Carbonate Selective Ca2Ru2O7-y Pyrochlore Enabling Room Temperature Carbonate Fuel Cells: I. Synthesis and Physical Characterization

Abstract

Synthesis of a carbonate selective Ca2Ru2O7-y catalyst was investigated using solid-state and hydrothermal routes. The resulting materials were physically characterized by x-ray diffraction, BET gas adsorption, scanning electron microscopy and temperature programmed desorption. The solid-state reaction of precursor oxides, CaO and RuO2, led to the formation of a perovskite phase. A hydrothermal route using O2 as an oxidizing agent yielded a mostly amorphous phase primarily comprised of unreacted precursor. The use of low concentration KMnO4 (10 mM) as a replacement for O2 in the hydrothermal synthesis, in combination with high pH (∼14) and moderate temperature (200°C), yielded a highly crystalline, thermally stable Ca2Ru2O7-y pyrochlore. The increase in crystallinity was attributed to the ability of permanganate to maintain ruthenium in the +5 oxidation state required for formation of the pyrochlore phase. Micron-sized primary particles with a high density of ∼ 50 nm surface nanocrystallites were obtained. The presence of the nanocrystallites gave the pyrochlore a high surface area, 174 m2/g. The pyrochlore showed preferential surface adsorption of CO2 compared to H2O, making it a feasible candidate for a carbonate selective catalyst. This preferential adsorption was attributed to the use of calcium, an alkaline earth metal, which gave the pyrochlore a high surface basicity.