Acta Phys. -Chim. Sin. ›› 1985, Vol. 1 ›› Issue (03): 222-231.doi: 10.3866/PKU.WHXB19850306

• ARTICLE • Previous Articles     Next Articles

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

Gui Linlin; Zhong Bianxiao; Tang Youqi   

  1. Institue of Physical Chemistry; Peking University; Beijing; China
  • Received:1984-09-07 Revised:1984-12-08 Published:1985-06-15

Abstract: 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.