Selective Hydrogenation of Acetylene over Cu-Pd and Ag-Pd Catalysts

The selective hydrogenation of acetylene is used to purify ethylene streams prior to polymerization in petrochemical processes. Although Pd catalysts are active for this process, bimetallic catalysts have been suggested to improve the selectivity to ethylene while avoiding over-hydrogenation to ethane.

In this work, we synthesized CuPd and AgPd catalysts on both TiO₂ and SiO₂ supports using controlled surface reactions. The deposition of the Pd precursor, cyclopentadienyl Pd allyl, was monitored using UV vis spectroscopy. Uptake of Pd increased in the presence of the Cu or Ag, compared to the supports alone, indicating that the Pd deposition is partially selective to the parent metal nanoparticles. The resulting catalysts were characterized using chemisorption measurements, STEM-EDS, infrared spectroscopy, and X-ray absorption spectroscopy. It was determined that the structure of the nanoparticles and the surface Pd species varies between the CuPd and AgPd systems and between the TiO₂ and SiO₂ supports. From IR spectra, we observe evidence for isolated Pd sites in the presence of CO and contiguous Pd species in the presence of hydrocarbons. These results indicate the dynamic nature of the catalyst surface depending on the gas environment.

The CuPd, AgPd, and Pd catalysts were investigated for acetylene hydrogenation. Although all catalysts achieve high selectivity (>92%), the ethylene selectivity is enhanced for the bimetallic catalysts (>97%). The rate of acetylene conversion is improved over the TiO₂ supported bimetallic catalysts while the bimetallic catalysts on SiO₂ have lower rates and turnover frequencies. The differences in rates can be explained by differences in the Pd surface structure as well as contributions of Cu and Ag to the overall activity. By synthesizing well-controlled bimetallic catalysts, we have developed an improved understanding of the relationships between catalyst structure, activity, and selectivity for acetylene hydrogenation.