Activation Energy For Oxidation Research Articles

Insights into carbon monoxide oxidation in supercritical H2O/CO2 mixtures using reactive molecular dynamics simulations

Carbon monoxide is an important intermediate in supercritical water oxidation processes. Carbon monoxide oxidation in supercritical H2O/CO2 mixtures was investigated using reactive molecular dynamics simulations. The results show that CO mainly converts into CO2 through the reaction CO + OH → HOCO → CO2 + H. The intermediate HOCO can also react with OH or O2 to form CO2. The overall activation energies of CO oxidation in the supercritical medium range from 125.3 ± 4.0–159.4 ± 3.6 kJ/mol, which are roughly consistent with the experimental results. Supercritical water not only provides reactive free radicals for CO conversion but also acts as a third body to induce some chemical events. The high concentration of CO2 inhibits the production of OH but highlights the oxidizing action of O and HO2. Oxygen increases the amounts of OH and HO2 through the reaction H2O + O2, thereby promoting the conversion of CO.

A promising new class of high-entropy ceramics: High-entropy oxycarbides with good oxidation resistance

(Hf0.25Zr0.25Nb0.25Ti0.25)(C0.5O0.5) high-entropy oxycarbides (HECO-1) with good oxidation resistance, as a promising new class of high-entropy ceramics, were synthesized by a facile carbothermic reduction method for the first time. The results showed that the as-synthesized HECO-1 samples had the single-phase rock-salt structure and high compositional uniformity. Compared to the as-synthesized (Hf0.25Zr0.25Nb0.25Ti0.25)C high-entropy carbides, the as-synthesized HECO-1 samples possessed the higher initial oxidation temperature and slower oxidation rate to show the better oxidation resistance, making them the promising applications in extreme environments, which was attributed to their higher oxidation activation energy and Gibbs free energy of oxidation reaction.

Mechanistic study of low-temperature CO oxidation over CuO/Cu2O interfaces with oxygen vacancy modification

Copper oxides have attracted great attention in the oxidation of CO due to their high activity at low temperatures. In this work, novel CuO/Cu2O heterojunction catalysts for low-temperature CO oxidation was prepared via hydrothermal in-situ oxidizing in H2O2. Based on studies of surface composition and the corresponding catalytic activities, the CuO/Cu2O catalysts contain rich oxygen vacancies (OVs), which improve the activation and migration ability of O2. The coupling effect between OVs and heterojunction reduces the activation energy of CO oxidation, leading the temperature of CO completed oxidation temperature from 250 °C to 140 °C. In addition, the in-situ Fourier transform infrared spectroscopy (in-situ FTIR) and Density functional theory (DFT) studies confirm two reaction paths under Langmuir-Hinshelwood (LH) mechanism: a low barrier path takes place at low temperature, and another new reaction path with a barrier of 1.8 eV can take place at a high temperature. The new reaction site formed at the CuO/Cu2O interface resulted in different activation modes of CO and O2. The modification of OVs significantly reduces the reaction temperature required. This work provides a method to improve the performance of co-catalytic oxidation via the coupling effect of OVs and heterojunctions.

A kinetic study on mercury oxidation by HCl over typical Mn-based SCR catalysts

A kinetic approach was adopted to analyze the reaction mechanism of elemental mercury (Hg0) oxidation by HCl over three Mn-based low-temperature SCR catalysts, MnOx/TiO2, Fe-MnOx/TiO2 and CeMnO3. Experimental results validated that Hg0 oxidation efficiencies of the catalysts were promoted by HCl at different temperatures. The kinetic models for the heterogeneous Hg0 oxidation were established and verified based on the experimental data. The verification results demonstrated that Hg0 oxidation over the Mn-based catalysts follows Langmuir-Hinshelwood mechanism. The kinetic parameters of reaction rate constant and HCl adsorptionconstant were calculated from the model fitting equations. The reaction rate constant was raised with the increase of temperature, while the HCl adsorptionconstant presented the opposite trend. The Mn-based catalysts with the reaction rate constant of 84.17–335.77 s−1 showed the advantage over some other catalysts such as commercial V2O5-(WO3)/TiO2 and CeO2/TiO2. The kinetic parameters were further improved in the presence of O2. The apparent activation energy for Hg0 oxidation over the catalysts that was derived from the kinetic parameters was 4.13–12.53 kJ/mol, which was much lower than that of the other approaches. The advantageous kinetic parameters and apparent activation energy were favorable for the Mn-based catalysts to act as low-temperature SCR catalyst for synergistic Hg0 removal. The kinetic study results were of fundamental significance for designing the reactor according to the known flue gas conditions, used catalyst and required Hg0 oxidation efficiency.

Sn-based atokite alloy nanocatalyst for high-power dimethyl ether fueled low-temperature polymer electrolyte fuel cell

Next-generation fuels are defined as those produced from non-food resources. A leading member in this group is dimethyl ether− DME (C2H6O), which is a high-energy, non-toxic gas, produced from a wide range of carbon feedstocks and wastes. We explored the oxidation of DME on a highly active catalyst based on Pt3Pd3Sn2 with an atokite structure in comparison to Pt3Sn and Pd3Sn. Following a comprehensive characterization of the new ternary catalyst by electron microscopy, X-ray diffraction, and photoelectron spectroscopy, the DME anodic reaction was analyzed by electrochemical online mass spectrometry of fuel cell gas emission product and supported by density functional theory (DFT) calculations. Pt3Pd3Sn2 catalyst exhibits optimal binding energy (−0.21 eV) and the lowest activation energy for electrochemical oxidation of DME (48.7 kJ mol−1 at 0.80 V). A few preferred oxidation routes were examined at different potentials corroborating with the identified CO2, formic acid, methanol, and methyl-formate by in-operando online mass spectrometry. Fuel-cell constructed using a Pt3Pd3Sn2/C anode catalyst and commercial Pt/C cathode catalyst, delivered an open circuit voltage of 0.9 V, a peak power density of 220 mW cm−2 at 0.40 V and a gravimetric power density of 135 mW mgpgm−1 at ambient pressure and 80 °C, which exceeded the highest values reported so far for direct DME fuel cells.

Fabrication and oxidation behavior of β-SiAlON powders in presence of trace Y2O3

β-SiAlON powders were fabricated using Al, Si, α-Al2O3 and trace Y2O3 powders as starting materials in flowing nitrogen atmosphere. The effect of Y2O3 on oxidation behavior of the prepared β-SiAlON powders has been investigated. The results show that the Y2O3 contributed to the generation of β-SiAlON during the fabricated process. The weight gain ratio of β-SiAlON in presence of Y2O3 is higher than that of without Y2O3 during non-isothermal oxidation up to 1723 K, which may be due to the Y2O3 dissolving into the lattice of β-SiAlON resulting in lattice deformation. At the initial oxidation period, the incorporation of Y2O3 increases the oxidation rate and decreases the oxidation activation energy (Ea) of β-SiAlON due to the lattice deformation increasing the reactivity of β-SiAlON. However, at the final oxidation period, denser glass film formed on the surface of β-SiAlON because of the incorporation of Y2O3, which reduces the opportunity for O2 contact with β-SiAlON, thus decreasing the oxidation rate and increasing the Ea of β-SiAlON.

ENERGY ACTIVATION OF DECOMPOSITION OF CASTINGS OF PURE ALUMINUM-STANNUM ALLOY

Thermogravimetry analyser is a tool to perform thermal analysis where the mass of the test material will be inversely or directly proportional to the increasing temperature rate and a function of time (constantly increasing temperature). The results of the TGA test are a comparison of mass to times, mass to DTA, and the ln k to 1/t; all relations to determine the oxidation point and activation energy required for each sample can be known. The samples used were 4: pure aluminium, pure aluminium mixed with 2% Sn, pure aluminium mixed with 6% Sn, and pure aluminium mixed with 10% Sn. The activation energy required for each sample is as follows, pure Aluminium of 64.24 kJ/mol, pure Aluminium mixed with 2% Sn of 58.70 kJ/mol, pure aluminium combined with 6% Sn of 16.63 kJ/mol and Aluminium pure mixed with 10% Sn at 47.68 kJ/mol.

A Quantitative Relationship between Oxidation Index and Chalcopyrite Flotation Recovery

The surface oxidation of chalcopyrite is one of the most important factors affecting its flotation performance. In this study, a critical oxidation degree is proposed to define “slight” and “significant” oxidation in terms of surface species and chalcopyrite flotation recovery. Slight oxidation enhanced chalcopyrite hydrophobicity, but significant oxidation reduced its recovery apparently. Microthermokinetic measurements indicated that the apparent activation energy (Ea) of chalcopyrite oxidation was reduced from around 173 kJ·mol−1 to 163 kJ·mol−1 when the reaction changed from slight oxidation to significant oxidation when applying H2O2. The surface oxidation degree was defined as the ratio of hydrophilic species to hydrophobic species. The highest recovery (94.8%) and contact angle (93°) were achieved at a concentration of 0.1 vol.% H2O2, with the lowest oxidation degree of 0.388 being observed. The oxidation degree was correlated to the flotation recovery, with a quantitative relationship (y = −298.81x + 213.05, y and x represent flotation recovery and oxidation degree, respectively, 0.388 ≤ x ≤ 0.618) being established, thereby giving a guideline to better manage chalcopyrite flotation by controlling its surface oxidation and SBX adsorption on chalcopyrite surfaces.

Impact of sulfur functional groups on physicochemical properties and oxidation reactivity of diesel soot particles

Soot particles with sulfur functional groups (SFGs) are one of the main concerns from shipping emissions, especially when they are produced from fuel combustion with high fuel sulfur content (FSC). The present study investigates the physicochemical characteristics of soot particles generated from different FSC fuels on a marine auxiliary diesel engine. A comprehensive set of experimental techniques including Fourier transform infrared (FT-IR), inductively coupled plasma optical emission spectroscopy (ICP-OES), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA) are employed to characterize SFGs, chemical elements, morphology, nanostructure, volatile organic fraction (VOF) content and oxidation behavior, respectively. Results reveal that, FSC has negligible influences on combustion performance, yet its high content increases the concentrations of SFGs (–SH, C–S–S, C–S, SO42−) and sulfur element VOF content and primary particle size (dp) of soot particles. Linear correlations between soot fringe length (La), oxidation activation energy (Ea) and relative concentrations of –SH and C–S are observed, respectively. Higher relative concentration of C–S bond corresponds to shorter La. The relative amounts of –SH and C–S could improve soot oxidation reactivity, associated with lower Ea. In addition, metal-sulfates are formed from the combination of SO42− with metal elements contained in fuel or lube oils, among which CaSO4 is shown an enhancing factor for soot oxidation.

Enhancing cleaner biomass-coal co-combustion by pretreatment of wheat straw via washing versus hydrothermal carbonization

Slagging, fouling and fuel segregation issues with co-combustion of wheat straw and coal could be mitigated by clean pretreatments on wheat straw such as washing or hydrothermal carbonization (HTC). Water washing (WW) could remove upwards of 90% of potassium and phosphorus, thus reducing the risk of slagging and fouling. The dehydration and demethylation reactions during HTC increased the heating value of wheat straw to 5.4%–35.7%, simultaneously more organically-bonded calcium and magnesium were removed. Conversely, the potassium retention of hydrochars was improved owing to the relatively higher specific surface area (3.153–4.693 m2g-1) than WS (1.181 m2g-1) and WW (1.375 m2g-1). Moreover, the retention of alkali metals and porous structure showed a dual effect on the co-oxidation property, varying with HTC temperatures. Hydrochars produced at 200 °C had similar ignition temperatures to a representative bituminous coal. Hydrochars produced at 220 °C showed enhanced reactivity, whereas 240 °C hydrochars had decreased reactivity and potentially problematic ash content. The activation energy of the HTC200 blends (40.71 kJ/mol for 20% ratio and 39.24 kJ/mol for 40% ratio) are lower than the WW blends. Overall, blends of 20 wt% hydrochars produced at 200 and 220 °C showed the optimum performance in terms of minimizing fuel segregation and inhibition, lowering the oxidation activation energy barrier, and reducing slagging and fouling.

Oxidation kinetics of SPS-densified U3Si2 fuels—Microstructure impact

U3Si2 is a potential candidate for accident tolerant fuels because of its high uranium density and excellent thermal conductivity in comparison to UO2. However, U3Si2 suffers from oxidation, steam corrosion, and subsequent disintegration/pulverization. The detailed investigation of kinetics that incorporates fundamental treatment of oxidation of U3Si2 is scarcely reported, and the oxidation mechanisms have not been fully elucidated. In this paper, the oxidation behavior of microcrystalline (mc-) and nanocrystalline (nc-) U3Si2 have been systematically investigated using a thermogravimetric analysis (TGA) apparatus through a series of isothermal and non-isothermal kinetic studies. The isothermal kinetic study with a model-fitting approach indicates oxidation activation energy of 85 kJ/mol for dense mc-U3Si2 and 96.4 kJ/mol for nc-U3Si2 pellets, while the isoconversional approach leads to an activation energy in the range of 70–85 kJ/mol for mc-U3Si2 and 75–86 kJ/mol for nc-U3Si2 with three most common model-free methods, including Kissinger–Akahira–Sunose, Flynn–Wall–Ozawa, and Friedman methods. The derivation of oxidation activation energies using both isothermal and isoconversional methods highlights the approach to evaluate the oxidation resistance of nuclear materials using TGA quantitatively and makes it possible to compare among various nuclear fuels.

Synthesis of ZrB2–ZrC hybrid powders via boro-carbothermal reduction of ZrO2 by B4C and carbon black

ZrB2–ZrC hybrid powders were synthesized by a novel two-step reduction on basis of ZrO2 + B4C + C→ ZrC + ZrB2 + CO reaction in Ar atmosphere, using ZrO2, B4C, and carbon black powders as starting materials. Thermodynamics of relevant reactions were evaluated. Effects of excess additions of B4C and C on phase constituents were investigated. Morphology and chemistry of the powder products were characterized by scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS) and transmission electron microscopy (TEM). The results showed that ZrB2–ZrC hybrid powders with no obvious impurity content could be obtained after heating at 1350 °C for 1 h followed by further reaction at 1700 °C for 1 h with 16 wt% B4C + 8 wt% C excess addition. Relative contents of the ZrB2: ZrC phase in the product powders could be conveniently regulated by varying the B4C and C content in the starting compositions. The resultant powders had good oxidation resistance with an oxidation activation energy value of 433 kJ/mol. Good sinterability of the powder products was demonstrated by hot pressing at 1950 °C for 60min under 30 MPa pressure, which resulted in fully dense ZrB2–ZrC composite ceramics with Vickers hardness value larger than 18.3 ± 0.6 GPa.

On the catalytic activity of InN/InGaN quantum dots

The electrocatalytic activity of epitaxial InN/InGaN quantum dots (QDs) is understood by the QDs electric dipole, generated by the positively charged surface donors and the zero-dimensional quantum states, reducing the activation energy for oxidation

Nitrogen-rich energetic polymer powered aluminum particles with enhanced reactivity and energy content

Aluminum particles are of significant interest in enhancing the energy release performance of explosives. One of the major impediments to their use is that Al2O3 shell significantly decreases overall performance. To address this issue, we investigate creating aluminum particles with a glycidyl azide polymer (GAP) coating to improve their reactivity while retaining their energy content. We found that the aluminum particles were coated with a GAP layer of thickness around 8.5 nm. The coated aluminum particles were compared to non-coated powder by the corresponding reactivity parameters obtained from simultaneous differential scanning calorimetry, thermal gravimetric analysis, coupled with mass spectral and infrared spectral analyses. Besides, the comparison on the energy content was also conducted based on P–t tests and a laser-induced air shock from energetic materials (LASEM) technique. It was found that GAP shifted the oxidation onset of aluminum particles to a lower temperature by ~ 10 °C. Besides, the oxidation activation energy of aluminum particles was also reduced by ~ 15 kJ mol−1. In return, aluminum particles reduced the activation energy of the second stage decomposition of the GAP by 276 kJ mol−1. And due to the synergistic effect between aluminum and GAP, the decomposition products of GAP were prone to be oxycarbide species rather than carbonitride species. In addition, the P–t test showed the peak pressure and pressurization rate of GAP coated aluminum particles were separately 1.4 times and 1.9 times as large as those of non-coated aluminum particles. Furthermore, the LASEM experiment suggested the shock wave velocity of the GAP coated aluminum particles was larger than that of non-coated aluminum particles, and the largest velocity difference for them could be 0.6 km s−1. This study suggests after coating by GAP, the aluminum particles possess enhanced reaction performance, which shows potential application value in the fields of aluminized explosives and other energetic fields.

Selective oxidation of methane into formaldehyde and carbon monoxide catalyzed by supported thermally stable iron oxide subnanoclusters prepared from a diiron-introduced polyoxometalate precursor

Direct oxidative conversion of methane (CH4) into useful C1 products remains challenging due to the low reactivity of CH4 and its facile overoxidation into carbon dioxide (CO2) under high temperature conditions. Iron oxide (FeOx) cluster catalysts are promising because of their low activation energy for CH4 oxidation. However, preliminary results revealed that FeOx clusters were easily aggregated and deactivated under CH4 oxidation conditions at 873 K. In this study, we used a diiron-introduced polyoxometalate as a precursor to form thermally stable FeOx subnanoclusters on SiO2, which selectively converted CH4 into formaldehyde (HCHO) and carbon monoxide (CO) (CH4 conversion, 2.3%; HCHO and CO selectivity, 87% at 873 K after 1 h). The FeOx subnanocluster catalyst maintained catalytic activity even after 72 h. Various characterizations, such as STEM, X-ray absorption spectroscopy, and X-ray diffraction, revealed that the in situ formed FeOx subnanoclusters were stabilized by WOx nanoclusters originating from the polyoxometalate frameworks.

Effect of particle morphology on reactivity, ignition and combustion of boron powders

Spherical boron powders with 2–20 µm sized particles were prepared by ball milling commercial boron in the presence of emulsion with hexane and acetonitrile serving as the continuous and droplet phases, respectively. A small amount of Fluorel®, a fluorocarbon serving as binder was dissolved in the acetonitrile. For comparison, an irregularly-shaped boron powder was prepared by milling commercial boron with the same acetonitrile/Fluorel® solution. Reactivity of the spherical boron powders was compared to those of milled and as received commercial powders with irregularly-shaped particles. Thermo-gravimetric measurements in Ar/O2 gas flow showed that milling boron with or without Fluorel® shifts the onset of boron oxidation to lower temperatures. The rate of initial oxidation was greater for the finer spherical powders compared to coarser spheres and to irregularly shaped milled powder. The milling did not appreciably change the activation energy of oxidation of boron and the observed accelerated oxidation was attributed to its increased specific surface area. Two of the finer prepared spherical powders could be ignited when coated on an electrically heated filament in the temperature range of ca. 600 – 1000 °C. Commercial powders, irregularly-shaped milled powder, and coarser spherical powders did not ignite. All boron powders aerosolized and ignited in an enclosed vessel in air generated similar pressures. In another experiment, blended boron and potassium nitrate powders were heated to ignition in room air using a laser beam. The ignition delays for the blends made with spherical powders were substantially shorter vs. similar blends with commercial powder. Combustion of the blends with spherical powders was more vigorous than with commercial boron. Particles were ejected more rapidly, and had shorter apparent burn times.

Green synthesis of Ni3S2 nanoparticles from a nontoxic sulfur source for urea electrolysis with high catalytic activity

Urea electrolysis receives great attention due to its capability to tackle with urea−containing wastewater and allow energy–saving hydrogen production simultaneously. Ni3S2 has favorably metallic property due to the short Ni−Ni bonds in its structure and the most abundant nickel content in the stoichiometric formula among nickel sulfides, thus it is considered an effective electrocatalyst for urea oxidation. However, the current Ni3S2−based catalysts usually derived from toxic sulfur sources, posing safety concerns in production. Herein, we reveal Ni3S2 nanoparticles (ca. 10 nm) can be directly deposited onto Ni substrate in a facile hydrothermal reaction from nontoxic L–cysteine sulfur source without any additives. Ni3S2 serves as a highly efficient and reliable electrocatalyst for urea oxidation with high current responses, low Tafel slope, and reduced resistance. Moreover, the apparent activation energy of urea oxidation for the prepared Ni3S2 electrocatalyst is evaluated as 3.2 kJ mol−1, which is much lower than that of Ni(OH)2 (15.7 kJ mol−1). As a result, the obtained Ni3S2 demonstrates remarkable catalytic activity of 280.2 A g–1 outperforming the Ni(OH)2 electrocatalyst (61.1 A g–1). In addition, Ni3S2 exhibits reliable catalytic performance, as examined by chronoamperometric, chronopotentiometric, and multi–current steps analyses.

Effect of natural carbon filler on thermo-oxidative degradation of thermoplastic-based composites

Thermo-oxidative degradation of thermoplastic-based composites filled with two types of carbon filler including bituminous (Pittsburgh No.8, P8) and sub-bituminous (Powder River Basin, PRB) coals were investigated using differential scanning calorimetry (DSC) and thermogravimetric analysis. Oxidation induction time (OIT) and activation energy (AE) for coal plastic composites (CPCs), HDPE, and commercially available wood plastic composites (WPCs) were determined using isothermal and isoconversional kinetic methods. OIT values for CPC increased with coal content, indicating higher thermo-oxidative stability. AE values obtained using isothermal method were 52–116 kJ/mol for all CPC formulations, 32 kJ/mol for HDPE, and 33–114 kJ/mol for WPCs. AEs for CPCs obtained using isoconversional methods were not constant but rather dependent on the degree of degradation with CPC/ P8 coal possessing higher overall AE. Incorporating coal into HDPE increased thermo-oxidative stability of the resulting composite, indicating coal potentially acts as a natural primary and secondary antioxidant for polymer materials.

Influence of coating parameters on oxidation behavior of Cr-coated zirconium alloy for accident tolerant fuel claddings

The Cr coatings (4.5–9.0 µm-thick) with porous/columnar and dense/columnar microstructure were deposited on Zr alloy. Oxidation was performed at 900, 1050, 1200 and 1400 °C in steam to define the role of coating microstructure and thickness on oxidation behavior of Cr-coated Zr alloy. The activation energy for chromium oxidation decreases from 249 to 124 kJ/mol when the coating microstructure changes from dense to porous. Both coatings types are consumed by Cr-Zr interdiffusion resulting in the similar thickness of residual Cr. The Cr-Zr interdiffusion can prevent forming the Cr oxide film and cause direct oxidation of the alloy at 1400 °C.

CaMn0.9Ti0.1O3 based redox catalysts for chemical looping – Oxidative dehydrogenation of ethane: Effects of Na2MoO4 promoter and degree of reduction on the reaction kinetics

Reduction kinetics and stability of 20 wt% Na2MoO4-promoted CaMn0.9Ti0.1O3 were investigated for its applications in Chemical Looping – Oxidative Dehydrogenation (CL-ODH) of ethane, a potential alternative for ethylene production with higher efficiency and lower emissions. The present work reports a kinetics model and parameters for a Na2MoO4-promoted, Ti-doped CaMnO3 (CaMn0.9Ti0.1O3) redox catalyst under H2 and C2H4. A first-order reaction model provides the best fit for the reduction of Na2MoO4/CaMn0.9Ti0.1O3 under H2, while the C2H4 reduction is well described by an Avrami–Erofe’ev model. The activation energy for C2H4 oxidation is approximately three times higher than that for H2 conversion, showing that the activation of C2H4 is significantly more difficult on the surface of the redox catalyst. The reduction rate of Na2MoO4/CaMn0.9Ti0.1O3 under H2 at 750 °C is more than two orders of magnitude greater than that under C2H4, while the reduction rate of unpromoted CaMn0.9Ti0.1O3 is comparable under H2 and C2H4, showing that the addition of Na2MoO4 effectively suppresses C2H4 combustion relative to H2 oxidation. The kinetics results for Na2MoO4/CaMn0.9Ti0.1O3 confirm its excellent selectivity towards hydrogen combustion, making it a promising candidate under CL-ODH. Additionally, the stability of the CaMn0.9Ti0.1O3@ Na2MoO4 core-shell structure, which was the underlying reason for the excellent selectivity, was examined under both shallow and deep reductions. It was determined that deep reduction of the redox catalyst, e.g. higher than 80% solid conversion, would lead to loss of sodium and hence to decreased selectivity for hydrogen combustion. In contrast, the core-shell structure was well-maintained, exhibiting excellent performance after 50 redox cycles when deep reduction of the redox catalyst was avoided. This study offers a basis for both the CL-ODH reactor design and redox catalyst optimizations.