(718e) Ni-Based Bimetallic Catalysts for Acetylene Semi-Hydrogenation

Spanjers, C., Penn State
Held, J., Penn State
Jana, S., Pennsylvania State University
Janik, M. J., Pennsylvania State University

The removal of trace acetylene in
ethylene streams destined for polyethylene production is a high-volume
industrial catalytic-based process that must demonstrate high selective for
alkyne hydrogenation over alkene hydrogenation. 
The current industrial catalyst is a Pd-Ag alloy; Ag dilutes Pd
ensembles that are unselective toward acetylene/ethylene hydrogenation.  The inclusion of Ag substantially reduces
activity but increases selectivity to acetylene semi-hydrogenation.  Recent research efforts to replace Pd-Ag
catalysts with base-metal catalysts demonstrated Ni-Zn alloy catalysts had comparable
activity and selectivity [1].  Additionally,
the computational aspects of this work demonstrated the activity and selectivity
were highly sensitive to the composition of the Ni-Zn alloy catalyst.

We report on the synthesis and
characterization of Ni-Zn catalysts that exhibit a higher selectivity than Pd-Ag
for acetylene semi-hydrogenation in excess ethylene.  Ni nanoparticles (NPs) with an average
diameter of 3, 6 and 10 nm synthesized by a high-temperature colloidal method
were converted into Ni-Zn NPs by the injection of diethylzinc
into hot solvent containing Ni NPs. 
Wide-angle XRD demonstrates Ni NPs ? regardless of their initial size ?
incorporated Zn in a 1:1 atomic ratio with respect to Ni.  The atomic composition of the Ni-Zn sample
could not be altered by changing the amount of diethylzinc
addition.  The Ni-Zn NPs were subsequently
adsorbed from solution on the surface of mesoporous
SBA-15 silica and further characterized.

X-ray absorption near-edge structure
(XANES) and extended x-ray absorption fine structure (EXAFS) were used to
determine the oxidation state and local coordination environment of the metal
atoms in Ni-Zn NPs.  XANES results
demonstrate reduced Ni and Ni-Zn NPs form during colloidal syntheses with no required
reductive pretreatment of the supported NPs.  This is an inherent advantage of the colloidal
route compared to Ni-Zn catalysts prepared by traditional simultaneous or
co-impregnation synthetic routes.  The
latter catalysts must be reduced and the disparate thermal stability of Ni and
Zn lead to catalysts with uncontrollable Ni:Zn ratios. 

Ni-Zn NPs are much more selective than
Ni NPs for acetylene semi-hydrogenation which is in agreement with previous
experimental and theoretical results [1]. 
DFT-based reaction energy calculations for acetylene and ethylene
hydrogenation were compared over Ni and Ni-Zn surfaces to determine the origin
of the experimentally-observed selectivity enhancement on the bimetallic
catalyst.  A sequential hydrogenation
mechanism from acetylene to ethane is not sufficient to capture the selectivity
enhancement over Ni-Zn compared with Ni. 
Additional in-situ
characterization of the Ni and Ni-Zn catalysts was conducted to rectify the
apparent contradictions between the DFT calculations and experimental

Studt et al.,
Science 320 (2008) 1320-1322

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