Substrate–Support Interactions Mediate Hydrogenation of Phenolic Compounds by Pd/CeO₂ Nanorods

Ceria-supported palladium (Pd/CeO₂) has spawned significant attention in recent years due to its ability to catalyze selective hydrogenation of phenolic compounds to cyclohexanones and cyclohexanols at a mild temperature and pressure. However, the mechanistic basis by which ceria enhances catalytic conversion is still unclear. Here, we use the increase in the ¹³C transverse relaxation rate upon the addition of nanoparticles (NPs) (¹³C ΔR₂) to investigate the adsorption of phenolic compounds on the surface of the Pd/CeO₂ catalyst by solution NMR. We show that hydroxyphenols adsorb on the support more efficiently than underivatized phenol and methoxyphenols and that phenol derivatives with an oxygen atom at position 2 (i.e., 2-hydroxyphenol and 2-methoxyphenol) form very stable interactions with the Pd site of Pd/CeO₂. An analysis of the kinetics of hydrogenation revealed that catalytic conversion is linearly correlated with the ability of the substrate to form interactions with the CeO₂ support and is inhibited by the formation of stable substrate–Pd adducts. Our data suggest that CeO₂–substrate interactions mediate phenol hydrogenation more efficiently than Pd–substrate interactions and explain the exceptional catalytic performance reported for Pd/CeO₂.