A reliable method to design site isolated catalysts is through the synthesis of intermetallic compounds where a small number of active atoms are isolated in an inert matrix of a second metal. Unlike alloys, intermetallics have long range periodic atomic order and are more likely resistant to surface segregation effects making them an ideal model system for studying structure-function relations. We have identified the Pd-Zn γ-brass phase as a starting point to study the catalytic effect of active site nuclearity, a key parameter in selective catalyst design. The prototypical Pd8Zn44 structure contains only isolated Pd atoms, but as the Pd concentration is increased by Zn substitution (Pd8+xZn44-x, x = 1-3) within the bounds of the γ-phase, a fixed number of Pd-Pd-Pd trimers are formed in the bulk. To prove this transition from monomer to trimer sites is apparent on the surface and catalytically impactful, H-D exchange and ethylene hydrogenation are used as probe reactions. The reaction kinetics of Pd8Zn44 for both reaction is distinct from the other Pd-Zn catalysts and this discrepancy is consistent with first principle calculations on model monomer and trimer Pd sites. The performance of these catalysts are also evaluated for acetylene semi-hydrogenation in excess ethylene. The trimer containing Pd9Zn43 and Pd10Zn42 have ~1000 times higher activity compared to trimer-free Pd8Zn44, but the latter leads to net ethylene gain by only semi-hydrogenating acetylene, consistent with theoretical predictions. Acetylene semi-hydrogenation is considered one of the most challenging reactions in terms of maintaining selectivity, Pd8Zn44 is likely to show high selectivity for several other similar chemistries as well. The Pd-Zn γ-brass phase also provides flexibility in modifying the composition of the active sites through incorporation of a third metal (Pt, Ni, Au or Ag). Rietveld refinement and DFT calculations suggest that Ni group elements form Pd-Pd-M clusters while Cu group elements can form Pd-M’-Pd clusters. Several Pd-M alloys involving these metals (notably Au, Cu and Ag) are known to be selective hydrogenation catalysts. Within the well-defined intermetallic γ-brass framework we now have the unprecedented opportunity to accurately probe how the presence of these heteroatoms affects the Pd active sites to alter reaction pathways and catalyst performance.

Intermetallics enable rational control of catalytic active site nuclearity and composition

256th National Meeting (2018)

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