Mechanistic Studies on flat and stepped Palladium Single Crystal Surfaces: Experimental Evidence of Reaction Intermediates for the Decomposition of Formic Acid and Oxidation of Ammonia

by Jan Fingerhut
Cumulative thesis
Date of Examination:2024-08-05
Date of issue:2024-10-04
Advisor:Prof. Dr. Alec M. Wodtke
Referee:Prof. Dr. Alec M. Wodtke
Referee:Prof. Dr. Theofanis N. Kitsopoulos
Referee:Prof. Dr. Dirk Schwarzer
Referee:Prof. Dr. Burkhard Geil
Referee:Prof. Dr. Daniel Obenchain
Referee:Prof. Dr. Jürgen Troe
Persistent Address: http://dx.doi.org/10.53846/goediss-10761

 

 

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Abstract

English

In this work, experimental and theoretical investigations of formic acid desorption, decomposition and ammonia oxidation on single crystal palladium surfaces are presented. The formic acid system is investigated using a helium seeded molecular beam of formic acid, isotopic labeling and universal femtosecond laser ionization. Ammonia oxidation is investigated using crossed molecular beams, resonanced-enhanced multiphoton ionization and laser-induced desorption. Ion-imaging detection is used for both systems. All experimental results are supported by density functional theory (DFT) based kinetic modeling. First, the thermal desorption rate of formic acid from Pd(111) between 228 to 273K using four isotopologoues (HCOOH, DCOOH, HCOOD, DCOOD) is investigated. Upon molecular adsorption, formic acid undergoes decomposition to CO2 and H2 and thermal desorption. The contribution of these processes to the measured rates is disentangled by quantification of the desorption probability using mass conservation. From the experimentally derived thermal desorption rate constants, the isotope-independent binding energy of 0.639 ± 0.008 eV is obtained using the detailed balance rate model. The experimental binding energy is compared to predictions of different DFT functionals. Second, the intermediates in the formic acid decomposition reaction on Pd(111) and Pd(332) are identified using isotopologue specific CO2 formation rates. At surface temperatures around ∼400K CO2 formation shows on both facets two temporally distinct channels which exhibit unique and large kinetic isotope effects (KIEs) upon isotopic labeling. Supported by rate constants derived from transition-state theory (TST) using DFT, the fast channel is assigned to the carboxyl intermediate (C*OOH) and the slow channel to the bidentate formate intermediate (HCO*O*). This is the first experimental observation of the carboxyl intermediate in any surface science experiment. On Pd(332) CO formation is observed which is only possible from the carboxyl intermediate. Furthermore, the experimental derived rate constants for bidentate formate dissociation show a large KIE. A new pathway based on TST-DFT derived rate constants is presented which predicts the correct KIE. Third, a mirco-kinetic model (MKM) within the mean-field (MF) approximation based on the identified reaction intermediates is presented. The MF-MKM is constructed using a one-dimensional grid which is separated into terrace and step-sites. The model is evaluated against the experimental CO2 formation rates and desorption yields of HCOOH, CO2 and CO on Pd(111) and Pd(332). While the initial prediction based on TST-DFT rate constants shows qualitatively correct results, a fitting routine to adjust the DFT calculated barrier heights is employed. The fitting routine improves the quantitative agreement by a factor of 2. The results of the MF-MKM are analyzed using the degree of rate control analysis. Predictions about other reaction products that have not experimentally been investigated and the high coverage regime are made. Last, results on ammonia oxidation on Pd(332) are presented. Ammonia is oxidized by atomic oxygen on the surface to form NO and H2O. NO formation rates feature a temporally resolved onset and decay which is fitted using the simple model I∗→ N∗O→ NO. Novel experimental data on the temporal evolution of the concentration of nitrogen atoms on the surface using laser-induced desorption restrict the simple kinetic model to formation of N*O being slower than desorption. A more complex kinetic model is developed based on oxygen assisted hydrogen abstraction from NHx (x = 3, 2, 1) intermediates which indicates that the rate determining step of ammonia oxidation is N*O formation from atomic nitrogen and atomic oxygen. Scaling of the effective rate constant of N*O formation indicates that the active species is oxygen bound to the up-step site. TST-DFT rate constant modeling shows reasonable agreement with the experimental second order rate constant but indicates an error in the energy barrier of at least 0.19 eV.
Keywords: surface science; reaction kinetics; physcial chemistry
 
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