(265d) Selective Hydrogenation of Acetylene over Cu-Pd and Ag-Pd Catalysts

The selective hydrogenation of acetylene in ethylene-rich streams is used to purify ethylene prior to polymerization reactions¹. This hydrogenation is typically carried out using Pd-based catalysts, but selectivity remains a challenge since over-hydrogenation of acetylene to ethane is undesirable². Often, the selectivity of Pd based catalysts can be improved by forming bimetallic catalysts with a group IB metal such as Ag, Au, or Cu^2–4. In this work, we have investigated AgPd and CuPd catalysts, synthesized by controlled surface reactions, for the selective hydrogenation of acetylene.

The bimetallic catalysts were synthesized by first preparing Cu/TiO₂ and Ag/TiO₂ catalysts. Pd was then deposited using controlled surface reactions^5,6 and the extent of Pd precursor (cyclopentadienyl Pd allyl) uptake was monitored using UV vis spectroscopy. Although Cp(Pd)allyl is partially deposited on the TiO₂ support alone during control experiments, additional uptake is observed during deposition onto Cu or Ag indicating formation of bimetallic particles. STEM-EDS analysis also indicates the formation of bimetallic particles. The catalysts were characterized using CO chemisorption and FTIR spectra of adsorbed CO and adsorbed C₂H₄. On AgPd and CuPd catalysts, CO adsorbs primarily on Pd top sites, indicating that the surface Pd is primarily isolated sites, separated from other Pd by the Ag or Cu. On a monometallic Pd catalyst, however, CO adsorbs on top, bridging, and 3-fold sites, indicative of contiguous Pd ensembles. The IR spectra of ethylene adsorbed on both the bimetallic and monometallic catalysts show ethylene adsorbed in a di-σ configuration, which requires neighboring Pd atoms. These results suggest that the surface Pd structure is mobile and influenced by the surface adsorbates. Additionally, some ethylidyne surface species are observed on the monometallic Pd catalyst but not on the AgPd or CuPd catalysts.

Acetylene hydrogenation reactions were carried out over AgPd/TiO₂, CuPd/TiO₂, and Pd/TiO₂ catalysts at 313 K and 1 atm. A molar ratio of 1:25:25 C₂H₂:C₂H₄:H₂ was used for all reactions. At acetylene conversions less than 2%, the monometallic Pd/TiO₂ catalysts is 92% selective to ethylene and the bimetallic catalysts are more than 99% selective to ethylene. These results indicate that the ethylene selectivity is enhanced by diluting the Pd in the Ag or Cu, likely due to a weakened binding of intermediates on the Pd sites. The rate of acetylene conversion per gram of Pd is more than 2 times higher on the CuPd catalysts than on the AgPd catalysts. This difference in activity between the bimetallic compositions may be due to the relative contributions of Cu and Ag to the hydrogenation activity. In both cases, although acetylene and ethylene adsorb on Pd, these species can also potentially adsorb on Cu and Ag. The binding energy of ethylene is weaker on Ag than on Cu, however, and therefore Ag may not interact sufficiently strongly with ethylene and acetylene to contribute to the overall conversion.

We observe enhancement in both the rate and selectivity for CuPd/TiO₂ catalysts compared to Pd/TiO₂ catalysts for the selective hydrogenation of acetylene. AgPd/TiO₂ catalysts are less active but still offer an improved selectivity to ethylene compared to monometallic Pd. By synthesizing well controlled catalysts and using a range of catalyst characterization techniques, we develop an understanding of the active site and the relationships between catalyst structure, activity, and selectivity.

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