(632c) Tuning Gold Nanoparticle Surface Site Accessibility and Electronic State Using Organic Ligands for Oxidation and Reduction Catalysis

The environment around active sites in enzymes is controlled by the different organic functional groups are proximally located near the active site. These functional groups are responsible for the high activity and selectivity of enzymes. In this work, we demonstrate the role of bound organic ligands in determining the stability, number of active sites, and catalytic activity of gold nanoparticle catalysts. A series of gold nanoparticles is synthesized using bulky phosphine and thiol ligands which impart stability to the nanoparticles which are less than 2 nm in diameter. 2-napthalenethiol titrations are used to measure the number of binding sites on the Au nanoparticle surfaces. The Au nanoparticles stabilized by bis(diphenylphosphino)methane produced the highest accessibility where 61% of the total gold atoms bind 2-napthalenethiol. This represents the highest fraction reported for an organic ligand-stabilized metallic nanoparticle in solution. The Au nanoparticles synthesized with phosphine ligands are all active for resazurin reduction and the reaction rate scales with the number of binding sites on the nanoparticle surface.

Triphenylphosphine (TPP)-stabilized Au nanoparticles are compared to triphenylmethyl mercaptan (TPMT)-stabilized Au nanoparticles in three different reactions: benzyl alcohol oxidation, CO oxidation, and resazurin reduction. The nanoparticles are similarly sized and have a similar number of sites accessible to 2-napthalenethiol. The catalytic activities of the TPP- and TPMT-stabilized Au nanoparticles are similar for benzyl alcohol oxidation and CO oxidation. However, when the TPP- and TPMT-stabilized Au nanoparticles are used in resazurin reduction, the TPP-stabilized nanoparticles are active while the TPMT-stabilized nanoparticles are in active. It is hypothesized that this difference in activity, which is not due to differences in nanoparticle size or number of active sites, originates in electronic differences of the Au nanoparticles surfaces as measured by X-ray photoelectron spectroscopy and FTIR of bound CO molecules.