Light weight and cheap electrolytes with fast multi-valent ion conductivity can pave the way for future high-energy density solid-state batteries, beyond the lithium-ion battery.

Nanoporous gold (np-Au) is a catalytically highly active material, prepared by selectively dealloying silver from a gold–silver alloy. It can promote aerobic CO oxidation and a range of other oxidation reactions. It has been debated whether the remarkable catalytic properties of np-Au are mainly due to its structural features or whether the residual Ag remaining in the material after dealloying is decisive for the activity, especially for the activation of O2. Recent theoretical studies provided evidence that Ag impurities can facilitate the adsorption and dissociation of O2 on np-Au. However, these studies predicted quite a high activation barrier for O2 dissociation on Au–Ag alloy catalysts, whereas experimentally reported activation energies are much lower. In this work we use the stepped Au(321) surface with Ag impurities, which is arguably a realistic model for np-Au material as well as for Au–Ag catalysts in general. We present alternative routes for O2 activation via its direct reaction with adsorbed CO or H2O. In all of the reactions considered, surface atomic O is generated via a sequence of elementary steps with calculated low activation energies of <0.4 eV with respect to coadsorbed reactants. Ag impurities are shown to increase the adsorption energy of O2 and hence the probability of a surface-mediated reaction versus desorption. We considered four possible mechanisms of CO oxidation in dry and humid environments in a microkinetic modeling study. We show that via the proposed mechanisms water indeed promotes O2 dissociation; nevertheless, the “dry” mechanism, in which CO directly reacts with O2, is by far the fastest route of CO2 formation on pure Au and on Au with Ag impurities. Ag impurities lead to significantly higher turnover rates; thus, calculations point to the key role of Ag in promoting the catalytic activity of Au–Ag alloy systems.

Nanoporous gold (np-Au) has recently emerged as a highly selective environmentally friendly catalyst for low-temperature applications. Despite the seeming simplicity of this material, which consists of almost pure gold, its surface chemistry turns out to be more complex than anticipated. Interactions among gold, chemisorbed O atoms generated and consumed during catalysis, and trace amounts of Ag impurities present in np-Au lead to complex surface dynamics. In this work, theoretical modeling by means of ab initio molecular dynamics (AIMD) is combined with an Auger electron spectroscopic study to investigate oxygen-driven Ag surface diffusion on Au model surfaces exhibiting structural characteristics of np-Au. AIMD simulations reveal that surface O atoms dynamically form −(Au−O)− chain structures on the stepped Au(321) surface and lead to surface restructuring, but no chain formation is found on the flat Au(111). Ag impurities at low concentration lower the activation barrier for −(Au−O)− chain formation, whereas the formation of −O−Ag−O− links is energetically slightly unfavorable, especially at high Ag concentration. Furthermore, our study reveals the migration of subsurface Ag atoms onto the surface toward O-rich areas. Using the stepped Au(332) surface with Ag impurities under UHV conditions as a model system, we show that atomic oxygen is able to induce surface segregation of Ag at 200 K. Our results suggest that atomic surface oxygen should be one of the driving forces leading to the ligament coarsening in np-Au.

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Nanoporous gold (NPG) is characterized by a bicontinuous network of nanometer-sized metallic struts and interconnected pores formed spontaneously by oxidative dissolution of the less noble element from gold alloys. The resulting material exhibits decent catalytic activity for low-temperature, aerobic total as well as partial oxidation reactions, the oxidative coupling of methanol to methyl formate being the prototypical example. This review not only provides a critical discussion of ways to tune the morphology and composition of this material and its implication for catalysis and electrocatalysis, but will also exemplarily review the current mechanistic understanding of the partial oxidation of methanol using information from quantum chemical studies, model studies on single-crystal surfaces, gas phase catalysis, aerobic liquid phase oxidation, and electrocatalysis. In this respect, a particular focus will be on mechanistic aspects not well understood, yet. Apart from the mechanistic aspects of catalysis, best practice examples with respect to material preparation and characterization will be discussed. These can improve the reproducibility of the materials property such as the catalytic activity and selectivity as well as the scope of reactions being identified as the main challenges for a broader application of NPG in target-oriented organic synthesis.