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English: catalysis, palladium, gallium, hydrogenation, intermetallic compound, acetylene German: Katalyse, Palladium, Gallium, Hydrierung, intermetallische Verbindung, Acetylen,
38, Pd intermetallic compounds
Abstract:
Intermetallic compounds of palladium with gallium (PdGa and Pd3Ga7) are introduced as selective catalysts for the hydrogenation of acetylene. Both intermetallic compounds are selected as site-isolated palladium catalysts because of the increased average interatomic Pd-Pd distances and decreased Pd-Pd coordination numbers in their structures. Detailed investigations of PdGa and Pd3Ga7 by thermal analysis, in situ X-ray diffraction, and in situ X-ray absorption spectroscopy during thermal treatment under various inert or reactive gas atmospheres showed a high thermal stability. The long-range and short-range order structures of the materials remained intact up to temperatures of about 600 K with no phase transitions or decomposition detectable. In addition to high thermal stability, no incorporation of hydrogen in the intermetallic compounds under reducing conditions was detected. Both intermetallic compounds studied exhibit sufficient thermal stability, isolation of Pd centers under reaction conditions, and no incorporation of hydrogen in the bulk. Therefore, besides being interesting model systems, palladium gallium intermetallic compounds may also be promising candidates for application as highly selective hydrogenation catalysts. The surface properties of PdGa and Pd3Ga7 were characterized by X-ray photoelectron spectroscopy, ion scattering spectroscopy, and CO chemisorption. Catalytic activity, selectivity, and long-term stability of PdGa and Pd3Ga7 were investigated under both acetylene hydrogenation reaction conditions in absence and in an excess of ethylene in temperature-programmed and isothermal long-term experiments. Compared to Pd/Al2O3 and Pd20Ag80 reference catalysts, PdGa and Pd3Ga7 exhibited a comparable activity per active Pd site, a higher selectivity, and a reduced deactivation with time on stream. The superior catalytic properties are attributed to the isolation of active Pd sites in the crystallographic structure of PdGa and Pd3Ga7 according to the active-site isolation concept. In addition, structural and catalytic properties of ternary intermetallic compounds of palladium with gallium and platinum and of a binary Pd-Sn intermetallic compound were investigated and the results were discussed and evaluated with respect to the Pd-Ga intermetallic compounds and the concept of active-site isolation.
