The mechanistic roles of free hydronium ions, surface hydrides, and interfacial protons during guaiacol hydrodeoxygenation (HDO) on ruthenium nanoparticles have been established. As guaiacol adsorbs on Ru, it loses its strong aromaticity and undergoes a rapid H-shift from its hydroxyl to meta carbons (in relation to its hydroxyl group), causing adsorbed enol and keto surface isomers to exist in chemical equilibrium. HDO occurs via a hydridic H-adatom (H*) attack on the enol, followed by a kinetically relevant C?O bond rupture step, during which water shuttles the hydroxyl proton, enabling its intramolecular attack on the methoxy, evolving to a highly charged [Ru<
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transition state. The competing hydrogenation (HYD) begins with a rapid H* attack on the keto form, before a second, kinetically relevant H* attack without proton involvement. Water, despite shifting the thermodynamics toward the more polar surface keto, promotes HDO to a much greater extent than HYD, because of its dual catalytic roles in reducing the activation free energies?(i) it mobilizes the hydroxyl proton of partially saturated guaiacol (Br�nsted acid) and functions cooperatively with the Ru metal surface (base) in rupturing the C?O bond and stabilizing the resulting cationic carbon-ring fragment and (ii) water layers solvate the charged [Ru<
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transition state. Free hydronium ions do catalyze a separate homogeneous enol?keto isomerization, but this reaction is kinetically unrelated to HDO catalysis. Furthermore, this mechanistic picture explains the strong effects of a polar protic solvent in hydrodeoxygenation, highlighting the requirements of surface hydrides and interfacial protons acting in tandem to complete a HDO turnover and the cooperative role of the protic solvent and the metal surface in breaking the aromaticity and preferentially stabilizing charged transition states.