Microwave Reduction Research Articles

Study of microwave reduction of graphene oxide suspension: structure and functional groups

Study of microwave reduction of graphene oxide suspension: structure and functional groups

Microwave-triggered low temperature thermal reduction of Zr-modified high entropy oxides with extraordinary thermochemical H2 production performance

Solar-driven two-step thermochemical H2O splitting has emerged as a promising strategy for hydrogen production, but the conversion efficiency of solar-to-fuel has to be improved to make it more economically viable. Here, we present insights on microwave-triggered low temperature (600 °C) oxygen exchange for water splitting based on using Zr-modified high entropy oxides. The as-synthesized nanoparticles were characterized by XRD, FTIR, SEM, TEM, BET and EIS. The characterization results showed that the introduction of Zr4+ enlarged the Metal-Oxygen bond length of the spinel phase of FeMgCoNiOx and produced more oxygen vacancies with the increase of Zr4+ concentration, which greatly improved the thermochemical performance of FeMgCoNiOx. Water was splitted via reaction with FeMgCoNiOx/Zry that was previously reduced by microwave irradiation. By tuning the level of Zr4+ content from 0.0 to 1.0, FeMgCoNiOx/Zr0.6 was found to show the best trade-off, giving rise to a H2 yield of 4.84 mmol/g that was two times higher than that of FeMgCoNiOx (2.35 mmol/g) and outstanding thermodynamic energy efficiency (50 %). This work demonstrates that designing materials with abundant oxygen vacancies is a very efficient strategy to improve their thermochemical H2 evolution activity. Moreover, the energy efficiency will be further improved through tighter control of microwave reduction energetics.

Highly Efficient Reduction of 4‐Nitrophenol by Cu Nanoparticle Decorated Graphdiyne

AbstractCatalytic reduction of 4‐nitrophenol (4‐NP) to 4‐aminophenol (4‐AP) over high‐performance and stable metal nanoparticle catalyst is highly desirable. This study experimentally demonstrated that graphdiyne (GDY), an all‐carbon two‐dimensional material composed of two hybrid carbon atoms (sp and sp2), can uniformly disperse copper nanoparticles through simple microwave reduction and has excellent 4‐NP reduction ability. Cu/GDY shows an extraordinary reaction rate constant of 0.9984 min−1, Moreover, the catalytic activity is maintained almost intact within five cycles. These encouraging results demonstrated that the unique pores and electronic structure of GDY enhance the stability of anchored metal nanoparticles and have great potential to degrade 4‐NP as a promising support for copper nanoparticles.

Noble Metal (Pt, Rh, Pd, Ir) Doped Ru/CNT Ultra-Small Alloy for Acidic Hydrogen Evolution at High Current Density.

There are still great challenges to prepare high-efficiency Ru-based catalysts that are superior to Pt/C under acidic conditions, especially under high current conditions. In this work, a series of surfactant-free noble metal doped Ru/CNT (M-Ru/CNT, M = Pt, Rh, Pd, Ir, CNT stands for carbon nanotube) are prepared by microwave reduction method in 1minute with ≈3-3.5nm in size for the first time. In 0.5 m H2 SO4 , the overpotential of Pt-Ru/CNT (Pt: 4.94 at %) is only 12mV. What's more, it also has much larger electrochemical surface area and intrinsic activity than Pt/C. Pt-Ru/CNT still has an ultra-small overpotential under high current density (113mVat 500mA cm-2 , 155mV at 1000mA cm-2 ). At the same time, it possesses excellent stability regardless of high current or low current after the durability test of 100 h. Theoretical calculation also deeply reveals that Ru is the main adsorption site of H+ . The comparison of the electronic structure of a series of noble metals adjusted by Ru shows that Pt has the most excellent Gibbs free energy of the adsorbed hydrogen and promotes the desorption of the product.

Rapid and large-scale synthesis of ultra-small immiscible alloy supported catalysts

It is an insurmountable challenge to synthesize carrier-supported ultra-small immiscible alloy catalyst with simple one-step method today. Here, we creatively report the first preparation of immiscible Ru-based alloys (RuRE-rGO NPs, RE=Gd, Er, Yb, La) with ~5 nm by solvent-free microwave reduction. Additionally, in alkaline electrocatalytic hydrogen evolution reaction (HER), RuGd-rGO NPs performed admirable (η = 12 mV at 10 mA cm−2, TOF=30.6 H2 s−1 at 0.1 V), which also can be stable for 500 h even if it provides industry-related current density of 500 mA cm−2. Density functional theory further indicates that from initial to final state of the reaction, RuGd-rGO NPs have large exothermic energy (−1.34 eV), while the dissociation energy barrier of H2O is relatively low, causing the most likely occurrence of HER. This work provides sufficient space for the synthesis of ultra-small immiscible nano-alloy supported catalysts.

Antibacterial and Antibiofouling Activities of Antimicrobial Peptide-Functionalized Graphene-Silver Nanocomposites for the Inhibition and Disruption of Staphylococcus aureus Biofilms.

Owing to the emergence of antibiotic-resistant strains, bacterial infection and biofilm formation are growing concerns in healthcare management. Herein, we report an eco-benign strategy for the synthesis and functionalization of graphene-silver (rGOAg) nanocomposites with an antimicrobial peptide (AMP) for the treatment of Staphylococcus aureus infection. The synthesis of rGOAg nanocomposites was carried out by simple microwave reduction, and the as-synthesized rGOAg was covalently functionalized with an AMP. As a natural AMP, poly-l-lysine (PLL) functionalization of rGOAg enhanced the antibacterial efficacy and target specificity against the S. aureus biofilm. The robust bactericidal efficiency and biofilm disruption by AMP-functionalized rGOAg (designated as GAAP) occurred through the "contact-kill-release" mode of action, where the electrostatic interaction with bacterial cells together with intracellular ROS generation induced physical disruption to the cell membrane. The internalization of GAAP into the cytoplasm through the damaged cell membrane caused an outburst of intracellular proteins and DNA. Crystal violet staining along with fluorescence and confocal microscopic images showed an effective inhibition and disruption of the S. aureus biofilm upon treatment with GAAP. PLL functionalization also prevented the dissolution of Ag+ ions and thereby minimized the in vitro toxicity of GAAP to the 3 T6 fibroblast and human red blood cells. The ex vivo rat skin disinfection model further demonstrated the potency of GAAP in eliminating the biofilm formation and disruption of the S. aureus biofilm. The obtained results demonstrated a general approach for designing a functional nanocomposite material to disrupt the mature biofilm and provided a promising strategy for treating bacterial infection.

Hierarchically porous rGO synthesized by microwave reduction propagation for highly efficient adsorption and enrichment of lindane

Gamma-hexachlorocyclohexane, also known as lindane, is one of persistent organic pollutants that are harmful to humans and ecosystems. In order to detect and remove lindane, effective adsorption and enrichment of lindane are necessary. Porous materials can be used to treat pollutants, but their removal rate, adsorption capacity and desorption separation efficiency still need to be improved. In this work, hierarchically porous graphene materials (M-HPRGO) were prepared by microwave reduction and used for rapid and efficient adsorption of lindane. The obtained M-HPRGO exhibited secondary structure and high specific surface area due to its morphology with micron-scale stacked pores and nanoscale in-plane holes. These structural characteristics and residual oxygen-containing functional groups on the defects efficiently increased the contact of M-HPRGO with aqueous solution and exposed sufficient hydrophobic graphene domains to adsorb lindane by hydrophobic effect. In the adsorption process, M-HPRGO showed excellent removal ability, fast adsorption rate and high uptake capacity for lindane. Meanwhile, pH, temperature, metal ions and organic matters showed no significant influence on the adsorption performance of M-HPRGO. Moreover, the desorption process of adsorbed lindane was simple and complete, and the recycled M-HPRGO performed well in reusability. In addition, based on the outstanding adsorption and desorption performance of M-HPRGO for lindane, efficient enrichment of ultra-trace lindane was realized with over 100-fold concentration amplification, which could be used in sample pretreatment to significantly push down the detection limit of analytical equipment. The current results indicate that M-HPRGO not only has important implications for pollutant treatment, but also has practical application potential for ultra-trace pollutant detection.

Graphene oxide synthesis using modified Tour method

Graphene oxide (GO) colloidal solution has been synthesised by the modified Tour method, in which the pH of the reaction medium has been increased to 2.0–2.2 at the final stage of graphite oxidation by adding NaOH solution. The solid-phase graphene oxide consists of multilayered graphene particles with a thickness of about 7.5 nm (9–10 layers of graphene) and an average size of about 7.7 nm. Reduced graphene oxide (rGO) has been prepared by hydrazine and microwave reduction. A comparative study of the structure, morphology, and electrical transport properties of rGO samples obtained by various methods has been carried out using XRD, SAXS, Raman spectroscopy, low-temperature nitrogen adsorption, and impedance spectroscopy. Structural analysis has shown the presence of two fractions of plate-like rGO particles for each reduction method, which consist of 4–6 layers of graphene in stacks and have a lateral size in the range of 7.1–7.6 nm. The BET specific surface area of the microwave-reduced rGO was higher than that of the chemically reduced one (296 and 237 m2 g−1 respectively). The frequency dependence (10−2–105 Hz) of the AC conductivity of the GO and rGO samples has been analysed in the temperature range of 25 °C–175 °C. For the GO sample, the proton exchange conductivity mechanism dominates. A Drude-like response of electrical conductivity at frequencies above 103 Hz with transition to Johnscher’s law response at 175 °C has been observed for rGO samples obtained using both chemical and microwave routes. Changes in activation energies and relaxation times have been interpreted using a model of a thermally activated frequency-dependent electron hopping mechanism between randomly coupled conducting sp 2 rGO packages separated by disordered sp 3 rGO regions with correspondingly higher resistivity.

Determination of Chromium(VI), Chromium(III), Arsenic(V), Aluminum(III), Iron(II), and Manganese(II) by Colorimetry and Surface-Enhanced Raman Scattering (SERS) Using Ferulic Acid Functionalized Silver Nanoparticles

Plasmonic nanoparticles having localized surface plasmon resonance in the visible and infrared region are potential candidates for the fabrication of simple, rapid, selective and specific metal ion detection systems based on colorimetry and surface-enhanced Raman spectroscopy. The study describes simple approaches for selective colorimetric and surface-enhanced Raman scattering determination of aluminum(III), chromium (VI), chromiun(III), manganese (II), iron(II) and arsenic (V) ions in aqueous medium using ferulic acid functionalized silver nanoparticles (F-AgNPs) synthesized by chemical reduction and microwave assisted method. The analysis of photographs reveals that ferulic acid functionalized silver nanoparticles synthesized by chemical reduction (C-F-AgNPs) sensitively detect Al(III) ions in aqueous media while nanoparticles synthesized by microwave reduction method can be used to determine As(V), Mn(II) and Fe(II) ions. In addition, C-F-AgNPs also enhance the Raman spectral profile of the water samples spiked by Cr(III) ions giving a peculiar vibration spectral feature that assists in the detection of Cr(III) ion in aqueous medium. Moreover, ferulic acid functionalized silver nanoparticles synthesized by the microwave assisted method (M-F-AgNPs) increase the intensity of Raman bands in the spectral profile of the water samples spiked with Cr(VI) and Mn(II) ions. The specific and selective interactions of F-AgNPs with the Cr(VI), Cr(III), Mn(II), Fe(II), Al(III) and As(V) leading to the aggregation of the dispersed silver nanoparticles enable their determination in aqueous media . The results of the present study shows the potential of F-AgNPs based sensor probes toward conceptualizing and fabricating simple, rapid and cost effective detection tools onsite determination of metal ions in aqueous media.

Basic study on microwave carbon-thermal reduction senarmontite (Sb2O3) to produce antimony: High-temperature dielectric properties and a microwave reduction mechanism

Minerals with high thermal conductivities and outstanding dielectric properties have tremendous potential for microwave metallurgy. The thermogravimetric and dielectric properties and heating characteristics of a mixture ore of Sb2O3 and senarmontite were investigated at various temperatures. The mechanism of action for senarmontite and carbon powders during microwave heating is proposed. Based on the results, the authors divided microwave reduction of senarmontite by carbon powder into three stages: 25 °C–705 °C, 705 °C–800 °C, and 800 °C–1000 °C. Between 700 °C and 800 °C, the dielectric properties of the mixture ore of Sb2O3 reached the optimal level. Moreover, the microstructure and the structure of the reduced metal from microwave carbon-thermal reduction were characterized via XRD and SEM. According to the results of this study, microwave carbon-thermal reduction of senarmontite (Sb2O3) is a promising approach for producing antimony with excellent properties.

Luminescence spectra of reduced graphene oxide obtained by different initial heat treatments under microwave irradiation

When solid-state graphene is exposed to microwave irradiation, it often emits intense luminescence; however, whether the luminescence spectra of reduced graphene oxide (RGO) remains the same at different temperatures is not known. In this study, the luminescence spectra of RGO prepared by different initial heat treatments were measured and studied under microwave irradiation. The results showed spectral characteristics that varied with the initial heat treatment temperature. In particular, a new spectral line appeared at 670 nm, whose intensity increased with increasing initial heat treatment temperatures above 500 °C. Besides, the other notable difference in the spectra is the change in the spectral lines at 588 and 765 nm (769 nm) with the initial heat treatment temperature, and the luminescence spectrum of RGO-400 exhibited the weakest intensity at 588 and 765 nm (769 nm). We speculate that these changes may have affected the results of the microwave reduction. Further studies indicate that an initial heat treatment of 400 °C facilitated the preparation of high-quality graphene with smaller defects, larger domain size, and higher conductivity with microwave irradiation. Overall, this study is helpful for the preparation of high-quality graphene samples by microwave reduction.

Comparison of the Electrode Properties of Graphene Oxides Reduced Chemically, Thermally, or via Microwave Irradiation

Samples prepared via chemical, thermal, and microwave reduction of graphene oxide have been characterized by elemental analysis, BET measurements, scanning electron microscopy, and IR spectroscopy and tested as supercapacitor electrode materials. A supercapacitor having electrodes from chemically reduced graphene oxide has been shown to have the highest specific capacitance and the best long-term cycling stability.

Synthesis and Characterization of Graphene Sheets from Graphite through Electrochemical Exfoliation and Microwave Reduction

Graphene as a wonder material has received great attention and importance due to its fascinating properties. Here in this study, we also demonstrate a simple two step process to prepare graphene sheets (GSs) from the electrochemical exfoliated graphene oxide (EE-GO) followed by microwave irradiation reduction. The properties and structure of the resulted product samples were studied by Ultraviolet spectroscopy (UV), Fourier-transform infrared spectroscopy (FTIR), Thermogravimetric analysis (TGA), X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy-dispersive X-ray (EDX or EDS), Raman spectroscopy and Atomic force microscopy (AFM) for validation of their distinguishing characteristics. UV spectra of microwave irradiation reduced graphene oxide (MI-RGO) and EE-GO exhibited an intense and sharp absorption peak concentrated at wavelengths of 236 and 266nm, respectively. The elimination of oxygen functionalities present in the EE-GO plane was illustrated by FTIR as a result of microwave irradiation treatment. The relative layer structures of MI-RGO and EE-GO were confirmed by XRD. Similarly, the Raman spectra revealed the difference in between the EE-GO and MI-RGO characteristic reflection bands. The thin, crumpled and curved type morphology of the obtained graphene sheets (GSs) was also displayed by SEM while the composition of a few layer graphene sheets with atomic mass percents of carbon (75%) and oxygen (25%) was established by EDX. The Synthesis procedure is simple, rapid and eco-friendly with high yield.

Microwave-intensified treatment of low-zinc EAF dust: A route toward high-grade metallized product with a focus on multiple elements

This study reports treatment of low-zinc EAF dust for obtaining high-grade metallized product with a focus on multiple valuable elements via microwave reduction followed by magnetic separation. In reduction, the conversions of components of iron, zinc, lead, manganese and chromium, mainly in the form of spinel phases, to Fe, Zn, PbCl2, MnO and Cr2O3, respectively, were partially restrained by the formation of complex silicates, e.g., Ca3Al0.84Fe1.16Si3O12. After microwave reduction under the conditions of C/Fe mass ratio of 0.25, temperature of 1323 K, dwell time of 15 min, heating rate of 25 K/min and microwave power of 2200 W, the reduced briquette had the iron metallization degree of 96.20%, with volatilization of 98.38% of Zn and 80.22% of Pb. By magnetic separation, a high-grade metallized product containing 81.77 wt% Fe (79.97 wt% metallic iron) and 0.04 wt% Zn, satisfying the Chinese standard (YB/T 4272–2012) of the first-grade metallized product from dust, was obtained.

Platinum quantum dots enhance electrocatalytic activity of bamboo-like nitrogen doped carbon nanotubes embedding Co-MnO nanoparticles for methanol/ethanol oxidation

Interaction of multi-active components can effectively maximize the overall catalytic ability of alcohol fuel cells. Herein, the self-assembled nitrogen doped carbon nanotubes (NCNTs) containing Co-MnO composite (Co-MnO/NCNTs) are successfully synthesized using dihydrodiamine as carbon and nitrogen source through one-step synthesis. In order to further improve the catalytic activity of Co-MnO/NCNTs for alcohol oxidation, small amounts of platinum quantum dots are uniformly loaded on Co-MnO/NCNTs formation of quaternary hybrid (named Pt/Co-MnO/NCNTs) during microwave reduction stage. Notably, the prepared Pt/Co-MnO/NCNTs hybrids possess the excellent methanol and ethanol oxidation mass current density of 1775.4 and 1112.8 mA mg−1 in alkaline condition, which are 3.6 and 2.25 times higher than that of Pt/C catalysts, respectively. The current density of ethanol catalytic oxidation is lower than that of methanol, which may be due to the partial oxidation of acetyl (the intermediate product of ethanol) on the Pt (1 1 1) crystal surface. More importantly, CO oxidation experiments reveal that strong electronic synergistic effect between MnO and Pt quantum dot can greatly improve the CO anti-poisoning ability. Another significant advantage of Pt/Co-MnO/NCNTs is that low platinum loading leads to low cost effective, which demonstrates that the modification non-noble metal catalysts with a few noble metals quantum dots is a promising choice to mass produce high performance catalyst with remarkably boosting electrocatalytic activity for alcohol oxidation.

Production of Metallic Iron from the Pudo Magnetite Ore using End-of-Life Rubber Tyre as Reductant: The Role of an Underlying Ankerite Ore as a Fluxing Agent on Productivity

This research work investigated the nature of a nonmagnetic ore from Pudo in the Upper West Region of Ghana and its fluxing effect on the extent of reduction of the Pudo titaniferous magnetite ore using pulverised samples of charred carbonaceous materials generated from end-of-life vehicle tyres (ELT) as reductants. Reduction studies were conducted on composite pellets of the Pudo titaniferous magnetite iron ore containing fixed amounts of charred ELT and varying amounts (0%, 10%, 15%, 20%, 30%, 40% and 50%) of the nonmagnetic fluxing material in a domestic microwave oven and the extent of reduction was calculated after microwave irradiation for 40 minutes. Analyses by XRF, SEM/EDS and XRD of the nonmagnetic ore revealed an Ankerite type of ore of the form Ca0.95Fe0.95Mn0.1 (CO3)2. From the microwave reduction studies it was observed that premium grade metallic iron could be produced from appropriate blends of the Pudo iron ores using ELT as reductant, with a measured extent of reduction up to 103.8%. Further, the extent of reduction was observed to increase with an increase in the amount of the nonmagnetic fluxing material (Ankerite) that was added as fluxing agent.
 
 Keywords: Ankerite, End-of-life Rubber Tyres, Fluxing Agent, Extent of Reduction

Microwave-assisted synthesis of Cr3C2@C core shell structure anchored on hierarchical porous carbon foam for enhanced polysulfide adsorption in Li-S batteries

In this paper, we use microwave reduction strategy to synthesize a new bi-functional sulfur host material at the service of cathode substrate for lithium-sulfur batteries (LSBs), the composite is made of hierarchical porous carbon foam supported carbon-encapsulated chromium carbide nano-particles (Cr3C2@C/HPCF), in which the well-distributed conductive Cr3C2 nano-particles can act as powerful chemical adsorbent and are effective in restraining the shuttle effect of lithium polysulfides (LiPSs). Test results show that the Cr3C2@C/HPCF based sulfur electrodes with 75 wt.% of sulfur exhibit a high initial discharging capacity of 1,321.1 mAh·g−1 at 0.1 C (3.5 mg·cm−2), and a reversible capacity can still maintain stability at 1,002.1 mAh·g−1 after 150 cycles. Even increasing the areal sulfur loading to 4 mg·cm−2, the electrodes can still deliver an initial discharging capacity of 948.0 mAh·g−1 at 0.5 C with ultra-slow capacity decay rate of 0.075% per cycle during 500 cycles. Furthermore, the adsorption energy between the Cr3C2 surface and LiPSs as well as theoretic analysis based on first-principles is also investigated.

Microwave-assisted in-situ incorporation of nanostructured HyWO3-x on cotton cellulose for enhanced ultraviolet shielding applications

Nanostructured materials modified cotton cellulose could be a versatile and cost-effective solution for developing wide range of protective fabrics. In this background, cotton cellulose was surface modified with hydrated form of non-stoichiometric tungsten oxide (HyWO3-x) using microwave reduction technique for the development of UV radiation blocking and thermo-chromic fabrics. Herein, the surface modification and characterization of cotton cellulose having ligand to metal charge transfer (LMCT) transition via reduction process with enhanced UV blocking features are presented. X-ray diffraction patterns confirmed the poly-crystalline and amorphous nature of unmodified and surface modified cotton fabrics. Scanning electron micrographs revealed the growth of nanostructured HyWO3-x on cotton cellulose and surface analysis showed the existence of tungsten, carbon, oxygen elements. Differential scanning calorimetry of treated fabric exhibited a change of exothermic behavior from endothermic at the characteristic onset temperature. Thermogravimetric analysis revealed the least weight loss up to 600°C of the surface treated cotton cellulose. Further, Ultraviolet Protection Factor (UPF) of the modified fabric was measured using an in-vitro method following the AATCC 183:2004 standard employing UV transmittance analyzer. The UPF was estimated to be 2000, which is excellent for protective clothing applications according to UPF standard for fabrics. Also, the ligand to metal charge transfer characteristics of WO3-x modified cotton cellulose revealed the selective transition towards microwave irradiation at different reduction power.

Investigation of carbothermic microwave reduction followed by acid leaching for recovery of iron and aluminum values from Indian red mud

The red mud is an abundantly generated by-product of the aluminum industry rich in iron values mainly in the form of hematite and goethite phase. The favorable dielectric properties of iron oxide enable efficient microwave heating of red mud, and ~750 °C is achieved in 10 min with 10.7% C at 800 W. The heating rate in the mixture depends on the charcoal dosage and phase composition. The theoretical thermal decomposition kinetics suggest that the hematite to magnetite conversion in the red mud is chemically controlled and requires activation energy of 20.95 kJ/mol. The carbon gasification findings suggest that the CO formation is initiated below the bulk temperature of 700 °C during microwave heating, possibly because of accelerated microwave-induced heating in carbon having a significant effect on the reduction reaction mechanism. Volumetric heating enables efficient reduction and iron enrichment in magnetic concentrate with iron grade 52% and ~98% recovery constituting the magnetite phase majorly in 10 min, consuming 478.8 kJ energy. The iron metallization is dependent on the charcoal dosage, and ~15% metallic iron is formed in 30 min with 1435.4 kJ energy expenditure. The non-magnetic fraction is leached in 0.5 M HCl to recover alumina values in the mullite phase with an Al purity of 48% suitable for refractory applications. The non-magnetic residue containing iron values can be mixed with magnetic fraction to increase the Fe-product yield to ~84%.

Kinetics characteristics and microwave reduction behavior of walnut shell-pyrolusite blends

Combining biomass pyrolysis with microwave heating technologies provides a novel and efficient approach for low-grade pyrolusite reduction. The microwave reduction behavior and pyrolysis kinetic characteristics of walnut shell-pyrolusite blends were explored. Results indicated the optimal reduction parameters were: reduction temperature of 650 °C, holding time of 30 min, Mbio/More of 1.8:10, and microwave power of 1200 W. The co-pyrolysis characteristics of the blends included four stages: dehydration, pre-pyrolysis, intense pyrolysis and reduction, and slow pyrolysis and reduction. Fitting analysis based on Coats-Redfern method revealed that chemical reaction was the control step of the process of reducing pyrolusite by biomass, which the finding matched to the isothermal kinetic analysis results determined through unreacted shrinking nuclear model. The activation energies and pre-exponential factors were determined at 5.62 kJ·mol−1–16.69 kJ·mol−1 and 0.0426 min−1–0.515 min−1. The work provides sound references for promoting the industrial application of the combined method on minerals reduction.