Photodissociation Of Water Research Articles

Water loss on Venus: The role of carbon monoxide

The abundance of water on Venus is four to five orders of magnitude lower than on the Earth. This difference may reflect initial differences in the bulk volatile contents of the two planets, or may be the result of processes for massive water loss on Venus. A series of thermodynamic calculations are performed on the heterogeneous system C-O-H-N-S, varying C/H upward from its terrestrial value of 0.033 in order to evaluate the extent to which either of these two possibilities may account for the low water content on Venus. Relative abundances of 84 molecular gaseous species are computed as functions of temperature, total pressure, oxygen fugacity, and bulk C/H. As bulk C/H increases, the complement of atmospheric H 2O decreases, but C/H would have to be raised to an improbably high value to account for the low water abundance on Venus by initial deficiency alone. Increasing C/H also results in a rapid increase in CO/H 2O, however, and enhances water loss by the reaction CO + H 2O = CO 2 + H 2 or by reaction of carbon monoxide with the free oxygen liberated by photodissociation of water. Other water loss mechanisms have relied on crustal FeO as an oxygen sink. Such mechanisms, however, should have operated on both Venus and the Earth and may be insufficient to account for the major differences in present H 2O abundance between the two planets. Calculations performed in this study suggest that if the outgassed C/H on Venus was higher than on Earth by even less than a factor of 5, it would have been sufficient to make carbon monoxide competitive with FeO as a sink for oxygen. Together with the lower initial water abundance that follows from a higher C/H, water loss involving CO may have been a major factor in determining the present low abundance of water on Venus.

Catalyzed photodissociation of water ? The first step in inorganic photosynthesis?

Catalyzed photodissociation of water ? The first step in inorganic photosynthesis?

The Characterization of Doped Iron Oxide Electrodes for the Photodissociation of Water: Stability, Optical, and Electronic Properties

A characterization of optical and electronic properties is presented for p‐type (Mg‐doped) and n‐type (Si‐doped) iron oxides used in the photoelectrolysis of water. Photocurrent vs. wavelength spectra for these electrodes indicate that is the active optical component for both p‐type and n‐type materials. Band‐edge locations for p‐type and n‐type iron oxides in sodium hydroxide aqueous solution are determined from differential capacitance measurements. The thermodynamic feasibility of the catalytic photodissociation of water without external potential is demonstrated for a short‐circuited p/n diode assembly on an energy level diagram of the electrode/electrolyte interfaces. The open‐circuit voltage and short‐circuit current generated by the p/n assembly as a function of the intensity of laser irradiation indicate that these doped iron oxides are low mobility, high carrier density semiconductors. Photo‐oxidation of water at the n‐type anode is verified through oxygen detection. Gas evolution is monitored from an operating diode assembly using mass spectrometry and isotopically labeled water . Photocurrents from these p/n assemblies show excellent long‐term stability in aqueous solution and Auger analysis of the semiconductor surfaces indicates no evidence of electrode dissolution.

The Catalyzed Photodissociation of Water

Abstract Chemical interactions between molecules in excited electronic, vibrational, or rotational states and surfaces is a new field of catalytic science. Until recently, catalysis of chemical reactions has only been considered for molecules in their thermodynamic ground states. Most of the surface reactions to be catalyzed were exothermic or thermodynamically downhill. In carrying out endothermic reactions, the only source of energy considered has been the addition of heat. This also assured that the molecules maintained an equilibrium energy distribution throughout the reaction.

The photodissociation of water by doped iron oxides: The unblased p/n assembly

Mott-Schottky capacitance measurements are used to locate semiconductor band edges for a short-circuited p-type/n-type α-Fe 2O 3 assembly in aqueous solution. The thermodynamic feasibility of the catalytic photodiscussion of water without external bias is verified for this assembly from an energy level diagram obtained for the electrode/electrolyte interfaces. Photocurrent stability and Auger analysis of the electrode show no evidence of electrode dissolution. Oxygen evolution is monitored from an assembly using mass spectrometry and H 2 18O enriched water.

Nuclear and electron dynamics in the photodissociation of water

The photodissociation of water in its first absorption band is studied by photolyzing H2O at 157 nm with an excimer laser. This dissociation proceeds directly to produce the electronic ground states of H and OH. Both nascent internal state distributions and alignment of the product OH (2Π) are probed by laser induced fluorescence. This is done with both warm (300 K) and cold (∼10 K) water. About 88% of the excess energy is translation, 10% vibration, about 2% rotation. The first three vibrational levels 0, 1, 2 have population ratios 1:1:0.15, respectively. The rotational distributions depend strongly upon the H2O temperature and are very different for the upper and lower energy components of the Λ doublets, which are measured via Q and P, R lines, respectively. For Q lines, the distributions can be described by rotational temperatures which are 930 K for warm and 475 K for cold water, a surprising difference. For P,R lines strong deviations from Boltzmann behavior are found for cold H2O. The spin distribution is almost statistical. A strong J dependent Λ-doublet population inversion is found from cold H2O, but there is no inversion from warm H2O. The inversion provides a possible pump mechanism for the astronomical OH maser and is simply explained by approximate symmetry conservation. The orientation of the unpaired pπ lobe in OH in the upper Λ-doublet state is measured to be perpendicular to the OH rotation plane. The J dependence of the inversion is explained by Λ-doublet mixing in OH and quantitatively described in terms of the singly occupied pπ-lobe in the excited water and the orientation of the corresponding singly occupied pπ-lobe in OH. The alignment of OH is measured by polarizing both lasers. The large polarization effects are strongly dependent upon J and also upon the temperature of H2O. It is shown that the dependence is related both to Λ-doublet mixing and hyperfine structure of OH. For the cold H2O the data indicate, despite the strong J dependence of both polarization and Λ-doublet inversion, a completely planar dissociation process. It is shown that due to Λ-doublet mixing the transition moment of Π molecules has a J dependent angle relative to the OH rotation plane which approaches the high J limit at the same rate that the molecule shifts from Hund’s case (a) to case (b). The model for the J dependence of the Λ-doublet population and the polarization is important for chemical reactions, surface scattering and other processes where Π molecules are analyzed with LIF.

Photochemical dehydrogenation of ethanol in dilute aqueous solution

The need for alternative sources of energy has stimulated research into the storage of sunlight as chemical potential. Many systems have been investigated but photogeneration of H2 seems to be the most practical. The cyclic photodissociation of water has yet to be realized in homogeneous solution using visible light, although heterogeneous photosystems are known1–3. Efficient photogeneration of H2 in homogeneous conditions can be achieved4–7 at the expense of an added electron donor but such systems have little application in practical devices. Replacement of the sacrificial donor with waste material, for example sulphide8, offers a route for improving these systems and, here we describe efficient H2 photogeneration from a system using aqueous ethanol, as available from low-grade fermentation, as donor. To couple the photoproduction of H2 to the oxidation of ethanol we have used NADH/alcohol dehydrogenase as a relay.

The Photodissociation of Water

The development of artificial photosynthesis units capable of converting solar energy into useful chemical products is still at a very preliminary stage, although some progress has been made during the last few years. Most work concerns the photodissociation of water, and this paper considers the two basic strategies that are being followed. Both of these require the use of a catalyst and, to date, platinum metal catalysts have proved to be the most efficient. With many laboratories actively investigating photodissociation, rapid progress should ensue.

Synergistic effects for the TiO 2/RuO 2/Pt photodissociation of water

Compressed discs of naked TiO 2 or TiO 2 coated with a thin film of a noble metal (e.g. Pt) do not photodissociate water upon illumination with UV light but small amounts of H 2 are generated if the TiO 2 has been reduced in a stream of H 2 at 600°C. Discs prepared from mixtures of TiO 2/RuO 2 facilitate the UV photodissociation of water into H 2 and O 2 although the yields are very low. When a thin (ca. 9nm) film of Pt is applied to the TiO 2/RuO 2 discs the yields of H 2 and O 2 observed upon irradiation with UV light are improved drastically.

The preparation and selected properties of Mg-doped p-type iron oxide as a photocathode for the photoelectrolysis of water using visible light

Stable Mg-doped iron oxides that exhibit p-type character have been synthesized. These have been utilized in the form of sintered polycrystalline disks for the photodissociation of water. We report the influence of sintering temperature, surface oxygen deficiency, Mg-doping level, and solution chemistry on the photoactivity of the Mg-doped iron oxide photocathodes.

Sulphur isotope abundances in Aphebian clastic rocks: implications for the coeval atmosphere

The evolution of the atmosphere has been of interest to many scientists because it relates to the history of the ocean, the sedimentary cycle, the geochemical cycle of volatile elements, and the development of life. It also has a key role in the genesis of iron formations and gold and uranium deposits. Although it is generally accepted that the primitive atmosphere was reducing, a debatable aspect is when and how the oxygen content increased. A rise of atmospheric oxygen pressure about 2,000 Myr BP is suggested by the appearance of redbeds and the banded iron formations1,2. On the other hand, high atmospheric oxygen in very early Precambrian time, as a result of photodissociation of water, has been proposed3. This is supported by occurrences in Precambrian rocks of sulphates4,5, volatile elements6, detrital minerals5,7 and palaeosols8, which are similar to those found in younger rocks. Because the change of sulphur valency is often accompanied by isotopic fractionation, sulphur isotope abundances in minerals have been used to infer the environment at the time of their formation. It occurred to us that additional sulphur isotope analyses of pyrite from continental sedimentary rocks might provide evidence relative to the question of oxygen development in the atmosphere. Accordingly, a major study of sulphur isotope abundances in pyrite from fluviatile sedimentary rocks in the Elliot Lake area- Canada, and for comparison, data from occurrences in Northwest Territories, Canada, Witwatersrand, South Africa, and Jacobina, Brazil, are now reported and implications respecting the coeval atmosphere discussed.

Photocatalytic production of hydrogen from water by a p- and n-type polycrystalline iron oxide assembly

The photodissociation of water to produce hydrogen has been accomplished with light in the solar range of the electromagnetic spectrum and in the absence of any external potential at 300 K. The catalyst for the reaction is a polycrystalline p/n diode assembly made out of Mg- and Si-doped iron oxide. In O.1 M Na/sub 2/SO/sub 4/ solution (pH 6) the device produces hydrogen catalytically with rates of 1-2 H/sub 2/ molecules per site per minute and its power conversion efficiency is about 0.05%. Iron oxide containing minerals could have played important roles in the photochemical evolution of the planet in the prechlorophyl era.

Photodissociation of water by p - and n -type polycrystalline iron oxides by using visible light (≤2.7 eV) in the absence of external potential

A polycrystalline p/n diode assembly, consisting of pressed magnesium- and silicon-doped iron oxide powders, has been shown to photodissociate water by using visible light in the absence of any external potential. In the investigated pH range (8-14) the device produces hydrogen catalytically in amounts that are readily detectable by gas chromatography. The relatively low-power conversion efficiency of 0.05% is believed to be due to the poor change-transfer properties of the p-type iron oxide used.

The Influence of Coulombic Potential Energy on the Efficiency of Photoinduced Charge Separation in Aqueous Solution

A promising system for the photodissociation of water into H2 and O2 involves the zinc porphyrin photosensitised reduction of methyl viologen and, for practical purposes, it is essential that the yield of primary redox products is maximal. The yield of redox products depends upon the partitioning of an ion-pair, and the efficiency of charge separation increases with increased electrostatic repulsion between the ion products. Where there is strong electrostatic repulsion within the ion-pair, the ionic strength of the aqueous solution influences the rate of ionic separation and high yields of redox products are favoured at low ionic strengths. These experimental observations are explained in terms of a kinetic model.

Membrane polarographic detectors for determination of hydrogen and oxygen produced by the photodissociation of water

Two-electrode membrane polarographic detectors (MPDs) were used as detector units for monitoring the concentration of H/sub 2/ and O/sub 2/ produced from the photodissociation of water. Responses included good stability, short response times, and linear correlations with the concentration of H/sub 2/ or O/sub 2/ over 3 orders of magnitude. These units provide rapid, facile, and inexpensive methods for following the concentration of evolved gas from such photochemical systems.

Photodissociation of water

Photodissociation of water

The Archean atmosphere of the earth (Aldan Time)

The Archean atmosphere has left certain traces in the sediments during the accumulation of primary sedimentary and volcanic-sedimentary rocks of the Aldan shield. The analysis of these traces makes it possible to establish the composition of the atmosphere of that time. Wide variation in Fe 2O 3: FeO from ∞ to O, presence of Mn 3+ and SO 4 −2, and considerable development of carbonates and organic carbon in the rocks suggest the presence of free oxygen in the atmosphere. It was generated by photolytic organisms and photo-dissociation of water in the upper layers of the atmosphere. The presence of oxygen in the atmosphere excludes the presence of carbon dioxide, ammonia, hydrocarbon, free hydrogen and hydrogen sulphide. The nature of the carbonaceous rocks, Mg:Ca in the sediments, and the wide development of organisms in the Aldean Archean indicate an important role of CO 2 in the atmosphere. Precipitation of carbonates excludes the presence of “acid” gases (hydrogen chloride, sulphur trioxide, etc.) in the Archean atmosphere. As compared with the present, the Early Archean atmosphere contained considerably more carbon dioxide and water vapour, and very little oxygen. Nitrogen was the major component. The atmosphere of the earth has undergone irreversible changes, repeatedly renewing its composition as the result of the exchange of its gases with the lithosphere, hydrosphere and space, and the influx of gases from the mantle and deeper zones.

Photochemistry of a surfactant derivative of tris(2,2′-bipyridyl)ruthenium(II)

Contrary to a previous report, it has been found that surfactant derivatives of tris(2,2′-bipyridyl)ruthenium(II)(2) do not sensitise the photodissociation of water into its elements.

Hydrogen Escape in the Terrestrial Atmosphere at Low Oxygen Levels: A Photochemical Model

It has already been shown that a decrease of the oxygen level in the atmosphere of the earth would lower significantly the temperature of the exosphere, thus limiting the escape of hydrogen. The reduction of the escape flux would have important consequences on the evolution of oxygen in the last two billion years if O2 comes entirely from photodissociated water. The evaluation of the escape flux is possible only if the mixing ratios of atomic and molecular hydrogen are known at the homopause. In this paper a detailed photo-chemical model including transport by eddy and molecular diffusion is developed for an oxygen level of 0.1 PAL (preexisting atmospheric level). Jeans flux is assumed to be the dominant escape mechanism. The exospheric temperatures used in the model are 500 and 1000 K representing the expected range of variability at 0.1 PAL. It is shown that for stratospheric mixing ratios of total hydrogen comparable to those observed in our epoch, the escape flux is reduced to about 5 × 106 cm−2 s−1 for 5 ppm and 500 K and to 2.3 × 108 for 10 ppm and 1000 K. For comparison results are presented at 1 PAL for exospheric temperatures of 500 and 1000 K. As a by-product ozone densities resulting from such models are also presented taking into account both odd hydrogen chemistry and other possible sinks. Within the assumptions of low exospheric temperature and stratospheric humidity, it is argued that the role of water photodissociation as one of the major sources of atmospheric oxygen might have been overestimated.

The photochemical formation of cometary radicals

The present state of our knowledge of the photochemical formation of cometary free radicals is reviewed. The evidence shows that the photodissociation of water can explain the formation of O, OH, H and H 2O +. Some of the other radicals such as CN and NH 2 can also be explained by the photodissociation of stable compounds such as HCN, CH 3CN and NH 3. Other radicals such as NH, C 2, and C 3 do not appear to originate from the direct photolysis of any stable parent compound. The photolysis of free radicals that are formed as primary products in the photolysis of stable parent molecules has been suggested as an alternate source for these radicals. It is shown that such a process would be consistent with the currently available laboratory data and with cometary observations.