The Synthesis of Novel Metal and Metal Oxide Nanoparticles with Applications Towards Catalysis
This dissertation describes the synthesis of novel nanoparticles that are interesting for catalytic applications. The decomposition of RhCp(C2H4)2 and Rh(hfacac)(CO)2 were investigated, and the complex RhCp(C2H4)2 was successfully shown to decompose to rhodium nanoparticles. Analysis of the decomposition chemistry was used to control nanoparticle seed formation and growth. New stabilizer ligands, both polymeric and molecular, were attempted for the synthesis of rhodium nanoparticles. Polymeric stabilizers were screened as replacements for the widely used polyvinylpyrollidone (PVP) surfactant, however none afforded the high degree of control exhibited by PVP. However, molecular stabilizers were screened and small, monodisperse rhodium nanoparticles were synthesized with the stabilizer octadecylphosphonic acid, with a size and size dispersity of 1.92 +/-0.16 nm.
A concurrent hydrogenation catalytic process was also utilized for the synthesis of small rhodium seed particles. In this nanoparticle synthesis, a rhodium precursor and a stabilizer were combined in the presence of an olefin and hydrogen, which aids in decomposition of the rhodium precursor to nanoparticles, and also catalytically converts the olefin to a saturated compound. The rate of hydrogen uptake was monitored and fit to a two-step autocatalytic mechanism correlated to nanoparticle formation and growth. Two new rhodium complexes were synthesized that contained a stabilizer ligand, however the most successful attempt to produce small, monodisperse rhodium nanoparticles by this process was with the rhodium source [(COD)Rh(NCCH3)2]BF4, and the stabilizer (Bu4N)2HPO4 in the presence of an equivalent of Proton Sponge. Rhodium nanoparticles synthesized by this process have a size and size distribution of 1.88 +/-0.27 nm. The presence of olefin and hydrogen pressure of 42 psi was found to be ideal for the stabilization of nanoparticles during their formation. Also, reactant concentrations and the rate of the cyclohexene consumption are crucial to yield nanoparticles with this excellent size dispersity. Growth reactions with these small rhodium nanoparticles have been successful the synthesis of larger nanoparticles under conditions involving alternate stabilizers.
The small nanoparticles were then tested and found to be useful as seed particles in the synthesis of larger rhodium nanoparticles. For each procedure, a mixture of 1-hexadecylamine, adamantane carboxylic acid, and 1,2-hexadecanediol was used to stabilize the nanoparticles. The use of synthesized seed particles allowed for the formation of tetrahedral (average edge length: 4.77 +/- 0.72 nm) or icosahedral shaped particles, depending on reaction temperature. Subsequent characterization revealed that approximately half of the tetrahedrally shaped nanoparticles are in fact triangular flat rafts, where one corner of the tetrahedron appears to be "cut off." However, the use of in situ seeds resulted in the formation of multipod structures. The multipods are single crystals with 2-8 arms per multipod, that propagate both the (110) and (111) directions.
The synthesis and characterization of mixed-metal oxide spinel nanoparticles was then attempted for water oxidation catalysis. Nanoparticles of the compositions MnFe2O4 and CoFe2O4 (5.7 nm and 6.1 nm respectively) were synthesized according to a literature procedure with the stabilizers oleic acid and oleylamine, however they were characterized by ICP-OES to have low M:Fe (M = Mn, Co) ratios of 1:5 and 1:4 respectively. Nanoparticles of NiFe2O4 (8.0 nm) were also synthesized by a similar approach, and had the expected Ni:Fe ratio of 1:2 by ICP-OES. Cubic nanoparticles of Co3O4 were also synthesized, and through a subsequent cation exchange reaction with this material, CuxCo3-xO4 and NixCo3-xO4 nanoparticles could be synthesized with varying degrees of copper or nickel incorporation. Linear scan voltammograms were conducted on anodes modified with these nanoparticle materials. For the mixed-metal ferrites, CoFe2O4 showed the lowest overpotentials in the water oxidation reaction in the range of 0-100 mA cm-2. Copper modified Co3O4 nanoparticles had a lower onset potential than Co3O4 and performed with lower overpotentials at low current densities (<20 mA cm-2), however they suffered from higher overpotentials when compared to Co3O4 at higher current densities ( >20 mA cm-2). The nickel modified Co3O4 nanoparticles were superior to the other MxCo3-xO4 materials at all current densities measured (0-100 mA cm-2).