Study on Novel Method of Catalyst Preparation Using Molecule Assembly and Control of Catalyst Microstructure

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The purpose of this work is a microstructure control of supported catalysts for improving the catalytic activity and the product selectivities. Thus, we proposed a novel preparation method of the supported catalysts using microemulsions, and studied the control of metal micropartticle size, regardless of metal content, and the improvement of the radio of metal particle surface exposed on surface to total metal particle surface. The results are as follows. [1] The silica- or zirconia-supported noble metal catalysts prepared by the microemulsion method exhibited an extremely higher activity than those prepared by the impregnation method for the CO and CO_2 hydrogenation. [2] The control of Rh particle size irrespective of Rh content has been studied on the Rh/SiO_2 catalysts. It was found that the Rh particle size in the range between 3 and 6 nm was controlled by the water core size inside the invert micelle and the concentration of RhCl_3. Furthermore, the Rh particle size in the wide range of 1.5 to 9 nm could be controlled by the synthesis temperature of the Rh particles in the microemulsion, the chain length of the hydrophilic group of the surfactant and the chain length of the organic solvent. For the Pd/ZrO_2 catalysts, the Pd particle size could also be controlled in the same manner. [3] The Rh/SiO_2 catalyst with the Rh particle size of about 5 nm had the highest turnover frequency for the CO hydrogenation. [4] It was suggested that some of the Rh particles prepared by the microemulsion method were partly or wholly buried in the silica support. Thus we investigated the effect of the catalyst preparation conditions on the ratio of the Rh surface area exposed to the gas phase to the total Rh surface area. The Rh exposure ratio of about 80% was obtained by the shortening of hydrolysis time of alkoxide and the increase of the water content during hydrolysis period. The increase of the Rh exposure ratio enhanced the catalytic activity for the CO hydrogenation.