Metal catalyzed hydrogenations are frequently reported to exhibit solvent effects, with aqueous environments affording the highest rates of reaction. The Horuiti-Polanyi mechanism is ubiquitous for hydrogenations on metal surfaces; hydrogen adatoms are sequentially added across an unsaturated moiety adsorbed on the metal surface. Deciphering the impact of the solvent environment on the Horuiti-Polanyi mechanism, however, has proven challenging due to the existence of multiple convoluting factors. While intuitive, investigating the effect of solvents in a bulk liquid environment results in thermodynamic, transport based and catalyst restructuring challenges. To understand the impact of a solvent locally at the metal surface, isolating the interactions of adsorbed solvents with the catalytic cycle, a more thermodynamically ideal vapor phase environment avoids a majority of convoluting factors faced in the liquid phase. Taking ethylene hydrogenation over Pt as a well-studied model chemistry, we present a vapor phase investigation of the impact of solvent molecules on the Horuiti-Polanyi mechanism. Through a combination of solvents varying in functionality, isotopic labelling and in-situ titration studies, we decouple the various mechanisms at play through which solvents alter the energetic landscape of alkene hydrogenation on the surface of Pt. An overall framework is developed through which to extend results to other relevant catalytic hydrogenations, where we find that true solvent effects on a metal surface can be broken down to either thermodynamic coverage effects and altering the surface energy of kinetically relevant intermediates.

Leveraging the curious case of solvents in the vapor phase: Alkene hydrogenation over Pt

ACS Spring Meeting 2021

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