Ethylene is vital in the large-scale synthesis of polyethylene (PE) and a variety of other practical substances. Significant amounts of acetylene in ethylene feed streams can poison the ethylene polymerization catalyst used in the production of PE. Our systems of ligand-coordinated supported catalysts (LCSCs), which are composed of palladium single atoms and 1,10-phenanthroline-5,6-dione (PDO) ligands on powder ceria (CeO<sub>2</sub>) supports, catalyze reaction of acetylene and excess hydrogen to achieve complete acetylene consumption. Despite an excess of hydrogen, these LCSCs have been shown to be highly selective for the desired partial hydrogenation of acetylene to ethylene, however, it is still unclear why PDO is an effective ligand. Applying ligands of similar structure to PDO, but with varying functional groups, helps in determining the most vital aspects of a ligand for defining and stabilizing the active metal center for the selective hydrogenation of acetylene. These LCSC systems are characterized by XPS, DRIFTS, ICP-MS, and flow reactor hydrogenation reactions. Through these techniques, it is made clear that the chelate effect and a rigid pocket in which the metal can sit are important for the stability of single sites and prevention of aggregation, whereas electron donation and withdrawal are less vital to selective hydrogenation. Interestingly, three-ring pi-systems that lack well-defined polydentate characteristics like Acridine and Benzo[h]quinoline have contributed to low-temperature ethylene production at the expense of high-temperature, long-term stability. Moreover, the success of these ligands and failures of smaller, pyridine-based ligands leads to the hypothesis that larger, heavier ligands are needed to effectively load onto the catalyst surface and interact with Pd single sites. Insight provided from this study informs the design and ligand selection of future LCSCs and expands our understanding of single-atom catalysts.

Ligand effects in defining single metal atom catalysts: Dissecting ligand into its base motifs

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