共查询到20条相似文献，搜索用时 218 毫秒
1.
Experimental evidence is presented which demonstrates a chemically produced, mass-independent isotopic fractionation of oxygen. The effect is thought to result from self-shielding by the major isotopic species (16)O(2), but other possible mechanisms such as molecular symmetry cannot be ruled out. In a three-isotope plot, the experimentally produced fractionation line is essentially equal in slope to the observed carbonaceous chondrite mixing line. The implications for the early history of the solar system are discussed. 相似文献
2.
From measurements of the photochemical rate of production of CO(2)(16,18) and CO(2)(16,16), produced from the low intensity photolysis of mixtures of CO, H(2)O, Ar, and O(2)(18,18), the rate constant for the reaction HO(2) + CO --> CO(2) + OH has been determined at 300 degrees K to be less than or equal to 10(-20) cubic centimeter per molecule per second. These measurements indicate that the reaction of thermalized HO(2) is of negligible importance as a sink mechanism for converting CO to CO(2) in either the troposphere or the stratosphere. 相似文献
3.
Westenberg AA 《Science (New York, N.Y.)》1972,177(4045):255-256
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. 相似文献
4.
Measurements of the oxygen isotope ratios (18O/16O and 17O/16O) in atmospheric nitrous oxide (N2O) from La Jolla, Pasadena, and the White Mountain Research Station (elevation, 3801 meters) in California and the White Sands Missile Range in New Mexico show that N2O has a mass-independent composition. These data suggest the presence of a previously undefined atmospheric process. The La Jolla samples can be explained by a mixing between an atmospherically derived source of mass-independent N2O and biologically derived mass-dependent N2O. Possible origins of the mass-independent anomaly in N2O are discussed. 相似文献
5.
Zhai Y Pierre D Si R Deng W Ferrin P Nilekar AU Peng G Herron JA Bell DC Saltsburg H Mavrikakis M Flytzani-Stephanopoulos M 《Science (New York, N.Y.)》2010,329(5999):1633-1636
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. 相似文献
6.
GI Gellene 《Science (New York, N.Y.)》1996,274(5291):1344-1346
Application of a theory of nuclear symmetry-based reaction restrictions to the O2 + O --> O3 reaction provides a potential explanation for the symmetry-induced isotopic enrichment observed for laboratory and atmospherically produced O3. Within this theory, the rate of formation of O3 from collisions of O and isotopically homonuclear O2 depends on whether the O2 molecule is in an f (allowed) or an e (restricted) parity label state. The restriction can be relaxed by various potential energy surface coupling terms, and the assumption that approximately 78 percent of the restricted O2(e) levels produce O3 with the same efficiency as the allowed O2(f) levels can account for laboratory-observed isotopic fractionation. In particular, the theory explains the special enhanced formation of the completely asymmetric isotopomer 16O17O18O. 相似文献
7.
The O((1)D) + H(2) --> OH + H reaction, which proceeds mainly as an insertion reaction at a collisional energy of 1.3 kilocalories per mole, has been investigated with the high-resolution H atom Rydberg "tagging" time-of-flight technique and the quasiclassical trajectory (QCT) method. Quantum state-resolved differential cross sections were measured for this prototype reaction. Different rotationally-vibrationally excited OH products have markedly different angular distributions, whereas the total reaction products are roughly forward and backward symmetric. Theoretical results obtained from QCT calculations indicate that this reaction is dominated by the insertion mechanism, with a small contribution from the collinear abstraction mechanism through quantum tunneling. 相似文献
8.
Irradiation of gaseous carbon disulfide [CS2(g)] at 313 nanometers produces a dark brown aerosol of (CS2)x. Its thermal decomposition products include disulfur (S2), carbon monosulfide (CS), and (CS)x. The photopolymerization process is accompanied by a large mass-independent isotopic fractionation of sulfur (a 5 to 10 per mil sulfur-33 excess and a 61 to 84 per mil sulfur-36 deficit). Excess sulfur-33 has been observed in several classes of meteorites. Photochemical production of (CS2)x may be important in the origin and evolution of cosmochemical environments such as the presolar nebula, meteorites, asteroids, and planetary atmospheres. 相似文献
9.
用电导仪测定植物组织呼吸速率 总被引：8，自引：0，他引：8
Ｂａ（ＯＨ）２溶液在０．０００５～０．００４５Ｎ范围内、浓度（Ｎ）和电导率（μΩ－１成直线相关ｒ＝０．９６２１５３＊＊。根据Ｂａ（ＯＨ）２＋ＣＯ２→ＢａＣＯ３↓＋Ｈ２Ｏ这一当量反应，植物组织呼吸释放的ＣＯ２量，可由Ｂａ（ＯＨ）２待测液下降的电导率显示，用此电导率查Ｂａ（ＯＨ）２标准曲线浓度，可计算出呼吸速率ＣＯ２ｍｇ／ｇ·ｈ。 相似文献
10.
Dunning TH Harding LB Wagner AF Schatz GC Bowman JM 《Science (New York, N.Y.)》1988,240(4851):453-459
Computational studies of basic chemical processes not only provide numbers for comparison with experiment or for use in modeling complex chemical phenomena such as combustion, but also provide insight into the fundamental factors that govern molecular structure and change which cannot be obtained from experiment alone. We summarize the results of three case studies, on HCO, OH + H(2), and O + C(2)H(2), which illustrate the range of problems that can be addressed by using modern theoretical techniques. In all cases, the potential energy surfaces were characterized by using ab initio electronic structure methods. Collisions between molecules leading to reaction or energy transer were described with quantum dynamical methods (HCO), classical trajectory techniques (HCO and OH + H(2)), and statistical methods (HCO, OH + H(2), and O + C(2)H(2)). We can anticipate dramatic increases in the scope of this work as new generations of computers are introduced and as new chemistry software is developed to exploit these computers. 相似文献
11.
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. 相似文献
12.
Chueh WC Falter C Abbott M Scipio D Furler P Haile SM Steinfeld A 《Science (New York, N.Y.)》2010,330(6012):1797-1801
Because solar energy is available in large excess relative to current rates of energy consumption, effective conversion of this renewable yet intermittent resource into a transportable and dispatchable chemical fuel may ensure the goal of a sustainable energy future. However, low conversion efficiencies, particularly with CO(2) reduction, as well as utilization of precious materials have limited the practical generation of solar fuels. By using a solar cavity-receiver reactor, we combined the oxygen uptake and release capacity of cerium oxide and facile catalysis at elevated temperatures to thermochemically dissociate CO(2) and H(2)O, yielding CO and H(2), respectively. Stable and rapid generation of fuel was demonstrated over 500 cycles. Solar-to-fuel efficiencies of 0.7 to 0.8% were achieved and shown to be largely limited by the system scale and design rather than by chemistry. 相似文献
13.
Self-shielding of carbon monoxide (CO) within the nebular disk has been proposed as the source of isotopically anomalous oxygen in the solar reservoir and the source of meteoritic oxygen isotopic compositions. A series of CO photodissociation experiments at the Advanced Light Source show that vacuum ultraviolet (VUV) photodissociation of CO produces large wavelength-dependent isotopic fractionation. An anomalously enriched atomic oxygen reservoir can thus be generated through CO photodissociation without self-shielding. In the presence of optical self-shielding of VUV light, the fractionation associated with CO dissociation dominates over self-shielding. These results indicate the potential role of photochemistry in early solar system formation and may help in the understanding of oxygen isotopic variations in Genesis solar-wind samples. 相似文献
14.
Sze ND 《Science (New York, N.Y.)》1977,195(4279):673-675
Present anthropogenic emissions of CO are apparently large enough to perturb the natural CO-OH-CH(4) cycle, which plays a crucial role in the self-cleansing processes in the troposphere. A significant increase in global concentrations of CO, CH(4) CH(3)Cl, and other trace gases may result from a decrease in the OH concentration caused by continued CO emissions. Even if the CO emissions were maintained at the present rate, increases in CO and CH(4) by the year 2025 might be as large as 50 and 25 percent, respectively. The time constants associated with the perturbations of the CO-OH-CH(4) cycle are of the order of a few decades. Perturbation of this cycle may also indirectly affect stratospheric chemistry. 相似文献
15.
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. 相似文献
16.
Friedman I O'neil JR Adami LH Gleason JD Hardcastle K 《Science (New York, N.Y.)》1970,167(3918):538-540
The water content of the breccia is 150 to 455 ppm, with a deltaD from-580 to -870 per mil. Hydrogen gas content is 40 to 53 ppm with a deltaD of -830 to -970 per mil. The CO(2) is 290 to 418 ppm with delta (13)C = + 2.3 to + 5.1 per mil and delta(18)O = 14.2 to 19.1 per mil. Non-CO(2) carbon is 22 to 100 ppm, delta(13)C = -6.4 to -23.2 per mil. Lunar dust is 810 ppm H(2)O (D = 80 ppm) and 188 ppm total carbon(delta(13)C = -17.6 per mil). The (18)O analyses of whole rocks range from 5.8 to 6.2 per mil. The temperature of crystallization of type B rocks is 1100 degrees to 1300 degrees C, based on the oxygen isotope fractionation between coexisting plagioclase and ilmenite. 相似文献
17.
不同处理方法对棉花秸秆营养价值影响的研究 总被引：4，自引：0，他引：4
用浓度为2％石灰＋2％尿素、4％石灰＋4％尿素、5％高锰酸钾、1．5％双氧水、2．5％石灰＋3．5％尿素＋3％食盐、3．5％尿素＋2．5％氢氧化钠＋3％食盐分别处理棉花秸秆15d，分析棉花秸秆粗纤维（CF）、酸性洗涤纤维（ADF）、中性洗涤纤维（NDF）及半纤维素（HC）含量变化。结果表明：与对照组相比，棉秆经上述六种处理后CF、ADF、NDF、HC有明显下降，其中CF平均下降19．45％（P〈0．01）、NDF平均下降18．31％（P〈0．01）、ADF平均下降11．81％（P〈0．01）、HC平均下降7．24％（P〈0，01），显示棉花秸秆经上述化学处理后，营养价值均有不同程度的提高，其中以浓度为2．5％石灰＋3．5％尿素＋3％食盐溶液处理效果最为明显。 相似文献
18.
The oxygen-18 (18O) content of atmospheric carbon dioxide (CO2) is an important indicator of CO2 uptake on land. It has generally been assumed that during photosynthesis, oxygen in CO2 reaches isotopic equilibrium with oxygen in 18O-enriched water in leaves. We show, however, large differences in the activity of carbonic anhydrase (which catalyzes CO2 hydration and 18O exchange in leaves) among major plant groups that cause variations in the extent of 18O equilibrium (theta(eq)). A clear distinction in theta(eq) between C3 trees and shrubs, and C4 grasses makes atmospheric C18OO a potentially sensitive indicator to changes in C3 and C4 productivity. We estimate a global mean theta(eq) value of approximately 0.8, which reasonably reconciles inconsistencies between 18O budgets of atmospheric O2 (Dole effect) and CO2. 相似文献
19.
Effects of Active Modified Atmosphere Packaging on Postharvest Quality of Shiitake Mushrooms (Lentinula edodes) Stored at Cold Storage 
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下载免费PDF全文 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. 相似文献
20.
Miller RL Suits AG Houston PL Toumi R Mack JA Wodtke AM 《Science (New York, N.Y.)》1994,265(5180):1831-1838
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. 相似文献