Investigation of Graphene Oxide as a Cooperative Catalytic Support and Coating

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Graphene and graphene oxide (GO), a highly oxidized form of graphene, are materials with incredibly interesting chemical and mechanical properties. These materials have high surface areas and electrical properties that can be tuned by reducing the amount of oxygenates on the surface. They have already demonstrated their importance in the future of nanotechnology, while their application as a catalyst is just beginning to emerge both as a catalytic support for metal nanoparticles as well as carbon based metal-free catalysts for a variety of chemical transformations. As a metal-free catalyst, we explored using a 3D graphene oxide framework to create catalytic “pockets” in which we could engineer a micro- or nano- environment to influence catalytic performance. The design target for these ‘pockets' was to create a partially nanoconfined or sterically hindered environment in which functional groups are placed near each other to work cooperatively to influence the catalytic active site or the reaction mechanism. We synthesized a 3D GO structure with tunable d-spacing that retained free amine functional groups in the nano-environments. We were also interested in gaining an understanding of GO’s influence on metal nanoparticle catalysis. Gold nanoparticles (AuNPs) were chosen as the metal catalyst of study because they are active for a wide variety of reactions and the reaction rate and selectivity are often highly dependent on the size of the nanoparticle (NP) and the nature of the support. We hypothesized that the catalytic activity of AuNPs on a metal oxide support could be influenced by a graphene oxide over-coat by: 1) reducing metal leaching during reaction, 2) controlling the monodisperisity of the AuNPs, 3) and by modifying catalytic activity by electron donation/withdrawal to the active site or by altering the nanoparticle environment. In this work small (<5 nanometers) AuNPs were formed on an amine decorated nonporous silica support and submicron GO was electrostatically wrapped around the silica effectively coating the gold and silica. The catalytic activity for these gold nanoparticles versus non coated particles was tested for the catalytic reduction of p-nitrobenzaldehyde, the oxidation of cyclohexene, and oxidation of cyclooctene. After catalytic testing the GO was determined to be unsuccessful in reducing metal leaching, or preventing the agglomeration of AuNPs during synthesis or reaction, and no catalytic difference was observed in these reactions for catalysts with or without a GO coating. While a GO coating didn’t have an impact on catalysis for cyclohexene and cyclooctene oxidation reactions some interesting results by other members of the Kung group led to a more in-depth investigation on the role and nature of Au active site in these reactions. Dissolved gold atoms were found to be the catalytically active species for cyclooctene oxidation. We hypothesized that dissolved gold clusters could be the active species for other cycloalkane oxidation reactions and that the small gold species generated in one reaction system could be used to catalyze other reactions. Dissolved gold clusters from a cyclooctene oxidation reaction were active for cyclohexene oxidation. However dissolved gold clusters recovered from a cyclohexene oxidation reaction could not catalyze cyclooctene oxidation if all of the hydroperoxide species in the product was reduced. It is possible that this is a result of the hydroperoxide reductant also acting as a ligand for the dissolved gold and rendering the dissolved gold as inactive in the reaction network.

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  • 10/28/2018
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