Isolated Pd sites on the intermetallic PdGa(111) and PdGa(111) model catalyst surfaces.

Catalytic reactions are of highest technological and economic importance, especially in the field of chemical synthesis and exhaust gas treatment. About 80% of the reactions in chemical industry rely on catalysts, thus giving motivation for research in this area. There has been enormous progress in heterogeneous catalysis through the understanding of reaction mechanisms and the relation of catalyst surface structures to the reaction rate and selectivity. A major step forward has been achieved by comparing single-crystal model catalysts to real catalysts and by combining the ultrahighvacuum (UHV) surface-science approach with high-pressure studies of the reaction kinetics. This development has led to a paradigm shift in catalyst development. Amongst the most ingenious and innovative catalyst materials are intermetallic compounds (IMCs) as they enable spatial separation of the catalytically active sites and thus the achievement of both high activity and selectivity. Additionally, it is also possible to tune the electronic properties of IMCs by changing their chemical composition. Recent results revealed that this new class of catalytic materials might be of industrial relevance, and outperform known catalysts, especially when highly selective processes are demanded. PdGa IMCs have shown remarkable catalytic properties for an important reaction in polyethylene production, namely the partial hydrogenation of acetylene to ethylene. However, the reaction pathway is largely unknown. We address this question on single-crystal IMC PdGa model systems. Their bulk crystal and electronic structure have been reported earlier. Herein, we determine and explore the stable surface terminations because they define the activity and selectivity of the catalyst.