The morphology of TiO2 microcrystals and their adsorptive properties toward CO: a HRTEM and IR study

High resolution transmission electron microscopy (HRTEM) shows that TiO₂ microcrystals obtained by flame hydrolysis of TiCl⁴ exhibit a virtually perfect crystalline habit with only a few crystallographic planes most abundantly exposed. CO adsorbs at 77 K on the Ti⁴⁺ five-coordinated ions emerging on these faces yielding, at maximum coverage, a strong ir band at 2179.5 cm^-1. The very low hand-band width of this peak ( <1.5 cm^-1) indicates that the adsorbed layer possesses a high degree of bidimensional order. By decreasing the CO coverage the main peak becomes broader and shifts gradually to 2192.5 cm^-1: this is due to changes in the lateral (dynamic and static) interactions. Other less intense peaks can be related to CO adsorbed on sites of higher Lewis acidity, to CO hydrogen bonded to residual OH groups and to the formation of tilted species.