Investigating Aqueous-Phase Adsorption for Enhancing (Electro)catalytic Hydrogenation

Abstract

Bio-oil derived from biomass and plastic wastes is a promising substitute to traditional petroleum for use as transportation fuels and chemical precursors and as such it has the potential to combat global warming. However, bio-oil must be upgraded as soon as it is produced to address its instability, primarily caused by high oxygen content and excess water. In this dissertation, we investigate electrocatalytic hydrogenation (ECH) in the aqueous phase to sustainably stabilize bio-oil molecules. ECH still exhibits poor performance for many bio-oil molecules and catalyst materials, which is in part due to a poor understanding of aqueous-phase adsorption and how it governs ECH activity and catalyst design. The chapters in this dissertation investigate aqueous adsorption for model bio-oil molecules on metals and alloy catalyst materials to link their adsorption energies in the aqueous phase to ECH activity. We first explore the role of the aqueous environment on the adsorption energies and enthalpies of C5/C6 organics on Pt and Rh metal surfaces. We show that the aqueous adsorption enthalpies measured from experimental adsorption isotherms are 50–250 kJ mol−1 lower than their gas-phase values. The lower aqueous adsorption enthalpy is due to organic solvation and solvent displacement, which introduces a large enthalpic penalty for adsorption in the aqueous phase. Secondly, the molecules display similar aqueous adsorption enthalpies on Pt and Rh despite a huge difference in their gas-phase values. The lower aqueous adsorption strengths explain why ECH of these molecules can occur with appreciable rates in the aqueous phase at room temperature. We also show that Pt and Rh have similar ECH activity toward phenol and benzaldehyde because Pt and Rh adsorb these molecules with similar adsorption energies. We next explore the entropy of the displaced interfacial water molecules upon phenol adsorption by examining the temperature dependence of phenol adsorption. We have initially assumed that the displaced interfacial water only results in an enthalpic penalty but retains its structure and does not influence the aqueous adsorption entropy. We show that the aqueous adsorption entropy is slightly positive in contrast to the negative adsorption entropy in the gas phase. This is because the water molecules upon displacement gain about half of the entropy of bulk liquid water. Consequently, temperature would have a less negative impact on coverages in aqueous phase compared to gas phase. Considering the importance of obtaining accurate aqueous-phase adsorption energies to explain ECH activity correctly, we developed a mathematical model that accounts for solvation and solvent displacement to aid the prediction of aqueous adsorption energies for molecules of any shape and size. This model uses gas-phase adsorption energies along with values from thermochemistry tables to calculate aqueous adsorption energies that are in semi-quantitative agreement with experiments. Finally, we synthesized Pt and PtxCoy alloys to elucidate the impact of catalyst structure and composition on phenol ECH turnover frequency and current efficiency. PtxCoy alloys have weaker hydrogen adsorption energies than Pt and are more active toward hydrogen evolution reaction than Pt, resulting in lower ECH current efficiencies. However certain PtxCoy alloys are more active at certain potentials due to a higher ECH barrier with Co fraction. By using kinetic modeling, we capture qualitative trends in the measured ECH turnover frequency as a function of Co fraction and potential. Ultimately, this dissertation advances our knowledge of aqueous-phase adsorption and ECH activity.