Effects of Pd ensemble size in dilute and single atom alloy PdAu catalysts for one-pot selective hydrogenation and reductive amination†

In the one-pot reaction between nitroarenes, aldehydes, and hydrogen, the desired outcome is the selective hydrogenation of nitroarenes to form aminoarenes that condense with aldehydes to yield pharmaceutically relevant imines and N-alkylamines. One approach to facilitate the selective hydrogenation of nitroarenes over aldehydes involves using bimetallic catalysts with near equimolar ratios. However, structural characterization of metallic ensembles on the nanoparticle surface is challenging at such high alloying ratios, which hinders the elucidation of clear structure–property relationships. Here, we prepared a well-controlled series of dilute Pd-in-Au alloy catalysts with a fixed nanoparticle size as a model system to investigate the effects of surface Pd ensemble size, from single atoms to dimers and trimers, in the one-pot hydrogenation reaction between nitrobenzene and benzaldehyde as our probe reaction. The highest (near unity) selectivity to condensation products was achieved using the catalyst with the lowest Pd content prepared (Pd₂Au₉₈/SiO₂), which predominantly exposed Pd single atoms on the nanoparticle surface as verified by surface-sensitive spectroscopy. Theoretical calculations reveal that Pd single atoms were inactive for benzaldehyde adsorption and thus enabled selective nitrobenzene hydrogenation. On the contrary, the adsorption of benzaldehyde became stronger than nitrobenzene for Pd trimers and larger ensembles, explaining the enhanced competitive adsorption from benzaldehyde in catalysts with increasing Pd content. Our results demonstrate that the commonly used (near equimolar) alloying ratio is rather arbitrary and may not necessarily produce the highest selectivity to condensation products. Instead, we illustrate how controlling the nanoscale Pd ensemble size on the nanoparticle surface tunes competitive kinetics to steer selectivity towards forming the desired condensation products.