Characterization of Solid Materials and Heterogeneous Catalysts: From Structure to Surface Reactivity

By Michel Che and Jacques C. Vedrine

This two-volume book provides an overview of physical techniques used to characterize the structure of solid materials, on the one hand,
and to investigate the reactivity of their surface, on the other. Therefore this book is a must-have for anyone working in fields related to surface
reactivity. Among the latter, and because of its most important industrial impact, catalysis has been used as the directing thread of the book.
After the preface and a general introduction to physical techniques by M. Che and J.C. Védrine, two overviews on physical techniques are
presented by G. Ertl and Sir J.M. Thomas for investigating model catalysts and porous catalysts, respectively.
The book is organized into four parts: Molecular/Local Spectroscopies, Macroscopic Techniques, Characterization of the Fluid Phase (Gas and/
or Liquid), and Advanced Characterization. Each chapter focuses upon the following important themes: overview of the technique, most important parameters to interpret the experimental data, practical details, applications of the technique, particularly during chemical processes,
with its advantages and disadvantages, conclusions.

• Language: English
• Publisher: Wiley
• Release date: Apr 16, 2012
• ISBN: 9783527645336

Book preview

For our mentor Dr. Boris Imelik, and our Families

About the Editors

Michel Che

After a chemical engineering degree from Ecole Supérieure de Chimie Industrielle (Lyon, F), M. Che joined the Institut de Recherches sur la Catalyse (Lyon) as member of CNRS (National Center of Scientific Research). After a Doctorat ès Sciences in 1968 (Université de Lyon), he was postdoctoral fellow (1969-1971) at Princeton University. Between 1972 and 1982, he frequently worked as visiting scientist at the Atomic Energy Research Establishment at Harwell (UK). He became Professor at Université Pierre & Marie Curie-Paris 6 in 1975, and Senior Member of Institut Universitaire de France in 1995.

His research concerns the reactivity of solid surfaces investigated from a molecular standpoint based on the combined use of transition metal complexes, specific isotopes and physical techniques. His work, which led to 450 publications and 5 patents, has contributed to improve our understanding of the elementary processes developing at solid/liquid (gas) interfaces and to bridge the gap between homo- and heterogeneous catalysis.

Michel Che was President-Founder of EFCATS, the European Federation of Catalysis Societies (creating the biennial EuropaCat congresses), and later President of the International Association of Catalysis Societies. He received awards in France (A. Joannides and P. Sue), Netherlands (J. H. Van't Hoff), Poland (M. Sklodowska-Curie & P. Curie lectureship), Germany (Von Humboldt - Gay-Lussac Award, and GDCh Grignard-Wittig lectureship), UK (RSC Centenary lectureship), USA (Frontiers in Chemical Research lectureship, Texas), Japan (Japanese Society for the Promotion of Science lectureship), China (Gold Medal of Chinese Academy of Sciences, Friendship Award and International Science and Technology Cooperation Award) and Europe (François Gault EFCATS lectureship). His work earned him several honorary doctorates and fellowships (German Academy of Sciences-Leopoldina, Academia Europaea, Hungarian Academy of Sciences, Polish Academy of Arts and Sciences).

Jacques C. Védrine

After a chemical engineering degree from Ecole Supérieure de Chimie Industrielle (Lyon, F), J.C. Védrine joined the Institut de Recherches sur la Catalyse (IRC) in Lyon as member of CNRS. After a Docteur ès Sciences degree in 1968 (Université de Lyon), he was post-doc in USA at Varian Ass., Palo Alto (1969-1970) and Princeton University (1970-1971). He then returned to IRC and became deputy director in 1988. In 1998, he moved to the University of Liverpool, UK as Chair Professor and Deputy Director of the Leverhulme Centre for Innovative Catalysis. In 2003, he returned to France and was chargé de mission at the Ministry of National Education and Research. In 2006, he joined the Laboratory of Surface Reactivity at Université Pierre & Marie Curie, Paris.

His scientific interests cover heterogeneous catalysis, especially selective oxidation on mixed metal oxides, acid catalysis on oxide-based systems and acidity strength and nature determination. He worked on combinatorial catalysis (high throughput technique) and contributed in the 1990s to the EUROCAT group activities in standardizing heterogeneous catalyst characterization. He co-authored over 350 publications and a few patents, and co-edited 7 books. He is one of the Editors of Appl. Catal. A: General.

One of his major contributions was to organize in the 1980s regular training sessions to help researchers use complementary physical techniques to improve the characterization of solid catalysts, including under working conditions. This led to two books on Physical Characterization of Solid Catalysts (Technip, Paris, 1988 and Plenum Press, New York, 1994).

He was awarded the Grand Prix Pierre Sue of the French Chemical Society (SCF) in 2001. He was elected President of Catalysis Division of SCF (1994-1997), President of EFCATS (1997-1999) and President of the Acid-Base World Organization (2005-2009). He holds honorary doctorate from the University of Lisbon.

List of Contributors

Sandra Alves

Université Pierre et Marie Curie, CNRS

Institut Parisien de Chimie

Moléculaire

Laboratoire de Chimie Biologique

Organique et Structurale

4 place Jussieu

75252 Paris

France

Masakazu Anpo

Osaka Prefecture University

Graduate School of Engineering

Department of Applied Chemistry

1-1 Gakuen-cho, Naka-Ku

Sakai-City

Osaka 599-8531

Japan

Aline Auroux

Université Lyon 1, CNRS

Institut de Recherches sur la Catalyse et

l'Environnement de Lyon

2 avenue Albert Einstein

69626 Villeurbanne

France

Andrew M. Beale

Utrecht University

Debye Institute for

NanoMaterials Science

Inorganic Chemistry and

Catalysis Group

Sorbonnelaan 16

3584 CA Utrecht

The Netherlands

Malte Behrens

Fritz-Haber-Institut der

Max-Planck-Gesellschaft

Faradayweg 4–6

14195 Berlin

Germany

Thomas Bein

Ludwig-Maximilians University of Munich

Department of Chemistry

Center for Nanoscience and Center for

Integrated Protein Science

Butenandtstraße 11

81377 Munich

Germany

Fabrice Bertoncini

IFP Energies Nouvelles

établissement de LYON

Catalysis and Separation Division

Rond-Point de l'échangeur de Solaize

69360 Solaize

France

Emily Bloch

Université Aix-Marseille, CNRS

Centre de St. Jérôme

Laboratoire Chimie Provence

avenue Normandie-Niemen

13397 Marseille

France

Sandrine Bourrelly

Université Aix-Marseille, CNRS

Centre de St. Jérôme

Laboratoire Chimie Provence

avenue Normandie-Niemen

13397 Marseille

France

Christoph Bräuchle

Ludwig-Maximilians University of

Munich

Department of Chemistry

Center for Nanoscience and Center

for Integrated Protein Science

Butenandtstraße 11

81377 MunichGermany

Michel Che

Institut Universitaire de France

Université Pierre et Marie Curie, CNRS

Laboratoire de Réactivité de Surface

4 place Jussieu

75252 Paris

France

Sergey Chenakin

Université Libre de Bruxelles (ULB)

Chimie-Physique des Matériaux

CP 243 Campus Plaine

1050 Brussels

Belgium

Marion Courtiade

IFP Energies Nouvelles

établissement de LYON

Physics and Analysis Division

Rond-Point de l'échangeur de Solaize

69360 Solaize

France

Caterina Ducati

University of Cambridge

Department of Materials Science and

Metallurgy

Pembroke Street

Cambridge CB2 3QZ

UK

Thomas Dutriez

IFP Energies Nouvelles

établissement de LYON

Physics and Analysis Division

Rond-Point de l'échangeur de Solaize

69360 Solaize

France

Angelos M. Efstathiou

University of Cyprus

Chemistry Department

Heterogeneous Catalysis Laboratory

University Campus

1678 Nicosia

Cyprus

Gerhard Ertl

Fritz-Haber-Institut der

Max-Planck-Gesellschaft

Faradayweg 4–6

14195 Berlin

Germany

Fengtao Fan

Chinese Academy of Sciences

Dalian Institute of Chemical Physics

State Key Laboratory of Catalysis

457 Zhongshan Road

Dalian 116023

China

Zhaochi Feng

Chinese Academy of Sciences

Dalian Institute of Chemical Physics

State Key Laboratory of Catalysis

457 Zhongshan Road

Dalian 116023

China

Karin Föttinger

Vienna University of Technology

Institute of Materials Chemistry

Getreidemarkt 9 BC

1060 Vienna

Austria

Christophe Geantet

Université Lyon 1, CNRS

Institut de Recherches sur la Catalyse et l'Environnement de Lyon

2 avenue Albert Einstein

69626 Villeurbanne

France

Elio Giamello

Università di Torino

Dipartimento di Chimica IFM and NIS

Centre of Excellence

via P. Giuria 7

10125 Turin

Italy

Lynn F. Gladden

University of Cambridge

Department of Chemical Engineering

and Biotechnology

Pembroke Street

Cambridge CB2 3RA

UK

John T. Gleaves

Washington University in St. Louis

Department of Energy, Environmental

and Chemical Engineering

1 Brookings Drive

St. Louis, MO 63130

USA

Wolfgang Grünert

Ruhr-Universität Bochum

Lehrstuhl Technische Chemie

Universitätsstraße 150

44801 Bochum

Germany

Friederike C. Jentoft

University of Oklahoma

School of Chemical, Biological and

Materials Engineering

Sarkeys Energy Center T-335

100 East Boyd Street

Norman, OK 73019

USA

Hervé Jobic

Université Lyon 1, CNRS

Institut de Recherches sur la Catalyse et

l'Environnement de Lyon

2 avenue Albert Einstein

69626 Villeurbanne

France

Norbert Kruse

Université Libre de Bruxelles (ULB)

Chimie-Physique des Matériaux

CP 243 Campus Plaine

1050 Brussels

Belgium

Timo Lebold

Ludwig-Maximilians University of

Munich

Department of Chemistry

Center for Nanoscience and Center for

Integrated Protein Science

Butenandtstraße 11

81377 Munich

Germany

Can Li

Chinese Academy of Sciences

Dalian Institute of Chemical Physics

State Key Laboratory of Catalysis

457 Zhongshan Road

Dalian 116023

China

Philip L. Llewellyn

Université Aix-Marseille, CNRS

Centre de St. Jérôme

Laboratoire Chimie Provence

avenue Normandie-Niemen

13397 Marseille

France

Michal Lutecki

University of Cambridge

Department of Chemical Engineering

and Biotechnology

Pembroke Street

Cambridge CB2 3RA

UK

Masaya Matsuoka

Osaka Prefecture University

Graduate School of Engineering

Department of Applied Chemistry

1-1 Gakuen-cho, Naka-Ku

Sakai-City

Osaka 599-8531

Japan

Françoise Maugé

ENSICAEN–Université de Caen, CNRS

Laboratoire Catalyse et Spectrochimie

6 boulevard Maréchal Juin

14050 Caen

France

James McGregor

University of Cambridge

Department of Chemical Engineering

and Biotechnology

Pembroke Street

Cambridge CB2 3RA

UK

Adrien Mekki-Berrada

Université Lyon 1, CNRS

Institut de Recherches sur la Catalyse et

l'Environnement de Lyon

2 avenue Albert Einstein

69626 Villeurbanne

France

Christophe Méthivier

Université Pierre et Marie Curie, CNRS

Laboratoire de Réactivité de Surface

4 place Jussieu

75252 Paris

France

Jens Michaelis

Ludwig-Maximilians University of

Munich

Department of Chemistry

Center for Nanoscience and Center for

Integrated Protein Science

Butenandtstraße 11

81377 Munich

Germany

Tomoaki Nishino

Osaka Prefecture University

Research Organization for the

21st century

Nanoscience and Nanotechnology

Research Center

Sakai

Osaka 599-8570

Japan

Matthew G. O'Brien

Utrecht University

Debye Institute for

NanoMaterials Science

Inorganic Chemistry and

Catalysis Group

Sorbonnelaan 16

3584 CA Utrecht

The Netherlands

Christophe Pichon

IFP Energies Nouvelles

établissement de LYON

Physics and Analysis Division

Rond-Point de l'échangeur de Solaize

69360 Solaize

France

Piotr Pietrzyk

Jagiellonian University

Faculty of Chemistry

ul. Ingardena 3

30-060 Krakow

Poland

Claire-Marie Pradier

Université Pierre et Marie Curie, CNRS

Laboratoire de Réactivité de Surface

4 place Jussieu

75252 Paris

France

Günther Rupprechter

Vienna University of Technology

Institute of Materials Chemistry

Getreidemarkt 9 BC

1060 Vienna

Austria

Masakazu Saito

Osaka Prefecture University

Graduate School of Engineering

Department of Applied Chemistry

1-1 Gakuen-cho, Naka-Ku

Sakai-City

Osaka 599-8531

Japan

Philippe Sautet

Université Lyon 1, CNRS

Ecole Normale Supérieure de Lyon

Institut de Chimie

15 parvis Descartes

69342 Lyon

France

Robert Schlögl

Fritz-Haber-Institut der

Max-Planck-Gesellschaft

Faradayweg 4–6

14195 Berlin

Germany

Zbigniew Sojka

Jagiellonian University

Faculty of Chemistry

ul. Ingardena 3

30-060 Krakow

Poland

Lorenzo Stievano

Université Montpellier 2

Institut Charles Gerhardt, CNRS

place Eugène Bataillon

34095 Montpellier

France

Jean-Claude Tabet

Université Pierre et Marie Curie, CNRS

Institut Parisien de Chimie

Moléculaire

Laboratoire de Chimie Biologique

Organique et Structurale

4 place Jussieu

75252 Paris

France

Frédéric Thibault-Starzyk

ENSICAEN–Université de Caen, CNRS

Laboratoire Catalyse et Spectrochimie

6 boulevard Maréchal Juin

14050 Caen

France

Didier Thiebaut

CNRS

Laboratoire Physicochimie des

Electrolytes Colloïdes et Sciences

Analytiques

ESPCI ParisTech

10 rue Vauquelin

75231 Paris

France

John Meurig Thomas

University of Cambridge

Department of Materials Science and

Metallurgy

Pembroke Street

Cambridge CB2 3QZ

UK

Jacques C. Védrine

Université Pierre et Marie Curie, CNRS

Laboratoire de Réactivité de Surface

4 place Jussieu

75252 Paris

France

Friedrich E. Wagner

Technische Universität München

Physik-Department E15

James-Franck-Strasse 1

85748 Garching

Germany

Bert M. Weckhuysen

Utrecht University

Debye Institute for

NanoMaterials Science

Inorganic Chemistry and

Catalysis Group

Sorbonnelaan 16

3584 CA Utrecht

The Netherlands

Christian Weilach

Vienna University of Technology

Institute of Materials Chemistry

Getreidemarkt 9 BC

1060 Vienna

Austria

Gregory S. Yablonsky

Saint Louis University

Parks College of Engineering

Department of Chemistry

3450 Lindell Boulevard

St. Louis, MO 63130

USA

Preface

Michel Che and Jacques C. Védrine

The spectacular progress achieved in chemistry is largely due to the use of physical techniques implemented at the level of the element, molecule, or phase with reliability and accuracy unattainable a few decades ago. Moreover, microscopic (molecular) and macroscopic (molar) information can be obtained by small-scale and often non-destructive experiments. Many of these techniques are now in routine use, essentially because of the progress of technology and availability of always more powerful and user-friendly computers.

We therefore thought that it was timely to provide a survey of the major techniques used to characterize solid materials and investigate their surface reactivity, a domain of chemistry, relevant to a variety of fields including adsorption, geochemistry, coatings, electrochemistry, corrosion, formation of biofilms, toxicity and catalysis. Those fields however do not require the same surface reactivity: for corrosion, the latter has to be inhibited, or even suppressed, because of its dramatic consequences on metals, while for catalysis not only it has to be enhanced but also selectively oriented to obtain the desired product.

From all the fields related to surface reactivity, catalysis appears to be unique because i) it has a large industrial impact, ii) it lies at the core of chemistry, i.e., starting with chemistry to prepare the catalytic system and ending with chemistry to promote a specific reaction, and iii) it involves physical and chemical processes developing mostly at liquid-solid, solid-solid, and/or gas-solid interfaces present at the successive steps of catalyst life, from its preparation to its use in the catalytic reaction. For those reasons, catalysis will be used as the directing thread of this book.

Investigations on solid materials have shown that their surfaces may change with the chemical environment to which they are exposed and that the more divided the solid, the more reactive it becomes. This book title illustrates this paradigm, with its dual aspects, the structure of the material on one hand and its surface reactivity on the other. For instance, for metals, it is known that metal-metal bond distances at the surface often contract under vacuum with respect to the bulk, while they relax in the presence of gaseous molecules reaching values close to those characteristic of the bulk. For alloys or mixed oxides, surface enrichment in one component is often observed under reaction conditions. For some reactions, e.g. selective oxidation of olefins on metal oxide catalysts, the surface atoms of the solid catalyst may react, and even be incorporated into reactant molecules. For such redox-type reactions, surface atoms have to be mobile enough to allow the redox process to occur.

This book is intended to consider all those aspects with the objective to offer a general survey on the Characterization of Solid Materials: From Structure to Surface Reactivity, useful to junior and senior research scientists, engineers and industrialists. We deemed it essential to present a portofolio of the techniques most frequently used and to dwell on those which appear to be most promising. For this reason, the space allocated to each chapter is different. Although still used, some techniques are not discussed in this book, because little improvement has been achieved since the publication of earlier books in 1988 [1] and 1994 [2].

Because of the large number of chapters/authors, consistency and homogeneity were felt to be essential. Therefore, the following format was suggested to authors:

1. Introduction covering the discovery and development of the technique,

2. Description of the basic phenomenon with a theoretical background including, where appropriate, its dimension/time/energy scales, energy states, terminology, units (those conventionally used in the field but also SI units), the strategy used and the essential parameters necessary to interpret experimental data,

3. Experimental considerations/constraints, relative to the characterization technique and to the surface reactivity,

4. Uses of the technique for the characterization of both model and real solid materials at different stages of their life (i.e., during preparation, functionalization, chemical or thermal activation, surface reactivity) with emphasis on the coupling with other techniques with its advantages and disadvantages,

5. Key examples of application of the technique to surface reactivity. For the field of catalysis, the reaction, deactivation, ageing, and regeneration steps had to be considered with emphasis on the identification and implication of intermediates in reaction mechanisms,

6. Conclusions including information gained with the technique, its advantages, limitations, and latest developments,

7. References.

We have tried to offer a book presenting a unique set of features:

it deals with an ensemble of physical techniques commonly used at present to i) characterize solid materials and ii) investigate their surface reactivity,

it provides overviews written by two outstanding scholars who have largely contributed to the development of physical techniques to investigate solid materials (single crystals and porous catalysts),

each chapter aims at being both general and concise enough for the readers to understand the technique, and the meaning of the essential key parameters,

it gives general data for each technique, with its historical background, domains of energy involved, spatial and time resolution, experimental constraints (vacuum or presence of gaseous phase, temperature,...), atomic/molecular or macroscopic aspects,

it emphasises the characterization of the solid material throughout its life: from its preparation to its application in surface reactivity-related domains,

it includes the use of both experimental and theoretical approaches as a guide in designing experiments and interpreting results,

it deals with both model and real solid materials,

it aims at being a toolbox from which any researcher should be able to find the appropriate technique(s) for solving a specific problem.

To conclude, this book aims at being pedagogical, illustrative and practical, hoping that after having read the book, the reader be in a position to identify the most appropriate technique(s) able to answer his questions.

References

1. Imelik, B. and Védrine, J.C. (Eds.)(1988) Les Techniques Physiques de Caractrisation des Catalyseurs, Technip, Paris.

2. Imelik, B. and Védrine, J.C. (Eds.)(1994) Catalyst Characterization: Physical Techniques for Solid Materials, Plenum Press, New York.

General Introduction

Michel Che and Jacques C. Védrine

The two main goals of the book are to show how physical techniques can be used to characterize solid materials and to investigate their surface reactivity. The first goal corresponds to establishing the identity card of the material, including its structure, morphology, porosity, and chemical composition, and the second to obtaining characteristics of the surface related to its reactivity (nature and number of surface sites, subsequent modification upon functionalization, nature and number of adsorbed species and possible intermediates in surface-promoted phenomenon/reaction).

1 Basic Phenomenon and Classification of Physical Techniques

All techniques are based on the same phenomenon, often referred to as the Propst diagram (Figure 1): an incident beam hits the sample, giving rise to an emitted beam which is detected and analyzed because of the information it contains, leading to the fingerprint of the solid and/or of species or reaction intermediates adsorbed on it. The incident beam can be composed of photons, electrons, ions, neutrals, or magnetic, electric, acoustic, or thermal fields, and also the emitted beam.

Figure 1 Basic phenomenon of physical techniques. The incident beams are defined by arrows oriented towards the sample while the emitted beams are defined by arrows oriented away from the sample. Adapted from [1].

Table 1 presents the main acronyms of the physical techniques presented or mentioned in this book and Table 2 gives the classification of typical techniques as a function of the nature of the incident and emitted beams. In Table 2, one distinguishes the diagonal techniques, for which incident and emitted beams are identical in nature and the information comes from the analysis of the modifications in intensity, energy, or frequency of the incident beam, from the off-diagonal techniques (shaded areas) for which those two beams are different in nature [2].

Table 1 Acronyms and names of the techniques presented or mentioned in this book.

Table 2 Classification of typical techniques as a function of the nature of the incident and emitted beams. Adapted from [2].

Table 3 lists the main spectroscopic techniques and associated events as a function of the characteristics of the incident beam (energy, domain of the electromagnetic spectrum, wavelength, and frequency).

Table 3 Main spectroscopic techniques and associated events as a function of the characteristics of the incident beama). Adapted from [2].

2 Coupling of Physical Techniques

The coupling of physical techniques has been a major advance in physical chemistry in the past decades and has become increasingly popular in chemical, environmental, biochemical, biological, and forensic laboratories, successfully solving most difficult problems such as speciation and trace analysis, which any single technique cannot solve.

Tandem GC–MS is one of the earliest and best examples – combining the separation power of gas chromatography with the identification ability of modern mass spectrometry is a powerful means to analyze complex mixtures. Calorimetry is another example: while it easily leads to the adsorption heat versus coverage, its coupling with laser Raman spectroscopy can simultaneously provide structural and thermal information. The quartz balance, which easily provides quantitative data, is a powerful addition to more qualitative techniques such as IR and IRAS. Coupling AFM and tunable lasers has also been developed for investigating living cell modifications upon incorporation of a foreign element. Thus, coupling AFM with IR spectroscopy can provide a chemical mapping of different structures absorbing in the IR domain at the sub-cell scale (AFM resolution ~ 10 nm) [3]. Many other couplings have been developed and are described in this book.

3 The Latest Challenge: Characterization of the Surface Reactivity of Solid Materials Under Working/Catalytic Reaction Conditions

The latest challenge in surface-related fields is to characterize the solid materials under working conditions, that is, in the real conditions of their applications (such as adsorption, geochemistry, coatings, electrochemistry, corrosion, formation of biofilms, and toxicity) involving most often the presence of gas–solid and/or liquid–solid interfaces. This implies a single cell where characterization and application concern the same sample with simultaneous control of the experimental parameters (e.g., temperature, pressure, concentration, flow, reactivity cell characteristics) which influence surface reactivity.

For the outstanding case of catalysis, the challenge is more involved because the application consists of a molecular reaction taking place at the catalyst surface. Hence characterization and reaction have to be performed in the same cell, under reaction conditions (reactants, flow, temperature, pressure, concentration, and reactor characteristics) and with simultaneous online analysis of reactants and products allowing the determination of the catalyst activity and selectivity. The cell also acts as a catalysis reactor.

In the past, the terms "in situ [4] and operando [5, 6], although originally precisely defined, have unfortunately often been misused, as discussed elsewhere [7–10], because characterization and catalytic reaction were not performed in the same cell and with simultaneous online analysis of reactants and products. To obtain truly meaningful catalytic data, it is worth recalling that the characterization–catalytic reaction" cell has to be designed so as to avoid any diffusion problems.

4 Book Content and Chapter Order

The solid material sees its properties change as one moves from the bulk to its surface. These changes are dramatically dependent on its morphology (particle size, specific surface area, porosity) and on the external conditions to which it is subjected (presence of a gas or liquid phase, with their associated parameters, for example, nature, composition, pressure, concentration, presence of ions, notably transition metal ions, etc.). In general terms, the more divided the solid is, the more reactive it becomes. The book title illustrates this paradigm, with its dual aspects, the material on the one hand and its surface reactivity on the other.

Between the solid material (Chapters 1–14) and the fluid phase (gas and/or liquid) (Chapters 20–22), inherent to most applications, lies the solid–fluid interface, the nature of which has been much investigated over the past decades. The intricacy of this interface is illustrated by Figure 2, in the case of transition metal complexes interacting with an oxide surface [11]. Depending on the experimental conditions, various types of interaction can be identified, resulting in a variety of species (with different ligands, symmetries, and nuclearities) referred to as speciation defined [12] as the distribution of an element (M in Figure 2) amongst defined chemical species in a given system. Speciation, which concerns both the aqueous solution and the oxide support [13], constitutes one of the most challenging problems in analytical chemistry: how to determine the nature and concentration of each species.

Figure 2 Transition metal complex [MLn]q+/q− (M = transition metal, L = ligand, n = number of ligands, and q = charge of the complex) interacting with an oxide surface after deposition [from the liquid (a) or gas phase (b)] and further thermal treatment (c). In models I–VIII, the oxide support acts as surface solvent (l), counter ion (II), surface bidentate ligand (III), framework ligand (IV), surface ligand (V–VII), and solid solvent (VIII). Note that the nuclearity of the final complex may differ from that of the starting complex. Designed from [11].

Figure 2 also shows that the coordination sphere of the transition metal ion can be very different from that of the original complex in the liquid or gas phase and that the oxide is a very versatile entity (e.g., solvent, counter ion, ligand, framework and solid solution).

The chapters in this book are arranged in four parts. Part one is devoted to molecular/local spectroscopies (Chapters 1–14). Part two is devoted to macroscopic techniques (Chapters 15–19). Part three is devoted to the characterization of the fluid phase (gas and/or liquid) (Chapters 20–22) in relation to the surface reactivity of solid materials. Part four (Chapters 23 and 24) is concerned with advanced characterization: Chapter 23 deals with techniques used in reaction conditions whereas Chapter 24 describes theoretical calculations and modeling to describe solid materials and their reactivity.

5 SI Units and Conversions

We have seen (Table 3) that, depending on the technique, the incident and emitted beams do not have the same nature and that their characteristics (energy, frequency, and wavelength) vary over wide ranges. From the various effects induced, valuable information on the solid material can be obtained.

In all chapters, the units used are those of the Système International d'Unités (SI). However, for historical reasons, the practitioners of each technique have adopted certain habits, which have persisted, even after the IUPAC recommendations. Thus, it is common practice to see units such as wavenumber (cm−1) in IR and Raman wavelength (nm) in UV–visible and energy (eV) in XPS and EXAFS. To help the reader to move from one system to the other, tables are provided which list the base units of the SI system (Table 4), the units derived from the latter (Table 5), the interconversion of energy and pressure units (Tables 6 and 7), and the fundamental physical constants (Table 8) based on the Bureau International des Poids et Mesures [14] and IUPAC [15] recommendations.

Table 4 Base SI units. Adapted from [14, 15].

Table 5 Units derived from the base SI units. Adapted from [15].

Table 6 Interconversion of energy units. Adapted from [15].

Table 7 Interconversion of pressure units. Adapted from [15].

Table 8 Fundamental physical constantsa. Adapted from [15].

Table 9, giving the prefixes used to denote decimal multiples and submultiples of SI units, is provided to remain in phase with the progress of the nanoworld (Submultiple column) and the huge progress made in computer memories (Multiple column). Finally, Table 10 gives a summary of the parameters related to the samples analyzed by physical techniques.

Table 9 SI prefixes and prefixes for binary multiples. From [15].

Table 10 Parameters relevant to selected physical techniquesa). Adapted from [16].

With all the techniques presented in this book, it now becomes possible to analyze samples of a great variety of solid materials related to surface reactivity at large, showing the immense potential of physical chemistry, even in unexpected domains such as music [17], painting [18], sparkling beverages [19], and gastronomy [20].

Acknowledgments

The authors are most grateful to Mrs. F. Sarrazin and Dr. F. Averseng (Université Pierre et Marie Curie – Paris 6) and Prof. L. Bonneviot (ENS Lyon) for their most valuable help in preparing some of the tables and figures.

References

1. Cover of the series Studies in Surface Science and Catalysis, Elsevier, Amsterdam.

2. Sojka, Z. and Che, M. (2008) J. Chem. Educ., 85, 934.

3. Dazzi, A. and Policar, C. (2011) in Biointerface Characterization by Advanced IR Spectroscopy (Eds. C-M. Pradier and Y.J. Chabal), Elsevier, Amsterdam, Chapter IX, p. 245.

4. Haw, J. (Ed.)(2002) In-Situ Spectroscopy in Heterogeneous Catalysis, Wiley-VCH Verlag GmbH, Weinheim.

5. Bañares, M. (Ed.)(2009) In situ to operando spectroscopy: from proof of concept to industrial application. Top. Catal., 52, 1301.

6. Weckhuysen, B.M. (2003) Phys. Chem. Chem. Phys., 5, 4351.

7. Ertl, G., Knözinger, H., and Weitkamp, J. (Eds.)(1997) Handbook of Heterogeneous Catalysis, vol. 2, Wiley-VCH Verlag GmbH, Weinheim.

8. Ertl, G., Knözinger, H., Schüth, F., and Weitkamp, J. (Eds.)(2008) Handbook of Heterogeneous Catalysis, 2nd edn., vol. 2, Wiley-VCH Verlag GmbH, Weinheim.

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Overview on Physical Techniques for Investigating Model Solid Catalysts

Gerhard Ertl

1 Why Model Systems?

A real catalyst consists usually of nanoparticles exposing different crystal planes and various structural defects with complex chemical composition. All these factors have their effect on the chemical reactivity. A possible strategy to solve this problem had been suggested by I. Langmuir already many years ago [1]. "Most finely divided catalysts must have structures of great complexity. In order to simplify our theoretical consideration of reactions at surfaces, let us confine our attentions to reactions on plane surfaces. If the principles in this case are well understood, it should then be possible to extend the theory to the case of porous bodies. In general, we should look upon the surface as consisting of a checkerboard..."

This surface science approach that Langmuir had in mind was not yet accessible in his days, but was beginning to become available only in the 1960s with the development of novel physical methods which enable chemical analysis as well as investigation of the structural, electronic, vibrational, and dynamic properties of solid surfaces [2–7]. In fact, low energy electron diffraction (LEED) was the first of these techniques which was used to follow the kinetics of chemical reactions on well-defined single crystal surfaces [52, 53]. In recent years, scanning probe techniques, in particular scanning tunneling microscopy (STM) proved to be most powerful for direct observation of processes on atomic scale [8–12]. Since many of these techniques involve the interaction of particles (e.g., electrons) with matter, the pressure in the apparatus has to be in the ultrahigh vacuum (UHV). The pressures have also to be low enough for keeping the clean surface free from adsorbing species for long enough time. This effect causes the so-called pressure gap between the conditions of real catalysis and surface science studies.

There exist various possibilities to overcome this pressure gap. First one should keep in mind that the surface concentration of a certain species in equilibrium depends not only on pressure but also on temperature, so that lowering the temperature may compensate the difference in pressure. Second, special high pressure enclosure cells permit the reaction to be performed at elevated pressure, while the sample may afterwards be transferred into the UHV region [13, 14], and finally, apart from probing the surface with electromagnetic radiation, investigations at elevated pressures may also be performed with devices in which the sample-probe distance is below the mean free path of gaseous species. For example, at 1 mbar an electron with 400 eV kinetic energy will travel about 4 mm before undergoing an inelastic collision [35]. Such in situ measurements can be achieved by scanning tunneling microscopy, electron microscopy and photoelectron spectroscopy.

Apart from the pressure gap there exists also a materials gap between well-defined single crystal surfaces and real catalysts. Attempts to approach to the latter systems are made by controlled introduction of surface defects or foreign atoms, as well as by the use of bimetallic surfaces, well-defined oxide films or small particles with controlled size.

Despite these apparent shortcomings, the surface science approach has been successfully applied to elucidate the molecular mechanisms of important catalytic reactions even under industrial conditions, among which ammonia synthesis on promoted iron catalysts is most notable [15]. These results would not have been attained otherwise and justify the investigations with model systems. In the following short overview the main physical techniques applied in this context will be presented.

2 Interactions of Molecules with Surfaces; Kinetics and Dynamics

Catalysis at surfaces is concerned with the rates of chemical reactions. Therefore investigation of the kinetic properties of molecules interacting with surfaces is mandatory for understanding the underlying mechanisms. An advantage of model systems consists in the fact that due to the large mean free path of gaseous particles transport processes can usually be neglected, and because of the low reaction rates the processes are essentially isothermal.

Energies of adsorption may be determined by sophisticated calorimetric methods [16, 17] or, less elaborate, through measurements during adsorption-desorption equilibrium. If in this case, the coverage θ is determined as a function of temperature T and partial pressure p of the gaseous species, the isosteric heat of adsorption Ead is derived through application of the Clausius-Clapeyron equation

equation

from plots of lnp over 1/T at the respective coverage. This technique requires accurate determination of the coverage under conditions of adsorption-desorption equilibrium.

More convenient, but less accurate data about adsorption energetics can be obtained from studies of the desorption kinetics through thermal desorption spectroscopy (TDS) or temperature programmed desorption (TPD) [6, 18]. The principle of this technique is rather simple and straightforward: After dosing an adsorbate to surface by gas adsorption, the sample temperature is continuously increased and the flux of desorbing particles is monitored by a quadrupole mass spectrometer (QMS) as a function of time. Detailed analysis yields the kinetics of desorption (including reaction order, as well as activation energy and preexponential) as a function of coverage. Since adsorption is frequently non-activated, in this way also the adsorption energy is derived. Instead of monitoring the desorbing species, in catalytic reactions also the kinetics of product release into the gas phase can be recorded in this way (temperature programmed reaction spectroscopy = TPRS). Investigation of more real surfaces is also frequently performed along these strategies.

Data on the kinetics of adsorption (= sticking coefficient) can again be derived from TPD data by monitoring the desorbed amount as a function of the preceding gas exposure. A convenient technique for the determination of fairly high sticking coefficients was developed by King and Wells [19] by measuring pressure changes in conjunction with molecular beam dosing.

More details and accurate results on the kinetics of adsorption and desorption may be obtained by the use of molecular beam techniques. Two types of molecular beams may be applied: Knudsen (or thermal) and supersonic beams. The former mimick the interaction with molecules carrying the normal thermal energy, while the latter may be used to study the influence of the translational energy of the impinging particles on the dynamics. By periodically modulating the flux of the incident molecules and recording the phase lag of the species coming off the surface the mean residence time before desorption or reaction, respectively, can be determined. In this way it was for example for the first time possible to unequivocally establish the operation of the Langmuir-Hinshelwood (LH) mechanism in CO oxidation on a Pd(111) surface by measuring the delay time between adsorption and product formation down to about 10−4 s, leading also to the activation energy of the LH step [20].

Full insight into the distribution of energy among the various degrees of freedom of the particles coming off the surface may be obtained in state resolved experiments combining molecular beam and laser spectroscopic techniques [21]. Time resolution down to the femtosecond region can eventually be reached by two-pulse correlation methods [22].

3 The Structure of Surfaces

The application of optical and electron microscopies is possible not only with model surfaces but also with real systems, whereby the second group, however, again is restricted to high vacuum conditions. Microscopy with atomic resolution, on the other hand, was achieved for the first time by field ion microscopy (FIM) [23] invented by E. W. Müller in 1951 [24]. This technique requires sharp (radius <50 nm) metal tips at low temperature in an atmosphere of an imaging gas (mostly He) as well as the application of a strong electric field of a few V/Å. These factors cause severe restrictions of the applicability of this technique and hence it is no longer widely in use.

It took several decades to reach a real breakthrough in this field by the development of the scanning tunneling microscope (STM) [25] which technique found already many applications in surface studies [8–10]. In this way it was for example possible to directly image atomic steps as active sites in dissociation [26] or to analyze surface diffusion of adsorbates on atomic scale [27]. The application of STM requires flat and conducting surfaces, but may operate also at high pressures [12, 28, 29] and even in liquids, for example, in electrochemical studies.

The restriction to conducting surfaces is no longer existent if instead of the STM the atomic force microscope (AFM) is applied [30]. With this technique, an atomically sharp tip is scanned across a surface and the force between the tip and the sample is determined by recording the deflection of a micromechanical cantilever spring. Thereby the surface topography can be studied even down to atomic resolution [31]. A recently developed technique thus enabled even visualization of the 3d distribution of water molecules at a mica-water interface [32]. However, routine applications under ambient conditions reduces the lateral resolution usually to a few nm.

The actual atomic coordinates at the surfaces of single crystals are accessible by the use of diffraction techniques. While x-rays penetrate deeply into the bulk of solids this probe is only applicable with special geometric arrangements, for example, with grazing-incidence configuration [3]. Most powerful in this respect, however, is the technique of low energy electron diffraction (LEED). The mean free path of electrons in the kinetic energy range between about 20 and 500 eV is of the order of less than 1 nm, so that the elastically scattered electrons of this kind carry information only from the few topmost atomic layers [2–6]. While the position of the diffraction spots provide the geometry of the unit cell of the surface lattice, the determination of the actual atomic positions requires analysis of the intensity/voltage data (I/V-curves) by application of an elaborate dynamical multiple scattering theory [33]. In the case of more disordered surfaces more local diffraction techniques such as photoelectron diffraction (PED) or surface extended X-ray absorption fine structure (SEXAFS) can be used to derive information about the local geometry of surface atoms. As a consequence of the complexity of analysis the number of quantitatively solved structures is still rather small if compared with the large amount of data for bulk structures. The NIST (NIST = National Institute of Standards and Technology) Surface Structure Data base listed only about 1400 quantitatively solved surface structures, mostly of elemental metals and derived from LEED [33].

Low energy electrons may also be used for imaging surface structures in the sophisticated low energy emission microscope (LEEM) [38].

4 Electron Spectroscopies

Various techniques based on analysis of electrons emitted from a surface serve mainly as sources for elemental analysis and for probing the valence state of the topmost layers. The small mean free paths of electrons with energies ≤500 eV as mentioned before render these techniques surface sensitive, while their application is again restricted to low pressure conditions.

Among these, Auger electron spectroscopy (AES) is the oldest and most versatile, because it can be readily incorporated into a LEED apparatus [2–7]. Typically electron bombardment of a sample with energies around a few keV creates a hole in the inner shells of a hit atom which is subsequently filled by transition of an electron from a higher level, whereby the energy is transferred to another (bound) electron which is then ejected. Measurement of the kinetic energies of these Auger electrons then provides information about the elemental nature of the emitting atoms, while quantitative analysis can be derived from the intensities. AES is in principle able to detect any element except H and He. However, with increasing atomic number, the competition by radiative relaxation of the core hole through x-ray fluorescence increases. The exact energies and lineshapes of Auger transitions are usually not taken into account since signals from different elements are mostly widely separated from each other, while, however, also overlaps may occur which render quantitative analysis more problematic. The composition of the probed surface near region can be derived with fair accuracy by use of the relative sensitive factors as tabulated in the Handbook of Auger Electron Spectroscopy [34].

Direct emission of an electron can be achieved by absorption of a photon whose energy hν exceeds the binding energy of the bond electron EB, whereby the kinetic energy of the emitted electron Ek = hν - EB is measured and provides EB. Core electrons are excited by absorption of X-rays either from laboratory sources or from a synchrotron, and this method is then denoted as X-ray photoelectron spectroscopy (XPS). The advantage of synchrotron radiation sources is the much higher photon flux and the possibility of continuously varying the photon energy. In addition, the high photon intensity permits focusing to small spots of few ten nanometers in diameter. Since the energies of core electrons are to a first approximation independent of the atomic environment, XPS is particularly useful for elemental (qualitative as well as quantitative) analysis. Typical variations of the binding energies are denoted as chemical shifts and are dependent on the bonding of the analyzed atom, in particular its oxidation state. This technique is not restricted to flat or even single crystal surfaces and is therefore frequently applied also to real surfaces. The still existing restriction to low pressures can to some extent be overcome by novel designs with differential pumping and additional electrostatic lens focussing of the emitted electrons so that the sample can be operated at ambient (i.e., up to the mbar) pressure region [35].

By scanning the photon energy with the use of synchrotron radiation a core level is resonantly excited when the photon energy matches the difference in energy between this level and the first unoccupied state. By recording the spectral region near the threshold in XANES (X-ray absorption near edge structure) or NEXAFS (near edge X-ray absorption fine structure) one can probe the electronic structure of unoccupied levels [36], while variations of the absorption cross section with photon energy over a wider range causes characteristic oscillations due to interference phenomena between outgoing electrons and those scattered by neighboring atoms. This is the basis of the powerful EXAFS technique which can be applied to any kind of samples even at higher pressures. If instead of fluorescence the electrons emitted by the sample are recorded the technique becomes again surface sensitive for flat model surfaces.

If instead of X-ray photons near ultraviolet radiation is used the emitted electrons originate from the valence levels of the sample: ultraviolet photoelectron spectroscopy (UPS). This technique is particularly well suited to study the bonding at surfaces. Usually a He lamp with photon energies of 21.2 or 40.8 eV is used to investigate the band structure of the topmost atomic layers of the solid as well as the levels of an adsorbate, whereby the full information on energy and momentum of the emitted electrons is derived from angle resolved measurements (ARUPS). Of course, again a synchrotron with variable photon energy may be used as light source.

By the application of very short laser pulses, multiphoton photoelectron spectroscopy may be performed which allows investigation of the dynamics of electronic excitations down to the femtosecond time domain [37].

In photoelectron spectroscopy the photon energy usually exceeds the work function of the sample significantly. If the photon energy is close to the work function the local intensity of photoemitted electrons will be affected by the dipole moments of adsorbed species. If this intensity distribution is probed by a laterally resolving technique in a photoemission electron microscope (PEEM) [39] the lateral distribution of adsorbed species can be imaged with a spatial resolution of better than 100 nm. In this way the formation and propagation of spatio-temporal concentration patterns of adsorbates during a catalytic reaction on mesoscopic length scale may directly be followed [40].

The ultimate step consists in combining electron microscopy with electron spectroscopy, where the energy distributions of electrons from individual small areas are recorded. An example of such a microspectrometer is presented by the SMART project installed at the Berlin BESSY synchrotron with an ultimate lateral resolution of 2 nm at an energy resolution of 100 meV [41].

Finally, a completely different electron spectroscopic technique will be mentioned which has been frequently applied with large area samples for identification of surface radicals: electron paramagnetic resonance (EPR), also known as electron spin resonance spectroscopy (ESR). Due to advanced experimental design features the sensitivity of this technique could recently be improved substantially so that now also low area single crystal surfaces become accessible for investigation [42].

5 Vibrational Spectroscopies

Vibrational spectroscopies are probably the most widely used techniques in chemistry to identify the molecular nature of a species and of its modification by bonding. This is also the case for surfaces and adsorbates including model systems [43].

Most common for real systems is infrared absorption spectroscopy which can also be adopted to small area single crystal surfaces by reflection of the primary beam at the surface near grazing incidence, a technique denoted as reflection absorption infrared spectroscopy (RAIRS) or infrared reflection absorption spectroscopy (IRAS). Apart from identifying characteristic bands, their relative intensities may also serve to derive the coverage of adsorbates. IR spectroscopy is, of course, only applicable if the radiation is not strongly absorbed by the substrate, and low frequency vibrations ≤1000 cm−1 are hence not accessible. On the other hand, the high spectral resolution allows also to monitor subtle effects on the vibrational spectra of adsorbates, such as the influence of temperature as a consequence of coupling to other modes [44]. No restriction with respect to pressure exist with this purely optical method.

This is no longer the case with a technique which has been widely in use with UHV single crystal studies: high resolution electron energy loss spectroscopy (HREELS) [45]. Here a collimated monochromatic beam of electrons is scattered at a surface whose energy distribution is analyzed afterwards. The impinging electrons may loose part of their energy by excitations of molecular vibrations either by dipole scattering like in IR spectroscopy or by impact scattering which mechanism is not governed by dipole selection rules so that also infrared inactive modes may be probed. An advantage of this technique is that it can also probe the low frequency range <1000 cm−1, whereby also the adsorbate-substrate bond and even phonons become accessible. On the other hand, it is only applicable in UHV, and the resolution is rather poor (~10 cm−1).

Another purely optical technique for probing vibrational spectra without any pressure restrictions is sum frequency generation (SFG) spectroscopy [46]. Typically a laser beam with constant frequency in the visible and another one tunable in the infrared range are directed onto the surface and the output beam is analyzed. By tuning the IR radiation over a resonance for excitation the vibration spectrum is obtained. As a second-order nonlinear optical process, no signal is obtained from centrosymmetric media such as gases, liquids or most solids, so that this technique becomes highly surface sensitive. This method has for example been successfully applied to detect surface intermediates on single crystal surfaces at high pressures [47]. Since for intensity reasons short laser pulses are used instead of cw radiation, by means of pulse delay techniques even time resolved processes can be probed by this method down to a temporal resolution <1 ps [22].

Ordinary Raman spectroscopy is not sensitive enough for flat model surfaces, but the situation may be improved by many orders of magnitude with surface enhanced Raman spectroscopy (SERS) whereby even single molecules may be detected [48]. The underlying mechanism involves most likely the excitation of localized surface plasmons, and as a consequence the technique is essentially restricted to rough silver surfaces. A recent modification transfers this requirement to a Ag tip, which is approached to an arbitrary surface (tip enhanced Raman spectroscopy = TERS) [49]. By thus combining Raman spectroscopy with STM [50] even single molecule detection with a lateral resolution of 15 nm could recently be reached [51].

6 Conclusions

One can safely conclude that the introduction of surface physical methods into the investigation of model surfaces has revolutionized our knowledge about the molecular mechanisms underlying heterogeneous catalysis. The most recent milestone in the development of these methods is represented by 4D electron microscopy [54] in which ultrafast laser techniques are combined with electron microscopy to follow the motion of individual atoms on a timescale of femtoseconds. Further progress of this field is to be expected during the coming years by which still more complex (and even biological) systems undergoing reactions at surfaces will become accessible for detailed investigation.

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Overview on Physical Techniques for Investigating Porous Catalysts

John Meurig Thomas

1 Why Porous and Nanoporous Systems?

Although model systems using single crystals, as described [1] in the preceeding chapter, have enormously enlarged our knowledge of the individual processes known to occur during the course of catalysis by polycrystalline solids composed of complex chemical systems, the great advantage that most porous, and all nanoporous (i.e., open structure) catalysts, possess is that they are amenable to in situ characterization under the most severe of ambient conditions. Because such solids have, in effect, three-dimensional surfaces and have high pore volumes, all the techniques of X-ray spectroscopy, and all the variants of infra red and Raman spectroscopy, Mössbauer spectroscopy, as well as all the techniques of nuclear magnetic, and electron paramagnetic resonance may be deployed to characterize such solid catalysts [2, 3] – see Figure 1. Moreover, as Llewellyn et al.