Role of intermolecular interactions in the coverage- and configuration-dependent adsorption of carboxylic acids on Pt(111)

Carboxylic acids derived from biomass can be upgraded via heterogeneous catalytic processes to replacement petrochemicals or green hydrogen. A limiting factor in the catalytic upgrading of biomass-derived carboxylic acids is the varied composition of reactant mixtures and consequential competitive adsorption effects between acids that ultimately control reactivity. To address this limitation, the combined effects of intermolecular interactions and acid molecular structure on the dominant adsorbed acid configurations at catalytically relevant coverages must be explored. Here, we determine the coverage- and configuration-dependent adsorption behavior of seven carboxylic acids on Pt(111) using density functional theory and ab initio molecular dynamics simulations. The carboxylic acids—ranging from formic to lactic acid—were chosen to vary carbon chain length and terminal end substituents. The results show that at moderate to high coverages, carboxylic acids preferentially form dimers on Pt(111), regardless of the individual acid's molecular structure. This is due to strongly attractive intermolecular interactions through hydrogen bonding between neighboring R-COOH substituents. Dimer stability was further influenced by carbon chain length and the number and chain placement of R-OH substituents. Finally, the observed trends in adsorption energy with acid molecular structure were used to develop and validate a general additivity model for predicting the adsorption energies of carboxylic acid dimers on Pt(111). This additivity model sheds light on the relative contributions of various substituents to adsorption strength: –COOH > –OH > –CH3. Overall, this work elucidates the important role of intermolecular interactions in the coverage- and configuration-dependent adsorption of carboxylic acids on transition metal surfaces. Furthermore, we provide a predictive tool for easily and rapidly rationalizing competitive adsorption effects during the catalytic upgrading of multi-component carboxylic acid mixtures.