Structure and phase changes of alumina produced by flame hydrolysis

Area of Science:

• Materials Science
• Nanotechnology
• Solid-State Chemistry

Background:

• Alumina nanoparticles are crucial in various industrial applications.
• Understanding the phase transformations and structural properties of alumina during synthesis is essential for controlling material performance.
• Fumed alumina produced by combustion offers unique nanoscale properties.

Purpose of the Study:

• To characterize fumed alumina nanoparticles produced from AlCl3 combustion.
• To investigate the phase evolution from gamma- to delta- and theta-alumina.
• To elucidate the structural and coordination environment of aluminum in different alumina phases.

Main Methods:

• Combustion synthesis of fumed alumina from aluminum chloride (AlCl3).
• Characterization using powder X-ray diffraction (XRD), 27Al solid-state Nuclear Magnetic Resonance (NMR), and transmission electron microscopy (TEM).
• Analysis of specific surface area using Brunauer-Emmett-Teller (BET) method.

Main Results:

• Nanoparticles with specific surface areas ranging from 30 to 220 m2/g were produced.
• Gamma-alumina transformed into a mixture of delta- and theta-alumina during synthesis.
• Five-coordinated aluminum was observed in high-surface-area gamma-alumina, partly on the surface.
• Water adsorption reversibly increased aluminum coordination in the particle surface.
• 27Al MQMAS NMR revealed 8 crystallographic sites in delta- and theta-alumina, with 4 distinct AlO4 sites unique to the delta phase.
• The delta-alumina phase was identified as an ordered structure.

Conclusions:

• The synthesis process yields alumina nanoparticles with varying surface areas and distinct phase compositions.
• The structural ordering of the delta-alumina phase was confirmed, offering insights into its formation mechanism.
• Aluminum coordination is sensitive to surface area and adsorbed water, impacting surface properties.