Abstract: 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 ...