Initial Stages Of Oxidation Research Articles

Mass transfer coefficient and mass diffusivity of O2 and CO2 during oxidation of macerated CTC and rolled orthodox leaves in black tea manufacturing

AbstractOxidation of withered and macerated crush‐tear‐curl (CTC) and rolled orthodox tea leaves of TV25 clone was carried out in a closed chamber and oxygen and carbon dioxide concentration inside the chamber was measured at small time intervals. From the O2 consumption and CO2 production pattern during oxidation, mass transfer coefficient (ky′) and mass diffusivity (DAB) of O2 and CO2 was calculated considering equimolar counterdiffusion of the gases. Change in mass transfer coefficient and mass diffusivity of O2 and CO2 with duration of oxidation was fitted into exponential equation. Value of R2 for all the relationships were greater than .80 and reduced chi‐square values was less than 1. Values of ky′ and DAB at the initial stage of oxidation are higher for macerated CTC leaves and they reduce at a faster rate for oxidation of CTC tea than orthodox leaves.Practical applicationFermentation (oxidation) in black tea processing is the most important processing step responsible for characteristics tea quality in terms of color, flavor, and aroma. This step consists of several biochemical reactions which are associated with consumption of O2 and production of CO2 gas. Mass transfer coefficient and mass diffusivity of O2 and CO2 gas during oxidation in CTC granules and rolled orthodox leaves were estimated in this study. This would help with the control of air flow rate in designing continuous fermentation machines and also the number of changes in room air required for floor/gumlah/trough fermentation and thereby improved tea quality.

Cyclic oxidation behavior of a multilayer composite coating for single-crystal superalloys

A multilayer composite coating, comprised of alternating Ni + CrAlYSiHfN sub-layers and AlxN sub-layers, was manufactured by magnetron sputtering. The multilayer is composed of β-NiAl, α-Cr and AlN phases after annealing, and their volume fractions are 72 vol.%, 12 vol.% and 16 vol.%, respectively. Due to doping with nitrides, the CTE of multilayer coating is reduced to around 12.7*10−6/K. The lower CTE mismatch allows multilayer coating to exhibit good resistance to oxide scale spalling in thermal cycling at 1100 °C. The θ-to-α Al2O3 transformation in oxide scale caused a small drop of residual stresses on multilayer coating in initial oxidation stage.

The catalytic oxidation of formaldehyde over palygorskite-supported copper and manganese oxides: Catalytic deactivation and regeneration

The emission of volatile organic compounds (VOCs) from industrial and commercial processes is detrimental to human health and the environment. Though numerous catalysts have been reported to eliminate the pollution of VOCs, their deactivation and regeneration are still largely unknown. In this study, the prepared palygorskite-supported Cu-Mn oxides displays slight decrease of catalytic activity in the initial stage of formaldehyde oxidation. After the thermal treatment of spent catalyst at 400 °C, the catalytic activity is partially recovered. A deactivation and regeneration mechanism is proposed, based on the stability test and the comprehensive characterizations of fresh and spent catalysts. The accumulation of formate species and the depletion of oxygen on the surface of Cu-Mn oxides are responsible for the decline of activity. Thermal treatment of deactivated catalyst at 400 °C recovers the catalytic activity because it gasifies the carbonaceous intermediates and replenishes surface oxygen. The obtained results will be of great significance for the application of transition metal oxide-based catalysts in the abatement of VOCs.

Mechanisms of oxidation of pure and Si-segregated α-Ti surfaces

Using first-principles molecular dynamics, we identified the mechanisms of the oxidation of α-Ti surfaces. Si segregation was found to suppress α-case formation in Ti, which was also confirmed experimentally. Charge transfer from the metal atoms to the gas molecules drives the initial stages of oxidation on the pure and Si-segregated α-Ti (0001) surfaces, while during the later stages, oxidation proceeds via oxygen penetration into the slab. Growth of the oxide network was strongly dependent on the oxidation state of the surface Ti atoms. Oxide growth in the Si-segregated material was retarded with the formation of TiOx (0.5 ≤ x ≤ 1) on the surface, which corresponds to the +1.5 oxidation state of the Ti atoms. The simulations and experiments clearly showed that Si reduces the ingress of oxygen into Ti, even at high temperatures. The primary and critical steps of identifying, understanding, and controlling the mechanisms of oxidation of Ti surfaces at high temperatures, as performed here, are expected to aid the design of new alloys with improved oxidation resistance.

The antioxidant mechanism of Maillard reaction products in oil-in-water emulsion system

Maillard reaction products (MRPs) were widely used to stabilize oil-in-water (O/W) emulsions to inhibit lipid oxidation due to their great antioxidant capacity, but the detailed antioxidant mechanism was still uncertain. In this study, MRPs produced by dry-heating whey protein isolate with dextran for different reaction time were used to prepare O/W emulsions. The peroxide value (POV value) decreased from 171.44 meq/kg to 30.63 meq/kg verified that the oxidation stability of emulsions was significantly improved by MRPs compared with unmodified protein. The reaction time was found to be positively related to the layer thickness and grafting degree, showing that MRPs with longer reaction time could form thicker interfacial layer. Comparing the antioxidant ability of MRPs with unadsorbed subnatants, it was indicated that MRPs with longer reaction time presented higher antioxidant effect at the interface. POV value of emulsions with subnatants replaced by water was compared with un-replaced emulsions to illustrate the antioxidant roles of MRPs at interfacial layer and in the continuous phase. The results showed that in the initial oxidation stage, thicker interfacial layer would provide critical physical steric hindrance function to inhibit free radicals while MRPs played an unobvious role. In the next stage, MRPs adsorbed at the interface had a pivotal impact on preventing the free radical chain reactions. At the later stage, the un-adsorbed MPRs started to manifest their influence on protecting oil from oxidation.

Evolution of Surface Morphology of Oxide Formed During Initial-Stage Oxidation of γ-TiAl Alloy Using In Situ Environmental SEM

Initial-stage oxidation characteristics of a γ-TiAl alloy were investigated by using in situ environmental scanning electron microscopy for both heating and isothermal oxidation processes. Selective oxidation led to primary nucleation and growth on the aluminum-rich γ phase. Under different isothermal oxidation times, the oxide morphology changed to a linear structure between 750 °C and 774 °C and to an acicular structure at 800 °C. The initial oxidation process of γ-TiAl alloy could be divided into three stages: nucleation; growth; and morphological transformation and subsequent growth stages.

Oxidation Behavior of β-Sialon Ultrafine Powders Prepared by the Combined Sol-Gel and Microwave Carbothermal Reduction Nitridation Method

Ultrafine powders of β-Sialon were prepared by the combined sol-gel and microwave carbothermal reduction nitridation method, and their oxidation process was studied by a non-isothermal thermogravimetry method. The results indicated that two different mechanism functions respectively corresponded to the initial and final oxidation stages. The reverse Jander equation with activation energy of 240.5 kJ/mol and the Avrami-Erofeev equation with activation energy of 410.7 kJ/mol were respectively identified as the most probable mechanism function for the initial and final oxidation stages in the temperature range of 1423-1623 K.

Oxidation Kinetics Researches under the Condition of Compressive Loading

High temperature oxidation behavior of alloys is mostly affected by an external loading. The growth of oxide scale, oxide products, interfacial reactions and other characteristics have be affirmed to be affected by the external loading. Here, it provided the oxidation kinetics of several metals under the compressive loading. And the growth of oxide scale was distinguished in the form of thin oxide layer and thick oxide layer. Different compressive effect on the growth of oxide scale was summarized. At the initial oxidation stage, the applied compressive stress induced more defects on the surface of alloy, supplying more oxide neclei, thereby increased the growth of the thin oxide layer. In the case of the growth of the thick oxide layer, it was attrbuted to the short circuit paths of the oxide grain boudary inherited from the initial oxidation stage and the cracks or other defects deduced by the applied compressive stress. Thus the oxide kinetics of alloys was promoted. However, considering the interfacial reaction at scale/metal, the coarsens of vacancies would occur after a long time of oxidation, decreasing the growth of oxide scale.

Quasi-in-situ observing the growth of native oxide film on the FeCr15Ni15 austenitic alloy by TEM

Element Cr imparts non-crystallinity effect on the oxide film on iron. Therefore, in contrast with the oxidation of iron, the oxidation process of Fe-Cr alloys is more complicated. By means of high angle annular dark field scanning transmission electron microscopic (HAADF-STEM) imaging combined with electron-energy loss spectroscopy (EELS) and super-X electron dispersed spectroscopy (Super-X EDS) techniques in an aberration-corrected TEM, we have clarified the evolution in the oxide film formed on an FeCr15Ni15 alloy. The oxide film is identified to be amorphous at the initial oxidation stage, but tends to become crystalline in the inner layer after aging in air.

New insight on early oxidation stages of austenitic stainless steel from in situ XPS analysis on single-crystalline Fe–18Cr–13Ni

In situ X-ray photoelectron spectroscopy real-time measurements and angular-dependent high resolution core level analysis were used for the first time to investigate the Cr enrichment and oxide growth mechanisms on a model 304 austenitic stainless steel surface in the very initial stages of oxidation leading to pre-passivation. The oxidation kinetics was followed for increasing oxygen exposure and temperature, revealing an early nucleation regime (for exposure <10 L) leading to the formation of a strongly Cr-enriched Cr3+/Fe3+ mixed layer followed by an oxide growth regime where preferential iron oxidation takes over and mitigate the initial chromium enrichment.

A new experimental apparatus for emissivity measurements of steel and the application of multi-wavelength thermometry to continuous casting billets.

An experimental apparatus has been designed for measuring the emissivity of a steel surface in both vacuum and oxidation atmosphere. The sample is heated with the method of electromagnetic induction in order to ensure the temperature uniformity. The radiance emitted from a sample is measured using a fiber-optic Fourier transform infrared spectrometer. Using this unique apparatus, we investigated the spectral (2-6 μm) and directional (0°-86°) emissivity of stainless steel 304 with different degrees of surface oxidation at temperatures ranging from 800 to 1100 °C. The experimental results show that the emissivity increases slightly with increasing temperature, which accords with the Hagen-Rubens relation. The emissivity increases rapidly at the initial stage of oxidation, but gradually reaches to a constant value after 20 min. In addition, the directional emissivity has a maximum value at the measuring angle of about 75°. The maximum uncertainty of emissivity is only 3.0% over all the measuring ranges, indicating that this experimental apparatus has a high reliability. In order to measure the surface temperature of casting billets based on multi-wavelength thermometry, the bivariate emissivity function with the two variables, wavelength and temperature, is determined. Temperature measurement results based on our technique are compared with those from common dual-wavelength radiation thermometry. Our approach reduces the measured temperature fluctuation from ±20.7 °C to ±2.8 °C and reflects the temperature variation with the changes of production parameters in real time.

The free radical chain mechanism of the initial stages of crude oil oxidation in term of SARA fractions

The formation and decomposition of hydro-peroxides are the key stages of combustion. These stages strongly depend on the several factors accelerating or slowing this process. The aim of this work is to estimate experimentally which oil components act as inhibitors of initial stages of oxidation and which accelerate the process. The next aim is to explore the process of adsorption of oil components on the grain of rock, which turned to be also a key process in the low temperature oxidation.The work includes experimental part where differential scanning calorimeter (PDSC) experiments with pure saturates, mixtures of saturates and aromatic oil fractions and mixtures of saturates, aromatic fractions and rock samples are considered. Effects of inhibition and acceleration of the initial oxidation stages are explored.

A Dynamic Impedance Study of the Initial Stages of Nickel Oxidation

The nature of the species that form on oxidizing Ni electrode in alkaline solution has been studied for a long time. Bode summarized the formation and interconversion of these hydroxide and oxyhydroxide phases in terms of time and potential [1], but their structure and kinetics remain an active area of study [2]. The initially formed oxide is believed to be a hydrated hydroxide, α-Ni(OH)2, which irreversibly interconverts to a less hydrated β-Ni(OH)2 at higher potentials. The formation and interconversion of these hydroxide phases is studied here by dynamic electrochemical impedance spectroscopy (dEIS) [3,4], which applies a multi-frequency waveform during a cyclic voltammetry sweep or during a chronoamperometry experiment. It enables the study of non-steady states of the surface that cannot be accessed by conventional potentiostatic EIS. It is particularly suited to study the irreversible α-Ni(OH)2 to β-Ni(OH)2 interconversion. An electrochemical polishing procedure was used to remove the oxide layer in order to have a clean and oxide-free surface before doing the experiment. This step consisted of galvanostatic holding in phosphoric acid solution. Impedance data were collected during voltammograms at different slow sweep rates, with selected hold periods before, during, or after the peaks, and were correlated to charges under the voltammograms. The impedance data were fitted to equivalent circuits. In most cases the faradaic impedance approximated a charge-transfer resistance, but the use of complex capacitance plots showed evidence for additional structure at low frequencies. Comparison of the the polarization resistance determined from the slope of the voltammograms with the estimated low-frequency intercept of the impedance showed a discrepancy in the peak region, which also suggests that a slow process exists that whose kinetics is not determinable by dEIS. The sweep-hold experiments followed the conversion of the α phase to the β phase, and found that the oxidation is not complete in the potential range of 0.2-0.3 V. Voltammograms of the admittance at selected frequencies were found to be sensitive to the surface condition, and showed that the surface condition did not return to the same value after cycling. This research was conducted as part of the Engineered Nickel Catalysts for Electrochemical Clean Energy project administered from Queen’s University and supported by Grant No. RGPNM 477963-2015 under the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Frontiers Program. [1] H. Bode, K. Dehmelt and J. Witte. Electrochim. Acta., 11, 8 (1966). [2] M. Alsabet, M. Grden and G. Jerkiewicz, Electrocatalysis, 2, 4 (2011). [3] R.L. Sacci, F. Seland and D.A. Harrington, Electrochim. Acta., 131, 13 (2014). [4] R.L. Sacci and D.A. Harrington, ECS Transactions, 19, 31 (2009).

Kinetics of Initial Stages of Pt Oxidation from Electrochemistry and Surface X-Ray Diffraction

The kinetics of the electroxidation of Pt to Pt "oxide" have been studied for many years, initially on polycrystalline Pt. Early studies showed that the oxide charge increases linearly with log(time) at constant potential, the so-called direct logarithmic growth law. Later, Harrington showed empirically that the underlying differential equation, with a Tafel potential dependence of the rate constant, predicted also the current plateau behavior in cyclic voltammetry [1]. The Tafel dependence is suggestive of electron-transfer kinetics and was also found in the oxidation of Pt(111) [2]. Despite this, the classical Conway model [3] invokes field-driven dipole flipping, and other literature models invoke bulk film models that cannot be applicable for a few-monolayer-thick oxide. An early atomistic model stressed the role of surface diffusion in leading to irreversibility and predicted a three-layer surface restructuring and the direct logarithmic law, but with the wrong slope [4]. Our recent Surface X-ray Diffraction studies [5] have shown that the number of place-exchanged Pt atoms also follows the direct logarithmic law. We here extend this result to an extended data set that systematically steps into the oxide peak in the region 1-1.2 V vs RHE, which is the peak in which the place-exchange initiates. The intensity of the (1,1,1.5) reflection shows that the place-exchange process initiates in the early stages of this peak, earlier than the peak maximum at 1.05 V where it sharply changes in a 50 mV/s intensity voltammogram. There is a systematic change in the slope of the direct logarithmic law over the range of the peak, with the largest slope at the peak maximum. The observation that both the current and the number of place-exchanged Pt atoms obey the direct logarithmic law suggests a common limiting process, though in principle the place-exchange and electron-transfer processes need not be coupled. The oxidation of adsorbed OH to O has also been suggested to occur in the same peak and there is some evidence for two processes. A careful comparison of the current density with the coverage of place-exchanged atoms is undertaken here to deduce the electron/place-exchange ratio and indirectly the nature of the accompanying species (O or OH), as well as determining how tightly coupled the exchange and oxidation processes are. The issue of double-layer charging correction is a significant issue in this determination. The place exchange is the initial step of the irreversible nanoscale restructuring of the surface that occurs on potential cycling. The later stages have recently been quantified using Grazing-Incidence Small-Angle X-ray Scattering [6-7]. A careful study of the kinetics is directed toward a detailed atomistic understanding of the evolution of the surface structure. The authors thank the European Synchrotron Radiation Facility, Deutsche Forschungsgemeinschaft, and the Natural Sciences and Engineering Research Council of Canada for support. [1] D.L. Heyd, D.A. Harrington, J. Electroanal. Chem., 335, 19 (1992). [2] A. Björling, J.M. Feliu, J. Electroanal. Chem., 662, 17 (2011). [3] H. Angerstein-Kozlowska, B.E. Conway, K. Tellefsen, B. Barnett, Electrochim. Acta, 34, 1045 (1989). [4] D.A. Harrington, J. Electroanal. Chem., 420, 101 (1997). [5] J. Drnec, M. Ruge, F. Reikowski, B. Rahn, F. Carlà, R. Felici, J. Stettner, O.M. Magnussen, D.A. Harrington, Electrochim. Acta, 224, 220 (2017). [6] M. Ruge, J. Drnec, B. Rahn, F. Reikowski, D.A. Harrington, F. Carlà, R. Felici, J. Stettner, O.M. Magnussen, J. Am. Chem. Soc., 139, 4532 (2017). [7] M. Ruge, J. Drnec, B. Rahn, F. Reikowski, D.A. Harrington, F. Carla, R. Felici, J. Stettner, O.M. Magnussen, J. Electrochem. Soc., 164, H608 (2017).

Thermal-mechanical coupling effect on initial stage oxidation of Si(100) surface

The initial stage oxidation of biaxially strained Si(100) at temperatures ranging from 300 K to 1200 K has been investigated by Reactive Force Field Molecular Dynamics simulations. We reported that the oxidation process involving the reaction rate and the amount of absorbed O atoms could be enhanced by the coupling effect of higher temperatures and larger external tension. By fitting the simulation results, the relationship between absorbed oxygen and the coupling of temperature and strain was obtained. In probing the mechanism, we observed that there was a ballistic transport of O atoms, displaying an enhancement of inward penetration by external tension. Since such an inward transport was favored by thermal actuation, more O atoms penetrated into deeper layers when the 9% strained Si oxidized at 1200 K. Moreover, the evolution of stress in the surface region during the oxidation process was discussed, as well as the related oxide structure and the film quality. These present results may provide a way to understand the thermally-mechanically coupled chemical reactions and propose an effective approach to optimize microscale component processing in the electronic field.

Influence of alloy element addition on the nucleation mechanism of the lead alloy surface and its oxide film properties

Oxidation film growth is an inextricable part of the nucleation mechanism at the initial stage of oxidation on the lead anode surface, which results from different degrees of corrosion resistance. In this paper, Pb, Pb-Sn-Ag, Pb-Ca, and Pb-Ca-Sr alloys were taken as raw materials to study the element doping effect on the nucleation growth mode of the oxide film on the lead alloy surface using CrO3 solution as an electrolyte. The results show that four kinds of alloy anodes behave in accordance with the Bewick 2D nucleation growth method during the oxidation process, while there are differences among anodes. Nucleation is remarkably related to the internal stress and structure of the lead alloy substrate; internal stress significantly influences the electrochemical properties of oxidation film. The oxidation film generated on the Pb-Sn-Ag alloy anodes is found to have a better corrosion resistance than those on the other anodes by means of electrochemical testing, as the lead anode features small internal stress and nucleates in a gentle manner, its thickness is the smallest as well.

Thermodynamic and Kinetic Parameters of Cerium(IV) Complexes with Some Dicarboxylic Acids

The thermodynamic and kinetic parameters of the cerium(IV) complexes formed in the initial stage of oxidation of dicarboxylic acids (H2L), like pentanedioic, butanedioic, propanedioic, and ethanedioic acids, by cerium(IV) sulfate were studied by the spectrophotometric and pH-potentiometric methods with the aid of integral kinetic methods at an ionic strength I = 2 mol/L within the pH range of–0.3–1.6 in a sulfuric acid medium and at temperature of 293.15 K. The composition of these complexes, the form of organic ligand existence therein, the thermodynamic parameters of their formation, and the kinetic parameters of their intramolecular redox decomposition were determined. Linear correlations between the found thermodynamic and kinetic parameters of the examined complexes [CeOHL]+ were obtained. The rate equation of the redox process occurring in the systems Ce4+–H2L was established and the corresponding reaction model was considered.

The initial stages of atomic force microscope based local anodic oxidation of silicon

In this paper, the initial stages of local anodic oxidation (LAO) process initiated by AFM probe are studied on the wide (∼100μm) terraces of the atomic-smooth Si (111) surface when creating dense array of local oxidation points. The dependence of LAO points height on the value of voltage initiating the oxidation is found to have a pronounced step-like feature with a characteristic period of 0.7 ± 0.1 nm. The presented analysis shows for the first time the realization of the step-layer mechanism of anodic oxide growth on the Si (111) surface.

Influence of surface roughness on the oxidation behavior of a Ni-4.0Cr-5.7Al single crystal superalloy

The high-temperature oxidation dynamics and mechanisms of a Ni-based single crystal superalloy with four kinds of surface roughnesses were investigated by virtue of XRD, OM, SEM and EDS at 1000 °C. In the initial oxidation stage, outer (Ni, Co)O was mainly produced on the surfaces of the samples with Ra = 90 nm and 19 nm. Correspondingly, outer Cr2O3 and transient θ-Al2O3 were mainly formed on the surfaces with Ra = 509 nm and 182 nm. After 180 min oxidation, the values of instantaneous parabolic mass gain coefficients (kp) of the samples with all surface roughnesses were gradually consistent with the data of the growth parabolic coefficient of α-Al2O3. The oxidation mechanisms of Ni-based superalloy with different surface roughnesses were discussed by a model. The external diffusion flux of Al (DAl) increases with the increases of surface roughness. Thus, the required Al concentration decreases with the increases of surface roughness when the selective oxidation of Al occurrs to form a protective single α-Al2O3 film.

First-Principles Investigation of Titanium Nanoparticle Oxidation

We perform first-principles calculations to investigate the initial stages of titanium nanoparticle oxidation. We determine the most stable structure of a 181-atom decahedral nanoparticle with various oxygen coverages ranging from a single atom to full oxidation of the surface. Linear Oad–Ti–Oad bonding configurations on the nanoparticle surface are found to be most stable for low oxygen coverage. The degree of lattice expansion is observed to gradually increase with increasing oxygen content up to 8.2% for full oxidation of the surface. To investigate likely mechanisms for subsequent subsurface oxidation, we calculate energy barriers for many inequivalent oxygen diffusion pathways. We find that the most favorable pathways involve penetration of oxygen into subsurface octahedral sites in the center of facets where the strain is largest. The results provide atomistic insight into the oxidation behavior of Ti nanoparticles and highlight the important role played by adsorption induced strain.