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One of the most important scientific challenges of the time is to design catalysts that produce H 2 from "minimum CO 2 " sources. One way to do this is by aqueous phase reforming (APR) of sugar alcohol molecules derived from biomass. However, to date, H 2 yields have been disappointing, indicating a need to optimize catalysts and reaction conditions to improve H 2 production. This requires a detailed understanding of the APR mechanism. There are three primary steps: dehydrogenation, decarbonylation, and water gas shift. However, the details of these steps remain unknown due to the large and complex structures of the reactant molecules, the aqueous reaction conditions, and the participation of multiple types of active sites in the mechanism. To begin to address these knowledge gaps, herein we study the effect of liquid H 2 O solvent and multiple types of active sites on the mechanism of CH 3 OH dehydrogenation. Specifically, we use a combination of multiscale modeling, microkinetic modeling, and Fourier transform infrared spectroscopy to determine the mechanism of CH 3 OH dehydrogenation on Pt/Al 2 O 3 catalysts. We investigate sites on the terraces of large Pt particles as well as sites at the Pt/Al 2 O 3 perimeter and the influence of liquid H 2 O on both. We show that the reaction is predominantly carried out on terrace sites due to inhibition by strongly bound H 2 O molecules at perimeter sites. We further show that water plays a significant role in the CH 3 OH dehydrogenation mechanism on Pt terrace sites but that these changes do not influence the observed rate of CH 3 OH consumption. [Display omitted] • The mechanism of methanol dehydrogenation in aqueous phase is explored with multiscale modeling. • Feedback looping between experiments and simulations reveals key mechanistic features. • Water molecules at the metal/support interface prevent methanol adsorption and facilitate hydrogenation. • The active sites are the sites on the Pt terraces. [ABSTRACT FROM AUTHOR]