted beams are to recombine.
TEM offer much help in the study of local structure, morphology and chemistry of
material in extremely high resolution. Nonetheless, TEM requires preparation of
samples that can be time consuming. In addition, there are some materials especially
- 37 -
Figure 2.1 Bruker XRD D8 advance
2.5 Transmission Electron Microscopy (TEM)
The transmission electron microscope was first developed in the 1930’s. In a
transmission electron microscope, high energy electron beam is collimated by
magnetic lenses and allowed to pass through the specimen under high vaccum.
25
This

results in the diffraction pattern that contained transmitted beam and a number of
diffracted beams, which will be then imaged on a fluorescent screen below the
specimen. In this case, one can obtain the lattice spacings and symmetry information
for the structure under consideration.
25
The transmitted beam or one of the diffracted beams can also form magnified image
of the sample on the viewing screen. One can obtain the high-resolution image that
contains information on the atomic structural of the materials if the transmitted beam
and one or more diffracted beams are to recombine.
TEM offer much help in the study of local structure, morphology and chemistry of
material in extremely high resolution. Nonetheless, TEM requires preparation of
samples that can be time consuming. In addition, there are some materials especially
- 37 -
Figure 2.1 Bruker XRD D8 advance
2.5 Transmission Electron Microscopy (TEM)
The transmission electron microscope was first developed in the 1930’s. In a
transmission electron microscope, high energy electron beam is collimated by
magnetic lenses and allowed to pass through the specimen under high vaccum.
25
This
results in the diffraction pattern that contained transmitted beam and a number of
diffracted beams, which will be then imaged on a fluorescent screen below the
specimen. In this case, one can obtain the lattice spacings and symmetry information
for the structure under consideration.
25
The transmitted beam or one of the diffracted beams can also form magnified image
of the sample on the viewing screen. One can obtain the high-resolution image that
contains information on the atomic structural of the materials if the transmitted beam
and one or more diffracted beams are to recombine.
TEM offer much help in the study of local structure, morphology and chemistry of

material in extremely high resolution. Nonetheless, TEM requires preparation of
samples that can be time consuming. In addition, there are some materials especially
- 38 -
polymers that lose their crystallinity and mass upon interaction with strong electron
beam. Imaging resolution is limited to about 0.2nm because of the great difficulties in
manipulating the magnetic field to get a higher resolution.
25
2.6 Scanning Electron Microscopy (SEM)
JEOL JSM-6700F Field Emission Scanning Electron Microscope (SEM), shown in
Figure 2.2, was used to observe the particle shape, size and morphology.
In SEM, electron beam is used to bombard on a sample, which generates secondary
electrons (that reveals surface morphology), backscattered electrons (that reveals
composition contrast), characteristic X-ray (use in elemental analysis), etc. All the
signals generated are detected simultaneously by the individual detectors that are
currently mounted on the JSM-6700F SEM.
Figure 2.2 JEOL JSM-6700F Field Emission Scanning Electron Microscope (SEM)
2.7 BET Measurement
The surface area of the catalysts was measured by the Brunauer-Emmett-Teller (BET).
- 38 -
polymers that lose their crystallinity and mass upon interaction with strong electron
beam. Imaging resolution is limited to about 0.2nm because of the great difficulties in
manipulating the magnetic field to get a higher resolution.
25
2.6 Scanning Electron Microscopy (SEM)
JEOL JSM-6700F Field Emission Scanning Electron Microscope (SEM), shown in
Figure 2.2, was used to observe the particle shape, size and morphology.
In SEM, electron beam is used to bombard on a sample, which generates secondary
electrons (that reveals surface morphology), backscattered electrons (that reveals
composition contrast), characteristic X-ray (use in elemental analysis), etc. All the
signals generated are detected simultaneously by the individual detectors that are

currently mounted on the JSM-6700F SEM.
Figure 2.2 JEOL JSM-6700F Field Emission Scanning Electron Microscope (SEM)
2.7 BET Measurement
The surface area of the catalysts was measured by the Brunauer-Emmett-Teller (BET).
- 38 -
polymers that lose their crystallinity and mass upon interaction with strong electron
beam. Imaging resolution is limited to about 0.2nm because of the great difficulties in
manipulating the magnetic field to get a higher resolution.
25
2.6 Scanning Electron Microscopy (SEM)
JEOL JSM-6700F Field Emission Scanning Electron Microscope (SEM), shown in
Figure 2.2, was used to observe the particle shape, size and morphology.
In SEM, electron beam is used to bombard on a sample, which generates secondary
electrons (that reveals surface morphology), backscattered electrons (that reveals
composition contrast), characteristic X-ray (use in elemental analysis), etc. All the
signals generated are detected simultaneously by the individual detectors that are
currently mounted on the JSM-6700F SEM.
Figure 2.2 JEOL JSM-6700F Field Emission Scanning Electron Microscope (SEM)
2.7 BET Measurement
The surface area of the catalysts was measured by the Brunauer-Emmett-Teller (BET).
- 39 -
The surface area of a powdered or porous solid can be calculated from the volume of
gas adsorbed onto the surface of the solid. The surface area, thus measured, includes
the entire surface accessible to the gas whether external or internal. In general, solids
adsorb gases weakly bound due to Van der Waals forces only. To cause sufficient gas
to be adsorbed for surface area measurement, the solid must be cooled — normally to
the boiling point of the gas. Most often, Nitrogen is the gas (adsorbate) and the solid
is cooled with liquid nitrogen (77.35K). Adsorption continues until the amount of N
2
adsorbed is in equilibrium with the concentration in the gas phase. Quantachrome

Autosorb-6B Surface Area and Pore Size Analyzer were used for BET tests. The
equipment setup is showed in Figure 2.3. The concept of the theory is an extension of
the Langmuir theory, which is a theory for monolayer molecular adsorption, to
multilayer adsorption with the following hypotheses: (a) gas molecules physically
adsorb on a solid in layers infinitely; (b) there is no interaction between each
adsorption layer; and (c) the Langmuir theory can be applied to each layer. The
resulting BET equation is expressed by (2.4):
P and P
0
are the equilibrium and the saturation pressure of adsorbents at the
temperature of adsorption, ν is the adsorbed gas quantity (for example, in volume
units), and vm is the monolayer adsorbed gas quantity. c is the BET constant, which is
expressed by (2.5):
E
1
is the heat of adsorption for the first layer, and E
L
is that for the second and higher
layers and is equal to the heat of liquefaction.
(2.4)
(2.5)
- 40 -
Figure 2.3 Quantachrome Autosorb-6B Surface Area and Pore Size Analyzer (BET)
2.8 Infrared Spectroscopy (IR)
The study of the formation and reactivity of surface intermediates is an important
subject of surface science and heterogeneous catalysis. One of the best tools for this
kind of investigation is no doubt infrared (IR) spectroscopy, which is capable of
detecting surface compounds in very low concentration under reaction conditions.
26
Fourier Transform-Infrared Spectroscopy (FTIR) probes the molecular vibrations of

bonds. Light of different energies is directed through a sample. When a particular
energy (or frequency) of light matches a vibrational frequency of the molecule, the
molecule absorbs the light and vibrates. Peaks in an infrared spectrum are upside
down compared to other forms of spectroscopy to convey that the peak is a decreased
intensity, or absorbance of light. The region of an infrared spectrum below
approximately 1600 wavenumbers is known as the fingerprint region. The remaining
area, usually from approximately 1600 to 3500 wavenumbers, is used to measure
- 40 -
Figure 2.3 Quantachrome Autosorb-6B Surface Area and Pore Size Analyzer (BET)
2.8 Infrared Spectroscopy (IR)
The study of the formation and reactivity of surface intermediates is an important
subject of surface science and heterogeneous catalysis. One of the best tools for this
kind of investigation is no doubt infrared (IR) spectroscopy, which is capable of
detecting surface compounds in very low concentration under reaction conditions.
26
Fourier Transform-Infrared Spectroscopy (FTIR) probes the molecular vibrations of
bonds. Light of different energies is directed through a sample. When a particular
energy (or frequency) of light matches a vibrational frequency of the molecule, the
molecule absorbs the light and vibrates. Peaks in an infrared spectrum are upside
down compared to other forms of spectroscopy to convey that the peak is a decreased
intensity, or absorbance of light. The region of an infrared spectrum below
approximately 1600 wavenumbers is known as the fingerprint region. The remaining
area, usually from approximately 1600 to 3500 wavenumbers, is used to measure
- 40 -
Figure 2.3 Quantachrome Autosorb-6B Surface Area and Pore Size Analyzer (BET)
2.8 Infrared Spectroscopy (IR)
The study of the formation and reactivity of surface intermediates is an important
subject of surface science and heterogeneous catalysis. One of the best tools for this
kind of investigation is no doubt infrared (IR) spectroscopy, which is capable of
detecting surface compounds in very low concentration under reaction conditions.

26
Fourier Transform-Infrared Spectroscopy (FTIR) probes the molecular vibrations of
bonds. Light of different energies is directed through a sample. When a particular
energy (or frequency) of light matches a vibrational frequency of the molecule, the
molecule absorbs the light and vibrates. Peaks in an infrared spectrum are upside
down compared to other forms of spectroscopy to convey that the peak is a decreased
intensity, or absorbance of light. The region of an infrared spectrum below
approximately 1600 wavenumbers is known as the fingerprint region. The remaining
area, usually from approximately 1600 to 3500 wavenumbers, is used to measure
- 41 -
characteristic vibrational absorbances of functional groups, or structural
characteristics of a molecule.
27
The IR spectra in this study were recorded on FTS-
575C, BIO-RAD Or NICOLET MAGNA 550) Fourier-transform spectrometer. (see
figure 2.4)
Figure 2.4 Digilab Excalibur FTIR
There are several techniques for acquiring IR spectra of solid samples: transmission,
diffuse-reflectance, attenuated total reflection and photoacoustic spectroscopy, etc.
Two primary techniques for the acquisition of spectra in this thesis are outlined below.
A) Transmission Spectroscopy
Transmission spectroscopy has been the most widely used technique. It works on
solids, liquids and gases. For powder samples which strongly absorb IR radiation, the
KBr pellet technique is employed. In this technique, the sample is ground to powder
to reduce its particle size, then diluted in an inert, infrared transparent material, e.g.
KBr, and pressed into a self-supporting disk. The disk is then placed perpendicular to
a beam of IR radiation, and the spectrum is recorded by observing the transmitted
beam intensity as a function of IR wavenumber.
B) Diffuse-reflectance Spectroscopy
- 41 -

characteristic vibrational absorbances of functional groups, or structural
characteristics of a molecule.
27
The IR spectra in this study were recorded on FTS-
575C, BIO-RAD Or NICOLET MAGNA 550) Fourier-transform spectrometer. (see
figure 2.4)
Figure 2.4 Digilab Excalibur FTIR
There are several techniques for acquiring IR spectra of solid samples: transmission,
diffuse-reflectance, attenuated total reflection and photoacoustic spectroscopy, etc.
Two primary techniques for the acquisition of spectra in this thesis are outlined below.
A) Transmission Spectroscopy
Transmission spectroscopy has been the most widely used technique. It works on
solids, liquids and gases. For powder samples which strongly absorb IR radiation, the
KBr pellet technique is employed. In this technique, the sample is ground to powder
to reduce its particle size, then diluted in an inert, infrared transparent material, e.g.
KBr, and pressed into a self-supporting disk. The disk is then placed perpendicular to
a beam of IR radiation, and the spectrum is recorded by observing the transmitted
beam intensity as a function of IR wavenumber.
B) Diffuse-reflectance Spectroscopy
- 41 -
characteristic vibrational absorbances of functional groups, or structural
characteristics of a molecule.
27
The IR spectra in this study were recorded on FTS-
575C, BIO-RAD Or NICOLET MAGNA 550) Fourier-transform spectrometer. (see
figure 2.4)
Figure 2.4 Digilab Excalibur FTIR
There are several techniques for acquiring IR spectra of solid samples: transmission,
diffuse-reflectance, attenuated total reflection and photoacoustic spectroscopy, etc.
Two primary techniques for the acquisition of spectra in this thesis are outlined below.

A) Transmission Spectroscopy
Transmission spectroscopy has been the most widely used technique. It works on
solids, liquids and gases. For powder samples which strongly absorb IR radiation, the
KBr pellet technique is employed. In this technique, the sample is ground to powder
to reduce its particle size, then diluted in an inert, infrared transparent material, e.g.
KBr, and pressed into a self-supporting disk. The disk is then placed perpendicular to
a beam of IR radiation, and the spectrum is recorded by observing the transmitted
beam intensity as a function of IR wavenumber.
B) Diffuse-reflectance Spectroscopy
- 42 -
An alternative to transmission spectroscopy is the use of the diffuse reflectance
infrared Fourier transform spectroscopy (DRIFT), in which, the pure catalyst powder
can be used without prior mechanical treatment and without the use of a diluting agent,
such as potassium bromide. By introducing a suitable experimental setup it is possible
to let reaction gases flow through the loosely packed catalyst bed. The collection and
analysis of diffuse-reflected electromagnetic radiation as a function of frequency or
wavelength obtain a reflectance-absorption spectrum.
When IR radiation is directed onto the surface of a solid sample, two types of
reflectance could occur. One is specular reflectance and the other is diffuse
reflectance. The former is the radiation reflected directly off the sample surface (the
IR energy not absorbed by the sample). The latter is the radiation penetrated into the
sample and then emerged. It is the diffuse reflected radiation that carries the
information about the sample surface as well as its bulk, while the specularly reflected
radiation will distort the IR spectrum. A diffuse reflectance accessory is designed so
that the diffusely reflected energy is optimized and the specular component is
minimized. DRIFT appears to have some surface specificity and can detect surface
species more easily than transmission spectra. In this work, a Harrick Scientific
Diffuse Reflection Attachment (DRA) with a CHC-CHA reaction chamber had been
used for in situ investigations. In our study, about 10 mg of powder catalyst was
placed in a shallow cup in the reaction chamber and exposed to a beam of IR radiation.

The reaction chamber was covered and sealed by a dome with KBr windows,
allowing the incident and reflected IR radiation to pass through. The diffusely
reflected lights were collected by an elliptical mirror, and focused onto the detector of
the spectrometer. The sample can be cooled through a cooling fringe by liquid
nitrogen or heated by a cartridge heater to enable that in situ experiments can be
- 43 -
performed in the temperature range from 95 to 773 K. Reaction mixture was fed
through the catalyst bed and exited through the tube assigned for this purpose. The
reduction of catalyst was carried out with 50% hydrogen at 100 Nml.min-1. The
temperature was ramped up at +5°C/min to 350°C and the reduction took 3 h before
ramping down (-10°C/min) to reaction temperature. Hydrogen gas flow was replaced
by Ar when catalyst temperature reached 100-150°C. IR spectra were recorded on an
FT-IR spectromete, accumulating 16 scans at a resolution of 2 cm
-1
. Prior to each
experiment, the sample was first activated in-situ by heating in a flow of O
2
(total
flow: 50 ml/min) at 673K for 2 hours, followed by cooling to the desired temperature
and purging in He for 30 min. Then, the spectrum of clean surface was recorded
which was used as the background of the in-situ experiment. The IR spectra of surface
species in a flow of various gas mixtures (total flow: 50 ml/min) were recorded as a
function of time. Detailed experimental setup referred to Figure 2.5.
Figure 2.5 Mass control system for in-situ DRIFT studies
- 44 -
2.9 Temperature-Programmed Reduction (TPR)
TPR was used to determine the temperature at which reduction of the samples with
hydrogen gas occurs. The amount of oxygen reacted can be obtained by calibrating
peak area under the TPR curve. H
2

consumption was determined using a Thermal
Conductivity Detector (TCD) with a computer data acquisition system. A TCD
detector consists of an electrically-heated wire or thermistor. The temperature of the
sensing element depends on the thermal conductivity of the gas flowing around it.
Testing gas must have very different thermal conductivity from carrier gas. Changes
in thermal properties of the system results in a change in heat loss from the wire, a
consequent change in wire temperature and wire resistance and the bridge becomes
out of balance. The out-of-balance signal is amplified and fed to a recorder.
The experiment was carried out in a vertical lug flow reactor. 50mg of sample was
used for reaction. The reactor was filled with a mixture of 5%H
2
/Ar, at a flow rate of
50ml/min. Temperature was hold at 50˚C for 60min to stabilize the gas flowing, and
then increased linearly at a heating rate of 10˚C/min to 800˚C.
The flow rate was controlled by a Brooks Microprocessor Control & Read Out Unit
Models 0154. The temperature program was controlled by Carbolite Temperature
Programmer Eurotherm 2416CG. (Figure 2.6)
- 45 -
Figure 2.6 Carbolite Temperature Programmer Eurotherm 2416CG
2.10 Micro-reactor for Catalytic Studies
Catalytic evaluations were carried out at atmospheric pressure in a continuous-flow
fixed-bed quartz micro-reactor (I.D. 4 mm) packed with samples and quartz wool.
Before testing, the catalysts were pre-treated in situ with a flow of air (100 ml min
-1
)
for 1 h at 200
o
C and 300
o
C respectively. For CO oxidation reactions, the feed gas was

a mixture of 90%He + 5%CO + 5%O
2
, which was introduced into the reactor at a gas
hourly space velocity (GHSV) of 60,000 cm
3
g
-1
h
-1
. For preferential oxidation of CO
in the presence of hydrogen, the feed gas was a 70%H
2
+ 1%CO + 2%O
2
mixture
balanced with helium, and was introduced into the reactor at a GHSV of 60,000 cm
3
g
-1
h
-1
. For both reactions, the reaction products were analyzed on-line using
Shimadzu GC-2010 gas chromatography equipped with a thermal conductivity
detector (TCD). The catalysts were evaluated for activity (in terms of CO conversion)
and CO
2
productivity in a temperature range of 25-200
o
C. Measurement readings
were taken after the system had been stabilized for at least 15mins for every

designated reaction temperature. Heating and cooling cycles are programmed via
temperature controller. Temperature profiles are recorded by an acquisition system.
K-type thermocouples are used in the furnace temperature control system as well as to
- 46 -
acquire reactor temperature profile. Thermocouple diameter is about 0.5 mm in the
case of reactor temperature measurement. The reactor is positioned at the center of the
furnace and the 0.5 mm thermocouple is inserted into the center of the quartz tube.
The gas flow rate is measured by the flow rate meter and calibrated by bubble flow
meter. The productive gases are analyzed by an on-line gas chromatography (GC).
Quantitative analysis involves the measurement of either peak heights or peak areas,
which are proportional to the total mass of the solute contained in the peak. The
proportional constant is obtainable from the measurement of standard reference
materials.
- 47 -
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