Reductibility and spectroscopic behaviour of the (NH 4) 4[H 6CuMo 6O 24] · 4H 2O anderson phase

Abstract

The vibrational spectrum of (NH 4) 4[H 6CuMo 6O 24] · 4H 2O was analyzed and is discussed on the basis of the different Mo-O bond lengths in the lattice. The temperature-programmed reduction (TPR) process was also studied by means of several techniques, such as X-ray diffraction, scanning electron microscopy, and infrared, Raman and electron spin resonance (ESR) spectroscopies. The results revealed that the MoO 2 oxide, together with Cu metal, is formed at ≈ 385 °C, while the final reduction to Mo 0 occurs at 783 °C. These reduction steps for molybdenum are at ≈ 350 and ≈ 70 °C, respectively, below those observed for ammonium heptamolybdate. The presence of Mo(V) and Cu(I) species was also suggested in the course of the thermal reduction by ESR spectroscopy. The TPR data of some other Anderson phases have been used for purposes of comparison. The effect of the redox potential of the heteroatom on the stability of these phases has been analyzed. The presence of metallic copper seems to increase the capability of the reducing agent to form Mo 0.