Distinguishing homogeneous and heterogeneous water oxidation catalysis when beginning with cobalt polyoxometalates

2013 Fall.Includes bibliographical references.Development of energy storage technologies is required prior to broad implementation of renewable energy sources such as wind or solar power. One of the leading proposals is to store this energy by splitting water into hydrogen and oxygen--that is, to store energy in chemical bonds. A major obstacle en route to this overall goal is the development of efficient, cost-effective water oxidation catalysts (WOCs). Due to the highly oxidizing environment needed to drive this reaction, one question which has arisen when dealing with homogeneous precatalysts is whether these precursors remain as intact, homogeneous WOCs, or whether they are transformed into heterogeneous metal-oxide catalysts. This problem, reviewed in Chapter II, addresses the methods and literature studies related to distinguishing homogeneous and heterogeneous water oxidation catalysts. Chapters III through V further develop the methodology for distinguishing homogeneous and heterogeneous water oxidation catalysis when beginning with the cobalt polyoxometalate [Co4(H2O)2(PW9O34)2]10- (Co4POM). In Chapter III, the investigation of Co4POM using electrochemical oxidation at a glassy carbon electrode reveals that under the conditions therein, an in-situ formed, heterogeneous cobalt-oxo-hydroxo (CoOx) material is the dominant catalyst and is formed from Co2+ leached from the Co4POM. In Chapter IV, investigation of whether the intact Co4POM could be a catalyst under other, more forcing conditions of higher electrochemical potentials and lower Co4POM concentrations is reported. Although the Co4POM shows different electrochemical properties relative to CoOx controls, the possibility that the Co4POM is being transformed into a meta-stable heterogeneous catalyst cannot be ruled out since the Co4POM degrades during the experiment. Lastly, Chapter V presents a kinetic and mechanistic study of the Co4POM when using a ruthenium(III)tris(2,2'-bipyridine) (Ru(III)(bpy)33+) chemical oxidant to drive the water oxidation reaction (i.e., rather than electrochemically driven oxidation). In this study, it was found that Co4POM catalyzes the oxidation of water as well as oxidation of the 2,2'-bipyridine ligand. In contrast, controls with in-situ formed CoOx catalysts more selectively promote the catalytic oxidation of water. The difference in reactivity and kinetics between the Co4POM and CoOx systems indicates that the active catalysts are fundamentally different when a chemical oxidant is employed. Overall, these studies demonstrate the need for careful experimental controls and highlight the importance which reaction conditions--in particular the source and electrochemical potential of the oxidant--can play in determining the active oxidation catalyst in water oxidation reactions

The habitability of stagnant-lid Earths around dwarf stars

The habitability of a planet depends on various factors, such as delivery of water during the formation, the co-evolution of the interior and the atmosphere, as well as the stellar irradiation which changes in time. Since an unknown number of rocky exoplanets may operate in a one-plate convective regime, i.e., without plate tectonics, we aim at understanding under which conditions planets in such a stagnant-lid regime may support habitable surface conditions. Understanding the interaction of the planetary interior and outgassing of volatiles with the atmosphere in combination with the evolution of the host star is crucial to determine the potential habitability. M-dwarf stars in particular possess a high-luminosity pre-main sequence phase which endangers the habitability of planets around them via water loss. We therefore explore the potential of secondary outgassing from the planetary interior to rebuild a water reservoir allowing for habitability at a later stage. We compute the boundaries of the habitable zone around M, K, G, and F-dwarf stars using a 1D cloud-free radiative-convective climate model accounting for the outgassing history of CO2 and H2O from an interior evolution and outgassing model for different interior compositions and stellar luminosity evolutions. The outer edge of the habitable zone strongly depends on the amount of CO2 outgassed from the interior, while the inner edge is mainly determined via the stellar irradiation, as soon as a sufficiently large water reservoir has been outgassed. A build-up of a secondary water reservoir for planets around M-dwarf stars is possible even after severe water loss during the high luminosity pre-main sequence phase as long as some water has been retained within the mantle. Earth-like stagnant-lid planets allow for habitable surface conditions within a continuous habitable zone that is dependent on interior composition.Comment: 15 pages, accepted by A&A, abstract shortene

Bone mineral density in human immunodeficiency virus-1 infected men with hypogonadism prior to highly-active-antiretroviral-therapy (HAART)

Alterations of bone metabolism have been observed in numerous studies of HIV-infected patients. Sex steroids are known to profoundly influence bone mass and bone turnover. Hypogonadism is common in HIV-infection. Therefore, we performed a cross sectional study of 80 male HIV-infected patients without wasting syndrome, and 20 healthy male controls, in whom we analyzed urine and serum samples for both calciotropic hormones and markers of bone metabolism and of endocrine testicular function. Bone mineral density (BMD) was assessed by dual-energy X-ray absorptiometry both in the lumbar spine and Ward's triangle of the left hip. None of the patients received highly-active-antiretroviral-therapy (HAART). Compared to eugonadal HIV-infected patients, subjects with hypogonadism (n = 32; 40%) showed statistically significant decrease of serum osteocalcin (p < 0.05) and elevated urinary excretion of crosslinks (p < 0.05). However, we found 13 and 15, respectively, patients with osteopenia (t-score -1.0 to -2.5 SD below normal) of the lumbar spine. The dissociation between bone formation and resorption and the reduction of of BMD (p < 0.05) is stronger expressed in patients with hypogonadism. Habitual hypogonadism appears to be of additional relevance for bone metabolism of male HIV-positive patients prior to HAART

The extrasolar planet Gliese 581 d: a potentially habitable planet? (Corrigendum to arXiv:1009.5814)

We report here that the equation for H2O Rayleigh scattering was incorrectly stated in the original paper [arXiv:1009.5814]. Instead of a quadratic dependence on refractivity r, we accidentally quoted an r^4 dependence. Since the correct form of the equation was implemented into the model, scientific results are not affected.Comment: accepted to Astronomy&Astrophysic

The habitability of a stagnant-lid Earth

Plate tectonics is a fundamental component for the habitability of the Earth. Yet whether it is a recurrent feature of terrestrial bodies orbiting other stars or unique to the Earth is unknown. The stagnant lid may rather be the most common tectonic expression on such bodies. To understand whether a stagnant-lid planet can be habitable, i.e. host liquid water at its surface, we model the thermal evolution of the mantle, volcanic outgassing of H$_2$O and CO$_2$, and resulting climate of an Earth-like planet lacking plate tectonics. We used a 1D model of parameterized convection to simulate the evolution of melt generation and the build-up of an atmosphere of H$_2$O and CO$_2$ over 4.5 Gyr. We then employed a 1D radiative-convective atmosphere model to calculate the global mean atmospheric temperature and the boundaries of the habitable zone (HZ). The evolution of the interior is characterized by the initial production of a large amount of partial melt accompanied by a rapid outgassing of H$_2$O and CO$_2$. At 1 au, the obtained temperatures generally allow for liquid water on the surface nearly over the entire evolution. While the outer edge of the HZ is mostly influenced by the amount of outgassed CO$_2$, the inner edge presents a more complex behaviour that is dependent on the partial pressures of both gases. At 1 au, the stagnant-lid planet considered would be regarded as habitable. The width of the HZ at the end of the evolution, albeit influenced by the amount of outgassed CO$_2$, can vary in a non-monotonic way depending on the extent of the outgassed H$_2$O reservoir. Our results suggest that stagnant-lid planets can be habitable over geological timescales and that joint modelling of interior evolution, volcanic outgassing, and accompanying climate is necessary to robustly characterize planetary habitability

The Quality Reference Framework for MOOC Design

This paper introduces "The Quality Reference Framework (QRF) for the Quality of MOOCs". It was developed by the European Alliance for the Quality of Massive Open Online Courses (MOOCs), called MOOQ that could involve in the QRF finalization more than 10,000 MOOC learners, designers, facilitators and providers. The QRF consists of three dimensions: Phases, Perspectives and Roles. It includes two quality instruments: the QRF Key Quality Criteria for MOOC experts and QRF Quality Checklist for MOOC beginners

Domains of depleted mantle : new evidence from hafnium and neodymium isotopes

Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 12 (2011): Q08001, doi:10.1029/2011GC003617.Isotope systematics of basalts provide information on the distribution of mantle components and the length scale of mantle heterogeneity. To obtain this information, high data and sampling density are crucial. We present hafnium and neodymium isotope data on more than 400 oceanic volcanics. Over length scales of several hundred to over one thousand kilometers hafnium and neodymium isotopes of mid-ocean ridge basalts are correlated and form an array of parallel trends on a global scale. On a larger scale these domains differ in the amount of highly depleted mantle material with radiogenic hafnium and neodymium isotope ratios. Compared to the Atlantic and Indian Ocean basins the asthenosphere of the Pacific basin seems to have a more uniform and a less radiogenic Hf isotopic composition for a given Nd isotopic composition. The parallel arrays of mid-ocean ridge basalts provide strong constraints on the makeup of the MORB mantle and are evidence for the presence of a highly depleted and highly radiogenic neodymium and hafnium component. This component, because of its highly depleted character, is unrecognized in the strontium-neodymium-lead isotope systems alone. Alternatively, the parallel arrays can have an ancient origin of systematic variations in the degree of depletion. Each array then represents the variations in this fossil melting regime. Individual ocean island basalt suites display different slopes in hafnium-neodymium isotope space, which are also best explained by varying amounts of highly residual mantle rather than isotopic differences in enriched mantle components as previously invoked. The ocean island basalt arrays diverge at the depleted end and project to radiogenic compositions that are similar to those of the asthenosphere through which they travel. This is strong evidence that the plume material interacts with its surrounding mantle as it ascends. The isotopic compositions of the ocean island and ridge basalts suggest that their systematics are influenced by a heretofore unrecognized depleted component.This work was supported by NSF grants EAR 0635864 and OCE0648484 to V.S. and OCE0351437 to S.H

Ancient refractory asthenosphere revealed by mantle re-melting at the Arctic Mid Atlantic Ridge

The upper mantle is a heterogeneous mixture of refractory and recycled crustal domains. The recycled portions, more fertile and thus preferentially melted, dominate the composition of the basalts erupted on the surface, whereas the imprint of melting of the refractory counterparts is more difficult to discern from the basalt chemistry. Contrasting radiogenic isotopic signatures of mid-ocean ridge basalts and oceanic mantle, however, show that Hf isotope ratios may provide hints for melting of refractory source materials despite ubiquitous magma mixing during ascent and stalling in the crust. This property may allow identifying contributions from depleted mantle materials unseen in other isotope systematics in basalts. Here, we show that basalts from Mohns and Knipovich ridges, two >500-km long oblique super-segments in the Arctic Atlantic, have distinctly high Hf isotope ratios, not mirrored by comparatively high Nd and low Sr and Pb isotope ratios. These compositions can be explained if a highly depleted asthenospheric mantle melts beneath this section of the Arctic Mid Atlantic Ridge. We argue that this depleted source consists of high proportions of ancient (>1 Ga), ultra-depleted mantle, previously drained of enriched components before being re-melted in its current location following a recent ridge-jump, allowing the identification of ultra-depleted mantle components in the arctic subridge mantle

A systematic survey in Arabidopsis thaliana of transcription factors that modulate circadian parameters

<p>Abstract</p> <p>Background</p> <p>Plant circadian systems regulate various biological processes in harmony with daily environmental changes. In <it>Arabidopsis thaliana</it>, the underlying clock mechanism is comprised of multiple integrated transcriptional feedbacks, which collectively lead to global patterns of rhythmic gene expression. The transcriptional networks are essential within the clock itself and in its output pathway.</p> <p>Results</p> <p>Here, to expand understanding of transcriptional networks within and associated to the clock, we performed both an <it>in silico </it>analysis of transcript rhythmicity of transcription factor genes, and a pilot assessment of functional phenomics on the <it>MYB</it>, <it>bHLH</it>, and <it>bZIP </it>families. In our <it>in silico </it>analysis, we defined which members of these families express a circadian waveform of transcript abundance. Up to 20% of these families were over-represented as clock-controlled genes. To detect members that contribute to proper oscillator function, we systematically measured rhythmic growth <it>via </it>an imaging system in hundreds of misexpression lines targeting members of the transcription-factor families. Three transcription factors were found that conferred aberrant circadian rhythms when misexpressed: <it>MYB3R2</it>, <it>bHLH69</it>, and <it>bHLH92</it>.</p> <p>Conclusion</p> <p>Transcript abundance of many transcription factors in Arabidopsis oscillates in a circadian manner. Further, a developed pipeline assessed phenotypic contribution of a panel of transcriptional regulators in the circadian system.</p