热处理对γ-Al2O3和θ-Al2O3性质的影响

摘要

采用不同的焙烧温度和时间对γ-Al2O3和θ-Al2O3进行热处理,并采用X射线衍射(XRD)、N2物理吸附和傅里叶红外光谱(FT-IR)等表征手段,揭示了热处理条件对这两种Al2O3性质的影响规律,并结合表征结果分析了Al2O3的相变机理.结果表明,γ-Al2O3和θ-Al2O3在低温处理时具有一定的结构稳定性,高温下均向稳定的α相转变.低于相变温度处理两种Al2O3时,其比表面积和总孔容随焙烧温度的升高有所减少,但经长时间热处理仍保持较高的比表面积和总孔容.在高温条件下,γ-Al2O3在900℃转变为θ相,在1200℃转变为α相;θ-Al2O3在1100℃逐渐向α相转变,且随着焙烧时间增加,α相的含量逐渐增加,在1200℃快速转变为单一的α相.两种Al2O3在高温条件下向α相转变时,孔结构完全坍塌导致比表面积和总孔容迅速降低,形成致密结构的α-Al2O3.%γ-Al2O3and θ-Al2O3were thermally treated under different calcination temperatures and time.The properties of these two kinds of alumina before and after treatments were characterized by X-ray diffraction(XRD),N2physical adsorption-desorption and fourier infrared(FT-IR),which revealed effects of the calcination temperatures and time on the properties of these two alumina.The phase transition mechanism of Al2O3was also analyzed based on the characterization results.The results showed that both γ-Al2O3and θ-Al2O3possessed a certain level of structural stability at relative low temperature.However,a transformation of γ-Al2O3and θ-Al2O3into α-Al2O3occurred at a high calcination temperature.When these alumina were calcined under phase transition temperature,the specific surface area and the total pore volume of the two kinds of Al2O3decreased with the increase of the calcination temperature.Nevertheless,the specific surface area and the total pore volume of the two kinds of Al2O3decreased with the prolonged heat treatment.With high temperature treatment,γ-Al2O3was transformed into θ-Al2O3and α-Al2O3at 900 ℃and 1 200 ℃,respectively.θ-Al2O3was gradually transformed to α phase from 1 100 to 1 200 ℃.With the increase of calcination time,the content of α phase gradually increased and rapidly transformed into a single α phase at 1 200 .℃ When the two kinds of Al2O3were transformed into α phase under the condition of high temperature,the pore structure completely collapsed to cause the specific surface area and the total pore volume to decrease rapidly to form the dense structure α-Al2O3.