INTERACTION BETWEEN MOLYBDENA AND SILICA - FT-IR/PA STUDIES OF SURFACE HYDROXYL-GROUPS AND PORE STRUCTURE ASSESSMENT

Silica impregnated in aqueous solutions of ammonium heptamolybdate to achieve different Mo loadings (3-15 wt.%) is studied by the Fourier transform infrared/photoacoustic (FT-IR/PA) technique. Special attention is given to the high frequency region of surface hydroxyl groups. Air calcination at 500 degrees C of the Mo loaded silica caused the evolution of defined features at 3720 and 3550 cm(-1) which are attributed to the free and hydrogen-bonded components of a pair of hydrogen-bonded silanol groups. The presence of such silanols on the surface of silica heated at 500 degrees C is attributed to the hydrolysis of open siloxane bridges by the action of constitutional water that is liberated from the bulk interior due to heating at 500 degrees C. The open siloxane represents a site of weakly attached Mo species that escape from the surface upon heating. These detached species are seen as the precursor of MoO3 detected on the surface (band at 994 cm(-1)). A surface polymolybdate phase is also detected (band at 970 cm(-1)) and considered to result from condensation between Mo species adsorbed on the surface. The effect of fluorination on the interaction between Mo and silica, and the pore structure of the Mo loaded silica are also discussed.