(745g) DFT Studies of Intermetallic Gamma-Brass Structured Catalysts for Selective Hydrogenation

Bimetallic compounds can offer tunable site electronics and ensemble structure for selective hydrogenation catalysis. We utilize the γ-brass phase (Cu₅Zn₈ prototype) to expose surfaces with controlled M_x nuclearity to control the selectivity for hydrogenation. DFT results are combined with microkinetic modeling to predict relative activities and selectivities for a series of hydrogenation reactions with varying metal site nuclearity and composition. Hydrogenation reactions discussed, with comparison to experimental testing, will include acetylene semi-hydrogenation, crotonaldehyde hydrogenation, and alkene/aromatic competitive hydrogenation. The γ-brass structure has a 52 atoms unit cell with 4 distinct symmetry sites —outer tetrahedral (OT), inner tetrahedral (IT) octahedral (OH) cuboctahedral (CO).The γ-brass structure has a 52 atoms unit cell with 4 distinct symmetry sites —outer tetrahedral (OT), inner tetrahedral (IT) octahedral (OH) cuboctahedral (CO). The Pd-Zn γ-brass atomic arrangement been extensively studied by Edstrom and Westman through x-ray diffraction analysis [1]. Our density functional theory (DFT) surface energy calculations indicate that the most stable Pd₈Zn₄₄ facet is (1 -1 0), which exposes only monomers for Pd₈Zn₄₄, but includes Pd trimers for Pd_9-11Zn_43-41. We use DFT to consider H₂ dissociation, acetylene hydrogenation and crotonaldehyde hydrogenation mechanisms on Pd monomer and trimer sites. DFT calculations agree with experimental results that H₂ activation is faster on trimer sites, substantiating the formation of Pd₃ trimer sites on Pd₉Zn₄₃ catalyst surfaces. The activation barrier for H₂ dissociation is nearly identical experimentally on Pd₉, Pd₁₀ and Pd₁₁, further substantiating the isolation of the Pd trimer sites. DFT calculations indicate that acetylene binds strongly on monomer and trimer sites, whereas ethylene binds less strongly on monomer site than trimer sites. H₂ dissociation and binding adjacent to ethylene is only possible on the trimer sites. DFT calculations showed the monomer apparent barrier of ethylene hydrogenation is higher than the ethylene desorption barrier. Pd_9-11 with trimer on surface, lowers the ethylene hydrogenation barrier, compared with Pd₈Zn₄₄. The full path of acetylene hydrogenation on these isolated sites, as well as a microkinetic model for acetylene hydrogenation on these intermetallics, will be presented. The gamma-brass intermetallic structures offer isolated active sites with controlled nuclearity, allowing both the design of active and selective catalysts as well as the elucidation of site requirements.

References

[1] V.A. Edstrom and S. Westman. Acta Chemica Scandinavica. 23 279 (1969)