Redox-active nickel in carbon nanotubes and its direct determination

The presence of residual metal-catalyst impurities in carbon nanotubes is responsible for their toxicity. It is important to differentiate between the total amount of impurities and the redox-active (bioavailable) amount of such impurities because only the bioavailable impurities exhibit toxic effects. Herein, we report a simple and specific method for quantifying the amount of redox-active Ni present in various commercial samples of CNTs. It is based on the electrochemical oxidation of Ni(OH)(2) that is formed in alkaline solutions when Ni impurities are opened to the surrounding environment. Metallic Ni impurities play an extremely active role in toxicological assays as well as in undesired catalytic processes, and thus a method to rapidly quantify the amount of redox-active Ni is of great importance.     

Direct Voltammetric Determination of Redox‐Active Iron in Carbon Nanotubes

With the advances in nanotechnology over the past decade, consumer products are increasingly being incorporated with carbon nanotubes (CNTs). As the harmful effects of CNTs are suggested to be primarily due to the bioavailable amounts of metallic impurities, it is vital to detect and quantify these species using sensitive and facile methods. Therefore, in this study, we investigated the possibility of quantifying the amount of redox‐available iron‐containing impurities in CNTs with voltammetric techniques such as cyclic voltammetry. We examined the electrochemistry of Fe₃O₄ nanoparticles in phosphate buffer solution and discovered that its electrochemical behavior could be affected by pH of the electrolyte. By utilizing the unique redox reaction between the iron and phosphate species, the redox available iron content in CNTs was determined successfully using voltammetry.     

Residual Metal Impurity Aids Facile In Situ Electrochemical Surface Derivatization of Single‐Walled Carbon Nanotubes

Residual metal impurities were exploited as reactants in the functionalization of the surface of single‐walled carbon nanotubes (SWCNT) with nickel hexacyanoferrate (NiHCF) by simple electrochemical cycling in ferricyanide solutions. This facile in situ electrochemical modification process provides intimate contact between NiHCF and SWCNTs that improves the stability of the redox property and reactivity of NiHCF. The characteristic redox behavior of NiHCF on SWCNT surfaces can be used as an electrochemical probe to access qualitative and quantitative information on unknown electroactive metal impurities in SWCNTs. Significantly, the NiHCF‐modified SWCNTs exhibit pseudocapacitive behavior, and the calculated specific capacitances are 710 and 36 F g^?1 for NiHCF‐SWCNTs and SWCNTs respectively. Furthermore, NiHCF‐SWCNTs were transformed into Ni(OH)₂/SWCNTs and used for enzymeless glucose oxidation.     

Control of the Transition between Ni‐C and Ni‐SIa States by the Redox State of the Proximal Fe?S Cluster in the Catalytic Cycle of [NiFe] Hydrogenase

[NiFe] hydrogenase catalyzes the reversible cleavage of H₂. The electrons produced by the H₂ cleavage pass through three Fe–S clusters in [NiFe] hydrogenase to its redox partner. It has been reported that the Ni‐SI_a, Ni‐C, and Ni‐R states of [NiFe] hydrogenase are involved in the catalytic cycle, although the mechanism and regulation of the transition between the Ni‐C and Ni‐SI_a states remain unrevealed. In this study, the FT‐IR spectra under light irradiation at 138–198 K show that the Ni‐L state of [NiFe] hydrogenase is an intermediate between the transition of the Ni‐C and Ni‐SI_a states. The transition of the Ni‐C state to the Ni‐SI_a state occurred when the proximal [Fe₄S₄]_p^2+/+ cluster was oxidized, but not when it was reduced. These results show that the catalytic cycle of [NiFe] hydrogenase is controlled by the redox state of its [Fe₄S₄]_p^2+/+ cluster, which may function as a gate for the electron flow from the NiFe active site to the redox partner.     

Control of the Transition between Ni‐C and Ni‐SIa States by the Redox State of the Proximal FeS Cluster in the Catalytic Cycle of [NiFe] Hydrogenase

[NiFe] hydrogenase catalyzes the reversible cleavage of H₂. The electrons produced by the H₂ cleavage pass through three Fe–S clusters in [NiFe] hydrogenase to its redox partner. It has been reported that the Ni‐SI_a, Ni‐C, and Ni‐R states of [NiFe] hydrogenase are involved in the catalytic cycle, although the mechanism and regulation of the transition between the Ni‐C and Ni‐SI_a states remain unrevealed. In this study, the FT‐IR spectra under light irradiation at 138–198 K show that the Ni‐L state of [NiFe] hydrogenase is an intermediate between the transition of the Ni‐C and Ni‐SI_a states. The transition of the Ni‐C state to the Ni‐SI_a state occurred when the proximal [Fe₄S₄]_p^2+/+ cluster was oxidized, but not when it was reduced. These results show that the catalytic cycle of [NiFe] hydrogenase is controlled by the redox state of its [Fe₄S₄]_p^2+/+ cluster, which may function as a gate for the electron flow from the NiFe active site to the redox partner.     

Supramolecular Photoinduced Electron Transfer between a Redox‐Active Hexanuclear Metal–Organic Cylinder and an Encapsulated Ruthenium(II) Complex

By using redox‐active nickel(II) ions as the connect nodes, a hexanuclear metal–organic cylinder (Ni‐YL) was achieved through self‐assembly with a large cavity and an opening windows capable to accommodate guest molecules. The suitable cavity of Ni‐YL provides an opportunity to encapsulate the anionic ruthenium bipyridine derivative [Ru(dcbpy)₃] (dcbpy=2,2′‐bipyridine‐4,4′‐dicarboxylic acid) as the photosensitizer for light‐driven reactions. The host–guest behavior between Ni‐YL and [Ru(dcbpy)₃] was investigated by mass spectrometry, NMR spectroscopy, and computational studies, revealing an effective binding of the guest [Ru(dcbpy)₃] within the cavity of Ni‐YL. Optical experiments suggested a pseudo‐intramolecular photoinduced electron transfer (PET) process between the [Ru(dcbpy)₃] and the host Ni‐YL, leading to an efficient light‐driven hydrogen production based on this system. Control experiments with a mononuclear Ni complex as a reference photocatalyst and the inactive [Fe(dcbpy)₃] as an inhibitor for comparison were also performed to confirm such a supramolecular photocatalysis process.     

Ni/ZSM‐5 Zeolite Modified Carbon Paste Electrode as an Efficient Electrode for Electrocatalytic Oxidation of Formaldehyde

In this study, we prepared a modified carbon paste electrode consisting of Nickel entrapped in synthesized ZSM‐5 zeolite (Ni/ZMCPE). Then Ni(II) ions were incorporated to electrode by immersion of modified electrode in 1 M Ni(II) ion solution. Cyclic voltammetry and chronoamperometry experiments were used for electrochemical study of this modified electrode; a good redox behavior of Ni(OH)₂/NiOOH couple at the surface of electrode can be observed, the excellent capability of this modified electrode for catalytic oxidation of formaldehyde was demonstrated during the anodic potential sweep in alkaline solution. The amount of transfer coefficient (α), surface coverage (Γ*) of the redox species and catalytic chemical reaction rate constant (k) for formaldehyde were evaluated. Thus, it can be a candidate as an anode for fuel cell application.     

Infrared Spectroscopy During Electrocatalytic Turnover Reveals the Ni‐L Active Site State During H2 Oxidation by a NiFe Hydrogenase

A novel in situ IR spectroscopic approach is demonstrated for the characterization of hydrogenase during catalytic turnover. E. coli hydrogenase 1 (Hyd‐1) is adsorbed on a high surface‐area carbon electrode and subjected to the same electrochemical control and efficient supply of substrate as in protein film electrochemistry during spectral acquisition. The spectra reveal that the active site state known as Ni‐L, observed in other NiFe hydrogenases only under illumination or at cryogenic temperatures, can be generated reversibly in the dark at ambient temperature under both turnover and non‐turnover conditions. The observation that Ni‐L is present at all potentials during turnover under H₂ suggests that the final steps in the catalytic cycle of H₂ oxidation by Hyd‐1 involve sequential proton and electron transfer via Ni‐L. A broadly applicable IR spectroscopic technique is presented for addressing electrode‐adsorbed redox enzymes under fast catalytic turnover.     

Promotion Effect of Bismuth on Nickel Electrodeposition and Its Electrocatalysis to Glucose Oxidation

Metallic Bi and Ni were co‐deposited onto the surface of glass carbon electrode (GCE) from the electrolyte solution containing their respective nitrate to fabricate a Bi/Ni alloy modified GCE (Bi/Ni‐GCE). The purpose is to study the influence of Bi³⁺ on the deposition of Ni and that of deposited Bi on the electrocatalytic performance of Ni to glucose in alkali solution. The results show that both redox signal of Ni(OH)₂/NiOOH and Ni(OH)₂/NiOOH mediated electrocatalysis to glucose is remarkably increased in the presence of Bi. It seems that there is a synergistic effect between Bi and Ni on each other’s redox electrochemistry. It’s possible that the firstly deposited Bi on GCE surface helps to the following nucleation and growth of Ni, leading to the deposition of more metallic Ni on GCE surface. An extremely attractive feature of Bi/Ni‐GCE is reflected by the fast response time to the electrocatalytic oxidation of glucose. The electrode nearly responses immediately after glucose is added and it reaches a steady‐state level within only 2 seconds, demonstrating a good electrocatalytic property of Bi/Ni‐GCE. The calibration plot is linear over the wide concentration range of 0–5.8 mM with a sensitivity of 33.96 µA/mM and a correlation coefficient of 0.9985. The detection limit of the glucose was found to be 0.59 µM at a signal‐to‐noise ratio of 3. The fabricated Bi/Ni‐GCE was successfully employed to analyze the glucose level in blood samples, exhibiting high accuracy, strong resistance against inference and good reliability in the practical applications.     

A Redox‐Active Nickel Complex that Acts as an Electron Mediator in Photochemical Giese Reactions

We report a simple protocol for the photochemical Giese addition of C(sp³)‐centered radicals to a variety of electron‐poor olefins. The chemistry does not require external photoredox catalysts. Instead, it harnesses the excited‐state reactivity of 4‐alkyl‐1,4‐dihydropyridines (4‐alkyl‐DHPs) to generate alkyl radicals. Crucial for reactivity is the use of a catalytic amount of Ni(bpy)₃²⁺ (bpy=2,2′‐bipyridyl), which acts as an electron mediator to facilitate the redox processes involving fleeting and highly reactive intermediates.     

Nickel‐Catalyzed Cross‐Coupling of Redox‐Active Esters with Boronic Acids

A transformation analogous in simplicity and functional group tolerance to the venerable Suzuki cross‐coupling between alkyl‐carboxylic acids and boronic acids is described. This Ni‐catalyzed reaction relies upon the activation of alkyl carboxylic acids as their redox‐active ester derivatives, specifically N‐hydroxy‐tetrachlorophthalimide (TCNHPI), and proceeds in a practical and scalable fashion. The inexpensive nature of the reaction components (NiCl₂?6 H₂O—$9.5 mol^?1, Et₃N) coupled to the virtually unlimited commercial catalog of available starting materials bodes well for its rapid adoption.     

Electrochemical Quartz Crystal Impedance Study of Glucose Oxidation on a Nickel Hydroxide Modified Au Electrode in Alkaline Solution

The electrochemical quartz crystal impedance (EQCI) technique has been applied to investigate glucose oxidation on bare and Ni(OH)₂‐modified Au electrodes in 0.2 mol L^?1 KOH aqueous solution. The EQCI responses suggest different contributions of H⁺‐release and OH^?‐incorporation reactions of the Ni(OH)₂‐film redox process in 0.2 mol L^?1 aqueous KOH at different potentials. Glucose adsorption on the Ni(OH)₂‐modified Au electrode was studied. A mechanism for potential cyclic redox process of glucose at Ni(OH)₂‐modified Au electrode is suggested, mainly based on a comparative EQCI analysis with direct glucose oxidation on bare gold and glucose ad‐/desorption on Ni(OH)₂ film.     

Nickel‐Catalyzed Barton Decarboxylation and Giese Reactions: A Practical Take on Classic Transforms

Two named reactions of fundamental importance and paramount utility in organic synthesis have been reinvestigated, the Barton decarboxylation and Giese radical conjugate addition. N ‐hydroxyphthalimide (NHPI) based redox‐active esters were found to be convenient starting materials for simple, thermal, Ni‐catalyzed radical formation and subsequent trapping with either a hydrogen atom source (PhSiH₃) or an electron‐deficient olefin. These reactions feature operational simplicity, inexpensive reagents, and enhanced scope as evidenced by examples in the realm of peptide chemistry.     

Mechanism of nickel extraction from sulfuric acid medium by synthesized α‐aminophosphonate derivative

A new compound, α‐aminophosphonate derivative containing pyridine ring (PPDE), was designed and synthesized as an extractant for the separation of nickel from metal impurities. Over 94.5% of Ni (II) extraction from sulfuric acid solution was achieved by using PPDE with an equilibrium pH of 4.02. Meanwhile, high separation coefficients of 23.8 for Ni to Co, 84.9 for Ni to Mn, 254.1 for Ni to Ca and 696.7 for Ni to Mg, respectively, were obtained. It was noted that PPDE exhibited an excellent regenerability, and the extraction of Ni with the recycled organic phase ranged from 92.5% to 93.9% during six circulations. The extracted nickel complex was determined as [Ni (PPDE)₂(DNNSA^?)₂(H₂O)₄], which was supported by the data obtained from Fourier transform‐infrared, UV–Vis and electrospray ionization‐mass spectrometry spectra.     

Electrocatalytic Oxidation of Some Carbohydrates by Nickel/Poly(o‐Aminophenol) Modified Carbon Paste Electrode

This study investigates the electrocatalytic oxidation of glucose and some other carbohydrates on nickel/poly(o‐aminophenol) modified carbon paste electrode as an enzyme free electrode in alkaline solution. Poly(o‐aminophenol) was prepared by electropolymerization using a carbon paste electrode bulk modified with o‐aminophenol and continuous cyclic voltammetry in HClO₄ solution. Then Ni(II) ions were incorporated to electrode by immersion of the polymeric modified electrode having amine group in 1 M Ni(II) ion solution. Cyclic voltammetric and chronoamperometric experiments were used for the electrochemical study of this modified electrode; a good redox behavior of Ni(OH)₂/NiOOH couple at the surface of electrode can be observed, the capability of this modified electrode for catalytic oxidation of glucose and other carbohydrates was demonstrated. The amount of α and surface coverage (Γ*) of the redox species and catalytic chemical reaction rate constant (k) for each carbohydrate were calculated. Also, the electrocatalytic oxidation peak currents of all tested carbohydrates exhibit a good linear dependence on concentration and their quantification can be done easily.     

Electrocatalytic Oxidation of Folic Acid on Carbon Paste Electrode Modified by Nickel Ions Dispersed into Poly(o‐anisidine) Film

Poly(o‐anisidine) (POA) was formed by successive cyclic voltammetry in monomer solution containing sodium dodecyl sulfate (SDS) at the surface of carbon paste electrode. Then Ni(II) ions were incorporated to electrode by immersion of the polymeric modified electrode having amine group in 0.1 M Ni(II) ion solution. Cyclic voltammetric and chronoamperometric experiments were used for the electrochemical study of this modified electrode; a good redox behavior of Ni(OH)₂/NiOOH couple at the surface of electrode can be observed. The capability of this modified electrode for catalytic oxidation of folic acid was demonstrated. The amount of α and surface coverage (Γ*) of the redox species and catalytic chemical reaction rate constant (k) for folic acid oxidation were calculated. The catalytic oxidation peak current of folic acid was linearly dependent on its concentration and a linear calibration curve was obtained in the range of 0.1 to 5 mM with a correlation coefficient of 0.9994. The limit of detection (3σ) was determined as 0.091 mM. This electrocatalytic oxidation was used as simple, selective and precise voltammetric method for determination of folic acid in pharmaceutical preparations.     

Chemical‐Looping Reforming of Methane Using Iron Based Oxygen Carrier Modified with Low Content Nickel

Fe₂O₃/Al₂O₃ and Fe₂O₃/Al₂O₃ modified by low content of Ni (below 2% in weight) oxygen carriers were prepared by mechanical mixing and impregnation method. The synthesized oxygen carriers were characterized by means of X‐ray diffraction (XRD), X‐ray fluorescence (XRF), scanning electron microscopy (SEM), BET‐surface area and temperature programmed reduction (TPR). Besides, redox cyclic reactivity and the performance of chemical looping reforming of methane of the oxygen carriers were studied in a thermal gravimetrical analysis (TGA) and fixed bed at 850°C. It was observed that the redox reactivity of the oxygen carriers is improved by Ni addition because synergic effect may occur between NiO and Fe₂O₃/Al₂O₃ to form NiFe₂O₄ and NiAl₂O₄ spinel phases. However, the improvement was not apparent as Ni addition reached 1 wt% or more because more nickel loaded resulted in methane decomposition into H₂ and carbon leading to carbon deposition. The SEM and BET analysis showed that NiFe₂O₄ and NiAl₂O₄ particles dispersed into the pores of the Fe₂O₃/Al₂O₃ particles in the course of preparation. In addition, the resistance to sintering of the modified samples increased with the Ni addition increasing. The results of successive redox cycles showed that the Ni modified Fe₂O₃/Al₂O₃ oxygen carriers have good regenerability. With integration of reactivity and carbon deposition, the content 1.04 wt% of nickel doping was an optimal amount in the three modified samples.     

Enhancing Capacity Performance by Utilizing the Redox Chemistry of the Electrolyte in a Dual‐Electrolyte Sodium‐Ion Battery

A strategy is described to increase charge storage in a dual electrolyte Na‐ion battery (DESIB) by combining the redox chemistry of the electrolyte with a Na⁺ ion de‐insertion/insertion cathode. Conventional electrolytes do not contribute to charge storage in battery systems, but redox‐active electrolytes augment this property via charge transfer reactions at the electrode–electrolyte interface. The capacity of the cathode combined with that provided by the electrolyte redox reaction thus increases overall charge storage. An aqueous sodium hexacyanoferrate (Na₄Fe(CN)₆) solution is employed as the redox‐active electrolyte (Na‐FC) and sodium nickel Prussian blue (Na_x‐NiBP) as the Na⁺ ion insertion/de‐insertion cathode. The capacity of DESIB with Na‐FC electrolyte is twice that of a battery using a conventional (Na₂SO₄) electrolyte. The use of redox‐active electrolytes in batteries of any kind is an efficient and scalable approach to develop advanced high‐energy‐density storage systems.     

Facile In Situ Synthesis of Hierarchical Porous Ni/Ni(OH)2 Hybrid Sponges with Excellent Electrochemical Energy‐Storage Performances for Supercapacitors

Herein, we report the in situ growth of single‐crystalline Ni(OH)₂ nanoflakes on a Ni support by using facile hydrothermal processes. The as‐prepared Ni/Ni(OH)₂ sponges were well‐characterized by using X‐ray diffraction (XRD), SEM, TEM, and X‐ray photoelectron spectroscopy (XPS) techniques. The results revealed that the nickel‐skeleton‐supported Ni(OH)₂ rope‐like aggregates were composed of numerous intercrossed single‐crystal Ni(OH)₂ flake‐like units. The Ni/Ni(OH)₂ hybrid sponges served as electrodes and displayed ultrahigh specific capacitance (SC=3247 F g^?1) and excellent rate‐capability performance, likely owing to fast electron and ion transport, sufficient Faradic redox reaction, and robust structural integrity of the Ni/Ni(OH)₂ hybrid electrode. These results support the promising application of Ni(OH)₂ nanoflakes as advanced pseudocapacitor materials.     

Development of a Ni–Pd/CeZrO<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst for the effective conversion of methane into hydrogen-containing gas

The effects of the Pd content (0–1 wt %) and the synthesis method (joint impregnation with Ni + Pd and Pd/Ni or Ni/Pd sequential impregnation) on the physicochemical and catalytic properties of Ni–Pd/CeZrO₂/Al₂O₃ were studied in order to develop an efficient catalyst for the conversion of methane into hydrogen-containing gas. It was shown that variation in the palladium content and a change in the method used for the introduction of an active constituent into the support matrix make it possible to regulate the redox properties of nickel cations but do not affect the size of NiO particles (14.0 ± 0.5 nm) and the phase composition of the catalyst ((γ + δ)-Al₂O₃, CeZrO₂ solid solution, and NiO). It was established that the activity of Ni–Pd catalysts in the reaction of autothermal methane reforming depends on the method of synthesis and increases in the following order: Ni + Pd < Ni/Pd < Pd/Ni. It was found that, as the Pd content of the Ni–Pd/CeZrO₂/Al₂O₃ catalyst was decreased from 1 to 0.05 wt %, the ability for self-activation, high activity, and operational stability of the catalyst under the conditions of autothermal methane reforming remained unaffected: at 850°C, the yield of hydrogen was ~70% at a methane conversion of ~100% during a 24-h reaction.