Penta-coordinated Al3+ ions as preferential nucleation sites for BaO on γ-Al2O3: An ultra-high-magnetic field 27Al MAS NMR study

We report the first observation of preferential anchoring of an impregnated catalytic phase onto penta-coordinated Al3+ sites on the surface of γ-Al2O3. The interaction of barium oxide with a γ-alumina support was investigated by high resolution solid state 27Al magic angle spinning NMR at an ultra-high magnetic field of 21.1 T and at sample spinning rates of up to 23 kHz. Under these experimental conditions, a peak in the NMR spectrum at ∼23 ppm with relatively low intensity, assigned to 5-coordinated Al3+ ions, is clearly distinguished from the two other peaks representing Al3+ ions in tetra-, and octahedral coordination. Spin-lattice 27Al relaxation time measurements clearly show that these penta-coordinated Al3+ sites are located on the surface of the γ-alumina support. BaO deposition onto this γ-alumina sample resulted in the loss of intensity of the 23 ppm peak. The intensity loss observed was linearly proportional to the amount of BaO deposited. The results of this study strongly suggest that, at least for BaO, these penta-coordinated Al3+ ions are the nucleation sites.