Nanoporous Metals for Heterogeneous Catalysis: Following the Success of Raney Nickel

Nanoporous Au Behavior in Methyl Orange Solutions

Nanoporous (NP) gold, the most extensively studied and efficient NP metal, possesses exceptional properties that make it highly attractive for advanced technological applications. Notably, its remarkable catalytic properties in various significant reactions hold enormous potential. However, the exploration of its catalytic activity in the degradation of water pollutants remains limited. Nevertheless, previous research has reported the catalytic activity of NP Au in the degradation of methyl orange (MO), a toxic azo dye commonly found in water. This study aims to investigate the behavior of nanoporous gold in MO solutions using UV-Vis absorption spectroscopy and high-performance liquid chromatography. The NP Au was prepared by chemical removal of silver atoms of an AuAg precursor alloy prepared by ball milling. Immersion tests were conducted on both pellets and powders of NP Au, followed by examination of the residual solutions. Additionally, X-ray photoelectron spectroscopy and electrochemical impedance measurements were employed to analyze NP Au after the tests. The findings reveal that the predominant and faster process involves the partially reversible adsorption of MO onto NP Au, while the catalytic degradation of the dye plays a secondary and slower role in this system.

The synthesis of fructose-based surfactants

This study describes the synthesis of a new class of surfactants that is based on the bioderived building blocks fructose, fatty acid methyl esters (FAME), and hydroxy propionitrile (cyanoethanol, 3-HP). The synthesis is scalable, is carried out at ambient conditions, and does not require chromatography. The produced surfactants have excellent surfactant properties with critical micelle concentrations and Krafft points comparable to current glucose-based surfactants.

Facet-Dependent Formation and Adhesion of Au Oxide and Nanoporous Au on Poly-Oriented Au Single Crystals

Nanoporous Au (NPG) has different properties compared to bulk Au, making it an interesting material for numerous applications. To modify the structure of NPG films for specific applications, e. g., the porosity, thickness, and homogeneity of the films, a fundamental understanding of the structure formation is essential. Here, we focus on NPG prepared via electrochemical reduction from Au oxide formed during high voltage (HV) electrolysis on poly-oriented Au single crystal (Au POSC) electrodes. These POSCs consist of a metal bead, with faces with different crystallographic orientations and allow screening of the influence of crystallographic orientation on the structure formation for different facets in one experiment. The HV electrolysis is performed between 100 ms and 30 s at 300 V and 540 V. The amount of Au oxide formed is determined by electrochemical measurements and the structural properties are investigated by scanning electron and optical microscopy. We show that the formation of Au oxide is mostly independent of the crystallographic orientation, except for thick layers, while the macroscopic structure of the NPG films depends on experimental parameters such as the Au oxide precursor thickness and the crystallographic orientation of the substrate. Possible reasons for the frequently observed exfoliation of the NPG films are discussed.

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.

High-Temperature Syngas Desulfurization and Particulate Filtration by ZnO/Ceramic Filters

Combustible gas (e.g., gasification syngas) cleaning at high temperatures can obtain further gains in energy efficiency for power generation and importantly leads to a simplified process and lower cost as a commercially viable source of clean energy. Thus, a feasibility study for high-temperature desulfurization (HTDS) and additional high-temperature particulate filtration (HTPF) of a raw syngas using ZnO sorbent-dispersed Raney CuO (ZnO/R-CuO) and ceramic filter (ZnO/CF) has been carried out. By synchrotron X-ray absorption near-edge structure (XANES) spectroscopy, mainly Zn(II) and Cu(II) are found in the ZnO/R-CuO sorbents. Both ZnO and R-CuO in the sorbents are involved in HTDS (1% H₂S) at 873 K to form ZnS, Cu₂S, and a small amount of CuS and reach relatively high HTDS efficiencies (82-90%). In addition, regeneration of the sulfurized sorbent by oxidation with O₂ at 873 K (HTRG) for 1 h can restore ZnO and CuO for continuous and repetitive HTDS-HTRG cycles. To facilitate the HTDS engineering applications by the ZnO/R-CuO sorbents, their reaction rate constant (8.35 × 10⁴ cm³/g/min) and activation energy (114.8 kJ/mol) at 873 K have also been determined. Furthermore, the ZnO/CF sorbent/filter can perform HTDS and additional HTPF at 873 K with very high particulate removal efficiencies (>98%). This demonstrates the feasibility for hot-syngas cleaning with a much better energy efficiency and lesser cost for cleaner power generation.

Nanoporous Au Formation on Au Substrates via High Voltage Electrolysis

Nanoporous Au (NPG) films have promising properties, making them suitable for various applications in (electro)catalysis or (bio)sensing. Tuning the structural properties, such as the pore size or the surface-to-volume ratio, often requires complex starting materials such as alloys, multiple synthesis steps, lengthy preparation procedures or a combination of these factors. Here we present an approach that circumvents these difficulties, enabling for a rapid and controlled preparation of NPG films starting from a bare Au electrode. In a first approach a Au oxide film is prepared by high voltage (HV) electrolysis in a KOH solution, which is then reduced either electrochemically or in the presence of H₂ O₂ . The resulting NPG structures and their electrochemically active surface areas strongly depend on the reduction procedure, the concentration and temperature of the H₂ O₂ -containing KOH solution, as well as the applied voltage and temperature during HV electrolysis. Secondly, the NPG film can be prepared directly by applying voltages that result in anodic contact glow discharge electrolysis (aCGDE). By carefully adjusting the corresponding parameters, the surface area of the final NPG film can be specifically controlled. The structural properties of the electrodes are investigated by means of XPS, SEM and electrochemical methods.

Photocatalytic dehydrative etherification of alcohols with a nanoporous gold catalyst

A simple and efficient photocatalytic approach for dehydrative etherification of alcohols has been developed by a nanoporous gold catalyst. This protocol features no requirement of addition of acids or bases, broad substrate generality, and excellent acid-sensitive functional group tolerance. The mechanistic studies demonstrate the heterogeneous nature of the catalytic system and the recyclability of the catalyst was demonstrated repeatedly.

Facile One-Pot Synthesis of Nickel Nanoparticles by Hydrothermal Method

The one-pot synthesis process has emerged as an economical synthesis method without the involvement of purification or formation of intermediate compounds. Therefore, nickel nanoparticles were selectively synthesized by a simple hydrothermal method using nickel(II) chloride hexahydrate and borane-ammonia complex as a precursor and reducing agent, respectively. The morphology and crystal growth were observed by controlling the precursor concentration ratio of Ni:AB from 1:0.1 to 1:4 under various temperatures ranging from 80 to 140 degrees. In addition, we observed that the crystal growth rate under the influence of NaCl and KCl resulted in spherical Ni particles with size distributions controlled in the range of 297.65 nm to 1082.15 nm and 358.6 nm to 605 nm, respectively.

Cu Nanowires and Nanoporous Ag Matrix Fabricated through Directional Solidification and Selective Dissolution of Ag-Cu Eutectic Alloys

Cu nanowires and a nanoporous Ag matrix were fabricated through directional solidification and selective dissolution of Ag-Cu eutectic alloys. Ag-39.9at.%Cu eutectic alloys were directionally solidified at growth rates of 14, 25, and 34 μm/s at a temperature gradient of 10 K/cm. The Cu phase in the Ag matrix gradually changed from lamellar to fibrous with an increase in the growth rate. The Ag matrix phase was selectively dissolved, and Cu nanowires of 300-600 nm in diameter and tens of microns in length were prepared in 0.1 M borate buffer with a pH of 9.18 at a constant potential of 0.7 V (vs. SCE). The nanoporous Ag matrix was fabricated through selective dissolution of Cu fiber phase in 0.1 M acetate buffer with a pH of 6.0 at a constant potential of 0.5 V (vs. SCE). The diameter of Ag pores decreased with increasing growth rate. The diameter and depth of Ag pores increased when corrosion time was extended. The depth of the pores was 30 μm after 12 h.

Catalysis of Alloys: Classification, Principles, and Design for a Variety of Materials and Reactions

Alloying has long been used as a promising methodology to improve the catalytic performance of metallic materials. In recent years, the field of alloy catalysis has made remarkable progress with the emergence of a variety of novel alloy materials and their functions. Therefore, a comprehensive disciplinary framework for catalytic chemistry of alloys that provides a cross-sectional understanding of the broad research field is in high demand. In this review, we provide a comprehensive classification of various alloy materials based on metallurgy, thermodynamics, and inorganic chemistry and summarize the roles of alloying in catalysis and its principles with a brief introduction of the historical background of this research field. Furthermore, we explain how each type of alloy can be used as a catalyst material and how to design a functional catalyst for the target reaction by introducing representative case studies. This review includes two approaches, namely, from materials and reactions, to provide a better understanding of the catalytic chemistry of alloys. Our review offers a perspective on this research field and can be used encyclopedically according to the readers' individual interests.

Sensitivity Detection of Uric Acid and Creatinine in Human Urine Based on Nanoporous Gold

Given the significance of uric acid and creatinine in clinical diagnostic, disease prevention and treatment, a multifunctional electrochemical sensor was proposed for sensitive detection of uric acid and creatinine. The sensitive detection of uric acid was realized based on the unique electrochemical oxidation of nanoporous gold (NPG) towards uric acid, showing good linearity from 10 μM to 750 μM with a satisfactory sensitivity of 222.91 μA mM−1 cm−2 and a limit of detection (LOD) of 0.06 μM. Based on the Jaffé reaction between creatinine and picric acid, the sensitive detection of creatinine was indirectly achieved in a range from 10 to 2000 μM by determining the consumption of picric acid in the Jaffé reaction with a detection sensitivity of 195.05 μA mM−1 cm−2 and a LOD of 10 μM. For human urine detection using the proposed electrochemical sensor, the uric acid detection results were comparable to that of high-performance liquid chromatography (HPLC), with a deviation rate of less than 10.28% and the recoveries of uric acid spiked in urine samples were 89~118%. Compared with HPLC results, the deviation rate of creatinine detection in urine samples was less than 4.17% and the recoveries of creatinine spiked in urine samples ranged from 92.50% to 117.40%. The multifunctional electrochemical sensor exhibited many advantages in practical applications, including short detection time, high stability, simple operation, strong anti-interference ability, cost-effectiveness, and easy fabrication, which provided a promising alternative for urine analysis in clinical diagnosis.

Palladium Supported on Porous Organic Polymer as Heterogeneous and Recyclable Catalyst for Cross Coupling Reaction

Palladium immobilized on an amide and ether functionalized porous organic polymer (Pd@AEPOP) is reported to be an effective heterogeneous catalyst for the Heck cross-coupling reaction of aryl iodides with styrene for the synthesis of diphenylethene derivatives. Excellent yields can be obtained using a 0.8 mol% Pd catalyst loading under the optimized reaction condition. The heterogeneous Pd@AEPOP catalyst can also be applied on the Suzuki reaction and the reduction of nitroarene.

Synthesis of Pd nanorod arrays on Au nanoframes for excellent ethanol electrooxidation

Au-Pd hollow nanostructures have attracted a lot of attention because of their excellent ethanol electrooxidation performance. Herein, we report a facile preparation of Au nanoframe@Pd array electrocatalysts in the presence of cetylpyridinium chloride. The reduced Pd atoms were directed to mainly deposit on the surface of the Au nanoframes in the form of rods, leading to the formation of Au nanoframe@Pd arrays with a super-large specific surface area. The red shift and damping of the plasmon peak were ascribed to the deposition of the Pd arrays on the surface of the Au nanoframes and nanobipyramids, which was verified by electrodynamic simulations. Surfactants, temperature and reaction time determine the growth process and thereby the architecture of the obtained Au-Pd hollow nanostructures. Compared with the Au nanoframe@Pd nanostructures and Au nanobipyramid@Pd arrays, the Au nanoframe@Pd arrays exhibit an enhanced electrocatalytic performance towards ethanol electrooxidation due to an abundance of catalytic active sites. The Au NF@Pd arrays display 4.1 times higher specific activity and 13.7 times higher mass activity than the commercial Pd/C electrocatalyst. Moreover, the nanostructure shows improved stability towards the ethanol oxidation reaction. This study enriches the manufacturing technology to increase the active sites of noble metal nanocatalysts and promotes the development of direct ethanol fuel cells.

Unraveling High Alkene Selectivity at Full Conversion in Alkyne Hydrogenation over Ni under Continuous Flow Conditions

Selective hydrogenation of alkynes into alkenes in continuous flow conditions over non-precious metal catalysts is an attractive prospect for the chemical industry, especially for the petrochemical and polymer industry. Achieving...

Capacity-Limited Na-M foil Anode: toward Practical Applications of Na Metal Anode

The practical applications of Na metal anodes are severely plagued by unstable Na plating/stripping. Here the fabrication of Na-rich Na-M (M = Au, Sn, and In) alloy anodes is reported as promising alternatives to address this issue. As compared to metallic Na foil anodes, the alloy foils exhibit improved electrolyte/electrode interface and provide abundant sodiophilic sites for efficient Na plating, while the self-evolved porous Na-M structures accommodate volume variation on cycling. Among three alloy foils, the Na-Au system shows the most promising performance. Under practical conditions such as capacity-limited anodes (5 mAh cm^-2 ) and large stripping/plating capacity (1.0 mAh cm^-2 ), the Na_0.9 Au_0.1 anodes demonstrate stable plating/stripping over 350 h. The proof-of-concept full cells assembled with hard carbon or Prussian blue materials and this thin Na_0.9 Au_0.1 anode also have much elongated cycling life than for pristine Na metal anodes. These findings confirm that the rational design of Na-M alloy anodes can be a promising strategy to promote the development of Na metal batteries.

Nanoporous Intermetallic Pd3 Bi for Efficient Electrochemical Nitrogen Reduction

Electrocatalytic nitrogen reduction at ambient temperature is a green technology for artificial nitrogen fixation but greatly challenging with low yield and poor selectivity. Here, a nanoporous ordered intermetallic Pd₃ Bi prepared by converting chemically etched nanoporous PdBi₂ exhibits efficient electrocatalytic nitrogen reduction under ambient conditions. The resulting nanoporous intermetallic Pd₃ Bi can achieve high activity and selectivity with an NH₃ yield rate of 59.05 ± 2.27 µg h^-1 mg_cat ^-1 and a Faradaic efficiency of 21.52 ± 0.71% at -0.2 V versus the reversible hydrogen electrode in 0.05 m H₂ SO₄ electrolyte, outperforming most of the reported catalysts in electrochemical nitrogen reduction reaction (NRR). Operando X-ray absorption spectroscopy studies combined with density functional theory calculations reveal that strong coupling between the Pd-Bi sites bridges the electron-transfer channel of intermetallic Pd₃ Bi, in which the Bi sites can absorb N₂ molecules and lower the energy barrier of *N₂ for N₂ adsorption and activation. Meanwhile, the intermetallic Pd₃ Bi with bicontinuous nanoporous structure can accelerate the electron transport during the NRR process, thus improving the NRR performance.

Electroreduction of nitrogen to ammonia on nanoporous gold

The electrochemical reduction of nitrogen (N2) to ammonia (NH3) has attracted attention as an emerging alternative to the traditional Haber-Bosch process to synthesize NH3. Unfortunately, electrocatalytic N2 reduction processes are still very inefficient. Here we report three-dimensional nanoporous gold (NPG) as an efficient and stable electrocatalyst for the N2 reduction reaction at room temperature and atmospheric pressure. NPG can deliver a high NH3 yield rate of 45.7 μg h-1 mg-1cat. and a high faradaic efficiency of 3.41% at an ultralow potential of -0.10 V versus the reversible hydrogen electrode, in 0.1 M HCl solution. These values are much higher than those obtained for most of the reported electrocatalysts under similar experimental conditions. Structural characterization and density functional theory calculations reveal that the excellent electrocatalytic activity of NPG mainly results from the high density of geometrically required surface steps and kinks.