Determination of Bulk and Surface Atomic Arrangement in Ni–Zn γ

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Determination of Bulk and Surface Atomic Arrangement in Ni−Zn γ‑Brass Phase at Different Ni to Zn Ratios Charles S. Spanjers,†,∥ Anish Dasgupta,†,∥ Melanie Kirkham,‡ Blake A. Burger,† Gaurav Kumar,† Michael J. Janik,† and Robert M. Rioux*,†,§ †

Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37931, United States § Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡

S Supporting Information *

ABSTRACT: 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, I43̅ m, 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.

1. INTRODUCTION

Finally, a large and distorted cuboctahedron is formed by the CO site (multiplicity of 24). One set of nested polyhedra (26 atoms) is shown in Figure 1. The coordination geometry for each site is presented in Table 1. Although the early literature refers to the γ-brass phase simply as an alloy, it is actually an intermetallic as its crystal structure is well-defined and periodic rather than a random distribution of atoms.3 Bradley and Thewlis were unable to conclusively identify the ordering scheme of Cu and Zn in the lattice of γ-brass Cu5Zn8. Heidenstam was the first to present an acceptable description of the atomic distribution in the γ-brass lattice.4 In the following decades, many other γ-brass bimetallic and trimetallics have been studied, and their atomic site distribution was determined.5−10

Westgren and Phragmen were the first to observe the existence of the γ-brass phase for several bimetallic alloy systems (Au− Zn, Ag−Zn and Cu−Zn) nearly a century ago.1 They identified a 52 atom unit cell in each case. Bradley and Thewlis properly processed the existing X-ray diffraction data to accurately describe the γ-brass crystal structure as a system of 26 interpenetrating bcc unit cells.2 They deduced the space group to be I4̅3m and also hypothesized the existence and location of four unique symmetry sites through comparison with the βbrass structure. These unique symmetry sites are referred to as outer tetrahedral (OT), inner tetrahedral (IT), octahedral (OH), and cuboctahedral (CO). These sites form two sets of nested polyhedra, one at the origin and one at the center of the unit cell, due to body-centering. The IT site has multiplicity of 8 and forms a tetrahedron at the center of each polyhedral set. The OT site also has multiplicity of 8 and forms another tetrahedron around each of the inner tetrahedra. These are enveloped by two octahedra formed by 12 OH positions. © XXXX American Chemical Society

Received: May 1, 2016 Revised: December 9, 2016 Published: December 11, 2016 A

DOI: 10.1021/acs.chemmater.6b01769 Chem. Mater. XXXX, XXX, XXX−XXX

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the model of Heidenstam.11 They, along with others, have justified further study into various γ-brass crystal phases because of their occurrence at compositions close to quasiperiodic structures.13−15 Dong has suggested the γ-brass phases may be used as quasicrystal approximants not only on the basis of similar chemical composition, but also because of its orientational relationships with quasicrystals.16 Due to the unique and periodic crystal structure, the γ-brass phase may facilitate opportunities for making site-isolated catalysts which are important in many chemical processes. Ni is known to be a hydrogenation catalyst17 capable of replacing noble metals18 such as Pd. However, large Ni clusters lower the selectivity toward commercially important semihydrogenation reactions19,20 because of strong Ni−hydrocarbon interactions which promote oligomerization. Reliable synthesis of siteisolated Ni catalysts and γ-brass Ni−Zn provides a viable option to study semihydrogenation catalyst development.19 The γbrass phase is an intermetallic which means that it has a perfectly periodic crystal structure, and can better ensure site isolation and defined composition than a randomly substituted semihydrogenation alloy catalyst like Pd−Ag. Since catalysis is a surface phenomenon, predicting catalytic activity requires determining both atom site distributions in the bulk and which facets are favored to be exposed and the distribution of atoms on those surfaces. To the best of our knowledge, this is the first effort to predict the arrangement of surface atoms in the γ-brass phase, with the ultimate objective of designing model multinuclear site-isolated catalysts using bulk intermetallics. In this paper, we use neutron diffraction to determine that, upon increasing the Ni concentration above 15.4% (Ni8Zn44), additional Ni atoms substitute Zn at the OH positions in Ni9Zn43 and Ni10Zn42. This leads to the formation of potentially catalytically relevant Ni−Ni−Ni trimers that are absent in Ni8Zn44. Density functional theory (DFT) calculations indicate that addition of excess Ni to OH positions is energetically most favorable, supporting our neutron diffraction results. We have also investigated the presence of these trimers on the surface (and hence the potential of varying catalytic properties through subtle changes in stoichiometry) using H−D exchange and ethylene hydrogenation. Ni11Zn41 (Ni11, 21.2% Ni) is also included in the catalytic study since it also has the γ-brass structure and determine the most probable distribution of Ni and Zn on the exposed surface with DFT surface energy calculations and Wulff construction.21

Figure 1. Illustration of the sites in the γ-brass structure: inner tetrahedral (IT, red), outer tetrahedral (OT, blue), octahedral (OH, green), and cuboctahedral (CO, orange).

These studies have revealed that, despite similarities in crystal structure, the site occupation as well as the substitution patterns vary widely between systems. Some illustrative examples are provided here. In Pd−Zn γ-brass (15.4%−24% Pd) with 15.4% Pd, the OT site was fully occupied by Pd, and the remaining sites were occupied by Zn. When the Pd concentration was increased, the excess Pd atoms occupied the OH positions by substituting for Zn.6 In the Fe−Zn γ-brass (17.5−31.5% Fe) system, Fe fully occupies the IT site and partially the OH site (35−70%).5 In the Cu−Zn γ-brass (30−43% Cu), Gourdon et al. modified Heidenstam’s original model and predicted that Cu occupies the OT and OH sites, and Zn is found in the IT and CO sites in the prototype Cu5Zn8 structure. Upon varying the composition toward either the Cu-rich or Zn-rich regimes of the γ-brass phase, substitution occurs only at the OH and CO sites.11 The structure of the Ni−Zn γ-brass phase was first investigated by Schramm et al., who determined the lattice parameter versus composition in the range of approximately 15−20 at. % Ni.12 Relatively little progress has been made further refining the structure of the Ni−Zn γ-brass phase (15.4−24% Ni) since the late 1960s when Johansson improved on the work of Schramm12 by identifying the ordering of Ni and Zn atoms in the Ni8Zn44 crystal structure.5 They were able to assign Ni occupation to the OT site, and the rest of the sites to Zn. These results imply that, in Ni8Zn44, the Ni atoms are coordinated by 12 Zn atoms. However, they failed to reach any conclusions for higher Ni concentrations. Recently, Gourdon et al. have rekindled interest in γ-brass structures by re-evaluating Cu5Zn8 using both neutron diffraction as well as theoretical calculations to improve upon

2. EXPERIMENTAL METHODS 2.1. Synthesis of Ni−Zn Catalysts. Four Ni−Zn samples with the γ-brass structure were synthesized by adding Ni powder (SigmaAldrich, 15.4%) γbrass intermetallics from a catalytic viewpoint. For a mirrored surface slab of identical stoichiometry with the bulk, the DFT surface energy can be calculated and referenced against the bulk energy.44 In our case, the mirrored surfaces are nonstoichiometric, and this discrepancy can be made up by the addition/subtraction of either Ni or Zn as indicated in eq 3. The right-hand side of eq 3 represents the nonstoichiometric mirrored surface formed by cleaving the optimized bulk structure. The values of f and x are calculated to ensure a balanced equation. The number of makeup atoms (x)

Figure 5. Relative bulk energy (with the lowest energy configuration representing the datum) of Ni10Zn42 when 8 Ni atoms are placed on the OT positions and the two excess Ni atoms are positioned in different combinations of OH, IT, and CO positions plotted against the distance between the excess Ni atoms normalized over their respective DFT optimized lattice constant.

DFT calculations support our neutron diffraction studies for the Ni10Zn42 sample. 3.3. Experimental Kinetic Results. It is extremely difficult to directly image the precise arrangement of atoms on the surface of a material, particularly when they are neighbors in the periodic table as in Ni−Zn intermetallics. Simple probe reactions can instead be used to characterize a surface as catalytic kinetics are directly affected by the properties of the surface. H2−D2 exchange is a well-documented probe reaction in understanding catalytic structure function relations in many related metallic and alloy systems.35−39 Since Ni is active for hydrogen dissociation and Zn is not, it is predicted that contiguous active (Ni) sites in Ni9Zn43, Ni10Zn42, and Ni11Zn41 would perform differently than Ni8Zn44 for a H2−D2 exchange reaction due to the presence of Ni−Ni−Ni trimers on the surface. The catalytic H2−D2 exchange reaction is described by the following equation: K eq

H 2 + D2 ←→ 2HD

rnet [HD]2 , where η = (1 − η) [H 2][D2 ]Keq

(1)

The forward reaction rate (rf) is defined in terms of the measured rate of reaction, rnet, and the equilibrium constant, Keq F

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Figure 6. Arrhenius plots for the catalytic H2−D2 exchange on (a) Ni8Zn44, (b) Ni9Zn43, (c) Ni10Zn42, and (d) Ni11Zn41 in a plug flow reactor with 10 mL/min H2 and 10 mL/min D2 at a conversion of 1−10% and a temperature ramp rate of 1 °C/min. Red points indicate data from first analysis, and blue points represent data from second analysis. Black lines are linear fits to the data.

Table 3. Catalytic Results for H2−D2 Exchange on Ni−Zn Catalysts sample Ni8Zn44 Ni9Zn43 Ni10Zn42 Ni11Zn41 Ni

BET surface area (m2/g) 1.06 1.38 1.33 1.26 0.45

± ± ± ± ±

0.08 0.12 0.04 0.09 0.005

Eapp (kJ mol−1) 55 53 55 57 37

± ± ± ± ±

2 2 2 2 0.2a

Table 4. Surface Energy and Surface Area Contribution to Equilibrium Shape

rate @ 150 °C (107 mol m−2 s−1) 4 4 8 6 42650

± ± ± ± ±

1 1 1 1 2270a

Physical mixture (100 mg) of Ni and SiC with a Ni concentration of 4600 ppm.

required is lower for Ni, and therefore, Ni was chosen over Zn to make up the stoichiometric discrepancy between the bulk and the surface in all the calculations. The surface energy (Esurface) for the mirrored slab normalized over surface area (2A) is calculated by eq 4. f (Ni 9Zn43)bulk + x(Ni/Zn)

⎯⎯⎯⎯→ (Ni nZn m)surface

energy (eV/Å 2)

fractional exposed area

(001) (11̅0) (111)

0.13 0.15 0.12

0.61 0 0.39

Ni; as such, trimers can only occur on {11̅0} facets, and no trimers are seen on {111} or {001} facets. Our DFT calculations suggest that exposure of Ni trimer sites is energetically unfavorable on the surface of Ni-rich (>15.4%) γ-brass Ni−Zn phases. This, in conjunction with our kinetic experimental results, strongly suggests that the exposed surface for both Ni8Zn44 as well as Ni-rich (>15.4%) γ-brass phases are essentially identical with completely isolated Ni atoms in Zn matrices and negligible Ni−Ni coordination.

a

bulk yields

facet

4. CONCLUSIONS We have identified, through neutron diffraction and DFT calculations, that additional Ni atoms (>8/unit cell) occupy the OH site in the Ni-rich (more than 15.4%) Ni−Zn γ-brass phase. This leads to the formation of Ni−Ni−Ni trimers which are not found in Ni8Zn44. However, we saw no evidence of change in catalytic performance for H−D exchange or ethylene hydrogenation as the Ni concentration increased from 15.4% to 21.1%, which suggests the catalyst surface remains unchanged and does not contain any trimer sites. Surface energy calculations using DFT show that this is due to the negligibly small contribution of the trimer containing {11̅0} surface to the equilibrium particle shape. We conclude that all γ-brass Ni−Zn phases have identical catalytic properties, and there is no effect,

(3)

[ENinZnm − f (ENi 9Zn43) − x(ENi)] (4) 2A DFT calculated surface energies for the three low index surfaces ((111), (001), and trimer containing (11̅0)) are shown in Table 4. A Wulff shape was constructed on the basis of these surface energies using Wulffmaker.30 The fractional contribution of each facet to the equilibrium particle shape (Figure S7) is also shown in Table 4. The high energy trimer containing the {11̅0} facet has no contribution to the equilibrium shape. A trimer is formed by replacement of Zn in OH position(s) by E surface =

G

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from a catalytic perspective, from the increase in Ni loading in these catalysts.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b01769. X-ray diffractograms and SEM images, description of Rietveld refinement procedure and additional results, and Arrhenius plots for ethylene hydrogenation (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (814) 867-2503. ORCID

Melanie Kirkham: 0000-0001-8411-9751 Michael J. Janik: 0000-0001-9975-0650 Robert M. Rioux: 0000-0002-6019-0032 Author Contributions ∥

C.S.S. and A.D. made equal contributions to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the donors of The American Chemical Society Petroleum Research Fund (ACS PRF #50794-DN15). C.S.S. acknowledges the National Science Foundation under Grant No. DGE1255832. Any opinions, findings, and conclusions expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. This work also used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the National Science Foundation Grant ACI1053575. XRD experiments were performed at the Materials Characterization Laboratory’s shared user facility at The Pennsylvania State University. The facility is supported in part by MRSEC, Center for Nanoscale Sciences, under NSF Award DMR-1420620. We would also like to acknowledge Mr. Zhifeng Chen (Department of Chemical Engineering, The Pennsylvania State University) for assisting us with the SEM images presented in Figure S5 and contributing to section 2.6,, of this manuscript.

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DOI: 10.1021/acs.chemmater.6b01769 Chem. Mater. XXXX, XXX, XXX−XXX