Microcalorimetric quantification of hydrogen adsorption thermodynamics in water-solvated systems on Pt/C

Abstract

Adsorption of simple gas phase molecules (e.g., H₂) on metal nanoparticles in the aqueous phase link thermo- and electrocatalysis communities through common elementary steps. Yet, a key facet of this linkage remains incomplete: the effects of water solvation on coverage-dependent adsorption thermodynamics, the migration and speciation of adsorbates across surfaces, and related electrical polarization, eludes current understanding but represents a necessary benchmark to relate computational and experimental investigations of such systems. Here, we describe an experimental approach to quantify adsorption thermodynamics of hydrogen, a species relevant for both thermo- and electrocatalysis in the condensed phase, utilizing volumetric adsorption uptakes, microcalorimetric assessments of adsorption enthalpies, and in situ measurements of catalyst open circuit potentials (E_cat) for water-wetted Pt nanoparticles dispersed on carbon supports as a model system. Precise control of H₂O thermodynamic activity and hydrogen fractional coverages reveals nearly constant molar enthalpies of adsorption (ΔH_ads = −32 vs. −27 kJ per mol-H) coupled with greater entropy losses (ΔS_ads = −100 vs. −62 J per mol-H per K) upon introducing H₂O. Additionally, hydrogen uptakes increase drastically in the presence of coadsorbed water and exceeds 20 mol-H per mol-Pt_surf, which indicates chemical species migrate from Pt nanoparticles to the carbon support. Analysis of adsorption free energies and E_cat measurements indicate that these migrated species remain bound as hydronium–electron pairs dispersed across the carbon support following the equilibrium of Tafel (H₂ + 2* ⇌ 2H*) and Volmer (H* + H₂O ⇌ H₃O⁺ + e⁻ + *) elementary steps commonly invoked in hydrogen evolution electrocatalysis alongside an electrostatic capacitive interaction step (H₃O⁺ + C⁻ ⇌ H₃O⁺⋯C⁻). Dissociative adsorption of H₂ proceeds more rapidly in the presence of co-adsorbed water as a consequence of hydronium shuttling enabled by the Volmer step. This case study illustrates a generalizable methodology to directly measure thermodynamic quantities for molecular and dissociative adsorption at solid–liquid interfaces at controlled thermodynamic activities of all species. We anticipate this form of measurement will serve as a foundation for connections between theory and experiment in pursuit of increasingly complex descriptions of chemical reactions in these environments.