Reactions of HMDS, TiCl₄, ZrCl₄, and AlCl₃ with Silica As Interpreted from Low-Frequency Diffuse Reflectance Infrared Spectra

Low-frequency diffuse reflectance infrared spectra are presented for silica samples reacted with hexamethyldisilazane (HMDS), TiCl 4, ZrCl 4, and AlCl 3 vapors under different process conditions. Si−O−M bonds were observed in Ti, Zr, and Al systems at 910−930, 950, and 920−900 cm -1, respectively, in the spectral window formed between the strongly absorbing SiO 2 bulk modes. Depending on the reaction temperature, at least in the case of the Ti and Zr systems the new bands were due to isolated species of type Si−O−M−Cl x formed in reactions with OH groups or due to Si−O−M bonds between silica and crystalline metal oxide particles formed in agglomeration reactions. Thus, the appearance of Si−O−M bonds in an IR spectrum does not unambiguously reveal the interaction mechanism between metal chlorides with silica, since two types of reaction are possible. In addition to allowing the identification of adsorbed species, the low-frequency region of HMDS and AlCl 3 spectra provided information about the bonding sites: thus the band of unreacted isolated OH groups, which in the high-frequency region had shifted to the region of H-bonded groups and was no longer discernible, was clearly seen in the low-frequency region.