Synthesis of Nanostructured Inorganic Materials for Enhanced Heterogeneous Catalysis
Thursday, March 21, 2002
Goddard Hall, Room 227
The synthesis of nanostructured inorganic materials by hydrodynamic
cavitation processing was investigated. The goal of this work was to
develop a general synthesis technique for nanostructured materials with
a control over crystallite size in the 1-20 nm
range. Materials with crystallite sizes in this range have shown
enhanced catalytic activity compared to materials with larger
Several supported and unsupported inorganic materials were studied to
understand the effects of cavitation on crystallite size. Cavitation
processing of calcium fluoride resulted in more spherical particles,
attached to one another by melted necks. This work produced the first
evidence of shock wave heating of nanostructured materials by
hydrodynamic cavitation processing.
Hydrodynamic cavitation synthesis of various catalytic support
materials indicated that their phase composition and purity could be
controlled by adjustment of the processing parameters.
Zirconia/alumina supports synthesized using hydrodynamic cavitation
and calcined to 1368 kelvin retained a high purity cubic zirconia
phase, whereas classically prepared samples showed a phase
transformation to monoclinic zirconia. Similarly, the synthesis of
alumina resulted in materials with varying Böhmite and Bayerite
contents as a function of the process parameters. High temperature
calcination resulted in stable alumina supports with varying amounts of
δ-, and θ-alumina.
Synthesis studies of palladium and silver showed modest
variations in crystallite size as a function of cavitation process
parameters. Calcination resulted in larger grain materials,
indicating a disappearance of intergrain boundaries.
Based on these results, a new synthesis method was studied involving
controlled agglomeration of small silver crystallites by hydrodynamic
cavitation processing, followed by deposition on alumina.
The optimal pH, concentration, and processing time for controlling the
silver crystallite size in the cavitation equipment were determined
using a statistical design of experiments approach. Three series of
alumina supported silver catalysts were prepared, with silver weight
loadings of 1%, 2%, and 5%. Variation of cavitation processing time
between 1-64 min allowed the systematic control of silver
crystallite size in the range of 3-19 nm.
The preferred oxidation of CO in hydrogen (PROX) was chosen as a
catalytic test reaction, because of its increasing importance for fuel
cell applications. It was found that the catalytic activity was
significantly increased for silver crystallite sizes below 5 nm.
This work is the first experimental evidence of independent crystallite
size control by hydrodynamic cavitation for alumina supported silver
catalysts. The synthesis method involving controlled agglomeration and
calcination is a general synthesis procedure that can be used to
synthesize a wide range of novel catalysts and advanced materials.