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

chi-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 chi-Al(2)O(3 )and chi-Al2O3@C core-shell materials in the temperature range of 650-1400 degrees C were studied by XRD and Al-27 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 chi-Al2O3 phase to 1100 degrees C. Relatively high surface area of the oxide core consisting of chi-and kappa-Al2O3, which was close to 80 m(2)/g after burning of the carbon shell, was preserved after the heat treatment at 1100 degrees C. Meanwhile, calcination of pure chi-Al2O3 at the same temperature yielded a sample consisting of alpha-and chi-Al2O3 with the surface are as low as 15 m(2)/g. Solid-state Al-27 NMR with magic-angle spinning proved to be an efficient tool for investigation of chi- -> kappa- -> alpha-Al2O3 phase transformations that can be used to obtain quantitative information on the alumina phase composition.