The synthesis, characterization and catalytic reaction studies of monodisperse platinum nanoparticles in mesoporous oxide materials

A catalyst design program was implemented in which Pt nanoparticles, either of monodisperse size and/or shape were synthesized, characterized and studied in a number of hydrocarbon conversion reactions. The novel preparation of these materials enables exquisite control over their physical and chemical properties that could be controlled (and therefore rationally tuned) during synthesis. The ability to synthesize rather than prepare catalysts followed by thorough characterization enable accurate structure-function relationships to be elucidated. Pt nanoparticles (1.7--7.1 nm) are synthesized by solution phase reduction methods in which Pt precursors are reduced in protic solvents in the presence of a surface templating polymer, which serves to stabilize the metal nanoparticles in solution. Particle size can be controlled during synthesis by altering either the PVP: Pt salt ratio, reaction media and by seeded growth methods. After Pt nanoparticles are synthesized, their size and morphology are confirmed with transmission electron microscopy and x-ray diffraction. Low power sonication in either aqueous or organic solvent was utilized to disperse Pt nanoparticles within the mesoporous metal oxide matrix. This method of catalyst synthesis is named capillary inclusion (CI). An alternative approach to catalyst synthesis combines the hydrothermal synthesis of mesoporous silica with Pt nanoparticle synthesis in the same solution. Synthesis under neutral conditions led to a catalyst in which the nanoparticles were highly dispersed throughout the catalyst matrix. This method of catalyst synthesis called nanoparticle encapsulation (NE) ensured that Pt nanoparticles were located on the internal pore surface of the mesoporous silica. Catalysts were characterized by transmission electron microscopy (TEM), x-ray diffraction (XRD), small angle x-ray scattering (SAXS) and physical adsorption to determine metal particle size and mesoporous structure. The surface chemistry of the nanoparticle surface was studied by infrared spectroscopy, selective gas adsorption (chemisorption) and catalytic reactivity studies. During nanoparticle synthesis, PVP is added to the solution to stabilize the platinum nanoparticles against aggregation. This polymer remains bound to the nanoparticle surface after catalyst synthesis and must be removed before catalytic reactions. Calcination of the catalyst at high temperature (623--723 K) for long time periods (24--36 hours) followed by reduction was initially used to clean the Pt surface. (Abstract shortened by UMI.)...