SURFACE AND NANOMOLECULAR CATALYSIS - PART 1 pot

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SURFACE
AND
NANOMOLECULAR
CATALYSIS
edited by
Ryan Richards
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Surface and nanomolecular catalysis / edited by Ryan Richards.
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Dedication
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Preface
Few terms have been more commonly used and abused in the scientific literature than nano.
However, if one is able to sift through the vast amounts of nano literature, there are also numerous

reports that are of both academic and commercial importance. This is particularly true for the field
of catalysis in which rapid progress is being made that has transformed this once black art into a sci-
ence, which is understood on a molecular and even atomic level. These gains have been particularly
driven by the fields of surface and nanomolecular science with improvements in instrumentation and
experimental techniques that have facilitated scientists’ observations on the nano-size scale.
While the field of catalysis has a dramatic impact on our daily lives, it does not receive a pro-
portional coverage in the typical undergraduate and graduate educations. This is possibly due to the
broad range of expertise involved in the field, which includes physics, chemical engineering, and
all subdisciplines of chemistry. The impact of catalysis in our current everyday lives cannot be un-
derstated. It was recently estimated that 35% of global GDP depends on catalysis. In addition, there
are major hurdles for mankind that may be overcome with developments in catalysis. In particular,
the goal of sustainability with regard to energy and environmental concerns will most certainly re-
quire significant contributions from catalysis.
Catalysts are materials that change the rate at which chemical equilibrium is reached without
themselves undergoing any change. Through the phenomenon of catalysis, very small quantities of a
catalytic material can facilitate several thousand transformations. In addition to the remarkable
increases in activity observed in the presence of a catalyst, an additional attribute of catalysts is that
there is often a selectivity toward certain reaction products. Often, this selectivity is of greater impor-
tance than activity since a highly selective process eliminates the generation of wasteful by-products.
The field of nanotechnology has generated a great deal of interest primarily because on this size
scale, numerous new and potentially useful properties have been observed. These size-dependent
properties include melting point, specific heat, surface reactivities, optical, magnetic, and catalytic
properties. In addition to the numerous proposed applications, there are also concerns regarding the
environmental and health implications associated with the use of these materials. These concerns
are, however, particularly difficult to address because the properties of nanoscale materials are dif-
ferent from both the molecular and bulk forms and can even change as a result of small differences
terials as a function of size and shape is necessary to address the concerns about nanomaterials and
their applications. A significant contribution to this understanding will be generated through stud-
ies of Surface and Nanomolecular Catalysis.
Surface and Nanomolecular Catalysis contains an overview of the field as given by several

chapter demonstrating how surface science can elucidate reaction mechanisms. The emerging field
of combinatorial approaches in catalysis is given by Schunk, Busch, Demuth, Gerlach, Haas, Klein,
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in size and shape. A general understanding of the chemical and physical properties of nanoscale ma-
5). Regalbuto (Chapter 6) follows with an insightful chapter on the preparation of supported metal
and application of traditional subclasses of heterogeneous catalysts. These include metal oxides by
catalysts. The engineering of catalytic processes is presented by Hocevar (Chapter 7) followed by
structure and reaction control by Tada and Iwasawa (Chapter 8). The chapter covering the texturo-
acterization methods. This is followed by four chapters highlighting preparation, characterization,
logical properties of catalytic systems by Fenelonov and Melgunov (Chapter 9) presents an in-
depth examination of this critical area. Wallace and Goodman (Chapter 10) then provide an excellent
leading international scientists. Chapter 1 by Ma and Zaera provides an excellent overview of char-
and Zech (Chapter 11). The final three chapters cover important specialized areas of catalysis.
Ranjit and Klabunde (Chapter 2), colloids by Bönnemann and Nagabhushana (Chapter 3), micro-
Pârvulescu and Marcu (Chapter 12) present an overview of heterogeneous photocatalysis.
porous and mesoporous materials by Schmidt (Chapter 4), and skeletal catalysts by Smith (Chapter
Overall, each chapter is designed to be able to stand alone as a short course. However, when
taken together, the contents form a comprehensive overview of Surface and Nanomolecular
Catalysis, appropriate for both a graduate course and as a reference text. In addition, each chapter
includes several questions appropriate for a graduate course, which should be particularly helpful
to instructors.
Other important aspects of modern catalysis including bio- and homogeneous catalysis are be-
Further, the emerging areas of computational catalysis and immobilized catalysts are not included
here but are covered dedicated texts in the literature.
It is the hope of the editor that this book forms the foundation of graduate-level courses in
Surface and Nanomolecular Catalysis and aids students in the understanding of this multidiscipli-
nary subject. Further, the editor thanks the contributors for their hard work.
Ryan M. Richards
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© 2006 by Taylor & Francis Group, LLC
Yamaguchi (Chapter 13). Finally, the developing field of enantioselective heterogeneous catalysis
yond the scope of the current book and are themselves the themes of several excellent books.
is presented by Coman, Poncelet, and Pârvulescu (Chapter 14).
Liquid-phase oxidations catalyzed by polyoxometalates are covered by Mizuno, Kamata, and
The Editor
Ryan M. Richards was raised near Flint, Michigan. In 1994, he
completed both B.A. in chemistry and B.S. in forensic science at
Michigan State University. He then spent 2 years as an M.S. student at
Central Michigan University working on organometallic chemistry with
Professor Bob Howell. He was awarded a Ph.D. in 2000 for investigat-
ing the properties of metal oxide nanoparticles in the laboratory of
Professor Kenneth Klabunde at Kansas State University. In 1999, he was
an invited scientist at the Boreskov Institute of Catalysis in Novosibirsk,
Russia where he began investigating the catalytic properties of
nanoscale metal oxides. In 2000, he joined the research group of
Professor Helmut Bönnemann investigating colloidal catalysts and het-
erogeneous catalysis as a research fellow at the Max Planck Institute für
Kohlenforschung, Mülheim an der Ruhr, Germany. He joined the engineering and science faculty
at the International University Bremen in 2002 and is leading a research group focusing on the
preparation of novel nanoscale materials and catalysis.
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Contributors
Helmut Bönnemann
MPI fuer Kohlenforschung
Heterogene Katalyse
Mülheim an der Ruhr, Germany
Oliver Busch
Hte Aktiengesellschaft

Heidelberg, Germany
Simona M. Coman
Department of Chemical Technology
and Catalysis
University of Bucharest
Bucharest, Romania
Dirk G. Demuth
Hte Aktiengesellschaft
Heidelberg, Germany
Vladimir B. Fenelonov
Boreskov Institute of Catalysis
Novosibirsk, Russia
Olga Gerlach
Hte Aktiengesellschaft
Heidelberg, Germany
D. Wayne Goodman
Department of Chemistry
Texas A&M University
College Station, Texas
Alfred Haas
Hte Aktiengesellschaft
Heidelberg, Germany
Stanko Hoc

evar
Laboratory of Catalysis and
Chemical Reaction Engineering
National Institute of Chemistry
Ljubljana, Slovenia
Yasuhiro Iwasawa

Department of Chemistry
Graduate School of Science
The University of Tokyo
Tokyo, Japan
Keigo Kamata
Core Research for Evolutional Science
and Technology
Japan Science and Technology Agency
Saitama, Japan
Kenneth J. Klabunde
Department of Chemistry
Kansas State University
Manhattan, Kansas
Jens Klein
Hte Aktiengesellschaft
Heidelberg, Germany
Zhen Ma
Department of Chemistry
University of California
Riverside, California
Victor Marcu
Israel Electric Corporation
Orot Rabin Power Station
Hadera, Israel
Maxim S. Mel’gunov
Boreskov Institute of Catalysis
Novosibirsk, Russia
Noritaka Mizuno
Department of Applied Chemistry
School of Engineering

The University of Tokyo
Tokyo, Japan
and Core Research for Evolutional Science
and Technology
Japan Science and Technology
Agency
Saitama, Japan
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K. S. Nagabhushana
Forschungszentrum Karlsruhe
ITC-CPV
Eggenstein Leopoldshafen
Germany
Vasile I. Pârvulescu
Department of Chemical Technology
and Catalysis
University of Bucharest
Bucharest, Romania
Georges Poncelet
Unité de Catalyse et Chimie des
Matériaux Divisée
Universite Catolique de Louvain
Louvain-la-Neuve, Belgium
Ranjit T. Koodali
Department of Chemistry
University of South Dakota
Vermillion, South Dakota
John R. Regalbuto
Department of Chemical Engineering

University of Illinois
Chicago, Illinois
Wolfgang Schmidt
MPI Fuer Kohlenforschung
Heterogene Katalyse
Mülheim an der Ruhr, Germany
Stephan Andreas Schunk
Hte Aktiengesellschaft
Heidelberg, Germany
Andrew J. Smith
School of Chemical Engineering and
Industrial Chemistry
The University of New South Wales
Sydney, Australia
Mizuki Tada
Department of Chemistry
Graduate School of Science
The University of Tokyo
Tokyo, Japan
W. T. Wallace
Department of Chemistry
Texas A&M University
College Station, Texas
Kazuya Yamaguchi
Department of Applied Chemistry
School of Engineering
The University of Tokyo
Tokyo, Japan
and Core Research for Evolutional Science
and Technology

Japan Science and Technology
Agency
Saitama, Japan
Francisco Zaera
Department of Chemistry
University of California
Riverside, California
Torsten Zech
Hte Aktiengesellschaft
Heidelberg, Germany
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Contents
Chapter 1
Characterization of Heterogeneous Catalysts 1
Zhen Ma and Francisco Zaera
Chapter 2
Catalysis by Metal Oxides 39
Ranjit T. Koodali and Kenneth J. Klabunde
Chapter 3
Colloidal Nanoparticles in Catalysis 63
Helmut Bönnemann and K.S. Nagabhushana
Chapter 4
Microporous and Mesoporous Catalysts 95
Wolfgang Schmidt
Chapter 5
Skeletal Catalysts 141
Andrew J. Smith
Chapter 6
A Scientific Method to Prepare Supported Metal Catalysts 161

John R. Regalbuto
Chapter 7
Catalysis and Chemical Reaction Engineering 195
Stanko Hoc

evar
Chapter 8
Structure and Reaction Control at Catalyst Surfaces 229
Mizuki Tada and Yasuhiro Iwasawa
Chapter 9
Texturology 257
Vladimir B. Fenelonov and Maxim S. Mel’gunov
Chapter 10
Understanding Catalytic Reaction Mechanisms: Surface Science Studies
of Heterogeneous Catalysts 337
W.T. Wallace and D. Wayne Goodman
Chapter 11
High-Throughput Experimentation and Combinatorial
Approaches in Catalysis 373
Stephan Andreas Schunk, Oliver Busch, Dirk G. Demuth, Olga Gerlach,
Alfred Haas, Jens Klein, and Torsten Zech
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Chapter 12
Heterogeneous Photocatalysis 427
Vasile I. Pârvulescu and Victor Marcu
Chapter 13
Liquid-Phase Oxidations Catalyzed by Polyoxometalates 463
Noritaka Mizuno, Keigo Kamata, and Kazuya Yamaguchi
Chapter 14

Asymmetric Catalysis by Heterogeneous Catalysts 493
Simona M. Coman, Georges Poncelet, and Vasile I. Pârvulescu
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CHAPTER 1
Characterization of Heterogeneous Catalysts
Zhen Ma and Francisco Zaera
CONTENTS
1.1 Introduction 2
1.2 Structural Techniques 2
1.2.1 X-Ray Diffraction 2
1.2.2 X-Ray Absorption Spectroscopy 3
1.2.3 Electron Microscopy 5
1.3 Adsorption–Desorption and Thermal Techniques 7
1.3.1 Surface Area and Pore Structure 7
1.3.2 Temperature-Programmed Desorption and Reaction 8
1.3.3 Thermogravimetry and Thermal Analysis 9
1.3.4 Microcalorimetry 10
1.4 Optical Spectroscopies 12
1.4.1 Infrared Spectroscopy 12
1.4.2 Raman Spectroscopy 13
1.4.3 Ultraviolet–Visible Spectroscopy 15
1.4.4 Nuclear Magnetic Resonance 16
1.4.5 Electron Spin Resonance 18
1.5 Surface-Sensitive Spectroscopies 19
1.5.1 X-Ray and Ultraviolet Photoelectron Spectroscopies 19
1.5.2 Auger Electron Spectroscopy 20
1.5.3 Low-Energy Ion Scattering 21
1.5.4 Secondary-Ion Mass Spectroscopy 21
1.6 Model Catalysts 22

1.7 Concluding Remarks 25
References 26
Chapter 1 Questions 32
1
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1.1 INTRODUCTION
Characterization is a central aspect of catalyst development [1,2]. The elucidation of the struc-
tures, compositions, and chemical properties of both the solids used in heterogeneous catalysis and
the adsorbates and intermediates present on the surfaces of the catalysts during reaction is vital for
a better understanding of the relationship between catalyst properties and catalytic performance.
This knowledge is essential to develop more active, selective, and durable catalysts, and also to op-
timize reaction conditions.
In this chapter, we introduce some of the most common spectroscopies and methods available
for the characterization of heterogeneous catalysts [3–13]. These techniques can be broadly grouped
according to the nature of the probes employed for excitation, including photons, electrons, ions,
and neutrons, or, alternatively, according to the type of information they provide. Here we have cho-
sen to group the main catalyst characterization techniques by using a combination of both criteria
into structural, thermal, optical, and surface-sensitive techniques. We also focus on the characteri-
zation of real catalysts, and toward the end make brief reference to studies with model systems.
Only the basics of each technique and a few examples of applications to catalyst characterization
are provided, but more specialized references are included for those interested in a more in-depth
discussion.
1.2 STRUCTURAL TECHNIQUES
1.2.1 X-Ray Diffraction
of heterogeneous catalysts with crystalline structures [14–16]. XRD analysis is typically limited
to the identification of specific lattice planes that produce peaks at their corresponding angular
positions 2, determined by Bragg’s law, 2d sin ϭ n. In spite of this limitation, the character-
istic patterns associated with individual solids make XRD quite useful for the identification of the
and after reduction [17]. These data indicate that, regardless of the starting point (MnO

2
,Mn
2
O
3
,
or Mn
3
O
4
), the structure of the catalyst changes after pretreatment with H
2
to the same reduced
MnO phase, allegedly the one active for selective hydrogenation. In situ XRD is particularly
suited to follow these types of structural changes in the catalysts during pretreatments or catalytic
reactions [18,19].
X-ray diffraction can also be used to estimate the average crystallite or grain size of catalysts
and become broader for crystallite sizes below about 100 nm. Average particle sizes below about
60 nm can be roughly estimated by applying the Debye–Scherrer equation, D ϭ 0.89/(B
0
2
ϪB
e
2
)
1/2
cos , where B
0
is the measured width (in radians) of a diffraction line at half-maximum, and B
e

the
displays an example of the application of this method for the characterization of anatase TiO
2
pho-
tocatalysts [21]. In that case, the line width of the (101) diffraction peak at 25.4° was used to cal-
culate the average grain sizes of samples prepared using different procedures: a significant growth
in particle size was clearly observed upon high-temperature calcination.
In spite of the large success of XRD in routine structural analysis of solids, this technique does
present some limitations when applied to catalysis [1,9]. First, it can only detect crystalline phases,
and fails to provide useful information on the amorphous or highly dispersed solid phases so com-
mon in catalysts [22]. Second, due to its low sensitivity, the concentration of the crystalline phase
in the sample needs to be reasonably high in order to be detected. Third, XRD probes bulk phases,
2 SURFACE AND NANOMOLECULAR CATALYSIS
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bulk crystalline components of solid catalysts. This is illustrated by the example in Figure 1.1,
X-ray diffraction (XRD) is commonly used to determine the bulk structure and composition
which displays XRD patterns obtained ex situ for a number of manganese oxide catalysts before
corresponding width at half-maximum of a well-crystallized reference sample [14,20]. Figure 1.2
[14,20]. The XRD peaks are intense and sharp only if the sample has sufficient long-range order,
and is not able to selectively identify the surface structures where catalytic reactions take place.
Finally, XRD is not useful for the detection of reaction intermediates on catalytic surfaces.
1.2.2 X-Ray Absorption Spectroscopy
X-ray absorption can also be used for both structural and compositional analysis of solid catalysts
[23–25]. In these experiments, the absorption of x-rays is recorded as a function of photon energy
in the region around the value needed for excitation of a core electron of the element of interest. The
region near the absorption edge shows features associated with electronic transitions to the valence
and conduction bands of the solid. Accordingly, the x-ray absorption near-edge structure (XANES,
also called NEXAFS) spectra, which are derived from these excitations, provide information about
the chemical environment surrounding the atom probed [26–28]. Farther away from the absorption

edge, the extended x-ray absorption fine structure (EXAFS) spectra show oscillatory behavior due to
the interference of the wave of the outgoing photoelectron with those reflected from the neighboring
atoms. In EXAFS, a Fourier transform of the spectra is used to determine the local geometry of the
The power of x-ray absorption spectroscopy for the characterization of catalysts is illustrated
exchanged molybdo(vanado)phosphoric acid (NbPMo
11
(V)pyr) active for light alkane oxidation
[31]. Specifically, the left panel of the figure displays an enlarged view of the Nb near-edge electronic
spectra of the NbPMo
11
(V)pyr catalyst at different temperatures and under the conditions used for
CHARACTERIZATION OF HETEROGENEOUS CATALYSTS 3
XRD data of MnO
x
samples
MnO
2
Mn
2
O
3
Mn
3
O
4
MnO
2
after reduction
Mn
2

O
3
after reduction
Mn
3
O
4
after reduction
9080706050403020100
Intensity (a.u.)
2θ (deg)
Figure 1.1 XRD patterns for different manganese oxides before and after pretreatment in H
2
at 420°C [17]. The
top three traces correspond to the original MnO
2
, Mn
2
O
3
, and Mn
3
O
4
solids used in these experiments,
while the bottom three were obtained after H
2
treatment. It can be seen here that the catalysts are all
reduced to the same MnO phase regardless of the nature of the starting material. It was
inferred that MnO is the actual working catalyst in all cases, hence the similarity in methyl benzoate

hydrogenation activity obtained with all these MnO
x
solids. (Reproduced with permission from Elsevier.)
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in Figure 1.3, where both XANES and EXAFS spectra are shown for a pyridine salt of niobium-
neighborhood around the atom being excited [25,29,30].
butane oxidation. The data indicate that below 350°C, the predominant species is Nb
5ϩ
, as deter-
mined by comparison with the spectrum from a reference Nb
2
O
5
sample. At higher temperatures,
however, the data resemble that of NbO
2
, indicating the predominance of Nb
4ϩ
ions . This change
in niobium oxidation state is directly related to the activation of the catalyst for alkane oxidation.
4 SURFACE AND NANOMOLECULAR CATALYSIS
XRD patterns of TiO
2
samples
(a) TiO
2
(hydrothermally treated at 80°C), 6 nm
(b) TiO
2

(hydrothermally treated at 180°C), 11 nm
(c) TiO
2
(thermally calcinated at 450°C), 21 nm
706050403020
Intensity (a.u.)
2θ (deg)
Figure 1.2 XRD patterns for three TiO
2
samples obtained by hydrothermal treatments at 80°C (a) and 180°C
(b) and after calcination at 450°C (c) [21]. From the six XRD peaks identified with the anatase phase,
the broadening of the (101) peak at 25.4° was chosen to estimate the average grain size of these
samples. Generally, the sharper the peaks, the larger the particle size. The differences in grain size
identified in these experiments were correlated with photocatalytic activity. (Reproduced with per-
mission from The American Chemical Society.)
RT
200°C
350°C
420°C
380°C
reaction
Nb K edge NbPMo
11
VO
40
pyr
NbPMo
11
VO
40

pyr
Absorption coefficient
18.99 19.0519.0419.0319.0219.0119
Energy (keV)
EXAFS
Fourier transform
Nb-O
0.04
Nb-Mo
4321056
R (Å)
Figure 1.3
Left
: Detailed view of the Nb K-edge XANES data of a pyridine salt of niobium-exchanged
molybdo(vanado)phosphoric acid (NbPMo
11
(V)pry) as a function of temperature [31]. A change in
niobium oxidation state, from Nb
5ϩ
to Nb
4ϩ
, is identified between 350 and 420°C by a relative in-
crease in absorption about 19.002 keV, and can be connected with the activation of the catalyst for
light alkane oxidation.
Right
: Radial Fourier-transform EXAFS function for the NbPMo
11
(V)pyr sam-
ple heated to 420°C [31]. The two peaks correspond to the Nb–O (1.5 Å) and Nb–Mo (3 Å) distances
in the heteropolymolybdate fragments presumed to be the active phase for alkane oxidation.

(Reproduced with permission from Elsevier.)
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k-weighed background-subtracted EXAFS data from the solid heated to 420°C [31]. This spectrum
shows two major peaks, one at about 1.5 Å associated with backscattering from O neighbors, and a
second at 3 Å related to the Nb–Mo pairs. The measured distances are consistent with a combina-
tion of niobium oxo species and heteropolymolybdate fragments, presumably the catalytically
active phase.
Several advantages and limitations are associated with the use of x-ray absorption spectroscopy
for catalyst characterization. On the positive side, no long-range order is needed in the samples
cuum environments, and can be employed in situ during catalysis [19]. However, XANES is not
very sensitive to variations in electronic structure, and the interpretation of the spectra is difficult,
often requiring the use of reference samples and high-level theory. EXAFS only provides average
values for the interatomic distances; it cannot be used to directly identify the chemical nature of the
neighboring atoms, and is not very sensitive to the coordination number. Finally, x-ray absorption
experiments typically require the use of expensive synchrotron facilities.
1.2.3 Electron Microscopy
Electron microscopy (EM) is a straightforward technique useful for the determination of the
morphology and size of solid catalysts [32,33]. Electron microscopy can be performed in one of two
modes — by scanning of a well-focused electron beam over the surface of the sample, or in a trans-
mission arrangement. In scanning electron microscopy (SEM), the yield of either secondary or
back-scattered electrons is recorded as a function of the position of the primary electron beam, and
the contrast of the signal used to determine the morphology of the surface: the parts facing the
detector appear brighter than those pointing away from the detector [34]. Dedicated SEM instru-
ments can have resolutions down to 5 nm, but in most cases, SEM is only good for imaging catalyst
particles and surfaces of micrometer dimensions. Additional elemental analysis can be added to
SEM via energy-dispersive analysis of the x-rays (EDX) emitted by the sample [34].
9 3 1.2
O
x

catalyst used in the selective oxida-
tion of acrolein to acylic acid [35]. Although SEM analysis of the fresh sample failed to reveal any
crystalline structure (data not shown), the images in Figure 1.4 clearly indicate the formation of
well-resolved crystals after activation of the catalyst in the reaction mixture. In addition, the EDX
lites of the catalyst. This analysis helped pin down the crystalline (MoVW)
5
O
14
-type structure as the
catalytically active phase. The EM images in this example were taken ex situ, that is, after transfer-
ring the used catalysts from the reactor to the microscope, but in situ imaging of working catalysts
is also possible [36,37].
Transmission electron microscopy (TEM) resembles optical microscopy, except that electro-
magnetic instead of optical lenses are used to focus an electron beam on the sample. Two modes are
available in TEM, a bright-field mode where the intensity of the transmitted beam provides a two-
dimensional image of the density or thickness of the sample, and a dark-field mode where the elec-
tron diffraction pattern is recorded. A combination of topographic and crystallographic information,
including particle size distributions, can be obtained in this way [32].
sized catalysts such as metal oxide particles, supported metals, and catalysts with nanopores
2
solid (A), and the particle size distribution estimated from statistical analysis of a number of simi-
lar pictures (B) [42]. Spherical Au particles, well dispersed on the surface of the round TiO
2
grains,
are clearly seen in the picture, with sizes ranging from 2 to 8 nm and averaging 4.7 nm. A good cor-
relation was obtained in this study between particle size and catalytic activity for CO
oxidation and acetylene hydrogenation reactions. High-resolution TEM (HRTEM), being capable of
CHARACTERIZATION OF HETEROGENEOUS CATALYSTS 5
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The right panel of Figure 1.3 displays the radial function obtained by Fourier transformation of the
Figure 1.4 shows SEM and EDX data for a Mo V W
spectra obtained from these samples point to variations in composition among the different crystal-
[38–41]. As an example, Figure 1.5 shows a TEM image of Au nanoparticles supported on a TiO
Since TEM has a higher resolution than SEM (down to 0.1 nm), it is often used to image nano-
under study, since only the local environment is probed. Also, this technique works well in nonva-
imaging individual planes in crystalline particles, can provide even more detailed structural infor-
mation on the surface of the catalysts [40,43].
Electron microscopy does have some limitations. For example, this technique usually requires spe-
cial sample preparations. Caution also needs to be exercised to minimize any electron beam-induced
6 SURFACE AND NANOMOLECULAR CATALYSIS
Mol: 61 at.%
VK: 29 at.%
WM: 11 at.%
Mol: 67 at.%
VK: 20 at.%
WM: 13 at.%
1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.001.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00
WL
WL
VK
VK
WM
WM
OK
OK
MoL
MoL
2 µm10 µm
AB

SEM from a M
O−V−W oxide catalyst
Figure 1.4 SEM images and EDX data from a Mo
9
V
3
W
1.2
O
x
catalyst after activation during the oxidation of
acrolein [35]. The pictures indicate that needle-like (A), platelet-like (B), and spherical (not shown)
particles are formed during exposure to the reaction mixture. EDX analysis at different spots, two of
which are exemplified here, point to V, Mo, and W contents that vary from 19 to 29, 60 to 69, and 11
to 13 atom%, respectively. It was determined that the
in situ
formation of a (MoVW)
5
O
14
-type phase
accounts for the increase in acrolein conversion observed during the initial reaction stages.
(Reproduced with permission from Elsevier.)
Particle size
distribution
b
25
20
15
10

5
0
12345678910
Particle size (nm)
Number of particles (%)
a
20 nm
20 nm
TEM, Au/TiO
2
Figure 1.5 Representative TEM image (a) and particle size distribution (b) obtained for a Au/TiO
2
catalyst pre-
pared by grafting of a [Au
6
(PPh
3
)
6
](BF
4
)
2
complex onto TiO
2
particles followed by appropriate re-
duction and oxidation treatments [42].The gold particles exhibit approximately spherical shapes and
an average particle size of 4.7 nm.The measured Au particle sizes could be well correlated with the
activity of the catalyst for carbon monoxide oxidation and acetylene hydrogenation. (Reproduced
with permission from Springer.)

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effects, such as changes in the specimen due to local heating, electronic excitations, or deposition of
contaminants during observation [40]. In addition, SEM and TEM work best for sturdy solids, and
are not well suited to detect reaction intermediates on catalyst surfaces. Finally, and importantly, sta-
tistical analysis of a large number of images is needed to get meaningful information on particle size
distributions. It is best to correlate such results with information obtained by other characterization
methods [38].
1.3 ADSORPTION–DESORPTION AND THERMAL TECHNIQUES
1.3.1 Surface Area and Pore Structure
Most heterogeneous catalysts, including metal oxides, supported metal catalysts, and zeolites, are
porous materials with specific surface areas ranging from 1 to 1000 m
2
/g [1]. These pores can display
fairly complex size distributions, and can be broadly grouped into three types, namely, micropores
(average pore diameter d Ͻ 2 nm), mesopores (2 Ͻ d Ͻ 50 nm), and macropores (d Ͼ 50 nm). The
surface area, pore volume, and average pore size of such porous catalysts often play a pivotal role in
determining the number of active sites available for catalysis, the diffusion rates of reactants and
products in and out of these pores, and the deposition of coke and other contaminants. The most com-
mon method used to characterize the structural parameters associated with pores in solids is via the
measurement of adsorption–desorption isotherms, that is, of the adsorption volume of a gas, typically
nitrogen, as a function of its partial pressure [44–48].
Given the complexity of the pore structure in high-surface-area catalysts, six types of adsorp-
tion isotherms have been identified according to a classification advanced by IUPAC [45–48]. Out
of these six, only four are usually found in catalysis:
●
Type II, typical of macroporous solids where the prevailing adsorption processes are the formation of
a monolayer at low relative pressures, followed by gradual and overlapping multilayer condensation
as the pressure is increased.
●

Type IV, often seen in mesoporous solids, where condensation occurs sharply at a pressure determined
by Kelvin-type rules.
●
Type I, characteristic of microporous solids, where pore filling takes place without capillary con-
densation, and is indistinguishable from the monolayer formation process.
●
Type VI, corresponding to uniform ultramicroporous solids, where the pressure at which adsorption
takes place depends on surface–adsorbate interactions, and shows isotherms with various steps each
corresponding to adsorption on one group of energetically uniform sites.
A number of models have been developed for the analysis of the adsorption data, including the
most common Langmuir [49] and BET (Brunauer, Emmet, and Teller) [50] equations, and others such
as t-plot [51], H–K (Horvath–Kawazoe) [52], and BJH (Barrett, Joyner, and Halenda) [53] methods.
The BET model is often the method of choice, and is usually used for the measurement of total sur-
face areas. In contrast, t-plots and the BJH method are best employed to calculate total micropore and
mesopore volume, respectively [46]. A combination of isothermal adsorption measurements can pro-
vide a fairly complete picture of the pore size distribution in solid catalysts. Many surface area ana-
lyzers and software based on this methodology are commercially available nowadays.
A recent example of the type of data that can be obtained with such instrumentation is presented
mesoporous silica, SBA-15, used as support for many high-surface-area catalysts. The isotherm,
identified as type IV according to the IUPAC classification, is typical of mesoporous materials.
Three regions are clearly seen with increasing nitrogen pressure, corresponding to monolayer–
multilayer adsorption, capillary condensation, and multilayer adsorption on the outer particle sur-
faces, respectively. A clear H1-type hysteresis loop, characterized by almost vertical and parallel
CHARACTERIZATION OF HETEROGENEOUS CATALYSTS 7
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in Figure 1.6 [54]. This corresponds to the nitrogen adsorption–desorption isotherm obtained for a
but displaced lines in the adsorption and desorption branches, is also observed in the adsorption–
desorption isotherm, indicating the presence of uniform cylindrical pore channels.
Aside from N

2
adsorption, Kr or Ar adsorption can be used at low temperatures to determine
low (Ͻ1m
2
/g) surface areas [46]. Chemically sensitive probes such as H
2
,O
2
, or CO can also be
employed to selectively measure surface areas of specific components of the catalyst (see below).
Finally, mercury-based porosimeters, where the volume of the mercury incorporated into the pores
is measured as a function of increasing (well above atmospheric) pressures, are sometimes used to
determine the size of meso- and macropores [1]. By and large, the limitations of all of the above
methods are that they only provide information on average pore volumes, and that they usually lack
chemical sensitivity.
1.3.2 Temperature-Programmed Desorption and Reaction
As stated above, when probes with specific adsorption characteristics are used, additional chemi-
cal information can be extracted from adsorption–desorption experiments. Temperature-programmed
desorption (TPD) in particular is often employed to obtain information about specific sites in cata-
lysts [55,56]. The temperature at which desorption occurs indicates the strength of adsorption,
whereas either the amount of gas consumed in the uptake or the amount of desorption upon heating
attests to the concentration of the surface sites. The most common molecules used in TPD are NH
3
and CO
2
, which probe acidic and basic sites, respectively, but experiments with pyridine, O
2
,H
2
,

CO, H
2
O, and other molecules are often performed as well [57–59]. As an example, the ammonia
8 SURFACE AND NANOMOLECULAR CATALYSIS
800
600
400
200
0
0 0.2 0.4 0.6 0.8 1
BET surface area 850 m
2
/g
Mesoporous silica
SBA-15
Relative pressure (P/P
0
)
5004003002001000
Pore size 89 Å
Pore volume 1.17 cm
3
/g
Pore diameter (Å)
8
6
4
2
0
Pore volume (cm

3
/g STP) Vol adsorbed (cm
3
/g STP)
Figure 1.6
Top
: Low-temperature nitrogen adsorption (•) and desorption (ϫ) isotherms measured on a calcined
SBA-15 mesoporous silica solid prepared using an EO
20
PO
70
EO
20
block copolymer [54].
Bottom
:
Pore size distribution derived from the adsorption isotherm reported at the top [54]. A high surface
2
volume (1.17 cm
3
/g) were all estimated from these data. These properties make this material suit-
able for use as support in the preparation of high-surface-area solid catalysts. (Reproduced with per-
mission from The American Chemical Society.)
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area (850 m /g), a uniform distribution of cylindrical nanopores (diameter ϳ90 Å), and a large pore
TPD data in Figure 1.7 show how special treatments of a V
2
O
5

/TiO
2
catalyst can influence its prop-
erties in terms of the strength and distribution of acid sites. These treatments can be used to tune se-
Some solid samples may decompose or react with the probe molecules at elevated temperatures,
causing artifacts in the TPD profiles [58]. However, this conversion can in some instances be used
to better understand the reduction, oxidation, and reactivity of the catalyst. In this mode, the tech-
nique is often called temperature-programmed reduction (TPR), temperature-programmed oxida-
tion (TPO), or, in general, temperature-programmed surface reaction (TPSR or TPR) [55,56,60].
The principles of TPR, TPO, and TPSR are similar to those of TPD, in the sense that either the up-
take of the reactants or the yields of desorption are recorded as a function of temperature.
Nevertheless, there can be subtle differences in either the way the experiments are carried out or the
scope of the application. TPSR in particular often requires the use of mass spectrometry or some
other analytical technique to identify and monitor the various species that desorb from the surface.
MoO
3
/Al
2
O
3
catalyst. There, the production of water, formaldehyde, and dimethyl ether was de-
tected above 100°C, around 250°C, and about 200°C, respectively [61]. Such information is key for
the elucidation of reaction mechanisms.
These TPD techniques reflect the kinetics (not thermodynamics) of adsorption, and are quite
useful for determining trends across series of catalysts, but are often not suitable for the derivation
of quantitative information on surface kinetics or energetics, in particular on ill-defined real cata-
lysts. Besides averaging the results from desorption from different sites, TPD detection is also com-
plicated in porous catalysts by simultaneous diffusion and readsorption processes [58].
1.3.3 Thermogravimetry and Thermal Analysis
Changes in catalysts during preparation, which often involves thermal calcination, oxidation,

and reduction, can also be followed by recording the associated variations in sample weight, as in
normal thermogravimetry (TG) or differential thermogravimetry (DTG); or in sample temperature,
CHARACTERIZATION OF HETEROGENEOUS CATALYSTS 9
NH
3
-TPD on V
2
O
5
/TiO
2
samples
H
2
760 torr
0.10 mol/kg
Evac.
0.084 mol/kg
O
2
760 torr
0.081 mol/kg
1000800600400
0
0.0005
0.0015
0.001
NH
3
concentration (mol/m

3
)
Temperature (K)
Figure 1.7 Ammonia TPD from a V
2
O
5
/TiO
2
catalyst after different pretreatments [59]. Two TPD peaks at 460
and 610 K are seen in the data for the oxidized sample, whereas only one is observed at 520 K for
the catalyst obtained after either evacuation or reduction. This indicates that the type of treatment
used during the preparation of the catalyst influences both the amount and the distribution of acidic
sites on the V
2
O
5
/TiO
2
surface. (Reproduced with permission from Elsevier.)
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lectivity in partial oxidation reactions [59].
Figure 1.8 shows an example of such application for the case of methanol adsorbed on a
as in differential thermal analysis (DTA) [62–64]. Although these thermal methods are quite tradi-
TG, DTG, and DTA techniques can be used to better understand and design procedures for catalyst
preparation [65]. In this case, a MgFe
2
O
4

spinel, used for the selective oxidation of styrene, was pre-
pared by co-precipitation from a solution containing Fe(NO
3
)
3
and Mg(NO
3
)
2
, followed by thermal
calcination. The data show that the initial amorphous precursor undergoes a number of transforma-
tions upon calcination, including the losses of adsorbed and crystal water around 110 and 220°C,
respectively, its decomposition and dehydroxylation into a mixed oxide at 390°C, and the forma-
tion of the MgFe
2
O
4
spinel at 640°C.
Besides the prediction of calcination temperatures during catalyst preparation, thermal analysis is
also used to determine the composition of catalysts based on weight changes and thermal behavior
during thermal decomposition and reduction, to characterize the aging and deactivation mechanisms
However, these techniques lack chemical specificity, and require corroboration by other characteriza-
tion methods.
1.3.4 Microcalorimetry
Another thermal analysis method available for catalyst characterization is microcalorimetry,
which is based on the measurement of the heat generated or consumed when a gas adsorbs and re-
acts on the surface of a solid [66–68]. This information can be used, for instance, to determine the
relative stability among different phases of a solid [69]. Microcalorimetry is also applicable in the
measurement of the strengths and distribution of acidic or basic sites as well as for the characteri-
for ammonia adsorption on H-ZSM-5 and H-mordenite zeolites [70], clearly illustrating the differ-

ences in both acid strength (indicated by the different initial adsorption heats) and total number of
acidic sites (measured by the total ammonia uptake) between the two catalysts.
10 SURFACE AND NANOMOLECULAR CATALYSIS
TPSR of methanol on MoO
3
/Al
2
O
3
m/e 32 (methanol)
m/e 18 (water)
(methanol +
formaldehyde)
m/e 28
400350300250200150100500
Temperature (°C)
70
60
50
40
30
20
10
0
−10
Mass spectral intensity (a.u.)
m/e 45
(dimethyl ether)
(formaldehyde)
m/e 30

Figure 1.8 TPSR spectra obtained after saturation of a MoO
3
/Al
2
O
3
catalyst with methanol at room tempera-
ture [61]. Seen here are mass spectrometry traces corresponding to methanol (
m
/
e
ϭ 28 and 32),
formaldehyde (
m
/
e
ϭ 28 and 30), water (
m
/
e
ϭ 18), and dimethyl ether (
m
/
e
ϭ 45).These data were
used to propose a mechanism for the selective oxidation of methanol on MoO
3
-based catalysts.
(Reproduced with permission from Elsevier.)
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© 2006 by Taylor & Francis Group, LLC
tional, they are still used often in catalysis research. In Figure 1.9, an example is provided on how
zation of metal-based catalysts [66–68]. For instance, Figure 1.10 presents microcalorimetry data
of catalysts, and to investigate the acid–base properties of solid catalysts using probe molecules.
Recent advances have led to the development of microcalorimeters sensitive enough for low-
surface-area (ϳ1cm
2
) solids [71]. This instrumentation has already been used in model systems to
determine the energetics of bonding of catalytic particles to the support, and also in adsorption and
reaction processes [72,73].
CHARACTERIZATION OF HETEROGENEOUS CATALYSTS 11
TG, DTG, and DTA data for the preparation
of a MgFe
2
O
4
spinel catalyst
35
30
25
20
15
10
5
0
−5
Weight loss (%)
0 200 400
Temperature (°C)
600 800 1000

TG
DTG
DTA
Exo
Figure 1.9 TG, DTG, and DTA profiles for an amorphous catalyst precursor obtained by coprecipitation of
Fe(NO
3
)
3
and Mg(NO
3
)
2
in solution [65]. This precursor is heated at high temperatures to produce a
MgFe
2
O
4
spinel, used for the selective oxidation of styrene. The thermal analysis reported here
points to four stages in this transformation, namely, the losses of adsorbed and crystal water at 110
and 220°C, respectively, the decomposition and dehydroxylation of the precursor into a mixed oxide
at 390°C, and the formation of the MgFe
2
O
4
spinel at 640°C. Information such as this is central in
the design of preparation procedures for catalysts. (Reproduced with permission from Elsevier.)
250
210
170

130
90
50
0 200 400 600 800 1000 1200
NH
3
coverage (µmol/g)
Microcalorimetry data for ammonia
adsorption on H-ZSM-5 ( ) and H-M ( )
Differential heat of adsorption (kJ/mol)
Figure 1.10 Differential heats of adsorption as a function of coverage for ammonia on H-ZSM-5 (o) and H-
mordenite (•) zeolites [70]. In both cases, the heats decrease with the extent of NH
3
uptake, indi-
cating that the strengths of the individual acidic sites on each catalyst are not uniform. On the other
hand, the H-ZSM-5 sample has a smaller total number of acidic sites. Also, the H-mordenite sam-
ple has a few very strong sites, as manifested by the high initial adsorption heat at low ammonia
coverage. These data point to a significant difference in acidity between the two zeolites. That may
account for their different catalytic performance. (Reproduced with permission from Elsevier.)
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1.4 OPTICAL SPECTROSCOPIES
1.4.1 Infrared Spectroscopy
In catalysis, infrared (IR) spectroscopy is commonly used to characterize specific adsorbates.
Because of the localized nature and particular chemical specificity of molecular vibrations, IR spec-
tra are quite rich in information, and can be used to extract or infer both structural and compositional
information on the adsorbate itself as well as on its coordination on the surface of the catalyst. In
some instances, IR spectroscopy is also suitable for the direct characterization of solids, especially
if they can be probed in the far-IR region (10–200 cm
Ϫ1

) [74–76].
Several working modes are available for IR spectroscopy studies [74–76]. The most common
arrangement is transmission, where a thin solid sample is placed between the IR beam and the
detector; this mode works best with weakly absorbing samples. Diffuse reflectance IR (DRIFTS)
offers an alternative for the study of loose powders, strong scatters, or absorbing particles.
Attenuated total reflection (ATR) IR is based on the use of the evanescent wave from the surface of
an optical element with trapezoidal or semispherical shape, and works best with samples in thin
flat reflecting surfaces, typically metals. In the emission mode, the IR signal emanating from the
heated sample is recorded. Finally, both photoacoustic (PAS) and photothermal IR spectroscopies
rely on temperature fluctuations caused by radiation of the sample with a modulated monochro-
matic beam. The availability of all these arrangements makes IR spectroscopy quite versatile for the
characterization of catalytic systems.
The applications of IR spectroscopy in catalysis are many. For example, IR can be used to di-
rectly characterize the catalysts themselves. This is often done in the study of zeolites, metal oxides,
displays transmission IR spectra for a number of Co
x
Mo
1Ϫx
O
y
(0 Յ x Յ 1) mixed metal oxides with
various compositions [79]. In this study, a clear distinction could be made between pure MoO
3
, with
its characteristic IR peaks at 993, 863, 820, and 563 cm
Ϫ1
, and the MoO
4
tetrahedral units in the
CoMoO

4
solid solutions formed upon Co
3
O
4
incorporation, with its new bands at 946 and 662 cm
Ϫ1
.
These properties could be correlated with the activity of the catalysts toward carburization and hy-
drodenitrogenation reactions.
Further catalyst characterization can be carried out by appropriate use of selected adsorbing
probes [80–83]. For instance, the acid–base properties of specific surface sites can be tested by
recording the ensuing vibrational perturbations and molecular symmetry lowering of either acidic
(CO and CO
2
) or basic (pyridine and ammonia) adsorbates. Oxidation states can also be probed by
using carbon monoxide [84,85]. For instance, our recent study of Pd/Al
2
O
3
and Pd/Al
2
O
3
–25%
ZrO
2
catalysts used for nitrogen oxide reduction indicated that the Pd component can be extensively
2
additive, but oxidized fully to PdO only in the

presence of the zirconia [86,87].
Another common application of IR is to characterize reaction intermediates on the catalytic sur-
ple in the form of a set of transmission IR spectra obtained as a function of temperature during
the oxidation of 2-propanol on Ni/Al
2
O
3
[90]. A clear dehydrogenation reaction is identified in
these data above 440 K by the appearance of new acetone absorption bands around 1378, 1472, and
1590 cm
Ϫ1
.
New directions have been recently advanced in the use of IR spectroscopy for the characteriza-
tion of adsorbates, including the investigation of liquid–solid interfaces in situ during catalysis.
Both ATR [91,92] and RAIRS [86,93] have been recently implemented for that purpose. RAIRS has
also been used for the detection of intermediates on model surfaces in situ during catalytic reactions
[94–96]. The ability to detect monolayers in situ under catalytic environments on small-area sam-
ples promises to advance the fundamental understanding of surface catalytic reactions.
12 SURFACE AND NANOMOLECULAR CATALYSIS
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and heteropolyacids, among other catalysts [77,78]. To exemplify this type of application, Figure 1.11
films. Reflection–absorption IR spectroscopy (RAIRS) is employed to probe adsorbed species on
faces, often in situ during the course of the reaction [76,78,88,89]. Figure 1.12 provides an exam-
reduced in both samples, with and without the ZrO
Owing to its great molecular specificity, good sensitivity, and high versatility, IR spectroscopy
is one of the most widely used techniques for catalyst characterization. Nevertheless, IR catalytic
studies do suffer from a few limitations. In particular, strong absorption of radiation by the solid
often limits the vibrational energy window available for analysis. For instance, spectra of catalysts
dispersed on silica or alumina supports display sharp cutoffs below 1300 and 1050 cm

Ϫ1
, respectively
[75]. Also, the intensities of IR absorption bands are difficult to use for quantitative analysis.
Finally, it is not always straightforward to interpret IR spectra, especially in cases involving com-
plex molecules with a large number of vibrational modes.
1.4.2 Raman Spectroscopy
Raman spectroscopy offers an alternative for the vibrational characterization of catalysts, and has
been used for the study of the structure of many solids, in particular of oxides such as MoO
3
,V
2
O
5
,
CHARACTERIZATION OF HETEROGENEOUS CATALYSTS 13
Infrared spectra of Co−Mo−O mixed oxides
MoO
3
993
946
863
820
563
418
613
662
Co/Mo = 0.25
Co/Mo = 0.5
Co/Mo = 0.67
Co/Mo = 1.0

Co
3
O
4
Transmittance (%)
1100 1000 900 800 700 600 500 400
664
575
Wavenumber (cm
−1
)
Figure 1.11 Transmission IR spectra from Co
x
Mo
1Ϫ
x
O
y
(0 Յ
x
Յ 1) samples obtained by addition of different
amount of Co
3
O
4
to pure MoO
3
[79]. As the Co/Mo ratio is increased from 0.25 to 1, the IR peaks
due to tetrahedral MoO
4

units (at 662 and 946 cm
Ϫ1
) grow at the expense of those associated with
the MoO
3
phase (at 563, 820, 863, and 993 cm
Ϫ1
), a trend that indicates the formation of CoMoO
4
.
This example shows how IR can be used to directly characterize solid catalyst samples.
(Reproduced with permission from Elsevier.)
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and WO
3
oxides such as SiO
2
,Al
2
O
3
, and zeolites give low Raman signals, this technique is ideal for the iden-
spectra of a series of transition metal oxides dispersed on high-surface-area alumina supports
[75,102]. A clear distinction can be made with the help of these data between terminal and bridging
oxygen atoms, and with that a correlation can be drawn between the coordination and bond type of
these oxygen sites and their catalytic activity. Data such as these can also be used to determine the
nature and geometry of supported oxides as a function of loading and subsequent treatment.
Surface-enhanced Raman spectroscopy (SERS) has also been employed to characterize metal
catalyst surfaces [103]. The low sensitivity and severe conditions required for the signal enhance-

ment have limited the use of this technique [104], but some interesting work has been published
over the years in this area, including studies on model liquid–solid interfaces [105].
14 SURFACE AND NANOMOLECULAR CATALYSIS
2-Propanol on 10% Ni/Al
2
O
3
IR vs. T in O
2
atmosphere
0.1
(7) 670 K in 10 torr O
2
(6) 530 K in 10 torr O
2
(5) 490 K in 10 torr O
2
(4) 450 K in 10 torr O
2
(3) 440 K in 10 torr O
2
(2) Heated at 400 K
(1) 10 Torr 2-ProOH at
300 K; then pumped
Transmittance
1200 1500 1800 2100 2400 2700 3000
Frequency (cm
−1
)
Figure 1.12 Transmission IR spectra obtained during the oxidation of 2-propanol on a Ni/Al

2
O
3
catalyst as a
function of reaction temperature [90]. A change in the nature of the adsorbed species from molecu-
lar 2-propanol to acetone is seen above 440 K. Experiments such as these allow for the identifica-
tion of potential reaction intermediates during catalysis. (Reproduced with permission from
Elsevier.)
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tification of oxygen species in covalent metal oxides. As an example, Figure 1.13 shows the Raman
[97–99], as well as for the investigation of a number of adsorbates [100,101]. Whereas