Facet-Dependent Water Inhibition of Alkanol Dehydration on TiO₂ via Distinct Water-Alkanol Complexes

Water is ubiquitous in biomass-derived feeds, yet its molecular impact on oxygen-elimination reactions remains poorly understood, particularly for catalysts exposing different facets. Here, we utilize well-defined TiO₂ nanocrystals with dominant (101) and (001) facets to reveal a pronounced facet-dependent effect of water, where inhibition for dehydration of isopropanol (IPA) on the TiO₂(001) surface is about four times more severe than TiO₂(101). Through a combination of in situ solid state NMR, in situ infrared spectroscopy, kinetics studies, and theoretical calculations, we demonstrate that this disparity arises from the formation of distinct alkanol-water complex intermediates. On TiO₂(001), IPA undergoes dissociative adsorption to form an isopropoxide-H₂O complex that readily drives the surface into a complex-dominated regime. This pathway increases the activation barrier for C-H cleavage by 40 kJ mol^-1 by inducing a disordered transition state. In contrast, TiO₂(101) favors molecular IPA adsorption with weak hydrogen bonding to water, resulting in a smaller complex formation constant and a much smaller activation barrier increase (25 kJ mol^-1). By quantitatively linking facet-dependent complex coverage to transition-state destabilization, this work moves beyond simple site-blocking models and provides a conceptual framework for designing catalysts that remain active in water-containing environments.