Basic concepts and recent advances in nitrophenol reduction by gold- and other transition metal nanoparticles

Synthesis and Catalytic Application of Silver Nanoparticles Supported on Lactobacillus kefiri S-Layer Proteins

Research on nanoparticles obtained on biological supports is a topic of growing interest in nanoscience, especially regarding catalytic applications. Silver nanoparticles (AgNPs) have been studied due to their low toxicity, but they tend to aggregation, oxidation, and low stability. In this work, we synthesized and characterized AgNPs supported on S-layer proteins (SLPs) as bidimensional regularly arranged biotemplates. By different reduction strategies, six AgNPs of variable sizes were obtained on two different SLPs. Transmission electron microscopy (TEM) images showed that SLPs are mostly decorated by evenly distributed AgNPs; however, a drastic reduction by NaBH4 led to large AgNPs whereas a smooth reduction with H2 or H2/NaBH4 at low concentration leads to smaller AgNPs, regardless of the SLP used as support. All the nanosystems showed conversion values between 75-80% of p-nitrophenol to p-aminophenol, however, the increment in the AgNPs size led to a great decrease in Kapp showing the influence of reduction strategy in the performance of the catalysts. Density functional theory (DFT) calculations indicated that the adsorption of p-nitrophenolate species through the nitro group is the most favored mechanism, leading to p-aminophenol as the only feasible product of the reaction, which was corroborated experimentally.

Synthesis of lignin-derived nitrogen-doped carbon as a novel catalyst for 4-NP reduction evaluation

In this study, nitrogen-doped carbon (NC) was fabricated using lignin as carbon source and g-C₃N₄ as sacrificial template and nitrogen source. The structural properties of as-prepared NC were characterized by TEM, XRD, FT-IR, Raman, XPS and BET techniques. Attractively, NC has proved efficient for reducing 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) using NaBH₄ as hydrogen donor with high apparent rate constant (k_app = 4.77 min^-1) and specific mass activity (s = 361 mol kgcat^-1 h^-1), which values are superior to the previously reported catalysts in the literature. Density functional theory (DFT) calculations demonstrate that four kinds of N dopants can change the electronic structure of the adjacent carbon atoms and contribute to their catalytic properties dependant on N species, however, graphitic N species has much greater contribution to 4-NP adsorption and catalytic reduction. Furthermore, The preliminary mechanism of this transfer hydrogenation reaction over as-prepared NC is proposed on the basis of XPS and DFT data. Astoundingly, NC has excellent stability and reusability of six consecutive runs without loss of catalytic activity. These findings open up a vista to engineer lignin-derived NC as metal-free catalyst for hydrogenation reaction.

Rhodium Nanoparticles Stabilized by PEG-Tagged Imidazolium Salts as Recyclable Catalysts for the Hydrosilylation of Internal Alkynes and the Reduction of Nitroarenes

PEGylated imidazolium (bromide and tetrafluoroborate) and tris-imidazolium (bromide) salts containing triazole linkers have been used as stabilizers for the preparation of water-soluble rhodium(0) nanoparticles by reduction of rhodium trichloride with sodium borohydride in water at room temperature. The nanomaterials have been characterized (Transmission Electron Microscopy, Electron Diffraction, X-ray Photoelectron Spectroscopy, Inductively Coupled Plasma-Optical Emission Spectroscopy). They proved to be efficient and recyclable catalysts for the stereoselective hydrosilylation of internal alkynes, in the presence or absence of solvent, and in the reduction of nitroarenes to anilines with ammonia-borane as hydrogen donor in aqueous medium (1:4 tetrahydrofuran/water).

Multifunctional Efficacy of Environmentally Benign Silver Nanospheres for Organic Transformation, Photocatalysis, and Water Remediation

Highly crystalline and monophasic silver nanospheres with a high specific surface area of 57 m²/g have been synthesized by an environmentally benign rapid chemical reduction using l-alanine for catalytic transformation, photocatalytic degradation, and bacterial disinfection, which can provide an ample strategy for water remediation. Electron microscopic analysis confirms the spherical morphology of as-prepared silver nanoparticles with an average grain size of 20 nm. Silver nanospheres showed excellent catalytic activity for the catalytic hydrogenation and conversion (95.6%) of 4-nitrophenol to 4-aminophenol. Significant photocatalytic degradation proficiency was also shown for methylene blue (94.5%) and rhodamine B (96.3%) dyes under solar irradiation. The antibacterial behavior of Ala-Ag nanospheres was demonstrated through the disk diffusion antibacterial assay against Gram-positive (Escherichia coli) and Gram-negative (Staphylococcus aureus) bacteria. Multifunctional efficiency of as-prepared Ala-Ag nanospheres for water remediation has also been established.

Renaissance of Stöber method for synthesis of colloidal particles: New developments and opportunities

Colloidal silica particles have received a widespread interest because of their potential applications in adsorption, ceramics, catalysis, drug delivery and more. Among many approaches towards fabrication of these colloidal particles, Stöber, Fink and Bohn (SFB) method, known as Stöber synthesis is an effective sol-gel strategy for production of uniform, monodispersed silica particles with highly tailorable size and surface properties. This review, after a brief introduction showing the importance of colloidal chemistry, is focused on the Stöber synthesis of silica spheres including discussion of the key factors affecting their particle size, porosity and surface properties. Next, further developments of this method are presented toward fabrication of polymer, carbon, and composite spheres.

Highly stable AgNPs prepared via a novel green approach for catalytic and photocatalytic removal of biological and non-biological pollutants

Increases in biological and non-biological pollutants pose a significant threat to environmental systems. In an effort to develop an effective means to treat such pollutants, the use of Phaseolus vulgaris (kidney beans) as reducing and capping agents is proposed for the green synthesis of highly stable silver nanoparticles (AgNPs) with a face-centered cubic (fcc) crystalline structure (size range: 10-20 nm). The potent role of the resulting AgNPs was found as triple platforms (photocatalyst, catalyst, and antimicrobial disinfectant). AgNPs were able to photocatalytically degrade approximately 97% of reactive red-141 (RR-141) dye within 150 min of exposure (quantum efficiency of 3.68 × 10^-6 molecule.photon^-1 and a removal reaction kinetic rate of 1.13 × 10^-2 mmol g^-1 h^-1). The role of specific reactive oxygen species (ROS) in the photocatalytic process and complete mineralization of dye was also explored through scavenger and chemical oxygen demand (COD) experiments, respectively. As an catalyst, AgNPs were also capable of reducing 4-nitrophenol to 4-aminophenol within 15 min. Overall, AgNPs showed excellent stability as catalyst and photocatalyst even after five test cycles. As an antimicrobial agent, the AgNPs are effective against both gram-positive (Bacillus subtilis) and -negative bacteria (Escherichia coli), with the zones of clearance as 15 and 18 mm, respectively. Thus, the results of this study validate the triple role of AgNPs derived via green synthesis as a photocatalyst, catalyst, and antimicrobial agent for effective environmental remediation.

Strategy to improve gold nanoparticles loading efficiency on defect-free high silica ZSM-5 zeolite for the reduction of nitrophenols

Catalytic reduction of toxic and aqueous stable nitrophenols by gold nanoparticles (Au NPs) is hot issue due to the serious environmental pollution in recent years. But the expensive price and poor recycling performance of Au NPs limit its further application. Defect-free high silica zeolite is suitable support for Au NPs due to its cheaper price, higher stability and stronger adsorbability, but the low alumina content and defect sites usually lead to poor Au NPs loading efficiency. Herein, we reported the improved Au NPs loading efficiency on defect-free high silica ZSM-5 zeolite through the additional surface fluffy structure. The fluffy structure was created through the addition of multi-walled carbon nanotubes (MWCNTs) and ethanol into synthesis gel. Highly dispersed ca. 4 nm Au NPs on zeolite surface are prepared by the green enhanced sol-gel immobilization method. The Au NPs loading efficiency on conventional ZSM-5 zeolite is 10.7%, in contrast, this result can arrive to 82.6% on fluffy structure ZSM-5 zeolite. The fluffy structure ZSM-5 zeolite and Au NPs nanocomposites show higher efficiency than traditional Au/ZSM-5 nanocomposites towards catalytic reduction of nitrophenols. Additionally, the experiments with different affecting factors (MWCNTs dosage, aging time, catalysts dosage, pH, initial 4-NP concentration, storage time and recycling times) were carried out to test general applicability of the nanocomposites. And the degradation of nitrophenols experiment was operated to explore the catalytic performance of the prepared nanocomposites in further environmental application. The detailed possible relationship between zeolite with fluffy structure and Au NPs is also proposed in the paper.

Multiple applications of polymers containing electron-reservoir metal-sandwich complexes

Ferrocene-containing polymers have been investigated for more than six decades, and more recently modern synthetic methods have allowed the fabrication of precise polymers that contain a variety of transition-metal complexes. Trends are now oriented towards applications, such as optics, energy conversion and storage, electrochemistry, magnetics, electric conductors and biomedicine. Metal-sandwich complexes such as those of ferrocene type and other related complexes that present redox-robust groups in polymers, i.e. that are isolable in both their oxidized and reduced forms, are of particular interest, because it is possible to address them using electronic or photonic redox stimuli for application. Our research groups have called such complexes Electron-Reservoirs and introduced them in the main chain or in the side chains of well-defined polymers. For instance, polymers with ferrocene in the main chain or in the side chain are oxidized to stable polycationic polyelectrolytes only if ferrocene is part of a biferrocene unit, because biferrocene oxidation leads to the biferrocenium cation that is stabilized by the mixed valency. Then a group of several redox-robust iron sandwich complexes were fabricated and incorporated in precise polymers including multi-block copolymers whose controlled synthesis and block incorporation was achieved for instance using ring-opening-metathesis polymerization. Applications of this family of Electron-Reservoir-containing polymers includes electrochemically induced derivatization of electrodes by decorating them with these polymers, molecular recognition and redox sensing, electrochromics with multiple colours, generation of gold and silver nanoparticles of various size by reduction of gold(iii) and silver(i) precursors and their use for nanocatalysis towards depollution and biomedicine.

Low density magnetic silicate-nickel alloy composite hollow structures: seed induced direct assembly fabrication and catalytic properties

An efficient seed induced direct assembly route is designed for the controlled synthesis of hollow microsphere supported catalysts (HMSCs) with nickel alloy as the active material. The inherent magnetic response of nickel alloy endows HMSCs magnetic separability, and the hollow interior of the support opens a new avenue for self-floating separation. It is found that the introduction of P and Co contributes largely to the improvement of the catalytic performance of the products, which may attributed to synergistic effects and electron transfer. Moreover, the loading amounts of alloy nanoparticles can be easily tailored through properly monitoring the reaction conditions. With the optimized loading of 2.68 wt%, the k_N of HMSC–NiCoP-2.68 wt% reaches 14.0 s⁻¹ g⁻¹ for the catalytic reduction of p-nitrophenol (4-NP), which is higher than commercial 5 wt% Pd/C of 11.6 s⁻¹ g⁻¹ under the same conditions. This work provides additional insights into preparation and property control of an easily separable supported non-noble metal catalyst, which holds potential to be extended to the preparation and property control of other metal nanocatalysts on various supports.

Synthesized HMSC–NiCoP-2.68 wt% catalyst exhibits excellent catalytic reduction activity of 4-NP, which can be separated by self-floating and external magnetic field.

Borophosphate glass as an active media for CuO nanoparticle growth: an efficient catalyst for selenylation of oxadiazoles and application in redox reactions

Herein, we report the preparation of CuO@ borophosphate nanoparticles (CuOnano@glass) and their wide catalytic applications. The glass annealing, under a controlled atmosphere, enables the growth of copper nanoparticles on the glass surface (not within) by an uncommon bottom-up process. Following the thermal annealing of metallic nanoparticles under air atmosphere, supported copper oxide nanoparticles CuONPs on the glass surface can be obtained. The approach enables the glass matrix to be explored as a precursor and a route for the synthesis of supported copper-based nanoparticles in a solvent-free process without immobilization steps or stabilizing agents. In order to demonstrate the wide synthetic utility of this CuONPs glass-based catalyst, one-pot three-component domino reactions were performed under an air atmosphere, affording the desired selenylated oxadiazoles in good to excellent yields. We also extended the application of these new materials as a glass-based catalyst in the phenol hydroxylation and the reduction of 4-nitrophenol.

Fly ash supported Pd-Ag bimetallic nanoparticles exhibiting a synergistic catalytic effect for the reduction of nitrophenol

Coal fly ash (FA) supported Pd-Ag bimetallic nanoparticles (FA-Pd-Ag) were prepared by reducing Pd(II) and Ag(I) salts together onto the dispersed solid support in aqueous medium. Electron microscope analysis (FE-SEM, HRTEM) in combination with elemental mapping (EDS) suggests that the nanoparticles are well dispersed on fly ash with an average diameter of 6-8 nm. The powder XRD analysis indicates that alloying of the interface occurs between Pd and Ag nanoparticles in FA-Pd-Ag, while XPS reveals that charge transfer takes place between the Pd and Ag moieties that come into contact with each other. The FA-Pd-Ag in aqueous NaBH4 solution exhibits an efficient catalytic reduction of 4-nitrophenol into 4-aminophenol and follows pseudo-first-order reaction kinetics (kPd-Ag = 0.7176 min-1). The higher rate constant for FA-Pd-Ag compared to that for their monometallic analogues (FA-Pd (kPd = 0.5449 min-1)) and (FA-Ag (kAg = 0.5572 min-1)) as well as their physical mixture ((FA-Pd + FA-Ag) (kPd+Ag = 0.4075 min-1)) suggests the synergistic catalytic effect of the bimetallic system. Moreover, the present bimetallic nanocatalyst exhibits the highest normalized rate constant (KPd-Ag ≈ 51 100 min-1 mmol-1) compared to the reported bimetallic Pd-Ag nanocatalysts.

Using a Nitrophenol Cocktail Screen to Improve Catalyst Down-selection

The catalytic reduction of 4-nitrophenol (4NP) with excess NaBH₄ is the benchmark model for quantifying catalytic activity of nanoparticles. Although broadly useful, the reaction can be very selective. This can lead to false positives and negatives when utilized for catalyst down-selection from a broader materials candidate pool. We report a multi-nitrophenol cocktail screening methodology incorporating 4NP and other amino-nitrophenols, utilizing Ag, Au, Pt, and Pd nanoparticles on carbon support. The reduction of the cocktail proceeds with no deleterious side reactions on the time-scale tested. The resulting kinetic rates provide an improved correlation of relative catalyst activity when compared to performance with other reducible moieties (e. g. azo bonds), or when compared to solely 4NP screening.

In situ immobilization of ultra-fine Ag NPs onto magnetic Ag@RF@Fe3O4 core-satellite nanocomposites for the rapid catalytic reduction of nitrophenols

Novel magnetic Ag@RF@Fe₃O₄ core-satellite (MCS) nanocomposites were prepared through in situ photoreduction upon bridging Fe(III) and Ag⁺ via hydroxyl groups in resorcinol formaldehyde (RF) resin by virtue of the coordination effect. The catalytic activity of MCS nanocomposites was evaluated based on catalytic 4-nitrophenol (4-NP) reduction with NaBH₄ as the reducing agent. It was noteworthy that the MCS-3 was beneficial to obtain a superior reaction rate constant of 2.27 min^-1 and a TOF up to 72.7 h^-1. Moreover, the MCS could be easily recovered by applying an external magnetic field and was reused for five times without significantly decrease in catalytic activity. Kinetic and thermodynamic study revealed that catalytic 4-NP reduction using MCS nanocatalysts obeyed the Langmuir-Hinshelwood mechanism and was controlled by the diffusion rate of substrates. Overall, the immobilization of ultra-fine Ag nanoparticles and the extremely negative potentials around MCS nanocomposites, which were effective for the diffusion of reactants, synergistically accelerated the catalytic reduction reactions.

Green synthesis of Ag/TiO2 composite coated porous vanadophosphates with enhanced visible-light photo-degradation and catalytic reduction performance for removing organic dyes

As environmental pollution and energy shortages have become global concerns, the construction of highly efficient catalysts using facile and green methods remains a long-term goal. In the present study, we proposed a facile catalyst preparation method in which Ag/TiO₂ composites were coated on the surface of the porous pure inorganic crystalline vanadium phosphates (VPO) by a one-step strategy. More importantly, the in situ reduction of Ag nanoparticles was achieved at room temperature without severe conditions or hydrogen atmosphere in which the porous VPO was employed as the reductant. The prepared Ag/VPO@TiO₂ composites act as a class of efficient bifunctional catalysts for visible light photodegradation of MB molecules and catalytic reduction of p-nitrophenol (4-NP). Among these samples, the 6.82%Ag/VPO@TiO₂ composite exhibited a superior photocatalytic activity in the degradation of MB and an ultrafast reduction rate for 4-NP of about 0.1 mM/40 s. The photocatalytic mechanistic studies revealed that the encapsulated VPO with a narrow band gap not only efficiently enhances the photosensitivity of the TiO₂ but also largely facilitates the photogenerated charge separation. The subsequent deposition of Ag NPs is able to further promote electron transfer ability, which leads to the higher photocatalytic activity. Moreover, the contact of Ag NPs with the surface of semiconductor TiO₂ can result in an electron-enhanced area in their interface that could effectively facilitate the uptake of electrons by the 4-NP molecules and then improve the reduction activity.

Core-shell microgels as thermoresponsive carriers for catalytic palladium nanoparticles

Responsive core-shell microgels are promising systems for a stabilization of Pd nanoparticles and control of their catalytic activity. Here, poly-N-n-propylacrylamide (PNNPAM) was copolymerized with methacrylic acid to yield microgel core particles, which were subsequently coated with an additional, acid-free poly-N-isopropylmethacrylamide (PNIPMAM) shell. Both core and core-shell systems were used as pH- and temperature-responsive carrier systems for the incorporation of palladium nanoparticles. The embedded nanoparticles were found to have a uniform size distribution with diameters at around 20 nm. Their catalytic activity was investigated by following the kinetics of the reduction of p-nitrophenol to p-aminophenol using UV-vis spectroscopy. For the PNNPAM microgel core, the temperature dependence of the rate constant followed the Arrhenius equation, which is an unusual behaviour for thermoresponsive carrier systems but common for passive systems such as polyelectrolyte brushes. In contrast, the catalytic activity of nanoparticles embedded in microgel core-shell systems decreased drastically at the volume phase transition temperature (44 °C) of the PNIPMAM shell. Accordingly, a promising architecture of passive nanoparticle-carrying core and thermoresponsive shell was realized successfully.

Kinetic Analysis of 4-Nitrophenol Reduction by "Water-Soluble" Palladium Nanoparticles

The most important model catalytic reaction to test the catalytic activity of metal nanoparticles is the reduction of 4-nitrophenol to 4-aminophenol by sodium borohydride as it can be precisely monitored by UV–vis spectroscopy with high accuracy. This work presents the catalytic reduction of 4-nitrophenol (4-Nip) to 4-aminophenol (4-Amp) in the presence of Pd nanoparticles and sodium borohydride as reductants in water. We first evaluate the kinetics using classical pseudo first-order kinetics. We report the effects of different initial 4-Nip and NaBH₄ concentrations, reaction temperatures, and mass of Pd nanoparticles used for catalytic reduction. The thermodynamic parameters (activation energy, enthalpy, and entropy) were also determined. Results show that the kinetics are highly dependent on the reactant ratio and that pseudo first-order simplification is not always fit to describe the kinetics of the reaction. Assuming that all steps of this reaction proceed only on the surface of Pd nanoparticles, we applied a Langmuir−Hinshelwood model to describe the kinetics of the reaction. Experimental data of the decay rate of 4-nitrophenol were successfully fitted to the theoretical values obtained from the Langmuir–Hinshelwood model and all thermodynamic parameters, the true rate constant k, as well as the adsorption constants of 4-Nip, and BH₄⁻ (K_4-Nip and K_BH4−) were determined for each temperature.

The supramolecular redox functions of metallomacromolecules

Abstract

Metallomacromolecules are frequently encountered in redox proteins including metal-tanned hide collagen and play crucial roles involving supramolecular properties in biological electron-transfer processes. They are also currently found in non-natural families, such as: metallopolymers, metallodendrimers and metallodendronic polymers. This mini-review discusses the supramolecular redox functions of such nanomaterials developed in our research group. Electron-transfer processes are first examined in mono-, bis- and hexa-nuclear ferrocenes and other electron-reservoir organoiron systems showing the influence of supramolecular and reorganization aspects on their mechanism. Then applications of electron-transfer processes using these same organoiron redox systems in metallomacromolecules and their supramolecular functions are discussed including redox recognition/sensing, catalysis templates, electrocatalysis, redox catalysis, molecular machines, electrochromes, drug delivery device and nanobatteries.

Graphical Abstract

Can Plasmonic Effect Cause an Increase in the Catalytic Reduction of p-nitrophenol by Sodium Borohydride over Au Nanorods?

The catalytic reduction of p-nitrophenol (4-NP) to 4-aminopyridine (4-AP) over Au nanoparticles can be increased by light illumination. Whether this is caused by the plasmonic effect remains unclear. The present research carried out a careful examination of the effects of light illumination and temperature on the catalytic conversion of 4-NP to 4-AP over Au nanorods. It was seen that light illumination has no effect on the apparent activation energy; this indicates that the catalytic mechanism is unchanged and the activity increase cannot be attributed to the effect of hot electrons. Based on the simulation of finite-difference time domain, the theoretical analysis also showed that plasmonic heating cannot play a major role. Thermographic mapping showed that the temperature of water solutions shows an increase under light illumination. By taking this temperature increase into consideration, the light-induced increase of the 4-NP to 4-AP conversion can agree well with dark catalysis, which cannot be attributed to the plasmonic effects of the Au nanorods.

Room-Temperature Nitrophenol Reduction over Ag–CeO2 Catalysts: The Role of Catalyst Preparation Method

Ag–CeO2 catalysts (20 mol % Ag) were synthesized using different techniques (co-precipitation, impregnation, and impregnation of pre-reduced ceria), characterized by XRD, N2 sorption, TEM, H2-TPR methods, and probed in room-temperature p-nitrophenol reduction into p-aminophenol in aqueous solution at atmospheric pressure. The catalyst preparation method was found to determine the textural characteristics, the oxidation state and distribution of silver and, hence, the catalytic activity in the p-nitrophenol reduction. The impregnation technique was the most favorable for the formation over the ceria surface of highly dispersed silver species that are active in the p-nitrophenol reduction (the first-order rate constant k = 0.656 min−1).

Highly Active Hydrogenation Catalysts Based on Pd Nanoparticles Dispersed along Hierarchical Porous Silica Covered with Polydopamine as Interfacial Glue

New catalysts based on Pd(0) nanoparticles (Pd NPs) on a bimodal porous silica of the UVM-7/polydopamine (PDA) support have been synthesized following two preparative strategies based on the sequential or joint incorporation of two components of the composite (Pd and PDA). We analyzed the role played by the PDA as ‘interfacial glue’ between the silica scaffold and the Pd NPs. The catalysts were tested for the hydrogenation of 4-nitrophenol using (NEt4)BH4 as the hydrogenating agent. In addition to the palladium content, the characterization of the catalysts at the micro and nanoscale has highlighted the importance of different parameters, such as the size and dispersion of the Pd NPs, as well as their accessibility to the substrate (greater or lesser depending on their entrapment level in the PDA) on the catalytic efficiency. Staged sequential synthesis has led to better catalytic results. The most active Pd(0) centers seem to be Pd NPs of less than 1 nm on the PDA surface. The efficiency of the catalysts obtained is superior to that of similar materials without PDA. A comprehensive comparison has been made with other catalysts based on Pd NPs in a wide variety of supports. The TOF values achieved are among the best described in the literature.

Synergism of transition metal (Co, Ni, Fe, Mn) nanoparticles and "active support" Fe3O4@C for catalytic reduction of 4-nitrophenol

Research reports, up to date, on supports for non-noble metal catalyst focus mainly on tuning their surface functionality and increasing surface area to maximize metal loading for high catalytic reduction of 4-nitrophenol. However, the "passive" role of these supports leads to inefficient hydride formation on the metal surface which limits catalytic activity. Herein, we present Fe₃O₄@porous-conductive carbon (Fe₃O₄@C-A) core-shell structure as an "active" support for non-noble metals (M = Co, Ni, Fe, and Mn) nanoparticles. Fe₃O₄@C-A was prepared by annealing Fe₃O₄@dense-carbon (Fe₃O₄@C) under N₂. The resultant M-Fe₃O₄@C-A catalysts show high catalytic performance at very low metal loading, while non-noble metals supported on a "passive" support (Fe₃O₄@C) shows very low activity even at high metal loading. The significant difference in catalytic activity is ascribed to the synergistic effect amongst Fe₃O₄, conductive carbon and metal nanoparticles which leads to efficient hydride formation. Amongst the prepared catalysts, Ni-Fe₃O₄@C-A and Co-Fe₃O₄@C-A show the best catalytic activity, completing 4-nitrophenol reduction within 50 s and 80 s, respectively, in the presence of NaBH₄. This result is comparable with previously reported noble-metal-based nanocomposites. In addition, Co-Fe₃O₄@C-A shows high recyclability in 5 consecutive catalytic reactions. In the broader context, our finding highlights how an "active support" together with non-noble metals can provide an efficient mechanism for hydride formation, subsequently accelerating the catalytic reduction of 4-nitrophenol.

Silver-Nanoparticles Embedded Pyridine-Cholesterol Xerogels as Highly Efficient Catalysts for 4-Nitrophenol Reduction

Two xerogels made of 4-pyridyl cholesterol (PC) and silver-nanocomposites (SNCs) thereof have been studied for their efficient reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of aqueous sodium borohydride. Since in-situ silver doping will be effective in ethanol and acetone solvents with a PC gelator, two silver-loaded PC xerogels were prepared and successive SNCs were achieved by using an environmentally benign trisodium citrate dehydrate reducing agent. The formed PC xerogels and their SNCs were comprehensively investigated using different physico-chemical techniques, such as field emission scanning electron microscopy (FE-SEM), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), powdered X-ray diffraction (XRD) and UV-Visible spectroscopy (UV-Vis). The FE-SEM results confirm that the shape of xerogel-covered silver nanoparticles (SNPs) are roughly spherical, with an average size in the range of 30–80 nm. Thermal degradation studies were analyzed via the sensitive graphical Broido’s method using a TGA technique. Both SNC-PC (SNC-PC-X1 and SNC-PC-X2) xerogels showed remarkable catalytic performances, with recyclable conversion efficiency of around 82% after the fourth consecutive run. The apparent rate constant (k_app) of SNC-PC-X1 and SNC-PC-X2 were found to be 6.120 × 10^-3 sec^-1 and 3.758 × 10^-3 sec^-1, respectively, at an ambient temperature.

Denatured proteins as a novel template for the synthesis of well-defined, ultra-stable and water-soluble metal nanostructures for catalytic applications

Abstract

The templated synthesis of noble metal nanoparticles using biomass, such as proteins and polysaccharides, has generated great interest in recent years. In this work, we report on denatured proteins as a novel template for the preparation of water-soluble metal nanoparticles with excellent stability even after high speed centrifugation or storage at room temperature for one year. Different noble metal nanoparticles including spherical gold and platinum nanoparticles as well as gold nanoflowers are obtained using sodium borohydride or ascorbic acid as the reducing agent. The particle size can be controlled by the concentration of the template. These metal nanoparticles are further used as catalysts for the hydrogenation reaction of p-nitrophenol to p-aminophenol. Especially, spherical gold nanoparticles with an average size of 2 nm show remarkable catalytic performance with a rate constant of 1.026 × 10− 2 L s− 1 mg− 1. These metal nanoparticles with tunable size and shape have great potential for various applications such as catalysis, energy, sensing, and biomedicine.

Graphical abstract

Adsorption characteristics and mechanism of p-nitrophenol by pine sawdust biochar samples produced at different pyrolysis temperatures

Biochar is becoming a low-cost substitute of activated carbon for the removal of multiple contaminants. In this study, five biochar samples derived from pine sawdust were produced at different pyrolysis temperatures (300 °C–700 °C) and used adsorbents to remove p-nitrophenol from water. Results indicate that, as the pyrolysis temperature increases, the surface structure of biochar grows in complexity, biochar’s aromaticity and number of functional group decrease, and this material’s polarity increases. Biochar’s physiochemical characteristics and dosage, as well as solution’s pH and environmental temperature significantly influence the p-nitrophenol adsorption behavior of biochar. p-nitrophenol adsorption onto biochar proved to be an endothermic and spontaneous process; furthermore, a greater energy exchange was observed to take place when biochar samples prepared at high temperatures were utilized. The adsorption mechanism includes physical adsorption and chemisorption, whereas its rate is mainly affected by intra-particle diffusion. Notably, in biochar samples prepared at low temperature, adsorption is mainly driven by electrostatic interactions, whereas, in their high-temperature counterparts, p-nitrophenol adsorption is driven also by hydrogen bonding and π–π interactions involving functional groups on the biochar surface.

Liquid Phase Hydrogenation of Pharmaceutical Interest Nitroarenes over Gold-Supported Alumina Nanowires Catalysts

In this work, Au nanoparticles, supported in Al₂O₃ nanowires (ANW) modified with (3-aminopropyl)trimethoxysilane were synthetized, for their use as catalysts in the hydrogenation reaction of 4-(2-fluoro-4-nitrophenyl)-morpholine and 4-(4-nitrophenyl)morpholin-3-one. ANW was obtained by hydrothermal techniques and the metal was incorporated by the reduction of the precursor with NaBH₄ posterior to superficial modification. The catalysts were prepared at different metal loadings and were characterized by different techniques. The characterization revealed structured materials in the form of nanowires and a successful superficial modification. All catalysts show that Au is in a reduced state and the shape of the nanoparticles is spherical, with high metal dispersion and size distributions from 3.7 to 4.6 nm. The different systems supported in modified-ANW were active and selective in the hydrogenation reaction of both substrates, finding for all catalytic systems a selectivity of almost 100% to the aromatic amine. Catalytic data showed pseudo first-order kinetics with respect to the substrate for all experimental conditions used in this work. The solvent plays an important role in the activity and selectivity of the catalyst, where the highest efficiency and operational stability was achieved when ethanol was used as the solvent.

Ultrafine bimetallic Ag-doped Ni nanoparticles embedded in cage-type mesoporous silica SBA-16 as superior catalysts for conversion of toxic nitroaromatic compounds

Highly active Ag-doped Ni nanoparticles are successfully fabricated within carboxylic acid (-COOH) functionalized mesoporous silica SBA-16 by a facile wet incipient technique for catalytic conversion of toxic nitroaromatics. The -COOH groups on SBA-16 play a crucial role by enhancing the electrostatic interactions with Ag(I)/Ni(II) cations, that control the crystal growth during the thermal reduction. Systematic characterizations of SBA-16C and Ag_x%Ni@SBA-16C are performed by different techniques including solid state ¹³C and ²⁹Si nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), N₂ sorption, X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM) and superconducting quantum interference device (SQUID). The highly dispersed ultrafine Ag-doped Ni NPs (∼3 nm) are well-confined within SBA-16C and exhibit magnetic properties that are extremely beneficial for recycling. The bimetallic Ag_2.4%Ni@SBA-16C shows exceptionally high catalytic activity during catalytic conversion of toxic nitroaromatics to environmentally friendly amino-aromatics. The enhanced catalytic activity could be ascribed to the combined effects of unique electronic properties, synergistic effects of Ag-doped Ni, ultra-small size, metal loading, and favorable textural properties. These magnetically separable nanocatalysts show excellent durability.

Platinum Nanoparticles Obtained at Mild Conditions on S-Layer Protein/Polymer Particle Supports

This work presents the synthesis of platinum nanoparticles supported on S-layer protein/polymeric particle systems, obtained by combining proteins isolated from Lactobacillus kefiri and an aqueous dispersion of acrylic particles. FTIR spectra of the protein/polymer supports did not show changes in the Amide I band of the proteins, suggesting that proteins maintained their conformation after adsorption. The SAXS spectra and DLS results are consistent with the formation of a protein corona around the polymer particles. After combining the supports with the platinum complex and subsequently reducing the combination with hydrogen at mild conditions, we obtained colloidal nanocomposite materials. In these, platinum nanoparticles with diameters around 3 nm located on the surface of the protein/polymer supports were observed by TEM. The obtained nanosystems showed catalytic activity in the reduction of p-nitrophenol with NaBH₄ at room temperature with conversions of 100% for reaction times of 50 to 70 min.

Tough, Stretchable, Compressive Double Network Hydrogel Using Natural Glycyrrhizic Acid Tailored Low-Molecular-Weight Gelator Strategy: In Situ Spontaneous Formation of Au Nanoparticles To Generate a Continuous Flow Reactor

Traditional solid supports of metal nanoparticles (MNPs) often suffer from the poor mechanical performance, the low recycling efficiency, and the mass loss in the regeneration process. To overcome this limit, in this work, we reported a natural triterpenoid-tailored low-molecular-weight gelator (LMWG) strategy to fabricate double network (DN) hydrogels with excellent mechanical properties for supporting MNPs. In this strategy, the supramolecular fibrillar structure of glycyrrhizic acid (GL) and the cross-linked polyacrylamide (PAAm) were used as the first physical network and the second chemical network, respectively. The resulting GL/PAAm DN gels possessed tough, stretchable, and compressive properties, as well as high fatigue resistance. In addition, the ice-templating technique has been used to recast the DN gel through the anisotropical growth of ice crystals for increasing the porosity and surface area. On account of the reductibility of the diglucuronic moiety of GL, gold nanoparticles (AuNPs) were in situ spontaneously reduced from Au(III) ions without external reducing reagents and anchored on the pore surface of Recast-GL/PAAm DN gel. This AuNP-anchored Recast-GL/PAAm DN gel can be used as a continuous flow reactor to catalyze the reduction of 4-nitrophenol to 4-aminophenol with high catalytic activity, good recyclability, and long-term stability. Our work provided an effective strategy to generate promising supports of MNPs with highly mechanical properties and excellent catalytic efficiencies.

Sea-Island-Like Morphology of CuNi Bimetallic Nanoparticles Uniformly Anchored on Single Layer Graphene Oxide as a Highly Efficient and Noble-Metal-Free Catalyst for Cyanation of Aryl Halides

Aryl nitriles are versatile compounds that can be synthesized via transition-metal-mediated cyanation of aryl halides. Most of the supported-heterogeneous catalysts are noble-metals based and there are very limited numbers of efficient non-noble metal based catalysts demonstrated for the cyanation of aryl halides. Herein, bimetallic CuNi-oxide nanoparticles supported graphene oxide nanocatalyst (CuNi/GO-I and CuNi/GO-II) has been demonstrated as highly efficient system for the cyanation of aryl halides with K₄[Fe(CN)₆] as a cyanating agent. Metal-support interaction, defect ratio and synergistic effect with the bimetallic nanocatalyst were investigated. To our delight, the CuNi/GO-I system activity transformed a wide range of substrates such as aryl iodides, aryl bromides, aryl chlorides and heteroaryl compounds (Yields: 95-71%, TON/TOF: 50-38/2 h^-1). Moreover, enhanced catalytic performance of CuNi/GO-I and CuNi/GO-II in reduction of 4-nitropehnol with NaBH₄ was also confirmed (k_app = 18.2 × 10^-3 s^-1 with 0.1 mg of CuNi/GO-I). Possible mechanism has been proposed for the CuNi/GO-I catalyzed cyanation and reduction reactions. Reusability, heterogeneity and stability of the CuNi/GO-I are also found to be good.

Facile Sonochemical Preparation of Au-ZrO2 Nanocatalyst for the Catalytic Reduction of 4-Nitrophenol

High-intensity ultrasonic waves have great potential for the green synthesis of various nanomaterials under mild conditions and offer an excellent alternative for hazardous chemical methods. Herein a facile approach for the eco-friendly synthesis of Au-ZrO2 nanocatalyst with a high catalytic activity using a facile ultrasonic method is presented. Gold (Au) in the nanosize regime was successfully deposited on the surface of solvothermally synthesized monodispersed ZrO2 nanoparticles (ZrO2 NPs) in a very short period of time (5 min) at room temperature. Spherical shape small size Au nanoparticles that are uniformly dispersed on the surface of ZrO2 nanoparticles were obtained. Notably, in the absence of ZrO2 nanoparticles, HAuCl4 could not be reduced, indicating that nano-sized ZrO2 not only acted as support but also helped to reduce the gold precursor at the surface. The as-prepared Au-ZrO2 nanocatalyst was characterized by various techniques. The Au-ZrO2 nanocatalyst served as a highly efficient reducing catalyst for the reduction of 4-nitrophenol. The reaction time decreased with increasing the amount of catalyst.

Orthogonal deposition of Au on different facets of Ag cuboctahedra for the fabrication of nanoboxes with complementary surfaces

We report the fabrication of Ag-Au cuboctahedral nanoboxes enclosed by {100} and {111} facets, respectively, through the orthogonal deposition of Au on two different facets of Ag cuboctahedra. Specifically, we titrate aqueous HAuCl₄ into an aqueous mixture containing Ag cuboctahedra, ascorbic acid, and NaOH (under basic conditions), in the presence of poly(vinylpyrrolidone) (PVP) and cetyltrimethylammonium chloride (CTAC), respectively. In the case of PVP, the oxidation of Ag was initiated from the {111} facets of the cuboctahedra through the galvanic replacement reaction between Au(iii) and Ag, accompanied by the deposition of Au onto the {100} facets. Because the dissolved Ag(i) ions could react with NaOH to form Ag₂O on the {111} facets and thus terminate the galvanic reaction, the Au(iii) ions would be further reduced by the ascorbate monoanion (HAsc^-) to generate Au atoms for their continuing deposition on the {100} facets, converting Ag cuboctahedra to Ag@Au_{100} cuboctahedra. Upon the etching of Ag from the core, we obtained Ag-Au cuboctahedral nanoboxes enclosed by {100} facets. In contrast, when CTAC was present, the oxidation of Ag through a galvanic reaction could continuously proceed on {100} facets as the dissolved Ag(i) ions would react with the excessive amount of Cl^- ions derived from CTAC to produce soluble AgCl₂^- ions rather than insoluble Ag₂O. As a result, the dissolved Ag(i) and Au(iii) ions would be co-reduced by HAsc^- for the generation of Ag and Au atoms, followed by their co-deposition onto {111} facets for the generation of Ag@Au_{111} concave cuboctahedra. After the removal of Ag from the core by etching, we obtained Ag-Au_{111} cuboctahedral nanoboxes enclosed by {111} facets. Both samples of cuboctahedral nanoboxes exhibited strong optical absorption in the infrared region. Interestingly, the cuboctahedral nanoboxes enclosed by {111} facets showed significantly enhanced catalytic activity toward the reduction of 4-nitrophenol by NaBH₄ relative to their counterparts encased by {100} facets.

Optimization of Cu catalysts for nitrophenol reduction, click reaction and alkyne coupling

Earth-abundant nanocatalysts are actively searched to replace expensive noble metal catalysts for a number of essential processes.