Active sites and mechanism of aqueous phase methanol dehydrogenation on Pt/Al2O3 catalysts from multiscale modeling, microkinetic modeling, and operando spectroscopy

One of the most important scientific challenges of the time is to design catalysts that produce H₂ from “minimum ${\mathrm{CO}}_2$” sources. One way to do this is by aqueous phase reforming (APR) of sugar alcohol molecules derived from biomass. However, to date, H₂ yields have been disappointing, indicating a need to optimize catalysts and reaction conditions to improve H₂ 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 CH₃OH dehydrogenation. Specifically, we use a combination of multiscale modeling, microkinetic modeling, and Fourier transform infrared spectroscopy to determine the mechanism of CH₃OH dehydrogenation on Pt/Al₂O₃ catalysts. We investigate sites on the terraces of large Pt particles as well as sites at the Pt/Al₂O₃ 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 CH₃OH dehydrogenation mechanism on Pt terrace sites but that these changes do not influence the observed rate of CH₃OH consumption.