DARWIN Digitale Dissertationen German Version Strich

FU Berlin
Digitale Dissertation

Christian Sachs :
Investigation of Reaction Mechanisms on Surfaces with Scanning Tunneling Microscopy
Hydrogen Oxidation on the Platinum(111) and the Rhodium(111) surfaces
Untersuchung von Reaktionsmechanismen auf Oberflächen mittels Rastertunnelmikroskopie

 FU Logo

|Abstract| |Table of Contents| |More Information|

Abstract

Abstract of doctoral thesis

During the catalyzed oxidation of hydrogen on a Pt(111) surface, reaction fronts develop below the desorption temperature of water (170 K). The reaction fronts travel at constant velocity over the surface; their width (from 10 nm till 100 nm) is in the mesoscopic range. The spatiotemporal evolution of the fronts was investigated by means of scanning tunneling microscopy (STM) at temperatures between 108 K and 134 K. The reaction was modeled by a reaction diffusion system, which contains the autocatalytic reaction system (OH + H -> H2O; 2 H2O + O -> 3 OH + H) and the diffusion of water. The numeric solutions of the kinetic equations qualitatively reproduce the experimental findings. Circular reaction fronts develop from a starting water nucleus and travel over the surface in the form of an OH maximum, thereby transforming the oxygen to water. A lower limit for the front velocity was derived by an analysis of the differential equations. For a quantitative comparison between experiment and theory the rate constants of the involved reactions and the diffusion coefficient of H2O on the oxygen-covered Pt(111) surface were determined by additional STM and LEED experiments. The comparison with the experimentally determined front velocities and widths reveal a good agreement for the velocities. For the front width the agreement between theory and experiment is much less good. It is shown that these deviations result from the inability of the reaction-diffusion system to describe reaction fronts on the mesoscopic scale. In RD systems interactions of the diffusing water molecules with each other and with OH islands and the complex chemical processes in the front are not included. The transferability of the reaction model for the low temperature mechanism of the hydrogen oxidation to other transition metal surfaces was tested on the Rh(111) surface. STM data of the oxygen phases on Rh(111) confirm the existence of a (2x1)O structure, which has not been observed by STM so far. STM and LEED experiments show that in contrast to Pt(111) at 145 K on Rh(111) no reaction of oxygen with hydrogen occurs. It is suggested that the higher binding energy of oxygen to Rh(111) compared to Pt(111) leads to an increase for the onset temperature of the involved reactions; therefore, the low temperature mechanism for the oxidation of hydrogen fails on rhodium.

Table of Contents

Download the whole PhDthesis as a zip-tar file or as zip-File

For download in PDF format click the chapter title

Inhaltsverzeichnis Titelblatt

Inhaltsverzeichnis

Abkürzungen 5
1 Einleitung 9
2 Experimentelles 15
2.1 Rastertunnelmikroskopie 15
2.2 Das Tieftemperatur-STM 16
2.3 Das Video-STM 18
2.4 Präparation der Einkristall-Oberflächen 19
3 Wasserstoffoxidation auf Platin(111) 23
3.1 Überblick über die bisherigen Arbeiten 24
3.2 Zeitaufgelöste Abbildung von Reaktionsfronten mit dem STM 28
3.2.1 Durchführung der Experimente 28
3.2.2 Ergebnisse 30
3.2.3 Diskussion 37
3.3 Modellierung der Reaktion 44
3.3.1 Das Reaktions-Diffusions-System 44
3.3.2 Numerische Integration des RD-Systems 49
3.3.3 Analytische Untersuchungen des RD-Systems 53
3.4 Experimente zur Bestimmung der kinetischen Parameter der Einzelreaktionen 60
3.4.1 Die Geschwindigkeitskonstante der Reaktion 2 H2O + O -> 3 OH+H 61
3.4.2 Die Geschwindigkeitskonstante der Reaktion OH + H -> H2O 64
3.4.3 Die Diffusionskonstante von H2O 65
3.5 Quantitativer Vergleich: Experiment - RD-Modell 66
3.6 Diskussion 70
3.6.1 Der Reaktionsmechanismus 70
3.6.2 Qualitativer Vergleich: Experiment - RD-Modell 73
3.6.3 Grenzen des RD-Systems und Ausblick für die Modellierung 75
4 Wasserstoffoxidation auf Rhodium(111) 79
4.1 Sauerstoffphasen auf Rhodium(111) 80
4.1.1 Bisherige Untersuchungen der Sauerstoffphasen 80
4.1.2 Charakterisierung der Sauerstoffphasen durch STM 83
4.2 Die Reaktion zu Wasser 89
4.2.1 Stand der Forschung 89
4.2.2 STM- und LEED-Untersuchungen der Reaktion 91
4.3 Diskussion 95
5 Zusammenfassung 99
Anhang 103
A: Bestimmung der minimalen Frontgeschwindigkeit 103
B: Kinetik der Reaktion 2 H2 + O -> 3 OH + H 107
C: RD-System für das alternative Reaktionsschema 107
Abbildungsverzeichnis 111
Literaturverzeichnis 113
Kurzzusammenfassung 121
Danksagung 123

More Information:

Online available: http://www.diss.fu-berlin.de/2001/253/indexe.html
Language of PhDThesis: german
Keywords: stm, oxygen, hydrogen, water, front, traveling wave, reaction-diffusion
DNB-Sachgruppe: 30 Chemie
Classification PACS: 82.40.Np
Date of disputation: 29-Nov-2001
PhDThesis from: Fachbereich Biologie, Chemie, Pharmazie, Freie Universität Berlin
First Referee: Prof. Dr. Gerhard Ertl
Second Referee: Prof. Dr. Klaus Christmann
Contact (Author): sachs@fhi-berlin.mpg.de
Contact (Advisor): wintterlin@fhi-berlin.mpg.de
Date created:10-Dec-2001
Date available:12-Dec-2001

 

|| DARWIN|| Digitale Dissertationen || Dissertation|| German Version|| FU Berlin|| Seitenanfang ||

Mail-Icon Fragen und Kommentare an:
darwin@inf.fu-berlin.de

© Freie Universität Berlin 1999