Raman spectroscopic studies on the sulfation of cerium oxide

Abstract

In the present study, we have examined sulfation of cerium oxide via impregnation of (NH 4) 2SO 4, followed by heating in the temperature range of 220–720°C, using Raman Spectroscopy. 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°C, a mixture of (NH 4) 2SO 4 crystals, SO 2− 4(aq) and HSO 1− r(aq) is found to have formed on ceria's surface, whereas complete conversion of (NH 4) 2SO 4 to SO 2− 4(aq) and HSO 1− 4(aq) occurs at 280°C. At 350°C, formation of a mixture of surface pyrosulfate S 2O 2− 7(surf.0, consisting of two S O oscillators and a bulk type compound identified as Ce(IV)(SO 4) x(SO 3) 2−x (0 < x < 2) have been observed. Upon introduction of moisture, the former transforms to HSO 1− 4(surf.), whereas the latter remains unchanged. At 400°C, only the bulk type compound can be observed. At 450°C, only Ce 2(SO 4) 3 is generated and remains stable until 650°C. Further increase in the temperature to 720°C results in the formation of CeOSO 4. 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.