Phase Progression of γ‑Al₂O₃ Nanoparticles Synthesized in a Solvent-Deficient Environment

Abstract

Our simple and uniquely cost-effective solvent-deficient synthetic method produces 3–5 nm Al₂O₃ nanoparticles which show promise as improved industrial catalyst–supports. While catalytic applications are sensitive to the details of the atomic structure, a diffraction analysis of alumina nanoparticles is challenging because of extreme size/microstrain-related peak broadening and the similarity of the diffraction patterns of various transitional Al₂O₃ phases. Here, we employ a combination of X-ray pair-distribution function (PDF) and Rietveld methods, together with solid-state NMR and thermogravimetry/differential thermal analysis-mass spectrometry (TG/DTA-MS), to characterize the alumina phase-progression in our nanoparticles as a function of calcination temperature between 300 and 1200 °C. In the solvent-deficient synthetic environment, a boehmite precursor phase forms which transitions to γ-Al₂O₃ at an extraordinarily low temperature (below 300 °C), but this γ-Al₂O₃ is initially riddled with boehmite-like stacking-fault defects that steadily disappear during calcination in the range from 300 to 950 °C. The healing of these defects accounts for many of the most interesting and widely reported properties of the γ-phase