Investigation of Surface Dynamics on Oxidized Metal Surfaces Using Ion Imaging Techniques
by Arved Cedric Dorst
Date of Examination:2025-11-04
Date of issue:2025-11-12
Advisor:PD Dr. Tim F. Schäfer
Referee:PD Dr. Tim F. Schäfer
Referee:Prof. Dr. Daniel Obenchain
Referee:Prof. Dr. Dirk Schwarzer
Referee:Prof. Dr. Burkhard Geil
Referee:Prof. Dr. Jürgen Troe
Referee:Prof. Dr. Martin A. Suhm
Persistent Address:
http://dx.doi.org/10.53846/goediss-11624
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Abstract
English
Heterogeneous catalysis plays a pivotal role in modern life, with applications ranging from the Haber-Bosch process over ethylene epoxidation to exhaust conversion in modern three-way catalysts in automobiles. While these reactions are well understood on the macroscopic level, the microscopic details are often unclear. This knowledge gap is where surface science experiments can provide valuable insights, enabling a deeper understanding of reactions at the nanoscopic level. In this work, surface reaction products which desorb from oxidized rhodium and silver surfaces were examined. The oxidized metal surfaces were characterized, and typical reactions on these surfaces were studied with a special emphasis on the effects of subsurface oxygen (Osub) formation on reaction dynamics. For that, the velocity distributions 𝑓(𝑣) of desorbing surface reaction products were chosen as a fingerprint of the dynamics. Recombinative desorption of oxygen from oxidized metal surfaces was studied on Rh(111) and on Ag(111). The experimental method of choice was a combination of temperature-programmed desorption (TPD) and velocity map imaging (VMI). By combining these two methods, the velocity distributions 𝑓(𝑣) deduced by VMI were assigned to distinct desorption features in TPD experiments. Additionally, molecular beam surface scattering experiments were conducted to study two different model reactions: styrene epoxidation on silver and CO oxidation on rhodium. The results from in total four experimental series are presented in this thesis. Chapters 4 and 5 report the velocity and angular distributions for recombinative desorption of oxygen atoms from Rh(111) and Ag(111), as well as the impact of Osub formation on Rh(111). The recombinatively-desorbing oxygen atoms display hyperthermal velocity distributions, and the angular flux distribution is narrower than predicted by a thermal cos(𝜗) distribution. Application of detailed balance provides a framework for interpreting these deviations. Chapter 6 presents the results for styrene epoxidation on Ag(111). Molecular beam surface scattering experiments demonstrate that an electrophilic oxygen (Oelec) phase, coexisting with Osub, exclusively produces styrene oxide. Once Oelec is depleted, only styrene combustion by nucleophilic oxygen (Onuc) occurs in titration experiments. Chapter 7 focuses on the dynamics of CO oxidation on a bifaceted Rh(111)/Rh(332) crystal. Velocity-resolved kinetics (VRK) measurements reveal that without Osub, only hyperthermal CO2 forms on both facets. Upon Osub formation, however, a thermal reaction channel emerges, whose contribution increases with the amount of Osub until exclusively thermal CO2 is observed. This behavior indicates that Osub does not simply act as an oxygen atom reservoir but also modifies the underlying potential energy surface (PES). First experimental findings on the reaction kinetics are presented, and preliminary theoretical models are shown which explain the observed velocity shift of CO2 induced by Osub incorporation. The findings of this dissertation have significant implications for our understanding of heterogeneous catalysis and the role of Osub in reaction dynamics. The results also provide a foundation for further experiments in the future and allow for the development and refinement of improved theoretical models. This could lead to a deeper understanding of oxidized metal surfaces.
Keywords: Surface Science; Heterogeneous Catalysis; Surface Dynamics; Ion Imaging; Velocity Map Imaging; Temperature-Programmed Desorption; Rhodium; Silver; Styrene epoxidation; CO oxidation; Velocity-Resolved Kinetics; Rh(111); Ag(111); Rh(332)
