The structurally well-defined intermetallic compounds PdGa and Pd 3 Ga 7 constitute suitable catalysts for the selective hydrogenation of acetylene. The surface properties of PdGa and Pd 3 Ga 7 were characterized by X-ray photoelectron spectroscopy, ion scattering spectroscopy and CO chemisorption. Catalytic activity, selectivity and long-term stability of PdGa and Pd 3 Ga 7 were investigated under different acetylene hydrogenation reaction conditions, in absence and in excess of ethylene, in temperature-programmed and isothermal long-term experiments. Chemical treatment with ammonia solution -performed to remove the gallium oxide layer introduced during the milling procedure from the surface of the intermetallic compounds -yielded a significant increase in activity. Compared to Pd/Al 2 O 3 and Pd 20 Ag 80 reference catalysts, PdGa and Pd 3 Ga 7 exhibited a similar activity per surface area, but higher selectivity and stability. The superior catalytic properties are attributed to the isolation of active Pd sites in the crystallographic structure of PdGa and Pd 3 Ga 7 according to the active-site isolation concept.
The intermetallic compounds PdGa and Pd 3 Ga 7 are introduced as selective catalysts for the hydrogenation of acetylene. Single phase PdGa and Pd 3 Ga 7 can readily be prepared by the appropriate thermal treatment of the stoichiometric mixtures of the corresponding elements. The initial low surface areas of the as-prepared materials can be increased by careful mechanical treatment without decomposition. Detailed investigations of PdGa and Pd 3 Ga 7 by DSC/TG, in situ powder 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 in the crystal structures remained intact up to temperatures of about 600 K. Neither phase transitions nor decomposition were detectable. In addition to high thermal stability -preserving the active-site isolation under reaction conditions -no incorporation of hydrogen or carbon in the intermetallic compounds under reducing conditions was observed. Besides being interesting model systems, palladium gallium intermetallic compounds are promising candidates for the application as highly selective hydrogenation catalysts.
A series of ZnO/Al 2 O 3 mixed oxide samples with varying Zn/Al ratio is prepared by coprecipitation, ageing, drying, and calcination. Samples are investigated in the state after drying and calcination. The applied methods include X-ray diffraction, solid-state 27Al magic-angle spinning nuclear magnetic resonance spectroscopy, transmission electron microscopy and thermogravimetric experiments coupled with evolved gas analysis. Phases present in the dried precursor samples include hydrozincite, zaccagnaite, and an unknown phase. After calcination zinc oxide and spinel can be found. All results indicate the substitution of Al ions for Zn ions in zinc oxide of zinc-rich samples.
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