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.

While a vast body of knowledge exists about adsorption energies of catalytic reaction intermediates on solid surfaces in gas or vacuum conditions based on experimental studies and calculations using quantum mechanics, much less is known about adsorption energies in the presence of liquid solvents. We present here a method for estimating adsorption energies in the liquid phase based on the gas-phase adsorption energy, the solvent’s adhesion energy to the solid surface, and the gas-phase adsorbate’s solvation energy. A simple bond-additivity model was recently developed for approximating the change in adsorption energy (relative to gas phase) due to the additional presence of liquid solvents using the solvent’s adhesion energy and the gaseous adsorbate’s solvation energy, but that model was limited to adsorbates whose thickness is much smaller than its lateral dimension (parallel to the surface). Here we present a simple extension of that model to adsorbates of finite thickness and general shape. We propose a model to convert the experimental solvation energy of a gaseous molecule into a molecule–solvent adhesion energy by assuming isotropic interaction of the molecule with the solvent. This adhesion energy allows us to estimate the fraction of this solvation energy that is retained when the molecule is adsorbed, based on the molecule’s shape, size, and adsorption geometry. As in the earlier bond-additivity model, adsorption energies in solvent are lower in magnitude than in the gas phase by an amount approximately equal to the adhesion energy of the solvent to the surface times the surface area of the solvent molecules displaced upon adsorption. We also report the predicted effects of different solvents for molecules on metal surfaces where solvation energies, gas-phase adsorption energies, and solvent/surface adhesion energies are available in the literature.

Condensed/aqueous phase surface reactions such as electrocatalytic hydrogenation of bio-oil often involve reactant adsorption and displacement of adsorbed solvent molecules. The enthalpy and entropy of these adsorption processes will influence the kinetics of surface reactions in the condensed/aqueous phase. The value of the adsorption entropy will have a significant effect on how the reactant coverages vary as a function of temperature. Here, adsorption isotherms from 10 to 40 °C and van't Hoff plots were constructed to directly extract the adsorption entropy and enthalpy of phenol, a bio-oil model compound, on Pt and Rh in aqueous media. We show that the effective adsorption entropy of phenol on Pt and Rh in aqueous phase is positive, in contrast to the negative entropy expected in gas phase. The positive entropy values in the aqueous phase are consistent with adsorbed water gaining a fraction of the entropy of bulk liquid water upon displacement by adsorbed phenol. Consequently, the phenol surface coverage is less dependent on temperature in the aqueous phase compared to the gas phase. The results here give insight to the way in which temperature impacts reaction rates for aqueous-phase phenol hydrogenation reaction.

Phenol is an important model compound to understand the thermocatalytic (TCH) and electrocatalytic hydrogenation (ECH) of biomass to biofuels. Although Pt and Rh are among the most studied catalysts for aqueous-phase phenol hydrogenation, the reason why certain facets are active for ECH and TCH is not fully understood. Herein, we identify the active facet of Pt and Rh catalysts for aqueous-phase hydrogenation of phenol and explain the origin of the size-dependent activity trends of Pt and Rh nanoparticles. Phenol adsorption energies extracted on the active sites of Pt and Rh nanoparticles on carbon by fitting kinetic data show that the active sites adsorb phenol weakly. We predict that the turnover frequencies (TOFs) for the hydrogenation of phenol to cyclohexanone on Pt(111) and Rh(111) terraces are higher than those on (221) stepped facets based on density functional theory modeling and mean-field microkinetic simulations. The higher activities of the (111) terraces are due to lower activation energies and weaker phenol adsorption, preventing high coverages of phenol from inhibiting hydrogen adsorption. We measure that the TOF for ECH of phenol increases as the Rh nanoparticle diameter increases from 2 to 10 nm at 298 K and −0.1 V vs the reversible hydrogen electrode, qualitatively matching prior reports for Pt nanoparticles. The increase in experimental TOFs as Pt and Rh nanoparticle diameters increase is due to a larger fraction of terraces on larger particles. These findings clarify the structure sensitivity and active site of Pt and Rh for the hydrogenation of phenol and will inform the catalyst design for the hydrogenation of bio-oils.