Systematic synthesis of mixed-metal oxides in NiO–Co3O4, NiO–MoO3, and NiO–CuO systems via liquid-feed flame spray pyrolysis

Abstract

We report here on the systematic synthesis of three nanopowder series along the NiO–Co₃O₄, NiO–MoO₃, and NiO–CuO tie lines. Sixteen individual samples were produced via liquid-feed flame spray pyrolysis (LF-FSP) and analyzed by SSA, SEM, EDX, FTIR, TGA-DTA, and XRD. The LF-FSP 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 propionate and ammonium molybdate precursors synthesized by reacting the metal nitrate with propionic acid. Particular ratios of the precursors were selected to control the compositions of the samples produced.

The powders typically consist of single crystal particles <35 nm in diameter and with specific surface areas (SSAs) of 20–50 m² g⁻¹. X-Ray powder diffraction (XRD) studies show a gradual change in their patterns from pure NiO to pure MO_x (M = Co, Mo, Cu). Most compositions yielded single phase materials but mixed phase materials were also detected. We believe that higher vapor pressures of the ions produced for the transition metals studied resulted in SSAs about a third lower than those measured typically in nanopowders containing NiO.

The partial pressure of O₂ in LF-FSP affects the formation of particular oxide phases and controls to a certain degree the morphologies of the as-produced materials. We observe in the NiO–MoO₃ system preferential growth of certain crystallographic planes in MoO₃, due to the relatively high vapor pressure of MoO_x species. Unusual particle morphologies seen in the NiO–Co₃O₄ system are attributed to some phase separation in the as-produced materials. TGA studies combined with diffuse reflectance infrared Fourier transform (DRIFT) spectroscopic studies indicate that both physi- and chemi-sorbed H₂O are the principal surface species present in the as-processed nanopowders.