Investigation of Electrocatalytic Oxidation Reactions: Insights From Electrochemical Mass Spectrometry

Tugce Yilmaz

Research output: Book/ReportPh.D. thesis

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Abstract

In recent years, we are witnessing the escalating impacts of climate change, including increasingly frequent heatwaves, flooding, and wildfires. These events emphasize the necessity of urgent actions to address global warming. One significant way of addressing this challenge is replacing fossil fuels with renewable energy sources. Notably, the chemical industry ranks among the leading contributors to CO2 emissions. Therefore, electrification of the chemical industry emerges as a strategy to tackle the climate change.
However, to compete with the current industrial methods, electrochemical methods must become more economically feasible. This goal requires the design of highly active, selective, and stable catalysts for the electrochemical synthesis of substances. To design catalysts rationally, a fundamental understanding of the reaction mechanisms is a necessity. Therefore, gaining real-time insights into the reactions is critically valuable. In this regard, electrochemical mass spectrometry (EC-MS) is used for the investigation of two important reactions for the chemical industry in this study: electrochemical propylene epoxidation and chlorine evolution reaction.
In this study, EC-MS is applied for the detection of propylene oxide, and electrochemical propylene epoxidation is measured in real-time. The selectivity and activity trends are investigated on PtOx and PdOx catalysts to obtain insights into the mechanism of electrochemical propylene epoxidation. The results reveal that the selectivity for epoxidation peaks at 1.3 V vs. RHE on PdOx, whereas it steadily increases on PtOx up to 1.6 V vs. RHE. However, further anodic potentials could not be explored due to conductivity issues arising from bubble formation in the stagnant thin-layer cell in the EC-MS system. Furthermore, mass spectrometry-derived Tafel plots indicate a consistent rise in epoxidation activity without a change in slope on PtOx as potentials become more anodic. Conversely, a notable change in slope at 1.3 V vs. RHE on PdOx suggests a shift in the reaction mechanism at this potential. This phenomenon aligns with a previously published theoretical study [1], which proposes a Langmuir-Hinselwood mechanism for PtO2 and a transition from Langmuir-Hinselwood to Mars-Van Krevelen mechanism with the formation of oxygen vacancies on PdO. Moreover, EC-MS reveals the formation of CO2 through surface oxide reduction, serving as an indicator of surface oxidation under the reaction conditions.
Furthermore, chlorine evolution reaction (CER) is explored with EC-MS during my three months of external stay in Spectro Inlets, ApS. EC-MS provided a great sensitivity for Cl2, resulting in the lowest detection potentials for CER to the best of our knowledge. This finding raised questions regarding equilibrium potential definitions in the literature. Consequently, this thesis conducts a comprehensive review of equilibrium potential definitions in the literature. A method for the dynamic equilibrium potential determination by utilizing EC-MS is proposed and its limitations are described. 
Moreover, CER activity evaluations are conducted for various catalysts including RuO2, IrO2, and Pt-based catalysts being Pt stub, Pt film on Ti substrate, and 2 nm TiO2 on Pt film on glassy carbon substrate. The results revealed that the activity of Pt-based catalysts surpasses that of RuO2 and IrO2. It is suggested that the oxidation of Pt is retarded in the presence of Cl– which leads to a reaction mechanism involving direct Pt- l intermediate with faster kinetics rather than Pt-OCl. This hypothesis is supported with the CO stripping experiments, conducted both with and without Cl . These experiments revealed a ≈600 mV shift in the CO oxidation potential towards more anodic values in the presence of Cl . This shift signifies the inhibition of oxygen species adsorption onto the catalyst surface due to competitive Cl adsorption. 
Original languageEnglish
PublisherDepartment of Physics, Technical University of Denmark
Publication statusPublished - 2023

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