Result Of Isotope Exchange Research Articles

Preparation of La2Zr2O7 composite oxides with fluorite/pyrochlore phases by excluding element influences for catalyzing oxidative coupling of methane.

To elucidate the effect of the A2B2O7 phase on the oxidative coupling of methane (OCM) while excluding elemental influences, La2Zr2O7 compounds with a disordered defect fluorite (La2Zr2O7-F) structure and an ordered pyrochlore phase (La2Zr2O7-P) have been synthesized. Irrespective of their element composition, the catalytic performance of La2Zr2O7-F exceeds that of La2Zr2O7-P. Furthermore, the La2Zr2O7-F surface has more oxygen vacancies/defects than the La2Zr2O7 surface because La2Zr2O7-F exhibits a higher lattice disorder degree and lower B-O bond strength, which leads to the formation of more reactive oxygen anions (O2- and O22-) and basic sites for OCM. Isotopic exchange results have testified that surface-active oxygen sites are generated due to the gaseous O2 adsorption/activation occurring on the surface vacancies via both simple and multiple hetero-exchange mechanisms. In conclusion, crystal structure is the primary factor that governs the catalytic performance of A2B2O7 compounds, with the disordered defect fluorite phase being the most optimal structure for OCM.

Petrographic and isotopic investigations of two unusual Ca-Al-rich inclusions from primitive CO3 chondrites

We performed high-precision SIMS (secondary ion mass spectrometry) 26Al-26Mg and oxygen isotope analyses of two unique CAIs, “Mesquite” and “Y24”, found in the CO3.05 chondrites Northwest Africa 7892 and Yamato-81020, respectively. Mesquite is unusually large (∼5 × 3 mm) for a CAI from any CO chondrite and exhibits a layered texture comprising a melilite-rich core surrounded by hibonite- and spinel-rich mantle layers and a semi-continuous spinel-dominated rim. The CAI Y24 stands out because of its distinct mineralogy: grossite, hibonite, and spinel are accompanied by abundant ultra-refractory-element-rich phases such as warkite, kangite, and perovskite. Silicates are absent in Y24.Negatively fractionated δ25Mg values of phases in the core and mantle layers of Mesquite suggest that the inclusion as a whole was never molten and, hence, represents an aggregate of condensates. The relatively large grain sizes of melilite in the core (up to ∼300 µm) most likely are the result of solid-state recrystallization and coarsening of melilite in the course of a heating event occurring in the solar nebula. This heating event, however, did not disturb the Al-Mg systematics of Mesquite. Regardless of their position within Mesquite and the phases analyzed, spots analyzed for Al-Mg plot on a single isochron characterized by an initial 26Al/27Al of (4.95 ± 0.08) × 10–5 and a δ26Mg*0 of –0.14 ± 0.05‰. We suggest that this initial 26Al/27Al ratio corresponds to the formation of Mesquite in the solar nebula that was slightly heterogeneous with respect to Mg isotopes. Spinel in the rim is uniform in Δ17O (∼–25‰); in contrast, hibonite in the core and mantle layers, albeit also 16O-rich, show variable oxygen isotope ratios (Δ17O ∼ –15‰ to –23‰), which would be consistent with hibonite condensation in a gas with quickly-changing oxygen isotope compositions. The 16O-poor composition of melilite (Δ17O ∼ –1‰ to 0‰) in the core could be the result of isotope exchange with an 16O-poor gas, perhaps during the heating event that caused the solid-state recrystallization and coarsening of melilite or the result of oxygen isotope exchange with a fluid on the parent body. Abundant calcite, phyllosilicates, and sodalite are witnesses to late-stage and low-temperature alteration of the Mesquite CAI; calcite and phyllosilicates most likely are of terrestrial origin, but sodalite could have formed in the parent body.Inclusion Y24 is irregularly-shaped, indicating a condensation origin. Completely enclosing other phases, warkite forms the matrix of Y24, which could be the result of simultaneous condensation and growth of warkite, grossite, and hibonite. Possibly, spinel formed by replacing grossite or hibonite or both minerals in a gradually cooling gas before any silicates condensed. SIMS analyses indicate that condensation occurred in an 16O-rich gas when 26Al/27Al was (5.4 ± 1.0) × 10–5. Oxygen isotope exchange with an 16O-poor fluid in the parent body or with an 16O-poor gas in a nebular setting caused the 16O-poor compositions in grossite and kangite.

Hydrogeochemistry, isotopes and geothermometry of Ixtapan de la Sal–Tonatico hot springs, Mexico

The Ixtapan de la Sal–Tonatico hot springs are located in the geological setting of the Trans-Mexican Volcanic Belt, on the border of the Morelos–Guerrero Basin, which is carbonated region with limestone and silicic rocks. The temperature, electrical conductivity and pH of the thermal waters are 25.7–37.2 °C, 9520–13,920 µS/cm and 5.90–6.88, respectively. The hydrochemical family is NaCl (Cl− mean concentration of 2198 mg/L and Na+ mean concentration of 1396 mg/L). The minor and trace elements that are notable for their high concentrations are B (19.69–23.58 mg/L), Li (6.09–8.06 mg/L), Sr (4.80–6.18 mg/L) and As (0.30–1.97 mg/L). Isotope data (δ18O, δ2H) show δ18O enrichment of the thermal waters, which may be a result of isotopic exchange with the host rock at high temperatures, although the value was very small, which may be due to mixing with groundwater near the surface. The water samples were oversaturated with calcite, dolomite and quartz, verified in the field by the presence of precipitated rocks (travertines). The samples were undersaturated with chalcedony, silica gel, cristobalite, celestite and strontianite. Several chemical geothermometers and the results from multicomponent equilibrium suggest a reservoir temperature between 90 and 130 °C. Meanwhile, a temperature of 129 °C was estimated with the Cl–enthalpy mixing model. According to the conceptual model of the functioning of this system, considering that flow is characterized by the heating of water at deep zones due to the high geothermal gradient caused by recent tectonic activity. This water rises to the surface through fractures and faults. As it rises, thermal water mixes in different proportions with Ca-HCO3 groundwater.

Hydrogeological and geochemical studies of the Efteni and Derdin geothermal areas, Turkey

The Efteni and Derdin geothermal areas are located in northwestern Turkey. Relatively low-temperature springs emerge from the Duzce Fault, a normal-component-dominated fault segment of the North Anatolian Fault System. The thermal waters of the Efteni and Derdin Springs show distinct geochemical and isotopic characteristics since they originate from different geothermal reservoirs and reflect the effects of different water–rock interaction processes. Geothermometry revealed higher reservoir temperatures for the Efteni system, however a strong δ 18O shift, interpreted as being the result of isotopic exchange at high temperatures, was observed in the Derdin system. Hydrogeological and geochemical techniques are applied to identify recharge mechanisms, water–rock interaction processes and to construct conceptual models of these geothermal systems.

Catalytic Activity and Characterization of Oxygen Mobility on Pt/Ce0.75Zr0.25O2 Catalyst by Isotopic Exchange with 18O

Abstract The Ce0.75Zr0.25O2-supported Pt catalysts with different Pt loadings were prepared by the impregnation method. The results of isotopic exchange of 18O, temperature-programmed surface reaction of ethanol (C2H5OH-TPSR), temperature-programmed desorption of CO (CO-TPD), and temperature-programmed reduction of H2 (H2-TPR) show that the Pt phase is amorphous and well dispersed on the support surface. The C2H5OH-TPSR and CO-TPD results show a good correlation of the amount of desorbed CO2 with the catalytic activity for ethanol or CO oxidation. The Pt/Ce0.75Zr0.25O2 catalyst has improved activity for ethanol or CO oxidation, and the catalytic activity is related to the Pt loading. The results of the ethanol or CO oxidation and isotopic exchange of 18O2 show that the oxygen exchange capacity is responsible for the improved catalytic activity. The sample with the best catalytic activity has the highest surface oxygen diffusion rate.

Hydrogeochemistry and groundwater circulation in the Xi’an geothermal field, China

Geothermal waters from the Tertiary aquifers located at 1000–3000 m beneath Xi’an city, Shaanxi Province, China, show unique isotopic composition as compared to local groundwaters from shallower Quaternary aquifers. Positive oxygen shifts of as much as 8‰ VSMOW are observed, while the corresponding δ 2H values remain essentially constant at about −80‰ VSMOW, which is significantly different from those of waters in the Quaternary aquifers with a mean δ 2H value of −60‰ VSMOW. The strong 18O shift is a result of isotope exchange between geothermal water and carbonate minerals such as calcite over a residence time of several thousand years up to 30,000 years, based on 14C dating. A comparison of the isotopic composition of geothermal waters with neighbouring groundwater units on both sides of the Guanzhong Basin indicates that the geothermal reservoirs are recharged by rain that falls on the northern slope of the Qinling Mountains, south of the Xi’an geothermal field, but not from the North Mountains to the north of the field. Based on chemical geothermometers the highest temperature estimated for the Tertiary aquifers of the Xi’an area is around 130 °C.

Supernova graphite in the NanoSIMS: Carbon, oxygen and titanium isotopic compositions of a spherule and its TiC sub-components

Presolar graphite spherules from the Murchison low-density separate KE3 contain a large number of internal TiC crystals that range in size from 15 to 500 nm. We have studied one such graphite grain in great detail by successive analyses with SEM, ims3f SIMS, TEM and NanoSIMS. Isotopic measurements of the ‘bulk’ particle in the ims3f indicate a supernova origin for this graphite spherule. The NanoSIMS measurements of C, N, O and Ti isotopes were performed directly on TEM ultramicrotome sections of the spherule, allowing correlated studies of the isotopic and mineralogical properties of the graphite grain and its internal crystals. We found isotopic gradients in 12C/ 13C and 16O/ 18O from the core of the graphite spherule to its perimeter, with the most anomalous compositions being present in the center. These gradients may be the result of isotopic exchange with isotopically normal material, either in the laboratory or during the particle’s history. No similar isotopic gradients were found in the 16O/ 17O and 14N/ 15N ratios, which are normal within analytical uncertainty throughout the graphite spherule. Due to an unusually high O signal, internal TiC crystals were easily located during NanoSIMS imaging measurements. It was thus possible to determine isotopic compositions of several internal TiC grains independent of the surrounding graphite matrix. These TiC crystals are significantly more anomalous in their O isotopes than the graphite, with 16O/ 18O ratios ranging from 14 to 250 (compared to a terrestrial value of 499). Even the most centrally located TiC grains show significant variations in their O isotopic compositions from crystal to crystal. Measurement of the Ti isotopes in three TiC grains found no variations among them and no large differences between the compositions of the different crystals and the ‘bulk’ graphite spherule. However, the same three TiC crystals vary by a factor of 3 in their 16O/ 18O ratios. It is not clear in what form the O is associated with the TiC grains and whether it is cogenetic or the result of surface reactions on the TiC grains before they accreted onto the growing graphite spherule. The presence of 44Ca from short-lived 44Ti (t 1/2 = 60y) in one of the TiC subgrains confirms the identification of this graphite spherule as a supernova condensate.

Coral calcification: autoradiography of a scleractinian coral Galaxea fascicularis after incubation in 45 Ca and 14 C

The uptake of 45Ca and/or 14C by the skeleton of coral colonies has been commonly used to investigate the processes of calcification. This study reports the differential uptake of these tracers within different regions of the skeleton and tissues of individual corallites and polyps of the hermatypic coral Galaxea fascicularis. Incubation in 45Ca in the light resulted in 80 percent of the 45Ca taken up being deposited in the skeleton. Autoradiography of transverse and longitudinal slices of freeze-substituted polyps and corallites showed that in the light 45Ca was incorporated into the exsert septa, the outside of the thecal walls of the corallite and the inner edges of the septa. Incorporation did not occur in the costae. The radioactivity in the skeleton was considerably greater than in the tissues. In the dark, or in the presence of the photosynthetic inhibitor Diuron, 45Ca was taken up by the exsert septa and was patchily distributed in the corallite walls which suggests that it was not a result of isotopic exchange. The differential incorporation of 45Ca onto the exsert septa was confirmed by scintillation counting. Negligible radioactivity remained in the extrathecal coelenteron after a brief 5 min rinse in non-radioactive seawater. Only 0.1% of 14C taken up in the light was incorporated into the skeleton and this was confirmed by autoradiography. In the presence of Diuron or in the dark, very little 14C was incorporated into tissues or skeleton and in autoradiographs was either not evident in the skeleton or the distribution was similar to that seen in autoradiographs of 45Ca uptake. These results show that the deposition of 45Ca, and therefore calcium carbonate, occurs at specific loci on the skeleton of a corallite. In the dark, deposition occurs specifically at the growing points of the corallite. Differential deposition of calcium carbonate within individual corallites has not been previously reported.

Uptake of Atmospheric Tritium by Market Foods

In this paper uptake of tritium by market foods from tritiated water vapor in the air is investigated using cereals and beans purchased in Deep River, Canada. The concentrations of tissue free water tritium (TFWT) and organically bound tritium (OBT) range from 12 to 79% and from 10 to 38% respectively, of that estimated for atmospheric water vapor of the sampling month. The specific activity ratios of OBT to TFWT were constant for cereals, but variable for beans. The elevated OBT was shown to be the result of isotopic exchange of labile hydrogen by the fact that washing the foods with tritium free-water reduced their tritium contents to levels characteristic of their production sites.

Influence of hydroxyl-bearing minerals on the isotopic composition of ice from the basal zone of an ice sheet

ONE of the important questions in ice-core studies is the extent to which isotope measurements made on ice loaded with bedrock debris can be trusted when interpreting the climate record from ice-core data. Here we show that the isotopic characteristics of basal ice containing debris from beneath the Greenland ice sheet can be understood only if isotope exchange between hydroxyl-bearing minerals and water is taken into account. Sub-glacial abrasion of clays and micas increases the number of broken bonds at the border of the mineral structure, promotes cleavage and so increases the rate at which these minerals exchange oxygen and hydrogen isotopes with water. One implication of our results is to call into question the recent suggestion1 that the basal ice of the Greenland ice sheet is 'superimposed ice', and that therefore extensive melting must have taken place during the last inter-glacial. The enrichment in 18O of the basal ice, thought to be acquired during the formation of the superimposed ice, may in fact be the result of isotope exchange with hydroxyl-bearing minerals.

Reactions involving isotopic exchange of heterocyclic compounds with D2O in the presence of transition metals (review)

The results of isotopic exchange with D2O of various O-, N-, and S-containing heterocycles and some hydrocarbons on transition metals are discussed from the positions of the theory of π-complex adsorption. The effect of various fragments of the molecules (the aromatic or alicyclic ring, electron-donor or electron-acceptor substituents, double bonds, and the number and nature of the heteroatoms) on the character of the substrate-catalyst interaction and on the reaction as a whole is examined on the basis of kinetic data from 1H and 13C NMR spectroscopy and Chromatographic mass spectrometry. Isotopic exchange mechanisms are discussed.

Isotopic exchange between deuterium and cycloalkane on palladium catalysts

Results of isotopic exchange between deuterium and the hydrocarbons, bicyclo[3.3.1]nonane, cis- and trans-1,2-dimethylcyclopentane, on palladium-alumina catalysts at about 50°C are reported. An intermediate common to the exchange of both dimethylcyclopentanes gives nearly completely exchanged cis- and trans-1,2-dimethylcyclopentane in the ratio of one to about four. In addition to this two-set exchange process, there is a one-set exchange process unaccompanied by epimerization in which nearly all of the hydrogen atoms in one set exchange. Other processes lead to the exchange of a smaller number of hydrogen atoms.