SYNTHESIS OF GOLD NANOPARTICLE CATALYSTS USING A BIPHASIC LIGAND EXCHANGE METHOD AND STUDY OF THEIR ELECTROCATALYTIC PROPERTIES
Noble metal nanoparticles have been studied extensively as heterogeneous catalysts for electrocatalytic and thermal reactions. In particular, the support material for the catalytic species is known to play a role in influencing the geometric and electronic properties of the active site as well as its catalytic performance. Polycrystalline gold electrodes have been used as a support to modify the electrocatalytic behavior of adsorbed molecular species. Here, we have studied two electrocatalytic processes- the hydrogen evolution reaction (HER) and the oxygen reduction reaction (ORR), using Au nanoparticle-based catalysts.
Transition metal dichalcogenides are well-known HER catalysts that show structure-sensitive catalytic activity. In particular, undercoordinated sulfur sites at the edges of bulk materials as well as amorphous clusters and oligomers tend to show the highest reactivity. The hydrogen adsorption energy of MoSx nanoclusters can be further tuned through the metallic support. Here, we synthesize colloidal Au@MoS42-, Au@WS42-and Au@MoS42--WS42- using a biphasic ligand-exchange method. The MoS42- and WS42- complexes show higher HER activity when supported on Au nanoparticles than on to a carbon control, illustrating the electronic role played by the support material.
In the second project, Au nanoparticle cores are utilized as supports for Pd submonolayer and monolayer surfaces in order to catalyze the two-electron reduction of O2 to generate hydrogen peroxide. Bulk surfaces of Pt and Pd are excellent catalysts for the four-electron reduction of O2 to H2O. In order to achieve high selectivity for H2O2, we postulate that the ensemble geometry of the Pd surface must be reduced to small islands or single atoms based on literature studies that have shown that large Pd ensembles are required for O–O bond cleavage. In this study, we synthesize several submonolayers surface coverages of Au@Pd core-shell nanoparticles using a biphasic ligand-exchange method. As the Pd coverage decreases from monolayer to submonolayer, the peroxide selectivity rises but is accompanied by an increase in catalytic overpotential. The highest peroxide selectivity was observed for 0.1 layers of Pd on Au, which likely exhibits the highest fraction of isolated atom and small cluster geometric ensembles of Pd.