In situ laser Raman study of the effect of water on undoped and caesium-doped silica-supported 12-molybdophosphoric acid catalysts

Abstract

The surface chemistry of silica-supported 12-molybdophosphoric acid (HPMo) catalysts and the caesium salts, in dehydrated and hydrated form, has been investigated by laser Raman spectroscopy (LRS). The Raman spectra were observed to be dependent on both the pretreatment temperature and the environment of the sample during heating. A sample containing 23 wt.% HPMo, equivalent to 0.12 of the molybdophosphate anions (Keggin unit, KU) per nm², a loading only marginally greater than the monolayer loading found from X-ray photoelectron spectroscopy (XPS), produced an unidentified species and/or MoO₃ on heating to temperatures higher than 300 °C. Upon exposure to water vapour, the unknown species is reconverted to the KU. With loadings of HPMo less than the monolayer equivalent no decomposition into MoO₃ is observed at temperatures up to 550 °C. The stable species found on the silica surface after heating is believed to be the dehydrated or defect KU formed on removal of an oxygen ion in the process of the desorption of water.