MoO3/γ-AI2O3, MoO3/TiO2, MoO3/SiO2的氨溶性质及表面状态的研究

本文用浸渍法制备了不同含量的MoO_3/γ-Al_2O_3、MoO_3/TiO_2、MoO_3/SiO_2系列样品, 用氨溶、水溶, XPS、XRD等实验手段对这三个体系进行了研究。氨溶实验表明MoO_3在γ-Al_2O_3和TiO_2表面呈单层分散, 其分散阈值(即最大分散量)与XPS强度比法, XRD相定量法所测结果基本一致。这二个体系氨溶曲线在阈值前斜率分别为0.71和0.66, 说明分散在γ-Al_2O_3或TiO_2上的单层MoO_3分为氨水可溶和不可溶二种状态, 这二种状态以确定的比例共存于阀值前的整个浓度区间。这一现象也为氮溶前后样品XPS强度比的比值所证实。实验还表明MoO_3/γ-Al_2O_3样品的氨溶残渣经脱水加热可产生大量可溶的MoO_3, 随着烘烤温度的提高, 时间的延长, 新生成的可溶MoO_3分率趋近样品氨溶前的可溶分率。我们认为分散在γ-Al_2O_3或TiO_2上的单层MoO_3分为二种结构状态, 一种是Mo~(6+)处在氧八面体空隙中, 另一种则处在氧四面体空隙中。二种状态相互邻接, 共用O~(2-)离子, 以特定连结方式(类似于γ-氧化铝Mo_4O_(11)形成“单层聚集小片”。这样的小片是热力学稳定态。由于二种结构状态与载体的相互作用强弱不同而造成了氨水可溶或不可溶二种状态。MoO_3/SiO_2体系, 对氨溶而言, 只有一种状态, 分散在SiO_2上的MoO_3几乎全部溶解。由此可见, MoO_3与SiO_2表面的作用比上述二个体系弱得多。

STUDY ON THE EXTRACTION OF MOLYBDENUM AND SURFACE STATE FOR MoO3 SUPPORTED ON THE SURFACE OF γ-Al2O3, TiO2 AND SiO2

MoO_3/γ-Al_2O_3, MoO_3/TiO_2 and MoO_3/SiO_2 samples prepared by impregnation method have been studied by various techniques, including XPS, XRD, and extraction of molybdenum with ammonium solution or water. The result of extraction of molybdenum with ammonium solution shows that if the MoO_3 content is lower than certain amount, a constant fraction (71% for MoO_3/γ-Al_2O_3 system and 66% for MoO_3/TiO_2 system) of MoO_3 can be dissolved in 3wt.% solution of NH_4OH and if the MoO_3 content is higher than that amount, surplus MoO_3 can be dissolved completly. The turning point of both systems is in line with the dispersed threshold value obtained by XPS and XRD methods. It means that MoO_3 dispreses as monolayer on these carriers before the turning point and forms crystalite grain after that. The constant dissolved fraction of MoO_3 in ammonium solution shows that there are two different surface species in monolayer state, one is soluble and another is not. Two kinds of species coexist in monolayer with constant ratio, which have also been confirmed by XPS experiment for samples before and after extraction. The samples which soluble species was thoroughly removed were recalcined at certain temperture and again soaked into ammonium solution a lot of soluble species were reproduced then. The dissolved fraction of them approched that of the samples before extraction with a rise in temperature and a prolonging of time. We suggest that the monolayer dispersed MoO_3 on γ-Al_2O_3 or TiO_2 surface can be divided into two kinds of surface species, one is located in octahedral vacancy of O~(2-) and another in tetrahedra vacancy. They join each other by sharing oxygen ions and form monolayer aggregate patch′ in a special way (similar to γ-molybdena Mo_4O_(11)). It is a thermodynamic favorable state in both systems. For MoO_3/SiO_2 samples, however, the situation is quite different. All of MoO_3 species in the monolayer is soluble thoroughly in ammonium solution. It shows that the mutual interaction between MoO_3 in monolayer state and the surface of SiO_2 carrier is much weaker than that of two other systems.