Reduction catalysis by metal complexes confined in a polymer matrix

Are Polyaniline and Polypyrrole Electrocatalysts for Oxygen (O2) Reduction to Hydrogen Peroxide (H2O2)?

In this report, we present results on the electrocatalytic activity of conducting polymers [polyaniline (PANI) and polypyrrole (PPy)] toward the electrochemical oxygen reduction reaction (ORR) to hydrogen peroxide (H₂O₂). The electropolymerization of the polymers and electrolysis conditions were optimized for H₂O₂ production. On flat glassy carbon (GC) electrodes, the faradaic efficiency (FE) for H₂O₂ production was significantly improved by the polymers. Rotating disc electrode (RDE) studies revealed that this is mainly a result of blocking further H₂O₂ to the water reduction pathway by the polymers. PPy on carbon paper (CP) significantly increased the molar production of H₂O₂ by over 250% at an average FE of above 95% compared to bare CP with a FE of 25%. Thus, the polymers are acting as catalysts on the electrode for the ORR, although their catalytic mechanisms differ from other electrocatalysts.

The Influence of Materials, Heterostructure, and Orientation for Nanohybrids on Photocatalytic Activity

In this work, different structures based on electrodeposited n-type ZnO nanorods and p-type Cu₂O, CuSCN, and NiO nanostructures are fabricated for the degradation of methyl orange (MO). The influence of materials, heterostructure, and orientation for nanohybrids on photocatalytic activity is discussed for the first time. The heterojunction structures show remarkable enhancement compared to the bare semiconductor. The morphology of nanostructure has mainly an influence on the photocatalytic activity. NiO has the highest catalytic activity among the four pristine semiconductor nanostructures of ZnO, Cu₂O, CuSCN, and NiO. The greatest enhancement of the photocatalytic activity is obtained using a ZnO/NiO (1 min) heterostructure attributed to the heterojunction structure and extremely higher specific surface area, which can degrade MO (20 mg/L) into colorless within 20 min with the fastest photocatalytic speed among homogeneous heterojunction structures. Meanwhile, the methodology and data analysis described herein will serve as an effective approach for the design of hybrid nanostructures for solar energy application, and the appropriate nanohybrids will have significant potential to solve the environment and energy issues.

Selectivity of quantum dot sensitized ZnO nanotube arrays for improved photocatalytic activity

QD-sensitized ZnO heterostructures with different kinds and cycles of QDs exhibit different photocatalytic activity and the selectivity of the best QDs sensitizing ZnO for high photocatalytic activity is realized.

Polymer coordination promotes selective CO2 reduction by cobalt phthalocyanine

Cobalt phthalocyanine (CoPc) is a known electrocatalyst for the carbon dioxide reduction reaction (CO2RR) that, when adsorbed onto edge-plane graphite (EPG) electrodes, shows modest activity and selectivity for CO production along with co-generation of H2. In contrast, electrodes modified with CoPc immobilized in a poly-4-vinylpridine (P4VP) film show dramatically enhanced activity and selectivity compared to those modified with CoPc alone. CoPc-P4VP films display a faradaic efficiency of ∼90% for CO, with a turnover frequency of 4.8 s-1 at just -0.75 V vs. RHE. Two properties of P4VP contribute to enhancing the activity of CoPc: (1) the ability of individual pyridine residues to coordinate to CoPc and (2) the high concentration of uncoordinated pyridine residues throughout the film which may enhance the catalytic activity of CoPc through secondary and other outer coordination sphere effects. Electrodes modified with polymer-free, five-coordinate CoPc(py) films (py = pyridine) and with CoPc catalysts immobilized in non-coordinating poly-2-vinylpyridine films were prepared to independently investigate the role that each property plays in enhancing CO2RR performance of CoPc-P4VP. These studies show that a synergistic relationship between the primary and outer coordination sphere effects is responsible for the enhanced catalytic activity of CoPc when embedded in the P4VP membrane.

Ultra-thin coating of g-C3N4 on an aligned ZnO nanorod film for rapid charge separation and improved photodegradation performance

Aligned 1D ZnO/g-C₃N₄ films were fabricated by simple refluxing with thermal vapor condensation for rapid charge separation and recyclable test.

Oxidation mechanism of molecular oxygen over cyclohexene catalyzed by a cobalt l-glutamic acid complex

We introduce a new strategy towards the cyclohexene oxidation reaction, which is driven by molecular oxygen with a cobalt(ii)/amine acid complex as the catalyst without any solvents.

Solvent-thermal preparation of a CuCo2O4/RGO heterocomposite: an efficient catalyst for the reduction of p-nitrophenol

The catalytic activity of an as-synthesized CuCo₂O₄/RGO composite was researched in the reduction of p-nitrophenol to p-aminophenol by NaBH₄, which exhibited complete conversion in 6 min.

Photoelectrode characteristics of partially hydrolyzed aluminum phthalocyanine chloride/fullerene C₆₀ composite nanoparticles working in a water phase

Photoelectrochemical measurements were used to study the photoelectrode characteristics of composite nanoparticles composed of fullerene C₆₀ and partially hydrolyzed aluminum phthalocyanine chloride (AlPc). In cyclic voltammetry measurements, the electrodes coated with the composite nanoparticles were found to have photoanodic [electron donor: 2-mercaptoethanol (ME)] and photocathodic (electron acceptor: O₂) characteristics similar to those of the vapor-deposited p/n junction electrode. Their photoanodic features were further investigated with respect to the transient photocurrent response to light irradiation and the dependence on ME concentration (under potentiostatic conditions), from which it was noted that there was a decrease in the initial spiky photocathodic current and saturation of the steady-state photoanodic current at a higher ME concentration. Thus, the reaction kinetics was probably dominated by charge transport process. Moreover, external and internal quantum efficiency spectrum measurements indicated that the composite nanoparticles responded to the full spectrum of visible light (<880 nm) for both the photoanodic and photocathodic current. The present research will assist comprehension of photocatalytic behavior of the composite nanoparticles.

Quenching of singlet photoexcited state of water soluble phthalocyanines and porphyrins by viologens interacting electrostatically

Quenching of photoexcited anionic and water-soluble phthalocyanines and 5,10,15,20-tetraarylporphyrins by viologens has been investigated. It was confirmed that the quenching of the singlet photoexcited state takes place, and that the mechanism is mostly a static one due to electrostatic interaction between the donor and the acceptor. The static mechanism was analyzed by curve-fitting of the relative emission intensity vs viologen concentration resulting in four kinds of mechanisms composed of static quenching accompanied partly by a dynamic one. The static mechanism was classified into two types: one mechanism is due to 1:n electrostatic interaction of the anionic sensitizer and the cationic acceptor, and other is a Perrin type for which the acceptor is incorporated into the quenching sphere around the sensitizer according to a Poisson distribution. The effect of micelles for the quenching was also studied including the effect of viologen with a long alkyl chain. The ionic micelles either incorporated or repulsed the ionic sensitizer and accepter resulting in either static quenching or prohibition of the quenching. Cationic phthalocyanines and porphyrins were also examined for the reaction with cationic viologens.

Voltammetric and spectroelectrochemical characterization and electrocatalytic application of metallophthalocyanines carrying pendant bulky units

In this work, voltammetric, spectroelectrochemical, and electrocatalytic properties of the metallophthalocyanines bearing four chloro and four biphenyl-malonic ester bulky groups were investigated. Voltammetric and spectroelectrochemical measurements of the metallophthalocyanines show that while cobalt phthalocyanine presents both metal-based and ring-based redox processes, all other complexes give only ring-based reduction and oxidation processes. The redox processes have generally diffusion-controlled, reversible and one-electron transferred mechanisms. Cobalt phthalocyanine electrocoated easily on the glassy carbon working electrode during the repeating cycles, a very useful feature for application in fabrication of thin films. Electrocatalytic activity of cobalt phthalocyanine to hydrogen evolution reaction was studied in solution titrated with perchloric acid and on glassy carbon electrode modified with cobalt phthalocyanine in aqueous solution. Electrocatalytic measurements show that cobalt phthalocyanine has significant catalytic activities towards hydrogen evolution reaction. Moreover, the catalytic activity of the complex enhanced significantly when the complex was incorporated into the cation exchange Nafion polymeric matrix.

Iron phthalocyanine supported on amino-functionalized multi-walled carbon nanotube as an alternative cathodic oxygen catalyst in microbial fuel cells

Amino-functionalized multi-walled carbon nanotube (a-MWCNT)-supported iron phthalocyanine (FePc) (a-MWCNT/FePc) has been investigated as a catalyst for the oxygen reduction reaction (ORR) in an air-cathode single-chambered microbial fuel cell (MFC). Cyclic and linear sweep voltammogram are employed to investigate the electrocatalytic activity of the a-MWCNT/FePc for ORR. The maximum power density of 601 mWm(-2) is achieved from a MFC with the a-MWCNT/FePc cathode, which is the highest energy output compared to those MFCs with other materials supported FePc, such as carbon black, pristine MWCNT (p-MWCNT), carboxylic acid functionalized MWCNT (c-MWCNT), and even with a Pt/C cathode. Furthermore, cyclic voltammetry performed on the a-MWCNT/FePc electrode suggests that the a-MWCNT/FePc has an electrochemical activity for ORR via a four-electron pathway in a neutral pH solution. This work provides a potential alternative to Pt in MFCs for sustainable energy generation.

Electrocatalytic reduction of oxygen at electrodes coated with a bimetallic cobalt(ii)/platinum(ii) porphyrin

Edge plane pyrolytic graphite (EPG) electrodes coated with 5-(4-pyridyl)-10,15,20-tris(3-methoxy-4-hydroxyphenyl)porphyrin and its Pt(II) and Co(II)/Pt(II) analogs undergo an electrochemical-chemical-electrochemical (ECE) reaction when anodically scanned in 1.0 M HClO4. The new redox couple formed from this anodic conditioning of the coated electrode is dependent on the pH of the solution. Roughened EPG electrodes coated with the Co(II)/Pt(II) bimetallic porphyrin show a catalytic shift of 500 mV for the reduction of O2 when compared to the reduction of O2 at a bare EPG electrode. An additional catalytic shift of ca. 100 mV is observed for O2 reduction at an EPG electrode coated with the Co(II)/Pt(II) porphyrin which has been oxidized in 1.0 M HClO4. In addition to the added electrocatalysis a significant percentage of O2 reduced at the oxidized Co(II)/Pt(II) EPG electrode is converted to H2O as determined by rotating disk electrode measurements.