Shili Zheng

Comparison of the Oxygen Reduction Reaction between NaOH and KOH Solutions on a Pt Electrode: The Electrolyte-Dependent Effect

The oxygen reduction reaction (ORR) on a polycrystalline Pt surface was studied using cyclic voltammetry techniques, and the influence of reaction media on the ORR is examined by comparing the ORR in NaOH and KOH solutions with concentration ranging from 0.5 to 14 M at 298 K. The results show that, in NaOH and KOH solutions, the ORR, a quasi-reversible diffusion-controlled reaction, is largely dependent on the electrolyte conditions, and KOH solutions are superior to NaOH solutions for the ORR process in both thermodynamic and kinetic consideration. As the alkaline concentration increases, the ORR performance frustrates, and the protonation of superoxide is suppressed; thus, the ORR shifts from a 2e reduction pathway to a 1e reduction pathway in both solutions.

Kinetics analysis of decomposition of vanadium slag by KOH sub-molten salt method

A novel process was developed for the decomposition of vanadium slag using KOH sub-molten salt under ambient pressure, and the effects of reaction temperature, alkali-to-ore mass ratios, particle size, and stirring speed on vanadium and chromium extraction were studied. The results suggest that the reaction temperature and KOH-to-ore mass ratio are more influential factors for the extraction of vanadium and chromium. Under the optimal reaction conditions (temperature 180 °C, initial KOH-to-ore mass ratio 4:1, stirring speed 700 r/min, gas flow 1 L/min, and reaction time 300 min), vanadium and chromium extraction rates can reach up to 95% and 90%, respectively. Kinetics analysis results show that the decomposing process of vanadium slag in KOH sub-molten salt can be well interpreted by the shrinking core model under internal diffusion control. The apparent activation energies for vanadium and chromium are 40.54 and 50.27 kJ/mol, respectively.

Indirect Electrochemical Cr(III) Oxidation in KOH Solutions at an Au Electrode: The Role of Oxygen Reduction Reaction

The indirect electro-oxidation of Cr(III) by in situ generated superoxide at a gold electrode has been investigated in KOH solutions using cyclic voltammetry and UV-vis spectroscopy. It is observed that the indirect Cr(III) oxidation behavior is substantially affected by the media pH and there is a pH-modulated oxygen reduction reaction (ORR) process to generate reactive oxygen species which promotes Cr(III) oxidation. The ORR in KOH solutions is attributed to a quasi-reversible diffusion-controlled reaction. In dilute KOH solution (0.2 M), 4e reduction occurs and no reactive oxygen species are generated for the indirect Cr(III) oxidation. Moreover, Cr(III) oxidation is inhibited due to competition for the electrode active sites. As the alkaline concentration increases (3.0 M), the protonation of superoxide is greatly suppressed, and thus, 1e ORR to generate superoxide is observed. This change in mechanism facilitates the indirect Cr(III) oxidation through the superoxide as a mediator to oxidize Cr(III) to Cr(IV), which is the rate-determining step of Cr(III) oxidation to Cr(VI).

Effect of mechanical activation on alkali leaching of chromite ore

Mechanical activation was used to improve the extraction of chromium in molten NaOH. It is observed that the extraction ratio reaches 97% after leaching for 200 min when chromite ore is mechanically activated for 10 min, but only 34% if not activated. Mechanical activation can decrease the particle size, increase the surface area, and enhance the lattice distortion. Further, the mechanisms for mechanical activation were exposed. The results show that the mechanical activation mainly focuses on chromite ore particle size decrease and the lattice distortion. The formation of aggregation weakens the strengthening effect of mechanical activation for releasing high surface energy.

Tuning α-Fe2O3 nanotube arrays for the oxygen reduction reaction in alkaline media

The oxygen reduction reaction plays a crucial role in alkaline fuel cells. Herein, an α-Fe2O3 nanotube array material was fabricated via a facile two-step electrochemical anodization method and employed as an efficient ORR catalyst in alkaline media. Due to its highly ordered open top architecture, the α-Fe2O3 nanotube array electrode exhibits excellent ORR catalytic activity with an onset potential of −0.39 V (vs. Hg/HgO) and high current density of 6.95 mA cm−2. Results show that the ORR exhibits quasi-reversible diffusion-controlled reaction characteristics and a well-defined four electron pathway. Moreover, as the crystalline structure transforms from α-Fe2O3 to Fe3O4 or as the alkaline concentration increases, the ORR activity is disturbed and the electron transfer number decreases. The as-prepared electrodes possess favorable alkaline tolerance as indicated from their chronoamperometric curves and Raman spectra. Accordingly, this novel α-Fe2O3 nanotube array material can be employed as efficient and low cost non-noble metal electrodes for electrochemical energy applications.

Research and prospect on extraction of vanadium from vanadium slag by liquid oxidation technologies

A series of innovative green metallurgical processes using novel reaction media including the NaOH/KOH sub-molten salt media and the NaOH-NaNO3 binary molten salt medium, for the extraction of vanadium and chromium from the vanadium slag have been developed. In comparison with the traditional sodium salt roasting technology, which operates at 850 degrees C, the operation temperatures of these new processes drop to 200-400 degrees C. Further, the extraction rates of vanadium and chromium utilizing the new approaches could reach 95% and 90%, respectively, significantly higher than those in the traditional roasting process, which are 75% and approximate zero, respectively. Besides, no hazardous gases and toxic tailings are discharged during the extraction process. Compared with the conventional roasting method, these new technologies show obvious advantages in terms of energy, environments, and the mineral resource utilization efficiency, providing an attractive alternative for the green technology upgrade of the vanadium production industries.

Research and industrialization progress of recovering alumina from fly ash: A concise review

Fly ash, a by-product of high temperature combustion of coal in coal-fired power plants, is one of the most complex and largest amount of industrial solid wastes generated in China. Its improper disposal has become an environmental problem. Now it is widely realized that fly ash should be considered as a useful and potential mineral resource. Fly ash is rich in alumina, making it a potential substitute for bauxite. With the diminishing reserves of bauxite resources, as well as the increasing demand for alumina, recovery of alumina from fly ash has attracted extensive attention world-wide. The present review describes, firstly, the generation and physicochemical properties of high alumina fly ash found in northern China and then focuses on the various alumina recovery technologies, the advantages and disadvantages of these processes, and in particular, the latest industrial developments. Finally, the directions for future research are also considered.

Hierarchical oxygen-implanted MoS2 nanoparticle decorated graphene for the non-enzymatic electrochemical sensing of hydrogen peroxide in alkaline media

Owing to the extensive applications of hydrogen peroxide (H2O2) in biological, environmental and chemical engineering, it is of great importance to investigate sensitive and selective sensing platform towards the detection of H2O2. Herein, oxygen-implanted MoS2 nanoparticles decorated graphene nanocomposite is synthesized via a facile one-pot solvothermal method for the sensitive detection of H2O2 in alkaline media. The structure and morphology of the MoS2/graphene nanocomposites were systematically characterized, showing that Mo-O bonds are formed and oxygen is implanted into the crystal structure in the nanocomposite. As a result, the MoS2/graphene composite exhibited enhanced electron transfer kinetics and excellent electro-reduction performance towards H2O2 in alkaline media. Under optimum conditions, the fabricated sensor demonstrated a wide linear response towards H2O2 in the range of 0.25-16mM with a low detection limit of 0.12μM and high sensitivity of 269.7μAmM-1cm-2. Besides, the constructed sensor presented a good selectivity to H2O2 with the presence of other interfering species. Therefore, the proposed sensor was successfully applied for the detection and determination of H2O2 in real sample, indicating great potential for the practical applications.

Adsorption study of selenium ions from aqueous solutions using MgO nanosheets synthesized by ultrasonic method

MgO nanosheets with thickness ranges of 3-10 molecule layers and high specific area (166.44m2g-1) were successfully fabricated by an ultrasound-assisted exfoliation method and used as adsorbent for the removal of both selenite (Se(IV)) and selenate (Se(VI)) from aqueous solutions. The resulting MgO nanosheets displayed high maximum adsorption capacities of 103.52 and 10.28mgg-1 for Se(IV) and Se(VI), respectively. ATR-FTIR and XPS spectroscopic results suggested that both Se(IV) and Se(VI) formed inner-sphere surface complexes on MgO nanosheets under the present experimental conditions. Furthermore, high adsorption capacity for Se(IV/VI) in the presence of coexistent anions (SO42-, PO43-, Cl-, and F-) and efficient regeneratability of adsorbent by NaOH solution were observed in the competitive adsorption and regeneration steps. The simple one-step synthesis process of MgO nanosheets and high adsorption capacities offer a promising method for Se(IV/VI) removal in water treatment.

Synthesis of nanostructured γ-AlOOH and its accelerating behavior on the thermal decomposition of AP

The hydrothermal treatment of γ-AlOOH agglomerates was introduced to synthesize nanostructured γ-AlOOH. Various characterizations were carried out to understand the synthesis procedure and the relationship between the shape of γ-AlOOH and its accelerating behavior for the thermal decomposition of ammonium perchlorate (AP). XRD, SEM and HRTEM showed that the γ-AlOOH nanoplates, nanorods and nanocubes with exposed (031), (110) and (001) facets, respectively, could be synthesized under controllable conditions. The DSC results showed that the addition of γ-AlOOH nanoplates, nanorods and nanocubes to AP remarkably decreased the decomposition temperature of AP from approximately 450 °C to 334 °C, 345 °C, and 357 °C, respectively. As the preferred additive, the γ-AlOOH nanoplate addition resulted in a decrease in the activation energy of AP from 346.29 kJ mol−1 to 144.73 kJ mol−1, determined by nonisothermal kinetic analysis. XRD, FT-IR and XPS results revealed that the γ-AlOOH phase transformed to γ-Al2O3 and α-AlOOH after being used as an additive for the AP decomposition; the excessive surface hydroxyls obtained by the exposed (031) facet of γ-AlOOH nanoplates, proved to be beneficial for accelerating AP decomposition.