AgPd and CuPd Catalysts for Selective Hydrogenation of Acetylene

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 that on 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 the adsorbed CO show that an increased fraction of isolated Pd species are present on the bimetallic catalysts compared to those on the monometallic catalysts. Adsorption of acetylene and ethylene, however, indicates 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 that over the monometallic Pd catalysts. An ethylene selectivity of 100% is observed over the CuPd0.08/TiO2 catalyst. The highest acetylene conversion rate per gram of Pd is observed over the CuPd0.02/TiO2 catalyst, while the highest turnover frequency is found over the AgPd0.64/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 modifications of Pd by the parent Cu or Ag metal.