(532cu) Aqueous Phase Heats of Adsorption of Phenolics in Aqueous Environments: Role of Chaotropic and Kosmotropic Species

Knowledge of adsorption thermodynamics is valuable to understand the chemical conversion rates of gas-phase molecules on solid surfaces, with broad applications across heterogeneous catalysis. However, the relationship between adsorption and kinetics is much less understood in the liquid (e.g., aqueous) phase because of the interference of the solvent. Compared with the gas phase, the solvent environment can change the coverage of organics on surfaces, stabilize intermediates via long range electrostatics or hydrogen bonding, or even participate directly in the reaction mechanism. This work aims to measure the adsorption energy of phenol, p-cresol, guaiacol, and catechol on platinum in mixed solvent systems to understand how co-solvents affect aqueous phase heats of adsorption. Specifically, we study the influence of chaotropic and kosmotropic species (e.g., acetic acid and K⁺, respectively) because of their differing ability to disrupt the hydrogen bonding network of water. We hypothesize chaotropic species will increase the heats of adsorption of phenolics by disrupting the water structure at the solid interface, in contrast to kosmotropic agents. We measure the surface coverage of phenolics at different bulk concentrations using a cyclic voltammetric technique known as hydrogen underpotential deposition. The aqueous phase heats of adsorption extracted from our experimental isotherms show agreement with values from a bond-additivity model^1,2 using density functional theory (DFT) calculations of gas-phase adsorption. The result from this study will elucidate how co-solvents modify the aqueous-phase heats of adsorption of common oxygenated aromatic molecules, as well as provide a starting point to link adsorption to kinetics for important reactions such as phenolic oxidation.

References:

1. Nirala Singh and Charles Campbell. ACS Catal. 2019, 9, 8116-8127.
2. Akinola, Charles Campbell, and Nirala Singh. Am. J. Phys. Chem. 2021, 125 (44), 24371-24380.