Well-defined Au-TiO_1tn2(110) model catalysts on fully oxidized substrates [Elektronische Ressource] : preparation, thermal stability and the influence of the substrate on the catalytic activity / Stefan Kielbassa

Stefan Löfving

Informations

• Publié par: universitat_ulm
• Publié le: 01 janvier 2008
• Nombre de lectures: 17
• Langue: Deutsch
• Poids de l'ouvrage: 8 Mo

Extrait

Well-defined Au/TiO (110) model catalysts on fully 2
oxidized substrates
-
Preparation, thermal stability and the influence of the substrate on the
catalytic activity


Universität Ulm
Institut für Oberflächenchemie und Katalyse


Dissertation
zur Erlangung des Doktorgrades Dr. rer. nat.
der Fakultät für Naturwissenschaften


Stefan Kielbassa
(Marburg / Lahn)

2008

















Amtierender Dekan: Prof. Dr. Klaus-Dieter Spindler

1. Gutachter: Prof. Dr. Rolf Jürgen Behm
2. Gutachter: Prof. Dr. Wolfgang Schmickler


Tag der Promotion: 21.10.2008





Der Wert davon, daß man zeitweilig eine strenge Wissenschaft streng
betrieben hat, beruht nicht gerade in deren Ergebnissen: denn diese
werden, im Verhältnis zum Meere des Wissenswerten, ein verschwindend
kleiner Tropfen sein. Aber es ergibt einen Zuwachs an Energie, an
Schlussvermögen, an Zähigkeit der Ausdauer; man hat gelernt einen
Zweck zweckmäßig zu erreichen. Insofern ist es sehr schätzbar, in
Hinsicht auf alles, was man später treibt, einmal ein wissenschaftlicher
Mensch gewesen zu sein.
(F. Nietzsche)

Table of contents 7
Table of contents
1. Introduction ................................................................................................................... 9
1.1. CO oxidation on Au/TiO .................................................................................... 12 2
1.2. Goals of this dissertation ..................................................................................... 27
2. Experimental................................................................................................................ 29
2.1. UHV System........................................................................................................29
2.2. In-situ XPS Measurements .................................................................................. 33
2.3. Crystal pretreatments...........................................................................................34
2.4. Preparation of Au particles by the micellar technique ........................................ 37
3. Set-up of new reactor systems .................................................................................... 39
3.1. Micro flow reactor............................................................................................... 39
3.2. Scanning mass spectrometer (SMS).................................................................... 53
4. Model catalysts prepared by thermal evaporation................................................... 75
4.1. Clean TiO and MgO substrates .......................................................................... 75 2
4.2. Growth of Au nanoparticles ................................................................................ 81
4.3. Stability of Au/TiO (110) model catalysts .......................................................... 90 2
4.4. Summary............................................................................................................102
5. Morphology and stability of Au nanoparticles on TiO (110) prepared from 2
micelle-stabilized precursors .................................................................................... 105
5.1. Optimization of the preparation technique ........................................................ 105
5.2. Long range order and particle shapes examined by SEM and TEM................. 112
5.3. Surface composition after preparation and UHV annealing.............................. 114
5.4. Variation of particle sizes and distances............................................................ 117
5.5. Stability in air at room temperature................................................................... 125
5.6. Thermochemical stability in O and CO/O ...................................................... 127 2 2
5.7. Summary............................................................................................................139
Table of contents 8
6. The influence of the support on the catalytic activity of Au/TiO (110) model 2
catalysts ...................................................................................................................... 141
6.1. Preparation of different Au/TiO (110) model catalysts .................................... 142 2
6.2. Initial CO oxidation activities ........................................................................... 149
6.3. Discussion.........................................................................................................153
6.4. Summary...........................................................................................................159
7. Summary and outlook............................................................................................... 161
8. References ..................................................................Fehler! Textmarke nicht definiert.
Appendix
A. Deutsche Zusammenfassung (German Summary)……………………...…………… 182
B. Short summary of literature on Au/TiO (110) model catalysts……………………... 185 2
C. Derivation of mathematical expressions…………………………………………….. 187
D. Danksagung (Acknowledgements)…………………………………………………... 189
E. Publications and Conference Contributions…………………………………………. 191
F. Lebenslauf (Curriculum Vitae)……………………………………………………… 193
G. Erklärung…………………………………………………………………………….. 194
1. Introduction 9
1. Introduction
Heterogeneous catalysis plays an important role in many industrial processes (like
synthesis of chemicals, fuel refinery etc.) and emission control (e.g., automotive three-way
catalyst). The catalyst itself is in the solid phase and serves to rapidly achieve chemical
equilibrium between the reactants in the gas or liquid phase. The catalytic reaction consists
of a sequence of the following elementary steps [1]:
• Adsorption on the surface
• Surface diffusion to the active centers
• Surface reaction
• Surface diffusion of the products
• Desorption of the products
A comprehensive knowledge of the surface chemistry between the reactants and the
catalyst, ideally on the atomic scale, is crucial for understanding and improving these
systems. From this point of view, the preparation and scientific investigation of
heterogeneous catalysts is basically a discipline of surface science [1,2]. The vast technical
improvements in this field during the last decades provide tools for a detailed
understanding of the morphology as well as the electronic and chemical structure of these
systems under various conditions [3].
A typical group of heterogeneous catalysts often used in commercial applications consists
of small metal particles of an active material (e.g. Rh, Pd, Pt) being in contact with a
support (e.g. carbon, metal oxides) in the form of powders or porous materials like
zeoliths [4]. Despite its name, the support is in many cases not only required to separate
and stabilize these metal particles, but it may also enhance the catalytic activity.
Furthermore, it is not only the chemical composition of the active material that has to be
considered for a fundamental understanding of the catalytic reaction, also the shape or the
size of the particles might play an important role [1]. Unfortunately, the high diversity of
these complex systems makes it difficult to obtain answers to very specific questions. To
overcome these problems, a different approach for studying catalyst systems has been
developed, the application of so-called model catalysts [5]. Starting from the most
simplified form of a catalyst, a planar metal single crystal surface of a certain orientation,
the system is made more and more complex by adding steps, kinks and defects and later by
using model samples consisting of metal particles of controlled sizes on planar single 1. Introduction 10
crystal or thin film supports [6]. Using this approach the influence of the metal surface
structure, metal particle size effects and chemical or structural properties of the support can
be determined in a controlled way. The catalytic performance of these systems is often
-3examined under vacuum conditions up to 10 mbar, e.g. with molecular beam techniques
or via temperature programmed desorption/reaction (TPD/TPR) experiments. However, the
results obtained from model catalysts may differ from those recorded on disperse powder
catalysts, which are usually examined in the pressure regime of 1-100 bar. To avoid
problems of this so called “pressure gap”, special reactors have to be used which allow the
study of model catalysts under high pressures despite their low active surface [7].
Au based catalysts are in the focus of the current work and there are several reasons why
especially model systems can help to obtain a better understanding of their performance.
Because of its noble nature gold was not thought to be a catalytically active material until
Haruta et al. revealed that very small Au particles indeed become excellent catalysts,
especially in low-temperature applications [8]. Despite numerous studies on this topic, the