Abstract
Cubic, octahedral, and rhombic dodecahedral gold nanocrystals enclosed by {100}, {111}, and {110} facets, respectively, were prepared by a seed-mediated growth method at the room temperature. Palladium thin films were coated on these Au nanocrystals by a redox replacement approach to explore their catalytic activities. It is revealed that formic acid and carbon monoxide oxidation in 0.1 M HClO4 on Au nanocrystals coated with one monolayer (ML) of Pd are facet-dependent and resemble those obtained from corresponding Pd single crystals and Pd films deposited on bulk Au single crystals, suggesting epitaxial growth of Pd overlayers on the Au nanocrystal surfaces. As the Pd film thickness increased, formic acid oxidation current density decreased and the CO oxidation potential moved to more negative. The catalytic activity remained largely unchanged after 3–5 MLs of Pd deposition. The specific adsorption of (bi)sulfate was shown to hinder the formic acid oxidation and the effect decreased with the increasing Pd film thickness. These observations were explained in the framework of the d-band theory. This study highlights the feasibility of engineering high-performance catalysts through deposition of catalytically active metal thin films on facet-controlled inert nanocrystals.
Highlights
Direct formic acid fuel cells (DFAFCs) have attracted much attention in the past decade because they have the advantages of high power density, fast oxidation kinetics, high theoretical cell potential, and mild fuel crossover as compared to direct methanol fuel cells [1,2,3,4,5]
The results reveal that cyclic voltammetric features of hydrogen adsorption/desorption, surface oxidation/reduction, and carbon monoxide (CO) stripping on Au nanocrystals covered with a monolayer of Pd resemble those from the corresponding bulk Pd single crystals in
The results show that the formic acid oxidation (FAO) peak current density and CO oxidation onset potential decreased with increasing Pd film thickness
Summary
Direct formic acid fuel cells (DFAFCs) have attracted much attention in the past decade because they have the advantages of high power density, fast oxidation kinetics, high theoretical cell potential, and mild fuel crossover as compared to direct methanol fuel cells [1,2,3,4,5]. One limitation in most of the previous studies of nanoparticles is that the particle surface structure is not controlled and the observed Pd film thickness dependent activity can at least partly arise from the surface structure difference It has been well-demonstrated that formic acid oxidation is structure-sensitive [7,23,24,25,26]. The results show that the FAO peak current density and CO oxidation onset potential decreased with increasing Pd film thickness These observations were explained with the strain and ligand effects from the d-band theory. This study demonstrates that depositing catalytically active materials as a thin film on facet-controlled nanocrystals is an effective means of obtaining high-performance catalysts because it combines the advantages of optimizing the d-band position by the substrate and the facet-dependent catalytic activity of the overlayer
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