Synthesis of nitrogen‐doped graphene catalyst by high‐energy wet ball milling for electrochemical systems

Platinum group metal‐based (PGM) catalysts are widely applied in many electrochemical systems such as fuel cells or metal–air batteries because of their excellent catalytic performance. But the high raw material cost of PGM catalysts has become a significant issue. In recent years, huge efforts have been made to reduce the material cost of electrochemical systems by developing non‐PGM catalysts, and as one of the promising non‐PGM catalysts, nitrogen‐doped graphene (N‐G) has emerged. In this research, nanoscale high‐energy wet ball milling methodology was investigated as an effective synthesis method for N‐G catalysts by using graphene oxide and melamine as raw materials. The main purpose is to study reaction mechanism of the synthesis process and the physical, chemical, and electrochemical properties of N‐G catalysts generated by this mechanochemical approach. The elemental composition, chemical bonding composition, and electron transfer number of the synthesized products were characterized. The results show that the electron transfer number of the N‐G catalyst with 23.2 at% nitrogen doping content, synthesized by the high‐energy wet ball milling method, has attained a value of 3.87, which is close to the number (3.95) of Pt/C catalysts, and the grinding time was found to be a significant factor in the properties of N‐G catalysts in the experiments. The results also show that the high‐energy wet ball milling developed in this research is a promising method to synthesize high‐performance N‐G catalysts with a simple and easy controllable approach. Copyright © 2016 John Wiley & Sons, Ltd.     

Effect of different grinding conditions on the dissociation and flotation of difficult-to-float coal

The recovery of clean coal can be improved through grinding. In this study, grinding and flotation experiments were conducted on difficult-to-float coal. Results show that the particle size obtained from ball milling is finer than that obtained from rod milling. However, the particle size distribution obtained from rod milling is more evenly distributed than that obtained from ball milling. The contact styles of steel rod and steel ball are “line contact” and “point contact,” respectively. Results from the flotation experiments are consistent with those from the orthogonal experiments.     

Crystallite size and lattice strain of lithiated spinel material for rechargeable battery by X‐ray diffraction peak‐broadening analysis

High‐energy ball milling is performed on Li_1.1Mn_1.95Fe_0.05O₄ spinel material, synthesized by sol‐gel method for lithium rechargeable battery, at different durations to obtain nanopowders of finite size distributions. The powders are investigated by means of scanning electron microscopy, particle size distribution, and X‐ray diffraction (XRD) measurements. The structural analysis of the powders is performed to investigate the effect of milling on the particle size, crystallite size, and lattice strain. The scanning electron micrographs and size distribution measurements show that the particle size decreases with the increase in milling duration. The XRD results show that the widths of the diffraction peaks increase with the decrease of particle size (increase of milling duration). This broadening is analyzed according to Scherrer, Williamson‐Hall, and Halder‐Wagner methods. Peak broadening is attributed to contributions of crystallite size and lattice strain. While reducing the particle and crystallite sizes is desirable to achieve higher specific capacity and energy density of the battery active material, lattice strain leads to material degradation and a reduced capacity retention. Thus, when performing mechanical milling, lattice strain should be taken seriously into consideration to optimize the milling parameters and to enhance the materials electrochemical performance.     

现代汽车发动机制造工艺的发展动向

本文叙述了发动机五大件加工工艺的发展动向：缸体与缸盖正在发展敏捷柔性生产线取代传统柔性生产线;曲轴的主轴颈和连杆颈的粗加工工艺已由车拉（含车一车拉）、内铣、单刀车削（转塔式）及高速外铣取代了过去多刀车削及普通内铣,并提出了怎样合理选用车拉、内铣、单刀车削及高速．外铣工艺;凸轮轴的凸轮磨削已由 ＣＮＣ无靠模磨削工艺取代了过去机械式靠模磨削工艺;凸轮轴（含曲轴）的主轴颈系统磨削工艺正在发展由高速点磨工艺取代传统磨削工艺,同时最近又开发出凸轮轴（含曲轴）的主轴颈系统和凸轮（曲轴的连杆颈）的集成磨削工艺;连杆分离面正在采用涨断工艺取代传统切削工艺。     

Hydrogen generation from the hydrolysis of LaMg12H27 ball-milled with LiH

The LaMg₁₂H₂₇ (LMH) and LMH-5 wt% LiH samples were prepared by different ball milling times, and the hydrogen generation performances of samples were investigated and compared. X-ray diffraction and scanning electron microscopy techniques were adopted to elucidate the performance improvement mechanisms. For the LMH, with the increase of ball milling time, the hydrogen generation yield and kinetics can be improved significantly. This may be caused by the change in particle size and crystallite size of the LMH sample after ball milling. However, for the LMH-5 wt% LiH, with the increase of the ball milling time, the initial kinetics increases firstly, and then decreases. This may be due to the gradual movement of LiH from the surface to the interior of LMH and to the covering of LMH after ball milling.     

A modified method for production of hydrogen from methane

The steam–methane‐reformation (SMR) reaction has been modified by including sodium hydroxide in the reaction. It is found that the reaction: 2NaOH+CH₄+H₂O = Na₂CO₃+4H₂ takes place at much lower temperatures (300–600°C) than the SMR reaction (800–1200°C). The reaction rate is enhanced with a nickel catalyst. We have studied the effect of variously ball‐milled nickel on the reaction rate and determined the optimum particle size of the catalyst. Best results were achieved by grinding the catalyst for 2 h. Prolonged ball milling caused the nickel platelets to coalesce and grow in size decreasing the reaction rate. Copyright © 2011 John Wiley & Sons, Ltd.     

Effect of mechanical activation process parameters on the properties of LiFePO4 cathode material

Pure, nano-sized LiFePO₄ and carbon-coated LiFePO₄ (LiFePO₄/C) positive electrode (cathode) materials are synthesized by a mechanical activation process that consists of high-energy ball milling and firing steps. The influence of the processing parameters such as firing temperature, firing time and ball-milling time on the structure, particle size, morphology and electrochemical performance of the active material is investigated. An increase in firing temperature causes a pronounced growth in particle size, especially above 600 °C. A firing time longer than 10 h at 600 °C results in particle agglomeration; whereas, a ball milling time longer than 15 h does not further reduce the particle size. The electrochemical properties also vary considerably depending on these parameters and the highest initial discharge capacity is obtained with a LiFePO₄/C sample prepared by ball milling for 15 h and firing for 10 h at 600 °C. Comparison of the cyclic voltammograms of LiFePO₄ and LiFePO₄/C shows enhanced reaction kinetics and reversibility for the carbon-coated sample. Good cycle performance is exhibited by LiFePO₄/C in lithium batteries cycled at room temperature. At the high current density of 2C, an initial discharge capacity of 125 mAh g⁻¹ (73.5% of theoretical capacity) is obtained with a low capacity fading of 0.18% per cycle over 55 cycles.     

Doping oxygen triggered electrocatalytic activity of carbon interpenetrating networks in acid electrolyte

The Fe–N–C catalysts may be promising candidates for replacing platinum group metal (PGM) catalysts to solve sluggish oxygen reduction reaction (ORR) kinetics in the proton exchange membrane fuel cells. However, the activity of Fe–N–C catalysts still has a certain gap compared with commercial Pt/C. Here, we provide a way to increase the intrinsic activity of Fe–N–C catalysts by designing active sites like ketone functional groups. A self-supporting interpenetrating network catalyst, composed of carbon nanotube (CNT) and carbon nanoparticle (CNP), is synthesized via multiple carbon sources (zinc-zeolitic imidazolate frameworks, polyaniline). The interpenetrating network features abundant ketone functional groups. The density functional theory (DFT) results prove that ketone groups can promote the ORR activity of FeN₄ active sites. This offers a new idea for improving the activity of Fe–N–C catalysts co-doped by oxygen and nitrogen in acidic systems.     

An iron ore-based catalyst for producing hydrogen and metallurgical carbon via catalytic methane pyrolysis for decarbonisation of the steel industry

Experiments to investigate the catalytic pyrolysis of methane using an iron ore-based catalyst were carried out to optimize catalytic activity and examine the purity of the carbon produced from the process for the first time. Ball milling of the iron ore at 300 rpm for varying times – from 30 to 330 min – was studied to determine the effect of milling time on methane conversion. Optimal milling for 270 min led to a five-fold increase in methane conversion from ca. 1%–5%. Further grinding resulted in a decline of methane conversion to 4% shown by SEM to correspond to an increase in particle size caused by agglomeration. Data from Raman and Mössbauer spectroscopy and H₂ temperature programmed reduction indicated a change in phase from magnetite to maghemite and hematite (at the particle surface) as the grinding time increased. Analysis of the carbon produced as a byproduct of the reaction indicated a highly pure material with the potential to be used as an additive for steel production.     

Effect of hydrogen on Fe and Pd alloying and physical properties

Metastable Fe–Pd powder samples were synthesized by mechanically activated solid-state diffusion using high-energy ball milling. The Fe and Pd alloying and the hydrogen effect on this process were followed by preparation of two samples: the A-sample was a mixture of Fe powder and of Pd powder pre-charged with hydrogen (PdH) and milled under Ar atmosphere, the B-sample was a mixture of the Fe and Pd powders milled under hydrogen atmosphere.The fundamental properties, i.e., chemical and phase composition, lattice parameters, microstructure, morphology, grain size, defect structure, and macro- and micro-magnetic properties, were monitored after several steps of the alloying at room and appropriately at elevated temperatures.The alloying of Fe and Pd in both samples begins already after 5 h of milling and two phases are formed, the dominating bcc-Fe(Pd) phase and a minor fraction of the fcc-Pd(Fe) phase. The occurrence of the fcc phase, not observed previously by solid-state diffusion under argon atmosphere, is ascribed to mainly a positive effect of hydrogen reducing the formation energy of lattice defects and facilitating their formation. Consequently, the moving defects during mechanical alloying make the solid-state diffusion of Pd into bcc-Fe lattice and Fe into fcc-Pd lattice easier. On the other hand, hydrogen used as atmosphere in the milling procedure is adsorbed on the particle surfaces and after the vial opening hydrogen atoms form water molecules with oxygen from air. This exothermic reaction causes a removal of hydrogen atoms from the particle surface which thus becomes more sensitive to oxidation. Nothing similar was observed after mechanical alloying under argon atmosphere having positive impact on the particle surface stability.     

Syntheses, characterization, and catalytic oxygen electroreduction activities of carbon-supported PtW nanoparticle catalysts

Carbon-supported PtW (PtW/C) alloy nanoparticle catalysts with well-controlled particle size, dispersion, and composition uniformity, have been synthesized by wet chemical methods of decomposition of carbonyl cluster complexes, hydrolysis of metal salts, and chemical reactions within a reverse microemulsion. The synthesized PtW/C catalysts were characterized by X-ray diffraction, high-resolution transmission electron microscopy, and energy-dispersive spectroscopy. The catalytic oxygen electroreduction activities were measured by the hydrodynamic rotating disk electrode technique in an acidic electrolyte. The influence of the synthesis method on PtW particle size, size distribution, composition uniformity, and catalytic oxygen electroreduction activity, have been investigated. Among the synthesis methods studied, PtW/C catalysts prepared by the decomposition of carbonyl cluster complexes displayed the best platinum mass activity for oxygen reduction reaction under the current small scale production; a 3.4-fold catalytic enhancement was achieved in comparison to a benchmark Pt/C standard.     

Characterization of graphite catalytic effect in reactively ball-milled MgH2-C and Mg-C composites

In the present work magnesium hydrogen sorption kinetics of different graphite nanocomposites prepared by low-energy ball milling is reported. A comparison is made between kinetic performance in a Sievert-type apparatus of pure Mg, Mg + xG (x = 5, 10, 20 wt.%, G = graphite) composites and a mixture obtained by adding 10 wt.% G to MgH₂ already synthesized via reactive mechanical milling. Moreover, the influence of two types of graphite morphology (flakes and fine powder) is evaluated. In addition, thermal dehydriding properties are analyzed by differential scanning calorimetry (DSC) measurements and morphological and microstructural characteristics are determined by X-ray diffraction (XRD), laser granulometry, BET surface analysis as well as scanning and transmission electron microscopies (SEM and TEM) for every material under study.Graphite catalytic effect is independent of its morphology and is more pronounced when the additive is incorporated from the beginning of milling, as well as in a higher weight proportion. Furthermore, the additive preserves material microstructure during a few hydriding and dehydriding cycles, i.e. particle size distribution and crystallite enlargement, and as a result kinetic properties remain practically unchanged.     

A metal-organic framework-derived Fe–N–C electrocatalyst with highly dispersed Fe–Nx towards oxygen reduction reaction

Metal and nitrogen co-doped catalysts have been promising alternatives to platinum group metal (PGM) catalysts for oxygen reduction reaction (ORR) over the past few decades. Herein, we have synthesized an efficient Fe–N–C catalyst by the co-calcination of NH₂-MIL-101@PDA and melamine. The best Fe–N–C shows a positive half-wave potential of 0.844 V, which is 14 mV higher than that of Pt/C catalyst, as well as superior methanol resistance and long-term durability in alkaline electrolyte. In addition, Fe–N–C also exhibits pronounced catalytic activity and a direct 4e⁻ reaction pathway in acid electrolyte. We ascribed the excellent ORR performance of Fe–N–C to its crumpled structure, large specific surface area (584.6 m² g⁻¹) and high content of Fe-Nx sites (1.22 at. %). This study provides a simple way for the fabrication of excellent PGM-free ORR catalysts.     

Controllable synthesis of carbon nanofiber supported Pd catalyst for formic acid electrooxidation

Carbon nanofiber (CNF) supported Pd nanoparticles are synthesized with sodium citrate and sodium borohydride served as stabilizing agent and reducing agent, respectively. The size and distribution of the supported Pd nanoparticles are controlled by adjusting the pH value of the synthesis solution. Analyses of the obtained Pd/CNF catalysts indicate that the supported Pd nanoparticles become more uniform in size and the average particle size is decreased from 5.85 to 3.62 nm with pH value of the synthesis solution increasing from 3.2 to 6.0. However, the further increasing of the pH value to 6.5 leads to an increased particle size and the formation of PdO phase in the synthesized Pd/CNF catalyst. The Pd/CNF catalyst synthesized at the pH value of 6.0 exhibits superior catalytic activity and stability for formic acid electrooxidation due to its small particle size and uniform size distribution.     

四甲基硅烷燃烧法制备气相白炭黑的研究

介绍了以四甲基硅烷为原料,采用气相燃烧的方法制备超细白炭黑的新工艺。探讨了火焰构型、四甲基硅烷给气速率、甲烷给气速率、氧气给气速率、载气(氮气)通气速率等反应条件对产物的理化特性的影响。结果表明,前驱体浓度、初级颗粒在火焰中停留时间和火焰温度是影响白炭黑颗粒粒径大小的主要因素,前驱体浓度大、火焰中停留时间长、火焰温度高、产物粒子粒径越大。通过调节适量载气,可以很好地控制白炭黑粒径大小,使其粒径分布更均匀,成功制取粒径为9.46nm的超细白炭黑。并指出二氧化硅纳米颗粒在扩散火焰中经历化学反应、成核、凝并及团聚等几个主要过程。     

The influence of mechanical milling parameters on hydrogen desorption from Mgh2-Wo3 composites

The influence of different milling conditions obtained using two high-energy mills on hydrogen desorption from MgH₂-WO₃ composites was investigated. The morphology, particle and crystallite size were studied as a function of milling speed, vial's volume, and ball-to-powder ratio. The vial's fill level, the number, and type of milling balls and additive's content kept constant. Changes in morphology and microstructure were correlated to desorption properties of materials. Higher milling speed reduced particle size but, there is no significant crystallite size reduction. On the other hand, additive distribution is similar regardless of the energy input. It has been noticed that different energy input on milling blend, which is the result of combined effects of above-mentioned factors, reflects on desorption temperature but not on the kinetics of desorption. In fact, desorption mechanism changes from 2D to 3D growth with constant nucleation rate, despite obtained changes in microstructure or chemical composition of the material.     

Catalytic co‐pyrolysis of Eichhornia Crassipes biomaѕѕ and polyethylene using waste Fe and CaCO3 catalysts

A wild aquatic plant, Eichhornia Crassipes, and polyethylene have been converted into liquid product thermo‐catalytically and cost effectively through co‐pyrolysis using batch steel pyrolyzer. The Fe and CaCO₃ catalysts were obtained as wastes from various mechanical processes. The catalytic process was compared with non‐catalytic pyrolysis. The effect of various reaction conditions was investigated in order to find out the optimized process conditions. It was found that the favorable reaction conditions were 450 °C temperature and 1‐h reaction time at a heating rate of 1 °C/s and 0.4‐mm biomass particle size. The bio‐oil yield was found to be 34.4% and 26.6% using Fe and CaCO₃ respectively with catalysts particle size of 0.4 mm at the optimized reaction conditions and 5 wt% of biomass. The non‐catalytic and catalytic co‐pyrolysis using Fe as catalyst produced 23.9% and 28.7% oil respectively. Thus the efficiency of processes in terms of bio‐oil production was found in order of: Fe > CaCO₃ > non‐catalytic pyrolysis. The GC/MS analysis of n‐hexane extract of bio‐oil shows that Fe catalyst favors formation of aliphatic hydrocarbons while CaCO₃ and non‐catalytic pyrolysis favors formation of aromatic hydrocarbons. Mostly unsaturated aliphatic hydrocarbons were formed in case of co‐pyrolysis reactions. The calorific value of bio‐oil was also measured in order to find out the fuel properties of the products. Copyright © 2016 John Wiley & Sons, Ltd.     

Relationship between lattice defects and phase transformation in hydrogenation/dehydrogenation process of the V60Ti25Cr3Fe12 alloy

The hydrogen storage capacities of vanadium-based solid solution alloys are determined by both bulk and surface characteristics. In this research, the influence of lattice defects and phase transformation of the ball-milled V₆₀Ti₂₅Cr₃Fe₁₂ alloy was investigated systematically. With the elongated ball milling time (0.5 h, 1 h, 2 h and 3 h), both the hydrogenation and dehydrogenation capacities decrease. XRD and TEM methods are applied to make the essential mechanism clearer. The XRD results demonstrate that after full hydrogenation, a large amount of VH₂ phase and some V₂H phase are observed, and the V₂H phase proportion rises with the elongated ball milling time. The alloy particle size is limited in several micrometers, however, the XRD and subsequent TEM results demonstrate that unnegligible amorphous phase is formed in the surface layer (about 5 nm). The PALS and subsequent HRTEM results conformed that some defects (lattice deformation and dislocations/vacancies) appear in the bulk of the milled alloy particles. It is because of the defects and amorphous phase caused by ball milling that the formation of the monohydride is not been suppressed effectively in the hydrogen absorbing and desorbing process, and the defects can trap the hydrogen atoms, leading to the decrease of the hydrogen capacity. In addition, the phase transformation between V₂H and VH₂ is confined, thus a decrease of effective hydrogenation capacity is observed. As to the cyclic property, the micro-strains cannot be released by refining the particle size of the alloy particles by ball milling, thus, the hydrogen storage cyclic durability is not ameliorated.     

Electrochemical hydrogen storage properties of ball-milled multi-wall carbon nanotubes

The structure changes of multi-wall carbon nanotubes (MWNTs) processed by mechanical ball milling and the influence on their electrochemical hydrogen storage capacities were studied. TEM micrographs show that MWNTs are shortened and open-ended after ball milling. The effects of different MWNT type and ball milling time on the discharging capacity were investigated. Among all the samples examined, the sample of short MWNTs with diameter of 5 nm and ball milling time of 12 h has the largest discharge capacity (741.1 mAh/g). According to the analysis of Raman spectra and nitrogen adsorption experiments, it can be inferred that the micropore volume, specific surface area and appropriate defects are crucial to the storage capacity. In the cyclic voltammograms, the hydrogen desorption peak appears prior to hydrogen oxidation peak, which is attributed to the slow reaction of hydrogen oxidation at MWNTs. The results also suggest the possible existence of the strong chemisorption of hydrogen.     

Direct synthesis of sodium alanate using mischmetal nanocatalyst

This study reports the synthesis of NaAlH₄ by ball milling of NaH and Al mixture along with 3 mol % Mischmetal (Mm) nanocatalyst under hydrogen atmosphere. It is observed that synthesis of the intermediate phase Na₃AlH₆ can be achieved by ball milling even under 1 atm hydrogen at room temperature. Ball milling of the NaH + Al with 3 mol % Mm with 3 atm hydrogen in excess of 40 h time did not lead to the formation of NaAlH₄ but charging of the milled material at 100 atm hydrogen pressure at 120 °C lead to formation of NaAlH₄ phase. Direct synthesis of NaAlH₄ was achieved by milling of NaH + Al with 3 mol % Mm under 100 atm hydrogen pressure. Direct synthesis is possible even without any catalyst by high pressure milling. However catalyst is required to improve the hydrogen sorption characteristics of the synthesized material. The as-prepared Mm catalyzed NaAlH₄ is also found to reversibly store hydrogen up to 4.2 wt% hydrogen. Catalytic activity is attributed to defects promoted by ball milling and catalysts.