Synthesis of Nanometer-size Inorganic Materials for the Examination of Particle Size Effects on Heterogeneous Catalysis

Emerson, Sean Christian

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Synthesis of Nanometer-size Inorganic Materials for the Examination of Particle Size Effects on Heterogeneous Catalysis

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The effect of acoustic and hydrodynamic cavitation on the precipitation of inorganic catalytic materials was investigated. The overall objective was to understand the fundamental factors involved in synthesizing nanometer-size catalytic materials in the 1â€“10 nm range in a cavitating field. Materials with grain sizes in this range have been associated with enhanced catalytic activity compared to larger grain size materials. A new chemical approach was used to produce titania supported gold by coprecipitation with higher gold yields compared to other synthesis methods. Using this approach, it was determined that acoustic cavitation was unable to influence the gold mean crystallite size compared to non-sonicated catalysts. However, gold concentration on the catalysts was found to be very important for CO oxidation activity. By decreasing the gold concentration from a weight loading of 0.50% down to approximately 0.05%, the rate of reaction per mole of gold was found to increase by a factor of 19. Hydrodynamic cavitation at low pressures (6.9â€“48 bar) was determined to have no effect on gold crystallite size at a fixed gold content for the same precipitation technique used in the acoustic cavitation studies. By changing the chemistry of the precipitation system, however, it was found that a synergy existed between the dilution of the gold precursor solution, the orifice diameter, and the reducing agent addition rate. Individually, these factors were found to have little effect and only their interaction allowed gold grain size control in the range of 8â€“80 nm. Further modification of the system chemistry and the use of hydrodynamic cavitation at pressures in excess of 690 bar allowed the systematic control of gold crystallite size in the range of 2â€“9 nm for catalysts containing (2.27 Â± 0.17)% gold. In addition, it was shown that the enhanced mixing due to cavitation led to larger gold yields compared to classical syntheses. The control of gold grain size was gained at the loss of CO activity, which was attributed to the formation of non-removable sodium titanate species. The increased mixing associated with cavitation contributed to the activity loss by partially burying the gold and incorporating more of the sodium titanate species into the catalysts. This work produced the first evidence of hydrodynamic cavitation influencing the gold crystallite size on titania supported gold catalysts and is the only study reporting the control of grain size by simple mechanical adjustment of the experimental parameters. Despite the low activity observed due to sodium titanate, the methodology of adjusting the chemistry of a precipitating system could be used to eliminate such species. The approach of modifying the chemical precipitation kinetics relative to the dynamics of cavitation offers a general scheme for future research on cavitational processing effects.

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