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.

PdGa, cubic, P2 13( no. 198), a =4.89695(6) Å, V =117.4 Å 3 , Z =4,R gt(F) =0.024, wRref(F 2 ) =0.035, T =295 K. Source of materialLarge single crystals of PdGa were obtained by Czochralski growth from the melt as described in [1]. The melt consisted of Pd (99.9 %, ChemPur) and Ga (99.99 %, ChemPur) in an atomic ratio of 45:55. The metals were pre-reacted in glassy carbon crucibles under inert argon atmosphere in ahigh-frequency induction furnace. Single crystals used in this study were obtained from the bottom of the as grown Czochralski crystal and were annealed at 800°Cfor 24 hindynamic vacuum at 10 -6 mbar prior to the X-ray diffraction experiments. For the lattice parameter determination a small part of the bottom of the large crystal was crushed and annealed as described above. WDXS measurements were performed on aC AMECA SX100 (W filament, 25 kV) with Pd50Ga50 (by chemical analysis via ICP-OES) as standard. Experimental detailsSingle crystal data were collected using aRigaku R-axis SPIDER diffractometer with monochromated Ag K a radiation. Determination and refinement of the crystal structure were performed with the SHELX-97 software [2]. Spence [3] defined the two absolute structures of the FeSi type as form A( Fe in 4a with x = 0.1358, Si in 4a with x =0.844) and form B(Fe in 4a with x = 0.8642, Si in 4a with x =0.156). Models of both absolute structures were refined for PdGa without twin model and resulted in considerable differences in R gt,A =0 .028 and Rgt,B =0 .040, respectively. Further refinement of form Aa si nversion twin resulted in aF lack parameter of 0.05(1), confirming the proper parameter set. Refining site occupancy for both atoms resulted in fully occupied sites within one e.s.d. This excludes mutual site occupation in the crystal as is expected due to the covalent interactions in the compound [4].

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The crystal structure and the electronic properties of YbGa 2 realising a CaIn 2 type atomic arrangement were characterised at ambient conditions using single crystal X-ray diffraction data and magnetic susceptibility measurements at ambient pressure. Pressure-induced changes of structural and electronic properties of YbGa 2 were measured by means of angle-dispersive X-ray powder diffraction and XANES at the Yb L III threshold. At pressures above 22(2) GPa, YbGa 2 undergoes a structural phase transition into a high pressure modification with a UHg 2 type crystal structure. Parallel to the pressure-induced structural alterations, ytterbium in YbGa 2 undergoes an increase of the oxidation state from +2 at ambient conditions to +3 in the high-pressure phase. Quantum chemical calculations of the Electron-Localisation-Function confirm that the phase transition is associated with a conversion of the three-dimensional gallium network of the low-pressure crystal structure into two-dimensional gallium layers in the high-pressure modification.