Kinetics of reduction of magnesium sulfate by carbon oxide

The thermodynamic analysis and the results of thermogravimetric investigation of the reduction of magnesium sulfate by carbon oxide are reported.The isothermal experiments were carried out in the temperature range 640 to 675°. A shrinking-core model was found to fit the reaction rate. An activation energy of 209.7±8.6 kJ/mol was obtained.

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Zusammenfassung Es wird über thermodynamische Analyse und Resultate einer thermogravimetrischen Untersuchung der Reduktion von Magnesiumsulfat mit Kohlendioxid berichtet. Die isothermen Experimente wurden im Temperaturbereich 640 bis 675° durchgeführt. Zur Erklärung der Reaktionsgeschwindigkeit wurde eine Shrinking-Modell gefunden. Es wurde eine Aktivierungsenergie von 209.7±8.6 kJ/mol erhlten.

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. 640–675°. , . 209,7±8,6 /.

     

Kinetics of reduction of magnesia with carbon

Reduction kinetics of magnesia with carbon powder was studied using the thermal gravimetric technic in the temperature range of 1743–1883 K. The reduction ratio was determined as a function of time. The effects of compact-forming pressure, composition, partial pressure of CO, sample height, and temperature on reduction ratio were investigated. Experimental results revealed that the gas diffusion including Mg vapor, CO and CO₂ through the porous medium was the rate-determining step of the overall reduction process. Activation energy of the gas diffusion was estimated to be about 30.59 kJ mol⁻¹.     

Some aspects of calcium sulphite reduction with carbon monoxide

The non-isothermal reductive decomposition reaction of crystalline CaSO₃·0.5H₂O and anhydrous CaSO₃ under CO, CO₂ and CO-CO₂ gas mixtures at the two heating rates of 25°C min⁻¹ and 6°C min⁻¹ up to 900°C, respectively, was investigated employing thermogravimetric analysis, X-ray powder diffraction, IR spectroscopy and continuous SO₂ measurement by on-line fluorescence spectroscopy. During the heating process the reaction involved a multistage weight loss, in both cases, with a corresponding multistage SO₂ release. The multistage weight loss and the corresponding SO₂ release were presumed to be the consequence of different sequential reactions taking place along with the direct reduction of CaSO₃ to CaS, including the formation and decomposition of CaCO₃ and CaSO₄, respectively. The corresponding SO₂ evolution was found to be considerably lower at slow heating rates. In the presence of additional CO₂, the SO₂ release was increased and the highest SO₂ concentration was found for a feed-gas mixture of 70% CO₂ and 30% CO, and also in pure CO₂. The CaCO₃ product also increased with increasing CO₂ concentration. The possible reaction pathway is discussed.     

Kinetics of nitric oxide reduction with pure and potassium-doped carbon

Summary The kinetics of the reduction of nitric oxide with pure and potassium-doped carbon, NO+C=1/2 N₂+CO, were investigated. For the reaction of NO with pure carbon, measurements were made in the temperature range from 1750 K to 2130 K and at initial NO pressures between 5×10^–3 Pa and 7×10^–2 Pa. The reaction was first order with respect to nitric oxide at NO pressures below 3×10^–2 Pa. The activation energy was 54 kJ/mol for temperatures below 2000 K, while at higher temperatures a second (parallel) reaction became noticeable with a definitely higher activation energy. Potassium-doped carbon was prepared by a molecular beam technique. AES studies verified that potassium was intercalated into the graphite surface and that the potassium-to-carbon ratio changed continuously with sample temperature. The reduction of NO with K-doped carbon was investigated in the temperature range from 710 K to 1080 K and at initial NO pressures between 7×10^–5 Pa and 6×10^–4 Pa while monitoring, in-situ using AES the K/C-ratio of the surface. The NO reduction rate rose linearly with K/C. Compared to pure carbon, the reaction rate for the NO reduction with K-doped carbon increased by a factor in the range of 10⁴. The activation energy for the NO reduction with K-doped carbon was found to be 82 kJ/mol.     

Nano/micro‐sized calcium phosphate coating on carbon/carbon composites by ultrasonic assisted electrochemical deposition

Nano/micro‐sized calcium phosphate (CaP) coating was prepared on carbon/carbon (C/C) composites by ultrasonic assisted electrochemical deposition. The coating obtained at different deposition voltages contained a mixture of hydroxyapatite (HA) and brushite (DCPD). The homogenous coating prepared at 2.4 V consisted of nano‐sized and needle‐like HA embedded in micro‐sized and plate‐like DCPD. An interlocking structure was formed along the depth direction of the coating. The internal stress may be released effectively through the interlocking structure of the coating. And the plate‐like crystals of the coating were inset in the grooves on the surface of C/C composites. This led to a better adhesive strength of the coating. Meanwhile, the formed interlocking structure could help enhance cohesive strength of the coating. It was found that the growth of CaP crystals in the coating under the voltage of 2.4 V consisted of the plate‐like crystals deposited initially. Then the plate‐needle‐like crystals of submicron size formed among the plate‐like crystals and developed needle‐like ones. The CaP‐coated C/C composites might improve the biological properties of coating for its unique morphology, structures and strong adhesion to the C/C substrate. Copyright © 2011 John Wiley & Sons, Ltd.     

Kinetics of mixed-controlled oxygen reduction at nafion-impregnated Pt-alloy-dispersed carbon electrode by analysis of cathodic current transients

The effects of Co alloying to Pt catalyst and Nafion pretreatment by NaClO₄ solution on the rate-determining step (RDS) of oxygen reduction at Nafion-impregnated Pt-dispersed carbon (Pt/C) electrode were investigated as a function of the potential step ΔE employing potentiostatic current transient (PCT) technique. For this purpose, the cathodic PCTs were measured on the pure Nafion-impregnated and partially Na⁺-doped Nafion-impregnated Pt/C and PtCo/C electrodes in an oxygen-saturated 1 M H₂SO₄ solution and analyzed. From the shape of the cathodic PCTs and the dependence of the instantaneous current on the value of ΔE, it was confirmed that oxygen reduction at the pure Nafion-impregnated electrodes is controlled by charge transfer at the electrode surface mixed with oxygen diffusion in the solution below the transition potential step |ΔE _tr| in absolute value, whereas oxygen reduction is purely governed by oxygen diffusion above |ΔE _tr|. On the other hand, the RDS of oxygen reduction at the partially Na⁺-doped Nafion-impregnated electrodes below |ΔE _tr| is charge transfer coupled with proton migration, whereas above |ΔE _tr|, it becomes proton migration in the Nafion electrolyte instead of oxygen diffusion. Consequently, it is expected in real fuel cell system that the cell performance is improved by Co alloying since the electrode reaches the maximum diffusion (migration) current even at small value of |ΔE|, whereas the cell performance is aggravated by Nafion pretreatment due to the decrease in the maximum diffusion (migration) current.     

Kinetics of chemical absorption of carbon dioxide into aqueous calcium acetate solution

Increasing energy demand in the world leads to more electricity generation mainly at fossil fuel power plants. Greenhouse gases are thus produced and mostly emitted to the atmosphere directly, resulting in global warming and climate change. Carbon dioxide is believed to be a main pollutant among greenhouse gases responsible from global warming. Conventional systems using mostly amine solutions to capture carbon dioxide at the source have some disadvantages, and alternatives are constantly being searched. In this work, a benign system of aqueous calcium acetate solution was investigated for this purpose. Calcium acetate is easy to produce, relatively cheap, environmentally friendly, nonhazardous, and noncorrosive. These properties make it a great alternative for use in capturing carbon dioxide. This absorption process is accompanied by chemical reaction. Therefore, the reaction kinetics needs to be investigated before its use in absorbers. A stirred cell reactor was used in the experiments using aqueous calcium acetate solution of different concentrations (2-20% w/w) and different carbon dioxide concentrations in gas mixtures (4.5-100% v/v dry carbon dioxide) at temperatures ranging from 286 to 352 K. The Gibbs free energy change for the overall reaction between carbon dioxide and aqueous calcium acetate solution was found to be –2.75 kJ/mol that shows the reaction is exergonic and occurs spontaneously. It was also found out that the reaction is pseudo–first order with respect to carbon dioxide which was also proven by calculating the Hatta number. Activation energy and Arrhenius (frequency) constant were also determined experimentally.     

Kinetics of nitric oxide reduction by carbon monoxide in the presence of cobalt(II) complexes

Kinetic studies of nitric oxide reduction by carbon monoxide in the presence of Co(II) complexes indicate that the reaction is first order with respect to catalyst, carbon monoxide, and nitric, oxide. Co(AC₂)(OH) ₂ ^– complexes have the highest catalytic activity. A reduction mechanism is proposed.

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Co(II). , , . Co(Ac₂)(OH) ₂ ^– . .

     

Kinetics of oxygen reduction by nanocomposite silver-ion exchanger

It was experimentally determined that the rate of reduction of molecular oxygen from water by the nanocomposite silver-ion exchanger KU-23 is substantially lower than by copper-containing composite and takes place with substantial kinetic inhibition. The kinetic parameters of the process were determined by solving the reverse kinetic problem; it was found that this was due to a reduction in the rate constant of the reaction of silver with oxygen at virtually constant internal diffusion coefficient of molecular oxygen. The transition from kinetic to diffusion control was determined to depend on the silver particle size.     

Removal of phosphate from solution by adsorption and precipitation of calcium phosphate onto monohydrocalcite

The sorption behavior and mechanism of phosphate on monohydrocalcite (CaCO₃?H₂O: MHC) were examined using batch sorption experiments as a function of phosphate concentrations, ionic strengths, temperatures, and reaction times. The mode of PO₄ sorption is divisible into three processes depending on the phosphate loading. At low phosphate concentrations, phosphate is removed by coprecipitation of phosphate during the transformation of MHC to calcite. The sorption mode at the low-to-moderate phosphate concentrations is most likely an adsorption process because the sorption isotherm at the conditions can be fitted reasonably with the Langmuir equation. The rapid sorption kinetics at the conditions is also consistent with the adsorption reaction. The adsorption of phosphate on MHC depends strongly on ionic strength, but slightly on temperature. The maximum adsorption capacities of MHC obtained from the regression of the experimental data to the Langmuir equation are higher than those reported for stable calcium carbonate (calcite or aragonite) in any conditions. At high phosphate concentrations, the amount of sorption deviates from the Langmuir isotherm, which can fit the low-to-moderate phosphate concentrations. Speciation–saturation analyses of the reacted solutions at the conditions indicated that the solution compositions which deviate from the Langmuir equation are supersaturated with respect to a certain calcium phosphate. The obtained calcium phosphate is most likely amorphous calcium phosphate (Ca₃(PO₄)₂?xH₂O). The formation of the calcium phosphate depends strongly on ionic strength, temperature, and reaction times. The solubility of MHC is higher than calcite and aragonite because of its metastability. Therefore, the higher solubility of MHC facilitates the formation of the calcium phosphates more than with calcite and aragonite.     

氯化钙催化纤维素热裂解动力学研究

用差示热重分析仪对氯化钙纤维素热裂解动力学催化影响进行了研究。结果表明，氯化钙对焦炭的形成具有强烈的促进效果，使热裂解最终残留物产率从5％提升到10％以上，氯化钙的存在影响到热失重初始阶段活性纤维素的生成，使热重曲线向低温侧移动，并在低温段产生了小的失重速率峰。通过热重分析发现，氯化钙催化条件下纤维素热裂解动力学参数被分为了三段，分别对应于活性纤维素的生成、炭化和活性纤维素转化为挥发分产物三个区间，并依次成为整体失重过程的控制步骤。结合Broido-Shafizadeh机理分析以及与纯纤维素热裂解动力学参数的对比，氯化钙对这三个主要反应步骤都产生了促进效果，其中以催化焦炭的生成最为明显，在促进焦炭化的过程中，降低了气体产物的生成比率。     

Kinetics of γ-alumina chlorination by carbon tetrachloride

The kinetics of the reaction between γ-Al₂O₃ and gaseous CCl₄ has been studied by isothermal TG measurements in the temperature range 700—1123 K. The reaction starts with a weight gain which can be attributed to the chemisorption of the reactive gas. The weight loss vs. time curves at relatively high temperatures can be described by the contracting cylinder equation and at relatively low temperatures by first-order kinetics. The dependence of the initial reaction rate on the CCl₄ partial pressure follows the Langmuir—Hinshelwood rate expression. At 700—723 K, chemical control is thought to be predominant and an apparent activation energy of 212 kJ mole^?1 is found for the chlorination process.     

Kinetics of carbon dioxide mechanosorption by Ca-containing silicates

Kinetics of mechanically induced CO₂ extensive sorption by silicate minerals (labradorite, diopside, okermanite, ghelenite and wollastonite) was considered. Mechanical activation of the silicates was carried out in a planetary mill in CO₂ at atmospheric pressure. Carbon dioxide was consumed by the silicates in the form of carbonate ions and its content in the samples after 30 min of mechanical treatment reached 3–12 mass% CO₂ depending on mineral composition. Equations that reasonably good represent kinetics of CO₂ mechanosorption by silicates were proposed. These equations enable to calculate mechanosorption coefficients that characterize the diffusivity of CO₂ into disordered silicate matrix under intensive mechanical impact. Thermal analysis of the mechanically activated silicates showed that there was positive correlation between temperature of complete carbonate decomposition and mechanosorption coefficient.     

Accelerating net-zero carbon emissions by electrochemical reduction of carbon dioxide

Electroreduction of CO₂ shows great potential for global CO₂ utilization and uptake when collaborated with renewable electricity. Recent advances have been achieved in fundamental understanding and electrocatalyst development for CO₂ electroreduction. We think this research area has progressed to the stage where significant efforts can focus on translating the obtained knowledge to the development of largescale electrolyzers, which have the potential to accelerat...     

Kinetics and mechanism of palladium(II) acetate reduction by hydrogen on the surface of a carbon support

The kinetics of the reduction of Pd(II) compounds by dihydrogen on the surface of a carbon support has been investigated for palladium acetate as an example. A kinetic model has been constructed for this reaction. An autocatalytic mechanism is suggested, in which the key role is played by Pd(0) compounds and their hydrides. The reaction occurring on the support surface is compared with the same reaction in solutions of palladium phosphine acetate complexes, where a similar mechanism is observed. One of the most important features of the surface reaction is the relatively slow reduction of the Pd(I) compounds to Pd(0). This makes it possible to obtain materials with a high Pd(I) content of 5% and above.