Recent strategy(ies) for the electrocatalytic reduction of CO2: Ni single-atom catalysts for the selective electrochemical formation of CO in aqueous electrolytes

To decrease the global carbon footprint concerns and to diminish the energy crisis, electrocatalytic reduction of CO₂ which results in the formulation of value-added chemicals is a potential solution. In this review, single-atom catalysts (SACs) which are rapidly growing and being developed as the stimulating catalytic materials for electrocatalytic reduction of CO₂ with improved selectivity, efficiency, and stability are considered. Various factors which are responsible for the efficient CO₂ reduction are discussed. The pyrolytic approach for the preparation of Ni-based SACs and the maximum atom utilization efficiency for the desirable production of CO from CO₂ are highlighted.     

Enhanced Four-Electron Selective Oxygen Reduction Reaction at Carbon-Nanotube-Supported Sulfonic-Acid-Functionalized Copper Phthalocyanine

In the present work, the oxygen reduction reaction (ORR) is explored in an acidic medium with two different catalytic supports (multi-walled carbon nanotubes (MWCNTs) and nitrogen-doped multi-walled carbon nanotubes (NMWCNTs)) and two different catalysts (copper phthalocyanine (CuPc) and sulfonic acid functionalized CuPc (CuPc-SO₃⁻)). The composite, NMWCNTs-CuPc-SO₃⁻ exhibits high ORR activity (assessed based on the onset potential (0.57 V vs. reversible hydrogen electrode) and Tafel slope) in comparison to the other composites. Rotating ring disc electrode (RRDE) studies demonstrate a highly selective four-electron ORR (less than 2.5 % H₂O₂ formation) at the NMWCNTs-CuPc-SO₃⁻. The synergistic effect of the catalyst support (NMWCNTs) and sulfonic acid functionalization of the catalyst (in CuPc-SO₃⁻) increase the efficiency and selectivity of the ORR at the NMWCNTs-CuPc-SO₃⁻. The catalyst activity of NMWCNTs-CuPc-SO₃⁻ has been compared with many reported materials and found to be better than several catalysts. NMWCNTs-CuPc-SO₃⁻ shows high tolerance for methanol and very small deviation in the onset potential (10 mV) between the linear sweep voltammetry responses recorded before and after 3000 cyclic voltammetry cycles, demonstrating exceptional durability. The high durability is attributed to the stabilization of CuPc-SO₃⁻ by the additional coordination with nitrogen (Cu-N_x) present on the surface of NMWCNTs.     

Nafion-multi-walled Carbon Nanotubes Supported Tris(bipyridyl)iron(II) for Nicotine Detection

This work reports an electrochemical sensing framework for nicotine determination based on glassy carbon electrode (GC) immobilized with Fe(bpy)₃²⁺ (where bpy is 2,2’-bipyridyl) supported by Nafion and multi-walled carbon nanotubes (Nf-MWCNTs). Fe(bpy)₃²⁺ immobilized Nf-MWCNTs modified GC (GC/Nf-MWCNTs/Fe(bpy)₃²⁺) manifests stable redox peaks, characteristics of Fe(bpy)₃²⁺. The GC/Nf-MWCNTs/Fe(bpy)₃²⁺ exhibits effective electrochemical oxidation of nicotine, diminishing the overpotential relative to GC/Nf-MWCNTs. The limit of detection is 0.1 μM (experimentally observed) with two different linear calibration ranges between 0.1 to 600 μM and 600 to 3000 μM. Electrocatalytic responses observed at GC/Nf-MWCNTs/Fe(bpy)₃²⁺ indicate superior performance for nicotine determination with acceptable selectivity, stability, and reproducibility. Additionally, the nicotine present in real samples such as beedi and tobacco are also analyzed with satisfactory recovery percentages.     

A Note on Pointwise Well-Posedness of Set-Valued Optimization Problems

Journal of Optimization Theory and Applications - Well-posedness for optimization problems is a well-known notion and has been studied extensively for scalar, vector, and set-valued optimization...     

Ion-exchange properties and electrochemical characterization of quaternary ammonium-functionalized silica microspheres obtained by the surfactant template route

Porous silica spheres functionalized with quaternary ammonium groups have been prepared by co-condensation of N-((trimethoxysilyl)propyl)-N,N,N-trimethylammonium chloride (TMTMAC) and tetraethoxysilane (TEOS) in the presence of cetyltrimethylammonium as a template and ammonia as a catalyst. The physicochemical characteristics of the resulting ion exchangers have been analyzed by various techniques and discussed with respect to the amount of organofunctional groups in the materials. For comparison purposes, both an ordered MCM-41 type mesoporous silica and two silica gels of different pore size have been grafted with TMTMAC. The ion-exchange capabilities were first evaluated from batch experiments (determination of anion-exchange capacities) and then by ion-exchange voltammetry at carbon paste electrodes modified with these hybrid materials. Effective concentration of Fe(CN)(6)(3)(-) species in the anion exchangers was pointed out, while no significant accumulation of Ru(NH(3))(6)(3+) was observed. The preconcentration efficiency was discussed on the basis of the organic group content in the materials as well as their structure and porosity. A second series of materials displaying zwitterionic surfaces was finally prepared and characterized with respect to their physicochemical properties and ion-exchange voltammetric behavior. They consisted of sulfonic acid-functionalized mesoporous silica samples resulting from the oxidation of thiol-functionalized silica spheres obtained by co-condensation of mercaptopropyl-trimethoxysilane (MPTMS) and TEOS, which were then grafted with TMTMAC at various functionalization levels. Possible interactions between the ammonium and sulfonate moieties in the confined medium were pointed out from X-ray photoelectron spectroscopy. The competitive accumulation-rejection of Fe(CN)(6)(3)(-) and Ru(NH(3))(6)(3+) redox probes was finally studied by cyclic voltammetry.     

Catalytic reduction of organic dyes at gold nanoparticles impregnated silica materials: influence of functional groups and surfactants

Gold nanoparticles (Au NPs) in three different silica based sol–gel matrixes with and without surfactants are prepared. They are characterized by UV–vis absorbance and transmission electron microscopic (TEM) studies. The size and shape of Au NPs varied with the organo-functional group present in the sol–gel matrix. In the presence of mercaptopropyl functionalized organo-silica, large sized (200–280 nm) spherical Au NPs are formed whereas in the presence of aminopropyl functionalized organo-silica small sized (5–15 nm) Au NPs are formed inside the tube like organo-silica. Further, it is found that Au NPs act as efficient catalyst for the reduction of organic dyes. The catalytic rate constant is evaluated from the decrease in absorbance of the dye molecules. Presence of cationic or anionic surfactants greatly influences the catalytic reaction. The other factors like hydrophobicity of the organic dyes, complex formation of the dyes with anionic surfactants, repulsion between dyes and cationic surfactant, adsorption of dyes on the Au NPs also play important role on the reaction rate.     

Isolation of the latent precursor complex in electron-transfer dynamics. Intermolecular association and self-exchange with acceptor anion radicals

Transient [1:1] complexes formed in the bimolecular interactions of electron acceptors (A) with their reduced anion radicals (A(-.)) are detected and characterized in solution for the first time. The recognition of such metastable intermediates as the heretofore elusive precursor complex (A(2)(-.)) in electron-transfer processes for self-exchange allows the principal parameters lambda (Marcus reorganization energy) and H(DA) (electronic coupling element) to be experimentally determined from the optical (charge-transfer) transitions inherent to these intermolecular complexes. The satisfactory correspondence of the theoretically predicted with the experimentally observed rate constants validates these ET parameters and the Marcus-Hush-Sutin methodology for strongly coupled redox systems lying in the (Robin-Day) Class II category. Most importantly, the marked intermolecular electronic interaction (H(DA)) within these precursor complexes must be explicitly recognized, since it dramatically affects the electron-transfer dynamics by effectively lowering the activation barrier. As such, the numerous calculations of the reorganization energy previously obtained from various self-exchange kinetics based on lambda = 4DeltaG must be reconsidered in the light of such a precursor complex, with the important result that ET rates can be substantially faster than otherwise predicted. On the basis of these studies, a new mechanistic criterion is proposed for various outer-sphere/inner-sphere ET processes based on the relative magnitudes of H(DA) and lambda.     

Synthesis,characterization, and catalytic application of ecofriendly Ca-bridged aminoclay

A novel ecofriendly Ca nanoparticle bridged aminoclay (AC) catalyst was prepared for the reduction of hazardous pollutants to treat the industrial wastewater. It was characterized by FT-IR spectroscopy, UV-visible spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectrum (EDX), differential scanning calorimetry (DSC), and thermogravimetry (TG) techniques. Water contact angle (WCA) measurement was used to confirm the hydrophobicity of AC after the structural modification reaction. Disappearance of free N−H stretching in the FT-IR spectrum confirmed the chemical modification of AC by Ca nanoparticle. The above-synthesized Ca nanoparticle bridged AC was used as a catalyst for the reduction of p-nitrophenol (NP), chromium (VI) and fluorescein (fluor) dye and also for oxidative Schiff base formation. The UV-visible spectrum confirmed the complex formation between chromium (VI) and NP by noting a peak at 390 nm. The k_app value was computed to assess the efficiency of the catalyst toward the reduction of hazardous pollutants in the industrial effluents. The catalytic reduction of a tricomponent system is disturbed due to the change in pH and complex formation between the pollutants, etc. The present catalyst system reduced the two-step reaction into a single-step reaction for the Schiff base formation.