Abstract
The catalytic properties of nanostructured Au and their physical origin were investigated by using the low-temperature CO oxidation as a test reaction. In order to distinguish between structural effects (structure–activity correlations) and bimetallic/bifunctional effects, unsupported nanoporous gold (NPG) samples prepared from different Au alloys (AuAg, AuCu) by selective leaching of a less noble metal (Ag, Cu) were employed, whose structure (surface area, ligament size) as well as their residual amount of the second metal were systematically varied by applying different potentials for dealloying. The structural and chemical properties before and after 1000 min reaction were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The catalytic behavior was evaluated by kinetic measurements in a conventional microreactor and by dynamic measurements in a temporal analysis of products (TAP) reactor. The data reveal a clear influence of the surface contents of residual Ag and Cu species on both O2 activation and catalytic activity, while correlations between activity and structural parameters such as surface area or ligament/crystallite size are less evident. Consequences for the mechanistic understanding and the role of the nanostructure in these NPG catalysts are discussed.
Highlights
Porous metallic materials with well-controlled morphologies and surface properties have attracted considerable attention in both fundamental research and technological applications owing to their unique physical and chemical properties, for applications, e.g., in optics, catalysis or as sensors [1,2]
Zielasek et al [4] and Xu et al [11] reported that nanoporous gold, prepared by the selective dissolution of Ag from a AuAg alloy, exhibits a remarkably high activity for CO oxidation with molecular oxygen at low temperatures, and recent experiments in our laboratory arrived at comparable conclusions [12,13]
Going to the Ag(3d) signals, we find that after reaction for 1000 min the doublet of the Ag(3d) X-ray photoelectron spectroscopy (XPS) peaks detected on the fresh samples for nanoporous gold (NPG)(Ag)-2 to NPG(Ag)-4 is replaced by a new peak with an intermediate binding energy (BE) of 368.0 eV
Summary
Porous metallic materials with well-controlled morphologies and surface properties have attracted considerable attention in both fundamental research and technological applications owing to their unique physical and chemical properties, for applications, e.g., in optics, catalysis or as sensors [1,2]. Measurements of the catalytic activity for CO oxidation of the NPG catalysts were performed in a conventional microreactor at atmospheric pressure and in a temporal analysis of products (TAP) reactor under ultrahigh vacuum (UHV) conditions. For testing the catalytic activity for CO oxidation in the TAP reactor, the samples were exposed to simultaneous pulses of CO/Ar and O2/Ar, with a CO/O2 ratio of 1:1, i.e., an excess of oxygen relative to stoichiometric reaction conditions Prior to these measurements it was checked that the gas mixing unit and the gas pipes containing the reaction mixture as well as the reactor and the dilution materials were inert; no conversion of CO or O2 was found under these conditions in control experiments. Using the Scherrer equation in the sense described before, we determined apparent ligament sizes of 21, 6.4, 4.1 and 4.1 nm for the samples NPG(Ag)-1, NPG(Ag)-2, NPG(Ag) and NPG(Ag)-4, respectively (Table 1)
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