Substrate-Dependent Selectivity in Alkyne Semihydrogenation Over a Hydrogen-Competent Pd₃Sn₂ Intermetallic Catalyst

A recurring challenge in heterogeneous hydrogenation is to maintain rapid H₂ activation while preventing the product from undergoing overhydrogenation. Site isolation can weaken adsorption but frequently compromises hydrogen competence, and selectivity is often presumed transferable across closely related substrates. Here we establish a substrate-dependent selectivity framework on Pd-Sn intermetallic catalysts by integrating theory-guided site identification with experimental validation. Theoretical calculations reveal that Pd₃Sn₂ hosts a distinctive surface structure in which Sn-bridged Pd-Pd dual sites cooperate with adjacent near-surface Pd to dissociate H₂ readily, while methyl substitution in C₃ intermediates weakens π-binding and kinetically favors propylene desorption over further hydrogenation. Guided by these predictions, phase-pure Pd₃Sn, Pd₃Sn₂, and PdSn₂ intermetallic catalysts with comparable particle sizes were synthesized and verified by comprehensive characterizations including aberration-corrected electron microscopy and x-ray absorption spectroscopy. Under excess-propylene conditions, the Pd₃Sn₂ catalyst achieves 98.0% propylene selectivity at 100% propyne conversion, whereas the Pd and Pd₃Sn catalysts suffer severe overhydrogenation and the PdSn₂ catalyst is intrinsically sluggish. Notably, the Pd₃Sn₂ catalyst performs poorly for acetylene hydrogenation, leading to significant ethane formation via overhydrogenation of both newly formed and co-fed ethylene. Complementary kinetic evidence supports the proposed mechanism by quantifying hydrogen activation competence and product residence on the intermetallic surfaces.