Low molybdenum silica (1.2 wt.% molybdenum) has been prepared by impregnation (pore filling
method) from an aqueous (NH4)6MoTO27 solution (pH ~ 6). The effect of temperature (373-
673 K) was followed by diffuse reflectance IR Fourier transform spectroscopy (DRIFTS). The
interaction between molybdenum species, MoO42-, and silica surface (OH pairs) is recognized at
373 K through the appearance of a band at 905 cm- 1 attributed to an Mo-O-Si linkage, that
shifted, and decreased in intensity, with the increase in temperature to appear at 915 cm -~ at
673 K. Bands at 555 (vsMo-O-Mo) and 881 cm-~ (vasMo--O-Mo) are correspondingly developed
with temperture, and only at 673 K a band due to MoO 3 (998 cm -~) is recognized. These
observations are used to propose a firm correlation between the detached molydenum species upon
the breaking of the Mo-O-Si bonds and MoO 3 formation. The spectral changes in the stretching
frequency region of surface hydroxyls indicated that as the separation between adjacent hydroxyls
(involved in interaction with MoO 2-) increases, the strength of the Mo-O-Si link decreases. The
presence of C1- or F on the surface of silica was found to decrease the extent of interaction
between the molybdenum species and silica through decreasing the probability of finding vicinal
hydroxyls. The formation of polymolybdate phase identified on the surface of molybdenum-supported
silica is also inhibited by halogenation. |
