Situ Phase Separation of NiAu Alloy Nanoparticles for Preparing
Highly Active Au/NiO CO Oxidation Catalysts
Shenghu Zhou, Hongfeng Yin, Viviane Schwartz, Zili Wu, David Mullins,
Bryan Eichhorn, Steven H. Overbury, and Sheng Dai
A new type of catalyst has been synthesized which has high activity
and a low propensity for deactivation by coalescence. The concept is
based upon an architecture in which a nanosized metal particle is stabilized
by its strong attachment to an oxide support particle of comparable
size. The resulting heterostructure can be manipulated and deposited
as a unit onto another high surface area oxide to anchor the particles
and isolate them from one another. Synthesis of these heterostructures
progresses by a novel pathway in which a bimetallic particle is first
prepared by a colloid synthesis using a mixture of Au and Ni precursors
dissolved along with a reducing agent and surfactants to control and
stabilize the colloidal particles. Crucially, kinetic limitations trap
the reduced Ni and Au into a meta-stable, fully mixed alloy, thereby
preventing separation into separate unmixed particles of Ni and Au.
This association is exploited in two subsequent treatments that lead
to a hetero-aggregate of Au and Ni oxide. Analysis techniques available
at the CNMS and at the National Synchrotron Light Source provided confirmation
of the structure of the particles at each step of the synthesis.
A key problem in catalysis is the stabilization of metallic and bi-metallic
particles under the high temperature conditions needed for pre-treatments
or at which they may be used. High temperature causes metal particles
to diffuse and then sinter together, a process that results in loss
of surface area and thus loss of catalytic activity. This study describes
the first solution-based synthesis of NiAu alloy nanoparticles by
way of a fast butyllithium reduction method. By supporting the particles
on SiO2 and subsequent conditioning, one obtains a NiO-stabilized
Au nanoparticle catalyst that exhibits remarkable resistance to sintering
and is highly active for CO oxidation. In contrast, the corresponding
NiO-free Au nanoparticles prepared by an analogous method show negligible
low-temperature catalytic activity and a high propensity for coalescence.
The method is general and demonstrates a key concept for producing
a new class of catalysts: hetero-aggregates based on alloy nanoparticle
precursors. This method could potentially be extended to other catalytic
systems plagued by serious sintering problems and to the rational
synthesis of bifunctional catalysts requiring a close spatial arrangement
between two active components.
S., Ma, Z., Yin, H., Wu, Z., Eichhorn, B., Overbury, S. H., Dai,
S., "Low-Temperature Solution-Phase Synthesis of NiAu Alloy
Nanoparticles via Butyllithium Reduction: Influences of Synthesis
Details and Application
as the Precursor to Active Au-NiO/SiO2 Catalysts Through Proper
Phys. Chem. C 113, 5758 (2009).
S. H., Yin, H. F., Schwartz, V., Wu, Z. L., Mullins, D., Eichhorn,
S. H., Dai, S., "In Situ Phase Separation of NiAu
Alloy Nanoparticles for Preparing Highly Active Au/NiO CO Oxidation
Catalysts," Chemphyschem 9, 2475 (2008).
work was conducted at Center for Nanophase Materials Sciences at
Oak Ridge National Laboratory, and was supported by Basic Energy
Sciences, DOE. S. Zhou and H. Yin acknowledge the ORNL postdoctoral
Research Associates Program. Bryan Eichhorn is supported by the
NSF (Grant No. 0401850).
nanoparticles consisting of nearly equal equimolar amounts of
Ni and Au are prepared in solution via a colloidal synthesis.
The method is based upon a fast butyl-lithium reduction that
simultaneously reduces the Ni and Au precursors and kinetically
traps the NiAu alloy phase. This process forms a metastable alloy
and circumvents the equilibrium immiscibility of Ni and Au in
alloy form. Subsequent high temperature reduction in H2 leads
to dealloying and Ni segregation, and subsequent oxidation leads
to formation of the NiO phase.
||Micrographs showing the uniformity of the nanosized NiAu particles.
TEM suggests that the nanoparticles are single phase alloy crystals,
a conclusion supported by x-ray diffraction and energy dispersive
analysis of the particles.
demonstrates that oxidizing the reduced NiAu particle at 300°C
causes a NiO phase to form.