Frequencies and Thermal Stability of Isolated Surface Hydroxyls on Pyrogenic TiO2 Nanoparticles

The surface reactivity of TiO2 is often governed by hydroxyl groups. Fourier transform infrared (FTIR) spectroscopy is the most commonly used method to study surface hydroxyls. However, interpretation of the observed bands of powder samples is not straightforward. In this work, we propose a facet-specific assignment of the surface hydroxyls of pyrogenic TiO2 (commonly known as P25 and P90) by comparison between experimentally observed FTIR bands of P90, rutile, and anatase with calculated vibrational frequencies for well-defined surface facets using density functional theory. Titania was calcined for extended periods in extremely dry O-2 to remove carbonates and water for diffuse reflectance infrared spectroscopy measurements of the most thermally stable hydroxyls remaining in the 300-400 degrees C range. Reactions of the dehydroxylated surfaces with H-2(D-2) provided further insight into hydroxyl formation. Theoretical assignments of hydroxyls were based on the calculated thermal stability of hydroxyls to dehydroxylation, agreement with calculated frequencies [scaled to be consistent with the experimental bridging hydroxyl frequency on rutile TiO2(110)], and the thermodynamic stability of the specific facets. Our assignments, combined with previous results, show that terminal and bridging hydroxyl frequencies overlap; therefore, the common assumption that terminal hydroxyls vibrate at higher frequencies is not valid.