Electrocatalytic oxidation of pyridoxine (vitamin B<Subscript>6</Subscript>) on aluminum electrode modified by metallic palladium particles/iron (III) hexacyanoferrate (II) film

The electrocatalytic activity of a Prussian blue (PB) film on the aluminum electrode by taking advantage of the metallic palladium characteristic as an electron-transfer bridge (PB/Pd–Al) for electrooxidation of 2-methyl-3-hydroxy-4,5-bis (hydroxyl–methyl) pyridine (pyridoxine) is described. The catalytic activity of PB was explored in terms of Fe^III [Fe^III (CN)₆]/Fe^III [Fe^II (CN)₆]¹⁻ system. The best mediated oxidation of pyridoxine (PN) on the PB/Pd–Al-modified electrode was achieved in 0.5 M KNO₃ + 0.2 M potassium acetate of pH 6 at scan rate of 20 mV s⁻¹. The mechanism and kinetics of the catalytic oxidation reaction of PN were monitored by cyclic voltammetry and chronoamperometry. The results were explained using the theory of electrocatalytic reactions at chemically modified electrodes. The charge transfer-rate limiting reaction step is found to be a one-electron abstraction, whereas a two-electron charge transfer reaction is the overall oxidation reaction of PN by forming pyridoxal. The value of α, k, and D are 0.5, 1.2 × 10² M⁻¹ s⁻¹, and 1.4 × 10⁻⁵ cm² s⁻¹, respectively. Further examination of the modified electrodes shows that the modifying layers (PB) on the Pd–Al substrate have reproducible behavior and a high level of stability after posing it in the electrolyte or Pyridoxine solutions for a long time.     

Comparison of electrocatalytic characterization of Ti/Sb-SnO<Subscript>2</Subscript> and Ti/F-PbO<Subscript>2</Subscript> electrodes

Electrocatalytic oxidation is a promising process for degrading toxic and biorefractory organic pollutants in wastewater treatment. Selection of electrode materials is crucial for electrochemical oxidation process. In this study, Ti/F-PbO₂ and Ti/Sb-SnO₂ electrodes were chosen to compare their electrocatalytic characterization, which were prepared by electrodeposition and thermal decomposition method, respectively. The surface morphology and crystal structure of two electrodes were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The linear polarization curves show that Ti/Sb-SnO₂ electrodes possess higher oxygen evolution overpotential than Ti/F-PbO₂ electrodes. But the stability and corrosion resistance ability of Ti/F-PbO₂ electrode was higher than that of Ti/Sb-SnO₂ electrode. The electrocatalytic activity of Ti/F-PbO₂ and Ti/Sb-SnO₂ electrodes was examined for the electrochemical oxidation of malachite green (MG). The bulk electrolysis shows that the Ti/Sb-SnO₂ electrodes exhibit the higher electrocatalytic activity for the degradation of MG than Ti/F-PbO₂ electrodes, and the degradation process is good fitting for the pseudo-first order reaction. The higher electrocatalytic activity of Ti/Sb-SnO₂ electrodes can be attributed to the higher oxygen evolution overpotential.     

Development of nano IrO<Subscript>2</Subscript> composite-reinforced nickel–phosphorous electrodes for hydrogen evolution reaction

Electroless and electroplated nickel electrodes are extensively used for hydrogen evolution reaction (HER). In the present work, TiO₂-supported IrO₂ mixed oxide composite was prepared and used to reinforce Ni–P electroless plates to be used as catalytic electrodes for HER. The electrodes exhibited high electrocatalytic activity when the electrodes were used for HER. All the parameters including particle size of the catalyst, surface roughness, and surface active sites were studied. The particle size of the IrO₂ catalyst in the mixed oxide was found to have high influence on the catalytic activity of the electrodes. Low overpotential as low as 70 mV at a current density of 200 mA cm⁻² was achieved with the mixed oxide-reinforced Ni–P electrodes.     

Kinetics of oxidation of methane to formaldehyde on Na<Subscript>4</Subscript>[PFeMo<Subscript>11</Subscript>O<Subscript>40</Subscript>]/SiO<Subscript>2</Subscript>

The kinetics of oxidation of CH₄ to formaldehyde on the catalytic system Na₄[PFeMo₁₁O₄₀]/SiO₂ were studied, and a significant role of the redox potential of the CH₄-O₂ system with respect to the catalyst was shown. The density of centers participating in the reaction was determined, and dissociative competitive adsorption of methane and oxygen was established. The equation was deduced in the framework of the Langmuir-Hinshelwood theory taking into account the side conversion of formaldehyde. Possible participation of lattice oxygen in the reaction was suggested.     

Tantalum oxide effect on the surface structure and morphology of the IrO<Subscript>2</Subscript> and IrO<Subscript>2</Subscript> + RuO<Subscript>2</Subscript> + TiO<Subscript>2</Subscript> coatings and on the corrosion and electrochemical properties of anodes prepared from these

Alterations in the phase composition, porosity, and surface morphology of coatings are examined following the insertion of a quantity of Ta₂O₅ into active coatings prepared from IrO₂ or IrO₂ + RuO₂ + TiO₂ (OIRTA). It is shown that even an insignificant concentration of Ta₂O₅ in a coating renders it substantially amorphous and leads to the appearance of a large number of wide protracted cracks in the coating. The latter extends the surface of anodes and boosts their apparent catalytic activity in the chlorine evolution reaction. In addition, this accelerates the diffusion of chloride ions toward the front surface of anodes, which noticeably reduces the overvoltage of the chlorine evolution reaction when manufacturing sodium chlorate. The coatings’ amorphization and the development of their surface substantially reduce the corrosion resistance of these anodes as compared with OIRTA.     

The physicochemical and catalytic properties of the Pt-CeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> system

The properties of the Pt-CeO _x system prepared by the oxidation of the Pt₂Ce intermetallic compound were studied. The sample was characterized by X-ray diffraction in situ, thermogravimetry, scanning electron microscopy (with an accessory for energy dispersion analysis), transmission electron microscopy, and temperature-programmed reduction with hydrogen. The catalytic properties of the sample were studied in the model reaction of toluene hydrogenation. The oxidation of the intermetallic compound caused the appearance of metallic platinum and cerium oxide phases and high-dispersity platinum particles encapsulated in cerium oxide. Metallic platinum on the surface of the catalyst experienced rapid deactivation in the presence of hydrogen sulfide; high-dispersity platinum particles encapsulated in cerium oxide exhibited enhanced stability toward sulfur compounds.     

Effects of peak current density on the structure and property of PbO<Subscript>2</Subscript>–CeO<Subscript>2</Subscript> nanocomposite electrodes prepared by pulse electrodeposition

PbO₂–CeO₂ nanocomposite electrodes were prepared by pulse electrodeposition method in the lead nitrate solution containing CeO₂ nanoparticles with different peak current density. The content of CeO₂ nanoparticles in the electrodes increase with the increase of peak current density. The effects of peak current density on the morphology and structure of PbO₂–CeO₂ nanocomposite electrodes were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The SEM and XRD results show that the increase of peak current density can make the morphology finer and more compact, and the crystal size decreases with the increase of peak current density. The oxygen evolution overpotential and stability of PbO₂–CeO₂ nanocomposite electrodes enhance with the increase of peak current density. The electrocatalytic property of PbO₂–CeO₂ nanocomposite electrodes was examined for the electrochemical oxidation of rhodamine B (RhB). The results show that the RhB removal efficiency on PbO₂–CeO₂ nanocomposite electrodes increase with the increase of peak current density, which can be attributed to the higher oxygen evolution overpotential and CeO₂ content in the composite electrodes.     

Effects of duty cycle on the preparation and property of PbO<Subscript>2</Subscript>-CeO<Subscript>2</Subscript> nanocomposite electrodes

PbO₂-CeO₂ nanocomposite electrodes were prepared by pulse electrodeposition in the lead nitrate solution containing CeO₂ nanoparticles with different duty cycles. The effects of duty cycle on the morphology and phase structure were investigated by scanning electronic microscopy (SEM) and X-ray diffraction (XRD), respectively. The SEM and XRD results show that the decrease of duty cycle can reduce the grain size of PbO₂-CeO₂ nanocomposite electrodes and make the electrodes more compact. The CeO₂ content in composite electrodes increases with the decrease of duty cycle. The steady-state polarization curves and accelerated life tests demonstrate that the oxygen evolution overpotential and service life of PbO₂-CeO₂ nanocomposite electrodes increase with the decrease of duty cycle. The service life of PbO₂-CeO₂ nanocomposite electrodes prepared with 25 % duty cycle reaches 218 h which is 1.8 times longer than that of PbO₂-CeO₂ nanocomposite electrodes prepared by direct electrodeposition. The bulk electrolysis shows that the degradation of malachite green (MG) on the PbO₂-CeO₂ nanocomposite electrodes is the pseudo-first-order reaction and the MG and chemical oxygen demand (COD) removal efficiency on PbO₂-CeO₂ nanocomposite electrodes increases with the decrease of duty cycle, which can be attributed to the higher oxygen evolution overpotentials, electrochemical active surface area, and CeO₂ content in the composite electrodes.     

Mechanisms of water photooxidation at n-TiO2 rutile single crystal oriented electrodes under UV illumination in competition with photocorrosion

Photoetching is known to compete with water photooxidation at n-TiO₂ rutile electrodes in contact with aqueous H₂SO₄ solutions under UV illumination and anodic bias. A mechanism based on the generation of bridging hydroxyl species from the adsorption of water molecules at photoinduced bridging oxygen vacancies is proposed in order to explain the competition between both photoreactions. This mechanism, designated as Redox Photooxidation (RP) Mechanism, correlates the atomic arrangement of the TiO₂ surface with its photocatalytic activity, considering that the first step for water photooxidation is the photogeneration of bridging oxygen/hydroxyl radicals associated with intrinsic bandgap surface states, via inelastic transfer of free valence band holes to bridging oxygen/hydroxyl groups, depending on the electrolyte pH. The critical distance between adjacent bridging oxygen/hydroxyl radicals allows their covalent bonding with generation of surface-bound peroxide species, which are further photooxidized leading to oxygen evolution. The RP mechanism allows to explain literature experimental results concerning surface modifications of n-TiO₂ rutile during photoetching in competition with water photooxidation, as well as their dependence on crystal orientation. The photogeneration of chemisorbed peroxo species, intermediates of the oxygen evolution reaction, detected by MIRIR spectroscopy, as well as experimental results obtained from PL and DEMS experiments are also interpreted in the light of the RP mechanism. A comparative analysis with the nucleophilic attack (NA) Mechanism, an alternative model proposed recently to explain photoelectrochemical water oxidation at n-TiO₂ rutile, is presented.     

Macroscopic Polarization Enhancement Promoting Photo‐ and Piezoelectric‐Induced Charge Separation and Molecular Oxygen Activation

Efficient photo‐ and piezoelectric‐induced molecular oxygen activation are both achieved by macroscopic polarization enhancement on a noncentrosymmetric piezoelectric semiconductor BiOIO₃. The replacement of V⁵⁺ ions for I⁵⁺ in IO₃ polyhedra gives rise to strengthened macroscopic polarization of BiOIO₃, which facilitates the charge separation in the photocatalytic and piezoelectric catalytic process, and renders largely promoted photo‐ and piezoelectric induced reactive oxygen species (ROS) evolution, such as superoxide radicals (^.O₂⁻) and hydroxyl radicals (^.OH). This work advances piezoelectricity as a new route to efficient ROS generation, and also discloses macroscopic polarization engineering on improvement of multi‐responsive catalysis.     

Acid-catalyzed isomerization of caryophyllene in the presence of SiO<Subscript>2</Subscript> and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> impregnated with sulfuric acid

The conversion of caryophyllene upon contact with Al₂O₃ and SiO₂ impregnated with sulfuric acid was carried out, and the components of the resulting mixtures were identified. Having in hands such “standard” mixtures greatly facilitates identification of components of sesquiterpene fractions of essential oils and other mixtures of natural origin. The catalytic activity of silica gel impregnated with sulfuric acid (H⁺-SiO₂) in the acid-catalyzed isomerization of caryophyllene is significantly higher than that for H⁺-Al₂O₃ and is comparable with the activity of concentrated sulfuric acid.     

A kinetic study of the oxidation of <Emphasis Type="SmallCaps">l</Emphasis>-methionine by water soluble colloidal MnO<Subscript>2</Subscript>

Aqueous solution of water soluble colloidal MnO₂ was prepared by Perez-Benito method. Kinetics of l-methionine oxidation by colloidal MnO₂ in perchloric acid (0.93 × 10⁻⁴ to 3.72 × 10⁻⁴ mol dm⁻³) has been studied spectrophotometrically. The reaction follows first-order kinetics with respect to [H⁺]. The first-order kinetics with respect to l-methionine at low concentration shifts to zero order at higher concentration. The effects of [Mn(II)] and [F⁻] on the reaction rate were also determined. Manganese (II) has sigmoidal effect on the rate reaction and act as auto catalyst. The exact dependence on [Mn(II)] cannot be explained due to its oxidation by colloidal MnO₂. Methionine sulfoxide was formed as the oxidation product of l-methionine. Ammonia and carbon dioxide have not been identified as the reaction products. The mechanism with the observed kinetics has been proposed and discussed.     

Kinetics and products derived from C<Subscript>60</Subscript> fullerene radiolysis in toluene

C₆₀ fullerene was radiolyzed in toluene solution both in presence of air and in vacuum at four different radiation doses 12, 24, 36, 48 and 96 kGy. Clear evidences of the addition of benzyl radicals to the fullerene cage derive from FT-IR and C¹³-NMR spectra of the reaction product. In presence of air the interference of oxygen is evident in the FT-IR spectra and from the elemental analysis. A detailed analysis of the kinetics of the multiple addition of benzyl radicals to the fullerene cage was made spectrophotometrically with the determination of the addition rate constants at the each addition step and the average number of benzyl groups added to the fullerene cage as function of the radiation dose.     

Kinetics for the Oxygen Evolution Reaction and Application of the Ti/SnO<Subscript>2</Subscript> + RuO<Subscript>2</Subscript> + MnO<Subscript>2</Subscript> Electrode

A Ti/SnO₂ + RuO₂ + MnO₂ electrode was prepared by thermal decomposition of their salts. Results from SEM and XPS analyses, respectively, indicate that the coating layer exhibits a compact structure and the oxidation state of Mn in the coating layer is +IV. The experimental activation energy for the oxygen evolution reaction, which increased linearly with increasing overpotential, is about 8 kJ⋅mol⁻¹ at the equilibrium potential (η=0). The electrocatalytic characteristics of the anode are discussed in terms of ligand substitution reaction mechanisms (Sn1 and Sn2). It was found that the transition state for oxygen evolution at the anode in acidic solution follows a dissociative mechanism (Sn1 reaction). The Ti/SnO₂ + RuO₂ + MnO₂ anode in conjunction with UV illumination was used to degrade phenol solutions, where the concentration of phenol remaining was determined by high-performance liquid chromatography (HPLC). The results indicate that the degradation efficiency of phenol on the anode can reach 96.3% after photoelectrocatalytic oxidation for 3 h.     

Study on the oxygen exchange capacities of Ce<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript> pyrochlore

Ce₂Sn₂O₇ pyrochlore was synthesized by a hydrothermal method. X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the composition and valence state of the sample. The oxygen exchange property of the Ce₂Sn₂O₇ phase was measured by an oxidation reaction in sealed air atmosphere and a followed reduction reaction in 5% H₂-95% N₂ atmosphere. Gas chromatography (GC) was used to analyze the oxygen change in the reaction. The results show that Ce₂Sn₂O₇ sample has excellent oxygen absorption capacity at 250°C as Ce³⁺ ions are oxidized to Ce⁴⁺ ions. The oxidized sample can be reduced by 5% H₂-95% N₂. The refreshed sample remains the capacity of oxygen absorption, while the oxygen exchange capacity degrades with the reduction times.     

Electrons and Hydroxyl Radicals Synergistically Boost the Catalytic Hydrogen Evolution from Ammonia Borane over Single Nickel Phosphides under Visible Light Irradiation

From the perspective of tailoring the reaction pathways of photogenerated charge carriers and intermediates to remarkably enhance the solar-to-hydrogen energy conversion efficiency, we synthesized the three low-cost semiconducting nickel phosphides Ni₂P, Ni₁₂P₅ and Ni₃P, which singly catalyzed the hydrogen evolution from ammonia borane (NH₃BH₃) in the alkaline aqueous solution under visible light irradiation at 298 K. The systematic investigations showed that all the catalysts had higher activities under visible light irradiation than in the dark and Ni₂P had the highest photocatalytic activity with the initial turnover frequency (TOF) value of 82.7 min⁻¹, which exceeded the values of reported metal phosphides at 298 K. The enhanced activities of nickel phosphides were attributed to the visible-light-driven synergistic effect of photogenerated electrons (e⁻) and hydroxyl radicals (^.OH), which came from the oxidation of hydroxide anions by photogenerated holes. This was verified by the fluorescent spectra and the capture experiments of photogenerated electrons and holes as well as hydroxyl radicals in the catalytic hydrogen evolution process.     

Catalytic and Mechanistic Insights of the Low‐Temperature Selective Oxidation of Methane over Cu‐Promoted Fe‐ZSM‐5

The partial oxidation of methane to methanol presents one of the most challenging targets in catalysis. Although this is the focus of much research, until recently, approaches had proceeded at low catalytic rates (<10 h^?1), not resulted in a closed catalytic cycle, or were unable to produce methanol with a reasonable selectivity. Recent research has demonstrated, however, that a system composed of an iron‐ and copper‐containing zeolite is able to catalytically convert methane to methanol with turnover frequencies (TOFs) of over 14 000 h^?1 by using H₂O₂ as terminal oxidant. However, the precise roles of the catalyst and the full mechanistic cycle remain unclear. We hereby report a systematic study of the kinetic parameters and mechanistic features of the process, and present a reaction network consisting of the activation of methane, the formation of an activated hydroperoxy species, and the by‐production of hydroxyl radicals. The catalytic system in question results in a low‐energy methane activation route, and allows selective C₁‐oxidation to proceed under intrinsically mild reaction conditions.     

Effect of metal substitution for cobalt on the oxygen adsorption properties of YBaCo<Subscript>4</Subscript>O<Subscript>7</Subscript>

YBaCo₄O₇ compound is capable to intake and release a large amount of oxygen in the temperature range of 200–400°C. In the present study, the effect of Zn, Ga and Fe substitution for Co on the oxygen adsorption/desorption properties of YBaCo₄O₇ were investigated by thermogravimetry (TG) method. Due to fixed oxidation state of Zn2+ ions, the substitution of Zn²⁺ for Co²⁺ suppresses the oxygen adsorption of YBaCo_4−xZn_xO₇. The substitution of Ga³⁺ for Co³⁺ also decreases the oxygen absorption capacity of YBaCo_4−xGa_xO₇. This can be explained by the strong affinity of Ga³⁺ ions towards the GaO₄ tetrahedron. Compared with Zn- and Ga-substituted samples, the drop of oxygen adsorption capacity is smallest for Fe-substituted samples because of the similar changeability of oxidation states of Co and Fe ions.     

Structural characterization and photocatalytic properties of novel Bi<Subscript>2</Subscript>FeVO<Subscript>7</Subscript>

Bi₂FeVO₇ was prepared by a solid-state reaction technique for the first time and the structural and photocatalytic properties of Bi₂FeVO₇ were studied. The results shows that this compound crystallized in the tetragonal crystal system with space group I4/mmm. Moreover, the band gap of Bi₂FeVO₇ was estimated to be about 2.22(6) eV. For the photocatalytic water splitting reaction, H₂ or O₂ evolution was observed from pure water with Bi₂FeVO₇ as the photocatalyst by ultraviolet light irradiation. Degradation of aqueous methylene blue (MB) dye by photocatalytic way over this compound was further studied under visible light irradiation. Bi₂FeVO₇ shows higher catalytic activity compared to TiO₂ (P-25) for MB photocatalytic degradation under visible light irradiation. Complete removal of aqueous MB was realized after visible light irradiation for 170 min with Bi₂FeVO₇ as the photocatalyst. The reduction of the total organic carbon (TOC) and the formation of inorganic products, SO ₄ ²⁻ and NO ₃ ⁻ revealed the continuous mineralization of aqueous MB during the photocatalytic course.     

Facile synthesis of Fe@Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> core-shell nanowires as O<Subscript>2</Subscript> electrode for high-energy Li-O<Subscript>2</Subscript> batteries

Fe@Fe₂O₃ core-shell nanowires were synthesized via the reduction of Fe³⁺ ions by sodium borohydride in an aqueous solution with a subsequent heat treatment to form Fe₂O₃ shell and employed as a cathode catalyst for non aqueous Li-air batteries. The synthesized core-shell nanowires with an average diameter of 50–100 nm manifest superior catalytic activity for oxygen evolution reaction (OER) in Li-O₂ batteries with the charge voltage plateau reduced to ～3.8 V. An outstanding performance of cycling stability was also achieved with a cutoff specific capacity of 1000 milliampere hour per gram over 40 cycles at a current density of 100 mA g^?1. The excellent electrochemical properties of Fe@Fe₂O₃ as an O₂ electrode are ascribed to the high surface area of the nanowires’ structure and high electron conductivity. This study indicates that the resulting iron-containing nanostructures are promising catalyst in Li-O₂ batteries.