Tuning the Strength of Molecular Bonds in Oxygenates via Surface-Assisted Intermolecular Interactions: Atomistic Insights

Abstract

Lateral interactions between coadsorbed hydrocarbon species play an important role in their chemical transformations on catalytic metal surfaces. In this report, we present a mechanistic study on mutual lateral interactions of the α-ketoester ethyl pyruvate adsorbed on a well-defined Pt(111) surface, resulting in a strong weakening of ester bonds. By employing a combination of surface-sensitive spectroscopic and microscopic techniques as well as theoretical calculations, we address the atomistic-level structure of surface assemblies containing several ethyl pyruvate species. We report formation of different types of surface oligomers comprising topologically different dimer, trimer, and tetramer species. Based on a combination of spectroscopic and microscopic observations, all species can be attributed to two large classes of oligomers exhibiting different types of intermolecular bonding. In the first class of species, the intermolecular interaction is realized via H-bonding between two acetyl groups of ethyl pyruvate, that is, a carbonyl and a methyl group of the neighboring molecules, while in the second type of species the bonding interaction involves the ester-O of one molecule and the acetyl group of a neighboring adsorbate. For the latter type of species, a strong IR frequency shift of the ester C–O vibration was observed pointing to a significant weakening of the related ester bonds, which might exert a strong impact on the chemical transformations involving this group. We demonstrate that the particular type of intermolecular interaction in ethyl pyruvate assemblies can be effectively tuned by controlling the adsorption parameters, such as surface coverage and the presence of coadsorbed hydrogen. Obtained results provide important insights into the details of lateral interactions of complex multifunctional molecules adsorbed on catalytically relevant surfaces. We show that the parameter space in a catalytic process involving ester compounds can be purposefully varied to tune the strength of the ester bond toward improving the catalytic performance.