Replacing noble metals in heterogeneous catalysts by low-cost substitutes has driven scientific and industrial research for more than 100 years. Cheap and ubiquitous iron is especially desirable, because it does not bear potential health risks like, for example, nickel. To purify the ethylene feed for the production of polyethylene, the semi-hydrogenation of acetylene is applied (80 × 10(6) tons per annum; refs 1-3). The presence of small and separated transition-metal atom ensembles (so-called site-isolation), and the suppression of hydride formation are beneficial for the catalytic performance. Iron catalysts necessitate at least 50 bar and 100 °C for the hydrogenation of unsaturated C-C bonds, showing only limited selectivity towards semi-hydrogenation. Recent innovation in catalytic semi-hydrogenation is based on computational screening of substitutional alloys to identify promising metal combinations using scaling functions and the experimental realization of the site-isolation concept employing structurally well-ordered and in situ stable intermetallic compounds of Ga with Pd (refs 15-19). The stability enables a knowledge-based development by assigning the observed catalytic properties to the crystal and electronic structures of the intermetallic compounds. Following this approach, we identified the low-cost and environmentally benign intermetallic compound Al(13)Fe(4) as an active and selective semi-hydrogenation catalyst. This knowledge-based development might prove applicable to a wide range of heterogeneously catalysed reactions.

Discussed are the recent experimental and theoretical results on palladium-based catalysts for selective hydrogenation of alkynes obtained by a number of collaborating groups in a joint multi-method and multi-material approach. The critical modification of catalytically active Pd surfaces by incorporation of foreign species X into the sub-surface of Pd metal was observed by in situ spectroscopy for X=H, C under hydrogenation conditions. Under certain conditions (low H2 partial pressure) alkyne fragmentation leads to formation of a PdC surface phase in the reactant gas feed. The insertion of C as a modifier species in the sub-surface increases considerably the selectivity of alkyne semi-hydrogenation over Pd-based catalysts through the decoupling of bulk hydrogen from the outmost active surface layer. DFT calculations confirm that PdC hinders the diffusion of hydridic hydrogen. Its formation is dependent on the chemical potential of carbon (reactant partial pressure) and is suppressed when the hydrogen/alkyne pressure ratio is high, which leads to rather unselective hydrogenation over in situ formed bulk PdH. The beneficial effect of the modifier species X on the selectivity, however, is also present in intermetallic compounds with X=Ga. As a great advantage, such PdxGay catalysts show extended stability under in situ conditions. Metallurgical, clean samples were used to determine the intrinsic catalytic properties of PdGa and Pd3Ga7. For high performance catalysts, supported nanostructured intermetallic compounds are more preferable and partial reduction of Ga2O3, upon heating of Pd/Ga2O3 in hydrogen, was shown to lead to formation of PdGa intermetallic compounds at moderate temperatures. In this way, Pd5Ga2 and Pd2Ga are accessible in the form of supported nanoparticles, in thin film models, and realistic powder samples, respectively

The structurally well-defined intermetallic compound PdGa -a highly selective catalyst for the semi-hydrogenation of acetylene -was characterized by Fourier transform infrared spectroscopy (FTIR), in situ X-ray photoelectron spectroscopy (XPS) and in situ Prompt Gamma Activation Analysis (PGAA). A strong modification of the electronic states in PdGa compared to elemental Pd was revealed as well as the complete isolation of the Pd atoms on the surface of PdGa. In situ investigations proved the high stability of the surface, thus excluding segregation phenomena (common for alloys) or sub-surface chemistry involving C and/or H atoms (known for elemental Pd). By suppressing the sub-surface chemistry, the electronic modification as well as the site isolation lead to the high selectivity and long-term stability of PdGa in the semi-hydrogenation of acetylene.