Constructing a Highly Active Pd Atomically Dispersed Catalyst for Cinnamaldehyde Hydrogenation: Synergistic Catalysis between Pd-N₃ Single Atoms and Fully Exposed Pd Clusters

Fabricating highly active catalysts with fully exposed metal atoms is necessary to greatly enhance the catalytic efficiency of selective hydrogenation. Here, we precisely constructed a carbon-nitride-nanosheet-supported fully exposed Pd atomically dispersed catalyst (PdSA+C/g-C3N4) by a simple low-temperature impregnation strategy. Importantly, the obtained PdSA+C/g-C3N4 includes Pd-N-3 single atoms and Pd subnanoclusters with atomic-layer thickness. Moreover, the PdSA+C/g-C3N4 exhibits a 100% cinnamaldehyde (CAL) conversion rate and a 97.3% phenylpropanal selectivity during the CAL hydrogenation, which is much better than most of the reported catalysts. Meanwhile, the turnover frequency of PdSA+C/g-C3N4 is 9.19 s(-1), about 12 times higher than that of Pd single-atom catalysts. Further mechanism studies show that the synergistic effect between Pd-N-3 single atoms and fully exposed Pd subnanoclusters in PdSA+C/g-C3N4 is the key to improve the selective hydrogenation activity of CAL. Specifically, hydrogen preferentially adsorbs and dissociates on Pd nanoclusters, while CAL preferentially adsorbs on Pd single atoms. After that, hydrogen could overflow from Pd clusters to Pd single atoms during the CAL hydrogenation, thus improving the reaction kinetics. This work develops a simple method to prepare catalysts with multimetal synergistic sites and provides an insight into the synergistic mechanism of atomically dispersed catalysts during hydrogenation reaction.