RAMAN-SPECTROSCOPIC STUDIES ON THE SULFATION OF CERIUM OXIDE

In the present study, we have examined sulfation of cerium oxide via i mpregnation of (NH4)(2)SO4, followed by heating in the temperature ran ge of 220-720 degrees C, using Raman Spec troscopy. Based on the S-O a nd S=O stretching frequencies in the range of 900-1400 cm(-1), a wide range of surface oxysulfur species and bulk cerium-oxy-sulfur species are identified. At 220 degrees C, a mixture of(NH4)(2)SO4 crystals, SO 4(aq2-) and HSO4(aq1-) is found to have formed on ceria's surface, whe reas complete conversion of (NH4)(2)SO4 to SO4(aq2-) and HSO4(aq1-) oc curs at 280 degrees C. At 350 degrees C, formation of a mixture of sur face pyrosulfate S2O7(surf.02-, consisting of two S=O oscillators and a bulk type compound identified as Ce(IV)(SO4)(x)(SO3)(2-x) (0 < x < 2 ) have been observed. Upon introduction of moisture, the former transf orms to HSO4(surf.1-), whereas the latter remains unchanged. At 400 de grees C, only the bulk type compound can be observed. At 450 degrees C , only Ce-2(SO4)(3) is generated and remains stable until 650 degrees C. Further increase in the temperature to 720 degrees C results in the formation of CeOSO4. The present study not only provides a more thoro ugh understanding of the sulfation of cerium oxide at a molecular leve l, but also demonstrates that Raman spectroscopy is a highly effective technique to characterize sulfation of metal oxides. (C) 1997 Elsevie r Science B.V.