Isotopologue enrichment factors of N2O reduction in soils

Isotopic signatures can be used to study sink and source processes of N₂O, but the success of this approach is limited by insufficient knowledge on the isotope fractionation factors of the various reaction pathways. We investigated isotope enrichment factors of the N₂O‐to‐N₂ step of denitrification (ε) in two arable soils, a silt‐loam Haplic Luvisol and a sandy Gleyic Podzol. In addition to the ε of ¹⁸O (ε_18O) and of average ¹⁵N (ε_bulk), the ε of the ¹⁵N site preference within the linear N₂O molecule (ε_SP) was also determined. Soils were anaerobically incubated in gas‐tight bottles with N₂O added to the headspace to induce N₂O reduction. Pre‐treatment included the removal of NO to prevent N₂O production. Gas samples were collected regularly to determine the dynamics of N₂O reduction, the time course of the isotopic signatures of residual N₂O, and the associated isotope enrichment factors. To vary reduction rates and associated fractionation factors, several treatments were established including two levels of initial N₂O concentration and anaerobic pre‐incubation with or without addition of N₂O. N₂O reduction rates were affected by the soil type and initial N₂O concentration. The ε_18O and ε_bulk ranged between ?13 and ?20‰, and between ?5 and ?9‰, respectively. Both quantities were more negative in the Gleyic Podzol. The ε of the central N position (ε_α) was always larger than that of the peripheral N‐position (ε_β), giving ε_SP of ?4 to ?8‰. The ranges and variation patterns of ε were comparable with those from previous static incubation studies with soils. Moreover, we found a relatively constant ratio between ε_18O and ε_bulk which is close to the default ratio of 2.5 that had been previously suggested. The fact that different soils exhibited comparable ε under certain conditions suggests that these values could serve to identify N₂O reduction from the isotopic fingerprints of N₂O emitted from any soil. Copyright © 2009 John Wiley & Sons, Ltd.     

Isotope fractionation factors of N2O diffusion

Isotopic signatures of N2O are increasingly used to constrain the total global flux and the relative contribution of nitrification and denitrification to N2O emissions. Interpretation of isotopic signatures of soil-emitted N2O can be complicated by the isotopic effects of gas diffusion. The aim of our study was to measure the isotopic fractionation factors of diffusion for the isotopologues of N2O and to estimate the potential effect of diffusive fractionation during N2O fluxes from soils using simple simulations. Diffusion experiments were conducted to monitor isotopic signatures of N2O in reservoirs that lost N2O by defined diffusive fluxes. Two different mathematical approaches were used to derive diffusive isotope fractionation factors for 18O (epsilon18O), average 15N (epsilonbulk) and 15N of the central (alpha(-)) and peripheral (beta(-)) position within the linear N2O molecule (epsilon15Nalpha, epsilon15Nbeta). The measured epsilon18O was -7.79 +/- 0.27 per thousand and thus higher than the theoretical value of -8.7 per thousand. Conversely, the measured epsilonbulk (-5.23 +/- 0.27 per thousand) was lower than the theoretical value (-4.4 per thousand). The measured site-specific 15N fractionation factors were not equal, giving a difference between epsilon15Nalpha and epsilon15Nbeta (epsilonSP) of 1.55 +/- 0.28 per thousand. Diffusive fluxes of the N2O isotopologues from the soil pore space to the atmosphere were simulated, showing that isotopic signatures of N2O source pools and emitted N2O can be substantially different during periods of non-steady state fluxes. Our results show that diffusive isotope fractionation should be taken into account when interpreting natural abundance isotopic signatures of N2O fluxes from soils.     

N2O and NO emissions during wastewater denitrification step: Influence of temperature on the biological process

The denitrification process occurring in wastewater treatment plants (WWTPs) is responsible for nitrous oxide (N₂O) and nitric oxide (NO) emissions. These compounds indirectly lead to the global warming. In this study, we investigated the impact of the temperature on N₂O and NO emissions. Experiments were achieved at PH 7 in a batch reactor with acetate as the carbon source. The nitrogen source was nitrates (NO₃⁻) and the COD/N ratio was set to three. Results showed that NO and N₂O emissions increased when the temperature decreased. NO emissions appeared only at 10 °C and 5 °C, with respectively 8% and 18% of the total denitrified nitrogen. N₂O emissions increased from 13 to 40 then 82% of the total denitrified nitrogen, respectively at 20, 10 and 5 °C. Several hypotheses were suggested to explain these results: a general enzymatic slow down, enzymatic inhibitions, electron donor competition between the different enzymes and metabolic pathway alterations.     

Photooxidation of polystyrene by O2 and N2O

Strips of polystyrene held in a flowing O₂ or N₂O atmosphere have been exposed to 240-600 nm radiation. The extent of photooxidation has been followed by x-ray photoelectron spectroscopy (XPS). Although N₂O is a more reactive gas than O₂, it produces a less oxidized polymer surface. This surprising observation can be correlated to the photochemistry occurring at the gas/polystyrene interface. © 1992 John Wiley & Sons, Inc.     

Die sechsgliedrigen Ringsysteme PIIISi2N2O, [PVSi2N2O]⊕ und PVSi2N2O

Compounds I-X of the sixmembered ring system PSi₂N₂O with phosphorus in different oxidation and bond numbers, collected in Schema 1, have been prepared for the first time and confirmed in their structure by elemental analysis as well as by infrared and¹H- and³¹P-spectroscopy.

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Mit Auszügen aus der DissertationK. P. Giesen, Techn. Univ. Braunschweig 1972.     

N,O-coupling towards the selectively electrochemical production of H2O2

Hydrogen peroxide (H₂O₂) synthesis generally involves the energy-intensive anthraquinone process. Alternatively, electrochemical synthesis provides a green, economical, and environmentally friendly route to prepare H₂O₂ via the two-electron oxygen reduction reaction, but this process requires efficient catalysts with high activity and selectivity simultaneously. Here, we report an N, O co-doped carbon xerogel-based electrocatalyst (NO-CX) prepared by a simple and economical method. The NO-CX catalyst exhibits a high H₂O₂ selectivity over 90% in a potential range of 0.2–0.6 V and a high H₂O₂ production rate of 1410 mmol g_cat^?1 h^?1. The density functional theory calculations demonstrate that the coupling effect between N and O can effectively induce the redistribution of surface charge and the edge carbon atom adjacent to an ether group and a graphite nitrogen atom is the active site. This work provides a straightforward and low-cost process to produce highly selective H₂O₂ catalysts, which is in place for the expansion of electrocatalytic synthesis of useful chemicals.     

Ab initio calculations of nitrogen oxide reactions: formation of N2O2, N2O3, N2O4, N2O5, and N4O2 from NO, NO2, NO3, and N2O

Ab initio computational methods were used to obtain Delta(r)H(o), Delta(r)G(o), and Delta(r)S(o) for the reactions 2 NO <=> N(2)O(2) (I), NO+NO(2) <=> N(2)O(3) (II), 2 NO(2) <=> N(2)O(4) (III), NO(2)+NO(3) <=> N(2)O(5) (IV), and 2 N(2)O <=> N(4)O(2) (V) at 298.15 K. Optimized geometries and frequencies were obtained at the CCSD(T) level for all molecules except for NO, NO(2), and NO(3), for which UCCSD(T) was used. In all cases the aug-cc-pVDZ (avdz) basis set was employed. The electronic energies of all species were obtained from complete basis set extrapolations (to aug-cc-pV5Z) using five different extrapolation methods. The [U]CCSD(T)/avdz geometries and frequencies of the N(x)O(y) compounds are compared with literature values, and problems associated with the values and assignments of low-frequency modes are discussed. The standard entropies are compared with values cited in the NIST/JANAF tables [NIST-JANAF Thermochemical Tables, J. Phys. Chem. Ref. Data Monograph No. 9, 4th ed. edited by M. W. Chase, Jr. (American Chemical Society and American Institute of Physics, Woodbury, NY, 1988)]. With the exception of I, in which the dimer is weakly bound, and V, for which thermodynamic data appears to be lacking, the calculated standard thermodynamic functions of reaction are in good agreement with literature values obtained both from statistical mechanical and various equilibrium methods. A multireference-configuration interaction calculation (MRCI+Q) for I provides a D(e) value that is consistent with previous calculations. The combined uncertainties of the NIST/JANAF values for Delta(r)H(o), Delta(r)G(o), and Delta(r)S(o) of II, III, and IV are discussed. The potential surface for the dissociation of N(2)O(4) was explored using multireference methods. No evidence of a barrier to dissociation was found.     

Stability of nitrogen-oxygen cages N12O2, N14O2, N14O3, and N16O4

Molecules consisting entirely of nitrogen have been studied extensively for their potential as high energy density materials (HEDM). However, many such molecules are too unstable to serve as practical energy sources. This has prompted many studies of molecules that are mostly nitrogen but which incorporate heteroatoms into the structure to provide additional stability. In the current study, cages of three-coordinate nitrogen are viewed as candidates for stabilization by insertion of oxygen atoms into the nitrogen framework. Cages of N12, N14, and N16 with four-membered rings are studied because four-membered rings have been previously shown to be a destabilizing influence. Insertion of oxygen atoms, which converts N-N bonds to N-O-N bonding groups, relieves ring strain and can potentially result in stable molecules. These molecules are studied by theoretical calculations, using Hartree-Fock and Moller-Plesset (MP3 and MP4) theories, to determine the dissociation energies of the molecules. The primary result of the study is that stable molecules can result from oxygen insertion but that oxygen-oxygen proximity destabilizes the insertion products.     

Quenching of I(2P1/2) by NO2, N2O4, and N2O

Quenching of excited iodine atoms (I(5p5, 2P1/2)) by nitrogen oxides are processes of relevance to discharge-driven oxygen iodine lasers. Rate constants at ambient and elevated temperatures (293-380 K) for quenching of I(2P1/2) atoms by NO2, N2O4, and N2O have been measured using time-resolved I(2P1/2) --> I(2P3/2) 1315 nm emission. The excited atoms were generated by pulsed laser photodissociation of CF3I at 248 nm. The rate constants for I(2P1/2) quenching by NO2 and N2O were found to be independent of temperature over the range examined with average values of (2.9 +/- 0.3) x 10(-15) and (1.4 +/- 0.1) x 10(-15) cm3 s(-1), respectively. The rate constant for quenching of I(2P1/2) by N2O4 was found to be (3.5 +/- 0.5) x 10(-13) cm3 s(-1) at ambient temperature.     

Simultaneous desulfurization and denitrification of flue gas by using urea/H2O2 solution

在填料塔上对尿素/H2O2溶液同时脱硫脱硝进行了实验研究.结果表明,尿素溶液对SO2具有很高的去除效率,但对NOx的去除受到多种因素的影响.实验主要研究了H2O2浓度、吸收反应温度、溶液pH值等因素对脱硝效率的影响,并确定了最佳实验条件,在此条件下脱硫脱硝效率分别达到100％和52.6％.同时,采用化学分析法和气相色谱法对尿素溶液同时脱硫脱硝的反应产物进行分析,推导出尿素/H2O2溶液同时脱硫脱硝的反应机理和总化学反应方程式.该技术可用于对现有湿法脱硫技术的改造,使其同时具有脱硫脱硝功能.     

Mechanism of N/O bond scission of N2O by an unsaturated rhodium transient

The mechanism of formation of triplet (PNP)RhO and (PNP)Rh(N(2)) (PNP = N(SiMe(2)CH(2)P(t)Bu(2))(2)) from reaction of two molecules of (PNP)Rh with N(2)O has been studied by DFT, evaluating mechanisms which (1) involve free N(2), and (2) which effect N/O bond scission in linearly coordinated (PNP)RhNNO. This work shows the variety of modes of binding N(2)O to this reducing, unsaturated metal fragment and also evaluates why a mechanism avoiding free N(2) is preferred. Comparisons are made to isoelectronic CO(2) in its reaction with (PNP)Rh.     

H_2O_2氧化N_2生成N_2O和H_2O机理的研究

采用量子化学计算方法研究了Ｈ２Ｏ２ 氧化Ｎ２ 生成Ｎ２Ｏ 和Ｈ２Ｏ 的机理．结果发现, Ｈ２Ｏ２ 氧化Ｎ２ 先通过１ 个四元环过渡态形成中间体Ｈ２Ｎ２Ｏ２ 分子,Ｈ２Ｎ２Ｏ２ 再通过一个五元环过渡态形成Ｎ２Ｏ和Ｈ２Ｏ．根据计算得到的每步反应的活化能,得知Ｈ２Ｏ２ 氧化Ｎ２ 生成中间体Ｈ２Ｎ２Ｏ２ 分子是整个反应的控制步骤．     

Photodissociative production of O(1S) and N(2D) from N2O in an argon matrix at 4 K

Vacuum ultraviolet photolysis of N₂O isolated in Ar matrices at 4 K gives direct luminescent evidence for the photodissociative production of both O(¹S) and N(²D). The matrix results are compared to relative atomic quantum yields measured in the gas phase.     

Modification of Hexatriacontane by O2–N2 Microwave Post-Discharges

Etch rates of hexatriacontane (HTC) as high as ~10 mg s⁻¹ m⁻² in late O₂ post-discharge are obtained at 333 K where no significant UV nor VUV irradiation occurs. Introducing N₂ in the gas mixture helps control the ratio of O/O₂ densities, which is shown to play a key role in the functionalization or etching of the HTC. The oxygen atoms are required for any further modification of the HTC because they initiate the formation of the radical chains by abstraction of one hydrogen. O(³P) atoms do not contribute directly to break the alkane chain close to room temperature but they can functionalise it. O₂ is the important reactive species for the etching because of the role played by the peroxide groups on the scission of the hydrocarbon chains.     

Ionization of O3 in Excess N2: A New Route to N2O via Intermediate N2O3+ Complexes

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