Catalytic reactions in condensed phase are important for applications such as biomass conversion and wastewater remediation. As is common in many heterogeneous catalytic reactions, adsorption of the reacting species on the catalyst surface can control the reaction kinetics, influencing adsorption/desorption rates as well as surface reaction barriers based on the energy of the transition states. However, the condensed phase brings with it additional challenges, both in characterizing the catalyst surface and mechanism, and the unclear role of the solvent on adsorption. In this talk we will address the challenge of identifying the reaction mechanism through the use of <i>in-situ/operando</i> X-ray absorption spectroscopy. To demonstrate these we will focus on two examples, the hydrogenation of bio-oil model compounds (e.g., phenol) and the electrocatalytic reduction of nitrate. We probe the catalyst using X-ray absorption, enabling us to determine coverages of adsorbed hydrogen and detect adsorbed phenol and nitrate, as well as identify the catalyst oxidation state and structure. We will also explore the effect of solvent on adsorption and on reaction kinetics using adsorption isotherms and traditional kinetic modeling in aqueous phase, focusing specifically on phenol adsorption and reaction on platinum and rhodium. We will discuss experimental methods in aqueous-phase to extract adsorption free energies, entropies, and enthalpies on these two metals. The measured values explain catalytic behavior observed in aqueous phase, for example how phenol can be hydrogenated at room temperature on metals in aqueous-phase, whereas in gas phase higher temperatures are required because of how strongly phenol is bound to the metal surface. We show that by accounting for the effect of solvent displacement we can explain thermodynamic and kinetic behavior for aqueous-phase adsorption of phenol and other organics on both Pt and Rh surfaces. This understanding opens up the opportunity to more accurately bring computational tools to bear on condensed phase catalysis and tune adsorption through the use of solvents.

Mechanism and kinetics of aqueous-phase hydrogenation on platinum group metal surfaces

ACS Spring Meeting 2021

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