PhD Thesis Defence Presentations - Dimitrios Zagoraios

Over the last four decades, the Electrochemical Promotion of Catalysis (EPOC effect) has been utilized successfully for more than 100 catalytic reactions to alter both catalytic activity and selectivity. This phenomenon is related to the modification of the work function of a catalyst, due to the electrochemical migration of ions/promoters from a solid electrolyte to the catalyst/gas interface, where an “effective” double layer is formed.

Up until now, the majority of EPOC studies have been carried out over noble metal catalytic films prepared via organometallic paste calcination or wet impregnation methods. This has resulted in poor metal dispersion and relatively low active surface area, increasing the total costs of the process. In view of the foregoing, this Doctoral Thesis deals with the use of supported metal nanoparticles, as well as the utilization of non-noble transition metals in electrochemical promotion experiments. The main goal of this Thesis is to study and implement practical applications and extensions of the EPOC effect in the modern catalytic industry.

The first part of the present Doctoral Thesis focuses on the synthesis, characterization and use of supported (nanodispersed) catalysts as electrodes in electrochemical promotion experiments. The novelty of this work based on the architecture of the working electrodes. We suggest the direct deposition of metal nanoparticles supported on a nanostructed semiconductor, providing mixed (ionic-electronic) conductivity at intermediate operating temperatures. This property allows the supported catalyst to behave as an electrode, thus satisfying the needs of the phenomenon. Experiments were performed on both oxidation (oxidation of methane (CH₄)) and hydrogenation (hydrogenation of carbon dioxide (CO₂)) reactions. More specifically, the supported catalyst 5% Pd/Co₃O₄ was studied for CH₄ oxidation, while 2% Ru/Co₃O₄ and free-standing Ru nanoparticles were studied for the hydrogenation of CO₂. According to the experimental results, the semiconductor acts as a "pathway" for the migration of ions from the solid electrolyte to the metal nanoparticles. At the same time, these catalysts can be considered as self-promoted (via the metal/support interactions) and their observed electro-promotion is a result of the higher and electrochemically controlled coverage of the electrode with promoting ions. Interestingly, the studied catalysts show excellent stability, as both the size and the dispersion of the metal nanoparticles do not significantly change, in contrast to the conventional methods of wet impregnation or organometallic paste calcination.

The second part of this Thesis aims to extensively study of non-noble transition metals as working electrodes in electrochemical promotion experiments. The objective of this study is twofold. On one hand, it aims to minimize the total costs of the EPOC effect, while at the same time utilizes the unique properties of this category of metals, such as the ability to easily alter their oxidation states. Experiments were performed over free-standing Co and Ni nanoparticles for the Reverse Water Gas Shift (RWGS) reaction. Both electro-catalytic experiments and physicochemical techniques (XPS and CV) reveals that (during EPOC) there is an in-situ formation of metal oxides, thus enhancing the catalytic activity and selectivity. Beyond their oxidation state, these oxides differ on terms of crystalline structure and work function. Reaction rate increase up to 700% was observed, while the polarization time of the sample seems to significantly affect their ratio of the surface oxides. The particular nature of the non-noble transition metals upon polarization seems to be attractive for the in-situ activation of catalysts through the EPOC phenomenon and constant current pulse operation.