Determination of Bulk and Surface Atomic Arrangement in Ni–Zn γ‑Brass Phase at Different Ni to Zn Ratios
journal contributionposted on 11.12.2016 by Charles S. Spanjers, Anish Dasgupta, Melanie Kirkham, Blake A. Burger, Gaurav Kumar, Michael J. Janik, Robert M. Rioux
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Previous attempts to characterize the γ-brass crystal structure of Ni–Zn (15.4–24% Ni) have failed to identify the location of the Ni and Zn atoms in the crystal lattice for more than 15.4% Ni content (Ni8Zn44) due to the similar X-ray diffraction cross sections of Ni and Zn. Ni8Zn44 is known to have a typical γ-brass crystal structure (space group 217, I4̅3m, 52 atom unit cell with four distinct symmetry positions: inner tetrahedral, outer tetrahedral, octahedral, and cuboctahedral) where Ni atoms reside in outer tetrahedral sites completely isolated from each other and coordinated by 12 Zn atoms. We utilize neutron diffraction to identify the substitution positions of Zn by Ni when the Ni content is increased above 15.4% and up to 19.2% (Ni10Zn42). Upon increasing the Ni content above 15.4% (Ni9Zn43 and Ni10Zn42), Zn in the γ-brass octahedral positions are replaced by Ni leading to the formation of Ni–Ni–Ni trimers, which are absent in Ni8Zn44. Density functional theory (DFT) calculations confirm our neutron diffraction results regarding the optimal position of excess Ni in the γ-brass unit cell. The well-defined atomic site distribution in γ-brass Ni–Zn provides an excellent opportunity for producing site-isolated base metal catalysts that may find application in selective semihydrogenation. We investigated the presence of Ni–Ni–Ni trimers on the surface using H–D exchange and ethylene hydrogenation as probe reactions, observing the influence of Ni concentration on catalysis. We conclude the catalytic performance is insensitive to Ni content. We provide a possible explanation for this observation using DFT calculations, which demonstrate that surface containing trimer sites are energetically unfavorable and therefore not exposed on Wulff reconstructions of γ-brass phase Ni–Zn particles.