Adsorption Energies of Oxygenated Aromatics and Organics on Rhodium and Platinum in Aqueous Phase

Accurately predicting adsorption energies of oxygenated aromatic and organic molecules on metal catalysts in the aqueous phase is challenging despite its relevance to many catalytic reactions such as biomass hydrogenation and hydrodeoxygenation. Here, we report the aqueous-phase adsorption enthalpies and free energies of phenol, benzaldehyde, furfural, benzyl alcohol, and cyclohexanol on polycrystalline Pt and Rh determined via experimental isotherms and density functional theory modeling. The experimental aqueous heats of adsorption for all organics are ∼50 to 250 kJ mol–1 lower than calculated gas-phase heats of adsorption, with a larger decrease for Rh compared with that for Pt. Unlike in gas phase, phenol and other aromatic organics adsorb with similar strength on Pt and Rh in aqueous phase. The similar aqueous adsorption strength of phenol and benzaldehyde on Pt and Rh explains their comparable aqueous-phase hydrogenation activities, which are rate-limited by a Langmuir–Hinshelwood surface reaction. A widely used implicit solvation model largely overpredicts the heats of adsorption for all organics compared with experimental measurements. However, accounting for the enthalpic penalty of displacing multiple water molecules upon organic adsorption using a bond-additivity model gives a much closer agreement between experimental measurements and predicted heats of adsorption. This bond-additivity model explains that the similar adsorption strength of organics on Pt and Rh in aqueous phase is due to the stronger adhesion of water to Rh than that on Pt, which offsets the stronger gas-phase organic adsorption energy on Rh. The data reported herein also provides a valuable resource for benchmarking methods for predicting aqueous-phase adsorption energies of C5/C6 organics on metal surfaces.