Great Oxidation Event | EPFL Graph Search

The Great Oxidation Event (GOE), also called the Great Oxygenation Event, the Oxygen Catastrophe, the Oxygen Revolution, the Oxygen Crisis, or the Oxygen Holocaust, was a time interval during the Early Earth's Paleoproterozoic era when the Earth's atmosphere and the shallow ocean first experienced a rise in the concentration of oxygen. This began approximately 2.460–2.426 Ga (billion years) ago, during the Siderian period, and ended approximately 2.060 Ga, during the Rhyacian. Geological, isotopic, and chemical evidence suggests that biologically-produced molecular oxygen (dioxygen or O2) started to accumulate in Earth's atmosphere and changed it from a weakly reducing atmosphere practically absent of oxygen into an oxidizing one containing abundant free oxygen, with oxygen levels being as high as 10% of their present atmospheric level by the end of the GOE. The sudden injection of highly reactive free oxygen, toxic to the then-mostly anaerobic biosphere, may have caused the extinction of many existing organisms on Earth — then mostly archaeal colonies that used retinal to utilize green-spectrum light energy and power a form of anoxygenic photosynthesis (see Purple Earth hypothesis). Although the event is inferred to have constituted a mass extinction, due in part to the great difficulty in surveying microscopic organisms' abundances, and in part to the extreme age of fossil remains from that time, the Great Oxidation Event is typically not counted among conventional lists of "great extinctions", which are implicitly limited to the Phanerozoic eon. In any case, Isotope geochemistry data from sulfate minerals have been interpreted to indicate a decrease in the size of the biosphere of >80% associated with changes in nutrient supplies at the end of the GOE. The GOE is inferred to have been caused by cyanobacteria who evolved porphyrin-based photosynthesis, which produces dioxygen as a byproduct. The increasing oxygen level eventually depleted the reducing capacity of ferrous compounds, hydrogen sulfide and atmospheric methane, and compounded by a global glaciation, devastated the microbial mats around the Earth's surface.

Methane accumulation and oxidation under-ice in various lakes in Quebec

Amélie Marie Corinne Séchaud

Les lacs, en tant que grands émetteurs de méthane (CH4) dans l'atmosphère, sont d'importants composants du cycle global du carbone. Malgré le comportement saisonnier marquée du méthane, peu de choses sont connues sur les processus hivernaux régulant le méthane dans les lacs couverts de glace. Ainsi, nous évaluons l'étendue de l'accumulation et l'oxydation du méthane dans 10 lacs gelées au Sud-Est du Québec, chaque mois entre janvier et mars 2018. Pour cela, nous examinons à la fois temporellement et verticalement les variations de concentration et signature isotopique du méthane, ainsi que les caractéristiques physiques et chimiques des lacs. Nous avons observé des concentrations de méthane relativement faibles (

2018

Design and Development of Functionalized Cyclometalated Ruthenium Chromophores for Light-Harvesting Applications

Michael Graetzel, Mohammad Khaja Nazeeruddin, Aswani Yella, Thomas Baumgartner

The syntheses and the electrochemical spectroscopic properties of a suite of asymmetrical bistridentate cyclometalated Ru(II) complexes bearing terminal triphenylamine (TPA) substituents are reported. These complexes, which contain structural design elements common to both inorganic and organic dyes that exhibit superior power conversion efficiencies in the dye-sensitized solar cell (DSSC), are broadly formulated as Ru-II(L-2,5'-thiophene-TPA-R-1)(L-R-2) [L = tridentate chelating ligand (e.g., 2,2':6',2 ''-terpyridine (tpy); deprotonated forms of 1,3-di(pyridin-2-yl)benzene (Hdpb) or 6-phenyl-2,2'-bipyridine (Hpbpy)); R-1 = -H, -Me, -OMe; R-2 = -H, -CO2Me, -CO2H]. The following structural attributes were systematically modified for the series: (i) electron-donating character of the terminal substituents (e.g., R-1 = -H, -Me, -OMe) placed para to the amine of the "L-2,5'-thiophene-TPA-R-1" ligand framework; (ii) electron-withdrawing character of the tridentate chelate distal to the TPA-substituted ligand (e.g., R-2 = -H, -CO2Me, -CO2H); and (iii) position of the organometallic bond about the Ru(II) center. UV-vis spectra reveal intense and broad absorption bands arising from a collection of metal-to-ligand charge-transfer (MLCT) and TPA-based intraligand charge-transfer (ILCT) transitions that, in certain cases, extend beyond 800 am. Electrochemical data indicate that the oxidative behavior of the TPA and metal chelate units can be independently modulated except in cases where the anionic phenyl ring is in direct conjugation with the TPA unit. In most cases, the anionic character of the cyclometalating ligands renders a metal-based oxidation event prior to the oxidation of the TPA unit. This situation can, however, be reversed with an appropriately positioned Ru-C bond and electron-rich R-1 group. This finding is important in that this arrangement confines the highest occupied molecular orbital (HOMO) to the TPA unit rather than the metal, which is optimal for sensitizing TiO2; indeed, a remarkably high power conversion efficiency (eta) in the DSSC (i.e., 8.02%) is measured for the TPA-substituted pbpy(-) chelate where R-1 = -OMe. These results provide a comprehensive strategy for improving the performance of bistridentate Ru sensitizers devoid of NCS- groups for the DSSC.

2011

Design and Development of Functionalized Cyclometalated Ruthenium Chromophores for Light-Harvesting Applications

Michael Graetzel, Mohammad Khaja Nazeeruddin, Aswani Yella, Thomas Baumgartner

2011