Catalysis by palladium–rare-earth-metal (REPd3) intermetallic compounds: hydrogenation of but-1-ene, buta-1,3-diene and but-1-yne

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Abstract
The hydrogenation reactions of but-1-ene, buta-1,3-diene and but-1-yne have been studied on a series of REPd3 intermetallic compounds (RE = La, Ce, Pr, Nd, Sm) in a powdered form. Because the catalytic pretreatments were found to change activities and selectivities, bulk and surface characterisations were performed by XRD and XPS, before and after the catalytic tests. After 15 h pretreatment with H2 at 573 K, these catalysts showed stable and reproducible activities and selectivities with characteristic changes throughout the series when compared to Pd/pumice catalysts. LaPd3 was the most easily decomposed and its behaviour resembled that of a Pd/La2O3 catalyst. CePd3 was only partly decomposed, the surface becoming partially transformed to metallic palladium and the catalytically active cerium compound, producing a highly active catalyst of low selectivity. Conversely, Pr(Nd,Sm)Pd3 were not decomposed in the bulk and they were more selective than Pd/pumice. Typical trends in the hydrogenation rate ratios of but-1,3-diene to but-1-yne were also observed. Different electronic effects were proposed to explain the different catalytic behaviour of these palladium catalysts.