Ethylene is a vital raw material with increasing global demand in the polymer, pharmaceutical and petroleum industries. Typically, ethylene is produced by the steam cracking of naphtha. The product stream from the steam cracking of naphtha contains ethylene and propylene along with a small concentration of unsaturated hydrocarbons. Effective ethylene polymerization requires that the ethylene feed stream contain minimal acetylene to avoid poisoning the polymerization catalyst. Our group has been working toward developing highly selective single-site catalysts using a metal-ligand coordination strategy by complexation of Pd with the ligand 1,10-phenanthroline-5,6-dione (PDO) on cerium oxide powder supports. In this presentation, we present a comparison of Pd-PDO and Pd-methyl-substituted PDO on pristine and defective CeO<sub>2</sub> supports. Methylation of the ligand at different positions on the molecule has a significant impact on metal-ligand complexation due to the steric and electronic effects of the added methyl groups. We also examine the role of oxygen vacancy defects on CeO<sub>2</sub> by comparing pristine ceria to ceria that has been annealed under reductive conditions. Higher defect densities improve the activity of the catalyst. Density Functional Theory calculations that capture the dynamic nature of the metal-ligand interaction at higher ligand ratios have provided insight into the ability of the ligand to transport hydrogen to the active site to enhance selectivity. These results will help in the design of next-generation single-site heterogeneous catalysts.

Reactivity and selectivity of single atom catalysts on metal oxide surfaces for the selective hydrogenation of acetylene

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