On the Role of Residual Ag in Nanoporous Au Catalysts for CO Oxidation: A Combined Microreactor and TAP Reactor Study

Nanoporous Gold: From Structure Evolution to Functional Properties in Catalysis and Electrochemistry

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.

Heterogeneity of oxygen reactivity: key for selectivity of partial methanol oxidation on gold surfaces

Recent evidence for low-temperature oxidation of methyl formate on Au(332) may affect the selectivity of gold catalysts during partial oxidation of methanol. Under isothermal conditions, overoxidation of methyl formate is significantly slower than methanol oxidation which can be attributed to special oxygen species required for overoxidation.

Methanol oxidation on Au(332): methyl formate selectivity and surface deactivation under isothermal conditions

Surface deactivation for partial methanol oxidation to methyl formate on Au(332) under oxygen-deficient conditions at low temperatures suggests a small number of highly active sites for methyl formate formation.

First-principles-based kinetic Monte Carlo simulations of CO oxidation on catalytic Au(110) and Ag(110) surfaces

At the core of the development of more efficient and reliable fuel cells (FCs), there are several essential chemical reactions, namely carbon monoxide (CO) oxidation. This reaction is a keystone in the cleaning of hydrogen fuel used in fuel cells due to strong poisoning by this species of the platinum catalyst used in these devices. The present work aims to provide insight regarding the activation of CO oxidation by gold or silver microfacets possessing low coordinated atoms. To achieve this, density functional theory (DFT) quantum calculations, which determined two competing reaction pathways for CO oxidation, i.e., by molecularly adsorbed oxygen, and by dissociated oxygen, are combined with first-principles kinetic Monte Carlo (1p-kMC) simulations, which employed the resulting DFT parameters in order to address the effect of temperature and partial pressures and the interplay of the elementary reaction events. The use of 1p-kMC is a step further from available works regarding the CO oxidation on gold- and silver-based catalysts for cleansing of hydrogen that is used as a fuel in FCs. Indeed, this research contributes to the conclusion that CO oxidation should preferentially occur on silver microfacets, while the obtained turnover frequencies (TOFs) reinforced such a conclusion.

Aerobic Methanol Oxidation over Unsupported Nanoporous Gold: The Influence of an Added Base

We studied the aerobic oxidation of methanol over nanoporous gold catalysts under neutral and alkaline conditions. We find that under neutral conditions the catalyst has an activation period of about 10 h while upon addition of a base the catalyst becomes active right away. After this activation period, however, the activity of the catalyst is in both cases similar. Moreover, the selectivity was not affected by the base. We tested different bases and found the largest effect when adding OH−. The cation, however, does not play a role. We conclude that it is OH−, which is impacting the reaction and propose a mechanism for the suppression of the activation period. While the catalytic cycle, i.e., the reaction of methanol on the catalyst surface seems unaffected, the transient adsorption of OH− onto the surface can facilitate the activation of molecular oxygen by donating electrons to the surface. Due to the intermediate formation of oxidic Ag species, an effective segregation of surface-near Ag can be induced, which increases the abundance of Ag being essential for the activation of oxygen at the surface. In this way, a more efficient pathway for the generation of active oxygen is opened, allowing the reaction to set in faster.

Independent control over residual silver content of nanoporous gold by galvanodynamically controlled dealloying

A new procedure was developed and characterized for the galvanodynamically controlled dealloying (GCD) of AuxAg100-x alloys to obtain nanoporous gold (npAu) mainly as an unsupported catalyst material for partial oxidation of alcohols. Such catalysts require residual Ag content of less than 1 at%. GCD was compared to the preparation of npAu by potentiostatically controlled deallyoing (PCD) and free corrosion (FC). The main advantage of GCD is the ability to obtain npAu with a predetermined residual Ag content including residual Ag contents below 1 at% while retarding the coarsening of the ligaments. For PCD and FC, there is a strong increase of ligament size with decreasing residual Ag content because the longer times required for dealloying unavoidably lead to coarsening of the npAu structure. On the other hand, GCD also prevented too high initial current density that leads to cracking of the samples and prevents formation of mechanically stable monoliths. GCD tolerates different compositions of the starting alloy for AuxAg100-x within the tested composition range (20 at% ≤ xAu ≤ 30 at%). The samples obtained by GCD were tested for methanol and ethanol oxidation and showed favorable characteristics for partial oxidation of methanol to methyl formate and of ethanol to ethyl acetate.

Self-activated surface dynamics in gold catalysts under reaction environments

Nanoporous gold (NPG) with sponge-like structures has been studied by atomic-scale and microsecond-resolution environmental transmission electron microscopy (ETEM) combined with ab initio energy calculations. Peculiar surface dynamics were found in the reaction environment for the oxidation of CO at room temperature, involving residual silver in the NPG leaves as well as gold and oxygen atoms, especially on {110} facets. The NPG is thus classified as a novel self-activating catalyst. The essential structure unit for catalytic activity was identified as Au–AgO surface clusters, implying that the NPG is regarded as a nano-structured silver oxide catalyst supported on the matrix of NPG, or an inverse catalyst of a supported gold nanoparticulate (AuNP) catalyst. Hence, the catalytically active structure in the gold catalysts (supported AuNP and NPG catalysts) can now be experimentally unified in low-temperature CO oxidation, a step forward towards elucidating the fascinating catalysis mechanism of gold.

Nanoporous gold (NPG) has gained significant attention, but its catalytically active structure has not yet been clarified. Here, the authors identify the catalytically active and dynamic structure in NPG by combining atomic-scale and microsecond-resolution environmental transmission electron microscopy with ab initio calculations.

Active site densities, oxygen activation and adsorbed reactive oxygen in alcohol activation on npAu catalysts

The activation of molecular O2 as well as the reactivity of adsorbed oxygen species is of central importance in aerobic selective oxidation chemistry on Au-based catalysts. Herein, we address the issue of O2 activation on unsupported nanoporous gold (npAu) catalysts by applying a transient pressure technique, a temporal analysis of products (TAP) reactor, to measure the saturation coverage of atomic oxygen, its collisional dissociation probability, the activation barrier for O2 dissociation, and the facility with which adsorbed O species activate methanol, the initial step in the catalytic cycle of esterification. The results from these experiments indicate that molecular O2 dissociation is associated with surface silver, that the density of reactive sites is quite low, that adsorbed oxygen atoms do not spill over from the sites of activation onto the surrounding surface, and that methanol reacts quite facilely with the adsorbed oxygen atoms. In addition, the O species from O2 dissociation exhibits reactivity for the selective oxidation of methanol but not for CO. The TAP experiments also revealed that the surface of the npAu catalyst is saturated with adsorbed O under steady state reaction conditions, at least for the pulse reaction.

CO adsorption and oxygen activation on group 11 nanoparticles – a combined DFT and high level CCSD(T) study about size effects and activation processes

The focus of this study lies in the activation of molecular oxygen and reaction with CO within density functional theory (DFT) and high level CCSD(T) calculations.

Recyclable magnetite-silver heterodimer nanocomposites with durable antibacterial performance

There is a significant need for magnetite-silver nanocomposites that exhibit durable and recyclable antimicrobial activity. In this study, magnetic iron oxide nanoparticles (Fe₃O₄ NPs) coated with ethylenediamine-modified chitosan/polyacrylic acid copolymeric layer (Fe₃O₄@ECS/PAA) were fabricated. Subsequently, directly deposited silver (Ag) NPs procedure was carried out to form the antibacterial heterodimers of Fe₃O₄@ECS/PAA-Ag NPs. The composition and morphology of the resultant nanostructures were confirmed by FT-IR, XRD, TEM and TGA. The overall length of the heterodimers was approximately 45 nm, in which the mean diameter of Fe₃O₄@ECS/PAA NPs reached up to 35 nm, and that of Ag NPs was around 15 nm. The mass fraction of silver NPs in the nanocomposites was about 63.1%. The obtained Fe₃O₄@ECS/PAA NPs exhibited good colloidal stability, and excellent response to additional magnetic field, making the NPs easy to recover after antibacterial tests. In particular, the Fe₃O₄@ECS/PAA-Ag NPs retained nearly 100% biocidal efficiency (10⁶–10⁷ CFU/mg nanoparticles) for both Gram-negative bacteria E. coli and Gram-positive bacteria S. aureus throughout ten cycles without washing with any solvents or water, exhibiting potent and durable antibacterial activity.

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Recyclable dual functional antibacterial heterodimer nanocomposites were fabricated.

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The nanostructures realized the combination of antibacterial ability and recyclable function.

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Biocidal efficiency retained nearly 100% throughout ten cycles.

Forty years of temporal analysis of products

A detailed understanding of reaction mechanisms and kinetics is required in order to develop and optimize catalysts and catalytic processes. Temporal analysis of products (TAP) is an instrument capable of providing such understanding.

Nanocatalysts Fabricated by a Dealloying Method

Two types of nanomaterials with different morphologies are described in this article: nanoporous metals and titanate nanowires. Both materials are fabricated by a dealloying method. In the former case, the catalytic properties of nanoporous gold and palladium are exemplified by many chemical transformations. The reactions proceed without any support, stabilizer, or ligands. The catalyst can be easily recovered by a simple separation process and reused many times without significant loss of catalytic activity. In the latter case, the dealloying of Ti-Al alloy is described as a new fabrication method for producing ultrafine titanate nanowires. This method does not require high-temperature conditions, which is advantageous for the construction of fine structures. The key to this process is achieving a fine dispersion of intermetallic TiAl3 nanocrystals in the Al matrix in the mother alloy. The resulting nanowires exhibit remarkable Sr(2+) ion-exchange properties.

Highly stable and dispersive silver nanoparticle-graphene composites by a simple and low-energy-consuming approach and their antimicrobial activity

A simple and low-energy-consuming approach to synthesize highly stable and dispersive silver nanoparticle-graphene (AgNP-GE) nanocomposites has been developed, in which the stability and dispersivity of the composites are varied greatly with the pH value and temperature of the reaction. The results demonstrate that the optimal reaction conditions are pH 11 at room temperature for 70 min. As-synthesized composites display excellent antimicrobial activity, and can completely inhibit the growth of Escherichia coli cells at a concentration of 20 mg L(-1) (20 ppm). After treatment with 10 ppm AgNP-GE composites, the cells are killed completely within 3 h. The unique structure imparts such good antimicrobial properties to the composites. Firstly, the sheetlike AgNP-GE tends to be adsorbed and accumulated onto the surface of cells, which can change the permeability and enhance the antimicrobial activity. Secondly, Ag(+) released from AgNPs can act on the cells effectively and fully, thereby resulting in cell death.

Nanoporous gold as an active low temperature catalyst toward CO oxidation in hydrogen-rich stream

Preferential CO oxidation (PROX) was investigated by using dealloyed nanoporous gold (NPG) catalyst under ambient conditions. Systematic investigations were carried out to characterize its catalytic performance by varying reaction parameters such as temperature and co-existence of CO2 and H2O, which revealed that NPG was a highly active and selective catalyst for PROX, especially at low temperature. At 20°C, the exit CO concentration could be reduced to less than 2 ppm with a turnover frequency of 4.1 × 10(-2) s(-1) at a space velocity of 120,000 mL h(-1) g(-1)cat. and its high activity could retain for more than 24 hours. The presence of residual Ag species in the structure did not seem to improve the intrinsic activity of NPG for PROX; however, they contributed to the stabilization of the NPG structure and apparent catalytic activity. These results indicated that NPG might be readily applicable for hydrogen purification in fuel cell applications.

Hierarchical nested-network nanostructure by dealloying

Applications of porous microstructures in functional materials often impose conflicting requirements on the pore size, which may be met by hierarchical structures that combine porosity on distinctly different length scales. Here we report an electrochemical dealloying strategy that yields bulk samples of porous gold with a hierarchical microstructure. A nanoscale network of solid ligaments forms the lower hierarchy level, which is nested within the geometrically similar, but much larger, network of the upper hierarchy level. Starting from a dilute solid solution of Au in Ag, controlled electrochemical corrosion yields nanoporous Ag-Au alloy as an intermediate product. Coarsening of the porous alloy creates the large ligaments of the upper hierarchy level. Those are then again dealloyed, which creates the fine ligaments of the lower hierarchy level. We show that the material exhibits enhanced charge transport kinetics while maintaining a large specific surface area.

Catalytic activity of nanostructured Au: Scale effects versus bimetallic/bifunctional effects in low-temperature CO oxidation on nanoporous Au

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.