PhD Thesis Defence Presentations - Nikoletta Kokkinou

The effect of Electrochemical Promotion of Catalysis (EPOC phenomenon) or non- Faradaic Electrochemical Modification of Catalytic Activity (NEMCA effect) has been studied in recent decades in many different catalytic reactions (oxidations, reductions, hydrogenations, dehydrogenations, isomerizations, gas reformations, etc.) using a lot of metal catalysts (such as Pt, Pd, Rh, Ag, Ni, Au, Fe, IrO₂, RuO₂) deposited on various solid electrolytes.

It has been proved that the catalytic activity and selectivity of conductive metal catalyst films deposited on solid electrolytes can be dramatically and reversibly changed due to application of a current or potential between the metal catalyst film and a second electrode (counter electrode) which is also deposited on the solid electrolyte. This phenomenon is based on the modification of the work function of a metal catalyst, due to the electrochemical migration of ions/promoters from the solid electrolyte to the metal/gas interface, where an effective double layer is formed.

The aim of this PhD thesis is to investigate the EPOC phenomenon for the CO2 hydrogenation on Rh films deposited on YSZ (i.e., O^2- ion conductors) and BZY (i.e., H⁺ cation conductors) solid electrolytes.

The CO₂ hydrogenation reaction is a reaction with various environmental and industrial advantages, such as the reduction of CO₂ emissions into the atmosphere due to its conversion into useful products and fuels. The main products of CO₂ hydrogenation under atmospheric pressure and median temperatures (200^oC<T<500^oC) are methane, CH₄, via the methanation reaction (Sabatier reaction), a strongly exothermic reaction, and carbon monoxide, CO, via of the Reverse Water Gas Shift (RWGS), an endothermic reaction which is favored as temperature increases.

In this PhD thesis, the EPOC phenomenon was investigated for the CO₂ hydrogenation reaction using a single pellet reactor at low temperature conditions and this is the innovation of this thesis compared to previous studies.

The First Chapter of the dissertation discusses the importance of managing CO₂, which is one of the main gasses responsible for the greenhouse effect. In this chapter, the Reverse Water Gas Shift (RWGS) and the Sabatier reaction (methanation reaction) are presented as well as the proposed mechanisms are discussed for the above reactions.

In the Second Chapter, solid electrolytes and their properties are described. A special report is placed on oxygen ion conductors (yttria stabilized zirconia, YSZ) and proton conductors (barium zirconate perovskite structure, BZY), which were used during the experiments of this PhD thesis.

Third Chapter is dedicated to the description of EPOC phenomenon, its history, mechanism, the rules that govern it, and its quantification. The chapter ends with a review of previous EPOC studies on the CO2 hydrogenation reaction that can be found in literature and with a correlation between the EPOC phenomenon and the phenomenon of metal support interactions (Metal Support Interactions, MSI).

In Fourth Chapter, the main functional parts of the experimental set-up and the single pellet reactor are thoroughly analyzed. This chapter also describes the method of preparing the electrochemical cells by depositing the working, counter and reference electrodes. In addition, information obtained via the physicochemical characterization techniques, namely Scanning Electron Microscopy (SEM), X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) are also discussed.

Chapter Fifth presents the results of the study on the Rh/YSZ catalyst film of loading 1.8 mg. The experimental operating conditions are under partial pressure ratios (P_CO2:P_H2) that ranged from 1:2 to 4:1, in the temperature range 350 - 430 ^oC, under atmospheric pressure and at a total flowrate of 50 cm³ min^-1. The results confirm that the application of positive and negative potentials and/or currents can change the selectivity of the reaction products. Furthermore, the catalyst surface morphological and electronic characteristics, before and after the reactions, using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) are described in detail in the same chapter.

The Sixth Chapter presents the results of the EPOC effect on the CO2 hydrogenation reaction on the Rh/BZY catalyst film of loading 0.9 mg. The study was caried out under partial pressure ratios (P_CO2:P_H2) that varied from 1:7 to 4:1, in the temperature range 380 - 430 ^oC, under atmospheric pressure, while a comparison of the results is made for the total flowrates of 50-100-150 cm³ min^-1. The chapter also studies the effect of the thickness and mass of the catalytic film both on catalytic rate and EPOC phenomenon. For this study, four different samples were prepared with different Rh loadings, 0.9, 3.6, 4.8 and 6.1 mg. Furthermore, the physicochemical characterization of the electrodes before and after the experiments is also presented.

Finally, the conclusions drawn from this study are presented.