281161 Ni-Based Bimetallic Catalysts for Acetylene Semi-Hydrogenation

Thursday, November 1, 2012: 1:50 PM
318 (Convention Center )
Charles Spanjers, Jacob Held, Subhra Jana, Michael J. Janik and Robert M. Rioux, Chemical Engineering, Pennsylvania State University, University Park, PA

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

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