In 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

Achievement
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.

Significance
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.

References:

1. Zhou, 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 Pretreatments," J. Phys. Chem. C 113, 5758 (2009).
2. Zhou, S. H., Yin, H. F., Schwartz, V., Wu, Z. L., Mullins, D., Eichhorn, B., Overbury, 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).
   

This 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).

NiAu 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.
EXAFS convincingly demonstrates that oxidizing the reduced NiAu particle at 300°C causes a NiO phase to form.