Levitated Droplets as Model System for Spray Drying of Complex Oxides: A Simultaneous in Situ X-ray Diffraction/Raman Study

Abstract

Levitated droplets can model spray drying, a process which is poorly understood despite its industrial importance. Monitoring the drying process by simultaneous synchrotron X-ray scattering and Raman spectroscopy provides insights into the processes starting from the molecular species up to larger crystalline particles forming the stable system. In this study, slurries of two different phosphate-containing ammonium iron molybdates with different Fe/Mo ratios which are used as precursors for catalysts were exemplarily studied. Next to the phases containing highly condensed Keggin anions other phases of less condensed molybdates and phosphates were formed during the drying depending on the composition of the slurry and the drying time. To validate the capability of this experimental approach, the stable samples obtained after the drying in the levitator were compared with samples obtained after conventional spray drying and filtration of the precipitates. The results of comprehensive characterization (X-ray diffraction (XRD), Raman, attenuated total reflection Fourier transformation infrared spectroscopy (ATR-FTIR), inductively coupled plasma optical emission spectroscopy (ICP-OES)) indicate that the setup can be used to acquire reliable information on the processes during spray drying of complex oxides by using minimal amounts of material with volumes of several micro liters.