Manganese–Schiff base complex immobilized silica materials for electrocatalytic oxygen reduction

Curtailment of platinum catalysts loading in fuel cell is a recent central issue. As substitutes, these days several organic metal chelate compounds having featured moieties of M–N₄ or M–N₂O₂ (M = transition metal ion) are being used as cathode catalysts in fuel cells. Here, in this study, we report in detail the electrocatalytic activity of manganese–Schiff base complexes for oxygen reduction reaction in 0·05 M HClO₄ at room temperature. Actually, [Mn(salen)]⁺: [N,N′-bis(salicylaldehyde) ethylenediimino manganese(III)]⁺ and [Mn(salophen)]⁺: [N,N′-bis(salicylaldehyde)-1,2-phenylenediimino manganese(III)]⁺ were introduced into/onto the MCM-41 type silica spheres and used for the electrocatalytic reduction of oxygen. Synthesized materials were characterized by UV–Vis, FT–IR and electrochemical techniques. Significant low overpotential for oxygen reduction in 0·05 M HClO₄ on [Mn(salen)]⁺- and [Mn(salophen)]⁺-incorporated silica-modified glassy carbon electrodes was observed.     

Facet-controlled Pt–Ir nanocrystals with substantially enhanced activity and durability towards oxygen reduction

Nanocrystals made of Pt–Ir alloys are fascinating catalysts towards the oxygen reduction reaction (ORR), but the lack of control over their surface atomic structures hinders further optimization of their catalytic performance. Here we report, for the first time, a class of highly active and durable ORR catalysts based on Pd@Pt–Ir nanocrystals with well-controlled facets. With an average of 1.6 atomic layers of a Pt₄Ir alloy on the surface, the nanocrystals can be made in cubic, octahedral, and icosahedral shapes to present Pt–Ir {1 0 0}, {1 1 1}, and {1 1 1} plus twin boundaries, respectively. The Pd@Pt–Ir nanocrystals exhibit not only facet-dependent catalytic properties but also substantially enhanced ORR activity and durability relative to a commercial Pt/C and their Pd@Pt counterparts. Among them, the Pd@Pt–Ir icosahedra deliver the best performance, with a mass activity of 1.88 A·mg⁻¹_Pt at 0.9 V, which is almost 15 times that of the commercial Pt/C. Our density functional theory (DFT) calculations attribute the high activity of the Pd@Pt–Ir nanocrystals, and the facet dependence of these activities, to easier protonation of O* and OH* under relevant OH* coverages, relative to the corresponding energetics on clean Pd@Pt surfaces. The DFT calculations also indicate that incorporating Ir atoms into the Pt lattice destabilizes OH–OH interactions on the surface, thereby raising the oxidation potential of Pt and greatly improving the catalytic durability.     

Biomass wood-derived efficient Fe–N–C catalysts for oxygen reduction reaction

Journal of Materials Science - Efficient and low-cost electrocatalysts for oxygen reduction reaction (ORR) were the key for scalable application of metal–air batteries and fuel cells. Herein,...     

Aluminium doped ceria–zirconia supported palladium-alumina catalyst with high oxygen storage capacity and CO oxidation activity

The Ce_0.5Zr_0.3Al_0.2O_1.9/Pd-γ-Al₂O₃ catalyst prepared by a mechanochemical route and calcined at 1000 °C for 20 h in air atmosphere to evaluate the thermal stability. The prepared Ce_0.5Zr_0.3Al_0.2O_1.9/Pd-γ-Al₂O₃ catalyst was characterized for the oxygen storage capacity (OSC) and CO oxidation activity in automotive catalysis. For the characterization, X-ray diffraction, transmission electron microscopy and the Brunauer–Emmet–Teller (BET) technique were employed. The OSC values of all samples were measured at 600 °C using thermogravimetric-differential thermal analysis. Ce_0.5Zr_0.3Al_0.2O_1.9/Pd-γ-Al₂O₃ catalyst calcined at 1000 °C for 20 h with a BET surface area of 41 m² g⁻¹ exhibited the considerably high OSC of 583 μmol-O g⁻¹ and good OSC performance stability. The same synthesis route was employed for the preparation of the CeO₂/Pd-γ-Al₂O₃ and Ce_0.5Zr_0.5O₂/Pd-γ-Al₂O₃ for comparison.     

WS2@Co9S8@N/C core–shell as multifunctional electrocatalysts for dye-sensitized solar cell,oxygen reduction reaction and oxygen evolution reaction

Journal of Materials Science - It is promising strategy in recent years to design and prepare metal/carbon-based functional materials with different structures using metal–organic framework...     

Co₃O₄ nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction

Catalysts for oxygen reduction and evolution reactions are at the heart of key renewable-energy technologies including fuel cells and water splitting. Despite tremendous efforts, developing oxygen electrode catalysts with high activity at low cost remains a great challenge. Here, we report a hybrid material consisting of Co?O? nanocrystals grown on reduced graphene oxide as a high-performance bi-functional catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Although Co?O? or graphene oxide alone has little catalytic activity, their hybrid exhibits an unexpected, surprisingly high ORR activity that is further enhanced by nitrogen doping of graphene. The Co?O?/N-doped graphene hybrid exhibits similar catalytic activity but superior stability to Pt in alkaline solutions. The same hybrid is also highly active for OER, making it a high-performance non-precious metal-based bi-catalyst for both ORR and OER. The unusual catalytic activity arises from synergetic chemical coupling effects between Co?O? and graphene.     

Current advances in precious metal core–shell catalyst design

Precious metal nanoparticles are commonly used as the main active components of various catalysts. Given their high cost, limited quantity, and easy loss of catalytic activity under severe conditions, precious metals should be used in catalysts at low volumes and be protected from damaging environments. Accordingly, reducing the amount of precious metals without compromising their catalytic performance is difficult, particularly under challenging conditions. As multifunctional materials, core–shell nanoparticles are highly important owing to their wide range of applications in chemistry, physics, biology, and environmental areas. Compared with their single-component counterparts and other composites, core–shell nanoparticles offer a new active interface and a potential synergistic effect between the core and shell, making these materials highly attractive in catalytic application. On one hand, when a precious metal is used as the shell material, the catalytic activity can be greatly improved because of the increased surface area and the closed interfacial interaction between the core and the shell. On the other hand, when a precious metal is applied as the core material, the catalytic stability can be remarkably improved because of the protection conferred by the shell material. Therefore, a reasonable design of the core–shell catalyst for target applications must be developed. We summarize the latest advances in the fabrications, properties, and applications of core–shell nanoparticles in this paper. The current research trends of these core–shell catalysts are also highlighted.     

Pt/Cr and Pt/Ni catalysts for oxygen reduction reaction: to alloy or not to alloy?

Bimetallic systems such as Pt-based alloys or non-alloys have exhibited interesting catalytic properties but pose a major challenge of not knowing a priori how the electronic and chemical properties will be modified relative to the parent metals. In this work, we present the origin of the changes in the reactivity of Pt/Cr and Pt/Ni catalysts, which have been of wide interest in fuel cell research. Using spin-polarized density functional theory calculations, we have shown that the modification of Pt surface reactivity in Pt/Ni is purely of geometric origin (strain). We have also found that the Pt-Ni bonding is very weak, which explains the observed instability of Pt-Ni catalysts under electrochemical measurements. On the other hand, Pt/Cr systems are governed by strong ligand effect (metal-metal interaction), which explains the experimentally observed reactivity dependence on the relative composition of the alloying components. The general characteristics of the potential energy curves for O2 dissociative adsorption on the bimetallic systems and the pure Pt clarify why the d-band center still works for Pt/Cr despite the strong Pt-Cr bonding and high spin polarization of Pt d-states. On the basis of the above clarifications, viable Pt-Cr and Pt-Ni structures, which involve nano-sized alloys and non-alloy bulk catalyst, which may strike higher than the currently observed oxidation reduction reaction activity are proposed.     

Highly effective Fe–N–C electrocatalysts toward oxygen reduction reaction originated from 2,6-diaminopyridine

Journal of Materials Science: Materials in Electronics - Fe–N–C electrocatalysts have been intensively studied due to their extraordinary catalytic activity toward oxygen reduction...     

Catalysis under shell: Improved CO oxidation reaction confined in Pt@h-BN core–shell nanoreactors

Core-shell nanostructures consisting of active metal cores and protective shells often exhibit enhanced catalytic performance,in which reactants can access a small part of the core surfaces through the pores in the shells.In this study,we show that Pt nanoparticles (NPs) can be embedded into few-layer hexagonal boron nitride (h-BN) overlayers,forming Pt@h-BN core-shell nanocatalysts.The h-BN shells not only protect the Pt NPs under harsh conditions but also allow gaseous molecules such as CO and O2 to access a large part of the Pt surfaces through a facile intercalation process.As a result,the Pt@h-BN nanostructures act as nanoreactors,and CO oxidation reactions with improved activity,selectivity,and stability occur at the core-shell interfaces.The confinement effect exerted by the h-BN shells promotes the Pt-catalyzed reactions.Our work suggests that two-dimensional shells can function as robust but flexible covers on nanocatalyst surfaces and tune the surface reactivity.     

Core–shell porphyrin·multi-walled carbon nanotube hybrids linked by multiple hydrogen bonds: nanostructure and electronic communication

In this paper, multiple hydrogen bonding interaction of ureidopyrimidinone (UPM) was employed as the linking bridge for the formation of a novel porphyrin·multi-walled carbon nanotube (MWNT) hybrid, which was then used to fabricate a layer-by-layer (LbL) film composed of porphyrin-UPM·MWNT-UPM. Both in the assembled hybrid and the LbL film, porphyrin was closely attached to the surface of the MWNT under the strong multiple hydrogen bonding interaction, forming the core–shell structure. In addition, strong electronic communication was observed between porphyrin and MWNT. The hydrogen bonding association behavior between porphyrin-UPM and MWNT-UPM was attentively analyzed by UV–Vis spectra, fluorescent spectra and transmission electron microscope experiments. It was found that the hydrogen bonding interactions were crucial for the formation of the supramolecular hybrid and the LbL film as well as the occurrence of interfacial electronic communication. This result indicated that the introduction of strong noncovalent bonding interaction between donor and acceptor is an appropriate way to control the material structure and facilitate the interfacial electronic communication in the hybrids which lay the groundwork for their application in electrochemical sensors or photovoltaic devices.     

Facile synthesis of FeCo@NC core–shell nanospheres supported on graphene as an efficient bifunctional oxygen electrocatalyst

Electrocatalytic conversion of oxygen holds great potential for clean energy technologies,including water electrolysis,regenerative fuel cells,and rechargeable metal-air batteries.The development of highly efficient and inexpensive oxygen electrocatalysts as replacements for precious metal-based catalysts is vitally important for large-scale practical application in the future.A bifunctional oxygen electrocatalyst based on FeCo nanoparticles/N-doped carbon core-shell spheres supported on N-doped graphene sheets was prepared via one-step pyrolysis of graphitic carbon nitride and acetylacetonates.The optimized product exhibited an oxygen electrode activity of 0.87 V and excellent durability.The remarkable performance is mainly attributed to the synergetic effect arising from the FeCo nanoparticles and N-doped carbon shell.This study introduces an inexpensive and simple way to develop highly active bifunctional oxygen electrocatalysts.     

Ultra-stable silica-coated chiral Au-nanorod assemblies: Core–shell nanostructures with enhanced chiroptical properties

Chiral nano-assemblies with amplified optical activity have attracted particular interest for their potential application in photonics, sensing and catalysis. Yet it still remains a great challenge to realize their real applications because of the instability of these assembled nanostructures. Herein, we demonstrate a facile and efficient method to fabricate ultra-stable chiral nanostructures with strong chiroptical properties. In these novel chiral nanostructures, side-by-side assembly of chiral cysteine-modified gold nanorods serves as the core while mesoporous silica acts as the shell. The chiral core–shell nanostructures exhibit an evident plasmonic circular dichroism (CD) response originating from the chiral core. Impressively, such plasmonic CD signals can be easily manipulated by changing the number as well as the aspect ratio of Au nanorods in the assemblies located at the core. In addition, because of the stabilization effect of silica shells, the chiroptical performance of these core–shell nanostructures is significantly improved in different chemical environments.

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Cooperative sensitization by reduction and chemical sensitization in AgX core–shell emulsions

Abstract

Cubic core–shell AgBr and AgBrI emulsions prepared by the twice emulsified method were used to observe the cooperative sensitization of reduction sensitization in the core and sulfur-plus-gold sensitization on the surface of core–shell grains through a space separation of different sensitizing centres by a AgBr shelled layer. The results showed: (1) that core–shell emulsions doped by Ag₂ centres in cores reduced by DMAB exhibited a remarkable increase in sensitivity relative to the corresponding undoped emulsions; (2) that I^? ions in AgBrI grains did not affect the sensitization of internal Ag₂ centres, but were beneficial to the inhibition of fog density; (3) cooperative sensitization by the internal reduced Ag₂ and the surface S+Au sensitizations was considered to result from two different mechanisms, i.e. trapping holes by the former and concentrating electrons by the latter, both of which were beneficial to the efficiency of latent image formation.     

Exploiting H-induced lattice expansion in β-palladium hydride for enhanced catalytic activities toward oxygen reduction reaction

We have developed an efficient strategy to synthesize an active and durable electrocatalyst of Pd hy-dride nanocubes(NCs).Instead of the traditional chemical me...     

Transition metal–nitrogen–carbon nanostructured catalysts for the oxygen reduction reaction: From mechanistic insights to structural optimization

Accelerating the rate-limiting oxygen reduction reaction (ORR) at the cathode remains the foremost issue for the commercialization of fuel cells.Transition metal-nitrogen-carbon (M-N/C,M =Fe,Co,etc.) nanostructures are the most promising class of non-precious metal catalysts (NPMCs) with satisfactory activities and stabilities in practical fuel cell applications.However,the long-debated nature of the active sites and the elusive structure-performance correlation impede further developments of M-N/C materials.In this review,we present recent endeavors to elucidate the actual structures of active sites by adopting a variety of physicochemical techniques that may provide a profound mechanistic understanding of M-N/C catalysts.Then,we focus on the spectacular progress in structural optimization strategies for M-N/C materials with tailored precursor architectures and modified synthetic routes for controlling the structural uniformity and maximizing the number of active sites in catalytic materials.The recognition of the right active centers and site-specific engineering of the nanostructures provides future directions for designing advantageous M-N/C catalysts.     

Preparation of nickel–silver core–shell nanoparticles by liquid-phase reduction for use in conductive paste

Nickel–silver (Ni–Ag) core–shell nanoparticles (NPs) were prepared by depositing Ag on Ni nanocores using the liquid-phase reduction technique in aqueous solution, and their properties were characterised using various experimental techniques. The core–shell NPs had good crystallinity, and the thicknesses of the Ag nanoshells could be tuned effectively. The oxidation resistance of the Ag surface and the electroconductive properties of the Ni core allowed these Ni–Ag core–shell NPs to be used in a conductive paste. Thick films composed of Ni–Ag core–shell NPs were screen-printed on a polycrystalline silicon substrate then sintered at temperatures ranging from 500 °C to 800 °C. Stable resistivity was obtained when the sintering temperature was higher than 650 °C, and the electrical properties of the Ni–Ag core–shell paste were close to those of pure Ag paste. Thus, the Ni–Ag NPs can partly replace pure Ag NPs in conductive pastes.     

Red@Black phosphorus core–shell heterostructure with superior air stability for high-rate and durable sodium-ion battery

Heterostructured electrodes have gained increasing attentions owing to the synergistic effects from individual building components and the unique interfaces. However, rational design and controllable fabrication of high areal capacity and durable phosphorus-based heterostructure anode for industry remains a critical challenge. Herein, a new red@black phosphorus core–shell heterostructure anchored on three-dimensional N-doped graphene (RP@BP/3DNG) has been prepared via a facile one-step solvothermal strategy. As demonstrated by experimental data and theoretical calculations, RP@BP/3DNG shows a superior high electronic conductivity and an extremely low Na⁺ diffusion barrier due to the build-in filed at the RP@BP heterointerface, thus RP@BP/3DNG delivers an ultra-high areal capacity of 3.46 mAh cm⁻² (1440.2 mAh/g at 0.05 A/g), impressive rate performance (521.3 mAh/g at 10.0 A/g) as well as unprecedented capacity retention rate of 89.3% after 1200 cycles at 10.0 A/g when evaluated as an anode for sodium ion batteries (SIBs). Furthermore, the internal electric field at the interfaces of RP@BP leads to the shift of electron cloud from BP to RP, which greatly suppresses the reaction activity of lone-pair electrons of BP atoms, and therefore RP@BP/3DNG shows much enhanced air stability. This work heralds a new insight for designing high-performance and stable P-based anodes for rechargeable batteries.     

Reassembling of Ni and Pt catalyst in the vapor–liquid–solid growth of GaN nanowires

The growth of GaN nanowires on sapphire substrates coated with Ni or Pt catalyst was investigated to address their behavior in a vapor–liquid–solid mechanism. Our observations revealed that both the two catalysts, which led to the growth of nanowires, behave rather complex, including diffusion and re-agglomeration from the coated films to the surface of the micro crystals that is formed in an early stage of growth by vapor–solid mechanism. GaN nanowires have a diameter and length of ~100 nm and several tens of micrometers, respectively, and tend to align epitaxially on the facets of the micro crystals.     

Phase evolution and reaction mechanism during reduction–nitridation process of titanium dioxide with ammonia

Titanium nitride powders were synthesized from titanium dioxide at 1173–1373 K in ammonia atmosphere. The reduction–nitridation products with various fractions obtained at various temperatures were analyzed by X-ray diffraction, Raman spectra, X-ray photoelectron spectroscopy, scanning electron microscope, transmission electron microscopy, and selected area electron diffraction. The reaction sequence from TiO₂ to TiN in ammonia atmosphere was changed by increasing the reaction temperature. The reaction sequence at 1173 K was found as TiO₂ → TiN_1?xO_x → TiN. When the reaction temperature was above 1273 K, the reaction sequence changed to as follows: TiO₂ → Ti₉O₁₇ → TiN_1?xO_x → TiN. Ti₃O₅ was not found as an intermediate phase on account of its instability in NH₃ atmosphere. The morphology of the synthesized TiN is closely related to that of the raw materials.