Apparent Activation Energy For Oxidation Research Articles

Oxidation and its effect on flexural strength of hot-pressed ZrB2-SiC composites with VC additives

ABSTRACT The oxidation and its effect on room-temperature flexural strength were investigated in the hot-pressed ZrB2-SiC composites with 3−7 wt% VC additives exposed to air within the temperature range of 1100−1300ºC, for up to 100 h. The parabolic weight gain versus oxidation time was measured for the period of 10−100 h. The apparent oxidation activation energy was determined to be 70−180 kJ/mol, dependent on the amount of VC. The oxidation products consisted of m-ZrO2, V2O5, and ZrSiO4 at 1100ºC, m-ZrO2, SiO2 (α- or β-cristobalite), ZrV2O7, and ZrSiO4 at 1200ºC and 1300ºC. Moreover, the room-temperature flexural strength of the postoxidized composites varied depending on the amount of VC as well as on oxidation temperature. The change in the strength of the postoxidized composites was associated with the formation of either outermost dense glassy scale or new defects in the oxide layer during the oxidation exposure.

Isothermal relaxation kinetics for the reduction and oxidation of SrFeO3 based perovskites.

The perovskite oxide SrFeO3 has favourable redox properties for oxygen exchange applications, including oxygen separation and oxygen production chemical looping cycles. For such applications, lower temperature operation can improve the energy demand and feasibility of the process, but can also lead to kinetic limitations. Here we investigate the oxidation and reduction reaction kinetics of SrFeO3 in the temperature range 450-750 K. Isothermal relaxation techniques are used to observe the reaction rates across this temperature range, using a thermogravimetric analysis system. Experimental data are analysed according to an isoconversional method and fit with a simple power law model to extract activation energies. The apparent activation energy of oxidation and reduction was found to be 92 ± 16 and 144 ± 17 kJ mol-1 respectively. Comparison of oxidation and reduction kinetics together with considerations of particle size indicate that the oxidation reaction rate may be limited by diffusion in the bulk, while the reduction reaction rate is limited by the surface reaction. Furthermore, we also investigated the mixed perovskite Sr0.93Ca0.07Fe0.9Co0.1O3, which exhibited a 4-fold increase in the oxidation rate.

Influence of praseodymium on a solid - state oxidation kinetics of Al +6%Li alloy

This article describes the results of thermogravimetric analysis of the oxidation kinetics of Al +6% Li alloy modified by praseodymium in amount up to 0.5%. It is demonstrated that the addition of praseodymium to the alloy or an increase in temperature provides an increase in the oxidation rate of the initial alloy in the solid state. The apparent oxidation activation energy of the initial (basic) alloy (35.2 kJ/mol) decreases to17.3 kJ/mol for the alloy with 0.5% Pr. The oxidation curves of Al +6% Li + Pr alloys in the solid state are described by equations evidencing a hyperbolic mechanism of this process.

Influence of praseodymium on a solid – State oxidation kinetics of Al + 6%Li alloy

This article describes the results of thermogravimetric analysis of the oxidation kinetics of Al + 6% Li alloy modified by praseodymium in amount up to 0.5%. It is demonstrated that the addition of praseodymium to the alloy or an increase in temperature provides an increase in the oxidation rate of the initial alloy in the solid state. The apparent oxidation activation energy of the initial (basic) alloy (35.2 kJ/mol) decreases to 17.3 kJ/mol for the alloy with 0.5% Pr. The oxidation curves of Al + 6% Li + Pr alloys in the solid state are described by equations evidencing a hyperbolic mechanism of this process.

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.

Preparation and oxidation resistance of SiC-coated graphite powders via microwave-assisted molten salt synthesis

Silicon carbide (SiC)-coated graphite composite powders was synthesized at 1423 K by a microwave-assisted molten salt synthesis, using graphite flakes and silicon powders as raw materials. The phase composition, morphology and microstructure of the coated graphite were characterized by X-ray diffraction, field emission scanning electron microscopy and high resolution transmission electron microscopy. The results showed that the formation temperature of SiC coating which consisted of lots of nanofibers with hundreds nanometers in length and several tens in diameter was reduced for about 150 K compared with conventional molten salts method. The water wettability and oxidation resistance of as-prepared coated graphite were greatly enhanced owing to the existence of SiC coating. The apparent oxidation activation energy of the coated graphite was 168.3 kJ/mol, which was higher than that 157.4 kJ/mol for raw graphite.

Solid-State Oxidation Kinetics of a Lanthanum-Modified Al + 6% Li Alloy

This article describes the results of thermogravimetric analysis of the oxidation kinetics of Al + 6% Li alloy modified by lanthanum in amount up to 0.5%. It is demonstrated that the addition of lanthanum to the alloy or an increase in temperature provides an increase in the oxidation rate of the initial alloy in the solid state. The apparent oxidation activation energy of the initial (basic) alloy (35.2 kJ/mol) decreases to 17.3 kJ/mol for the alloy with 0.5% La. The oxidation curves of Al + 6% Li + La alloys in the solid state are described by equations evidencing a hyperbolic mechanism of this process.

Effect of Tungsten on Precious Metal Extraction During Processing of Radio-Electronic Scrap

An increase in demand for metal creates a requirement for finding new ways of supply. Toughening of ecological requirements for ore recovery necessitates ways for intensifying treatment processes with the aim of more complete extraction of economic components. In the forefront is secondary metallurgy technology making it possible to extract nonferrous and precious metals from waste. Ecological standards become an important condition for processing radio-electronic engineering scrap. Enrichment product compositions obtained from radio-electronic industry scrap are provided and results are described for melting radio-electronic scrap and dissolution of anodes prepared from this waste. The compositions of concentrates entering processing are provided. These compositions are analyzed in a Lab Center XRF-1800 x-ray fluorescence spectrometer. Anode potential measurements are given. The degree of tungsten oxidation in relation to temperature is analyzed. Kinetics of tungsten oxidation are studied and tungsten apparent oxidation activation energy is determined. It is concluded that tungsten may be a collector for gold. Tungsten oxidation is a required condition for reducing gold loss.

Effect of purity and surface modification on stability and oxidation kinetics of boron powders

A commercial 95-% pure boron was ultrasonicated in acetonitrile and then in toluene to remove the surface layer of boron oxide and boric acid. The powder suspended in acetonitrile forms stratified layers, with the bottom layer containing most of the material with the least amount of boron oxide and boric acid. The surface of the processed boron powder was modified with a protective layer, making it more stable in room temperature humid oxidizing environment than the reference 95-% pure commercial boron. The stability of different powders was characterized using isothermal calorimetry employing a thermal activity monitor TAM III with a perfusion ampoule. Oxidation of the surface modified boron powders was studied using thermal gravimetry and compared to oxidation of both 99 and 95-% pure commercial powders. At room temperature, the 99-% pure boron powder was the most stable. Upon heating, the 99-% boron begins oxidizing at a lower temperature than all the 95-% pure powders. However, the apparent activation energy of oxidation at elevated temperatures obtained using a model-free isoconversional methodology for the 95-% pure powder processed in both acetonitrile and toluene is lowest among all tested powders and is close to 100kJ/mol. It is expected that the latter powder is the most attractive as a fuel, which will be characterized by the shortest ignition delays.

Solid-state oxidation kinetics of an yttrium-containing AK1M2 alloy

The interaction of an yttrium-modified AK1M2 alloy with atmospheric oxygen in the temperature range 723–823 K is studied by thermogravimetry. The kinetic parameters of oxidation are determined. An yttrium addition is shown to decrease the oxidation rate, which is accompanied by an increase in the apparent oxidation activation energy from 114.9 to 134.6 kJ/mol.

Oxidation kinetics of the germanium-alloyed AK7M2 + 0.05% Sr alloy

The oxidation kinetics of the germanium-alloyed AK7M2 + 0.05% Sr alloy in air at temperatures of 773 K, 798 K, and 823 K is investigated by the thermogravimetric method. It is revealed that the addition of germanium up to 0.6 wt % decreases the oxidation rate, as evidenced by an increase in the apparent activation energy of oxidation from 14.7 to 79.8 kJ/mol, which drops again to 25.2 kJ/mol as the Ge content increases to 1.0 wt %. The true oxidation rate varies in limits of (4.30–6.00) × 10−4 kg/(m2 s), depending on the amount of alloying component. Silicon dioxide SiO2 and germanium dioxide GeO2 are also revealed in oxidation products of alloys along with γ-Al2O3.

High Temperature Behavior of Si3N4 and Yb2SiO5 Coated Carbon Fibers for Silicon‐Nitride CMC

For oxide‐free ceramic matrix composites (CMC), with Si3N4 matrix and carbon fiber reinforcement, for extreme high temperature applications, protective coatings of the C‐fibers are investigated. Two different coatings are compared: reactive CVD‐derived pure Si3N4 coatings to investigate C‐fiber‐matrix reactions and powder based Yb‐silicate coatings to reveal potential reactions with the Yb‐silicate additive serving as sintering aid for Si3N4. The reactivity toward carbon in nitrogen atmosphere is studied in the temperature interval from 20 °C up to 1700 °C. A new ceramic phase – an Yb‐carbido‐nitiridosilicate, Yb2Si4CN6–is found as product of carbothermal reduction of the Yb‐silicate. The carbothermal reduction occurs also with other RE‐silicates, RE = Yb, Er, Y, Gd, and Sm while SiC is found as reaction product on carbon fibers coated with pure Si3N4. The oxidation resistance of the coated fibers in air was investigated in the temperature interval up to 1000 °C, and the apparent activation energy of oxidation was analyzed based on DTA‐EGA results. The oxidation kinetic reveals a significant increase of onset point of oxidation temperature by up to 150 K for Si3N4 coated short carbon fibers obtained from the reactive CVD coating process. Such fibers have a high application potential for carbon‐fiber reinforced Si3N4‐CMC. The role of Yb2Si4CN6 as reinforcement for Si3N4‐CMC is discussed based on bond strength comparison of carbides (SiC), nitride silicates (SiAlON), and nitrides (Si3N4).

Comparison between oxidation kinetics of HVOF sprayed WC–12Co and WC–10Co–4Cr coatings

In this study, the high temperature oxidation behavior of HVOF-sprayed WC–12Co and WC–10Co–4Cr coatings were investigated. To explore the oxidation mechanism, thermo-gravimetric analysis (TGA) was applied for isothermal treatments in the range of 500–800°C for 3h. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to evaluate the structural changes and microstructural evolutions during oxidation tests. The TGA experiments showed negligible oxidation mass gains at 500°C for both coatings. At higher temperatures, i.e. 700 and 800°C, the oxidation mass gains of WC–12Co were found to be much higher than those for WC–10Co–4Cr coating, respectively. The higher oxidation resistance of WC–10Co–4Cr coating probably results from the formation of compact chromium oxide layers and higher MWO4 type tungstate (M: Co and/or Cr) to tungsten trioxide (WO3) ratios which provide lower porosity and consequently more efficient passivation effect against oxidation. The time dependent mass gain of WC–12Co coating obeys the linear law within temperature range of 600–800°C with apparent oxidation activation energy of ~104kJ/mol. As for the oxidation of WC–10Co–4Cr coating, a negligible deviation from linear law was observed possibly due to the presence of chromium oxide and higher tungstate to tungsten trioxide ratio which hinders the diffusion process through the scales compared with WC–12Co coating. The apparent activation energy for oxidation of the WC–10Co–4Cr coating was found to be ~121kJ/mol.

Low-temperature catalytic oxidation of multi-walled carbon nanotubes

When in a pure form, carbon nanotubes are known to be stable in air up to ∼800K making them attractive for a large variety of applications. In this work, we report a significant decrease of ignition temperature (in some cases occurring at ∼500K) and a reduction in the apparent activation energy for oxidation in air as a result of impregnation with nanoparticles (<2nm) of metal (Pt, Pd, Ni and Co) acetylacetonates or by decoration with corresponding oxides. Surprisingly, defects introduced by partial oxidation of the carbon nanotubes do not in practice have any influence on the enhancement of further oxidation. Reduction temperatures of metal oxides with H2 were close to those of other carbon supported catalyst materials. However, the carbon nanotubes showed a tendency for low temperature gasification in the presence of hydrogenation catalyst metals (Pt, Pd).

Aliphatic C–H and oxygenated surface functional groups of diesel in-cylinder soot: Characterizations and impact on soot oxidation behavior

This paper reports the characteristics of surface functional groups (SFGs) and oxidation reactivity of in-cylinder soot during the diesel combustion process and discusses the correlation between SFGs and the oxidation reactivity of soot. Test soot samples were obtained from a total cylinder sampling system. SFGs and the apparent activation energy for the soot oxidation were determined by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and thermogravimetric analysis. Results reveal that the concentrations of aliphatic C–H, C–OH and CO groups are dependent on the combustion stages. The ratio of aromatic CC to aliphatic C–H groups rapidly increases in the premixed combustion phase, indicating significant loss of aliphatic C–H groups from the soot surface. As the combustion proceeds, the ratio of the two groups gradually increases, following an earlier drop during the diffusion combustion phase. In contrast, the concentration of both C–OH and CO groups shows a bimodal distribution during the combustion process, with the concentrations peaking in the premixed and diffusion combustion stages, respectively. The apparent activation energies for the oxidation of soot collected under the applied engine operating conditions are observed in the range of 122.12–158.2kJmol−1 at various crank angles, revealing a similar variation to that of aliphatic C–H groups. Aliphatic C–H groups are a more important factor in governing soot oxidation reactivity than the oxygenated SFGs.

The kinetics of oxidation of Diesel soots by NO 2

The kinetics of oxidation of three soots, from a Diesel engine fuelled by either Ultra-Low Sulphur Diesel (ULSD) or biodiesel, by NO 2 have been measured in a packed bed at various temperatures (300–550 °C) and [NO 2] (20–880 ppm) relevant to regenerating a Diesel Particulate Filter. Adsorbed hydrocarbons and oxygen accounted for a significant fraction (∼20% by mass) of the otherwise carbonaceous material. After pre-treatment (heating up to 550 °C in a flow of pure Ar and holding the temperature at 550 °C for 1 h) to ensure consistency between samples, they were subsequently burned at a fixed temperature in a flow of NO 2 + Ar. For this, a balance on oxygen atoms entering and leaving the packed bed showed that during oxidation in NO 2 any oxygen remaining in a soot after pre-treatment was not rapidly liberated as CO or CO 2. A mass balance on the element nitrogen demonstrated that no N 2 or N 2O was formed below 550 °C; mass balances on carbon and oxygen demonstrated that all the carbon ended up as CO or CO 2 and below 550 °C the nitrogen yielded only NO. The oxidation of soot in NO 2 was found to be first-order with respect to NO 2. Also, the soot derived from biodiesel was more reactive than soot from ULSD; nevertheless, the apparent activation energies for oxidation by NO 2 were the same (70 ± 18 kJ mol −1) for each carbon. When the distribution of diameters of the individual spherules of soot was taken into account, it was not possible to tell whether there was internal burning of porous spherules or, on the other hand, non-porous, solid spherules were burning on their exteriors.

Electro-oxidation of ethanol and ethylene glycol on carbon-supported nano-Pt and -PtRu catalyst in acid solution

Present paper reports kinetics of electro-oxidation of ethanol (EtOH) and ethylene glycol (EG) onto Pt and PtRu nanocatalysts of different compositions in the temperature range of 298–318 K. These catalysts have been characterized by SEM, EDX, XRD, CV and amperometry. It has been observed that apparent activation energies for oxidation of EtOH and EG pass through a minimum at about 15–20 at.% of Ru in the PtRu alloy catalysts. Anodic peak current vs. composition curve also shows a maximum around this composition. The results have been explained by a geometric model, which proposes requirement of an ensemble of three Pt atoms with an adjacent Ru atom onto PtRu surface for an efficient electro-oxidation of EtOH or EG. This is further supported from statistical data analysis of probability of occurrence of such ensembles onto PtRu alloy surface. Present results also suggest that electro-oxidation of EG onto nano-PtRu catalyst surfaces follows a different path from that of EtOH at alloy composition less than 15 at.% of Ru.

Oxidation behavior of polymer derived SiCO powders

Polymer derived SiCO ceramic powders were fabricated by pyrolysis of polycarbosilane precursor. The oxidation behaviors of these powders were studied by thermogravimetric analysis at the temperature range of 900–1400 °C. The oxidation parabolic constants at different temperatures were calculated. From these parabolic constants, the apparent activation energies for oxidation could also be obtained. These results indicated that SiCO powders have a close oxidation rate to that of pure silicon carbide. The crystallinity of SiCO ceramics and silica scales has little effect on the oxidation rates. The results also elucidated that the oxidation mechanism for amorphous and crystalline silica is the same.

Kinetics and Mechanism of High-Temperature Oxidation in Air of Au - Cu Alloy

We have used thermogravimetry to study the kinetics of high-temperature (up to 800 °C) oxidation of the alloy 58.3 mass% Au - 41.7 mass% Cu with isothermal heating of the specimens. Using petrographic analysis of the oxide layers, we determined the reaction products. We have shown that up to 200 °C, the indicated alloy is not oxidized at all. More rapid oxidation of the alloy is observed at temperatures above 400 °C. Up to 500 °C, an inner layer consisting of Cu2O predominates in the two-layer scale on the alloy, while the outer CuO layer has a significantly smaller thickness. At 600 °C, the upper layer of scale contains Cu2O while the lower layer contains Cu2O and gold. At higher temperatures, all the way up to 800 °C, the scale is two-layer as before but its upper layer contains CuO while its lower layer contains Cu2O and small gold rods distributed in that oxide. Thus we have established three oxidation regions characterized by different scale phase compositions and different mechanisms for the process, mainly due to transition from an ordered state of the alloy (intermetallic AuCu3) to a completely disordered solid solution of gold in copper. We used the Arrhenius equation to calculate the apparent activation energy for oxidation: E1 = 20.4 kJ/mole for the temperature range 400–500 °C and E2 = 9.5 kJ/mole for 600–800 °C.

Mechanism of acetylene oxidation on the Pt(111) surface using in situ fluorescence yield near-edge spectroscopy

In situ studies of acetylene oxidation on Pt(111) have been performed with both fluorescence yield near-edge spectroscopy (FYNES) and temperature-programmed FYNES (TP-FYNES) for temperatures up to 600 K and flowing oxygen pressures up to 0.009 Torr. Low-temperature spectroscopic (FYNES) results indicate that at 150 K acetylene adsorbs with the C C backbone tilted slightly up from the Pt(111) surface, consistent with the formation of an η 2 - μ 3 -CCH 2 surface intermediate. When acetylene is preadsorbed on the Pt(111) surface, forming the η 2 - μ 3 -CCH 2 intermediate, oxidation occurs in a single step over the 330–420 K temperature range. In the presence of excess oxygen, little effect on the rate of oxidation is seen when the oxygen pressure is increased, as observed previously for oxidation of adsorbed propyne on the Pt(111) surface. Comparison between the intensity of the C H σ ∗ resonance and the intensity in the carbon continuum clearly shows that the adsorbed hydrocarbon intermediate maintains a 1:1 C H stoichiometry throughout oxidation, suggesting that oxydehydrogenation and skeletal oxidation occur simultaneously above 330 K. Identical reaction temperature profiles observed for (1) reaction of coadsorbed acetylene and atomic oxygen, and (2) reaction of acetylene and oxygen from the gas phase signify that in both cases a direct, bimolecular surface reaction mechanism occurs during oxidation. For the catalytic studies, a large temperature hysteresis, which is associated with inhibition of oxygen adsorption by acetylene, is observed during thermal cycling. Detailed isothermal kinetic studies performed in flowing oxygen pressures indicate that the apparent activation energy for oxidation is 20.3 ± 2.0 kcal / mol with a pre-exponential factor of 10 10.3 ± 1.0 s −1 . Taken together, these results clearly indicate that surface properties and reactant properties both play major roles in controlling oxidation reactions over an extended range of reaction conditions.