TREATMENT AND HALOGENATION ON LOW MOLYBDENUM SILICA - DIFFUSE-REFLECTANCE IR FOURIER-TRANSFORM STUDY (DRIFTS)

Low molybdenum silica (1.2 wt.% molybdenum) has been prepared by impre gnation (pore filling method) from an aqueous (NH4)(6)Mo7O27 solution (pH approximate to 6). The effect of temperature (373-673 K) was follo wed by diffuse reflectance IR Fourier transform spectroscopy (DRIFTS). The interaction between molybdenum species, MoO42-, and silica surfac e (OH pairs) is recognized at 373 K through the appearance of a band a t 905 cm(-1) attributed to an Mo-O-Si linkage, that shifted, and decre ased in intensity, with the increase in temperature to appear at 915 c m(-1) at 673 K. Bands at 555 (nu(s)Mo-O-Mo) and 881 cm(-1) (v(as)Mo-O- Mo) are correspondingly developed with temperture, and only at 673 K a band due to MoO, (998 cm(-1)) is recognized. These observations are u sed to propose a firm correlation between the detached molydenum speci es upon the breaking of the Mo-O-Si bonds and MoO3 formation. The spec tral changes in the stretching frequency region of surface hydroxyls i ndicated that as the separation between adjacent hydroxyls (involved i n interaction with MoO42-) increases, the strength of the Mo-O-Si link decreases. The presence of Cl- or F- on the surface of silica was fou nd to decrease the extent of interaction between the molybdenum specie s and silica through decreasing the probability of finding vicinal hyd roxyls. The formation of polymolybdate phase identified on the surface of molybdenum-supported silica is also inhibited by halogenation.