Raman spectroscopic studies on the sulfation of cerium oxide

In the present study, we have examined sulfation of cerium oxide via impregnation of (NH4)(2)SO4, followed by heating in the temperature range of 220-720 degrees C, using Raman Spec troscopy. Based on the S-O and 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, SO4(aq2-) and HSO4(aq1-) is found to have formed on ceria's surface, whereas complete conversion of (NH4)(2)SO4 to SO4(aq2-) and HSO4(aq1-) occurs at 280 degrees C. At 350 degrees C, formation of a mixture of surface 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 transforms to HSO4(surf.1-), whereas the latter remains unchanged. At 400 degrees 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 thorough understanding of the sulfation of cerium oxide at a molecular level, but also demonstrates that Raman spectroscopy is a highly effective technique to characterize sulfation of metal oxides. (C) 1997 Elsevier Science B.V.