Rate constants for the reactions of hydroxyl and hydroperoxyl radicals with ozone

Chain decomposition of ozone by hydroxyl and hydroperoxyl radicals has been observed. The rate constant at 3000 degrees K for OH + O(3)-->HO(2) + O(2) is 8 x 10(-14) cubic centimeters per second. The rate constant for HO(2) + O(3)--> OH + 2O(2), is 3 x 10(-15) cubic centimeters per second. These results have implications concerning stratospheric ozone.     

Carbon monoxide and nitric oxide consumption in polluted air: the carbon monoxide-hydroperoxyl reaction

A recent laboratory measurement of the ratio of the rate constant for the reaction CO + HO (2) --> C0(2) + OH relative to that for H + HO(2) --> 2OH indicates that the former reaction is probably faster than CO + OH --> CO(2) + H. On this basis a simple analysis is given showing that the calculated lifetime of nitric oxide in polluted atmospheres would be appreciably longer than that estimated on the assumption that the carbon monoxide-hydroperoxyl reaction may be neglected. A fast carbon monoxide-hydroperoxyl reaction implies that the cyclic consumption of carbon monoxide (an atmospheric sink) could occur even with no nitric oxide present.     

Alkali-stabilized Pt-OHx species catalyze low-temperature water-gas shift reactions

We report that alkali ions (sodium or potassium) added in small amounts activate platinum adsorbed on alumina or silica for the low-temperature water-gas shift (WGS) reaction (H(2)O + CO → H(2) + CO(2)) used for producing H(2). The alkali ion-associated surface OH groups are activated by CO at low temperatures (~100°C) in the presence of atomically dispersed platinum. Both experimental evidence and density functional theory calculations suggest that a partially oxidized Pt-alkali-O(x)(OH)(y) species is the active site for the low-temperature Pt-catalyzed WGS reaction. These findings are useful for the design of highly active and stable WGS catalysts that contain only trace amounts of a precious metal without the need for a reducible oxide support such as ceria.     

Hydrogen radicals, nitrogen radicals, and the production of O3 in the upper troposphere

The concentrations of the hydrogen radicals OH and HO2 in the middle and upper troposphere were measured simultaneously with those of NO, O3, CO, H2O, CH4, non-methane hydrocarbons, and with the ultraviolet and visible radiation field. The data allow a direct examination of the processes that produce O3 in this region of the atmosphere. Comparison of the measured concentrations of OH and HO2 with calculations based on their production from water vapor, ozone, and methane demonstrate that these sources are insufficient to explain the observed radical concentrations in the upper troposphere. The photolysis of carbonyl and peroxide compounds transported to this region from the lower troposphere may provide the source of HOx required to sustain the measured abundances of these radical species. The mechanism by which NO affects the production of O3 is also illustrated by the measurements. In the upper tropospheric air masses sampled, the production rate for ozone (determined from the measured concentrations of HO2 and NO) is calculated to be about 1 part per billion by volume each day. This production rate is faster than previously thought and implies that anthropogenic activities that add NO to the upper troposphere, such as biomass burning and aviation, will lead to production of more O3 than expected.     

Removal of Stratospheric O3 by Radicals: In Situ Measurements of OH, HO2, NO, NO2, ClO, and BrO

Simultaneous in situ measurements of the concentrations of OH, HO(2), ClO, BrO, NO, and NO(2) demonstrate the predominance of odd-hydrogen and halogen free-radical catalysis in determining the rate of removal of ozone in the lower stratosphere during May 1993. A single catalytic cycle, in which the rate-limiting step is the reaction of HO(2) with ozone, accounted for nearly one-half of the total O(3) removal in this region of the atmosphere. Halogen-radical chemistry was responsible for approximately one-third of the photochemical removal of O(3); reactions involving BrO account for one-half of this loss. Catalytic destruction by NO(2), which for two decades was considered to be the predominant loss process, accounted for less than 20 percent of the O(3) removal. The measurements demonstrate quantitatively the coupling that exists between the radical families. The concentrations of HO(2) and ClO are inversely correlated with those of NO and NO(2). The direct determination of the relative importance of the catalytic loss processes, combined with a demonstration of the reactions linking the hydrogen, halogen, and nitrogen radical concentrations, shows that in the air sampled the rate of O(3) removal was inversely correlated with total NOx, loading.     

Chlorine chemistry on polar stratospheric cloud particles in the arctic winter

Simultaneous in situ measurements of hydrochloric acid (HCl) and chlorine monoxide (ClO) in the Arctic winter vortex showed large HCl losses, of up to 1 part per billion by volume (ppbv), which were correlated with high ClO levels of up to 1.4 ppbv. Air parcel trajectory analysis identified that this conversion of inorganic chlorine occurred at air temperatures of less than 196 +/- 4 kelvin. High ClO was always accompanied by loss of HCI mixing ratios equal to (1/2)(ClO + 2Cl(2)O(2)). These data indicate that the heterogeneous reaction HCl + ClONO(2) --> Cl(2) + HNO(3) on particles of polar stratospheric clouds establishes the chlorine partitioning, which, contrary to earlier notions, begins with an excess of ClONO(2), not HCl.     

A solid sulfur cathode for aqueous batteries

Because of its high resistivity and subsequent low electroactivity, sulfur is not normally considered a room-temperature battery cathode. An elemental sulfur cathode has been made with a measured capacity of over 900 ampere.hours per kilogram, more than 90 percent of the theoretical storage capacity of solid sulfur at room temperature, accessed by means of a lightweight, highly conductive, aqueous polysulfide interface through the electrocatalyzed reaction S + H(2)O + 2e(-) --> HS(-) + OH(-). This solid sulfur cathode was first used in a battery with an aluminum anode for an overall discharge reaction 2Al + 3S + 3OH(-) + 3H(2)O --> 2Al(OH)(3) + 3HS(-), giving a cell potential of 1.3 volts. The theoretical specific energy of the aluminum-sulfur battery (based on potassium salts) is 910 watt.hours per kilogram with an experimental specific energy of up to 220 watt.hours per kilogram.     

The "Ozone Deficit" Problem: O2(X, v ge 26) + O(3P) from 226-nm Ozone Photodissociation

Highly vibrationally excited O(2)(X(3)sigmag(-), v >/= 26) has been observed from the photodissociation of ozone (O(3)), and the quantum yield for this reaction has been determined for excitation at 226 nanometers. This observation may help to address the "ozone deficit" problem, or why the previously predicted stratospheric O(3) concentration is less than that observed. Recent kinetic studies have suggested that O(2)(X(3)sigmag(-), v >/= 26) can react rapidly with O(2) to form O(3) + O and have led to speculation that, if produced in the photodissociation of O(3), this species might be involved in resolving the discrepancy. The sequence O(3) + hv --> O(2)(X(3)sigmag(-), v >/= 26) + O; O(2)(X(3)sigmag(-), v >/= 26) + O(2) --> O(3) + O (where hv is a photon) would be an autocatalytic mechanism for production of odd oxygen. A two-dimensional atmospheric model has been used to evaluate the importance of this new mechanism. The new mechanism can completely account for the tropical O(3) deficit at an altitude of 43 kilometers, but it does not completely account for the deficit at higher altitudes. The mechanism also provides for isotopic fractionation and may contribute to an explanation for the anomalously high concentration of heavy O(3) in the stratosphere.     

Mass-independent oxygen isotope fractionation in atmospheric CO as a result of the reaction CO + OH

Atmospheric carbon monoxide (CO) exhibits mass-independent fractionation in the oxygen isotopes. An 17O excess up to 7.5 per mil was observed in summer at high northern latitudes. The major source of this puzzling fractionation in this important trace gas is its dominant atmospheric removal reaction, CO + OH --> CO2 + H, in which the surviving CO gains excess 17O. The occurrence of mass-independent fractionation in the reaction of CO with OH raises fundamental questions about kinetic processes. At the same time the effect is a useful marker for the degree to which CO in the atmosphere has been reacting with OH.     

Martian atmospheric erosion rates

Mars was once wet but is now dry, and the fate of its ancient carbon dioxide atmosphere is one of the biggest puzzles in martian planetology. We have measured the current loss rate due to the solar wind interaction for different species: Q(O+) = 1.6.10(23) per second = 4 grams per second (g s(-1)), Q(O+2) = 1.5.10(23) s(-1) = 8 g s(-1), and Q(CO+2) = 8.10(22) s(-1) = 6 g s(-1) in the energy range of 30 to 30,000 electron volts per charge. These rates can be propagated backward over a period of 3.5 billion years, resulting in the total removal of 0.2 to 4 millibar of carbon dioxide and a few centimeters of water. The escape rate is low, and thus one has to continue searching for water reservoirs and carbon dioxide stores on or beneath the planetary surface and investigate other escape channels.     

Activity of CeOx and TiOx nanoparticles grown on Au(111) in the water-gas shift reaction

The high performance of Au-CeO2 and Au-TiO2 catalysts in the water-gas shift (WGS) reaction (H2O + CO-->H2 + CO2) relies heavily on the direct participation of the oxide in the catalytic process. Although clean Au(111) is not catalytically active for the WGS, gold surfaces that are 20 to 30% covered by ceria or titania nanoparticles have activities comparable to those of good WGS catalysts such as Cu(111) or Cu(100). In TiO(2-x)/Au(111) and CeO(2-x)/Au(111), water dissociates on O vacancies of the oxide nanoparticles, CO adsorbs on Au sites located nearby, and subsequent reaction steps take place at the metal-oxide interface. In these inverse catalysts, the moderate chemical activity of bulk gold is coupled to that of a more reactive oxide.     

Reaction of plutonium dioxide with water: formation and properties of PuO(2+x)

Results show that PuO(2+x), a high-composition (x 相似文献   

Synthesis of Graphite and Hydrocarbons by Reaction between Calcite and Hydrogen

The reaction of calcite with hydrogen was investigated over a range of pressure, temperature, and time. The reaction initiates at about 500 degrees C. Its primarily temperature-dependent rateproceeds in a crystallographically anisotropic manner, and reaction products are CaO, Ca(OH)(2), H(2)O, CO, CH(4), C2H(6), and C (graphite), plus a black solid residue that may be hydrocarbon.     

不同气体组合对杏果实贮藏品质的影响

以银白和苹果白杏果实为试材,探讨了不同气调处理,即气体O2与CO2 6种比例组合:3% O2(CA1)、3% O2+2%CO2(CA2)、3%O2+4%CO2(CA3)、8%O2 (CA4)、8%O2+2%CO2(CA5)、8%O2+4%CO2(CA6)及对照(21% O2)对杏果实贮藏品质的影响.结果表明:在相同的贮藏时间内,气调贮藏可缓解银白杏果实硬度的下降,CA6处理的作用最明显,其次为CA3和CA4处理;冷藏苹果白杏果实的硬度在贮藏后期增加,气调贮藏果实的硬度降低,但处理间的差异不显著.在贮藏30 d内,冷藏银白杏果实的可溶性固形物和VC含量仍较高,贮藏30～45 d两者明显下降,气调贮藏可缓解其下降,CA3处理的效果最明显;气调贮藏对苹果白杏果实可溶性固形物和VC含量的影响不大.气调贮藏可缓解银白和苹果白杏果实可滴定酸含量的下降.可见,CA3处理对维持银白杏果实贮藏品质有较好的作用;气调贮藏对苹果白杏果实品质的影响表现在可滴定酸方面,CA1和CA2的效果较好.     

Carbynes in meteorites: detection, low-temperature origin, and implications for interstellar molecules

Carbon from the Allende meteorite is not graphite but carbyne (triply bonded elemental carbon), inasmuch as on heating to 250 degrees to 330 degrees C it releases mainly triply bonded fragments: -(C identical withC)(n),- with n = 1 to 5, and -(C identical withC)(n)-CN, with n = 1 to 3. Although carbynes have been known to form only by condensation of carbon vapor above 2600 K or by explosive shock of > 600 kilobars, it is found that they also form metastably by the reaction 2CO --> CO(2) + C (solid) at 300 degrees to 400 degrees C in the presence of a chromite catalyst. Such low-temperature formation by surface catalysis may be the dominant source of carbynes on the earth and in meteorites, and a major source of interstellar carbynes and cyanopolyacetylenes.     

Effects of Active Modified Atmosphere Packaging on Postharvest Quality of Shiitake Mushrooms (Lentinula edodes) Stored at Cold Storage

The effects of active modified atmosphere packaging (MAP) on the postharvest quality of shiitake mushrooms stored at cold temperature (4°C) were investigated. The gas components were 2% O2+7% CO2 (MAP1), 2% O2+10% CO2 (MAP2) and 2% O2+13% CO2 (MAP3), respectively. The results showed that active MAP could extend the shelf-life of shiitake mushrooms to 17 d and the concentration of carbon dioxide could influence the postharvest quality of shiitake mushrooms. MAP2 treatment inhibited the increase in respiration rate and malondinaldehyde (MDA) contents, delayed the decrease in firmness, soluble sugar and vitamin C, and obviously reduced the activity of polyphenol oxidase (PPO) and the degree of browning, therefore maintaining better quality.     

Carbon-13--rich diagenetic carbonates in miocene formations of california and Oregon

Carbon unusually rich in C(13)(deltaC(13) = +5.4 to +19.0 per mil relative to the Peedee belemnite carbonate standard of the University of Chicago) is characteristic of certain diagenetic limestones and dolomites in the Miocene Monterey Shale of California and the Nye Mudstone of Oregon. This heavy carbon may have originated through low-temperature equilibration between CO(3)- - and CO(2) in migrating carbonated waters or between CH(4) and CO(2) in natural gas. Light carbon (deltaC(13) = -5.6 to -18.2 per mil) derived through nonequilibrium oxidation of organic matter also occurs in the carbonate of Monterey Shale in some localities, but at most places it is much less common than heavy carbon.     

Gas bubbles in fossil amber as possible indicators of the major gas composition of ancient air

Gases trapped in Miocene to Upper Cretaceous amber were released by gently crushing the amber under vacuum and were analyzed by quadrupole mass spectrometry. After discounting the possibility that the major gases N(2), O(2), and CO(2) underwent appreciable diffusion and diagenetic exchange with their surroundings or reaction with the amber, it has been concluded that in primary bubbles (gas released during initial breakage) these gases represent mainly original ancient air modified by the aerobic respiration of microorganisms. Values of N(2)/(CO(2) + O(2)) for each time period give consistent results despite varying O(2)/CO(2) ratios that presumably were due to varying degrees of respiration. This allows calculation of original oxygen concentrations, which, on the basis of these preliminary results, appear to have changed from greater than 30 percent O(2) during one part of the Late Cretaceous (between 75 and 95 million years ago) to 21 percent during the Eocene-Oligocene and for present-day samples, with possibly lower values during the Oligocene-Early Miocene. Variable O(2) levels over time in general confirm theoretical isotope-mass balance calculations and suggest that the atmosphere has evolved over Phanerozoic time.     

Free energy and temperature dependence of electron transfer at the metal-electrolyte interface

The rate constant of the electron-transfer reaction between a gold electrode and an electroactive ferrocene group has been measured at a structurally well-defined metal-electrolyte interface at temperatures from 1 degrees to 47 degrees C and reaction free energies from -1.0 to +0.8 electron volts (eV). The ferrocene group was positioned a fixed distance from the gold surface by the self-assembly of a mixed thiol monolayer of (eta(5)C(5)H(5))Fe(eta(5)C(5)H(4))CO(2)(CH(2))(16)SH and CH(3)(CH(2))(15)SH. Rate constants from 1 per second (s(-1)) to 2 x 10(4) s(-1) in 1 molar HClO(4) are reasonably fit with a reorganization energy of 0.85 eV and a prefactor for electron tunneling of 7 x 10(4) s(-1) eV(-1). Such self-assembled monolayers can be used to systematically probe the dependence of electron-transfer rates on distance, medium, and spacer structure, and to provide an empirical basis for the construction of interfacial devices such as sensors and transducers that utilize macroscopically directional electron-transfer reactions.     

Biogenic methane, hydrogen escape, and the irreversible oxidation of early Earth

The low O2 content of the Archean atmosphere implies that methane should have been present at levels approximately 10(2) to 10(3) parts per million volume (ppmv) (compared with 1.7 ppmv today) given a plausible biogenic source. CH4 is favored as the greenhouse gas that countered the lower luminosity of the early Sun. But abundant CH4 implies that hydrogen escapes to space (upward arrow space) orders of magnitude faster than today. Such reductant loss oxidizes the Earth. Photosynthesis splits water into O2 and H, and methanogenesis transfers the H into CH4. Hydrogen escape after CH4 photolysis, therefore, causes a net gain of oxygen [CO2 + 2H2O --> CH4 + 2O2 --> CO2 + O2 + 4H(upward arrow space)]. Expected irreversible oxidation (approximately 10(12) to 10(13) moles oxygen per year) may help explain how Earth's surface environment became irreversibly oxidized.