(483d) Design of Heterogeneous Catalyst By Stoichiometric Tuning of Intermetallic Gamma-Brass Crystal Structure

DFT Studies of
Intermetallic Gamma-Brass Crystal Structures for Selective Hydrogenation

Haoran He, Anish Dasgupta, Gaurav Kumar, Robert M. Rioux, and Michael J. Janik

Department of
Chemical Engineering, The Pennsylvania State
University, University Park, PA 16802, USA

*mjanik@ psu.edu; (814) 863-9366

Bimetallic
compounds can offer tunable site electronics and ensemble structure for
selective hydrogenation catalysis. In this study, we consider the γ-brass
phase (Cu₅Zn₈ prototype), in
order to expose surfaces with controlled M_x
nuclearity to control the selectivity for
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)
as shown in Figure 1. In particular, the Pd-Zn
γ-brass atomic arrangement as well as the substitution pattern of Zn by Pd in the Pd-Zn γ-brass
phase (15.4-24%) has been extensively studied by Edstrom and Westman through x-ray diffraction analysis[1].
Surface energy calculations indicated that the most stable Pd₈Zn₄₄
facet is (1 -1 0), which exposes only monomers for Pd₈, but includes
Pd trimers for Pd_9-11. We vary the number
of Pd atoms per isolated active site and investigate
its effect on H₂ dissociation and acetylene hydrogenation
mechanisms. DFT calculations agreed 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 both monomer bridge and trimer
sites, whereas ethylene binds strongly on monomer atop and trimer side atop
sites. At the same time, H₂ dissociation and binding adjacent to
ethylene is only possible on the trimer sites. DFT calculations showed the
apparent barrier of ethylene hydrogenation is higher than the ethylene
desorption barrier, which indicates Pd₈Zn₄₄ is superior
catalyst in selectively hydrogenating acetylene to ethylene.  Pd_9-11 on the other hand, contains
trimers on the surface, which can lower 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. 

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).

References

1.       
Edstrom, V.A and S. Westman,
X-ray Determination of Structure of Cubic Gamma Pd,
Zn Phase. Acta Chemica Scandinavica, 1969. 23 (1):
p.279-&. ADDIN EN.REFLIST