Isotope Exchange Method Research Articles

Hydride Bed Isotopic Exchange

Savannah River Tritium Enterprise (SRTE) has used LaNi4.25Al0.75 (LANA75) hydride beds to store hydrogen isotopes for over two decades. A benefit of using LANA75 is that the 3He generated from tritium decay is retained in the hydride material, allowing the hydride beds to deliver high-purity product gas. A disadvantage is that the 3He accumulates in the LANA75 material over time, which forms a heel that cannot be removed under normal operating conditions. The heel traps hydrogen in the bed, slowly reducing the operational capacity of the bed as the heel grows. Eventually, the 3He begins to release from the material, preventing the delivery of high-purity product.The hydride beds are replaced when (1) operational capacity is reduced such that it is impactive to routine operations and/or (2) product purity is not maintained due to 3He release. Prior to replacing and disposing of the beds, it is necessary to isotopically exchange the gas on the bed to recover as much tritium as possible. Isotopic exchange involves repeatedly absorbing deuterium onto the bed and desorbing hydrogen isotopes from the bed until a predetermined criterion has been met. The isotopic exchange process represents a significant additional load on routine operations both in time and in the amount of waste gas that requires further processing.A set of beds was recently prepared for replacement. The isotopic exchange method used by SRTE is presented, along with results of the most recent isotopic exchange. Lessons learned during the recent isotopic exchange process led to modifications that reduce isotopic exchange duration and corresponding waste gas produced while increasing the amount of tritium recovered.

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.

Oxygen transport in Pr nickelates: Elucidation of atomic-scale features

Pr2NiO4+δ oxide with a layered Ruddlesden–Popper structure is a promising material for SOFC cathodes and oxygen separation membranes due to a high oxygen mobility provided by the cooperative mechanism of oxygen migration involving both interstitial oxygen species and apical oxygen of the NiO6 octahedra. Doping by Ca improves thermodynamic stability and increases electronic conductivity of Pr2−xCaxNiO4+δ, but decreases oxygen mobility due to decreasing the oxygen excess and appearing of 1–2 additional slow diffusion channels at x ≥ 0.4, probably, due to hampering of cooperative mechanism of migration. However, atomic-scale features of these materials determining oxygen migration require further studies. In this work characteristics of oxygen diffusion in Pr2−xCaxNiO4+δ (x = 0–0.6) are compared with results of the surface analysis by X-ray photoelectron spectroscopy and modeling of the interstitial oxygen migration by the plane-wave density functional theory calculations. According to the X-ray photoelectron spectroscopy data, the surface is enriched by Pr for undoped sample and by Ca for doped ones. The O1s peak at ~531 eV corresponding to a weakly bound form of surface oxygen located at Pr cations disappears at ~500 °C. Migration of interstitial oxygen was modeled for a I4/mmm phase of Pr2NiO4+δ. The interstitial oxygen anion repulses the apical one in the NiO6 octahedra pushing it into the tetrahedral site between Pr cations. The calculated activation barrier of this migration is equal to 0.585 eV, which reasonably agrees with the experimental value of 0.83 eV obtained by the oxygen isotope exchange method. At the same time, for the model compound Ca2NiO4+δ, obtained by isomorphic substitution of Pr by Ca in Pr2NiO4+δ, calculations implied formation of the peroxide ion comprised of interstitial and lattice oxygen species not revealed in the case of incomplete substitution (up to PrCaNiO4+δ composition).Hence, calculations in the framework of the plane-wave density functional theory provide a realistic estimation of specificity of oxygen migration features in Pr2NiO4+δ doped by alkaline-earth metals.

An overlooked pool of hydrogen stored in humic matter revealed by isotopic exchange: implication for radioactive 3H contamination

Tritium (3H), the radioactive isotope of hydrogen, is a contaminant occurring in most nuclear liquid releases from nuclear industries. The behavior of tritium in the environment is an ongoing societal concern, notably the storage of tritium in organic compounds in waters, soils and sediments. In particular, the fate of tritium depends on the availability of hydrogen for isotopic exchanges in natural organic matter, especially in lignin-like molecules. Therefore, we studied here H exchangeability using natural lignite and leonardite humic matter samples. We compared results from gas–solid isotopic exchanges (αiso) with those from widely used methods of functional group characterization by means of deprotonation (αdepro = αcarboxyl +αphenol +αalcohol). Since standard methods for titrating total acidity and determining protonated carboxyl groups are limited by the unconstrained slow deprotonation of humic material, we have modified methods to maximize exchangeable hydrogen reactivity by extending the reaction time in reagent solutions out to 3 weeks, while total OH groups are investigated by the standard acetylation method for alcohol group determination. Results show that more than two times less hydrogen remains available for deprotonation than for isotopic exchanges with an average of αiso = 35.9% versus αdepro = 15.5%. This finding reveals the existence of a neglected pool of hydrogen likely to play a key role in processes based on hydrogen exchange, such as metal complexation by natural ligands. Results also show the reliability of the isotopic exchange method for hydrogen speciation in soil-type matrices.

Highlights of C (sp2 )-H hydrogen isotope exchange reactions.

The highlights of C (sp2 )-H hydrogen isotope exchange (HIE) methods developed over the past 10 years are summarized in this review. Major developments include improved Ir(I) catalysts with greater functional group and solvent compatibility and the development of novel base metal catalysts for HIE. In addition, a number of novel Ru-based catalysts have been developed with promising activity. In the area of Pt- and Pd-catalysed exchange, in addition to new advances on heterogeneous Pt- and Pd-catalysed HIE by Sajiki and Shevchenko, a number of groups have reported on homogenous catalysts of Pt and Pd that show an interesting activity and selectivity.

Oxygen isotope exchange in proton-conducting oxides based on lanthanum scandates

A method of the oxygen isotope exchange with equilibration of the gas phase was used in order to obtain the temperature dependences of the oxygen surface exchange and diffusion coefficients of the proton-conducting oxides La1–xSrxScO3–δ (x = 0; 0.04; 0.09) in the temperature range of 600–900 °C and oxygen pressure of 1.01 kPa. The diffusion and oxygen surface exchange coefficients increase with the strontium content in oxides. The rates of the individual stages of the oxygen exchange process on the surface of the oxides were determined. The oxygen incorporation was revealed to be rate-determining for the undoped LaScO3 oxide. However, for the strontium-doped oxides La1–xSrxScO3–δ (x = 0.04; 0.09), the difference between the rates of oxygen dissociative adsorption and oxygen incorporation decreases with increasing the strontium concentration, in a way that for the La0.91Sr0.09ScO3−δ oxide these stages become competing. The possible reasons for these differences in oxygen surface exchange kinetics were analyzed in this paper. Using the obtained oxygen diffusion coefficients, the oxygen-ionic conductivities in accordance with the Nernst-Einstein equation, and the contributions of the oxygen-ion and proton components of the total conductivity of the La1–xSrxScO3–δ (x = 0.04; 0.09) oxides in the wet reducing atmosphere (pH2O = 2.35 kPa, pO2 = 10−15 Pa) were calculated. The proton transference numbers were found to be close to unit at the wet reducing atmospheres in the temperature range of 500–600°С.

Role of Lattice Oxygen in the Oxygen Evolution Reaction on Co3O4: Isotope Exchange Determined Using a Small-Volume Differential Electrochemical Mass Spectrometry Cell Design

This work demonstrates the roleof lattice oxygen of metal oxide catalysts in the oxygen evolution reaction (OER) as evidenced by isotope labeling together with the differential electrochemical mass spectrometry (DEMS) method. Our recent report assessed this role for Co3O4 using a flow-through DEMS cell, which requires a large volume of electrolyte. Herein, we extend this procedure to different Co3O4 catalyst loadings and particle sizes as well as the mixed Ag + Co3O4 catalyst. We introduce, for the first time, a novel small-volume DEMS cell design capable of using disc electrodes and only <0.5 mL of electrolyte. The reliability of the cell is demonstrated by monitoring gas evolution during OER in real time. This cell shows high sensitivity, high collection efficiency, and very short delay time. DEMS results reveal that only the interfacial part (∼0.2% of the total loading or 25% of surface atoms) of the catalyst is active for OER. Interestingly, the amount of oxygen exchanged on the mixed Ag + Co3O4 catalyst is higher than that on the single Co3O4 catalyst, which illustrates the improved electrocatalytic activity previously reported on this mixed catalyst. Furthermore, the real surface area of the catalysts is estimated using different methods (namely, the ball model, double layer capacitance, isotope exchange, and redox peak methods). The surface areas estimated from the Brunauer-Emmett-Teller (BET) and ball models are comparable but roughly three times higher than that of the redox peak method. Our method represents an alternative approach for probing the mechanism and real surface area of catalysts.

Interaction of O2, H2O and H2 with proton-conducting oxides based on lanthanum scandates

An interaction of components from the gas phase, containing gaseous oxygen, hydrogen and water, with the La1–xSrxScO3–α oxides in the temperature range of 300–950 °С and partial pressure of 6.1–24.3 kPa 8.1–50.7 kPa has been studied using a high temperature thermogravimetric analysis. The effects of partial pressure of the gaseous oxygen, water and hydrogen on the apparent uptake level of protons with oxides has been found. The studies of incorporation processes of hydrogen from molecular hydrogen atmosphere into the structure of proton-conducting oxides based on strontium-doped lanthanum scandates were performed in the temperature range of 300–800 °C and hydrogen pressure of 0.2 kPa by means of the isotope exchange method with the equilibration of isotope composition in the gas phase. The protons and deuterons concentrations were determined for the La0.91Sr0.09ScO3–α oxide. The existence of proton defects in the structure of studied oxides after the exposure in H2O and H2-containing atmospheres was revealed using the 1Н NMR. The role of oxygen vacancies in the proton incorporation processes is considered in the present work.

Highlights of aliphatic C(sp3 )-H hydrogen isotope exchange reactions.

This review summarizes the highlights of aliphatic C (sp3 )-H carbon hydrogen isotope exchange (HIE) methods developed in the last 10 years. In particular, new highly selective and reactive protocols in the areas of nanoparticle and metal-catalyzed homogeneous catalysis are reported.

Detailed description of pulse isotopic exchange method for analyzing oxygen surface exchange behavior on oxide ion conductors

A novel pulse 18O-16O isotopic exchange (PIE) technique for measurement of the rate of oxygen surface exchange of oxide ion conductors was presented. The technique employs a continuous flow packed-bed micro-reactor loaded with the oxide powder. The isothermal response to an 18O-enriched pulse passing through the reactor, thereby maintaining chemical equilibrium, is measured by on-line mass spectrometry. Evaluation of the apparent exchange rate follows from the uptake of 18O by the oxide at given reactor residence time and surface area available for exchange. The developed PIE technique is rapid, simple and highly suitable for screening and systematic studies. No rapid heating/quenching steps are required to facilitate 18O tracer anneal or analysis, as in other commonly used techniques based upon oxygen isotopic exchange. Moreover, the relative distribution of the oxygen isotopologues 18O2, 16O18O, and 16O2 in the effluent pulse provides insight into the mechanism of the oxygen exchange reaction. The PIE technique has been demonstrated by measuring the exchange rate of selected oxides with enhanced oxide ionic conductivity in the range of 350-900 ° C. Analysis of the experimental data in terms of a model with two consecutive, lumped steps for the isotopic exchange reaction shows that for mixed conductors Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) and La2NiO4+δ the reaction is limited by the apparent rate of dissociative adsorption of O2 molecules at the oxide surface. For yttria-stabilized zirconia (YSZ), a change-over takes place, from rate-limitations by oxygen incorporation below ~800 ° C to rate-limitations by O2 dissociative adsorption above this temperature. Good agreement is obtained with exchange rates reported for these materials in literature.

Interaction of H2O, O2 and H2 with Proton Conducting Oxides Based on Lanthanum Scandates

The research uses the method of high-temperature thermogravimetric analysis to study the processes of interaction of the gas phase in the temperature range 300–950 °C in the partial pressure ranges of oxygen 8.1–50.7 kPa, water 6.1–24.3 kPa and hydrogen 4.1 kPa with La1–xSrxScO3–α oxides (x = 0; 0.04; 0.09). In the case of an increase in the partial pressure of water vapor at a constant partial pressure of oxygen (or hydrogen) in the gas phase, the apparent level of saturation of protons is shown to increase. An increase in the apparent level of saturation of protons of the sample also occurs with an increase in the partial pressure of oxygen at a constant partial pressure of water vapor in the gas phase. The paper discusses the causes of the observed processes. The research uses the hydrogen isotope exchange method with the equilibration of the isotope composition of the gas phase to study the incorporation of hydrogen into the structure of proton-conducting oxides based on strontium-doped lanthanum scandates. The concentrations of protons and deuterons were determined in the temperature range of 300–800 °C and a hydrogen pressure of 0.2 kPa for La0.91Sr0.09ScO3–α oxide. The paper discusses the role of oxygen vacancies in the process of incorporation of protons and deuterons from the atmosphere of molecular hydrogen into the structure of the proton conducting oxides La1–xSrxScO3–α (x = 0; 0.04; 0.09). The proton magnetic resonance method was used to study the local structure in the temperature range 23–110 °C at a rotation speed of 10 kHz (MAS) for La0.96Sr0.04ScO3–α oxide after thermogravimetric measurements in an atmosphere containing water vapor, and after exposures in molecular hydrogen atmosphere. The existence of proton defects incorporated into the volume of the investigated proton oxide from both the atmosphere containing water and the atmosphere containing molecular hydrogen is unambiguously shown. The paper considers the effect of the contributions of the volume and surface of La0.96Sr0.04ScO3–α oxide on the shape of the proton magnetic resonance spectra.

Oxygen isotope exchange with La0.6Sr0.4Co1−yFeyO3−δ (y = 0.0, 0.2) oxides

Oxygen surface exchange kinetics of La0.6Sr0.4Co1−yFeyO3−δ (y = 0.0, 0.2) have been studied by the isotope exchange method with gas phase equilibration using a static circulation experimental rig in the oxygen pressure range of 0.13–8.53 kPa and temperature of 600–800 °C. The values of the oxygen diffusion coefficient and the rates of the elementary stages have been calculated. Doping of iron into the La0.6Sr0.4Co1−yFeyO3-δ has been shown to reduce the heterogeneous exchange rate, the oxygen diffusion coefficient and the rates of the elementary stages. Probable reasons for such behavior are discussed.

Oxygen Isotope Exchange in Proton-Conducting Oxides Based on Lanthanum Scandates

The method of oxygen isotope exchange with the gas phase equilibration have been used to obtain the temperature dependences of the oxygen surface exchange and diffusion coefficients with proton-conducting oxides La1–xSrxScO3–δ (x = 0; 0.04; 0.09) in the temperature range of 600−900°C at oxygen pressure 1.01 kPa. The paper determines that the diffusion and oxygen surface exchange coefficients increase with the increasing of the strontium content in the oxides. We have found out the rates of the individual stages of the oxygen exchange process on the surface of the oxides. It is shown that oxygen incorporation is rate-determining stage of the oxygen exchange on the surface of the undoped oxide, whereas for the strontium-doped oxides La1–xSrxScO3–δ (x = 0; 0.04; 0.09) with increasing of strontium concentration, the difference between the rates of dissociative adsorption and oxygen incorporation decreases so that for the oxide La0,91Sr0,09ScO3–δ the stage of dissociative adsorption of oxygen becomes rate-determining stage. The paper analyzes the possible reasons of these differences in oxygen surface exchange kinetics. Moreover, the paper using the obtained oxygen diffusion coefficients that have been recalculated in the oxygen-ionic conductivities according to the Nernst-Einstein equation performs the contributions of the oxygen-ion and proton components of the total conductivity of oxides La1–xSrxScO3–δ (x = 0; 0.04; 0.09) in the wet reducing atmosphere (pH2O = 2.35 kPa, pO2 = 10−15 Pa). Proton transference numbers are shown to be close to unit in the temperature range of 500–600 °С at the wet hydrogen-containing reducing atmosphere.

Oxygen isotope exchange in doped lanthanum zirconates

The interaction between oxygen in the gas phase and La2–xCaxZr2O7–α (x = 0; 0.05; 0.1) with a pyrochlore structure was studied by the isotope exchange method with the gas phase equilibration in the temperature range of 600–900 °C and the oxygen pressure of 1.01 kPa. The oxygen diffusion and oxygen surface exchange coefficients were calculated for the investigated oxides. The rate-determining stage of the oxygen surface exchange kinetics for La1.9Ca0.1Zr2O7–α was found to be different from that for La1.95Ca0.05Zr2O7–α and La2Zr2O7–α. The influence of the calcium content in La2–xCaxZr2O7–α (x = 0; 0.05; 0.1) oxides on the heterogeneous exchange rates and oxygen tracer diffusion coefficients were discussed.

Isotope Purification of Gases Containing Deuterium and Tritium by the Method of the Phase Isotopic Exchange of Water

The study of the method of the phase isotopic exchange (PIE) for the purification of gas flows containing deuterium and tritium is carried out. The experimental data obtained for a column with a diameter of 32 mm on a Sulzer copper oxidized packing showed that the mass transfer characteristics do not depend on the deuterium concentration. Based on these results, a mathematical model for separating three-isotope mixtures, which makes it possible to calculate the column parameters under different initial conditions, is developed.

Effect of AO Segregation on Catalytical Activity of La0.7A0.3MnO3±δ (A = Ca, Sr, Ba) Regarding Oxygen Reduction Reaction

Oxygen surface exchange kinetics of La0.7A0.3MnO3±δ has been studied by the isotope exchange method with gas phase equilibration using a static circulation experimental rig at the temperature of 850 °C and oxygen pressure of 1 kPa. Oxides of La0.7A0.3MnO3±δ with different dopant at one level of doping were considered. Rates of elementary acts of the oxygen exchange were found to change differently. So, the oxygen dissociative adsorption rate increased, the oxygen incorporation rate decreased, and the increase in the cation radius did not influence the interphase exchange rate. These trends are associated with the AO segregation processes at the surface and the appropriate mechanism of oxygen reduction is considered. The oxygen dissociative adsorption rate increased, the incorporation rate decreased, and the increase in the cation radius did not influence the interphase exchange rate. These trends are associated with the AO segregation processes at the surface.

Structural properties and thermal decomposition kinetics of beryllium deuteride by isotope exchange: Compared to beryllium hydride

The structural properties and thermal decomposition kinetics of beryllium deuteride (BeD2) synthesized from isotope exchange method were investigated compared to beryllium hydride (BeH2) using infrared spectroscopy (IR) and thermogravimetric analysis (TG) techniques. The results indicate that infrared characteristic bands of BeD2 shift to lower frequencies compared to corresponding bands of BeH2; Infrared absorption intensities of BeD2 increase with temperature increasing; BeD2 possesses relatively higher activity energy and much larger pre exponential factor compared to that of BeH2, which leads to macroscopic less thermal stability.

Surface defect chemistry and oxygen exchange kinetics in La2–xCaxNiO4+δ

Surface oxygen exchange kinetics and diffusion in La2–xCaxNiO4+δ (x = 0; 0.1; 0.3) have been studied by the isotope exchange method with gas phase equilibration in the temperature range of 600–800 °C and oxygen pressure range 0.13–2.5 kPa. Despite an enhanced electrical conductivity of La2–xCaxNiO4+δ theirs oxygen surface exchange (k*) and oxygen tracer diffusion (D*) coefficients were significantly lower in comparison with La2NiO4+δ. The rates of the elementary stages of oxygen exchange have been calculated. Upon Ca doping the change of the rate-determining stage was observed. The surface of the oxides was found to be inhomogeneous towards oxygen exchange process according to the recently developed model. The reasons of such inhomogeneity are discussed as well as Ca influence on the surface defect chemistry and oxygen surface exchange and diffusivity.

Oxygen isotope exchange in praseodymium nickelate

Oxygen surface exchange kinetics and diffusion were studied in Pr2NiO4 + δ (PNO) by the isotope exchange method with gas phase equilibration in the temperature range of 600–800 °C and oxygen pressure range of 0.33–1.62 kPa. The oxygen heterogeneous exchange rate (rH), oxygen diffusion coefficient (D), rates of oxygen dissociative adsorption (ra), and oxygen incorporation (ri) were calculated along with the apparent activation energies of oxygen surface exchange and diffusion processes. The temperature dependence of rH was found to benon-linear in Arrhenius coordinates. The apparent activation energy changed from 1.4 ± 0.2 eV at T > 700 °C to 2.0 ± 0.1 eV. This might be attributed to the change in the rate-determining stage of oxygen exchange for Pr2NiO4 + δ at T ~ 700 °C, because of a shift in the ratio between ra and ri caused by the difference in their activation energies. Possible reasons for the observed changes in the rate-determining stage are discussed.

Degradation kinetics of LSM–YSZ cathode materials for SOFC

Solid oxide fuel cells are perspective electrochemical devices for hydrogen energy applications and require long time performance. The paper studies the influence of the processes of particles coarsening and cation interdiffusion on the degradation kinetics of the LSM–YSZ cathode material catalytic activity as related to the interaction processes with oxygen from the gas phase. The exchange kinetics with the gas phase oxygen was studied by the oxygen isotope exchange method with the gas phase equilibration at the temperature of 850 °С and Po2 = 10−2 atm. The samples were exposed for 1000 h. The coefficient of gas phase and surface oxygen exchange was found to decrease at the exposure. The paper discusses the physical reasons, which lead to the oxygen surface exchange coefficient decrease in LSM–YSZ electrodes and degradation of the polarization conductivity measured by electrochemical impedance spectroscopy.