A sol–gel methodology for the preparation of lanthanide-oxide aerogels: preparation and characterization

Abstract

Lanthanide oxide-based aerogels were synthesized employing the so-called epoxide addition sol–gel method already successfully applied for main- and transition metal oxide aerogels. Using chlorides and nitrates as precursors, our aim was to test the transferability of this robust sol–gel methodology to the entire lanthanide series. By adding the proton scavenging organic epoxide, propylene oxide, to hydrated lanthanide trichloride dissolved in ethanol or methanol, uniform monolithic alcogels were obtained. Subsequent processing in supercritical CO2 resulted in monolithic aerogels. No gelling process could be induced by using nitrates, in contrast to previous results with iron oxide or alumina aerogels. All materials were characterized by nitrogen adsorption/desorption analysis, transmission electron microscopy, and powder X-ray diffraction. With the exception of cerium, for which fractions of crystalline CeO2 were found already in the as-prepared material, XRD analysis revealed that the other materials were mainly amorphous. Subsequent heat treatment of the aerogels above 650 °C resulted in nanocrystalline phases for all aerogel materials. However, except for ceria, more detailed TEM and XRD studies provided evidence that crystalline oxychloride phases are formed in addition to fractions of oxide phases. The trends and possible explanations are discussed in this contribution. A sol–gel methodology using organic epoxides as proton scavengers has been used to synthesize oxide aerogels of the entire lanthanide series. As precursors, nitrates and chlorides were used but only in the latter case, gelation set in. After supercritical drying, aerogels with high surface areas and mesoporous, nanoparticulate structures were obtained. Further characterization revealed that cerium plays a special role, since it contained crystalline CeO2 fractions already before calcination. After calcination it was the only material consisting of a pure oxide phase. The syntheses with the other lanthanides resulted in mixtures with varying fractions of crystalline oxychlorides in addition to an oxide phase. The trends and possible explanations are discussed in this contribution.A sol–gel methodology using organic epoxides as proton scavengers has been used to synthesize oxide aerogels of the entire lanthanide series. As precursors, nitrates and chlorides were used but only in the latter case, gelation set in. After supercritical drying, aerogels with high surface areas and mesoporous, nanoparticulate structures were obtained. Further characterization revealed that cerium plays a special role, since it contained crystalline CeO2 fractions already before calcination. After calcination it was the only material consisting of a pure oxide phase. The syntheses with the other lanthanides resulted in mixtures with varying fractions of crystalline oxychlorides in addition to an oxide phase. The trends and possible explanations are discussed in this contribution.