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.

Comparison of the photocatalytic activity of novel hybrid photocatalysts based on phthalocyanines, subphthalocyanines and porphyrins immobilized onto nanoporous gold.

A series of different singlet oxygen photosensitizers was immobilized onto nanoporous gold powder with a mean pore size of 40 nm copper catalyzed azide-alkyne cycloaddition. The attachment of phthalocyanine and porphyrin derivatives was performed on the peripheral substituent of the macrocycle, whereas the subphthalocyanine derivatives were attached the axial substituent with respect to the macrocyclic ring system. All obtained hybrid systems were studied in the photooxidation of 2,5-diphenylfuran as a chemical singlet oxygen quencher and showed increased photocatalytic activity compared to the same amount of the corresponding photosensitizer in solution due to photoinduced interactions of the plasmon resonance of the nanostructured gold support and the attached photosensitizer.

Nanoporous Gold Catalyst for the Oxidative N-Dealkylation of Drug Molecules: A Method for Synthesis of N-Dealkylated Metabolites.

A novel method for the selective catalytic N-dealkylation of drug molecules on a nanoporous gold (NPG) catalyst producing valuable N-dealkylated metabolites and intermediates is described. Drug metabolites are important chemical entities at every stage of drug discovery and development, from exploratory discovery to clinical development, providing the safety profiles and the ADME (adsorption, distribution, metabolism, and elimination) of new drug candidates. Synthesis was carried out in aqueous solution at 80 °C using air (oxygen source) as oxidant, in single step with good isolated yields.

A versatile heterogeneous photocatalyst: nanoporous gold powder modified with a zinc(II) phthalocyanine derivative for singlet oxygen [4 + 2] cycloadditions.

Nanoporous gold was functionalized with a photosensitizer, a zinc(II) phthalocyanine derivative. Such systems are active for the generation of reactive singlet oxygen which can be used for photocatalytic oxidation reactions. This study aims to demonstrate the versatility of such an approach, in terms of substrates and the employed solvent, only possible for a truly heterogeneous catalytic system.

Impact of photosensitizer orientation on the distance dependent photocatalytic activity in zinc phthalocyanine-nanoporous gold hybrid systems.

Nanoporous gold powder was functionalized in a two-step approach by an azide terminated alkanethiol self-assembled monolayer (SAM) and a zinc(ii) phthalocyanine (ZnPc) derivative by copper catalyzed azide-alkyne cycloaddition (CuAAC). A series of different hybrid systems with systematic variation of the alkyl chain length on both positions, the alkanethiol SAM and the peripheral substituents of the ZnPc derivative, was prepared and studied in the photooxidation of diphenylisobenzofuran (DPBF). An enhancement by nearly one order of magnitude was observed for the photosensitized singlet oxygen (O) generation of the hybrid systems compared to the same amount of ZnPc in solution caused by the interaction of the npAu surface plasmon resonance and the excited state of the immobilized sensitizer.

Photocatalytic coatings based on a zinc(ii) phthalocyanine derivative immobilized on nanoporous gold leafs with various pore sizes.

A series of singlet oxygen sensitizing hybrid materials is reported consisting of a zinc(ii) phthalocyanine (ZnPc) derivative immobilized on nanoporous gold leafs (npAu) with various pore sizes. The resulting photocatalytic coatings exhibit a thickness of around 100 nm and pore sizes between 9-50 nm. Herein, we report the synthesis and characterization of those hybrid materials which were synthesized by functionalization of npAu leafs by an azide terminated alkanethiol self-assembled monolayer (SAM) and subsequent copper catalyzed azide-alkyne cycloaddition (CuAAC).

A versatile nanoreactor for complementary in situ X-ray and electron microscopy studies in catalysis and materials science.

Two in situ `nanoreactors' for high-resolution imaging of catalysts have been designed and applied at the hard X-ray nanoprobe endstation at beamline P06 of the PETRA III synchrotron radiation source. The reactors house samples supported on commercial MEMS chips, and were applied for complementary hard X-ray ptychography (23 nm spatial resolution) and transmission electron microscopy, with additional X-ray fluorescence measurements. The reactors allow pressures of 100 kPa and temperatures of up to 1573 K, offering a wide range of conditions relevant for catalysis.

Nanoporous gold functionalized with praseodymia-titania mixed oxides as a stable catalyst for the water-gas shift reaction.

Dealloyed nanoporous metals hold great promise in the field of heterogeneous catalysis; however their tendency to coarsen at elevated temperatures or under catalytic reaction conditions sometimes limit further applications. Here, we report on a highly stable nanoporous gold catalyst (npAu) functionalized with praseodymia-titania mixed oxides as synthesized by a sol-gel method. Specifically, we used aberration-corrected transmission electron microscopy to study the morphology and the interface between the oxide deposits and the npAu substrate at the atomic level.

Influence of distortions of recorded diffraction patterns on strain analysis by nano-beam electron diffraction.

Images acquired in transmission electron microscopes can be distorted for various reasons such as e.g. aberrations of the lenses of the imaging system or inaccuracies of the image recording system.

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).

Correlative Multiscale 3D Imaging of a Hierarchical Nanoporous Gold Catalyst by Electron, Ion and X-ray Nanotomography.

Tomographic imaging of catalysts allows non-invasive investigation of structural features and chemical properties by combining large fields of view, high spatial resolution, and the ability to probe multiple length scales. Three complementary nanotomography techniques, (i) electron tomography, (ii) focused ion beam-scanning electron microscopy, and (iii) synchrotron ptychographic X-ray computed tomography, were applied to render the 3D structure of monolithic nanoporous gold doped with ceria, a catalytically active material with hierarchical porosity on the nm and μm scale. The resulting tomograms were used to directly measure volume fraction, surface area and pore size distribution, together with 3D pore network mapping.

Steam reforming of methanol over oxide decorated nanoporous gold catalysts: a combined in situ FTIR and flow reactor study.

Methanol as a green and renewable resource can be used to generate hydrogen by reforming, i.e., its catalytic oxidation with water.

In Situ Ptychography of Heterogeneous Catalysts using Hard X-Rays: High Resolution Imaging at Ambient Pressure and Elevated Temperature.

A new closed cell is presented for in situ X-ray ptychography which allows studies under gas flow and at elevated temperature. In order to gain complementary information by transmission and scanning electron microscopy, the cell makes use of a Protochips E-chipTM which contains a small, thin electron transparent window and allows heating. Two gold-based systems, 50 nm gold particles and nanoporous gold as a relevant catalyst sample, were used for studying the feasibility of the cell.

Potential-induced electronic structure changes in supercapacitor electrodes observed by in operando soft X-ray spectroscopy.

The dynamic physiochemical response of a functioning graphene-based aerogel supercapacitor is monitored in operando by soft X-ray spectroscopy and interpreted through ab initio atomistic simulations. Unanticipated changes in the electronic structure of the electrode as a function of applied voltage bias indicate structural modifications across multiple length scales via independent pseudocapacitive and electric double layer charge storage channels.

Catalysis by unsupported skeletal gold catalysts.

Catalysis is one of the key technologies for the 21st century for achieving the required sustainability of chemical processes. Critical improvements are based on the development of new catalysts and catalytic concepts. In this context, gold holds great promise because it is more active and selective than other precious metal catalysts at low temperatures.

Do cysteine residues regulate transient receptor potential canonical type 6 channel protein expression?

The regulation of calcium influx through transient receptor potential canonical type 6 (TRPC6) channel is mandatory for the activity of human monocytes. We submit the first evidence that cysteine residues of homocysteine (HC) or acetylcysteine (ACC) affect TRPC6 expression in human monocytes. We observed that patients with chronic renal failure had significantly elevated HC levels and TRPC6 mRNA expression levels in monocytes compared with control subjects.

ALD functionalized nanoporous gold: thermal stability, mechanical properties, and catalytic activity.

Nanoporous metals have many technologically promising applications, but their tendency to coarsen limits their long-term stability and excludes high temperature applications. Here, we demonstrate that atomic layer deposition (ALD) can be used to stabilize and functionalize nanoporous metals. Specifically, we studied the effect of nanometer-thick alumina and titania ALD films on thermal stability, mechanical properties, and catalytic activity of nanoporous gold (np-Au).

Silver residues as a possible key to a remarkable oxidative catalytic activity of nanoporous gold.

Recently, several forms of unsupported gold were shown to display a remarkable activity to catalyze oxidation reactions. Experimental evidence points to the crucial role of residual silver present in very small concentrations in these novel catalysts. We focus on the catalytic properties of nanoporous gold (np-Au) foams probed via CO and oxygen adsorption/co-adsorption.

Nanoporous gold: a new gold catalyst with tunable properties.

Nanoporous gold (np-Au) represents a novel nanostructured bulk material with very interesting perspectives in heterogeneous catalysis. Its monolithic porous structure and the absence of a support or other stabilizing agents opens up unprecedented possibilities to tune structure and surface chemistry in order to adapt the material to specific catalytic applications. We investigated three of these tuning options in more detail: change of the porosity by annealing, increase of activity by the deposition of oxides and change of activity and selectivity by bimetallic effects.

Nanoporous gold: a new material for catalytic and sensor applications.

Nanostructured materials are governed by their surface chemical properties. This is strikingly reflected by np-Au. This material can be generated by corrosion of bulk Ag-Au alloys.

Effect of surface chemistry on the stability of gold nanostructures.

Understanding the role of surface chemistry in the stability of nanostructured noble-metal materials is important for many technological applications but experimentally difficult to access and thus little understood. To develop a fundamental understanding of the effect of surface chemistry on both the formation and stabilization of self-organized gold nanostructures, we performed a series of controlled-environment annealing experiments on nanoporous gold (np-Au) and ion-bombarded Au(111) single-crystal surfaces. The annealing experiments on np-Au in ambient ozone were carried out to study the effect of adsorbed oxygen under dynamic conditions, whereas the ion-bombarded Au single-crystal surfaces were used as a model system to obtain atomic-scale information.

Nanoporous gold catalysts for selective gas-phase oxidative coupling of methanol at low temperature.

Gold (Au) is an interesting catalytic material because of its ability to catalyze reactions, such as partial oxidations, with high selectivities at low temperatures; but limitations arise from the low O2 dissociation probability on Au. This problem can be overcome by using Au nanoparticles supported on suitable oxides which, however, are prone to sintering. Nanoporous Au, prepared by the dealloying of AuAg alloys, is a new catalyst with a stable structure that is active without any support.

N-acetylcysteine improves arterial vascular reactivity in patients with chronic kidney disease.

Background: Patients with stage 5 chronic kidney disease show increased cardiovascular morbidity and mortality that are partly related to impaired arterial vascular reactivity. We investigated whether intravenous administration of the antioxidant acetylcysteine improves arterial vascular reactivity in these patients.

Methods: Arterial vascular reactivity was determined during reactive hyperemia by photoplethysmography of digital pulse waves in a randomized, prospective, placebo-controlled cross-over study of 24 patients with stage 5 chronic kidney disease with and without infusion of acetylcysteine during hemodialysis.