| Summary: | Oxides have been extensively used as high temperature catalysts in electrochemical devices and for gas conversion reactions, including solid oxide fuel cells (SOFC), solid oxide electrolysis cells (SOEC), ethane cracking, CO oxidation, and the oxidative coupling of methane. In hightemperature catalysis, the gaseous reactants undergo chemical or electrochemical reactions at the surfaces of the oxide catalysts. Therefore, understanding and controlling the surface chemistry of oxides is essential for maximizing their catalytic performance. Despite being of high importance and scientific interest, our current understanding of the surface chemistry of oxide catalysts remains limited. One of the reasons for this is that it is not trivial to investigate oxide catalysts in operando. High temperatures (typically 750-900˚C) and reactive gas environments limit the use of many surface analysis tools such as X-ray photoelectron spectroscopy and scanning probe microscopy. Aiming to fill the gap in our knowledge of the surface chemistry of oxides, this thesis investigates the surface chemistry of various oxide catalysts, including perovskite oxides, fluorite oxides, and amorphous oxides in different catalytic applications.
This thesis introduces five different studies on the surface chemistry of oxide catalysts used for different catalytic applications. The first study investigates the surface degradation of doped lanthanum manganite perovskite oxides which results from the formation of surface dopant precipitates during their operation as air electrodes in solid oxide fuel/electrolysis cells. The 4 effects of polarization on the surface degradation are explained by two different driving forces for dopant segregation to the surface of the air electrode. This dopant segregation and resulting surface degradation causes a significant decrease in the activity of the air electrode. In the second study, a method to reverse this surface degradation and regenerate the surface catalytic activity with an applied electrical potential is introduced. The mechanism of the surface reactivation under anodic potential is proposed. Next, a novel electrochemical method to control the size and number density of Au nanoparticles deposited onto perovskite oxides and fluorite oxides is demonstrated. This electrochemical method allows one to control the concentration of the surface oxygen vacancies and obtain small Au nanoparticles with a high density on the oxide surface. Such a fine distribution of metal nanoparticles is important to maximize their catalytic performance in gas conversion reactions such as CO oxidation and the water-gas shift reaction. In the fourth study, the surface chemistry of state-of-the-art amorphous oxide catalysts for the oxidative coupling of methane (OCM) is investigated. The degradation mechanisms of these OCM catalysts and various ways to inhibit such degradation by controlling the phase evolution of the catalyst surface are introduced. The fifth and final study introduces an electrochemical way to perform the OCM reaction using mixed ionic-electronic conductors (MIEC) as a backbone electrode and OCM-active secondary particles deposited onto the surface of the MIEC electrode. Optimizing the dispersion of the OCM-active secondary particles has been proposed as a promising way for improving the OCM selectivity of this material.
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