Apparent Activation Energy Of Reduction Research Articles

Zn Promotes Chemical Looping Ammonia Synthesis Mediated by LiH-Li2 NH Couple.

Chemical looping ammonia synthesis (CLAS) is a promising alternative route to ammonia production because of its advantages of avoiding competitive adsorption of N2 and hydrogen source (H2 O or H2 ) and intervening the scaling relations in the catalytic process. Our previous studies showed that NH3 can be synthesized at low temperatures via a CLAS mediated by an alkali or alkaline earth metal hydride-imide couple with the aid of transition metal catalysts. Herein, we demonstrate that a group-IIB metal Zn, which has rarely been studied in the thermal-catalytic process, can significantly promote the performance of the lithium hydride-lithium imide (LiH-Li2 NH)-mediated CLAS process (denoted as Zn-LiH-Li2 NH). The addition of Zn dramatically changes the reaction pathway of the LiH-Li2 NH mediated loop by forming a series of intermediates including Li2 NH, lithium zinc intermetallic compounds (LiZnx ), and a ternary metal nitride (LiZnN). LiZnN together with Li2 NH functions as nitrogen carrier in the Zn-LiH-Li2 NH-mediated CLAS. Because of these properties, the kinetics of N2 fixation is significantly enhanced with a reduction in apparent activation energy from 102 kJ mol-1 to 50 kJ mol-1 . The ammonia production rate reaches 956 μmol g-1 h-1 at 350 °C, which is 19 times higher than that of the neat LiH-Li2 NH-mediated CLAS.

Highly conductive polyacrylonitrile-based hybrid aqueous/ionic liquid solid polymer electrolytes with tunable passivation for Li-ion batteries

The rapid growth in demand for lithium-ion batteries that can deliver more energy and power has generated concerns over safety. Aqueous electrolytes are a strong candidate to alleviate this apprehension, however their ability to overcome the “cathodic challenge” is limited due to anion-dominated passivation at the anode. In this work, the recently developed “hybrid aqueous/nonaqueous” electrolyte (HANE) strategy was employed to tune the degree of passivation at the anode in solid polymer electrolytes (SPEs) by using various ionic liquids as the nonaqueous component. Whereas common HANE systems sacrifice ionic conductivity to create a more robust passivation layer at the cathodic limit, the “hybrid aqueous/ionic liquid” SPEs (HAILSPEs) investigated in this work do not. Two HAILSPE systems (H1, H2.5) were fabricated from a blend of polyacrylonitrile (PAN), water, lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), and either triethylsulfonium-TFSI (S2,2,2) or N-methyl-N-propylpyrrolidinium-TFSI (Pyr1,3). These HAILSPE systems demonstrated a remarkable improvement in transport properties compared to their predecessors, achieving room temperature ionic conductivities of up to 5.39 mS/cm. A reduction in apparent activation energy and nearly complete decoupling of ionic transport from polymer chain mobility were found to contribute to this increase. Stable and complete growth of a passivating layer at 2 V vs. Li/Li+ was also observed, which was tuned by changing the ionic liquid. The work presented here provides a potential route for overcoming the “cathodic challenge” in aqueous SPEs.

Effect of Surface Dissolution on Dislocation Activation in Stressed FeSi6.5 Steel.

The effects of surface dissolution on dislocation activation in FeSi6.5 steel are quantitatively studied by analyzing the stress relaxation data using the thermal activation theory of dislocation. The stressed FeSi6.5 steel sample in acid solutions (H2SO4 or HCl) exhibits a much higher rate of stress reduction with time compared with that in air or deionized water. As the stress relaxation time is prolonged to 20 min, the relaxation rates are 0.055 MPa·min-1 in water and 0.074, 0.1, 0.11 MPa·min-1 in H2SO4 solutions with pH 4, 3, and 2, respectively. In a NaCl solution, a slight increase in the relaxation rate compared with air was found. Higher acidity (lower pH) of the solution inducing higher stress relaxation rate implies the softening is associated with the anodic dissolution of the surface layer and the accelerated (additional) flow of dislocations. The analyses using the thermal activation theory of dislocation during relaxation reveal the mechanism for the accelerated plastic flow induced by the corrosive medium. The variations of these parameters are related to the relaxation of the stress field of dislocations and the weakening of interaction between slip dislocations and short-range obstacles. The chemomechanical effect, including a reduction in apparent activation energy and a decrease in waiting time for dislocation to obtain sufficient thermal activation energy to cross obstacles, causes an increase in the stress relaxation rate (plastic strain rate). The study confirms that surface dissolution accelerates the plastic flow of metals and supports the view that surface dissolution facilitates dislocation slip. It is helpful to improve the formability of brittle metals.

Exploration of LiCoO2-doped cobalt oxide composite for thermochemical energy storage at high temperature

Due to the high energy storage density, long-term energy storage and long-distance transport possibility, as well as operation possibility under open system, metal oxide-based thermochemical energy storage (TCS) was considered as a promising candidate for efficient energy utilization. Herein, lithium cobalt oxide (LiCoO2)-doped cobalt oxide composite was proposed and investigated for the metal oxide-based TCS application. Doping with LiCoO2 diminished the onset temperature of reduction and re-oxidation by 50 °C and 65 °C, respectively. Also, doping with appropriate amount of LiCoO2 contributed to improve the re-oxidation rate of cobalt oxide-based system under a high cooling rate, resulting the discharging heat (423.8 kJ/kg) increased significantly in comparison with pure cobalt oxide (289.8 kJ/kg) under temperature cooling rate of 20 °C/min. It was found that doping with LiCoO2 decreased the apparent activation energy of reduction and oxidation. Meanwhile, the results obtained from in-situ X-ray diffraction indicate that LiCoO2 was involved in the reduction/re-oxidation process. Moreover, excellent repeatability of the composite was attested in 30 cycles. Hence, doping with LiCoO2 was verified as an effective method to expanding the possibility of cobalt oxide for TCS application.

Effects of scCO2 foaming process and post‐vulcanization method on cellular structure, mechanical properties, and thermal stability of PMVQ foams

AbstractIn this paper, the microcellular foam materials of methyl vinyl phenyl silicone rubber (PVMQ) were prepared by supercritical foaming technology (SFT) and high temperature compression molding. The effect of supercritical carbon dioxide (scCO2) on the vulcanizing properties of PVMQ composite was investigated because vulcanization of microcellular PVMQ foams played an important role on the cellular structure and mechanical properties. It was found that the sulfurization reaction of PVMQ composite was inhibited by scCO2, which led to reduction of apparent activation energy (Ea) of PVMQ composite. Compared with the existing post‐vulcanization process, the introduction of the high temperature molding can effectively improve the degree of vulcanization of microcellular PVMQ foams, which can solve the problem that the cellular structure will be destroyed during the application of external force. What's more, the mechanical properties of microcellular PVMQ foam were obviously improved. The maximum value of tensile strength and the compressive strength of microcellular PVMQ foam with the high temperature molding respectively increase by 34.7% and 133.3% compared with that of microcellular PVMQ foams with existing post‐vulcanization.

Rapid Elemental Extraction from Ordered and Disordered Solutes by Acoustically-Stimulated Dissolution

Alkaline industrial wastes (e.g., slags: ordered crystalline solids, and fly ashes: disordered solids) represent abundant reservoirs of elements such as silicon and calcium. Rapid elemental extractions from these wastes, however, have often relied on the use of “stoichiometric additives” (i.e., acids or bases). Herein, we demonstrate that acoustic stimulation enhances the release of network-forming Si species from crystalline blast furnace slags and amorphous fly ashes at reaction temperatures less than 65 °C. These additive-free enhancements are induced by cavitation processes which reduce the apparent activation energy of solute dissolution (Ea, kJ/mol) by up to 40% as compared to unstimulated conditions. Because of the reduction in the apparent activation energy, acoustic stimulation features an energy intensity that is up to 80% lower in promoting dissolution, as compared to other additive-free methods such as enhancing the solute’s surface area, introducing heat, or convectively mixing the solvent. Based on atomic topology analysis, we show that the reduction in apparent dissolution activation energy upon acoustic stimulation scales with the number of weak topological constraints per atom in the atomic network of the dissolving solute, independent of their ordered or disordered nature. This suggests that sonication breaks the weakest constraints in the solute’s atomic network, which, in turn, facilitates dissolution. The results suggest the ability of acoustic stimulation to enhance waste utilization and circularity, by enabling efficient resource extraction from industrial wastes.

Photocarriers-enhanced photothermocatalysis of water-gas shift reaction under H2-rich and low-temperature condition over CeO2/Cu1.5Mn1.5O4 catalyst

Aiming for cost-efficient hydrogen purification for application of fuel cells, a low-temperature water-gas shift (WGS) reaction over noble-metal-free catalyst to attain completed CO conversion in H2-rich atmosphere is required but still a great challenge. Herein, we present a configuration of semiconductor-bridging active sites in CeO2/Cu1.5Mn1.5O4 catalyst to work for the photothermal WGS reaction under light irradiation; due to the injection of photocarriers from CeO2 into active sites, a 61 % reduction of apparent activation energy enabled the WGS reaction to occur at 225°C. Notably, the catalyst delivered a 96.6 % of CO conversion in 30 min, and subsequently, 0.18 vol.% of CO was left in the system, which matches the industrial standard (< 1 vol.%). This study provides a new design idea on the semiconductor-coupled photothermal catalyst to boost catalytic performance.

Comparison of microwave and conventional heating for CO2 desorption from zeolite 13X

This study investigates microwave irradiation as an alternative to conventional heating for temperature swing adsorption processes. The performance of microwave and conventional heating during sorbent regeneration was evaluated by measuring CO2 desorption from zeolite 13X at different temperatures. Experimentally, a fixed bed of zeolite 13X was saturated by a 150 sccm flow of 15 % CO2 at room temperature followed by sorbent regeneration under nitrogen at 55, 100, or 150 °C by applying either microwave irradiation or conventional heating. Microwaves reduced sorbent regeneration times by at least half compared to regeneration by conventional heating. Under conventional regeneration, desorption curves were resolved into two peaks representing physisorbed CO2 (mass diffusion limited) at low temperature and bicoordinated CO2 (thermally limited) with increasing temperature. Under microwave regeneration, only one desorption peak was observed suggesting that CO2 desorption was limited by mass diffusion through the porous zeolite 13X structure, rather than by temperature. Depending on microwave power, apparent activation energy of the microwave-assisted regeneration was 15.8–18.1 kJ/mol, compared to 41.5 kJ/mol for conventional regeneration. The reduction in apparent activation energy is mainly attributed to selective microwave heating of CO2 adsorption sites (Na+ sites) resulting in greater steady state temperatures of Na+ relative to framework atoms, suggesting greater heating efficiency due to microwaves compared to conventional heat transfer. Due to rapid cycling and efficient heat transfer to CO2 sites on zeolite 13X, microwave regeneration is found to increase adsorption/desorption cycling productivity and potentially reduce the energy penalty of temperature swing capture.

The co-combustion performance and reaction kinetics of refuse derived fuels with South African high ash coal

This research focuses on the co-firing of low-quality coal with refuse derived fuel (RDF) as a means to reduce the volume of waste dumped in landfill sites. The co-combustion behaviour and kinetics of various RDF/coal blends at different weight ratios, along with their physicochemical characteristics were investigated. The physicochemical analysis revealed that the run-of-mine and discard coal have relatively low calorific values of 21.7 MJ/kg and 16.7 MJ/kg, respectively. The RDF samples, plastic blend (31.2 MJ/kg) and paper blend (22.4 MJ/kg), were found to have higher energy contents. The thermogravimetric analysis was performed in an atmosphere of air, over a temperature range of 25–850 °C, and the results showed that the RDF samples had lower ignition, devolatilisation, and burnout temperatures compared to the coals. The ignition temperatures for the blended fuel occurs in the lower temperature region when RDF is added to the blend, likewise the peak temperatures and burnout temperature shifted to a lower temperature zone. The activation energies (Ea) were determined using the Coats-Redfern method. The Ea for the run-of-mine (ROM) coal of 104.4 kJ/mol, was found to reduce to 31.4 kJ/mol for the 75% PB + 25% ROM coal blend and 35 kJ/mol for the 75% PL + 25% ROM coal blend, respectively. The discard coal which had an Ea of 109.9 kJ/mol was reduced to 30.9 kJ/mol for the 75% PB + 25% discard blend, and 33.5 kJ/mol for the 75% PL + 25% discard coal blend. It was determined that the most favourable blend for co-combustion was 70% discard coal + 30% PL RDF due to the similarity of the combustion profile to that of 100% coal and the simultaneous reduction in apparent activation energy.

Enhancement of oxidation resistance of modified P91 grade ferritic-martensitic steel by surface modification using laser shock peening

In this paper, effect of laser shock peening (LSP) on high temperature oxidation behavior of P91 steel has been studied. P91 samples were subjected to single and triple LSP treatments at a constant laser power density of 3.9 GW.cm−2. Single and triple LSP treatment led to the formation of ultrafine subgrains of sizes ~300 ± 30 and ~105 ± 10 nm respectively at near surface comparing to unpeened P91 sample (~1200 ± 120 nm). Further, unpeened and LSP treated samples were oxidized at temperatures of 600, 650 and 700 °C for time 5 to 200 h in air. Compare to unpeened sample, single and triple peened samples exhibited two and five fold reduction in weight gain after 200 h of oxidation at highest test temperature (700 °C). Improvement of oxidation resistance in LSP treated samples is attributed to the formation of continuous layer of protective oxides such as (Fe,Cr)2O3, Cr2O3 and MnCr2O4 due to enhanced diffusivity of Cr and Mn from bulk during early stage of oxidation. The enhanced diffusivity Cr and Mn in LSP treated samples have been further supported by reduction in apparent activation energy of oxidation, which is evaluated by modelling the time-temperature dependence of weight gain data in purview of kinetics theory.

Thermogravimetric and kinetic analysis to discern synergy during the co-pyrolysis of microalgae and swine manure digestate

BackgroundCo-pyrolysis of wastes with other feedstock can synergistically improve the rate of biomass decomposition and also help to resolve the issues related to limited availability feedstock. In this regards, synergistic interaction between feedstock during co-pyrolysis is an important aspect of research. As the constituents of aquatic and lignocellulosic biomass are different, and the decomposition pattern of aquatic biomass is dissimilar when compared to lignocellulosic biomass, it is important to understand whether these two biomasses interact during co-pyrolysis.ResultsSynergism in the co-pyrolysis of microalgae (MA), swine manure digestate (SWD), and their blends (MA/SWD) (w/w %), 2.5/7.5 (MD-1), 5/5 (MD-2), and 7.5/2.5 (MD-3), was evaluated based on decomposition behavior, gas yields, extent of thermal degradation, and kinetics. Extractives and volatiles in biomass enhanced the reaction kinetics and products yields, as indicated by the reduction in apparent activation energy of the blends, accompanied by an increase in H2, total gas yield, and extent in degradation. Thermogravimetric data, via isoconversional methods, were interpreted to achieve the apparent activation energies for the thermal degradation of the MA, SWD, and their blends. The best fit reaction models were identified using compensation effect and generalized master plots methods. Semi-quantitative method was used to quantify the evolved gas species. H2, CO, and CO2 were noted to be the dominant gases, implying that tar cracking and reforming reactions were predominant.ConclusionsOverall, synergy was noticed with respect to the pyrolysis of SWD biomass to gas products in the presence of MA biomass, whereas synergy was witnessed up to 50 w/w % MA in view of kinetic parameters as evaluation criteria.

Kinetics and Mechanism of Low-Grade Manganese Ore Reduction by Natural Gas

The calcination and the reduction behaviors of a low-grade manganese ore by methane was studied at 973 K to 1273 K by several techniques. The onset temperatures of thermal decompositions of pyrolusite and calcite phases for this ore were found to be 868 K and 1056 K, and the rates of decompositions are obtained as 1.4 × 10−3 s−1 and 1.53 × 10−3 s−1, respectively. Because of the presence of manganese in the goethite structure of the ore, the goethite decomposition resulted in higher degrees of decomposition and magnetite formation at the studied temperature range. The exothermic reduction of manganese oxides to MnO started prior to the reduction of the coexisting iron oxides, and the apparent activation energy of reduction of Mn3O4 to MnO was determined. The reduction of manganese oxides resulted in CO2 gas release due to the reaction of H2O with the carbon supplied via methane cracking. Although manganese oxide minerals of the ore were not carburized at the experimental conditions, the manganese associated with iron oxides was carburized to cementite ((Fe,Mn)3C), and it was accompanied with CO gas release. Higher rates and extents of manganese oxides reduction were obtained with increase in temperature; however, the simultaneous formation of manganese silicates was more favorable at higher temperatures and hindered complete reduction.

Comparative kinetic analysis of conventional and ultrasound-assisted leaching of scheelite by sodium carbonate

Abstract For further comprehending the mechanism of the intensification of ultrasound for extraction, the difference of leaching kinetics between the ultrasound-assisted and conventional sodium carbonate leaching of scheelite was studied, and the corresponding kinetic equations were established. The results indicate that the obtained apparent activation energy with or without ultrasound is same 72 kJ/mol, and both reactions are controlled by surface chemical reaction. By comparing the leaching kinetics of scheelite by Na2CO3, NaOH and Na3PO4 reactants with or without ultrasound, a general conclusion for scheelite hydrometallurgy is obtained. For the process with the formation of compact product layer, the intensification of the tungsten extraction with ultrasound mainly reflects in the stripping and elimination of product layer, corresponding to the transformation of kinetic controlling step from diffusion controlling step and reduction of apparent activation energy. For the one without compact product layer or with a loose and porous product layer, the intensification mainly embodies in the increase of frequency factor, and the controlling step and apparent activation energy of the reaction always remain constant.

Effect of pre-oxidation on the kinetics of reduction of ironsand

The kinetics of reduction raw and pre-oxidized ironsand with hydrogen was studied using a combination of thermogravimetric analysis technique and microstructural characterization. Temperature range of 1073–1273 K (800–1000 °C) and H2 partial pressure of 0.1–1.0 atm were covered. It was found that the reaction is first order with respect to pH2 at 1173 K (900 °C). The reduction reaction was considerably faster for the pre-oxidized ironsand, owing to the presence of micro-cracks in the particles. The extent of reduction was quantified against time at different temperatures and the results were analyzed using both conventional kinetic models, as well as an iso-conversional method. The results indicated that for the raw ironsand, the reduction rate is first limited by nucleation and growth of products and later by first order chemical reaction. At last, it is controlled by three-dimensional diffusion of reactants. The apparent activation energy of reduction increased from ∼50 to 105 kJ mol−1 when the conversion degree increased from 0.2 to 0.6. On the other hand, the activation energy for the pre-oxidized ironsand remained stable at 67.2 kJ mol−1 over the entire process, following a first order chemical reaction control step. The diffusion-limited step does not appear to be limiting the rate for the pre-oxidized ironsand reduction process due to the micro-cracks formed in this material, facilitating the gas access to solids.

Dry reforming of methane to syngas over lanthanum-modified mesoporous nickel aluminate/γ-alumina nanocomposites by one-pot synthesis

La-modified NiAl2O4/γ-Al2O3−La composites with mesoporous structures were prepared by one-pot template-free strategy and applied for dry reforming of methane (DRM) to syngas. The characterization results confirmed that these materials possessed high specific surface areas, large pore volumes and narrow pore size distributions. The reduced catalysts exhibited excellent catalytic properties as well as long-term stability for DRM reaction. Addition of La showed little influence on the catalyst structure and the mean sizes of metal Ni particles, but could enhance the medium-strength basicity and the accumulation of Ni2+ on the catalyst surface, resulting in the enhancement of intrinsic activity, the reduction of apparent activation energy, and the suppression of carbon deposition for DRM reaction. The catalyst containing 3 wt% La possessed the best catalytic performance. The characterization of spent catalysts also demonstrated that La could effectively prevent the phase transformation of γ-alumina in the DRM process.

Kinetic studies on the reduction of iron ore nuggets by devolatilization of lean-grade coal

An isothermal kinetic study of a novel technique for reducing agglomerated iron ore by volatiles released by pyrolysis of lean-grade non-coking coal was carried out at temperature from 1050 to 1200°C for 10–120 min. The reduced samples were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and chemical analysis. A good degree of metallization and reduction was achieved. Gas diffusion through the solid was identified as the reaction-rate-controlling resistance; however, during the initial period, particularly at lower temperatures, resistance to interfacial chemical reaction was also significant, though not dominant. The apparent rate constant was observed to increase marginally with decreasing size of the particles constituting the nuggets. The apparent activation energy of reduction was estimated to be in the range from 49.640 to 51.220 kJ/mol and was not observed to be affected by the particle size. The sulfur and carbon contents in the reduced samples were also determined.

Influence of alkali metal amides on the catalytic activity of manganese nitride for ammonia decomposition

Strong promoting effect of alkali metal amides, i.e., LiNH2, NaNH2 and KNH2, on the catalytic activity of manganese nitride (MnN) for ammonia decomposition has been demonstrated, which is evidenced by ca. 100K drop in onset temperature and by ca. 50–60kJmol−1 reduction in apparent activation energy as compared with the neat MnN. The order of promotion capability of alkali metal amides can be ranked as LiNH2>KNH2≥NaNH2, either in term of NH3 conversion rate or turnover frequency (TOF), which is distinctly different from that of conventional alkali metal oxides or hydroxides, i.e., K>Na>Li. This phenomenon suggests that the promoting mechanism of alkalis depends on their chemical forms. When alkalis are in the form of amide or imide, they may function as co-catalysts by participating in the catalytic circle directly rather than by executing electronic promotion influence on transition metals.

Effect of external magnetic field on the Co3O4 reduction kinetics

The effect of external magnetic field on the kinetics of reduction of bulk Co3O4 and silica-supported cobalt oxide (Co3O4/SiO2) with hydrogen has been investigated. The magnetic field exerts an effect on the apparent activation energy of reduction of the bulk oxide and has no effect on the activation energy of reduction of Co3O4/SiO2. It is hypothesized that the changes in the kinetic parameters are due to the effect of the external magnetic field on the structure defects in the solid.

Investigation of the Reduction/Oxidation Behavior of Cobalt Supported on Nano-ceria

The oxidation and reduction behavior of cobalt catalysts supported on nano-ceria (5–8 nm) was investigated under hydrogen, oxygen and water atmospheres. A novel quantitative isothermal reduction (QIR) technique was introduced to analyze the kinetics and activation barrier of Co reduction. CoO to Co reduction was found to be of first order in the 250–350 °C temperature range. Temperature programmed reduction and oxidation experiments were conducted and the Kissinger method was used to obtain apparent activation energies for reduction and oxidation with O2. The apparent activation energy for CoO reduction was found to agree with that obtained from the QIR technique. Re-oxidation of Co metal was found to have a slightly lower activation energy barrier than the reduction. The reduction and oxidation behavior was further investigated with in situ XANES where the reaction order for reduction was observed to change at 450 °C. The pre-reduced samples were seen to re-oxidize under a water atmosphere, where the oxidation followed first order kinetics. Re-oxidation by water yielded a higher activation energy when compared to re-oxidation under oxygen.

Reduction Kinetics of Vanadium Titano-Magnetite Carbon Composite Pellets Adding Catalysts Under High Temperature

Experiments were carried out by adding CaF2 and NaF as catalysts in an Ar atmosphere to study the isothermal reduction kinetics of vanadium titano-magnetite carbon composite pellets under high temperature in the range from 1 473 to 1 673 K. The scanning electron microscope (SEM) was used to characterize the microstructure of product. By analyzing reduction mechanism, it was found that the rate controlling step was gas diffusion, and the activation energy was 178. 39 kJ/mol without adding any catalysts. Adding CaF2 or NaF of 3% to vanadium titano-magnetite carbon composite pellets can decrease the apparent activation energy of reduction, and the decrease extent was 14. 95 and 15. 79 kJ/mol, respectively. In addition, temperature was an important factor influencing on reaction rate.