Atomic control of active-site ensembles in ordered alloys to enhance hydrogenation selectivity.

Intermetallic compounds offer unique opportunities for atom-by-atom manipulation of catalytic ensembles through precise stoichiometric control. The (Pd, M, Zn) γ-brass phase enables the controlled synthesis of Pd-M-Pd catalytic sites (M = Zn, Pd, Cu, Ag and Au) isolated in an inert Zn matrix. These multi-atom heteronuclear active sites are catalytically distinct from Pd single atoms and fully coordinated Pd. Here we quantify the unexpectedly large effect that active-site composition (that is, identity of the M atom in Pd-M-Pd sites) has on ethylene selectivity during acetylene semihydrogenation. Subtle stoichiometric control demonstrates that Pd-Pd-Pd sites are active for ethylene hydrogenation, whereas Pd-Zn-Pd sites show no measurable ethylene-to-ethane conversion. Agreement between experimental and density-functional-theory-predicted activities and selectivities demonstrates precise control of Pd-M-Pd active-site composition. This work demonstrates that the diversity and well-defined structure of intermetallics can be used to design active sites assembled with atomic-level precision.

Supercritical water gasification of phenol over Ni-Ru bimetallic catalysts.

Incorporating Ru in a Ni catalyst for gasification of phenol in supercritical water at 450 °C and 30 min promoted formation of cyclohexanol via hydrogenation, which is a key step toward gasification. Both Ni and Ni-Ru catalysts were effective to reduce the formation of cyclohexanone and oligomerization products, compared with the case with no catalyst. H2 and CH4 yields increased as the Ru/Ni ratio increased, as did the carbon and hydrogen yields in the gas phase products. The Ni80Ru20/Al2O3 catalyst provided good gasification performance and it exhibits Ru (101), Ru (100) and Ni (111) facets and evidence of overlaid bimetallic particles. DFT calculations show that the presence of Ru (either as pure Ru or as a Ni-Ru alloy) reduces the energy barrier for phenol hydrogenation by close to 0.2 eV relative to pure Ni, and that the energy barrier is not as largely affected by the amount of Ru present, provided it is non-zero.