The surface structure of sulfated zirconia: Studies of XPS and thermal analysis

Sulfated zirconias were prepared using two kinds of amorphous zirconia gels, XZO 631 and 632 supplied by MEL Chemicals, and their thermal gravimetrical analyses were carried out. DTG of the former sample showed two peaks based on decomposition of the sulfate species on the surface, the first peak at 680 °C and the second broad one centered at 850 °C. The latter sample indicated only broad peak at 850 °C in the range from 700 to >1000 °C. The first peak for the former sample was ascribed to the decomposition of Zr(SO4)2 remained on the surface, and the broad one at 700 to >1000 °C for the both samples was attributed to the catalytically active species. The acidic character of sulfated zirconia calcined at 1000 °C was examined in acid-catalyzed reactions of cumene, ethylbenzene, and butane together with the adsorption heat of Ar, showing a solid acid with acidity higher than that of silica–alumina. It was indicated from the XPS analysis that the S species are composed of SO42−. The results led to a structural model of the active surface to be polysulfate species containing mainly three or four S atoms with two ionic bonds of SOZr in addition to coordination bonds of SO with Zr, the active site being Lewis sites on the S atoms.