Raman spectroscopy studies on the structure of MoO3/ZrO2 solid superacid

MoO3/ZrO2 prepared by different methods are distinctive in catalytic behavior. MoO3/ZrO2 (MZ) obtained by impregnating crystallized ZrO2 and then calcining at high temperature is a catalyst for partial oxidation. However, MoO3/ZrO2 (MZH) obtained by impregnating Zr(OH)(4) and then calcining at high temperature is a solid superacid. It has been proved by Raman spectroscopy that they have different structural characteristics. In the MZ sample MoO3 is present as a monolayer, i.e., two-dimensional polymolybdates on monoclinic ZrO2, whose characteristic broad band is at about 950 cm(-1). As MoO3 content is beyond its monolayer dispersion capacity (0.12 g MoO3/100 m(2) ZrO2), the surplus MoO3 is present as crystalline MoO3, whose intense peaks are at 820 and 994 cm(-1). As for MZH sample, MoO3 exists on met astable tetragonal ZrO2 (ZrO2(t)) in two kinds of surface states, i.e., two-dimensional polymolybdates and Mo-O-Zr surface species which have a broad band at about 814 cm(-1). Mo(VI) in Mo-O-Zr surface species is tetra-coordinated and bonds strongly with ZrO2(t). As MoO3 content in MZH sample exceeds a certain value, bulk Zr(MoO4)(2) appears, which is indicated by the appearance of sharp peaks at 750, 946, 326 and 1002 cm(-1). Quantitative Raman measurements for the three states of MoO3 in MZH samples show that the Mo-O-Zr surface species may be a preliminary compound of bulk Zr(MoO4)(2). There is a good corresponding relation between its content and the catalytic activity of the superacid, therefore it should be responsible for its superacidity.