Opening the structure-activity relationship black box in Pd-catalyzed nitroaromatic hydrogenation by quantifying reactive sites of Au@Pd catalysts

Abstract

Correlating the physical structure of reactive sites with catalysis is key to designing atomically precise heterogeneous catalysts. However, understanding the structural features of the various active sites at the nanoparticle level is vital for establishing a structure-activity relationship because multiple and unquantifiable reactive sites co-exist on these catalysts. Here, we analyzed the surface-enhanced Raman spectroscopy (SERS) spectra of chemisorbed probe, 4-iodo-2,6-dimethylphenylisocyanide, to obtain the site distribution of Pd sites on Au@Pd catalysts. Based on the new SERS-derived structure and distribution of Pd sites, the performance of Au@Pd in hydrogenation of nitroaromatics is directly correlated with the theoretical site-specific activity and selectivity. This fully understood structure-activity relationship allows accurate prediction of the performance of different catalysts. We enriched the most active and selective single-atom Pd sites on the Au surface with an Ag monolayer, reaching >99% conversion of 4-chloronitrobenzene into 4-chloroaniline with >99% selectivity during a 100-h continuous-flow reaction.