In this article we investigate the electrochemical reduction of CO2 at gold electrodes under mildly acidic conditions. Differential electrochemical mass spectroscopy (DEMS) is used to quantify the amounts of formed hydrogen and carbon monoxide as well as the consumed amount of CO2. We investigate how the Faradaic efficiency of CO formation is affected by the CO2 partial pressure (0.1–0.5 bar) and the proton concentration (1–0.25 mM). Increasing the former enhances the rate of CO2 reduction and suppresses hydrogen evolution from proton reduction, leading to Faradaic efficiencies close to 100%. Hydrogen evolution is suppressed by CO2 reduction as all protons at the electrode surfaces are used to support the formation of water (CO2 + 2H+ + 2e– → CO + H2O). Under conditions of slow mass transport, this leaves no protons to support hydrogen evolution. On the basis of our results, we derive a general design principle for acid CO2 electrolyzers to suppress hydrogen evolution from proton reduction: the rate of CO/OH– formation must be high enough to match/compensate the mass transfer of protons to the electrode surface.

The properties of Nanoporous Gold (NPG) obtained by the selective dissolution of Ag from an Au-Ag alloy can be tuned by the details of its fabrication, and specifically the residual Ag content is correlated to the ligament size of the material. We link this correlation to methanol electro-oxidation. Specifically, two different NPG types (obtained by potentiostatic dealloying) are compared with one obtained by free corrosion. They show remarkable differences in activity. Quantitative product analysis reveals that NPG shows nearly selective oxidation of CHOH to HCOO when NPG is used as an active electrode in contrast to planar Au. This trend can further be enhanced when applying finer nanoporous structures that are linked to a higher Ag content. X-ray photoelectron spectroscopy (XPS) reveals changes in the nature of residual Ag from which we conclude that Ag is not a passive component in the methanol oxidation process.

High-resolution elemental mapping in a transmission electron microscope shows that the residual silver in dealloying-made nanoporous gold (NPG) is aggregated in nanoscale clusters. Kinetic Monte Carlo simulation confirms that these regions are buried relics of the master alloy that have never been exposed to corrosion. The surface of as-dealloyed NPG is covered by at least one atomic monolayer of nearly pure gold. The preferential location of silver in the bulk is relevant when interfaces control the material's function, as in catalysis and sensing. Annealing in air homogenizes the alloy by surface diffusion. IMPACT STATEMENT The residual silver which is typically found in nanoporous gold made by dealloying is localized in clusters that are relics of the original master alloy which have evaded corrosion.

Nanoporous gold (NPG) made by selective corrosion, or dealloying, serves as a model system for the investigation of electrochemical and mechanical properties of nanomaterials. While various dealloying protocols are in use, it is typically assumed that the structural characteristics are identical and independent of the preparation technique. Yet, reported properties such as strength, Young's modulus, or catalytic behavior can vary widely. Here, we compare the microstructure and the mechanical behavior of NPG structures prepared by three different synthesis protocols reported in the literature. We find that corrosion rates, the content of residual sacrificial metal, the average ligament size and the densification by shrinkage strongly depend on the synthesis protocol and show the consequences on the mechanical properties. NPG exhibits an open-cell nanoscale network structure with a well-defined characteristic size of the metal phase, the so-called ligaments. These can be scaled ranging from as small as 5 nm up to 1 μm 17-21 so that NPG established as a model system for the investigation of nanoscale materials behavior.Studies of the catalytic or mechanical behavior of NPG by different authors do not report consistent observations in all instances. The role of the residual sacrificial metal content as an origin for the remarkable catalytic activity (e.g. for CO oxidation) is under discussion. 22-25As another instance, reports on the mechanical strength and stiffness of nominally comparable structures disagree by up to one order of magnitude. 26,27 Therefore it is significant to note that a number of different dealloying protocols are applied within these studies. Conceivably, diverging observations on the material's behavior may be related to the impact of these protocols on the microstructure. Here, we explore this issue by comparing the microstructures of NPG made by three common preparation routes.The elementary steps of dealloying are the removal of atoms of the less noble element from the parent phase and the redistribution of the nobler element atoms on the crystal lattice by surface diffusion. 21This creates a network structure, typically with sizes between 5 and 20 nm. 28,29 When assuming complete removal of the less noble element from a parent alloy (e.g. Ag x Au 1-x which is composed of similarly sized atoms), the solid volume fraction, ϕ, of the porous structure will ideally agree with the atom fraction of the more noble element in the master alloy. Yet, the shrinkage during dealloying may reduce the external sample dimensions, thereby increasing ϕ.29 Structural coarsening begins during dealloying 28 and its rate is affected e.g. by the composition of the electrolyte, 30 additional alloy elements 31,32 or the temperature. 33 These observations emphasize that details of the dealloying process may have a substantial effect on the microstructure of the nanoporous material. Table I lists several synthesis protocols for NPG; these are often implicitly expected to result in similar structures. As the object of...