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

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
observations. 

1.     
F.
Studt et al.,
Science 320 (2008) 1320-1322