Rate Of Oxygen Adsorption Research Articles

АГОРИТМ РАСЧЕТА СКОРОСТИ АДСОРБЦИИ КИСЛОРОДА С ИСПОЛЬЗОВАНИЕМ ГЛОБАЛЬНОЙ СТОХАСТИЧЕСКОЙ МОДЕЛИ

The paper proposes an algorithm for calculating the rate of oxygen adsorption on the crystal surface taking into account the global stochastic model. The constructed model follows from the microscopic model and takes into account the interaction stage of the contacting phases

Experimental study on oxygen adsorption capacity and oxidation characteristics of coal samples with different particle sizes

This study is aimed at investigating the effect of particle size on the oxygen adsorption capacity and spontaneous combustion and oxidation of coal, so as to provide guidance for the prevention of spontaneous combustion of residual coal in goafs. First, gas adsorption experiments, low-temperature oxidation (LTO) experiments and electron spin resonance (ESR) experiments were carried out to obtain the oxygen adsorption capacities of coal samples with different particle sizes (less than0.075 mm, 0.075–0.096 mm, 0.096–0.125 mm, 0.125–0.180 mm and 0.180–0.425 mm) and the variations of macroscopic and microscopic characteristic parameters (CO concentration, crossing-point temperature, g-factor, linewidth △H, free radical concentration Ng) during LTO. The results show that the cumulative amount of oxygen adsorption on coal surface presents grows linearly with the rise of adsorption equilibrium pressure. A smaller particle size corresponds to a higher growth rate and larger cumulative amount of oxygen adsorption. At the same temperature, coal samples with smaller particle size generate higher concentrations of CO at higher rates, and the crossing-point temperature decreases with the decrease in particle size. Such a result indicates that coal samples with smaller particle sizes are more capable of oxidizing and self-heating. During LTO, the free radical concentrations of different-sized coal samples increase continuously with the rise of temperature, and the increase differs in different stages. Specifically, after the temperature reaches 150 °C, the rate of coal-oxygen reaction accelerates, and the free radical concentration surges. For the coal sample with the particles size of less than 0.075 mm, the growth of free radical concentration in the temperature range of 150-250 °C accounts for 90.23 % of the total growth. In summary, a smaller particle size and a higher temperature conduce to accelerating the spontaneous combustion and oxidation of coal.

Enhancement of electrochemical performance for proton conductive solid oxide fuel cell by 30%GDC-LSCF cathode

In proton conducting solid oxide fuel cell (H-SOFC), the primary reaction occurs at the three-phase reaction boundary where protons, electrons and oxygen ions meet. In the present study, an oxygen ion conductor, Gd0.1Ce0.9O2-δ(GDC), added La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) as a cathode for anode-supported H-SOFC with BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) as the electrolyte. The addition of GDC could increase the oxygen vacancy concentration and three-phase reaction interface, which improved the adsorption and dissociation rate of oxygen, but led to the decrease of conductivity and the decrease of charge transfer rate. The single-cell with LSCF-30GDC cathode presented outstanding performance, and the maximum power density (MPD) was 56% higher than that of LSCF at 650 °C, while the performance of the cell with 50%GDC only increased by 10%, indicating that 30% GDC was the most suitable doping amount. The superb performance and stability imply that the LSCF-GDC composite material is a promising substitute cathode for H-SOFCs at low temperatures.

Design of Organic-Free Superhydrophobic TiO2 with Ultraviolet Stability or Ultraviolet-Induced Switchable Wettability.

Superhydrophobic TiO2 with great application potential is mainly obtained by surface modification with low surface energy organics, which is easily degraded under sunlight irradiation, which results in the loss of superhydrophobic properties. Herein, we developed a room-temperature pulsed chemical vapor deposition (pulsed CVD) method to develop amorphous TiO2-deposited TiO2 nanoparticles. The ultraviolet stability/ultraviolet-induced reversible wettability switch had been simultaneously realized by different and controllable deposition cycles of amorphous TiO2. The superhydrophobic properties of the organic-free TiO2 were determined by the micrometer-nanometer-sub-nanometer multiscale structure, the multiscale pore structure, and the large Young's contact angle resulting from carboxylic acid adsorption. Also, we found that the adsorption rate and adsorption stability of oxygen and water at the surface oxygen vacancies were the key to facilitate the reversible switching between superhydrophilic and superhydrophobic states, which was well demonstrated by experimental characterization and theoretical simulation. In addition, we also found that the resistance of dense amorphous TiO2 films on the TiO2 surface to the migration of photogenerated electrons and holes was the key to maintain the stable superhydrophobic properties of superhydrophobic TiO2 under ultraviolet illumination. The powders were strongly ground and the coating surface was rubbed on the surface of the sandpaper, which still maintained superhydrophobic properties, providing favorable conditions for the application of superhydrophobic TiO2. This work modulates the ultraviolet stability and dark/ultraviolet-induced switchable superhydrophobicity/superhydrophilicity of coated TiO2 by simply adjusting the number of deposition times in a pulsed CVD process for the first time, thus contributing to the development of organic-free superhydrophobic TiO2.

Oxide growth characteristics on Al (100), (110), and (111) surfaces: A chemo-mechanical evaluation

The oxide growth mechanism on Al (100), (110) and (111) and the corresponding oxide film properties were examined using all-atom molecular dynamics simulations. The growth kinetics of the aluminum oxide film at a constant dose rate of oxygen were analyzed. Al (110), with a structurally unstable surface and a high surface energy, showed the highest oxide growth rate, oxygen adsorption rate, and adsorption energy. However, as the crystallinity of the surface structure collapsed as oxidation progressed, the reaction energy of Al (110) with atomic oxygen converged to the same value as that of Al (100) and (111). Instead, a high anisotropic residual stress remained in the oxide layer on Al (110) with the aid of high surface energy and anisotropic surface structure. The results suggest that although the chemical bonding features of the as-prepared oxide layers were similar, the intermediate process of oxide film formation and the resultant mechanical properties were highly dependent on surface crystallinity. The oxidation kinetics model presented in this work showed consistency with other reported ab initio calculations. Moreover, it also successfully reproduced the experimental fact that the formation speed of oxygen islands on Al (100) is more delayed than that on Al (111).

Rate-Determining Steps of Oxygen Surface Exchange Kinetics on Sr2Fe1.5Mo0.5O6−δ

The oxygen surface kinetics of Sr2Fe1.5Mo0.5O6−δ was determined using the 16O2/18O2 isotope exchange method with gas phase analysis at 600–800 °C. The heterogeneous exchange rates (rH) and the oxygen diffusion coefficients (D) were calculated by processing the concentration dependences of the 18O fraction using Ezin’s model. The rates of oxygen dissociative adsorption (ra) and incorporation (ri) were calculated based on a model using the three exchange type rates. It has been established that the rates ra and ri were comparable in this temperature range. Assumptions were made about the effect of the chemical composition of the surface on the rate of oxygen adsorption. It was found that the oxygen exchange coefficient (k) of Sr2Fe1.5Mo0.5O6−δ is comparable to that of La0.6Sr0.4MnO3±δ oxide. High values of the oxygen diffusion coefficient were found for Sr2Fe1.5Mo0.5O6−δ. The values were comparable to those of the double cobaltite praseodymium-barium and exceed by more than an order those of lanthanum-strontium manganite.

Voltage Ripple of Electrode Sensor in Electrolyte Flow

Voltage ripple of electrode sensor that moves relative to electrolyte (flow noise) is interpreted in agreement with experimental data using variations in the adsorption rate of oxygen on the electrode surface upon variations in the thickness of the diffusion layer. A relationship of electrode potential and flow velocity jumps is derived. It is shown that the electrode voltage ripple is proportional to the pulsation of the liquid flow velocity and is inertial with respect to such a pulsation with a time constant of several milliseconds. It is also shown that the sensor sensitivity to velocity pulsations exponentially decreases with a time constant of several hours. The dependences of the ripple amplitude on the pulsation frequency of the velocity, electrolyte concentration, and storage time of electrodes in electrolyte that are obtained using the above relationship are proven in experiments.

Reactor scale study of self-heating and self-ignition of torrefied wood in contact with oxygen

This experimental work aims at investigating the potential development of self-heating when torrefied wood chips enter into contact with oxygen. Two kilograms of dried beech chips have been torrefied under nitrogen in a cylindrical reactor at 250 °C or 285 °C. The temperature was then leveled down between 100 °C and 160 °C and nitrogen flow was replaced by oxygen-containing flow. Influences of oxygen volumetric fraction and sweeping gas flow rate were investigated. Temperatures at different bed locations and gaseous product volumetric fractions at the bed output were monitored continuously. In some of the experiments a limited temperature rise was observed while in others a temperature runaway occurred. Thermal considerations indicate a clear enhancement of self-heating propensity with the severity of torrefaction. Gas phase analysis and molecule oxygen balance reveal an adsorption mechanism pathway and a higher rate of oxygen adsorption for the severely torrefied wood. Lowering the oxygen volumetric fraction or increasing the flow rate decrease self-heating behavior and was used to stop self-ignition of the bed.

Remarkable Decrease in the Oxidation Rate of Cu Nanocrystals Controlled by Alkylamine Ligands

Colloidal solutions of copper nanoparticles (7.2 ± 1.1 nm diameter), stabilized by alkylamine ligands, show a remarkably long persistence (several months) of the localized surface plasmon resonance (LSPR) when exposed to ambient air at room temperature. The oxidation kinetics of these nanoparticles have been investigated by optical spectroscopy and modeled using numerical simulations. Three distinct oxidation regimes are evidenced: (i) A fast regime in which oxygen is adsorbed and dissociated on the nanoparticle to form preoxide islands; (ii) a slower regime where the coalescence of the oxide islands takes place up to the formation of a complete Cu2O shell; (iii) and finally an extremely slow oxidation of the residual copper core and eventually the formation of hollow Cu2O nanoparticles. The adsorption rate of oxygen on copper nanoparticles is controlled by the amount of alkylamine ligands in solution (from 0.1 to 2 mol equiv).

Predicting the degree of surface oxidation on fine coals by measuring the oxygen transfer rate in coal suspensions

Coal surface oxidation plays a dominant role in differential coal flotation and the utilization of coal products. However, a robust and reliable tool to determine coal surface oxidation in coal preparation plants is not currently available. In this study, a novel technique was developed to determine the degree of coal surface oxidation by measuring the adsorption rate of oxygen on coal surfaces after understanding the nature of oxygen transfer in water and oxygen adsorption on oxidized and un-oxidized coal surfaces. In this study, coal samples with different extents of surface oxidation were prepared and the degree of coal surface oxidation was quantified by X-ray photoelectron spectroscopy (XPS) as the percentage of oxidized carbon. Oxygen was purged into the coal suspension at a constant flow rate and the change of dissolved oxygen (DO) concentration was monitored. It was found that the DO concentration increased with oxygen purging time and the rate of increase was dependent on the degree of coal surface oxidation. A faster increase in DO concentration was observed for more oxidized coals, which is related to a slower adsorption of oxygen on oxidized coal surfaces and, therefore, more dissolved oxygen remained in the suspension. The kinetics of the change of DO concentration was calculated using the oxygen transfer equation, based on which the rate of oxygen adsorption on coal surfaces was obtained. A linear relationship was found between the oxygen adsorption rate and the degree of coal surface oxidation. This technique may be implemented in coal preparation plants as a daily tool to closely monitor the coal oxidation status due to its simplicity and accuracy.

Non-Pt Catalyst Layer Operation in a PEM Fuel Cell: A Variable-thickness Regime

A recently pubilshed experimental polarization curve of a PEM fuel cell with the non–Pt cathode catalyst layer (CCL) exhibits unusual feature: in the region of small current densities, the curve is close to linear. We report a model for the CCL performance which explains this effect. The model includes finite rate of the oxygen adsorption on the catalyst surface. Qualitatively, due to a very high exchange current density of the non–Pt catalyst, the ORR rate close to the membrane is determined by the potential–independent oxygen adsorption rate. This leads to a specific regime of the CCL operation, when only part of the CCL thickness contributes to current production, while the rest part is completely inactive. With the growth of the cell current, the active part increases in width, while the inactive part shrinks. The resulting polarization curve appears to be close to linear.

Polarization curve of a PEM fuel cell with the account of a finite rate of oxygen adsorption on Pt surface

We report a model for performance of the cathode catalyst layer in PEM fuel cell; the model takes into account a finite rate of oxygen adsorption on Pt surface. An analytical polarization curve of the cathode catalyst layer is derived. The curve is fitted to recent experimental polarization curves; the equivalent current density of O2 adsorption on Pt surface is estimated to be 0.18 A cmPt−2. We show that decrease in the cell performance due to finite rate of O2 adsorption at a small catalyst loading can be mitigated by making a gradient electrode with the catalyst loading growing toward the membrane.

Nanoscale Electrochemical Processes On Cu(111) Surface Using Periodic DFT and Quantum/Classical Simulations

In this work, we explored the energetic and configurational properties of molecular oxygen adsorption on Cu(111) in water using classical mechanics and electronic structure based computations. It has been found that the structure of the water network surrounding the oxygen at the electrolyte/metal interface can be determinant in the adsorption configuration of the molecule on the metal surface, instead of the energetic ordering of possible adsorption configurations. An external electric field applied to the H2O/Cu(111) models to simulate the electrode potentials of electrochemical nature of the system are shown to affect the orientations of water molecules, thus affect the configuration, energetics and rate of oxygen adsorption on Cu(111).

Oxygen surface exchange kinetics of erbia-stabilized bismuth oxide

The surface oxygen exchange kinetics of bismuth oxide stabilized with 25 mol% erbia (BE25) has been studied in the temperature and pO2 ranges 773–1,023 K and 0.1–0.95 atm, respectively, using pulse-response 18O–16O isotope exchange measurements. The results indicate that BE25 exhibits a comparatively high exchange rate, which is rate determined by the dissociative adsorption of oxygen. Defect chemical considerations and the observed $$ p{{\hbox{O}}_2}^{1/2} $$ dependence of the rate of dissociative oxygen adsorption suggest electron transfer to intermediate superoxide ions as the rate determining step in surface oxygen exchange on BE25.

Enhancing the oxygen permeability of Ba 0.5Sr 0.5Co 0.8Fe 0.2O 5 + δ membranes by coating RBaCo 2O 5 + δ (R= Pr, Nd, Sm, Gd) layers

The effects of a thin RBaCo 2O 5 + δ (R= Pr, Nd, Sm, Gd) layer coating on the oxygen permeation flux through Ba 0.5Sr 0.5Co 0.8Fe 0.2O 5 + δ (BSCF) membrane were investigated. Due to the high oxygen adsorption and desorption rate constants k a and k d of the RBaCo 2O 5 + δ (R112) materials and the porous structure of the coating layers, the oxygen permeation flux through the BSCF membranes can be enhanced remarkably. It was found that the reaction between Pr112 and BSCF also has significant influence on the oxygen permeation flux. The reaction between Pr112 and BSCF forms impurities which may block oxygen permeation flux. However, Nd112, Sm112, and Gd112 do not react seriously with BSCF, therefore, coating layers of these materials can significantly enhance the oxygen permeation flux of BSCF membranes.

Mechanism of PdO thin film formation during the oxidation of Pd(1 1 1)

We investigated the mechanism by which a surface oxide layer on Pd(1 1 1) transforms to a PdO(1 0 1) thin film during oxidation with gaseous oxygen atoms in ultrahigh vacuum. Our results provide evidence that the precursor to bulk PdO formation is a distinct oxide phase that forms as small particles, referred to as PdO seeds, after the surface oxide saturates. With increasing oxygen coverage, the PdO seeds grow in size and eventually transform to more stable particles that agglomerate to yield a PdO film. Oxidation effectively ceases when the surface oxide layer is completely replaced by the bulk PdO film, demonstrating that the surface oxide is needed for PdO formation at the conditions studied. Both the kinetics of PdO formation and the final thickness of the PdO thin film depend strongly on the thermal stability of the PdO seeds. Below the decomposition temperature of the seeds (∼600 K), oxidation follows kinetics similar to Langmuirian adsorption and appears to be limited only by the rate of oxygen adsorption onto the surface oxide. In contrast, PdO formation above 600 K initially exhibits acceleratory kinetics, with the rates starting low but increasing steadily during the initial growth of PdO. We also observe a significant decrease in PdO(1 0 1) film thickness and improved crystallinity when oxidation is conducted below 600 K. We show that the trends observed in the oxidation kinetics and film thickness can be qualitatively explained within the context of a model in which the thermodynamic stability of PdO particles increases with increasing particle size and PdO seeds/particles coexist with a two-dimensional (2D) gas of oxygen atoms adsorbed on the surface oxide layer. This model suggests that the PdO particle-2D gas coexistence relation gives rise to three distinct growth regimes, namely, stable seed nucleation, metastable seed nucleation and oxygen dissolution into the subsurface where the latter is established at 2D gas coverages below the stability limit of a flat PdO surface.

Thermogravimetric study on perovskite-like oxygen permeation ceramic membranes

The oxygen permeation properties of mixed-conducting ceramics SrFeCo 0.5O 3− δ (SFCO), Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3− δ (BSCFO), La 0.2Sr 0.8Co 0.8Fe 0.2O 3− δ (LSCFO) and Ba 0.95Ca 0.05Co 0.8Fe 0.2O 3− δ (BCCFO) were studied by thermogravimetric method in the temperature range 600–900 °C. The results show that the oxygen adsorption rate constants k a of all material are larger than oxygen desorption rate constants k d and both k a and k d are not strongly dependent on temperature in the studied temperature range. The oxygen vacancy contents δ(N 2) and δ(O 2) in nitrogen and oxygen and their difference Δ δ = δ(N 2) − δ(O 2) play an important role in determining the temperature behavior of oxygen permeation flux J O 2 .

Oxygen adsorption on silver catalysts during the course of partial oxidation of ethylene

To explore oxygen adsorption kinetics on silver catalyst surfaces during the course of ethylene oxidation, relations between the amount of oxygen consumed by reaction with ethylene ( q) and oxygen partial pressure ( p O 2 ) at a given ethylene pressure ( p C 2 H 4 ), i.e., kinetic data, were measured using a pulse reaction technique on K 2SO 4-promoted Ag/α-Al 2O 3 catalyst and a flow reaction method on Cs and Re-copromoted Ag/(α-Al 2O 3-crystal) catalyst at 453–529 K. The kinetic data were analyzed by the use of three kinetic models derived on the basis of a redox model assuming that, under steady states, no oxygen desorption occurred and the rate of oxygen adsorption on silver surface was equal to the rate of surface reactions consisting of adsorbed oxygens and gaseous ethylene. One was the model that two-step consecutively dissociative oxygen adsorptions forming oxygen adatom and then admolecule were rate-determining (Model 1), and others were molecular oxygen adsorption rate-determining model (Model 2) and surface reaction rate-determining model when one-step dissociative oxygen adsorption forming oxygen adatom only lay in a complete equilibrium state (Model 3). The Model 1 showed the most excellent suitability to the kinetic data and reproduced accurately the distinctive features for experimental q – p O 2 curves. The coverage of oxygen admolecule estimated from the Model 1 was 0.48–0.96 and 0.12–0.34 on the K 2SO 4-promoted Ag/α-Al 2O 3 catalyst at p C 2 H 4 ≦ 0.03 atom and on the Cs and Re-copromoted Ag/(α-Al 2O 3-crystal) catalyst at p C 2 H 4 ≧ 0.30 atom, respectively. They were about thousand times bigger than those for oxygen adatom. These results indicated that the first dissociative adsorption on vacant active center was much slower than the second dissociative oxygen adsorption on adatom; the adatom seemed likely to act as active center for the second oxygen adsorption. Selectivity to ethylene oxide elevated almost linearly with increase in the coverage of oxygen admolecule. Activation energies and heats of adsorption obtained for dissociative oxygen adsorption were quite reasonable values compared to literature values and remarkedly reduced with increasing p C 2 H 4 .

Periodical forcing for the control of chaos in a chemical reaction

Control of the chaotic behavior of a chemical system can be achieved perturbing periodically some control parameters of the system. This procedure based on external forcing, which is based on the phenomenon of resonance, can change a chaotic behavior into a periodical one by means of the application of a sinusoidal perturbation. In this paper, the influence of a periodical modulation added to the parameter controlling the oxygen adsorption rate in a cellular automaton (CA) model studying CO oxidation is analyzed. This CA model considers the oxidation reaction of CO on a catalytic surface, taking into account the catalyst temperature variation in order to analyze the reaction time oscillatory behavior. Simulations of the CA model exhibit chaotic and quasiperiodical behaviors, and it can be shown that the periodical forcing strategy can suppress the chaotic dynamics by means of the stabilization of periodical solutions.

Dynamic Monte Carlo simulations of O 2 adsorption and reaction on Pt(1 1 1)

Temperature-programmed dynamic Monte Carlo simulations describe the time evolution of adsorption and desorption of molecular oxygen, its dissociation, and atomic oxygen diffusion. A hexagonal lattice containing six adsorption sites per unit cell represents the Pt(1 1 1) surface. The temperature is increased linearly with time, starting with an empty initial surface. Temperature effects on the rate of molecular oxygen adsorption are incorporated through the temperature dependence of the O 2 sticking coefficient and that of the flux of molecules impinging the surface at a given pressure. The results illustrate the mechanisms by which molecular oxygen cannot be adsorbed at temperatures above 155 K.