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

Nanoporous gold with microporous structure prepared by sodium dodecyl sulfate-mediated electrochemical dealloying

Nanoporous gold (NPG) is a promising catalytic material for the oxidation of CO and methanol applications. However, NPGs are prone to extensive macroscopic cracking that often decrease mechanic properties of NPGs and depresses their catalytic action. To produce crack-free NPG with an ultra-finer porosity in room temperature, the anionic surfactant sodium dodecyl sulfate (SDS) was added in electrochemical dealloying process. SDS has the effect of reducing the surface diffusion of gold which hinder the initial coarsening of ligaments and prevents interior silver atoms from being exposed and dissolved. As a result, the pore and ligment size are finer, but higher residual silver of NPG samples. NPG with pore size down to 2 nm and the ligament 4.0 nm was successfully fabricated with 13.32 mM SDS in perchloric acid solution. The surface diffusion coefficient of Au atoms was 1.6 × 10-24m2·s-1, nearly 3 orders of magnitude smaller than that of Au atoms in the absence of SDS (2.8 × 10-21m2·s-1). Nanoindentation results demonstrated that high residual silver content made NPG samples harder and stiffer，the specific surface areas of NPG with 6.66 mM SDS was 190 m2g-1by BET. This work provided very important clues on how to control the crack free ultrafine nanoporous structure of other materials.

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.

Electrochemical Discrimination of Salbutamol from Its Excipients in VentolinTM at Nanoporous Gold Microdisc Arrays

The emergence of specific drug–device combination products in the inhalable pharmaceutical industry demands more sophistication of device functionality in the form of an embedded sensing platform to increase patient safety and extend patent coverage. Controlling the nebuliser function at a miniaturised, integrated electrochemical sensing platform with rapid response time and supporting novel algorithms could deliver such a technology offering. Development of a nanoporous gold (NPG) electrochemical sensor capable of creating a unique fingerprint signal generated by inhalable pharmaceuticals provided the impetus for our study of the electrooxidation of salbutamol, which is the active bronchodilatory ingredient in Ventolin^TM formulations. It was demonstrated that, at NPG-modified microdisc electrode arrays, salbutamol is distinguishable from the chloride excipient present at 0.0154 M using linear sweep voltammetry and can be detected amperometrically. In contrast, bare gold microdisc electrode arrays cannot afford such discrimination, as the potential for salbutamol oxidation and chloride adsorption reactions overlap. The discriminative power of NPG originates from the nanoconfinement effect for chloride in the internal pores of NPG, which selectively enhances the electron transfer kinetics of this more sluggish reaction relative to that of the faster, diffusion-controlled salbutamol oxidation. Sensing was performed at a fully integrated three-electrode cell-on-chip using Pt as a quasi-reference electrode.

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.

Ag surface segregation in nanoporous Au catalysts during CO oxidation

The present study focuses on the modification of surface compositional profiles induced in nanoporous (NP) Au catalysts by the catalytic oxidation of carbon monoxide to carbon dioxide in the presence of oxygen. The phenomenon has deep implications concerning the catalytic behavior of NP Au foams in particular, and more in general for the design of more efficient catalysts. Aimed at gaining deeper insight into the mechanisms governing surface segregation, we exposed NP Au foams containing residual Ag to a mixture of gaseous carbon monoxide and oxygen at different temperature. Structural and surface composition analyses pointed out the concomitant occurrence of both NP Au coarsening and Ag surface segregation processes. Experimental findings suggest for Ag surface segregation a two-stage kinetics. During the initial, rapid coarsening of the NP Au structure, Ag surface segregation is mediated by surface rearrangements, which allow the Ag atoms to reach the surface at anomalously fast rate. As coarsening decelerates, the slower diffusion of buried Ag atoms towards the surface predominates, due to favorable chemical interactions with adsorbed oxygen. This novel mechanism’s understanding can benefit strategic areas of science and technology.

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.

The role of incidence angle in the morphology evolution of Ge surfaces irradiated by medium-energy Au ions

Germanium (Ge) surfaces have been irradiated with 26 keV gold (Au) ions at a constant fluence and at incidence angles varying from 0° to 85°. The evolution of the emerging nanostructures is studied by atomic force microscopy (AFM), scanning electron microscopy, x-ray photoelectron spectroscopy (XPS), and cross-sectional transmission electron microscopy. The obtained results are compared with findings reported in the literature. Periodic rippled patterns with the wave vector parallel to the projection of the ion beam direction onto the Ge surface develop between 30° and 45°. From 75° the morphology changes from parallel-mode ripples to parallel-mode terraces, and by further increasing the incidence angle the terraces coarsen and show a progressive break-up of the front facing the ion beam. No perpendicular-mode ripples or terraces have been observed. The analysis of the AFM height profiles and slope distributions shows in the 45°-85° range an angular dependence of the temporal scale for the onset of nonlinear processes. For incidence angles below 45°, the surface develops a sponge-like structure, which persists at higher incidence angles on the top and partially on the face of the facets facing the ion beam. The XPS and the energy-dispersive x-ray spectroscopy evidence the presence of Au nano-aggregates of different sizes for the different incidence angles. This study points out the peculiar behavior of Ge surfaces irradiated with medium-energy Au ions and warns about the differences to be faced when trying to build a universal framework for the description of semiconductor pattern evolution under ion-beam irradiation.

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.

Electrocatalytic methanol oxidation with nanoporous gold: microstructure and selectivity

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 CH₃OH 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.

How silver segregation stabilizes 1D surface gold oxide: a cluster expansion study combined with ab initio MD simulations

Gold surprises us again by the unusual stability of one-dimensional gold oxide structures supported on bimetallic surfaces of gold and silver.

Insights into the dominant factors of porous gold for CO oxidation

Three different porous Au catalysts that exhibit high catalytic activity for CO oxidation were prepared by the leaching of Al from an intermetallic compound, Al2Au, with 10 wt. %-NaOH, HNO3, or HCl aqueous solutions. The catalysts were investigated using Brunauer-Emmett-Teller measurements, synchrotron X-ray powder diffraction, hard X-ray photoelectron spectroscopy, field emission scanning electron microscopy, and transmission electron microscopy (TEM). Broad diffraction peaks generated during the leaching process correlated with high activity for all the porous Au catalysts. CO oxidation catalyzed by porous Au leached with NaOH and HNO3 is considered to be dominated by different mechanisms at low (< 320 K) and high (> 370 K) temperatures. Activity in the low-temperature region is mainly attributed to the perimeter interface between residual Al species (AlOx) and porous Au, whereas activity in the high-temperature region results from a high density of lattice defects such as twins and dislocations, which were evident from diffraction peak broadening and were observed with high-resolution TEM in the porous Au leached with NaOH. It is proposed that atoms located at lattice defects on the surfaces of porous Au are the active sites for catalytic reactions.

The exclusive response of LSPR in uncapped gold nanoparticles towards silver ions and gold chloride ions

LSPR of gold nanoparticles supported over glass or silica nanoparticles modulated using simple ionic treatment.

Atomic observation of catalysis-induced nanopore coarsening of nanoporous gold

Dealloyed nanoporous metals have attracted much attention because of their excellent catalytic activities toward various chemical reactions. Nevertheless, coarsening mechanisms in these catalysts have not been experimentally studied. Here, we report in situ atomic-scale observations of the structural evolution of nanoporous gold during catalytic CO oxidation. The catalysis-induced nanopore coarsening is associated with the rapid diffusion of gold atoms at chemically active surface steps and the surface segregation of residual Ag atoms, both of which are stimulated by the chemical reaction. Our observations provide the first direct evidence that planar defects hinder nanopore coarsening, suggesting a new strategy for developing structurally stable and highly active heterogeneous catalysts.