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Facet-specific interaction between methanol and TiO2 probed by sum-frequency vibrational spectroscopy

Significance Facet engineering has become a major strategy for designing crystalline catalysts, yet many fundamental issues, including facet-specific interaction with adsorbates, remain unsolved due to lack of experimental investigation. Using surface-specific sum-frequency vibrational spectroscopy, we have conducted an in-depth study of methanol adsorption on four different facets of TiO2 under ambient conditions. The spectra revealed that for the four facets investigated, dissociation of adsorbed methanol occurs only when surface defects are present. Adsorption kinetics and energetics appeared nearly the same on different facets, but the Fermi resonance coupling strength for CH3 of adsorbed methanol was found to depend sensitively on facets and methanol coverage, and could serve as a gauge for studying facet effects on molecular adsorption and surface reactions. The facet-specific interaction between molecules and crystalline catalysts, such as titanium dioxides (TiO2), has attracted much attention due to possible facet-dependent reactivity. Using surface-sensitive sum-frequency vibrational spectroscopy, we have studied how methanol interacts with different common facets of crystalline TiO2, including rutile(110), (001), (100), and anatase(101), under ambient temperature and pressure. We found that methanol adsorbs predominantly in the molecular form on all of the four surfaces, while spontaneous dissociation into methoxy occurs preferentially when these surfaces become defective. Extraction of Fermi resonance coupling between stretch and bending modes of the methyl group in analyzing adsorbed methanol spectra allows determination of the methanol adsorption isotherm. The isotherms obtained for the four surfaces are nearly the same, yielding two adsorbed Gibbs free energies associated with two different adsorption configurations singled out by ab initio calculations. They are (i) ∼−20 kJ/mol for methanol with its oxygen attached to a low-coordinated surface titanium, and (ii) ∼−5 kJ/mol for methanol hydrogen-bonded to a surface oxygen and a neighboring methanol molecule. Despite similar adsorption energetics, the Fermi resonance coupling strength for adsorbed methanol appears to depend sensitively on the surface facet and coverage.