Large efforts have been made trying to understand the origin of the high catalytic activity of dealloyed nanoporous gold as a green catalyst for the selective promotion of chemical reactions at low temperatures. Residual silver, left in the sample after dealloying of a gold-silver alloy, has been shown to have a strong influence on the activity of the catalyst. But the question of how the silver is distributed within the porous structure has not finally been answered yet. We show by quantitative energy dispersive X-ray tomography measurements that silver forms clusters that are distributed irregularly, both on the surface and inside the ligaments building up the porous structure. Furthermore, we find that the role of the residual silver is ambiguous. Whereas CO oxidation is supported by more residual silver, methanol oxidation to methyl formate is hindered. Structural characterisation reveals larger ligaments and pores for decreasing residual silver concentration
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.
Strain has a strong effect on the properties of materials and the performance of electronic devices. Their ever shrinking size translates into a constant demand for accurate and precise measurement methods with very high spatial resolution. In this regard, transmission electron microscopes are key instruments thanks to their ability to map strain with sub-nanometer resolution. Here we present a novel method to measure strain at the nanometer scale based on the diffraction of electron Bessel beams. We demonstrate that our method offers a strain sensitivity better than 2.5· 10 −4 and an accuracy of 1.5·10 −3 , competing with, or outperforming, the best existing methods with a simple and easy to use experimental setup. a) Electronic mail: giulio.guzzinati@uantwerpen.be 1 arXiv:1902.06979v3 [cond-mat.mtrl-sci]
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