No abstract

Distinct from inert bulk gold, nanoparticulate gold has been found to possess remarkable catalytic activity towards oxidation reactions. The catalytic performance of nanoparticulate gold strongly depends on size and support, and catalytic activity usually cannot be observed at characteristic sizes larger than 5 nm. Interestingly, significant catalytic activity can be retained in dealloyed nanoporous gold (NPG) even when its feature lengths are larger than 30 nm. Here we report atomic insights of the NPG catalysis, characterized by spherical-aberration-corrected transmission electron microscopy (TEM) and environmental TEM. A high density of atomic steps and kinks is observed on the curved surfaces of NPG, comparable to 3-5 nm nanoparticles, which are stabilized by hyperboloid-like gold ligaments. In situ TEM observations provide compelling evidence that the surface defects are active sites for the catalytic oxidation of CO and residual Ag stabilizes the atomic steps by suppressing {111} faceting kinetics.

source materials were heated to between 60 and 200 C. The pressure in the source vessels was between 80 and 196 hPa. The carrier gas was Ar and the flow rate was in the range 70±300 sccm, depending on the source materials, which were loaded individually into the source vessels. The pipes connected to the reactor were heated to above 200 C in order to avoid vapor condensation. Oxygen was supplied to the reactor at 36 hPa and the total pressure in the reactor was 65 hPa. The deposition temperature was 800 C. The as-prepared thin films had a cation composition of Bi/Sr/Ca/Cu = 1:1:1:(1.5±1.7). The composition and thickness of the thin films were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) (SPS 7700, Seiko Instruments Inc.). X-ray diffraction patterns (D500, Siemens) have shown that that films were epitaxial, c-axis aligned, and formed from the Bi 2 Sr 2 Ca 2 Cu 3 O 10±x phase. The morphology of the thin films was inspected by optical microscopy on large areas, and by atomic force microscopy (AFM) (SPA 300, Seiko Instruments Inc.) locally. The superconductivity of the thin films was checked by the standard four-probe method.

The improvement of catalysts for the four-electron oxygen-reduction reaction (ORR; O(2) + 4H(+) + 4e(-) → 2H(2)O) remains a critical challenge for fuel cells and other electrochemical-energy technologies. Recent attention in this area has centred on the development of metal alloys with nanostructured compositional gradients (for example, core-shell structure) that exhibit higher activity than supported Pt nanoparticles (Pt-C; refs 1-7). For instance, with a Pt outer surface and Ni-rich second atomic layer, Pt(3)Ni(111) is one of the most active surfaces for the ORR (ref. 8), owing to a shift in the d-band centre of the surface Pt atoms that results in a weakened interaction between Pt and intermediate oxide species, freeing more active sites for O(2) adsorption. However, enhancements due solely to alloy structure and composition may not be sufficient to reduce the mass activity enough to satisfy the requirements for fuel-cell commercialization, especially as the high activity of particular crystal surface facets may not easily translate to polyfaceted particles. Here we show that a tailored geometric and chemical materials architecture can further improve ORR catalysis by demonstrating that a composite nanoporous Ni-Pt alloy impregnated with a hydrophobic, high-oxygen-solubility and protic ionic liquid has extremely high mass activity. The results are consistent with an engineered chemical bias within a catalytically active nanoporous framework that pushes the ORR towards completion.