MoO3/γ-Al2O3, MoO3/TiO2和MoO3/SiO2的还原性质及表面状态研究

本文以XPS为检测手段研究了浸渍法制备的MoO_3/γ-Al_2O_3、MoO_3/TiO_2和MoO_3/SiO_2三个系列样品的还原性质。用XPS测定还原后与还原前强度比的比值, 实验结果表明阈值后的样品还原后强度比急剧增大, 说明晶相MoO_3从内孔向颗粒外表面大幅度迁移。而单层MoO_3的性质却各不相同, TiO_2上的单层MoO_3还原后部分凝聚, SiO_2上单层则部分迁移, 而γ-Al_2O_3上的单层MoO_3既不迁移也不凝聚, 亦即单层MoO_3的稳定性是Mo(Al)>Mo(Ti)>Mo(Si)。这也是MoO_3与这三个载体表面作用力强弱的顺序。用计算机对Mo3d谱峰解叠, 结果表明, 几乎所有样品还原后都只含有四价和五价钼。Mo(Al)和Mo(Si)体系还原后Mo~Ⅳ的百分数随负载量的增大而增大。我们认为反映了两种载体表面区域的不均匀性。Mo(Ti)体系还原后Mo~Ⅳ百分数在阈值附近出现转折点, 阈值前各点还原性质相同, 说明在应用还原为探测手段时, TiO_2表面表现为均匀的。以Mo~(Ⅳ)百分数衡量时, 三个体系的还原性为Mo(Al)

STUDY ON THE REDUCIBILITY AND SURFACE STATE OF MoO3 SUPPORTED ON THE SURFACE OF γ-AI2O3, TiO2 AND SiO2 BY XPS

MoO_3/γ-Al_2O_3, MoO_3╱TiO_2, and MoO_3╱SiO_2 samples prepared by impregnation method were reduced at 480 ℃ by H_2 flow(1 atm, 50 ml min~(-1)) for an hour and characterized by XPS without exposure to the air. The ratios of XPS intensities(I_(Mo3d)/I_(A12p), I_(Mo3d)/I_(Ti2p) and I_(Mo3d)/I(si2p) of reduced samples have been measured and compared with that of the oxidative pre cursors (Fig.1-3). For All samples which contain crystalline MoO_3, the ratios are dramatically increased after reduction. We consider that a lot of molybdena has migrated from internal to external surface of the carriers during reduction. The monolayer molybdena supported on γ-Al_2O_3, TiO_2 or SiO_2 carriers shows quite different behaviours. On TiO_2, it condensed slightly. On SiO_2, it migrated partly from internal to external surface of the carrier. On γ-Al_2O_3, it neither condensed and the surfaces of the carriers is: Mo(Al)>Mo(Ti)>Mo(Si). This is also the order of the stabilities of the monolayer of molybdena on these carriers.Computer fitting for Mo3d spectra shows that there are only Mo~V and ...