Abstract
Aluminum oxide is widely used as a catalyst carrier, including in internal combustion engine systems, where operating temperatures exceed 1000 °C. As such, aluminum oxide must exhibit enhanced thermal stability. This property is linked to the presence of pentacoordinated centers on the surface of the γ-phase of Al2O3. This paper examines the effect of the pH during aluminum hydroxide precipitation on the formation of pentacoordinated centers on the surface of aluminum oxide. The samples of aluminum hydroxide were synthesized via controlled double-jet precipitation, followed by thermal decomposition into oxides. Precipitation was carried out at constant pH levels, and for comparison, parallel samples were synthesized at pH values of 5, 6, 7, 8, and 9. The precursors for precipitation were a 1 M aluminum nitrate solution (Al3+) and a 10 wt. % ammonia solution (NH4OH). The solutions were introduced into the reactor in a dropwise mode with continuous stirring. The resulting aluminum oxide samples were analyzed using X-ray diffraction and nuclear magnetic resonance techniques. The data show a direct correlation between the pH of aluminum hydroxide precipitation and the presence of pentacoordinated centers on the aluminum oxide surface: the higher the pH, the lower the content of pentacoordinated atoms. Additionally, a relationship was observed between the pH value and the size of the coherent scattering region, with an increase in coherent scattering observed at higher pH levels.
Published Version
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