Oxygen Desorption Process Research Articles

BaCoO3−δ perovskite-type oxygen carrier for chemical looping air separation, part Ⅰ: Determination of oxygen non-stoichiometry and cyclic stability of oxygen carrier

Chemical looping air separation (CLAS) is a cost-effective high-temperature air separation technology, and finding suitable oxygen carriers with high oxygen selectivity and capacity is of vital importance to it. Perovskite-type oxides have been extensively studied as oxygen carriers due to their high oxygen selectivity. In this work, BaCoO3-δ perovskite oxide with large oxygen capacity was proposed as an excellent oxygen carrier for CLAS. BaCoO3-δ perovskite powder was synthesized through a sol–gel method, and the determination of oxygen non-stoichiometry and cyclic stability of BaCoO3-δ was investigated via thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS). Results indicated that the oxygen non-stoichiometry of BaCoO3-δ was a function of temperature and oxygen partial pressure, and the empirical formula was determined by linear fitting in the range of δ less than 0.5. The standard enthalpy change and standard entropy change of the oxygen desorption reaction were 169.43 kJ/mol and 156.19 kJ/(mol·K) at δ of 0.38, which increased to 292.58 kJ/mol and 298.81 kJ/(mol·K) at δ of 0.47, respectively. The oxygen desorption amount and desorption rate of BaCoO3-δ remained extremely stable during 50 cycles at 875℃, with an oxygen capacity of approximately 0.018 g O2/g BaCoO3 of each cycle. The kinetics of the oxygen desorption process of BaCoO3-δ perovskite was described using the 3-D diffusion Jander model, and the obtained values of apparent activation energy (Ea) and pre-exponential factor (A) were 209.79 kJ/mol and 1.93E06, respectively. Further results indicated that BaCoO3-δ demonstrated a hexagonal perovskite phase, the desorbed oxygen was provided by lattice oxygen in perovskite, the crystal structure remained stable during cycles, and the micro-morphology slightly changed after 50 cycles.

Degradation mechanisms of the Pb1−хBaхZr0,53Ti0,47O3−δ ferroelectric structure

The present paper is devoted to the study of the structural degradation mechanisms of ferroelectric having Pb 1−х BaхZr 0,53 Ti 0,47 O 3−δ composition. On the base of investigations of the influence of temperature-time synthesis modes on the structural perfection degree of the Pb 0,85 Ba 0,25 Zr 0,53 Ti 0,47 O 3−8 compound, it has been determined, that a shift of the position of the XRD reflex from the plane (112) and its broadening, which is caused by the amorphization and an increase of the elastic stresses in the material crystal lattice during the annealing of the compound. It has been demonstrated that at the increase of the annealing temperature up to 520 K, the crystal lattice of the ferroelectric is deformed by means of a formation of VPb,V0 and [V О •• 2Pb / i ] defects, which is realized in a decrease of the I(hkl) intensity relationship of the XRD reflexes from the planes (112) and (211). It has been supposed that an excess of lead is produced on the grain boundaries as a result of this annealing process, which promotes a local manifestation of the liquid phase. This phase which takes place in the transfer of the material components, not only increases the interaction of the reacting components, but lowers the stability of their crystal lattices due to the formation of point defects, as well. In this case, the lattice state is determined both by the emergence of the oxygen (VО) and lead (VPb) vacancies, as well as of the associates of the [V О •• 2Pb / i ] type, and by the emergence of various elastic stresses, which promotes the relaxation of the chemical bonds between atoms, caused by the deficiency introduced into the anionic sublattice of the Pb 0,85 Ba 0,25 Zr 0,53 Ti 0,47 O 3−8 solid solution at the oxygen desorption process.

Oxygen desorption behavior of sol-gel derived perovskite-type oxides in a pressurized fixed bed reactor

Perovskite-type oxides have been considered to be an effective oxygen carrier for producing O2/CO2 for oxy-fuel combustion application, which is a promising technique for carbon capture and storage. However, the main problem of common perovskite-type oxygen carriers is their relatively low oxygen desorption capacity at atmospheric pressure. Therefore, it is important to develop new oxygen carriers and enhance the reaction pressure to achieve high desorption capacity. In this study, a series of A/B-site substitution Ba1−xSrxCo1−yFeyO3−δ(x=0, 0.5, 1; y=0, 0.2, 1) perovskites were synthesized by sol-gel method, and the oxygen desorption performance of them was investigated in pressurized fixed bed. The perovskite samples were characterized using X-ray diffraction (XRD), X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS) and Transmission electron microscopy (TEM) measurements. The effect of reaction pressure, temperature and flow rate on the oxygen desorption performance were investigated in details. The results indicated that all of the Ba1−xSrxCo1−yFeyO3−δ perovskites showed an obvious improvement on oxygen desorption capacity at pressurized conditions. Besides, BaCoO3−δ showed the best oxygen desorption performance among the Ba1−xSrxCo1−yFeyO3−δ(x=0, 0.5, 1; y=0, 0.2, 1) perovskites. Oxygen desorption amounts of BaCoO3−δ increased by 57.2% and 62.7% under 2MPa and 3MPa conditions respectively, comparing to that of it under atmospheric pressure at 850°C. The optimal temperature and WHSV for BaCoO3−δ are 850°C and 176.78h−1. XRF and XRD results showed that the elemental contents of perovskites remained after reactions and the crystal structure of perovskites could recover after cyclic process under high pressure calcination. XPS results showed the evolution of oxygen species during oxygen desorption process.

Photoluminescence from porous textured ZnO films grown by chemical bath deposition

Porous textured ZnO films have been fabricated using the low temperature bath chemical deposition method. The photoluminescence (PL) spectra of the ZnO films show a broad emission band peaked at ~400nm. The PL exhibits sensitivity to the emission recording conditions. A decrease in temperature leads to separation of the PL into the UV and violet emission bands; at 10K they are located at 368.4 and 412nm, respectively. The observed violet emission is attributed to zinc vacancy related defects. The observed sensitivity of the emission to the recording conditions is due to oxygen desorption processes and variations in the width of a depletion region on the porous textured ZnO film surface.

Cobalt-free SrNbxFe1−xO3−δ (x = 0.05, 0.1 and 0.2) perovskite cathodes for intermediate temperature solid oxide fuel cells

The development of high performance perovskite cathode for solid oxide fuel cells (SOFCs) relies upon the knowledge and understanding of the interplay between the metal oxide components, structure, redox properties and conductivity. In this work, we partially substituted Fe on SrFeO3−δ with Nb. In particular, 3 Nb-doped compositions were prepared, e.g. SrNb0.05Fe0.95O3−δ (SNF0.05), SrNb0.1Fe0.9O3−δ (SNF0.1) and SrNb0.2Fe0.8O3−δ (SNF0.2). Mössbauer spectroscopy revealed decreasing ratio of Fe4+ to Fe3+ at the higher Nb doping content which translates to the gradual decrease of the average Fe oxidation state from 3.403 (for SNF0.05) to 3.375 (for SNF0.1) and to 3.291 (for SNF0.2). Likewise, the oxygen desorption process and the thermal expansion coefficients decreased with increasing Nb content, therefore providing evidence on their correlation with the thermal reduction of Fe4+. The temperature-dependent oxygen nonstoichiometry displayed two different regimes separated by a transition temperature of 625 °C, below which SNF0.2 showed the highest nonstoichiometry and above which SNF0.05 provided the highest nonstoichiometry. The analogous shifting in trends was reproduced for oxygen reduction reaction (ORR) performance which signifies oxygen nonstoichiometry as the main variable affecting ORR performance.

Dry Oxidation and Vacuum Annealing Treatments for Tuning the Wetting Properties of Carbon Nanotube Arrays

In this article, we describe a simple method to reversibly tune the wetting properties of vertically aligned carbon nanotube (CNT) arrays. Here, CNT arrays are defined as densely packed multi-walled carbon nanotubes oriented perpendicular to the growth substrate as a result of a growth process by the standard thermal chemical vapor deposition (CVD) technique.(1,2) These CNT arrays are then exposed to vacuum annealing treatment to make them more hydrophobic or to dry oxidation treatment to render them more hydrophilic. The hydrophobic CNT arrays can be turned hydrophilic by exposing them to dry oxidation treatment, while the hydrophilic CNT arrays can be turned hydrophobic by exposing them to vacuum annealing treatment. Using a combination of both treatments, CNT arrays can be repeatedly switched between hydrophilic and hydrophobic.(2) Therefore, such combination show a very high potential in many industrial and consumer applications, including drug delivery system and high power density supercapacitors.(3-5) The key to vary the wettability of CNT arrays is to control the surface concentration of oxygen adsorbates. Basically oxygen adsorbates can be introduced by exposing the CNT arrays to any oxidation treatment. Here we use dry oxidation treatments, such as oxygen plasma and UV/ozone, to functionalize the surface of CNT with oxygenated functional groups. These oxygenated functional groups allow hydrogen bond between the surface of CNT and water molecules to form, rendering the CNT hydrophilic. To turn them hydrophobic, adsorbed oxygen must be removed from the surface of CNT. Here we employ vacuum annealing treatment to induce oxygen desorption process. CNT arrays with extremely low surface concentration of oxygen adsorbates exhibit a superhydrophobic behavior.

Dry Oxidation and Vacuum Annealing Treatments for Tuning the Wetting Properties of Carbon Nanotube Arrays

In this article, we describe a simple method to reversibly tune the wetting properties of vertically aligned carbon nanotube (CNT) arrays. Here, CNT arrays are defined as densely packed multi-walled carbon nanotubes oriented perpendicular to the growth substrate as a result of a growth process by the standard thermal chemical vapor deposition (CVD) technique.1,2 These CNT arrays are then exposed to vacuum annealing treatment to make them more hydrophobic or to dry oxidation treatment to render them more hydrophilic. The hydrophobic CNT arrays can be turned hydrophilic by exposing them to dry oxidation treatment, while the hydrophilic CNT arrays can be turned hydrophobic by exposing them to vacuum annealing treatment. Using a combination of both treatments, CNT arrays can be repeatedly switched between hydrophilic and hydrophobic.2 Therefore, such combination show a very high potential in many industrial and consumer applications, including drug delivery system and high power density supercapacitors.3-5 The key to vary the wettability of CNT arrays is to control the surface concentration of oxygen adsorbates. Basically oxygen adsorbates can be introduced by exposing the CNT arrays to any oxidation treatment. Here we use dry oxidation treatments, such as oxygen plasma and UV/ozone, to functionalize the surface of CNT with oxygenated functional groups. These oxygenated functional groups allow hydrogen bond between the surface of CNT and water molecules to form, rendering the CNT hydrophilic. To turn them hydrophobic, adsorbed oxygen must be removed from the surface of CNT. Here we employ vacuum annealing treatment to induce oxygen desorption process. CNT arrays with extremely low surface concentration of oxygen adsorbates exhibit a superhydrophobic behavior.

Charge transfer and weak chemisorption of oxygen molecules in nanoporous carbonconsisting of a disordered network of nanographene sheets

The adsorption/desorption processes of oxygen are investigated innanoporous carbon (activated carbon fiber (ACF)) consisting of adisordered network of nanographene sheets. The heat-induced desorption at200 °C shows the decomposition of oxygen-including functional groups weakly bonded tonanographene edges. The removal of these oxygen-including negatively charged functionalgroups brings about a change in the type of majority carriers, from holes to electrons,through charge transfer from the functional groups to the interior of nanographene sheets.The oxygen adsorption brings ACF back to the electronic state with holes beingmajority carriers. In this process, a large concentration of negatively chargedO2δ − moleculeswith δ ∼ 0.1 are created through charge transfer from nanographene sheets to the adsorbedoxygen molecules. The changes in the thermoelectric power and the electricalresistance in the oxygen desorption process is steeper than that in the oxygenadsorption process. This suggests the irreversibility between the two processes.

Oxygen permeability and stability of asymmetric multilayer Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3 − δ ceramic membranes

Perovskite-type Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3 − δ (BSCF) is considered as one of most promising materials for the oxygen separation from air. In order to assess the impact of asymmetric architecture on oxygen transport properties and stability, dense ceramics and membranes consisting of two porous and one dense layers were fabricated and tested. The stability of Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3 − δ in atmospheres with low oxygen partial pressure is similar to that for SrCo 0.8Fe 0.2O 3 − δ . Analysis of the thickness dependence of oxygen permeation fluxes through symmetric dense membranes unambiguously showed that the permeation is limited by surface exchange kinetics at d ≤ 1 mm. The main limitations for the oxygen permeation through asymmetric membranes are associated with gas diffusion in pores and oxygen desorption process, despite a high open porosity (49%) of the porous layers. The asymmetric membranes demonstrated a stable performance if using inert sweep gas. On exposing to CO 2/H 2O containing atmospheres, the permeation decreases dramatically due to a deep decomposition process on the permeate side, resulting in the formation of carbonates and/or hydroxides causing blocking of oxygen transport.

Phase transitions in Sr 1 + xCo 0.8Fe 0.2O 3 − δ oxides

The phase transitions that take place in Sr 1 + x Co 0.8Fe 0.2O 3 − δ (− 0.2 ≤ x ≤ 0.1) oxides are reported here. Thermogravimetric analysis (TGA) showed that the oxides with − 0.2 ≤ x ≤ 0 were prone to undergo oxygen-vacancy disorder–order phase transitions, while others with x = 0.05, 0.1 had more stable crystal structures during oxygen-desorption processes in nitrogen. These results were further confirmed by high-temperature in-situ X-ray techniques. The changes in activation energies of three typical oxides, Sr 1 + x Co 0.8Fe 0.2O 3 − δ ( x = − 0.2, 0, 0.1), used as oxygen-permeable membranes were investigated. The phase transitions in Sr 1 + x Co 0.8Fe 0.2O 3 − δ ( x = − 0.2, 0) have also been detected in differential scanning calorimetry (DSC) profiles.

Kinetic Monte Carlo simulations of the interaction of oxygen with Pt(111)

The adsorption, dissociation, diffusion, and desorption of oxygen interacting with the Pt(111) surface have been studied using kinetic Monte Carlo simulations. This study has been motivated by uncertainties in the theoretical and the experimental derivations of O(2)Pt(111) reaction barriers. The simulations reproduce all known experimental data within basically one set of parameters, thus yielding microscopic insights into the elementary reaction steps occurring in the interaction of oxygen with Pt(111) and providing reliable estimates for adsorption energies and diffusion and desorption barriers. In particular, we confirm that the distance of oxygen atoms directly after dissociation is caused by ballistic hot atom motion rather than by diffusive motion. We address the equilibrium structure of oxygen atoms at high coverages. At low temperatures, chains of oxygen pairs are formed. We show that this mechanism can be explained by a lowered dissociation in the vicinity of already adsorbed atoms. Finally we discuss the role of the lateral interaction between the oxygen atoms in the oxygen desorption process.

Investigation of sputtering and oxygen desorption processes by binary collision approximation method

The peculiarities of the formation of the primary knock-on recoil (PKR) atoms and oxygen desorption processes under conditions of low-energy grazing ion bombardment of clean and O 2 covered Ag(1 1 0) surface have been investigated by computer simulation. The present computer code for a calculation of the ion and recoil trajectories is based on the binary collision approximation (BCA). The energy and angular distributions of particles sputtered and desorbed from the surface, as well as their yields have been calculated and studied in detail. It was shown that the main contribution to sputtering comes from first layer of the surface. In the case of grazing angles of incidence the energy of PKR is sufficiently low (1–10% of energy of incident ion). It was shown that due to movement of incident ion in surface channels formed by adsorbed molecules and atoms of surface layers the intensive non-dissociative desorption of adsorbed layer is observed. At low-energy grazing ion bombardment the adsorbed molecules are basically desorbed as molecule, the probability of dissociative desorption increases with increasing the initial energy.

Computer simulation of sputtering and oxygen desorption processes at grazing ion bombardment of Ag([formula omitted]) surface

The ion sputtering and oxygen desorption processes at 5 keV Ne ion bombardment of clean and O2 covered Ag(110) surface have been investigated by computer simulation. The trajectories of the colliding particles were constructed as a sequence of binary collisions. The influence of surface potential barrier on the non-dissociative desorption yield has been studied and it was observed that the basic part of ion-impact desorbed particles overcomes a potential barrier. The sputtering yield of Ag atoms and the non-dissociative O2 desorption yield versus the azimuth angle of incidence at grazing ion bombardment have been presented. It was shown that these dependencies correlate the orientation of crystal. At grazing ion bombardment the particles are ejected on azimuth angle mainly along the normal of plane of incidence.

Oxygen desorption process in CdS thin films studied by thermally stimulated current measurements

Thermally stimulated current (TSC) studies on CdS thin films, grown by the chemical bath deposition (CBD) technique as a function of several thermal cycles in vacuum are reported for the first time. The result can be explained as an oxygen desorption process, and the TSC spectrum due to this process is a sensitive technique for the characterization of trapping kinetics on the surface of the as-deposited films. Trap parameters, such as activation energy and capture cross section have been determined.

Structural changes of TI oxide system on non-isothermal thermogravimetry with slow heating rates

The oxygen desorption processes of Ta 2Ba 2CuO 6 and Ti 0.5Pb 0.5Sr 2CuO 5 with double and single TI layer structures, respectively were studied by non-isothermal thermagravimetry (TG) with slow brating rates of 10–30° /h in Ar and O 2 streams. The structural changes and the valence changes of the metals caused by the oxygen desorption were examined. The TG curve of TI 2Ba 2CuO 6 gave an inflection point around at 600 °C, indicating that the oxygen desorption occurred separately from decomposition. In this course, the tetragonal unit was kept and the cell dimension a increased. TI 0.5P 0.5Sr 2CuO 5 had no labile oxygen which is released separately from decomposition and can changed a .

The Dynamic Behavior of the Electrical Conductivity of CAD-PbSe Films

PbSe films prepared by chemical “anorganic” deposition (CAD) are investigated. The electrical resistivity (R) and the thermoelectric power (tep) are measured in a thermal dynamic regime which disturbs the equilibrium of the PbSe-oxygen system. A comparison with the behavior of PbSe films prepared by the usual chemical “organic” deposition (COD) and vacuum evaporation (VE) procedures is made. Peaks in the electrical resistivity around 170 °C (for some COD and the CAD films) and around 225 °C (for some COD and the VE films) are found, due to the super-position of the oxygen desorption process on the normal R-T dependence. The change in the sign of tep occurs before the peak in resistivity appears. These effects are discussed in terms of the “double layer” model. Based on the results from thermogravimetric and specific surface measure-ments and on oxygen isotherms for the powdered COD and CAD materials, the influence of the surface and of the bulk diffusion on the electrical properties is analysed.