Mechanistic details of C O bond activation in and H-addition to guaiacol at water-Ru cluster interfaces

•Guaiacol hydrodeoxygenation proceeds via competing CO cleavage and H-addition routes.•Rapid, initial H* addition to guaiacol leads to common, partially-saturated reactive intermediate.•Partially-saturated reactive intermediate undergoes kinetically relevant CO cleavage or H-addition.•Selectivity ratio of CO cleavage to H-addition increases linearly with *-to-H* coverage ratio.•Initial H* addition on guaiacol is reversible and leads to H-D exchange on the benzylic ring.