Palladium–gallium intermetallic compounds for the selective hydrogenation of acetylenePart I: Preparation and structural investigation under reaction conditions

Abstract

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.

In situ surface characterization of the intermetallic compound PdGa – A highly selective hydrogenation catalyst

“…Catalytic properties of as-cast [6], milled [6][7] and chemically etched [8] PdGa were reported by us recently. Catalytically active samples derived from as-cast PdGa powder were used in the XPS and PGAA measurements.…”

“…Details of the preparation can be found elsewhere [6][7][8]. Briefly, PdGa was prepared by melting the appropriate amounts of Pd (ChemPur 99.95 %) and Ga (ChemPur 99.99 %) under protective Ar atmosphere in a high frequency furnace and subsequent annealing of the obtained ingot in an evacuated quartz glass ampoule at 1073 K for 170 h. The phase purity was controlled by X-ray powder diffraction (Guinier camera, Huber G670, image plate, CuK 1 with  = 1.540562 Å).…”

“…Commercial Pd/Al 2 O 3 (Sigma-Aldrich 205710, 5 wt.-% Pd, BET surface: 114 m 2 /g, Pd active surface area 5.6 m 2 /g [7]) was used as self-supporting wafers. Prior to the measurements the samples of were treated in situ in a heatable section of the IR cell at 673 K for one hour in either 500 mbar of hydrogen (PdGa) or vacuum (Pd/Al 2 O 3 ).…”

“…Details of the catalytic measurements are published elsewhere [6][7][8]. Measurements were carried out in a plugflow reactor (30 mL/min) with a high excess of ethylene (0.5% acetylene, 5% hydrogen (99.999%) and 50% ethylene (99.95%) in helium (99.999%)).…”

“…The concept of using intermetallic compounds with covalent bonding rather than alloys is a suitable way to arrive at long-term stable catalyst with pre-selected electronic and local structural properties. Detailed in situ investigations of bulk structure of PdGa and Pd 3 Ga 7 by DSC/TG, XRD and EXAFS during thermal treatment under various inert or reactive gas atmospheres showed that the long-and shortrange ordered crystal structures of the materials are intact up to 600 K. Both intermetallic compounds exhibit a higher selectivity and enhanced long-term stability in acetylene hydrogenation than commercial supported Pd/Al 2 O 3 and an unsupported Pd-Ag alloy [6][7][8]. While the former studies explored the catalytic properties and the bulk stability of the compounds, we present here a careful characterization (including in situ) of the surface of PdGa and compare it with the bulk structure.…”