Catalytic reduction of 4-nitrophenol over Ni-Pd nanodimers supported on nitrogen-doped reduced graphene oxide

Reduction-immobilizing strategy of polymer-embedded sub-2 nm Cu nanoparticles with uniform size and distribution responsible for robust catalytic reactions

Polymer-embedded metal nanoparticles are in great demand owing to their unique features, leading to their use in various important applications, including catalysis reactions. However, particle sintering and aggregation are serious drawbacks, resulting in a drastic loss of catalytic activity and recyclability. Herein, a reduction-immobilizing strategy of polymer-embedded sub-2 nm Cu nanoparticles offered highly controlled distribution and nanoparticle size within polymer structures with high fidelity. This work sheds light on the high catalytic performance of nanoparticles that rely on their ultrasmall size and uniform distribution in polymer structures, generating more active sites that result in high efficiency reduction of organic compounds. A catalysis study was carried out for the hydrogenation of nitro compounds, achieving nearly 100% reduction in an extremely short time and remaining stable after 15 consecutive cycles. Furthermore, the catalytic mechanism was demonstrated by density functional theory (DFT) calculations. Notably, the discovery of this facile strategy may enable the remarkable cutting-edge design of catalyst materials with promising performance and stability.

Ultrasmall Bismuth Nanoparticles Supported Over Nitrogen-Rich Porous Triazine-Piperazine Polymer for Efficient Catalytic Reduction

The development of inexpensive and reusable nanocatalysts to convert the hazardous pollutant 4-nitrophenol (4-NP) into a valuable platform chemical 4-aminophenol (4-AP) is quite demanding due to environmental and public health concerns. Herein, we report a facile strategy for the preparation of supported Bi nanoparticles (NPs) over the surfaces of nitrogen-rich porous covalent triazine-piperazine-3D nanoflowers (BiNPs@3D-NCTP). SEM and TEM image analysis suggested 3D-flower-like morphology of the composite consisting of the self-assembly of interweaving and the slight bending of the nanoflakes. The powder X-ray diffraction (PXRD) analysis also confirmed the loading of Bi NPs. N2 sorption analysis suggested BET surface areas of 663 and 364 m2 g-1 for the 3D-NCTP and BiNPs@3D-NCTP materials, respectively. The large surface area, bimodal pores and 3D nanoflower architecture enable uniform loading of Bi nanoparticles, while its nitrogen-rich functionality stabilizes and acts as a capping agent restricting further nanoparticle expansion. BiNPs@3D-NCTP showed a 99.85 % conversion for the 4-NP to 4-AP within four minutes. The normalized rate constant of 38.3 min-1 mg-1 of BiNPs@3D-NCTP catalyst for the reduction of 4-NP suggested its superior catalytic efficiency. Nitrogen-rich functionality activates the catalytic site to accelerate the reaction, while bimodal pores can promote the diffusion of reactant molecules. After five catalytic cycles, the nanocatalyst showed high chemical stability and negligible activity loss.

Magnetically retrievable carbon-wrapped CNT/Ni nanospheres as efficient catalysts for nitroaromatic reduction

We present a facile strategy for synthesizing magnetically retrievable carbon-wrapped CNT/Ni nanospheres (C-wrapped CNT/Ni) that enhance the catalytic performance of metals for environmental pollutant reduction. Structural and compositional analyses using X-ray diffraction (XRD), Raman spectroscopy, energy-dispersive X-ray spectroscopy (EDS), and field emission scanning electron microscopy (FESEM) confirmed the phase purity, morphology, and structure of the C-wrapped CNT/Ni. XRD, Raman, and EDS data validate the formation of the nanospheres, while FESEM images reveal uniform Ni nanospheres wrapped with a carbon layer through interconnected, evenly dispersed CNTs. Initially, Ni nanoparticles were anchored onto multiwalled carbon nanotubes to form magnetic CNT/Ni nanospheres, which were then coated with a carbon layer to prevent aggregation, improve Ni particle stability, and introduce additional surface functionalities. The catalytic efficacy of C-wrapped CNT/Ni was assessed through the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The reaction rate constant (k app = 0.6167 min-1) with C-wrapped CNT/Ni is approximately six times higher than that with bare Ni nanospheres (k app = 0.1056 min-1). This enhanced catalytic activity is attributed to the synergistic effect between the spherical Ni core and the wrapped carbon layer, mediated by the interconnected CNT, which promotes efficient hydride formation. Additionally, C-wrapped CNT/Ni demonstrates exceptional reusability in the 4-NP reduction process. The integration of these features within a single framework suggests its significant potential for diverse engineering and environmental applications.

Coupling Fe-Co atomic pair to promote the selective reduction of nitroaromatics under mild conditions

Selectively reducing nitroaromatics into aromatic amines will not only remove nitroaromatic pollutants in waste effluents to reduce environmental risks, but also yield important feedstocks for chemical industrial manufactures. In this study, a FeCo-co-embedded N-doped Carbon (FeCo-N-C) catalyst with Fe-Co atomic pair has been identified with favorable activity, superior selectivity, excellent reusability, as well as outstanding performance in the treatment of real water. The combined results from theoretical study and experimental tests indicate that the improved catalytic performance of FeCo-N-C is owing to the narrowed band gap and electron delocalization caused by the Fe-Co atomic pair which can improve electron transport in its catalytic reaction. The results of isotope experiments and H* quenching experiments confirm that H2O is the source of hydrogen in catalytic reduction of PNP. FeCo-N-C is identified as a superior catalyst to replace multitudinous currently used noble-metal catalysts for the selective catalytic reduction of nitroaromatics in wastewater treatment.

Cobalt-nickel phosphide supported on reduced graphene oxide for sensitive electrochemical detection of bisphenol A

Bisphenol A (BPA) is a commonly utilized phenolic contaminant in several manufacturing processes, contributing to environmental pollution. Therefore, the detection of BPA holds significant importance for monitoring water quality. In this work, we report a robust electrochemical detection method for BPA utilizing cobalt-nickel bimetal phosphide nanoparticles (CoNiP) supported on reduced graphene oxide (rGO). The CoNiP@rGO-modified glassy carbon electrode exhibits remarkable electrochemical activity in BPA detection. The detection mechanism is controlled by adsorption-mediated electron transfer, showcasing a low limit of detection (LOD) at 0.38 nM and a high sensitivity of 96.4 A M-1 cm-2 within the linear range of 0.001-8 μM. Furthermore, our developed sensor demonstrates good reproducibility and successfully detected BPA in actual water samples. The electrochemical activity of CoNiP@rGO was also characterized for hydroquinone (HQ) detected through a diffusion-controlled mechanism, displaying an excellent sensitivity of 36.4 A M-1 cm-2 across a broad linear range. These findings underscore the promising potential of CoNiP@rGO as a candidate for electrochemical detection of phenolic contaminants, especially in the sensing of BPA in environmental water samples. This efficacy is attributed to the modulation of its electronic properties, combined with its large electroactive surface area and low electron-transfer resistance.

Electron transfer in heterojunction catalysts

Heterojunction catalysis, the cornerstone of the modern chemical industry, shows potential to tackle the growing energy and environmental crises. Electron transfer (ET) is ubiquitous in heterojunction catalysts, and it holds great promise for improving the catalytic efficiency by tuning the electronic structures or building internal electric fields at interfaces. This perspective summarizes the recent progress of catalysis involving ET in heterojunction catalysts and pinpoints its crucial role in catalytic mechanisms. We specifically highlight the occurrence, driving forces, and applications of ET in heterojunction catalysis. For corroborating the ET processes, common techniques with measurement principles are introduced. We end with the limitations of the current study on ET, and envision future challenges in this field.

Noble metal nanoparticles (Mx = Ag, Au, Pd) decorated graphitic carbon nitride nanosheets for ultrafast catalytic reduction of anthropogenic pollutant, 4-nitrophenol

We report an effective facile immobilization of noble nanoparticles (M_x = Ag, Au and Pd) assembled on g-C₃N₄ (g-CN) prepared via a simple ultra-sonication strategy. The M_x assembled g-CN nanocomposites were applied for the effective conversion of 4-nitrophenol (4-NP). As prepared nanocomposites were characterized by techniques of XRD, SEM-EDS, TEM, XPS, and FT-IR analysis to gain crystallographic structural, and morphological insights. The Pd@g-C₃N₄ (Pd@g-CN) nanocomposite exhibited best catalytic performance (k_app = 1.141 min^-1) toward the conversion of 4-NP to 4-aminophenol (4-AP), almost 100% within 4 min using aqueous sodium borohydride (NaBH₄). The higher catalytic efficiency of Pd@g-CN could be attributed to the surface electron density on the Pd and rapid electron transfer capacity. Interestingly, g-CN not only role as a stabilizer but also provided compatibility for noble metal deposition, which improves the chemical and morphological stability of noble metal nanoparticles. Different reaction parameters including concentrations of 4-NP, and catalyst amount were studied. These unique combinations make noble metal nanoparticles anchored g-CN nanosheets an ideal platform for catalysis applications and environmental remediation.

3-D porous cellulose nanofibril aerogels with a controllable copper nanoparticle loading as a highly efficient non-noble-metal catalyst for 4-nitrophenol reduction

Nitrophenols(NPs) are highly toxic compounds that occur in various industrial effluents. Herein, we investigated Cu nanoparticle-loaded cellulose nanofibril (CNF/PEI-Cu) aerogels as a catalyst for degrading 4-nitrophenol (4NP) in the wastewater. Non-noble metal based low-cost catalyst material and easily scalable preparation method make CNF/PEI-Cu aerogel as an appropriate catalyst for practical application in 4NP wastewater treatment. Our strategy to improve the loading amount of homogeneously distributed Cu nanoparticles was to functionalize a CNF aerogel using polyethylene imine (PEI), which can bind Cu²⁺ ions. Porous CNF aerogels with homogenously distributed 20-40 nm Cu nanoparticles were obtained by adsorbing Cu²⁺ ions and chemically reducing them to Cu metal. The FTIR, XRD, SEM, XPS and ICP-OES analysis were used to confirm the in-situ formation of Cu nanoparticles. In the presence of the CNF/PEI-Cu aerogels, 4NP was effectively reduced to 4-aminophenol (4AP) without loss of the Cu nanoparticles. The activation energy (E_a) and reaction rate constant (k_app) of the catalytic 4NP reduction reaction by the CNF/PEI2-Cu aerogels were calculated to be E_a = 39.56 kJ mol^-1 and k_app = 0.770 min^-1, respectively. The E_a is similar or even smaller than the E_a values of the corresponding reactions involving noble-metal catalysts, demonstrating that the CNF/PEI-Cu aerogels developed in the present study have strong potential as practical and economical catalysts.

Layered Double Hydroxide/Nanocarbon Composites as Heterogeneous Catalysts: A Review

The synthesis and applications of composites based on layered double hydroxides (LDHs) and nanocarbons have recently seen great development. On the one hand, LDHs are versatile 2D compounds that present a plethora of applications, from medicine to energy conversion, environmental remediation, and heterogeneous catalysis. On the other, nanocarbons present unique physical and chemical properties owing to their low-dimensional structure and sp2 hybridization of carbon atoms, which endows them with excellent charge carrier mobility, outstanding mechanical strength, and high thermal conductivity. Many reviews described the applications of LDH/nanocarbon composites in the areas of energy and photo- and electro-catalysis, but there is still scarce literature on their latest applications as heterogeneous catalysts in chemical synthesis and conversion, which is the object of this review. First, the properties of the LDHs and of the different types of carbon materials involved as building blocks of the composites are summarized. Then, the synthesis methods of the composites are described, emphasizing the parameters allowing their properties to be controlled. This highlights their great adaptability and easier implementation. Afterwards, the application of LDH/carbon composites as catalysts for C–C bond formation, higher alcohol synthesis (HAS), oxidation, and hydrogenation reactions is reported and discussed in depth.

Advances in Graphene/Inorganic Nanoparticle Composites for Catalytic Applications

Graphene-based nanocomposites with inorganic (metal and metal oxide) nanoparticles leads to materials with high catalytic activity for a variety of chemical transformations. Graphene and its derivatives such as graphene oxide, highly reduced graphene oxide, or nitrogen-doped graphene are excellent support materials due to their high surface area, their extended π-system, and variable functionalities for effective chemical interactions to fabricate nanocomposites. The ability to fine-tune the surface composition for desired functionalities enhances the versatility of graphene-based nanocomposites in catalysis. This review summarizes the preparation of graphene/inorganic NPs based nanocomposites and their use in catalytic applications. We discuss the large-scale synthesis of graphene-based nanomaterials. We have also highlighted the interfacial electronic communication between graphene/inorganic nanoparticles and other factors resulting in increased catalytic efficiencies.

Pd-Nanoparticles@Layered Double Hydroxide/ Reduced Graphene Oxide (Pd NPs@LDH/rGO) Nanocomposite Catalyst for Highly Efficient Green Reduction of Aromatic Nitro Compounds

A facile chemical method is developed to fabricate well-dispersed and an approx. 5 nm sized Pd-nanoparticles (Pd-NPs) deposited ZnAl-LDH/rGO nanocomposite (Pd NPs@LDH/rGO) as a highly efficient and stable catalyst for...

CoFe Nanoparticle-Decorated Reduced Graphene Oxide for the Highly Efficient Reduction of 4-Nitrophenol

High-performance, nonprecious metal catalysts with special morphologies and easy-to-recycle properties are essential for the treatment of environmental pollutants. Herein, CoFe nanoparticle-decorated reduced graphene oxide (RGO) catalysts were designed and successfully fabricated, and the catalyst was then used to reduce 4-nitrophenol into 4-aminophenol. Outstanding catalytic properties with a reduction rate constant of 4.613 min^-1 were achieved due to the synergistic properties of the CoFe metal alloy and the high-conductivity RGO components in the catalysts. In addition, the catalyst was conveniently recovered via magnets due to its inherent magnetic properties. The facile preparation, outstanding catalytic performance, structural stability, and low material costs make the CoFe/RGO nanocatalyst a promising candidate for potential applications in catalysis.

Size-Dependent Catalytic Activity of PVA-Stabilized Palladium Nanoparticles in p-Nitrophenol Reduction: Using a Thermoresponsive Nanoreactor

Palladium nanoparticles (Pd NPs) of various average global diameters (2.1-7.1 nm) encapsulated with hydrophilic polymer polyvinyl alcohol (PVA) have been synthesized and used as catalysts for sodium borohydride assisted reduction of p-nitrophenol to p-aminophenol. The synthesized catalysts exhibit excellent and typical size-dependent catalytic activity in the green protocol. UV-visible absorption spectroscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy were employed to characterize the prepared Pd NPs. The kinetics of this reaction was easily monitored by a UV-visible absorption spectrophotometer. The mechanism of the reaction is explained by the Langmuir-Hinshelwood model. The catalytic performance increases with decreasing size of the synthesized nanoparticles. The apparent rate constants (k app × 103/s-1) of the catalytic reduction in the presence of Pd NPs of average diameters of 2.1, 3.35, 6.2, and 7.1 nm are determined as 8.57, 7.67, 6.16, and 5.04, respectively, at 298 K by using 2.91 mol % palladium nanocatalyst in each case. Moreover, the estimated activation energy of 22.2 kJ mol-1 obtained for Pd NPs with the smallest average diameter of 2.1 nm is very low as reported in the literature for the reduction. The influences of catalyst dose and concentration of p-nitrophenol on catalytic reduction are fully investigated. The catalyst with the largest diameter shows a temperature-sensitive property that might be due to the presence of a very low amount of rapped PVA used as stabilizer during the fabrication process. Thus, the synthetic protocol provides a unique fabrication process of a catalytically active thermoresponsive nanoreactor consisting of Pd NPs encapsulated into a PVA stabilizing agent.

Highly Sensitive and Selective Eco-Toxic 4-Nitrophenol Chemical Sensor Based on Ag-Doped ZnO Nanoflowers Decorated with Nanosheets

Herein, we have developed a novel sensing electrode to detect the eco-toxic 4-nitrophenol (4-NP). Ag-doped-ZnO nanoflowers were synthesized by facile hydrothermal method and examined by several characterization techniques in order to understand the morphology, crystal structure, composition, and surface properties. Morphological results were confirmed by the formation of Ag-doped ZnO nanoflowers decorated with nanosheets. Ag-doped ZnO/glassy carbon electrode (GCE) electrode-material-matrix was used for electrochemical sensing of toxic 4-NP. Under optimized conditions, Ag-doped ZnO/GCE modified electrode exhibits high-sensitivity and selectivity compared to the bare GCE electrode. The Ag-doped ZnO/GCE modified electrode exhibits high electrocatalytic oxidation towards 4-NP. Anodic peak current of 4-NP is increased linearly by increasing the concentration of nitrophenol. Additionally, Ag-doped ZnO/GCE shows a wide range of sensitivity from 10 µM to 500 µM, and a linear calibration plot with a good detection limit of 3 µM (S/N = 3). The proposed Ag-doped ZnO/GCE modified electrode showed high sensing stability. In addition, the oxidation mechanism was studied. The obtained results revealed that the Ag-ZnO/GCE electrode could be the promising sensing electrode for 4-NP sensing.

In situ preparation of highly dispersed Pd supported on exfoliated layered double hydroxides via nitrogen plasma for 4-nitrophenol reduction

In this work, a simple and environmental-friendly nitrogen glow discharge plasma reduction method has been developed for synthesizing palladium nanoparticles (PdNPs) supported on exfoliated Mg-Al-layered double hydroxide (Pd/LDH) catalysts. The as-prepared catalysts were characterized by means of characterizations methods, which contain X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectrometry (XPS), and Fourier transform infrared (FT-IR). Highly dispersed ultrafine PdNPs were supported on exfoliated, defect-induced LDHs uniformly without agglomeration. The effects of treatment time of nitrogen plasma and Pd loading amount on structure, morphology, and catalytic performance of Pd/LDHs were investigated. The comparisons of structure and morphology between LDHs and Pd/LDHs were also discussed. The average particle size of as-synthesized PdNPs with face-centered cubic structure is 2.01 nm, which ranges from 1.18 to 3.01 nm. Nitrogen plasma cannot only reduce Pd²⁺, but also exfoliate LDHs, introduce defects, and even destroy the structure of LDHs. The Pd/LDH catalyst with 1 wt% Pd loading under nitrogen plasma treatment for 60 min showed the best catalytic performance in 4-nitrophenol reduction. The turnover frequency (TOF) of as-prepared catalyst is 20-fold higher than that of commercial Pd/C catalyst.

Cold Plasma Preparation of Pd/Graphene Catalyst for Reduction of p-Nitrophenol

Supported metal nanoparticles with small size and high dispersion can improve the performance of heterogeneous catalysts. To prepare graphene-supported Pd catalysts, graphene and PdCl₂ were used as support and Pd precursors, respectively. Pd/G-P and Pd/G-H catalysts were prepared by cold plasma and conventional thermal reduction, respectively, for the catalytic reduction of p-nitrophenol (4-NP). The reaction followed quasi-first-order kinetics, and the apparent rate constant of Pd/G-P and Pd/G-H was 0.0111 and 0.0042 s⁻¹, respectively. The graphene support was exfoliated by thermal reduction and cold plasma, which benefits the 4-NP adsorption. Pd/G-P presented a higher performance because cold plasma promoted the migration of Pd species to the support outer surface. The Pd/C atomic ratio for Pd/G-P and Pd/G-H was 0.014 and 0.010, respectively. In addition, the Pd nanoparticles in Pd/G-P were smaller than those in Pd/G-H, which was beneficial for the catalytic reduction. The Pd/G-P sample presented abundant oxygen-containing functional groups, which anchored the metal nanoparticles and enhanced the metal-support interaction. This was further confirmed by the shift in the binding energy to a high value for Pd3d in Pd/G-P. The cold plasma method operated under atmospheric pressure is effective for the preparation of Pd/G catalysts with enhanced catalytic activity for 4-NP reduction.

Pd Anchored on a Phytic Acid/Thiourea Polymer as a Highly Active and Stable Catalyst for the Reduction of Nitroarene

A kind of N, P, C, O-containing polymer was easily prepared via microwave heating of phytic acid and thiourea just for 90 s. After impregnation and reduction of H₂PdCl₄, highly dispersed Pd single atoms/sub-nano clusters loaded on the phytic acid/thiourea polymer (Pd-CNSP) were successfully obtained. Owing to the synergetic effect of the polymer support and Pd, the catalyst Pd-CNSP achieves a great atomic efficiency of Pd species and exhibits an outstanding catalytic ability in the reduction of 4-nitrophenol. The k value of the catalyst Pd-CNSP (2.17 min^-1 mg^-1) is about 19 times higher than that of the commercial Pd/C (5 wt %) catalyst. The turnover frequency value is as high as 848 min^-1, which is the highest value reported so far. Pd-CNSP also has good selectivity for the reduction of halogen-substituted (Cl and Br) nitroaromatics. It is expected to be mass-produced and used in other industrial hydrogenation reactions.

A facile and green synthesis of CuO/NiO nanoparticles and their removal activity of toxic nitro compounds in aqueous medium

In this present work, we report the green synthesis of mixed bimetal oxides (CuO/NiO) for the efficient reduction of toxic nitrophenols (NP, DNP and TNP) in aqueous medium. The CuO/NiO NPs were synthesized by green hydrothermal method combined calcination process. The physiochemical properties of the synthesized CuO/NiO NPs were systematically characterized by using XRD, XPS, FTIR, SEM, and HR-TEM techniques. The calcinated CuO/NiO NPs XRD pattern and SEM morphology show the high crystalline nature than the non-calcinated. Whereas, the XPS and FTIR results confirmed the formation of the metal oxide bonding and the interaction of the bimetals. The HR-TEM images showed the spherical crystals with average particle size about 25 nm. In addition, the SAED pattern confirmed the polycrystalline nature of CuO/NiO NPs. The catalytic reduction of nitro compounds to amino derivative was studied with reducing agent (NaBH₄). The CuO/NiO NPs showed the high catalytic activity and completed the reduction reaction of NP, DNP and TNP with in 2, 5 and 10 min respectively. In addition, CuO/NiO NPS exhibited the excellent kinetic rate constant k value about 1.519, 0.5102, 0.4601 min^-1 for NP, DNP and TNP respectively. Furthermore, the conversion product aminophenol was observed for these three nitro compounds. The proposed CuO/NiO NPs showed excellent crystal stability after the nitrophenol reduction reactions. An inexpensive CuO/NiO NPs is a promising catalysts for reduction of toxic nitro compounds to useful products in aqueous or non-aqueous medium.

Facile Use of Silver Nanoparticles-Loaded Alumina/Silica in Nanofluid Formulations for Enhanced Catalytic Performance toward 4-Nitrophenol Reduction

The introduction of toxic chemicals into the environment can result in water pollution leading to the degradation of biodiversity as well as human health. This study presents a new approach of using metal oxides (Al₂O₃ and SiO₂) modified with a plasmonic metal (silver, Ag) nanoparticles (NPs)-based nanofluid (NF) formulation for environmental remediation purposes. Firstly, we prepared the Al₂O₃ and SiO₂ NFs of different concentrations (0.2 to 2.0 weight %) by ultrasonic-assisted dispersion of Al₂O₃ and SiO₂ NPs with water as the base fluid. The thermo-physical (viscosity, activation energy, and thermal conductivity), electrical (AC conductivity and dielectric constant) and physical (ultrasonic velocity, density, refractive index) and stability characteristics were comparatively evaluated. The Al₂O₃ and SiO₂ NPs were then catalytically activated by loading silver NPs to obtain Al₂O₃/SiO₂@Ag composite NPs. The catalytic reduction of 4-nitrophenol (4-NP) with Al₂O₃/SiO₂@Ag based NFs was followed. The catalytic efficiency of Al₂O₃@Ag NF and SiO₂@Ag NF, for the 4-NP catalysis, is compared. Based on the catalytic rate constant evaluation, the catalytic reduction efficiency for 4-NP is found to be superior for 2% weight Al₂O₃@Ag NF (92.9 × 10⁻³ s⁻¹) as compared to the SiO₂@Ag NF (29.3 × 10⁻³ s⁻¹). Importantly, the enhanced catalytic efficiency of 2% weight Al₂O₃@Ag NF for 4-NP removal is much higher than other metal NPs based catalysts reported in the literature, signifying the importance of NF formulation-based catalysis.

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.

Templated synthesis of nickel nanoparticles embedded in a carbon layer within silica capsules

The encapsulation of small non-noble metal nanoparticles (NPs) within an inorganic layer has received considerable attention owing to their enhanced stability and high catalytic activity. Using a combination of emulsion-free polymerization, inner RF-Ni2+ and outer SiO2 coating, and subsequent carbonization treatment, herein, we have fabricated worm-like structured Ni-based composites in which a high density of nickel NPs are embedded in a carbon layer and also entrapped by SiO2 nanocages. We find that the carbonization temperature plays a vital role in adjusting the size of the Ni NPs. A detailed examination of the encapsulated nickel particles synthesized at 700 °C exhibited the best performance on the catalysis of the reduction of 4-NPs. Moreover, owing to the good alloying ability of the Ni NPs with noble metal NPs, the Ni-Pd alloy NPs are also entrapped in the SiO2 nanocages, which exhibit better performance on the catalysis than the Ni-based composites. The encapsulation of Ni-Pd alloys within SiO2 nanocages also improves stability against agglomeration and metal separation during catalytic operation.

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.

In situ formation of nitrogen-doped carbon-wrapped Co3O4 enabling highly efficient and stable catalytic reduction of p-nitrophenol

CNx@Co₃O₄ exhibits the highest activity reported to date for Co-based catalysts in the reduction of PNP to PAP with NaBH₄.

Synthesis of CoFe2O4/C nano-catalyst with excellent performance by molten salt method and its application in 4-nitrophenol reduction

CoFe₂O₄/C nano-sheets (NSs) have been synthesized by a facile molten salt method using cheap potassium fulvate as carbon source and sodium chloride as template. The morphology, crystallinity and composition of the materials were analyzed by TEM, XRD and XPS. The study on the catalytic performance of 4-nitrophenol (4-NP) shows that CoFe₂O₄/C-600 nano-catalyst has the highest catalytic activity and the corresponding apparent constant is 1.91 min^-1, this result is higher than that reported in most literatures. Catalytic kinetics of 4-NP reduction was studied in this article, and activation energy (E_a) was calculated to be 14.31 kJ mol^-1. The catalyst also shows good cycle performance and stability. This convenient method provides a reference for the synthesis of MFe₂O₄/C and other nano-metal oxides/C nanocomposite catalysts.

Synthesis and controlled morphology of Ni@Ag core shell nanowires with excellent catalytic efficiency and recyclability

Ni@Ag core shell nanowires (NWs) were prepared by in situ chemical reduction of Ag⁺ around NiNWs as the inner core. Different Ni@Ag NWs with controllable morphologies were achieved through the layer-plus-island growth mode and this mechanism was confirmed by scanning electron microscopy, X-ray fluorescence, and X-ray photoelectron spectroscopy analyses. When used as a catalyst, the synthesized Ni@Ag NWs exhibited high reduction efficiency by showing a high reaction rate constant k of 0.408 s^-1 in reducing 4-nitrophenol at room temperature. Besides, combining the magnetic property, including high saturation magnetization and low coercivity, the magnetic NiNW core contributes to excellent recyclability and long-term stability with only a 2.2% performance loss after 10 recycles by magnets. The Ni@Ag NWs proposed here show unprecedentedly high potential in applications requiring high efficiency and a recyclable catalyst.

Facile Aqueous-Phase Synthesis of an Ultrasmall Bismuth Nanocatalyst for the Reduction of 4-Nitrophenol

Bismuth metallic nanoparticles have evoked considerable interest in catalysis owing to their small size, high surface area-to-volume ratio, and low toxicity. However, the need for toxic reductants and organic solvents in their synthesis often limits their desirability for application development. Here, we describe a green strategy to synthesize bismuth nanodots via the redox reactions between bismuth nitrate and d-glucose, in the presence of poly(vinylpyrrolidone) in the basic aqueous phase. Both reagents play a crucial role in the formation of monodisperse bismuth nanodots acting as mild reducing and capping agents, respectively. We further demonstrate that the catalytic activity of these dots via the successful reduction of the environmental contaminant 4-nitrophenol to its useful 4-aminophenol analogue requiring only 36 μg/mL nanocatalyst for 20 mM of the substrate. Moreover, they can be recovered and recycled in multiple reactions before the onset of an appreciable loss of catalytic activity. The proposed facile synthetic route and inexpensive matrix materials lead the way to access bismuth nanodots for both the fundamental study of reactions and their industrial catalysis applications.

Reductive transformation of nitroaromatic compounds by Pd nanoparticles on nitrogen-doped carbon (Pd@NC) biosynthesized using Pantoea sp. IMH

Reductive transformation of nitroaromatic compounds is a central step in its remediation in wastewater, and therefore has invoked extensive catalytical research with rare metals such as palladium (Pd). Herein, we report Pantoea sp. IMH assisted biosynthesis for Pd@NC as an efficient catalyst for the reduction of nitroaromatics. Multiple complementary characterization results for Pd@NC evidenced the evenly dispersed Pd NPs on an N-doped carbon support. Pd@NC exhibited the superior catalytic activity in the reduction of nitroaromatic compounds (4-nitrophenol, 2-nitroaniline, 4-nitroaniline, and 2,6-dichloro-4-nitroaniline). The origin of the catalytic activity was explained by its unique electronic structure, as explored with X-ray absorption near-edge structure (XANES) spectroscopy and density functional theory (DFT) calculations. XANES analysis revealed an increase of 25.6% in the d-hole count in Pd@NC compared with Pd°, as the result of pd hybridization. In agreement with our experimental observations, DFT calculations suggested the formation of Pd-C bonds and charge re-distribution between Pd and the carbon layer, which contributed to the superior catalytic activity of Pd@NC.

Engineered iron-carbon-cobalt (Fe3O4@C-Co) core-shell composite with synergistic catalytic properties towards hydrogen generation via NaBH4 hydrolysis

Cobalt (Co) nanoparticle supported catalysts have better dispersion and recyclability than unsupported Co. However, the surface chemistry and limited surface area (SA) of supports limit their Co loading which lowers activity. Currently, supports with high SA and functionality which allow high Co loading are been developed. However, a smarter solution would be to develop "active" supports which can boost the activity of Co, even at low loading. The value of such a support lies in the ability to use low catalyst loading without scarifying activity. Herein, we demonstrate how via a simple annealing process the chemical properties of Fe₃O₄ and physico-electrical properties of carbon (C) in Fe₃O₄@C can be effectively combined to prepare an "active" support for Co. The unique properties of the "active" Fe₃O₄@C triggers a synergistic catalytic reaction involving Co, Fe₃O₄ and C during NaBH₄ hydrolysis. Consequently, the hydrogen generation rate (1746 ml g^-1 min^-1) and activation energy (47.3 kJ mol^-1) of Fe₃O₄@C-Co are significantly enhanced compared to reported catalyst even though its Co loading is significantly lower. Additionally, Fe₃O₄@C-Co is highly recyclable which demonstrates its stability. Our study gives a new perspective on the role supports can play in catalyst design.

Flower-like 3-dimensional hierarchical Co3O4/NiO microspheres for 4-nitrophenol reduction reaction

Aromatic nitro compounds are toxic and not biodegradable. Therefore, the elimination of nitro groups is very important. Metal catalysts play an important role in the catalytic transformation. We present here flower-like 3D hierarchical Co₃O₄/NiO microspheres, which are prepared by a chemical precipitation method. The as-prepared catalyst is characterized by FTIR, SEM, TEM, EDS, XRD, XPS and N₂ sorption isotherms. They have shown different morphologies such as flower, nanocubes, and hexagonal structure at different calcined temperatures. The synthesized catalyst is tested and used for the reduction of 4-nitrophenol to 4-aminophenol in the presence of sodium borohydride as a reducing agent. The reaction takes place in an aqueous medium at room temperature. The bimetallic catalyst Co₃O₄/NiO showed good performance and reusability.

Palygorskite Supported AuPd Alloy Nanoparticles as Efficient Nano-Catalysts for the Reduction of Nitroarenes and Dyes at Room Temperature

In this work, AuPd alloy palygorskite based Pal-NH₂@AuPd nano-catalysts were prepared and used as catalysts for the reduction of nitroarenes and dyes at room temperature. The surface of palygorskite (Pal) was first modified with 3-aminpropyltriethoxysilane, and then covered with AuPd alloy nanoparticles through co-reduction of HAuCl₄ and K₂PdCl₄. The morphology and structures of the Pal-NH₂@AuPd nano-catalysts were characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). The as-synthesized Pal-NH₂@AuPd nano-catalysts displayed excellent catalytic performance in reducing 4-nitrophenol (4-NP) and various other nitroaromatic compounds. Moreover, the catalytic activities of the Pal-NH₂@AuPd nano-catalysts were adjustable via changing the atomic ratio of AuPd alloy nanoparticles, leading to the Pal-NH₂@Au₄₈Pd₅₂ component as having the best atomic ratio. The Pal-NH₂@Au₄₈Pd₅₂ continued to display good catalytic stability after being reused for several cycles and there were no obvious changes, either of the morphology or the particle size distribution of the nano-catalysts. Furthermore, these Pal-NH₂@Au₄₈Pd₅₂ nano-catalysts also provided a convenient and accessible way for the degradation of dyes in artificial industrial wastewater.