Investigation of Au, Pd, and AuPd Nanoparticle Catalysts for Alcohol Oxidation Reactions
The focus of this thesis is on the synthesis, analysis, and applications of Au, Pd, and AuPd nanoparticle catalysts. The primary focus of this work is their use as catalysts for the oxidation of a variety of alcohols and the characterization of these materials by x-ray absorption spectroscopy to further understand their structure/property relationships. The first project of this thesis consists of the synthesis of a series of Au, Pd, and AuPd nanoparticles by chemical reduction for the use in nanoparticle-catalyzed oxidations of a variety of α,β-unsaturated alcohols. It was seen that Pd NPs, K2PdCl4, AuPd nanoparticles, and Au nanoparticles mixed with K2PdCl4 were active catalysts for the oxidation of α,β-unsaturated alcohols in water at 60°C, with no added base, using O2 as the oxidant. However, monometallic Au nanoparticles showed little to no activity for all substrates in this system. Monometallic Pd nanoparticles, in the absence of Au, showed high activities for reactions in ionic liquids due to the ease of Pd reduction in this environment. Structural analyses of the nanoparticles before and after reactions was performed by TEM and EXAFS. It was found that, during reactions catalyzed by Au nanoparticles and K2PdCl4, bimetallic AuPd nanoparticles were formed in-situ during the reaction. All of the other systems showed significant Ostwald ripening, indicating a redox mechanism for these reactions. The second project focuses on the analysis of the formation of bimetallic AuPd NPs in-situ during the Au nanoparticle/K2PdCl4 catalyzed oxidation of α,β-unsaturated alcohols. The room temperature oxidation of crotyl alcohol using the aforementioned catalyst system was studied by in-situ x-ray absorption spectroscopy (both x-ray absorption near edge spectroscopy iii and extended x-ray absorption fine structure spectroscopy). The mechanism by which this reaction occurs is poorly understood and remains unclear. Two proposed mechanisms for this system are a redox reaction involving Pd ions as the active species and a β-hydride elimination reaction. Studying the changes in Pd during the reaction by time-resolved, quick scan x-ray absorption spectroscopy allowed us to gain information about Pd speciation, and kinetics of the reduction reaction. It was determined that, upon addition of crotyl alcohol to an aqueous mixture of Au nanoparticles and K2PdCl4, the Pd(II) quickly and completely reduced onto the Au nanoparticles, forming bimetallic nanoparticles. The susceptibility of nanoparticles formed in-situ to oxidation was also tested. It was found that these particles are very stable, unaffected by exposure to molecular oxygen for up to seven hours. This suggests that Au in the system prevents the re-oxidation of Pd(0) from the surface of the bimetallic nanoparticles.
Master of Science (M.Sc.)