von Jorge Adrian Tapia Burgos ; Christoph Mahr ; Alex Ricardo Silva Olaya ; Lars Robben ; Marco Schowalter ; Thorsten M. Gesing ; Andreas Rosenauer ; Gunther Wittstock ; Arne Wittstock ; Marcus Bäumer
Nanoporous gold (Au) films are self-supported structures that possess a large surface area and extraordinary catalytic activity. Generally, nanoporous gold is obtained by solution-based dealloying where the less noble metal, often silver (Ag), is etched out. However, the residual amounts of the sacrificial metal are not well controlled, the impure samples show restructuring, and the residual metal prevents the study of the catalytic role of Au alone. Here, we fabricate impurity-free nanoporous gold films by a plasma-enabled dry synthetic route. The scheme does not include sacrificial metals or solution processing and is much more general. It is used to obtain self-supported ultra-pure nanoporous gold films with controllable pore sizes. The impurity-free nanoporous gold films possess highly curved ligaments, are remarkably robust, and stable over hundreds of electrochemical cycles. Furthermore, they contain many catalytically active sites, which is highly promising for electrocatalytic applications.
American Chemical Society ACS catalysis Washington, DC : ACS, 2011 13(2023), 17, Seite 11656-11665 Online-Ressource
Nanoporous gold (NPG) obtained by dealloying Ag75Au25 with an overall residual Ag content of less than 1% was investigated as an electrocatalyst for the oxidation of methanol, formaldehyde, and formate in aqueous 0.1 M NaOH solution. The NPG was used to fill cavity microelectrodes, which allowed the recording of well-resolved voltammetry from the porous material. NPG differs from polycrystalline Au (Au(poly)) by its microstructure and its residual Ag content and also behaves distinctly different than Au(poly). The residual Ag content is higher at the surface of the ligaments than in the bulk. By cycling the NPG electrodes in 0.1 M H2SO4, the surface concentration of Ag could be decreased. It could then be set to a defined value by underpotential deposition (UPD) of Ag. The surface structure, and specifically its evolution upon the removal of Ag from the surface, was analyzed by the characteristic voltammetric features of Pb UPD. The effect of Ag on the electrocatalytic methanol oxidation reaction (MOR) is different in different potential regions. Ag coverage shifts the onset of the methanol oxidation current to less positive potentials. In the range of the peak current density, only a defined low Ag concentration enhanced the MOR current density compared to the Ag-free NPG. The {1 0 0} and {1 1 1} facets contributed the largest current, as concluded from selective poisoning experiments. At a potential of 1.63 V vs RHE, Ag2O at the surface is oxidized to AgO. Those layers can oxidize methanol and formate to CO2. The oxidation of formaldehyde proceeds at a much higher reaction rate than the MOR and formate oxidation; the reaction leads to CO and CO2 depending on the applied potential. Given the high oxidation rate of formaldehyde, it would be immediately further oxidized should it be formed as an intermediate of MOR. This is an important difference to the methanol oxidation at Pt. The water oxidation that occurs at the same potential range in the methanol-free solution was suppressed during CO2 formation.
American Chemical Society ACS catalysis Washington, DC : ACS, 2011 12(2022), 8, Seite 4415-4429 Online-Ressource
Sonstige Körperschaft: Technische Universität Hamburg ; Sonstige Körperschaft: Technische Universität Hamburg, Institut für Werkstoffphysik und Werkstofftechnologie
Electrocatalysis; Methanol oxidation; Nanoporous gold; Surface structure; Under potential deposition
Nanoporous gold (NPG) obtained by potentiostatic dealloying of an AuAg master alloy was pulverized, filled into a cavity microelectrode and its surface electrochemistry was investigated in NaOH and KOH solutions. This method yielded highly resolved undistorted voltammograms of this polycrystalline material with clear evidences for surface segregation of Ag during storage in air. One cycle in 0.1 M H SO can remove most of the residual Ag from the surface as evidenced by voltammetry after back transfer to alkaline solution and by X-ray photoelectron spectroscopy (XPS). After removal of residual Ag from the surface, the surface undergoes rephasing forming wider 111 and 110 terraces as evidenced by Pb underpotential deposition (UPD). By adjustment of the bulk concentration of Pb species, a selective partial coverage of the high internal surface area of NPG could be intentionally adjusted. Lead species remain attached to the NPG surface at potentials positive of the UPD regions either as plumbates or Pb species as evidenced by XPS. Those tools enable to disentangle effects on the electrocatalysis of the methanol oxidation reaction (MOR) in different potential regions for 1 M methanol in different concentrations of NaOH and KOH. The MOR commences at lower potentials in the presence of residual Ag. At very high potential, the presence of Ag species promotes the oxidation to CO /CO . Tafel analysis after selective blocking of specific facets shows that the 110 terraces promote especially the first step of MOR while the 111 terraces enhance the rate of further steps that are rate-limiting at higher potentials. While high coverages by Pb UPD layers or adsorbed plumbate anions inhibit electrooxidation, the behavior of NPG electrodes with low coverages of Pb species is reminiscent to NPG electrodes with residual Ag. This could be caused by promoting the methoxide and/or OH adsorption at low overpotentials and catalyzing complete oxidation by a surface bound Pb species at higher potentials. The catalytic currents increase with base concentration and are higher for KOH compared to NaOH. Comparable effects of base concentration on the parameters of the Tafel lines suggest that this effect is a result of slightly stronger deprotonation of methanol in bulk KOH solutions. 2 4 2 3 II IV 2- - IV
Electrochimica acta New York, NY [u.a.] : Elsevier, 1959 383(2021), Artikel-ID 138348, Seite 1-15 Online-Ressource
