The phase stability of mesoporous -alumina modified with phosphate species was investigated by comparing the effects of phosphoric acid (PA) and ammonium dihydrogen phosphate (ADP).Both treatments delayed the -- phase transformation through the formation of aluminum phosphate; however, slight differences in the transformation and formation behaviors were observed.X-ray diffraction and solid-state nuclear magnetic resonance analyses reveal that PA induces more rapid formation and crystallization of aluminum phosphate than ADP, resulting in a more effective delay of the -- phase transformation.Infrared spectroscopy suggests that a fraction of NH 4+ species in ADP is oriented toward the -alumina surface, thereby reducing the effective interaction of POH groups with alumina and suppressing the formation of aluminum phosphate compared with PA.These results demonstrate that not only the presence of phosphorus but also the chemical form and acidity of phosphate species influence surface reactions, diffusion behavior, and phase stability of mesoporous -alumina.
Machida et al. (Thu,) studied this question.