FT-IR STUDIES ON THE EVOLUTION OF DIFFERENT PHASES AND THEIR INTERACTION IN FERRIC MOLYBDATE MOLYBDENUM TRIOXIDE CATALYSTS

Two samples of Fe-Mo catalysts have been prepared from ferric nitrate and ammonium paramolybdate, one following a conventional procedure and the other by a new precipitation procedure in a buffer medium. The evolution of different Fe Mo phases, from the fresh precipitate to the aged and then to the calcined state, has been followed by FT-IR spectroscopy. All the spectral vibrations have been related to different coordination spheres of Mo and Fe ions in the samples and a comparison with a commercial catalyst of similar composition has been made. The new preparation procedure leads to small, uniform (0.3 mum size) particles and a homogeneous morphology of phases as seen by scanning electron microscopy. Additionally, a higher surface area (10.3 m2 g-1), lower bulk density (0.66 g cm-3), a methanol conversion efficiency of > 99% and an HCHO selectivity of 95% distinguish this sample from the conventionally prepared one. Better thermal stability of this sample is inferred from the IR spectral and XRD analyses of the heat-treated (up to 800-degrees-C) samples and is related to the interaction between Fe2(MoO4)3 and MoO3 phases.