Mechanistic Studies of Hydrogen Recombination and Water Formation on Palladium Surfaces

by Michael Schwarzer
Doctoral thesis
Date of Examination:2024-03-01
Date of issue:2024-12-19
Advisor:Prof. Dr. Alec M. Wodtke
Referee:Prof. Dr. Alec M. Wodtke
Referee:Prof. Dr. Dirk Schwarzer
Referee:Prof. Dr. Jürgen Troe
Referee:Prof. Dr. Burkhard Geil
Referee:Prof. Dr. Martin A. Suhm
Referee:Prof. Dr. Jörg Behler
Persistent Address: http://dx.doi.org/10.53846/goediss-10960

 

 

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Abstract

English

In heterogeneous catalysis, one of the major challenges is to determine which microscopic processes are of importance for the reaction to occur with desired activity and selectivity. Efforts towards a fundamental understanding of the principles of catalysis can contribute to the development of superior catalytic materials in the future. In this work, the elementary steps of catalytic water formation from gaseous hydrogen and oxygen, exposed to single crystalline surfaces of palladium, are studied. Transient kinetics experiments at various surface temperatures and reactant coverages are realized using pulsed molecular beams, laser ionization and ion-imaging detection under ultra-high vacuum conditions. Experimental data is analyzed by means of kinetic modeling, assisted by density functional theory (DFT) calculations to achieve an atomic-scale interpretation. In the first part of this work, the energy landscape related with H-atom recombination and H-atom diffusion into the bulk of the crystal is determined. Compared to earlier studies, a more fundamental and consistent framework is developed. In the second part of this work, water formation experiments are presented and analyzed. The presence of atomic steps on the surface drastically accelerates water formation at elevated oxygen coverages, while under oxygen lean conditions the stepped surface is similarly active as the flat one. These observations are in agreement with DFT calculations which predict hydroxyl formation O + H <-> OH to be the rate-limiting reaction at low oxygen coverages with similar barriers on steps and terraces. However, at elevated oxygen coverages, the O-atoms form zig-zag structures along the step. O-atoms of the zig-zag structure have a reduced barrier for OH formation and therefore water formation is facilitated. Experimental water formation rates under oxygen rich conditions follow a second-order kinetics behavior which is assigned to the OH + OH <-> H2O + O reaction at steps. Due to the high reactivity of the zig-zag structure, OH formation is no longer rate-limiting. It is speculated that the high activity of O-atoms in the zig-zag structure is also of importance in other oxidation reactions.
Keywords: Chemical Kinetics; Surface Science; Heterogeneous Catalysis; Palladium; Hydrogen; Oxygen; Water
 
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