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Abstract:
The model Pd and Au mono- and bimetallic (Pd-Au) catalysts were prepared using vapor deposition of metals (Au and/or Pd) under ultra-high vacuum conditions on the defective highly oriented pyrolytic graphite (HOPG) surface. The model catalysts were investigated using the X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) at each stage of preparation procedure. For the preparation of bimetallic catalysts, different procedures were used to get different structures of PdAu particles – Aushell-Pdcore or alloyed. All prepared catalysts showed rather narrow particles size distribution with an average particles size in the range of 4-7 nm. Parahydrogen-enhanced nuclear magnetic resonance (NMR) spectroscopy was used as a tool for the investigation of Pd-Au/HOPG, Pd/HOPG and Au/HOPG model catalysts in propyne hydrogenation reaction. In contrast to Au sample, Pd, PdAualloy, and Aushell-Pdcore samples were shown to have catalytic activity in propyne conversion, and pairwise hydrogen addition routes were observed. Moreover, bimetallic samples demonstrated the 2- to 5-fold higher activity in pairwise hydrogen addition in comparison to the monometallic Pd sample. It was shown that the structures of bimetallic Pd-Au particles supported on HOPG strongly affected their activities and/or selectivities in propyne hydrogenation reaction: the catalyst with Aushell-Pdcore structure demonstrated higher pairwise selectivity than that with PdAualloy structure. Thus, the reported approach can be used as an effective tool for the synergistic effects investigations in hydrogenation reactions over model bimetallic Pd-Au catalysts, where the active component is supported on a planar support.
