Solid-state Reduction Research Articles

Electrochemically synthesized N-doped molybdenum carbide nanoparticles for efficient catalysis of hydrogen evolution reaction

An efficient catalyst for hydrogen evolution reaction (HER), nanostructured nitrogen-doped molybdenum carbide (β-Mo2C), is reported in this work. The catalyst is prepared via a novel one-step solid-state electrolytic reduction of a mixture of MoS2 and polypyrrole nanofiber in molten CaCl2 at 850 °C. As comparison, a series of Mo2C materials are prepared by the method using different carbon precursors. TEM tests show that the as-prepared N-doped Mo2C is composed of near-spherical nanoparticles with the particle sizes of about 40–60 nm. Nitrogen-doping of Mo2C is shown to facilitate the exposure of active centers on the surface of the sample, concurrently modify the electronic property of Mo2C to promote electron transfer for HER. As a result, the optimized N-doped Mo2C (Mo2C-PPyF-1/1) displays a low onset potential of 78.1 mV for HER and a Tafel slope of 59.6 mV dec−1 in 0.5 M H2SO4, meanwhile exhibits high activity and stability in a broad pH range (0–14). Furthermore, the synthesis method is extended to fabricate other N-doped transition metal carbides (NbC, TaC and TiC). All of the N-doped materials exhibit significantly enhanced activity compared to the undoped ones.

Ti3+-TiO2/Ce3+-CeO2 Nanosheet heterojunctions as efficient visible-light-driven photocatalysts

Novel Ti3+-TiO2/Ce3+-CeO2 nanosheet heterojunctions are successfully prepared via a facile hydrothermal approach combined with a wet-chemical deposition precipitation and an in situ solid-state chemical reduction strategy. The structures of the as-prepared samples are characterized in detail by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy and UV–vis diffuse reflectance spectroscopy. The results show that Ti3+ and Ce3+ are both doped into the lattice of TiO2 and CeO2, respectively, which narrow the band gap to 2.7 eV and extend the photoresponse to visible light region. The photocatalytic degradation rate of methyl orange and methylene blue are as high as 93.3% and 97.1% under visible-light illumination, respectively. The high degradation rate is attributed to the efficient doping of Ti3+ and Ce3+, formation of surface oxygen vacancy and heterojunction, which favors the absorption of visible light and separation of photogenerated charge carriers.

Surface plasmon resonance-enhanced visible-light-driven photocatalysis by Ag nanoparticles decorated S-TiO2− nanorods

Abstract A novel plasmonic Ag decorated S-TiO2−x nanorods are successfully fabricated under 700 °C calcination, then followed by an in-situ solid-state chemical reduction approach with NaBH4 in a mild calcination (350 °C) and combined with photo-deposition approach. The resultant Ag/S-TiO2−x sample with high crystallinity and narrow band gap of ∼2.39 eV, extends the photoresponse to near-infrared region. The visible-light-driven photocatalytic degradation rate of phenol and hydrogen production rate for Ag/S-TiO2−x nanorods are as high as 98.67% and 209.2 µmol h−1 g−1, which is about 2 and 5 times higher than that of S-TiO2. Especially, first-order reaction rate constant of Ag/S-TiO2−x nanorods is 14 times higher than that of S-TiO2. The excellent photocatalytic property can be attributed to the synergistic effect of Ti3+ self-doping, S doping and the enhanced surface plasmon resonance of Ag nanoparticles, which narrows the band gap, favors the visible light utilization, and promotes the separation of photogenerated charge carriers.

Ti3+-TiO2/g-C3N4 mesostructured nanosheets heterojunctions as efficient visible-light-driven photocatalysts

Ti3+ self-doped TiO2/g-C3N4 mesostructured nanosheets heterojunctions (Ti3+-TiO2/Meso-g-C3N4) have been prepared via calcination-sonication assisted method using amino cyanamide as precursors, combined with a solid-state chemical reduction method. The Ti3+-TiO2/Meso-g-C3N4 heterojunctions with narrow band gap of ∼2.21 eV possess relative high surface area of ∼73.8 m2 g−1, and large pore size of ∼10 nm. Remarkably, the Ti3+-TiO2/Meso-g-C3N4 heterojunctions exhibit excellent visible-light-driven photocatalytic activity for degradating phenol in wastewaters. The photocatalytic hydrogen production rate of Ti3+-TiO2/Meso-g-C3N4 heterojunctions rise to ∼290.2 μmol h−1 g−1, which is about 4 and 14 times higher than that of the bare Meso-g-C3N4 and TiO2, respectively. The enhanced photocatalytic activity may be ascribed to the synergetic effect of the mesoporous structure providing sufficient surface active sites, and Ti3+ self-doping and the formed heterojunctions promoting the spatial separation of photogenerated electron-hole pairs.

Mesoporous black N-TiO2−x hollow spheres as efficient visible-light-driven photocatalysts

Mesoporous black N-TiO2−x hollow spheres are successfully fabricated through a facile evaporation-induced self-assembly (EISA) process, coupled with an etching procedure and an in-situ solid-state chemical reduction strategy. The resultant black N-TiO2−x hollow spheres with high crystallinity possess a narrow band gap of ∼2.37 eV, a large specific surface area of ∼128 m2 g−1, and a well defined hollow sphere structure, which exhibit excellent visible-light-driven photocatalytic performance for degradation of phenol, reduction of Cr(IV), and hydrogen production. The photocatalytic reaction apparent rate constants (k) of the black N-TiO2−x for phenol degradation and Cr(IV) reduction are ∼5 and 6 times higher than that of pristine TiO2, and hydrogen production rate for N-TiO2−x is ∼160 μmol h−1, which is ∼7 folds higher than that of pristine one. The excellent photocatalytic and photoreduction property can be attributed to the synergy effect of N and Ti3+ codoping narrowing the band gap, the high specific surface area offering more surface active sites, and the mesoporous hollow structure favoring light harvesting and refraction.

Pilot-scale plant study on solid-state metalized reduction–magnetic separation for magnesium-rich nickel oxide ores

An innovative technology named solid-state metalized reduction–magnetic separation (SSMRMS) was developed to produce ferronickel concentrates from magnesium-rich nickel oxide ores. A pilot-scale plant with a daily processing capacity of 500kg of dry ores was assembled and tested. SSMRMS involves four steps: feed preparation, solid-state metalized reduction, quenching and ball milling, and magnetic separation. After 40days of continuous tests, the operational stability of the proposed technology was good, and accretion did not form in a rotary kiln. Results revealed that (i) an appropriate positive pressure in the kiln terminal was beneficial to metallization; (ii) the overall recoveries of nickel and iron could reach 91.3% and 73.8%, respectively, whereas the nickel and iron grades of the produced ferronickel concentrate could be 7.4% and 69.6%, respectively; (iii) residual nickel to tailings was 0.16%; and (iv) the return ratio of dusts was approximately 8%. Notably, nickel could be released and sufficiently metalized at an appropriate temperature once the structures of the Ni-bearing silicates were destroyed in the presence of fluorite. The metalized nickel aggregated with the metalized iron surrounding the margins of the minerals. Therefore, fluorite could promote the generation and growth of ferronickel alloy particles, thereby increasing the recoveries of nickel and iron. Preliminary calculation showed that the electricity consumption of the solid-state metalized process was 52.5kWh/t-ore. Hence, SSMRMS is a competitive strategy for the processing of magnesium-rich nickel oxide ores.

Thermodynamic Analysis of the Selective Reduction of a Nickeliferous Limonitic Laterite Ore by Hydrogen

AbstractNickeliferous limonitic laterite ores are becoming increasingly attractive as a source of metallic nickel as the costs associated with recovering nickel from the sulphide ores increase. Unlike the sulphide ores, however, the laterite ores are not amenable to concentration by conventional mineral processing techniques such as froth flotation. One potential concentrating method would be the pyrometallurgical solid state reduction of the nickeliferous limonitic ores at relatively low temperatures, followed by beneficiation via magnetic separation. A number of reductants can be utilized in the reduction step, and in this research, a thermodynamic model has been developed to investigate the reduction of a nickeliferous limonitic laterite by hydrogen. The nickel recovery to the ferronickel phase was predicted to be greater than 95 % at temperatures of 673–873 K. Reductant additions above the stoichiometric requirement resulted in high recoveries over a wider temperature range, but the nickel grade of the ferronickel decreased.

Mesoporous black TiO2-x/Ag nanospheres coupled with g-C3N4 nanosheets as 3D/2D ternary heterojunctions visible light photocatalysts

3D mesoporous black TiO2-x/Ag nanosphere coupled with 2D g-C3N4 sheet ternary heterojunctions are successfully fabricated through a facile evaporation-induced self-assembly (EISA) process and photodeposition method, followed by a mild calcination (350°C) under an argon atmosphere after an in situ solid-state chemical reduction strategy. The resultant mesoporous black TiO2-x/Ag/g-C3N4 ternary heterojunctions with narrow band gap of∼2.27eV possess a relative high specific surface area of∼100m2g−1, main pore size of 6.2nm and the highest visible-light-driven photocatalytic property for degradation of methyl orange (97%) and methylene blue (99%). The apparent reaction rate constants (k) of mesoporous black TiO2-x/Ag/g-C3N4 for methyl orange and methylene blue are∼9 and 11 times higher than that of pristine TiO2. The possible mechanism is proposed, and the excellent photocatalytic property can be ascribed to the introduction of Ti3+ self-doping and g-C3N4, which favor the visible light absorption and the separation of electron-hole pairs, the surface plasma resonance effect of Ag nanoparticle, and the mesoporous networks offer more surface active sites.

In-situ C-N-S-tridoped single crystal black TiO2 nanosheets with exposed {001} facets as efficient visible-light-driven photocatalysts

In-situ C-N-S-tridoped single crystal black TiO2 nanosheets with exposed {001} facets are successfully fabricated via a facile hydrothermal method combined with an in-situ solid-state chemical reduction approach, followed by calcination at 450°C. The crystal structure, crystallinity, morphology and composition of the as-prepared samples are investigated through X-ray diffraction, Raman, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and UV–vis diffuse reflectance spectroscopy, respectively. The results suggest that the obtained C-N-S-tridoped single crystal black TiO2 nanosheets exhibit uniform nanosheet-like structure with exposed {001} facets and average length of ∼300–400nm, thickness of ∼20nm. The visible-light photocatalytic degradation rate of methyl orange is up to 92.13%, and photocatalytic hydrogen evolution is 149.7μmolh−1g−1. The excellent photocatalytic activity can be ascribed to the synergetic effect of surface heterojunction between {001} and {101} facets, the C-N-S-tridoping, and the formed Ti3+ species in photocatalyst, which can not only favor the visible-light absorption but also facilitate the separation and transfer of photogenerated charges.

Fabrication and Characterization of Magnetic Nanocomposite Powders in Hematite-Ca System by Mechanical Alloying

We have applied mechanical alloying technique to produce magnetic nanocomposite material using a mixture of Fe2O3 and Ca powders at room temperature. An optimal ball milling and heat treatment conditions to obtain magnetic α-Fe/CaO composite with fine microstructure were investigated by X-ray diffraction, scanning electron microscope and vibrating sample magnetometer measurements. We have revealed that the magnetic α-Fe /CaO nanocomposite powders can be produced by solid state reduction during ball milling. It is found that α-Fe/CaO nanocomposite powders in which CaO is dispersed in α-Fe matrix with a grain size of 45 nm are obtained by mechanical alloying of Fe2O3 with Ca for 5 hours. The saturation magnetization of ball-milled powders increases with increasing milling time and reaches to a maximum value of 65 emu/g after 7 hours of MA. The average grain size of a-Fe in 5 hours MA powders estimated by diffraction line-width are gradually decreased with increasing milling time, and tend to reach at 45 nm. The magnetic hardening due to the reduction of the α-Fe grain size by MA is also observed.

In Situ Ti3+/N-Codoped Three-Dimensional (3D) Urchinlike Black TiO2 Architectures as Efficient Visible-Light-Driven Photocatalysts

In situ Ti3+/N-codoped 3D urchinlike black TiO2 (b-N-TiO2) is synthesized via hydrothermal treatment with an in situ solid-state chemical reduction method, followed by annealing at 350 °C in argon. The results indicate that N and Ti3+ was codoped into the lattice of anatase TiO2. The prepared b-N-TiO2, with a narrow bandgap of ∼2.43 eV, possesses a three-dimensional (3D) urchinlike nanostructure, which is composed of fiberlike architecture with a length of 200–400 nm and a width of 25 nm. The visible-light-driven photocatalytic degradation rate of Methyl Orange and hydrogen evolution rate for b-N-TiO2 are 95.2% and 178 μmol h–1 g–1, respectively, which are ∼3 and ∼8 times higher than those of pristine TiO2. The excellent photocatalytic activity is mainly attributed to synergistic effect of the N and Ti3+ codoping narrowing the bandgap, and unique 3D urchinlike architecture favors the separation and transport of photogenerated charge carriers and offers more surface-active sites.

Solid-State Metalized Reduction of Magnesium-Rich Low-Nickel Oxide Ores Using Coal as the Reductant Based on Thermodynamic Analysis

The solid-state metalized reduction of magnesium-rich low-nickel oxide ore using coal as a reductant was studied based on thermodynamic analysis. The major constituent minerals of the ore were silicates and goethite. The former was the main nickel-bearing mineral, and the latter was the main iron-bearing mineral. Single factor tests were conducted to investigate the effects of reduction temperature, duration, and coal dosage on the beneficiation of nickel and iron such that optimal conditions were achieved. Considering the low recoveries of nickel and iron (Ni, 13.9 pct; Fe, 30.3 pct) under the obtained optimal conditions, an improved process, adding CaF2 before the reaction, was proposed to modify the solid-state metalized process. The results showed that the recoveries of nickel and iron reached to 96.5 and 73.4 pct, respectively, and that the grades of nickel and iron in the concentrate increased from 2.5 and 62.6 wt pct to 6.9 and 71.4 wt pct, respectively. Nickel and iron in the absence of CaF2 were metalized; nevertheless, the size of ferronickel particles was only 1 μm. Furthermore, alloys in the presence of CaF2 aggregated and exhibited bands with a length greater than 200 µm. These observations suggested that CaF2 could effectively reduce the surface tension of the newly generated alloy interface and promote the migration and polymerization of the alloy particles, which improves the beneficiation of nickel and iron by magnetic separation.

Black TiO2 nanobelts/g-C3N4 nanosheets Laminated Heterojunctions with Efficient Visible-Light-Driven Photocatalytic Performance

Black TiO2 nanobelts/g-C3N4 nanosheets laminated heterojunctions (b-TiO2/g-C3N4) as visible-light-driven photocatalysts are fabricated through a simple hydrothermal-calcination process and an in-situ solid-state chemical reduction approach, followed by the mild thermal treatment (350 °C) in argon atmosphere. The prepared samples are evidently investigated by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, N2 adsorption, and UV-visible diffuse reflectance spectroscopy, respectively. The results show that special laminated heterojunctions are formed between black TiO2 nanobelts and g-C3N4 nanosheets, which favor the separation of photogenerated electron-hole pairs. Furthermore, the presence of Ti3+ and g-C3N4 greatly enhance the absorption of visible light. The resultant b-TiO2/g-C3N4 materials exhibit higher photocatalytic activity than that of g-C3N4, TiO2, b-TiO2 and TiO2/g-C3N4 for degradation of methyl orange (95%) and hydrogen evolution (555.8 μmol h−1g−1) under visible light irradiation. The apparent reaction rate constant (k) of b-TiO2/g-C3N4 is ~9 times higher than that of pristine TiO2. Therefore, the high-efficient laminated heterojunction composites will have potential applications in fields of environment and energy.

Solid-state reduction of an ilmenite concentrate with carbon

Solid-state reduction of an ilmenite concentrate with carbon

The Effect of Carbonaceous Reductant Selection on Chromite Pre-reduction

Ferrochrome (FeCr) production is an energy-intensive process. Currently, the pelletized chromite pre-reduction process, also referred to as solid-state reduction of chromite, is most likely the FeCr production process with the lowest specific electricity consumption, i.e., MWh/t FeCr produced. In this study, the effects of carbonaceous reductant selection on chromite pre-reduction and cured pellet strength were investigated. Multiple linear regression analysis was employed to evaluate the effect of reductant characteristics on the aforementioned two parameters. This yielded mathematical solutions that can be used by FeCr producers to select reductants more optimally in future. Additionally, the results indicated that hydrogen (H)- (24 pct) and volatile content (45.8 pct) were the most significant contributors for predicting variance in pre-reduction and compressive strength, respectively. The role of H within this context is postulated to be linked to the ability of a reductant to release H that can induce reduction. Therefore, contrary to the current operational selection criteria, the authors believe that thermally untreated reductants (e.g., anthracite, as opposed to coke or char), with volatile contents close to the currently applied specification (to ensure pellet strength), would be optimal, since it would maximize H content that would enhance pre-reduction.

Facile synthesis of water‐dispersible magnetite nanorings from surfactant‐free hematite nanorings

Surfactant-free α-Fe2O3 nanorings (NRs) were directly converted into water-dispersible Fe3O4 NRs through a simple and straightforward approach. In this process, uniform α-Fe2O3 NRs obtained through hydrothermal synthesis method are firstly thermally reduced to hydrophobic Fe3O4 NRs in the presence of hydrophobic oleic acid surfactant and non-polar trioctylamine solvent (under a flow of 5% H2/95% Ar). Subsequently, oxidative cleavage of oleic acid capping agent using sodium periodate oxidising agent is imparted to yield water-dispersible Fe3O4 NRs. As compared with the traditional solid-state reduction in tube furnace, the obtained Fe3O4 NRs using solution-based reduction process can retain its original uniform ring shape with good crystallinity and well-defined magnetic vortex domain. Moreover, with the presence of azelaic acid on the surface of hydrophilic Fe3O4 NRs, antibody or drugs can be easily conjugated, which further boost its potential theranostics application. This work provides a convenient approach to convert surfactant-free nanoparticle (NP) powders into water-dispersible NPs colloidal solutions for various biomedical applications.

Direct Alloying Steel with Chromium by Briquettes Made from Chromite Ore, Mill Scale, and Petroleum Coke

In this work, the effectiveness of using briquettes made from chromite ore, mill scale, and petroleum coke for direct chromium alloying is tested by induction furnace trials carried out in three different scales. The experimental results show that steel scrap can be alloyed with chromium by the chromite ore in the briquettes and the Cr yield from the chromite ore increases with the increase in mill scale addition to the briquettes: the more mill scale is added to the briquettes, the lower the mass ratio of Cr to (Cr + Fe) would be, leading to a higher Cr yield from the chromite ore. Specifically, the maximum Cr yield from the chromite ore is 99.9% when the mass ratio of Cr to (Cr + Fe) in the briquettes is 0.05, and being 93.0% when the ratio is 0.10. However, when the ratio of Cr to (Cr + Fe) in the briquettes reaches 0.20, the maximum Cr yield is only 67.1%. The reduction of chromite ore under the present experimental conditions is promoted by a solid-state reduction mechanism.

Fabrication of 3D flower-like black N-TiO2-x@MoS2 for unprecedented-high visible-light-driven photocatalytic performance

As is well-known, it is a great challenge that the smooth TiO2 nanospheres are coated by MoS2 nanosheets to form the core-shell nanostructure owing to their poor interaction. Herein, we report 3D black N-TiO2-x@MoS2 core-shell nanostructures synthesized by a mild and effective strategy combined with a typical hydrothermal reaction and an in situ solid-state chemical reduction method followed by 350°C calcination under an argon atmosphere. The prepared samples are characterized in detail by X-ray diffraction, Raman, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The results suggest that the 3D N-TiO2-x@MoS2 photocatalyst is successfully doped with N and Ti3+, and simultaneously coupling with MoS2 to form the core-shell heterojunction nanostructure. The N and Ti3+ co-doped and hybrid heterostructures can effectively utilize visible-light and solar energy to degrade methyl orange and produce hydrogen. The degradation rate of methyl orange and the hydrogen production rate are as high as 91.8% and 1.882mmolh−1g−1. To the best of our knowledge, this work is the first instance of combining MoS2 with N and Ti3+ co-doped TiO2, and the proposed photocatalytic mechanism will provide a new perspective for high activity photocatalyst in future.

Mechanisms and Kinetics of Solid State Reduction of Titano Magnetite Ore with Methane

Sustainable development aims to reduce energy demand and carbon footprint in all fields of industry. In this study, a new approach to decrease these has been performed. Methane, a hydrocarbon gas, has been employed as a reductant with better reducing properties than solid carbon and hydrogen separately. Reduction of a titano magnetite ore has been executed at temperature ranging from 800 to 1200 °C using methane contents between 10 and 30 vol% in the hydrogen–methane gas mixture. Due to the high carbon activity reaching up to approximately aC = 200, higher reduction degrees has been achieved at lower temperatures compared to ordinary carbothermic reductions. Kinetically, the reduction of titano magnetite ore was most likely mix controlled regardless of the methane contents used. The reduction process especially at the early stages displayed shrinking core behaviour. The calculated activation energies, based on the obtained experimental metallisation data, varied when assuming the reaction is controlled by diffusion (iron 25–28 kJ/mol, titanium: 335–355 kJ/mol) or chemical reaction rate (iron 21–25 kJ/mol, titanium 178–184 kJ/mol) indicating a likely mixed control process. Iron was fully reduced at 1000 °C after 1 h and at 1200 °C in half an hour. The calculated rate constants for metallisation of titanium, vanadium and iron varied too, i.e. Fe from 0.00454 to 0.01150. The possible reduction mechanism is presented and discussed based on the kinetics results achieved and SEM–EDS observations.

Solid-State Redox Switching of Magnetic Exchange and Electronic Conductivity in a Benzoquinoid-Bridged Mn(II) Chain Compound.

We demonstrate that incorporation of a redox-active benzoquinoid ligand into a one-dimensional chain compound can give rise to a material that exhibits simultaneous solid-state redox switching of optical, magnetic, and electronic properties. Metalation of the ligand 4,5-bis(pyridine-2-carboxamido)-1,2-catechol ((N,O)LH4) with Mn(III) affords the chain compound Mn((N,O)L)(DMSO). Structural and spectroscopic analysis of this compound show the presence of Mn(II) centers bridged by (N,O)L(2-) ligands, resulting partially from a spontaneous ligand-to-metal electron transfer. Upon soaking in a solution of the reductant Cp2Co, Mn((N,O)L)(DMSO) undergoes a ligand-centered solid-state reduction to [Mn((N,O)L)](-), as revealed by a suite of techniques, including Raman and X-ray absorption spectroscopy. The ligand-based reduction engenders a dramatic modulation of the physical properties of the chain compound. An electrochromic response, evidenced by a color change from dark green to dark purple is accompanied by a nearly 40-fold increase in magnetic coupling strength, from J = -0.38(1) to -15.6(2) cm(-1), and a 10,000-fold increase in electronic conductivity, from σ = 2.33(1) × 10(-12) S/cm (Ea = 0.64(1) eV) to 8.61(1) × 10(-8) S/cm (Ea = 0.39(1) eV). Importantly, the chemical reduction is reversible: treatment of the reduced compound with [Cp2Fe](+) regenerates the oxidized chain. Taken together, these results highlight the ability of benzoquinoid ligands to facilitate solid-state ligand-based redox reactions in nonporous coordination solids, giving rise to reversible switching of optical properties, magnetic exchange interactions, and electronic conductivity.