Effect Of Phosphorus On The Properties And Structure Of γ-Alumina And Research Of Phosphorus Doping Mechanism

The structural physical and chemical properties of surface as well as thermal stability of phosphorus modifiedγ-alumina were investigated. A novel producedure, aluminium phosphide (AlP) hydrolysis method was developed to synthesize phosphorus crystal littice modified alumina. The effect and mechanism of phosphorus dopant on the alumina structure and thermal properties was studied. Finally, the interaction between chemical environment of phosphorus and alumina properties and mechanism of thermal stable of phosphorus in alumina were explored.Four series aluminas, GI (surface modified), GP (shell modified), PG (core modified) and G (particle bulk modified) samples, were synthesized by impregnation and gel method through change the introduced ways of phosphorus into alumina. The nitrogen adsorption, XRD, FTIR etc. technique were used to determine the pore texture, crystallography, acid/base properties, IEP and high thermal or hydrothermal stability. The results showed that the alumina, with 5 wt% phosphorus, has the best thermal stability. Below 1000℃, GI sample has better stability to resist sintering; when calcination temperature was higher than 1000℃,G sample performs higher phase transformation temperature. The Br?nsted acid appeared on the alumina surface when phosphorus doped alumina as well as the surface base decreased.The aluminium phosphide hydrolysis was used to synthesize the crystal lattice modified alumina, which is a new process to produce active alumina. The influence of procedure parameters of AlP hydrolysis on alumina performance was investigated and optimized. The properties and structure of the alumina was studied using HRTEM, XRD, in situ DRIFT, NMR, XPS and EDS. It was showed that theαphase transformation temperature of the alumina was enhanced to 1300℃, due to 1 wt% phosphorus anchored in the crystal lattice of alumina. Valence of phosphor changed from -3 to +5 during calcination, which increased the anions vacancies and enhanced the surface base.The location and chemical environment of phosphorus in alumina was studied using NMR, XRD, EDS and DRIFT techniques and related to the properties of alumina. Additionally, the mechanism of thermostable of phosphorus was explored. It was proved that the more homogeneous of phosphorus in alumina and the stronger interaction between P and alumina, the more stable of alumina was; additionally, the coordination of Al3+ ions might be changed with the introduced ways of phosphorus to alumina, which can modified the stability of alumina. Octahedral Al³⁺ ions can improve the thermal stability of active alumina. Phosphorus, as form of amorphous phosphate on the surface of alumina, can modify the amounts and nature of surface hydroxyl groups, which inhibited the dehydroxyl sintering and prevent the particles growth inhibited the decrease of surface area; phosphorus, as mono-phosphate, in the bulk of alumina, prevented the Al3+ ions diffusion in the bulk, which enhanced the alumina phase transformation temperature. Furthermore, the hydrothermostability of alumina was improved effectively, due to the Al-OHs of particles surface was replaced by P-OHs, which was more difficult to remove than Al-OHs.