AgPd and CuPd Catalysts for Selective Hydrogenation of Acetylene

In this work, the selective hydrogenation of acetylene has been studied over AgPd and CuPd catalysts. Controlled surface reactions were used to synthesize these bimetallic nanoparticles on both TiO₂ and SiO₂ supports. Chemisorption measurements of the bimetallic catalysts indicate that Pd prefers to be on the nanoparticle surface with a Cu parent catalyst while Pd prefers to be subsurface with a Ag parent catalyst. From energy dispersive X-ray spectroscopy analysis, the composition of the nanoparticles is determined to be more uniform on the SiO₂ support compared to the TiO₂ support. X-ray absorption spectroscopy results indicate that, after reduction, the CuPd bimetallic catalysts have some Pd-Pd bonds but the average number of Pd-Pd bonds decreases after reaction. Infrared spectra of adsorbed CO show an increased fraction of isolated Pd species are present on the bimetallic catalysts compared to the monometallic catalysts. Adsorption of acetylene and ethylene, however, indicate adsorbed surface species that require contiguous Pd ensembles. These results suggest that the surface structure of the catalyst is highly dynamic and influenced by the gas environment as well as the support. The catalysts are active for the selective hydrogenation of acetylene in an ethylene-rich environment under mild conditions. Over all catalysts, the ethylene selectivity is greater than 92%, however improved selectivity is observed over the bimetallic catalysts compared to monometallic Pd catalysts. An ethylene selectivity of 100% is observed over the CuPd_0.08/TiO₂ catalyst. The highest acetylene conversion rate per gram of Pd is observed over the CuPd_0.02/TiO₂ catalyst, while the highest turnover frequency is found over the AgPd_0.64/TiO₂ catalyst. The bimetallic SiO₂-supported catalysts have lower rates than Pd/SiO₂ but still show improved selectivity. The combined characterization measurements and reaction kinetics studies indicate that the performance improvements of the bimetallic catalysts may be attributed to both electronic and geometric modification of Pd by the parent Cu or Ag metal.