(750d) Adsorption of Oxygenated Aromatics and Organics on Pt and Rh in Aqueous Phase

Adsorption thermodynamics play a key role in understanding and predicting the catalytic activity for many reactions, including electrocatalytic hydrogenation of bio-oil compounds to fuels and chemicals.¹ Relative to the gas phase, adsorption energies in the aqueous phase are much lower because of the enthalpic penalty of displacing multiple water molecules.² The aqueous-phase heats of adsorption and the role of water displacement on metals are unknown for many oxygenated aromatic and organic species of relevance to bio-oil catalysis, which prevents understanding of how adsorption strength of reactants correlates with catalytic activity in the aqueous-phase.

In this work,³ we study the effect of water displacement on adsorption energies for phenol, furfural, benzaldehyde, benzyl alcohol, and cyclohexanol on Pt and Rh and show how the reduced aqueous adsorption energies compared to gas-phase explain why several bio-oil hydrogenation reactions proceed readily at room temperature. Specifically, we extract adsorption free energies using hydrogen underpotential deposition (H_upd) inhibition on Pt and Rh to construct adsorption isotherms. These free energies are consistent with energies extracted from microkinetic modeling for aqueous-phase hydrogenation. We will discuss how water adhesion energies on Pt and Rh impact the measured aqueous heats of adsorption on Pt compared to Rh.³ This will be used to explain that Pt and Rh have similar turnover frequencies for aqueous phenol and benzaldehyde hydrogenation because Pt and Rh have similar aqueous adsorption energies for these molecules, despite Pt and Rh adsorbing these organics with very different gas-phase adsorption energies.

References

(1) Song, Y. et al. J. Catal. 2018, 359, 68.

(2) Singh, N. et al. ACS Catal. 2019, 9, 8116.

(3) Akinola, J. et al. ACS Catal. 2020, 10, 4929.