Morphological and microstructural evolution in BZY/Ni catalyst materials

Abstract

Yttria doped barium zirconate (BZY, BaZr 1-xYxO3-δ) is of interest for its potential uses as a catalyst and in protonic ceramic fuel cells. BZY is often doped with transition metals, such as Ni, which can form into metallic nanoparticles and greatly increase the catalytic performance of the material. The process of precipitating Ni nanoparticles during a reduction treatment, termed 'exsolution', is utilized to produce stable catalytic nanoparticles. Studies are presented here which aim to further the understanding of morphological and microstructural evolution in BZY/Ni, focusing on how that evolution will affect catalytic performance. To begin, BZY/Ni is analyzed as a catalyst in the water-gas-shift (WGS) reaction, demonstrating bi-functionality of BZY as a WGS support for the first time. The loss of catalytic surface area through the coarsening of metallic nanoparticles is a major degradation mechanism for supported metal catalysts, and has not been examined in BZY/Ni. To study Ni coarsening in BZY/Ni, the kinetics of Ni nanoparticle growth are analyzed, allowing for a determination of the dominant coarsening mechanisms. In addition, the morphology in Ni particles produced through exsolution and those produced through metal deposition and dewetting are compared; it is demonstrated that Ni particle morphology is controlled by the thermodynamics of surfaces and interfaces. Finally, the potential of in situ HRTEM as a technique for studying exsolution in BZY/Ni is demonstrated in preliminary experiments. Epitaxial BZY thin films are used to provide more control for fundamental studies into the relationships between the BZY support and Ni nanoparticles. BZY thin films have been studied in the literature for their excellent protonic conductivity when compared to bulk BZY, but the morphological and microstructural evolution of these films at high temperatures has not been examined thoroughly. Here, two studies are presented that describe the decomposition of BZY thin films, beginning with the formation of crystallographically oriented barium carbonate grains from the BZY film. Subsequently, the addition of Fe and Ni are observed to have different effects on the decomposition of BZY thin films, and analysis is provided to explain the effects of the dopants.