Brominated Pd-on-Au Nanostructures Enable Reductive Relay Isomerization of Alkynes to E-alkenes

Achieving stereoselective alkyne semi-hydrogenation to E-alkenes remains a persistent challenge due to inherent limitations of conventional catalysts in controlling stereochemistry and suppressing over-hydrogenation. Herein, we resolve this fundamental dilemma through a rationally designed brominated Pd-on-Au nanocatalyst (Pd_0.03-Br₁^Au/TiO₂) featuring spatially segregated active sites operating via reductive relay isomerization. This sophisticated architecture enables unprecedentedly efficient E-alkene synthesis (>96% selectivity for trans-stilbene at near-quantitative conversion). Fabricated by sequentially depositing Au nanoparticles on TiO₂, with tiny Pd loading on Au, and controlled surface bromination, the catalyst leverages synergistic cooperativity: The TiO₂-Au interface primarily activates formic acid (FA) to generate reactive surface-bound hydride species (H*) while minimizing unproductive H₂ formation; concurrently, atomically dispersed Pd₁ sites on Au nanoparticles exclusively mediate rapid Z-to-E isomerization, whereas bromide-capped Pd nanoclusters kinetically regulate FA dissociation kinetics at TiO₂-Au interface and sterically block overhydrogenation adsorption geometries. This spatially orchestrated multisite system decisively overcomes classical activity-selectivity trade-offs, establishing a universally applicable framework for decoupling and optimizing individual catalytic functions in heterogeneous design. Our work delivers both a sustainable strategy for scalable trans-alkene production and fundamental mechanistic insights into complex cooperative reaction networks.