Effect of carbon coating on the thermal stability of nanocrystalline χ-Al2O3

Abstract χ-Al2O3 is a rarely studied low-temperature alumina phase that can be prepared in a finely dispersed form and attracts interest as a potential catalyst or catalyst support. Phase transformations of nanocrystalline χ-Al2O3 and χ-Al2O3@C core-shell materials in the temperature range of 650–1400 °C were studied by XRD and 27Al solid-state NMR. Deposition of the carbon shell was demonstrated to stabilize the oxide core size and extend the stability range of the low-temperature χ-Al2O3 phase to 1100 °C. Relatively high surface area of the oxide core consisting of χ-and κ-Al2O3, which was close to 80 m2/g after burning of the carbon shell, was preserved after the heat treatment at 1100 °C. Meanwhile, calcination of pure χ-Al2O3 at the same temperature yielded a sample consisting of α-and κ-Al2O3 with the surface are as low as 15 m2/g. Solid-state 27Al NMR with magic-angle spinning proved to be an efficient tool for investigation of χ- → κ- → α-Al2O3 phase transformations that can be used to obtain quantitative information on the alumina phase composition.