Construction of Isolated Pd₃ Geometry on GaO_x-Modified Pd/Al₂O₃ as a Highly Active and Selective Catalyst for Semihydrogenation of Acetylene

Site isolation is an effective strategy for achieving high selectivity for acetylene semihydrogenation, yet often at the cost of reduced activity. Moreover, most of the site isolation approaches involve single-atom catalysts (SACs), less is known about the active site ensemble that can behave even superior to SACs in both activity and selectivity. Herein, by decoration of partially reduced GaO_x on the Pd particles in Pd/Al₂O₃ and followed by in situ treatment in reaction gas, we are able to construct isolated Pd₃ sites and identify the structure by using in situ CO–DRIFTS and DFT calculations. Reaction tests for acetylene semihydrogenation under simulated industrial feed gas conditions show that the isolated Pd₃ sites offer ∼99% ethylene selectivity at acetylene conversion of 95% at very mild reaction conditions (50 °C and 1 bar), with a turnover frequency of 13.5-fold that of Pd₂Ga intermetallic compound, outperforming most of the state-of-the-art Pd-based catalysts. The combined TPD experiments and theoretical calculations reveal that the partially reduced GaO_x on the Pd surface not only isolates and stabilizes the Pd₃ geometry but also alters the electronic properties of Pd via electron transfer from Ga to Pd atoms, thereby enhancing acetylene adsorption while weakening ethylene adsorption, leading to greatly increased activity and selectivity. This work opens up a distinct avenue from the prevalent SAC strategy toward site-isolation, highlighting the great potential of manipulating the active site ensemble in overcoming the activity-selectivity trade-off in selective hydrogenation reactions.