Electrochemical and spectroscopic investigation of the reduction of dimethylglyoxime at mercury electrodes in the presence of cobalt and nickel

How to overcome the difficulty in assaying Ni in biodiesel samples? Extraction induced by microemulsion breaking and square wave adsorptive stripping voltammetry could be the answer

Herein, a simple, green, and relatively inexpensive approach to determine nickel (Ni) in biodiesel samples by square wave adsorptive cathodic stripping voltammetry (SWAdCSV) is presented. A method based on the accumulation of Ni as Ni(II)-dimethylglyoxime (Ni(II)(HDMG)2) on the glassy carbon electrode was carried out in a solution containing the aqueous phase extract (APhEx) obtained from an extraction induced by microemulsion breaking (EIMB), which was achieved by adding a few microliters of ultrapure water to a microemulsion composed of biodiesel, n-propanol and a diluted HNO3 solution. The LOD and LOQ were 0.2 μg L-1 and 0.8 μg L-1, respectively, and the accuracy was evaluated by recovery assays of spiked samples and by analyzing a standard reference material. Results obtained from a comparative method (HR-CS GF AAS) were also used for this evaluation. The method was applied to biodiesel samples produced from different feedstocks. To the best of the authors knowledge, it is the first time that: 1) Ni in biodiesel is determined by a voltammetric method; 2) EIMB is applied to extract Ni from this matrix and 3) this type of sample preparation method is used with adsorptive stripping voltammetry.

Bismuth-Chitosan Nanocomposite Sensors for Trace Level Detection of Ni(II) and Co(II) in Water Samples

Trace minerals play an essential role in methane production via anaerobic digestion (AD). It is important to monitor Ni(II) and Co(II) concentrations and the Ni/Co concentration ratio for the rapid diagnosis of the ecological status or activity of methanogens in AD. Electrochemical detection of Ni(II) and Co(II) was investigated by coating the Bi-chitosan nanocomposite on a glassy carbon electrode (GCE) via the electrodeposition technique. A square-wave adsorptive cathodic stripping voltammetry technique (SWAdCSV) was applied and optimized when dimethylglyoxime (DMG) was used as the chelating agent for Ni(II) and Co(II) measurements. The SWAdCSV results showed that the current peaks for Co(II) detection are 6.1 times greater than the current peaks for Ni(II) measurements, probably due to the different affinity of DMG molecules between Ni(II) and Co(II). DMG molecules demonstrated higher selectivity toward Co(II) cations compared to Ni(II). The modified Bi-chitosan GCE developed in this study showed a relatively wide range of the Ni(II) and Co(II) concentrations (2–100 µg L−1) with a limit of detection of 3.6 µg L−1 for Ni(II) and 2.4 µg L−1 for Co(II), respectively. The developed sensor was applied to Ni(II) and Co(II) spiked natural water samples and showed good performance of detection with 12 consecutive measurements. Overall, the fabricated sensor showed excellent sensitivity toward Ni(II) and Co(II) in natural water samples.

Understanding the Performance of NiO Photocathodes with Alkyl-Derivatized Cobalt Catalysts and a Push-Pull Dye

Mesoporous NiO photocathodes containing the push-pull dye PB6 and alkyl-derivatized cobaloxime catalysts were prepared using surface amide couplings and analyzed for photocatalytic proton reduction catalysis. The length of the alkyl linker used to derivatize the cobalt catalysts was found to correlate to the photocurrent with the highest photocurrent observed using shorter alkyl linkers but the lowest one for samples without linker. The alkyl linkers were also helpful in slowing dye-NiO charge recombination. Photoelectrochemical measurements and femtosecond transient absorption spectroscopic measurements suggested electron transfer to the surface-immobilized catalysts occurred; however, H₂ evolution was not observed. Based on UV-vis, X-ray fluorescence spectroscopy (XRF), and X-ray photoelectron spectroscopy (XPS) measurements, the cobalt catalyst appeared to be limiting the photocathode performance mainly via cobalt demetallation from the oxime ligand. This study highlights the need for a deeper understanding of the effect of catalyst molecular design on photocathode performance.

Sustainable removal of N2O by mediated electrocatalytic reduction at ambient temperature electro-scrubbing using electrogenerated Ni(I) electron mediator

Direct catalysis is generally proposed for nitrous oxide (N₂O) abatement but catalysis is expensive, requires high temperatures, and suffers from media fouling, which limits its lifetime. In the present study, an ambient temperature electroscrubbing method was developed, coupling wet-scrubbing with an electrogenerated Ni(I) ([Ni(I)(CN)₄]^3-) mediator, to enable N₂O reduction in a single process stage. The initial studies of 10 ppm N₂O absorption into 9 M KOH and an electrolyzed 9 M KOH solution showed no removal. However, 95% N₂O removal was identified through the addition of Ni(I) to an electrolyzed 9 M KOH. A change in the oxidation/reduction potential from -850 mV to -650 mV occurred following a decrease in Ni(I) concentration from 4.6 mM to 4.0 mM, which confirmed that N₂O removal was mediated by an electrocatalytic reduction (MER) pathway. Online analysis identified the reaction product to be ammonia (NH₃). Increasing the feed N₂O concentration increased NH₃ formation, which suggests that a decrease in electrolyzed solution reactivity induced by the increased N₂O load constrained the side reaction with the carrier gas. Importantly, this study outlines a new regenerable method for N₂O removal to commodity product NH₃ at ambient temperature that fosters process intensification, overcomes the limitations generally observed with catalysis, and permits product transformation to NH₃.

Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes

The synthesis of renewable fuels from abundant water or the greenhouse gas CO2 is a major step toward creating sustainable and scalable energy storage technologies. In the last few decades, much attention has focused on the development of nonprecious metal-based catalysts and, in more recent years, their integration in solid-state support materials and devices that operate in water. This review surveys the literature on 3d metal-based molecular catalysts and focuses on their immobilization on heterogeneous solid-state supports for electro-, photo-, and photoelectrocatalytic synthesis of fuels in aqueous media. The first sections highlight benchmark homogeneous systems using proton and CO2 reducing 3d transition metal catalysts as well as commonly employed methods for catalyst immobilization, including a discussion of supporting materials and anchoring groups. The subsequent sections elaborate on productive associations between molecular catalysts and a wide range of substrates based on carbon, quantum dots, metal oxide surfaces, and semiconductors. The molecule-material hybrid systems are organized as "dark" cathodes, colloidal photocatalysts, and photocathodes, and their figures of merit are discussed alongside system stability and catalyst integrity. The final section extends the scope of this review to prospects and challenges in targeting catalysis beyond "classical" H2 evolution and CO2 reduction to C1 products, by summarizing cases for higher-value products from N2 reduction, C x>1 products from CO2 utilization, and other reductive organic transformations.

Complexation-Based Detection of Nickel(II) at a Graphene-Chelate Probe in the Presence of Cobalt and Zinc by Adsorptive Stripping Voltammetry

The adsorptive stripping voltammetric detection of nickel and cobalt in water samples at metal film electrodes has been extensively studied. In this work, a novel, environmentally friendly, metal-free electrochemical probe was constructed for the ultra-trace determination of Ni2+ in water samples by Adsorptive Cathodic Stripping Voltammetry (AdCSV). The electrochemical platform is based on the adsorptive accumulation of Ni2+ ions directly onto a glassy carbon electrode (GCE) modified with dimethylglyoxime (DMG) as chelating agent and a Nafion-graphene (NGr) nanocomposite to enhance electrode sensitivity. The nafion-graphene dimethylglyoxime modified glassy carbon electrode (NGr-DMG-GCE) shows superior detection capabilities as a result of the improved surface-area-to-volume ratio and enhanced electron transfer kinetics following the incorporation of single layer graphene, while limiting the toxic effects of the sensor by removal of the more common mercury, bismuth and lead films. Furthermore, for the first time the NGr-DMG-GCE, in the presence of common interfering metal ions of Co2+ and Zn2+ demonstrates good selectivity and preferential binding towards the detection of Ni2+ in water samples. Structural and morphological characterisation of the synthesised single layer graphene sheets was conducted by Raman spectrometry, HRTEM and HRSEM analysis. The instrumental parameters associated with the electrochemical response, including accumulation potential and accumulation time were investigated and optimised in addition to the influence of DMG and graphene concentrations. The NGr-DMG-GCE demonstrated well resolved, reproducible peaks, with RSD (%) below 5% and a detection limit of 1.5 µg L-1 for Ni2+ reduction at an accumulation time of 120 s., the prepared electrochemical sensor exhibited good detection and quantitation towards Ni2+ detection in tap water samples, well below 0.1 mg L-1 set by the WHO and EPA standards. This comparable to the South African drinking water guidelines of 0.15 mg L-1.

Sn(IV) Metalloporphyrin/Co(III) Complex: An All-Abundant-Element System for the Photocatalytic Production of H2 in Aqueous Solution

A new, molecular system for the light-driven production of hydrogen in aqueous solution was developed by combining a water-soluble tin porphyrin ([Sn(IV)Cl2TPPC], A) acting as photosensitizer with a cobalt-based proton-reduction catalyst ([Co(III)Cl(dmgH)2(py)], C). Under visible light illumination and with triethanolamine (TEOA) as electron source, the system evolves H2 for hours and is clearly catalytic in both dye and catalyst. A detailed analysis of the relevant redox potentials in combination with time-resolved spectroscopy resulted in the development of a Z-scheme type model for the flow of electrons in this system. Key intermediates of the proposed mechanism for the pathway leading to H2 are the porphyrin dye's highly oxidizing singlet excited state (1)A* (E ∼ +1.3 V vs NHE), its strongly reducing isobacteriochlorin analogue (E ∼ +0.95 V), and the Co(I) form of C (E ∼ -0.8 V), acting as catalyst for H2 formation. Among other results, the suggested reaction sequence is supported by the detection of a shortened excited-state lifetime for singlet (1)A* (τ ∼ 1.75 ns) in the presence of TEOA and the ultraviolet-visible detection of the Sn(IV) isobacteriochlorin intermediate at λ = 610 nm. Thus, a molecular, conceptually biomimetic, and precious-metal-free reaction chain was found which photocatalytically generates H2 in a 100% aqueous system from an electron donor with a high oxidation potential (E(TEOA) ∼ +1.1 V). On the other hand, at identical conditions, this photoreaction chain yields H2 markedly slower than a system using the photosensitizer [Re(I)(CO)3(bpy) (py)](+), probably due to the much longer excited-state lifetime (τ ∼ 120 ns) of the rhenium dye and better electron-transfer rates caused by its simple single-electron photoreduction chemistry.

A nickel dimethyl glyoximato complex to form nickel based nanoparticles for electrocatalytic H2 production

We report on the electrochemical alteration of a nickel(II) bis-glyoximato complex into nickel-based nanoparticles at a glassy carbon electrode under acid reducing conditions. These particles show electrocatalytic activity towards hydrogen production at +410 mV compared to the bare glassy carbon electrode. Mechanistic insights are discussed based on DFT calculations.

Studies of cobalt-mediated electrocatalytic CO2 reduction using a redox-active ligand

The cobalt complex [Co(III)N4H(Br)2](+) (N4H = 2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]-heptadeca-1(7),2,11,13,15-pentaene) was used for electrocatalytic CO2 reduction in wet MeCN with a glassy carbon working electrode. When water was employed as the proton source (10 M in MeCN), CO was produced (fCO= 45% ± 6.4) near the Co(I/0) redox couple for [Co(III)N4H(Br)2](+) (E1/2 = -1.88 V FeCp2(+/0)) with simultaneous H2 evolution (fH2= 30% ± 7.8). Moreover, we successfully demonstrated that the catalytically active species is homogeneous through the use of control experiments and XPS studies of the working glassy-carbon electrodes. As determined by cyclic voltammetry, CO2 catalysis occurred near the formal Co(I/0)redox couple, and attempts were made to isolate the triply reduced compound ("[Co(0)N4H]"). Instead, the doubly reduced ("Co(I)") compounds [CoN4] and [CoN4H(MeCN)](+) were isolated and characterized by X-ray crystallography. Their molecular structures prompted DFT studies to illuminate details regarding their electronic structure. The results indicate that reducing equivalents are stored on the ligand, implicating redox noninnocence in the ligands for H2 evolution and CO2 reduction electrocatalysis.

Cobalt-based particles formed upon electrocatalytic hydrogen production by a cobalt pyridine oxime complex

An open-coordination-sphere cobalt(III) oximato-based complex was designed as a putative catalyst for the hydrogen evolution reaction (HER). Electrochemical alteration in the presence of acid occurs, leading to the formation of cobalt-based particles that act as an efficient catalyst for HER at pH 7. The exact chemical nature of these particles is yet to be determined. This study thus raises interesting issues regarding the fate of molecular-based complexes designed for the HER, and points to the challenging task of identifying the real catalytic species. Moreover, understanding and rationalizing the alteration pathways can be seen as a new route to reach catalytic particulates.

Adsorptive stripping voltammetric determination of cobalt in the presence of dimethylglyoxime and cetyltrimethylammonium bromide

A sensitive procedure for determination of micro-traces of Co(II) by adsorptive stripping voltammetry is proposed. The procedure exploits the enhancement of the cobalt peak obtained by use of the system Co(II)-dimethylglyoxime-piperazine-1,4-bis(2-ethanesulfonic acid)-cetyltrimethylammonium bromide. Using the optimized conditions, a detection limit (based on the 3 sigma criterion) for Co(II) of 1.2 x 10(-11) mol L(-1) (0.7 ng L(-1)) was achieved. The calibration plot for an accumulation time of 30 s was linear from 5 x 10(-11) to 4 x 10(-9) mol L(-1). The procedure was validated by analysis of certified reference materials and natural water samples.