Synthesis and Characterization of Mixed‐Metal Oxide Nanopowders Along the CoOx–Al2O3 Tie Line Using Liquid‐Feed Flame Spray Pyrolysis

Abstract

We report here the use of liquid‐feed flame spray pyrolysis (LF‐FSP) to produce a series of nanopowders along the CoOx–Al2O3 tie line. The process is a general aerosol combustion synthesis route to a wide range of lightly agglomerated oxide nanopowders. The materials reported here were produced by aerosolizing ethanol solutions of alumatrane [Al(OCH2CH2)3N] and a cobalt precursor, made by reacting Co(NO3)2·6H2O crystals with propionic acid. The compositions of the as‐produced nanopowders were controlled by selecting the appropriate ratios of the precursors. Nine samples with compositions (CoO)y(Al2O3)1−y, y=0−1 along the CoOx–Al2O3 tie line were prepared and studied. The resulting nanopowders were characterized by X‐ray fluorescence, BET, scanning electron microscopy, high‐resolution transmission electron micrographs, X‐ray diffraction (XRD), thermogravimetric analysis (TGA), and FTIR. The powders typically consist of single‐crystal particles <40 nm diameter and specific surface areas (SSAs) of 20–60 m2/g. XRD studies show a gradual change in powder patterns from δ‐Al2O3 to Co3O4. The cobalt aluminate spinel phase is observed at stoichiometries (21 and 37 mol%) not seen in published phase diagrams, likely because LF‐FSP processing involves a quench of >1000°C in microseconds frequently leading to kinetic rather than thermodynamic products. Likewise, the appearance of Co3O4 rather than CoO as the end member in the tie line is thought to be a consequence of the process conditions. TGA studies combined with diffuse reflectance FTIR spectroscopic studies indicate that both physi‐ and chemi‐sorbed H2O are the principal surface species present in the as‐processed nanopowders. The only sample that differs is Co3O4, which has some carbonate species present that are detected and confirmed by a sharp mass loss event at ∼250°C. The thermal behavior of the high cobalt content samples differs greatly from the low cobalt content samples. The latter behave like most LF‐FSP‐derived nanopowders exhibiting typical 1%–4% mass losses over the 1400°C range due mostly to loss of water and some CO2. The high cobalt content samples exhibit a sharp mass loss event that can be attributed to the decomposition of Co3O4 to CoO.