The existence and stability of the silica-supported 12-molybdophosphoric acid keggin unit as shown by Raman, XPS, and 31P NMR spectroscopic studies

Abstract

Silica-supported 12-molybodophosphoric acid catalysts developed for partial oxidation of methane have been characterized by laser Raman spectroscopy (LRS), X-ray photoelectron spectroscopy, and 31P NMR spectroscopy after preparation, calcination, and use in catalytic tests performed at high-temperature (570°C) and atmospheric pressure, It is shown that the 12-molybdophosphoric acid (HPMo) Keggin unit (KU) can be deposited uniformly on the silica surface in a highly dispersed form up to a coverage of 0.04 KU nm −2. Aggregates are then formed for loading up to about 0.12 KU nm −2. At higher loading these aggregates are likely to grow to give particles of 12-molybdophosphoric acid. Both LRS and 31P NMR spectroscopic results demonstrate that the highly dispersed HPMo is stable up to calcination or reaction temperatures of about 580–600°C. On the contrary, the particles of HPMo present on the highly loaded catalysts disappear after catalytic testing to give M003 as shown by LRS. The increase in stability of the supported HPMo KU compared with that of bulk HPMo is proposed to be the result of the interaction of the KU with some surface site of the silica, giving the highly dispersed and dehydrated HPMo species thought to be responsible for the catalytic activity for methane partial oxidation.