IR spectroscopic measurement of diffusion kinetics of chemisorbed pyridine through TiO₂ particles

The chemisorption and surface diffusion/desorption of pyridine from TiO₂ powder (P25) have been measured as a function of temperature using transmission IR spectroscopy. Two classes of diffusion have been measured with activation energies of 36 kJ/mol (fast) and 90 kJ/mol (slow). By comparing density functional theory (DFT) calculations of bonding energies on TiO ₂ rutile (110) and anatase (101), the dominant crystal planes expected in P25 TiO₂, it is found that fast diffusion would be expected on the rutile phase and that much slower diffusion would be expected on the anatase phase. In addition, the presence of oxygen defect sites will produce more strongly bound pyridine than either of the model crystal planes selected for investigation. To both types of TiO₂ surfaces, pyridine bonding occurs to coordinatively unsaturated Ti cation sites through the N lone pair of the pyridine molecule. More than 85% of the molecules exhibit slow surface diffusion, and this is attributed to diffusion on the dominant anatase crystallites as well as to diffusion on defective sites. Diffusing molecules exhibit a v_19b ring-breathing mode, with the rapidly diffusing molecules exhibiting a mode frequency of ∼1438 cm^-1 and the slowly diffusing species exhibiting a mode frequency of ∼1445 cm ^-1. Electron stimulated desorption ion angular distribution (ESDIAD) studies of the C-D bond directions for pyridine-d₅ on the rutile TiO₂(110)-1 × 1 surface show that the ring plane is rotated by 37 ± 1° with respect to the [001] azimuth on the crystal surface, in good agreement with the 39° rotational angle calculated by DFT. Several ring-breathing modes of pyridine are slightly broadened on rutile sites compared to anatase sites, and this may be due to more freedom for librational vibrations of the more weakly bound pyridine molecules.