Ph.D. Defense:
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 crystallite sizes.

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.