Resolving the Adsorption Geometries of Methanol on Anatase Titania (101) via Facet-Controlled Nanocrystals

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Bridging the gap between single-crystal studies and technical catalysts is critical for understanding structure–function relationships in oxide catalysis. Here, we utilize hydroxyl- and partially water-covered anatase TiO2 nanocrystals with an exposure of the (101) facet exceeding 90% to resolve long-standing controversies in methanol adsorption geometry. We combine in situ DRIFTS conducted at atmospheric pressure with first-principles calculations to pinpoint species-specific vibrational signatures and reveal adsorbate geometries that are inaccessible to experiments alone. Specifically, we distinguish distinct vibrational signatures of bridging methoxy (CH3Ob*) and terminal methoxy (CH3Ot*) species across isotopologues (CH3OH, CD3OD, CHD2OH). Across (2)δas(CH3), νs(CH3), νas(CD3), νs(CD3), ν(C–H), νas(CD2), and νs(CD2) modes, the vibrational frequency ordering is molecular methanol > bridging methoxy > terminal methoxy. Across (2)ν(C–O) modes, the vibrational frequency ordering is terminal methoxy > bridging methoxy > molecular methanol, although (2)ν(C–O) modes were not always experimentally resolved. Binding energies were computed from first principles. Simulations reveal that molecularly adsorbed methanol and methoxy adopt significant tilt angles (14–46°) relative to the surface normal, while the CHD2OH spectra are consistent with rapid rotational averaging of the methyl group at 355 K. Together, these results establish coordination-specific spectroscopic signatures that enable molecular methanol, bridging methoxy, and terminal methoxy populations to be followed independently on facet-enriched anatase nanocrystals. More broadly, the work demonstrates how controlled nanocrystal morphology, isotopic substitution, in situ spectroscopy, and periodic modeling can connect atomistic surface structures with experimentally observed adsorbate populations under catalytically relevant conditions.