Isotope Exchange Technique Research Articles

Dissociative Oxygen Adsorption and Incorporation in Co3O4-Dispersed BaZr0.9Sc0.1O2.95 for PCFC Cathode.

The development of air electrodes with superior surface oxygen exchange properties at intermediate temperatures is crucial for improving the efficiency of protonic ceramic fuel cells. This study evaluated the surface exchange properties of Co3O4 dispersed protonic conductors, BaZr0.9Sc0.1O2.95. Although Co3O4 is widely acknowledged as superior dissociative adsorption catalysts, there is still ambiguity regarding the enhancement mechanisms of their surface exchange properties by Co3O4, as well as their optimal composition to achieve high catalytic activity. To overcome these difficulties, this study elucidated the effect of the chemical states and composition of composites on their surface exchange properties by evaluating their chemical states and surface exchange reaction rates with several compositions prepared at different temperature conditions using a vibrating-sample magnetometer and the pulse isotope exchange technique. For samples annealed at a high temperature, it became evident that the surface exchange activity became the most active by adding only 1 vol % Co3O4 and indicated an abrupt decline above this composition despite an increase in the volume of the catalysts. This was attributed to the combined effect of the high dissociative adsorption activity of the Co-containing solid solutions formed at a high temperature and a decrease in oxygen vacancies due to hole compensation. For samples annealed at intermediate temperature, their chemical states remained unchanged from those of the original milled powders, and their surface exchange properties monotonically improved with an increase in the volume of Co3O4. Based on the results, different chemical states of composites derived from different preparation conditions lead to completely different activation behavior of the surface exchange reaction.

Effect of light-induced changes in leaf anatomy on intercellular and cellular components of mesophyll resistance for CO2 in Fagus sylvatica.

Mesophyll resistance for CO2 diffusion (rm) is one of the main limitations for photosynthesis and plant growth. Breeding new varieties with lower rm requires knowledge of its distinct components. We tested new method for estimating the relative drawdowns of CO2 concentration (c) across hypostomatous leaves of Fagus sylvatica. This technique yields values of the ratio of the internal CO2 concentrations at the adaxial and abaxial leaf side, cd/cb, the drawdown in the intercellular air space (IAS), and intracellular drawdown between IAS and chloroplast stroma, cc/cbd. The method is based on carbon isotope composition of leaf dry matter and epicuticular wax isolated from upper and lower leaf sides. We investigated leaves from tree-canopy profile to analyse the effects of light and leaf anatomy on the drawdowns and partitioning of rm into its inter- (rIAS) and intracellular (rliq) components. Validity of the new method was tested by independent measurements of rm using conventional isotopic and gas exchange techniques. 73% of investigated leaves had adaxial epicuticular wax enriched in 13C compared to abaxial wax (by 0.50‰ on average), yielding 0.98 and 0.70 for average of cd/cb and cc/cbd, respectively. The rIAS to rliq proportion were 5.5:94.5% in sun-exposed and 14.8:85.2% in shaded leaves. cc dropped to less than half of the atmospheric value in the sunlit and to about two-thirds of it in shaded leaves. This method shows that rIAS is minor but not negligible part of rm and reflects leaf anatomy traits, i.e. leaf mass per area and thickness.

Isotope exchange of ND3 on Pt catalyst-loaded 13X molecular sieve

During D-T fusion operations the capture, purification, and recycling of unburned tritium will be crucial, as the formation of tritium containing molecules require additional processing. Removing the tritium can require costly processing to be unbound from the tritium-containing molecules and improvements to these processes will be necessary moving forward. Existing techniques using sorbent material beds to remove tritium-containing molecules from process gas streams undergo repeated high-heat cycling which leads to diminished bed lifespans, necessitating replacement and associated downtime. This work demonstrates the capture and isotopic exchange technique of deuterated ammonia (ND3), used as a surrogate for tritium, at ambient temperature using a Pt catalyst-loaded 13X molecular sieve. Unmodified 13X molecular sieve is capable of adsorbing and retaining the ND3 however, incorporation of a catalyst facilitates the isotopic exchange of the hydrogen isotopes. The effluent gas streams were analyzed in conjunction with desorbed ammonia isotopologues post-exchange to verify these results. Isotopically exchanging and removing heavier hydrogen isotopes using this technique provides an alternative to traditional removal methods.

Comprehensive study of O2 and H2O interaction with La0.9Sr0.1ScO3–δ oxide

In this paper we study the processes of O2 and H2O interaction with La0.9Sr0.1ScO3−δ. Using high temperature neutron powder diffraction (HT-NPD), the evolution of crystal structure and water uptake was investigated in the temperature range 25–700 °C at 21.3 kPa in O2 and O2 + D2O atmospheres. The results of HT-NPD show the influence of the water uptake by La0.9Sr0.1ScO3–δ bulk on the thermal expansion of the material. The kinetics of oxygen surface exchange was studied using pulsed isotope exchange technique. The measurements were carried out in the temperature range 300−900 °C at pO2 = 5.1, 12.2 and 21.3 kPa in dry and humid (pH2O = 2.6 kPa) conditions. For the first time the physical model describing the kinetics of oxygen surface exchange between the mixture of oxygen and water in the gas phase and a solid oxide has been suggested. The model includes following steps: oxygen dissociative adsorption, oxygen incorporation, hydrogen surface diffusion, water adsorption and dissociation. The rate-determining step of the exchange is considered. The influence of O2 humidification on the oxygen surface exchange is discussed.

Consistent interpretation of isotope and chemical oxygen exchange relaxation kinetics in SrFe0.85Mo0.15O3-δ ferrite.

This paper is devoted to the study of phase composition and kinetic and thermodynamic characteristics of Mo-doped strontium ferrite SrFe0.85Mo0.15O3-δ (SFM15) under oxygen-conducting membrane working conditions. Single-phase SFM15 with a cubic Pm3̄m structure was synthesized using a ceramic method. It was shown that the molybdenum introduction stabilizes the perovskite cubic structure over a wide range of oxygen pressures and temperatures, preventing the bulk phase transition at high temperatures. Oxygen exchange constants, diffusion coefficients and activation energy of oxygen exchange were obtained using oxygen relaxation and isotopic exchange techniques, and the obtained values are consistent with known literature data. It was shown that the surface reaction rates obtained using chemical and tracer relaxation methods are quantitatively comparable with each other, despite significantly different experimental conditions. This result not only confirms the reliability of the data obtained by independent methods, but also allows one to expand the area of physical conditions for studying the kinetics of oxygen transfer where another method has technical or methodological limitations.

Design of Mixed Ionic-Electronic Materials for Permselective Membranes and Solid Oxide Fuel Cells Based on Their Oxygen and Hydrogen Mobility.

Oxygen and hydrogen mobility are among the important characteristics for the operation of solid oxide fuel cells, permselective membranes and many other electrochemical devices. This, along with other characteristics, enables a high-power density in solid oxide fuel cells due to reducing the electrolyte resistance and enabling the electrode processes to not be limited by the electrode-electrolyte-gas phase triple-phase boundary, as well as providing high oxygen or hydrogen permeation fluxes for membranes due to a high ambipolar conductivity. This work focuses on the oxygen and hydrogen diffusion of mixed ionic (oxide ionic or/and protonic)-electronic conducting materials for these devices, and its role in their performance. The main laws of bulk diffusion and surface exchange are highlighted. Isotope exchange techniques allow us to study these processes in detail. Ionic transport properties of conventional and state-of-the-art materials including perovskites, Ruddlesden-Popper phases, fluorites, pyrochlores, composites, etc., are reviewed.

Assessing the Lability and Environmental Mobility of Organically Bound Copper by Stable Isotope Dilution.

The environmental mobility of Cu and therefore its potential toxicity are closely linked to its attachment to natural organic matter (NOM). Geochemical models assume full lability of metals bound to NOM, especially under strong oxidizing conditions, which often leads to an overestimation of the lability of soil metals. Stable isotope dilution (SID) has been successfully applied to estimate the labile (isotopically exchangeable) pool of soil metals. However, its application to study the lability of NOM-Cu required development of a robust separation and detection approach so that free Cu ions can be discriminated from (the also soluble) NOM-Cu. We developed a SID protocol (with enriched 65Cu) to quantify the labile pool of NOM-Cu using size exclusion chromatography coupled to a UV detector (for the identification of different NOM molecular weights) and ICP-MS (for 65Cu/63Cu ratio measurement). The Cu isotopic-exchange technique was first characterized and verified using standard NOM (SR-NOM) before applying the developed technique to an "organic-rich" podzol soil extract. The developed protocol indicated that, in contrast to the common knowledge, significant proportions of SR-NOM-Cu (25%) and soil organic-Cu (55%) were not labile, i.e., permanently locked into inaccessible organic structures. These findings need to be considered in defining Cu interactions with the reactive pool of NOM using geochemical models and risk evaluation protocols in which complexed Cu has always been implicitly assumed to be fully labile and exchangeable with free Cu ions.

Oxygen Isotope Exchange Reaction for Untargeted LC-MS Analysis.

LC-MS is a key technique for the identification of small molecules in complex samples. Accurate mass, retention time, and fragmentation spectra from LC-MS experiments are compared to reference values for pure chemical standards. However, this information is often unavailable or insufficient, leading to an assignment to a list of candidates instead of a single hit; therefore, additional features are desired to filter candidates. One such promising feature is the number of specific functional groups of a molecule that can be counted via derivatization or isotope-exchange techniques. Hydrogen/deuterium exchange (HDX) is the most widespread implementation of isotope exchange for mass spectrometry, while oxygen 16O/18O exchange is not applied as frequently as HDX. Nevertheless, it is known that some functional groups may be selectively exchanged in 18O enriched media. Here, we propose an implementation of 16O/18O isotope exchange to highlight various functional groups. We evaluated the possibility of using the number of exchanged oxygen atoms as a descriptor to filter database candidates in untargeted LC-MS-based workflows. It was shown that 16O/18O exchange provides 62% (median, n = 45) search space reduction for a panel of drug molecules. Additionally, it was demonstrated that studying the fragmentation spectra after 16O/18O can aid in eliminating false positives and, in some cases, help to annotate fragments formed with water traces in the collisional cell.

ZSM-5 core–shell structured catalyst for enhancing low-temperature NH3-SCR efficiency and poisoning resistance

By constructing core–shell ZSM-5@CeO2 support rather than bulk phase for loading copper, the enhanced NH3-SCR performance as well as H2O and SO2 tolerance was achieved. Cu/(ZSM-5@CeO2) catalyst exhibited the superior activity with the lowest T90 at 215 °C, which is due to the interaction between CeO2 shell and copper ions that promotes the redox properties as evidenced by oxygen isotopic exchange technique. The ZSM-5 core provides more acid sites to improve the ammonia adsorption. Thus, the highest activity of Cu/(ZSM-5@CeO2) catalyst can be ascribed to the synergistic effect of redox ability and acidity. The CeO2 shell can react with SO2 preferentially and lead to a high sulfur tolerance. This study provides a strategy for design and the practical applications of NH3-SCR zeolite catalysts.

Carburization Kinetics of Solid Iron at 1523 K via the CO Gas Isotope Exchange Technique

Carburization Kinetics of Solid Iron at 1523 K via the CO Gas Isotope Exchange Technique

Determination of Oxide Ion Conductivity in Ba-Doped LaYbO3 Proton-Conducting Perovskites via an Oxygen Isotope Exchange Method

Proton-conducting oxides have garnered much attention as electrolyte materials of electrochemical devices operating at intermediate temperatures ranging 400–700 °C. For the implementation, quantifying the transport number of oxygen ions is crucial to predict the fuel utilization efficiency of solid oxide fuel cells (SOFCs). The acceptor-doped LaYbO3 proton conductors are one of the candidates for the electrolyte material of electrochemical devices such as SOFCs because of their high pure proton conduction. However, the oxide ion conductivity in this material is not quantified yet. Here, we evaluate the tracer diffusion coefficient (D*) of 5 mol % Ba-doped LaYbO3 by the oxygen isotope exchange and depth profile technique using secondary ion mass spectrometry. It is found that the oxygen ion conductivities in this material are lower than 10–4 S cm–1 below 800 °C, less than 2% to the total conductivity. We also visualize the domain of dominant conduction in this material. It reveals that proton conduction is dominant in fairly wide areas of the atmospheric condition, in particular below 600 °C, suggesting that the Ba-doped LaYbO3 is suitable for the SOFC electrolyte in terms of transport number.

Low-Temperature Operation of CeO2-ZrO2-Based Oxygen Storage Materials

CeO2-ZrO2-based oxides are widely used in automobiles to remove harmful components in exhaust gas. Ce0.5Zr0.5O2 (CZ55) shows an excellent oxygen storage capacity (OSC); however, it is difficult to keep its OSC at low temperatures such as 400 °C. This is because of limited oxygen transport in this temperature range. Our previous research revealed that the addition of CoFe2O4 (CFO) was effective to improve its OSC at low temperature. The purpose of this study is to clarify the role of CFO on the phases present and OSC properties including surface kinetics of CZ55. CFO-added CZ55 was prepared by Pechini and solid-state reaction techniques. OSC was evaluated by thermogravimetric analysis (TGA) at 400°C in 5%H2-Ar and synthetic air. Surface exchange reaction rates were analyzed by a pulse isotope exchange (PIE) technique. CFO was found to be soluble in CZ55 up to 5 vol%. The 5 vol%CFO-added CZ55 showed an excellent OSC at 400 °C. The activation energy of surface exchange kinetics was also affected by CFO addition.

Isotope Exchange Measurements of the Interfacial Reaction Rate Constant of Nitrogen on Fe-Mn alloys and an Advanced High-Strength Steel

Precise control of dissolved nitrogen is required for producing Third-Generation Advanced High-Strength Steels (AHSS). Compared with well-documented thermodynamic data, the data on the kinetics of nitrogen dissolution in these steels are limited. The isotope exchange technique (15N–14N exchange reaction) was applied to measure the nitrogen reaction rate at different temperatures, on pure liquid iron, liquid iron containing different levels of manganese, and liquid Fe-Mn-Al-Si steel with a Third-Generation AHSS composition. The increase in the interfacial reaction rate with increased temperature corresponded to an activation energy of 144.2 kJ/mol for pure iron. For Fe-Mn alloy, the interfacial reaction rate increased with the increased temperature and increased manganese content. For liquid AHSS (Fe-Mn-Al-Si), the interfacial reaction rate was approximately one third of the rate for pure liquid iron, at the same temperature, with a corresponding activation energy of 160.5 kJ/mol.

Operando Isotopic Exchange in Solid Oxide Fuel Cells: Oxygen-Transport Dependency on Applied Potential.

The oxygen isotopic exchange technique is a powerful tool to investigate the oxygen transport kinetics in an oxide solid. In a solid oxide fuel cell, isotopic surface exchange and diffusion coefficients are classically determined by using the Isotopic Exchange Depth Profiling method followed by ex situ SIMS characterizations. Despite its relevance, the utilization of in situ or operando techniques to measure the isotopic exchange under an electrical bias remains marginal. We developed here a set-up which enables operando monitoring of oxygen exchange in SOFC type cells under polarization. The system has been used for studying the oxygen mobility dependency upon polarization on a symmetrical Pt/YSZ/Pt cell (YSZ: yttria-stabilized zirconia). Homomolecular and heterolytic exchange reactions were undertaken to investigate the oxygen activation step and discriminate the limiting step among the sequence of elementary steps which constitute the oxygen transport process in the SOFC system. Oxygen ions incorporation into the dense ionic conductor was identified to be the rate determining step, and its first order rate constant dependency on applied potential was established.

Determination of relevant factors affecting the surface oxygen exchange coefficient of solid oxide fuel cell cathode with ionic conducting oxide coating

In order to investigate the relevant factors affecting the transport properties of solid oxide fuel cell cathodes upon coating with Ce0.9Gd0.1O1.95 (GDC), the surface oxygen exchange coefficient, k*, and oxide ion diffusivity, D*, of La0.6Sr0.4CoO3-δ (LSC) and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) were measured by a combination of isotope exchange technique and secondary ion mass spectrometry (SIMS). The k* of LSCF/GDC enhanced compared to a bare LSCF for all measured temperatures. In contrast, the k* of LSC/GDC only showed an enhancement compared to the bare LSC at relatively low temperatures (below 873 K) but not at higher temperatures. From these results, GDC has a less significant effect on the k* of LSC compared to LSCF. This suggests that the enhancement of k* is possibly induced only when the oxygen vacancy concentration, δ, of the cathode is lower than that of GDC, although another factor may affect the enhancement. The enhancement of k* is attributed to the formation of triple-phase boundary where a spill-over mechanism controlled the oxygen reduction reaction. The change of rate-determining step of LSCF due to the GDC coating suggests that the enhancement of the k* is not only with respect to the incorporation process, but it is possibly enhancing the reaction steps that involve δ as well.

Influence of Sr and Co deficiency on the transport properties and oxygen reduction reaction of La0.6Sr0.4Co0.2Fe0.8O3-δ

The surface oxygen exchange coefficient (k*) and the oxide ion diffusivity (D*) in the La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) related perovskite materials with various strontium (Sr) and cobalt (Co) compositions were determined by a combination of isotope exchange (IE) technique and secondary ion mass spectrometry (SIMS). Results show that the derived values of k* and D* decrease with decreasing Sr and Co content in those LSCF samples. To corroborate the results, electrochemical impedance spectroscopy (EIS) was performed on porous LSCF samples with the same Sr and Co deficiency; obtained impedance spectra were analyzed by the transmission line model (TLM) to derive k* and D*. It showed that those k* and D* values obtained from the isotope exchange experiments are in good agreement with the electrochemically determined values. For further analysis, thermogravimetry was performed to evaluate the oxygen vacancy concentration (δ) for respective samples. It has been found that changes in log D* can be well correlated with the log (δ/(3-δ)) as expected from the vacancy mechanism for the oxide ion diffusion. On the other hand, changes in log k* are not correlated only with the ionic properties like log (δ/(3-δ)) but also with the electron properties; this can be seen in the h plane coordinated with log D* and log k*.

Catalytic activity for dissociative oxygen adsorption of Co-based oxides at high temperature evaluated by a modified pulse isotopic exchange technique

A modified pulse isotopic exchange technique is proposed to quantitatively evaluate the dissociative adsorption rate of Co-based oxides at high temperature. Their catalytic activity is significantly enhanced by the formation of Co-ion clusters.

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.

Effects of pH, competing ions and aging on arsenic(V) sorption and isotopic exchange in contaminated soils

Cattle tick control in Australia using arsenicals from early 1900s to 1955 has led to the existence of some 1600 contaminated sites in northern New South Wales alone. Sorption processes play key roles in controlling arsenic (As) accessibility and subsequent mobility in these dip soils. As(V) sorption and accessibility in three As-contaminated soils and two uncontaminated soil types (ferralitic and sandy soils) are investigated utilizing batch sorption experiments and isotopic exchange techniques. The aged contaminated soils displayed little or no ability to sorb additional As(V), and increasing the soil pH caused a substantial reduction in As(V)-sorption and resulting in As(V)-release. Isotope exchange experiments further supported that any further exposure of the aged-contaminated-soils to additional As(V) increased As-mobilization potential. Amendments of phosphate greatly decreased As(V) sorption in aged-contaminated-soils where As-sorption sites were more highly saturated, whereas phosphate had little effect on As(V) sorption in pristine soils. Similarly, sulfate reduced As(V) sorption, but these effects were less marked than those for phosphate, hence, the application of both PO43− and SO42− in As(V)-contaminated-soils may lead to potential As(V)- mobilization. Conversely, Ca2+ increases As(V)-sorption, which is consistent with expected changes in the surface charge characteristics from Ca2+ sorption, and/or Ca-AsO4 precipitations, consequently Ca2+ amendments may improve As-retention, thereby decreasing As accessibility from cattle dip soils. Therefore, the detailed knowledge presented here provides new insights that may be useful for the assessment and management of the As-contaminated soils.

Analysis of oxygen-18 labeled phosphate to study positional isotope experiments using LC-QTOF-MS

A method is proposed in this paper for the determination of oxygen-18 labeled phosphate so that positional isotope experiments using sensitive and rapid liquid chromatography–QTOF–mass spectrometry (LC-QTOF-MS) experiments can be carried out. The positional isotope exchange technique is a useful tool in understanding the mechanisms and kinetics of many enzyme-catalyzed reactions. Detection of the positions and concentration of these exchanged isotopes is the key. Gas chromatography–mass spectrometry (GC-MS) and nuclear magnetic resonance imaging are commonly used analytical techniques for measurement of 18O/16O, 31P and 15N isotope enrichment. Since these techniques either require a time-consuming derivatization step or have a limited sensitivity, an LC and accurate mass-based method for monitoring 18O/16O exchange was developed and compared with a standard GC-MS method. Our results showed that the LC-QTOF-MS method developed was not only as accurate as the standard GC-MS method, but also a sensitive and robust analytical platform for the simultaneous determination of isotope enrichment and the analysis of positional isotopes without chemical derivation. The LC-QTOF-MS method developed was successfully applied to the measurement of 18O/16O in the reversibility study of ATP hydrolysis by Lon proteases.