Application of Laser Induced Desorption to study Velocity-Resolved Kinetics

by Kim Papendorf
Doctoral thesis
Date of Examination:2022-05-09
Date of issue:2023-05-04
Advisor:Prof. Dr. Alec M. Wodtke
Referee:Prof. Dr. Alec M. Wodtke
Referee:Prof. Dr. Dirk Schwarzer
Referee:Prof. Dr. Claus Ropers
Referee:Prof. Dr. Theofanis N. Kitsopoulos
Referee:Prof. Dr. Jürgen Troe
Referee:Prof. Dr. Thomas Zeuch
Persistent Address: http://dx.doi.org/10.53846/goediss-9861

 

 

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Abstract

English

In this work, I present a new method to measure velocity-resolved kinetics (VRK) of chemical reactions on surfaces. Ultra-fast laser-induced desorption (LID) is utilized as a tool to directly measure coverages of adsorbates. This highly non-linear process leads to a very high heating rate at the surface, which favors the desorption reaction, and other reactions are interrupted or suppressed. In principle, all kind of adsorbates are desorbed simultaneously and particle selection is achieved afterwards during the detection. Gating of the detector allows for mass selection and resonance-enhanced multi photon ionization (REMPI) can be used to selectively ionize. Resolving the velocity of the desorbing flux is achieved by using the ion-imaging technique with slice imaging. Scanning the delay between the detection and the molecular beam pulse, which initiates a reaction, yields the kinetic trace of the selected particle. In this work, I have chosen to study the VRK of the thermal desorption reaction of carbon monoxide, oxygen and ammonia from platinum. The results are compared to previous measurements and serve as a proof-of-concept for this new method. I have discovered that this method works especially well for slow reactions at low temperatures where traditional methods struggle because they measure the thermally desorbing flux, which simply is too weak if the reaction is slow. This allows the extension of the temperature range of CO thermal desorption kinetics towards lower temperatures, which shows Arrhenius-like behavior over a range of more than 150 K. In principle, this method can be applied to chemical reactions on surfaces with more reactants as well, by dosing the surface with one adsorbate and initiating the reaction with a molecular beam pulse of a different adsorbate. One could choose to measure the kinetic trace of the reactants or products. It should also be possible to record the kinetic trace of reaction intermediates, which are not stable in the gas phase, and therefore can not be measured in the thermal desorption flux. Although measuring the kinetic trace of reaction intermediates is beyond the scope of this work, I am able to show that it is possible to induce the desorption of O-atoms, which are unstable particles that play a role as reaction intermediate in many oxidation reactions. Important examples of catalytic oxidations are the CO oxidation, hydrogen oxidation and the Ostwald process. For those examples, the kinetic of the elementary steps are not entirely clear and are still in discussion to this day. Measuring kinetic traces of reaction intermediates with the LID-VRK method has the potential to provide invaluable information about the kinetics of elementary steps in catalytic reactions.
Keywords: velocity-resolved kinetics; laser-induced desorption; surface detection; ion imaging; adsorbate; molecular beam
 
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