Improvement of Selectivity in Acetylene Hydrogenation with Comparable Activity over Ordered PdCu Catalysts Induced by Post-treatment

In this work, a simple post-treatment has been carried out on a solid palladium-copper alloy to enhance the ethylene selectivity without any loss of activity. In all catalysts, PdCu/C catalysts post-treated at 375 °C exhibit improved ethylene selectivity (86%) compared to the solid PdCu/C catalysts (61%) at 100% acetylene conversion with comparable catalytic activity. During the post-treatment, the average size of PdCu nanoparticles is maintained at 6.6-6.8 nm, and no obvious segregation is observed. X-ray photoelectron spectroscopy and in situ extended X-ray absorption fine structure (EXAFS) results display that Pd is in a metallic state for all PdCu catalysts before and after post-treatment. Moreover, the EXAFS fitting results show that the Pd-Pd bond is gradually replaced by the Pd-Cu bond. The good separation of Pd atoms by Cu is also proven by XRD characterization, which shows that a body-centered cubic PdCu structure with uniform distribution of Pd and Cu in a unit cell forms under 375 °C post-treatment. The rearrangement of Pd and Cu atoms has a limited impact on the surface Pd dispersion, avoiding the activity loss due to the decrease in Pd sites. The improved selectivity could be attributed to the isolation of Pd and the accompanied d-band center downshifting, which favors the desorption of π-bonded ethylene species.