Shock-tube studies of the reactions of NO2 with NO2, SO2, and CO

The thermal decomposition of NO₂ and its atom-transfer reactions with SO₂ and CO have been studied behind incident shock waves using photometric detection methods. From the decomposition study it is possible to obtain information on the rate of the reaction 2NO₂ → antisymmetric-NO₃ + NO. The results on the reaction, NO₂ + SO₂ → NO + SO₃ extend the earlier work of Armitage and Cullis to about 2000°K. The reaction with CO [NO₂ +] [CO NO + CO₂] at shock temperatures is somewhat faster than predicted from available low-temperature data and provides a modification of the rate-constant expression that is applicable over a wide temperature range.     

Kinetic modeling of the CO/H2O/O2/NO/SO2 system: Implications for high‐pressure fall‐off in the SO2 + O(+M) = SO3(+M) reaction

Flow reactor experiments were performed to study moist CO oxidation in the presence of trace quantities of NO (0–400 ppm) and SO₂ (0–1300 ppm) at pressures and temperatures ranging from 0.5–10.0 atm and 950–1040 K, respectively. Reaction profile measurements of CO, CO₂, O₂, NO, NO₂, SO₂, and temperature were used to further develop and validate a detailed chemical kinetic reaction mechanism in a manner consistent with previous studies of the CO/H₂/O₂/NO_X and CO/H₂O/N₂O systems. In particular, the experimental data indicate that the spin‐forbidden dissociation‐recombination reaction between SO₂ and O‐atoms is in the fall‐off regime at pressures above 1 atm. The inclusion of a pressure‐dependent rate constant for this reaction, using a high‐pressure limit determined from modeling the consumption of SO₂ in a N₂O/SO₂/N₂ mixture at 10.0 atm and 1000 K, brings model predictions into much better agreement with experimentally measured CO profiles over the entire pressure range. Kinetic coupling of NO_X and SO_X chemistry via the radical pool significantly reduces the ability of SO₂ to inhibit oxidative processes. Measurements of SO₂ indicate fractional conversions of SO₂ to SO₃ on the order of a few percent, in good agreement with previous measurements at atmospheric pressure. Modeling results suggest that, at low pressures, SO₃ formation occurs primarily through SO₂ + O(+M) = SO₃(+M), but at higher pressures where the fractional conversion of NO to NO₂ increases, SO₃ formation via SO₂ + NO₂ = SO₃ + NO becomes important. For the conditions explored in this study, the primary consumption pathways for SO₃ appear to be SO₃ + HO₂ = HOSO₂ + O₂ and SO₃ + H = SO₂ + OH. Further study of these reactions would increase the confidence with which model predictions of SO₃ can be viewed. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 317–339, 2000     

Influence of NO on the Reduction of NO2 with CO over Pt/SiO2 in the Presence of O2

Reduction of NO2 with CO in the presence of NO and excess oxygen, a model mixture for flue gas, over a 0.1% Pt/SiO2 catalyst was studied. The related reaction mechanisms, such as oxidation of CO and NO, were discussed. It was found that there was a narrow temperature window （180-190 ℃） for the reduction of NO2 by CO. When the temperature was lower than the lower limit of the window, the reduction hardly occurred, while when the temperature was higher than the upper limit of the window, the direct oxidation of CO by O2 occurred and thereby NO2 could not be effectively reduced by CO. The presence of NO shifted the window to higher temperatures owing to the inhibition effect of NO on the activation of O2 on Pt, which made it possible to reduce NO2 by CO in flue gas.     

Reduction of NO2 in Flue Gas by CO and Propylene over CuO‐CeO2/SiO2 in the Presence of O2

Catalytic reduction of NO₂ with CO and/or propylene in the presence of NO and excess oxygen, a model mixture for flue gas, was studied over a series of CuO‐CeO₂/SiO₂ catalysts between 120–260 °C. The effect of HCl, an impurity in flue gas, on the activity of the catalysts was evaluated. It was found that a binary oxide catalyst, 2% CuO‐8% CeO₂/SiO₂, was active for the reduction of NO₂ by CO and/or propylene. CO was effective for selective reduction of NO₂ in the presence of NO and O₂ in a temperature window between 160–200 °C while propylene was effective at temperature higher than 200 °C. In the presence of HCl, the activity of the catalyst for reduction of NO₂ with CO was irreversibly deactivated. However, the activity for reduction of NO₂ with propylene was not influenced by HCl.     

Structural and Magnetic Study of N2, NO,NO2, and SO2 Adsorbed within a Flexible Single‐Crystal Adsorbent of [Rh2(bza)4(pyz)]n

The crystalline one‐dimensional compound, [Rh^II₂(bza)₄(pyz)]_n ( 1 ) (bza=benzoate, pyz=pyrazine) demonstrates gas adsorbency for N₂, NO, NO₂, and SO₂. These gas‐inclusion crystal structures were characterized by single‐crystal X‐ray crystallography as 1 ?1.5 N₂ (298 K), 1 ?2.5 N₂ (90 K), and 1 ?1.95 NO (90 K) under forcible adsorption conditions and 1 ?2 NO₂ (90 K) and 1 ?3 SO₂ (90 K) under ambient pressure. Crystal‐phase transition to the P space group that correlates with gas adsorption was observed under N₂, NO, and SO₂ conditions. The C2/c space group was observed under NO₂ conditions without phase transition. All adsorbed gases were stabilized by the host lattice. In the N₂, NO, and SO₂ inclusion crystals at 90 K, short interatomic distances within van der Waals contacts were found among the neighboring guest molecules along the channel. The adsorbed NO molecules generated the trans‐NO???NO associated dimer with short intermolecular contacts but without the conventional chemical bond. The magnetic susceptibility of the NO inclusion crystal indicated antiferromagnetic interaction between the NO molecules and paramagnetism arising from the NO monomer. The NO₂ inclusion crystal structure revealed that the gas molecules were adsorbed in the crystal in dimeric form, N₂O₄.     

Experimental and kinetic modeling study of the effect of NO and SO2 on the oxidation of CO?H2 mixtures

The effect of NO and SO₂ on the oxidation of a CO? H₂ mixture was studied in a jet‐stirred reactor at atmospheric pressure and for various equivalence ratios (0.1, 1, and 2) and initial concentrations of NO and SO₂ (0–5000 ppm). The experiments were performed at fixed residence time and variable temperature ranging from 800 to 1400 K. Additional experiments were conducted in a laminar flow reactor on the effect of SO₂ on CO? H₂ oxidation in the same temperature range for stoichiometric and reducing conditions. It was demonstrated that in fuel‐lean conditions, the addition of NO increases the oxidation of the CO? H₂ mixture below 1000 K and has no significant effect at higher temperatures, whereas the addition of SO₂ has a small inhibiting effect. Under stoichiometric and fuel‐rich conditions, both NO and SO₂ inhibit the oxidation of the CO? H₂ mixture. The results show that a CO? H₂ mixture has a limited NO reduction potential in the investigated temperature range and rule out a significant conversion of HNO to NH through reactions like HNO + CO ?? NH + CO₂ or HNO + H₂ ?? NH + H₂O. The chain terminating effect of SO₂ under stoichiometric and reducing conditions was found to be much more pronounced than previously reported under flow reactor conditions and the present results support a high rate constant for the H + SO₂ + M ?? HOSO + M reaction. The reactor experiments were used to validate a comprehensive kinetic reaction mechanism also used to simulate the reduction of NO by natural gas blends and pure C₁ to C₄ hydrocarbons. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 564–575, 2003     

Study of chemiluminescence in O + NO elementary reaction by a crossed beam technique

Chemiluminescent spectra of nitrogen dioxide in the visible region have been observed in the O + NO elementary reaction by a crossed beam technique. Dependences of the emission intensity on both the nitric oxide and atomic oxygen fluxintensities were first order, and the emission was concentrated near the crossin point. These results show that the chemiluminescence observed is due to the chemically excited NO₂ formed in the binary reaction between NO and O.     

Kinetic Analysis and Structural Interpretation of Competitive Ligand Binding for NO Dioxygenation in Truncated Hemoglobin N

The conversion of nitric oxide (NO) into nitrate (NO₃^?) by dioxygenation protects cells from lethal NO. Starting from NO‐bound heme, the first step in converting NO into benign NO₃^? is the ligand exchange reaction FeNO+O₂→FeO₂+NO, which is still poorly understood at a molecular level. For wild‐type (WT) truncated hemoglobin N (trHbN) and its Y33A mutant, the calculated barriers for the exchange reaction differ by 1.5 kcal mol^?1, compared with 1.7 kcal mol^?1 from experiment. It is directly confirmed that the ligand exchange reaction is rate‐limiting in trHbN and that entropic contributions account for 75 % of the difference between the WT and the mutant. Residues Tyr 33, Phe 46, Val 80, His 81, and Gln 82 surrounding the active site are expected to control the reaction path. By comparison with electronic structure calculations, the transition state separating the two ligand‐bound states was assigned to a ²A state.     

Effect of NO, SO2, and O2 on the conversion of nitrous oxide on iron-containing zeolite catalysts

Conditions were found for facilitation of the conversion of nitrous oxide in the presence of Fe-containing zeolite catalysts by oxidants (NO, SO₂, and O₂). The results were interpreted in the framework of a mechanism involving decomposition of N₂O. The effect of NO_x on the reduction of nitrous oxide by C₃-C₄ alkanes was established. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 4, pp. 241–245, July–August, 2006.     

A kinetic study of the gas‐phase reactions of OIO with NO,NO2, and Cl2

The kinetics of iodine dioxide (OIO) reactions with nitric oxide (NO), nitrogen dioxide (NO₂), and molecular chlorine (Cl₂) are studied in the gas‐phase by cavity ring‐down spectroscopy. The absorption spectrum of OIO is monitored after the laser photodissociation, 266 or 355 nm, of the gaseous mixture, CH₂I₂/O₂/N₂, which generates OIO through a series of reactions. The second‐order rate constant of the reaction OIO + NO is determined to be (4.8 ± 0.9) × 10^?12 cm³ molecule^?1 s^?1 under 30 Torr of N₂ diluent at 298 K. We have also measured upper limits for the second‐order rate constants of OIO with NO₂ and Cl₂ to be k < 6 × 10^?14 cm³ molecule^?1 s^?1 and k < 8 × 10^?13 cm³ molecule^?1 s^?1, respectively. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 688–693, 2007     

Characterization and catalytic behavior of La2 - x (Sr, Th)xCuO4 ± λ in reaction NO+CO

Two mixed oxide systems La₂-_xSr_xCuO_{4± λ} (0.0⩽x⩽1. 0) and La_2-xTh_xCuO_{4± λ} (O. O⩽x⩽ 0.4) with K₂NiF₄ structure were prepared by varyingx values. Their crystal structures were studied by means of XRD and IR spectra. The average valence of Cu ion at B site, nonstoichiometric oxygen (λ) and the chemical composition in the bulk and on the surface of the catalysts were measured by means of chemical analysis and XPS. The catalytic behavior in reaction CO+NO was investigated under the regular change of average valence of Cu ion at B site and nonstoichiometric oxygen (λ). Meanwhile, the adsorption and activation of the small molecules NO and the mixture of NO+CO over the mixed oxide catalysts were studied by means of MS-TPD. The catalytic mechanism of reaction NO+CO over these oxide catalysts were proposed; and it has been found that, at lower temperatures the activation of NO is the rate determining step and the catalytic activity is related to the lower valent metallic ion and its concentration, while at higher temperatures the adsorption of NO is the rate determining step and the catalytic activity is related to the oxygen vacancy and its concentration. Project supported by the National Natural Science Foundation of China.     

Nitrite and Nitrate Production by NO and NO2 Dissolution in Water Utilizing Plasma Jet Resembling Gas Flow Pattern

This study investigated the reactive dissolution of nitric oxide (NO) and nitrogen dioxide (NO₂) mixtures in deionized water. The dissolution study was carried out in a flat surface type gas–liquid reaction chamber utilizing a gas flow-pattern resembling plasma jets which are often used in biomedical applications. The concentration of NO and NO₂ in the gas mixtures was varied in a broad range by oxidizing up to 800 ppm of nitric oxide in Ar carrier gas with variable amount of ozone. The production of nitrite (NO₂^?) and nitrate (NO₃^?) in the water was proportional to treatment time up to 50 min. The concentration of NO₃^? was a power function of gas phase NO₂ while the concentration of NO₂^? increased approximately linearly with gas phase NO₂. The formation of NO₂^? and NO₃^? could be described by reactions between dissolved NO₂ and NO in the water while the production rate was determined by diffusion-limited mass transport of nitrogen oxides to the bulk of the liquid. At higher NO₂ concentrations, the formation of dinitrogen tetraoxide (N₂O₄) increased the formation rate of NO₂^? and NO₃^?. The identified mass transport limitation by diffusion suggests that convection of water created by the gas jet is insufficient and dissolution of nitrogen oxides can be increased by additional mixing. In respect of practical applications, the ratio of NO₂^? /NO₃^? in water could be varied from 0.8 to 5.3 with treatment time and gas phase NO₂ and NO concentrations.

     

The reactions of isotopically labeled N15NO+ with CO,NO, O2, N2, NO2, and N2O

The reactions of labeled N¹⁵NO⁺ with CO, NO, O₂, ¹⁸O₂, N₂, NO₂, and N₂O have been investigated using a tandem ICR instrument. In each case the total rate coefficient, product distribution, and kinetic energy dependence were measured. The results indicate that very specific reaction mechanisms govern these reactions. This conclusion is suggested by the lack of isotopic scrambling in many cases and by the complete absence of energetically allowed products in almost all of the systems. The kinetic energy studies indicate that most of the reaction channels proceed through an intermediate complex at low energies and via a direct mechanism at higher kinetic energies. Such direct mechanisms include long range charge transfer and atom or ion transfer.     

Selective catalytic reduction of NO with hydrocarbon on Cu2+-exchanged pillared clay: An IR study of the NO decomposition mechanism

In an earlier study, it has been found that Cu²⁺ ion-exchanged pillared clay (Cu-PILC) has a substantially higher activity for the selective catalytic reduction of NO by ethylene over Cu-ZSM-5. Moreover, it is not significantly deactivated by water vapor and SO₂. In this study, the activity for direct NO decomposition in the presence of O₂ on Cu-PILC was studied and an in situ IR study for the key intermediates and the reaction mechanism was made. The direct NO decomposition activities for Cu-PILC and Cu-ZSM-5 were similar. Under in situ NO and O₂ reaction conditions at temperatures up to 300°C, IR absorption bands at well-defined peak positions are identified. The band at 1699 cm⁻¹ is assigned to a dinitrosyl species on Cu⁺. The bands with peaks at 1609, 1530–1480 and in the region of 1440–1335 cm⁻¹ are assigned to bidentate nitrate, monodentate nitrate and nitro species bonded to Cu²⁺. A redox mechanism is proposed for NO decomposition. The limiting step is thought to be the N–N coupling between surface nitrate and gaseous nitric oxide to form nitrogen. The existence of substantial amounts of nitrate formed from NO alone indicates the important role of the large amount of lattice oxygen that is available on Cu-PILC. As a result, the role of external oxygen supply is only to replenish the consumed lattice oxygen. The proposed NO decomposition mechanism suggests that the redox property of Cu-PILC is crucial for this reaction.     

Thermal Decomposition of CF3C(O)O2NO2, CClF2C(O)O2NO2, CCl2FC(O)O2NO2, and CCl3C(O)O2NO2

Haloacetyl, peroxynitrates are intermediates in the atmospheric degradation of a number of haloethanes. In this work, thermal decomposition rate constants of CF₃C(O)O₂NO₂, CClF₂C(O)O₂NO₂, CCl₂FC(O)O₂NO₂, and CCl₃C(O)O₂NO₂ have been determined in a temperature controlled 420 l reaction chamber. Peroxynitrates (RO₂NO₂) were prepared in situ by photolysis of RH/Cl₂/O₂/NO₂/N₂ mixtures (R = CF₃CO, CClF₂CO, CCl₂FCO, and CCl₃CO). Thermal decomposition was initiated by addition of NO, and relative RO₂NO₂ concentrations were measured as a function of time by long-path IR absorption using an FTIR spectrometer. First-order decomposition rate constants were determined at atmospheric pressure (M = N₂) as a function of temperature and, in the case of CF₃C(O)O₂NO₂ and CCl₃C(O)O₂NO₂, also as a function of total pressure. Extrapolation of the measured rate constants to the temperatures and pressures of the upper troposphere yields thermal lifetimes of several thousands of years for all of these peroxynitrates. Thus, the chloro(fluoro)acetyl peroxynitrates may play a role as temporary reservoirs of Cl, their lifetimes in the upper troposphere being limited by their (unknown) photolysis rates. Results on the thermal decomposition of CClF₂CH₂O₂NO₂ and CCl₂FCH₂O₂NO₂ are also reported, showing that the atmospheric lifetimes of these peroxynitrates are very short in the lower troposphere and increase to a maximum of several days close to the tropopause. The ratio of the rate constants for the reactions of CF₃C(O)O₂ radicals with NO₂ and NO was determined to be 0.64 ± 0.13 (2σ) at 315 K and a total pressure of 1000 mbar (M = N₂). © 1994 John Wiley & Sons, Inc.     

NOx-Sorbing Metal Oxides, MnOx–CeO2. Oxidative NO Adsorption and NOx–H2 Reaction

(n)MnO_x–(1?n)CeO₂ binary oxides have been studied for the sorptive NO removal and subsequent reduction of NO_x sorbed to N₂ at low temperatures (≤150 °C). The solid solution with a fluorite-type structure was found to be effective for oxidative NO adsorption, which yielded nitrate (NO^? ₃) and/or nitrite (NO^? ₂) species on the surface depending on temperature, O₂ concentration in the gas feed, and composition of the binary oxide (n). A surface reaction model was derived on the basis of XPS, TPD, and DRIFTS analyses. Redox of Mn accompanied by simultaneous oxygen equilibration between the surface and the gas phase promoted the oxidative NO adsorption. The reactivity of the adsorbed NO_x toward H₂ was examined for MnO_x–CeO₂ impregnated with Pd, which is known as a nonselective catalyst toward NO–H₂ reaction in the presence of excess oxygen. The Pd/MnO_x–CeO₂ catalyst after saturated by the NO uptake could be regenerated by micropulse injections of H₂ at 150 °C. Evidence was presented to show that the role of Pd is to generate reactive hydrogen atoms, which spillover onto the MnO_x–CeO₂ surface and reduce nitrite/nitrate adsorbing thereon. Because of the lower reducibility of nitrate and the competitive H₂–O₂ combustion, H₂–NO reaction was suppressed to a certain extent in the presence of O₂. Nevertheless, Pd/MnO_x–CeO₂ attained 65% NO-conversion in a steady stream of 0.08% NO, 2% H₂, and 6% O₂ in He at as low as 150 °C, compared to ca. 30% conversion for Pd/γ–Al₂O₃ at the same temperature. The combination of NO_x-sorbing materials and H₂-activation catalysts is expected to pave the way to development of novel NO_x-sorbing catalysts for selective deNO_x at very low temperatures.     

Release of nitric oxide from nitrated fatty acids: Insights from computational chemistry

Nitrated fatty acids (NO₂‐FAs) exhibit a variety of important biological attributes, including a nitric oxide (˙NO) donor and a cell‐signaling molecule. We investigated the mechanisms of fatty‐acid nitration, and the release of ˙NO from NO₂‐FAs. NO₂‐FAs are formed effectively by the addition of ˙NO₂, followed by either hydrogen abstraction or addition of a second NO₂. The latter reaction results in a vicinal nitronitrite ester form of FA, which isomerizes into vicinal nitrohydroxy FA via hydronium ion catalysis. The nitrohydroxy FAs exist in equilibria with NO₂‐FAs. Nitration of conjugated linoleic acid (cLA) was proved to be significantly more efficient than that of LA. In a nonaqueous environment, release of ˙NO from nitrite ester (ONO‐FA) was facilitated by ˙NO₂. Furthermore, the release of ˙NO from NO₂‐cLA is the most favorable in the nitrite ester mechanism. In an aqueous environment, the modified Nef reaction was shown to be feasible. In addition, the release of ˙NO from 10‐ and 12‐NO₂‐LA involves a larger reaction barrier and is more endergonic than those from 9‐ and 13‐NO₂‐LA.     

Formation of nitrous acid and nitric oxide in the heterogeneous dark reaction of nitrogen dioxide and water vapor in a smog chamber

The dark reaction of NO_x and H₂O vapor in 1 atm of air was studied for the purpose of elucidating the recently discussed unknown radical source in smog chambers. Nitrous acid and nitric oxide were found to be formed by the reaction of NO₂ and H₂O in an evacuable and bakable smog chamber. No nitric acid was observed in the gas phase. The reaction is not stoichiometric and is thought to be a heterogeneous wall reaction. The reaction rate is first order with respect to NO₂ and H₂O, and the concentrations of HONO and NO initially increase linearly with time. The same reaction proceeds with a different rate constant in a quartz cell, and the reaction of NO₂ and H₂¹⁸O gave H¹⁸ONO exclusively. Taking into consideration the heterogeneous reaction of NO₂ and H₂O, the upper limit of the rate constant of the third-order reaction NO + NO₂ + H₂O → 2HONO was deduced to be (3.0 ± 1.4) × 10^?10 ppm^?2-min^?1, which is one order of magnitude smaller than the previously reported value. Nitrous acid formed by the heterogeneous dark reaction of NO₂ and H₂O should contribute significantly to both an initially present HONO and a continuous supply of OH radicals by photolysis in smog chamber experiments.     

EPR study of the mechanism of interaction of NO and NO2 molecules with zeolite surfaces

The adsorption of the paramagnetic molecules of NO and NO₂ by zeolites in the alkali and alkaline earth cationic forms has been studied by EPR and reflectance spectroscopic methods. The change in the EPR spectra of adsorbed nitric oxide with increase in the degree of covering of the surface of the alkali cationic form of the zeolites, and also the nature of the change in the spectra when oxygen is adsorbed on zeolites on which NO has previously been adsorbed, indicate the existence of two types of adsorption center. At low degrees of covering of the surface, on the order of 10¹⁸ g^–1, as can be judged from the EPR spectra, the adsorbed NO molecule is strongly polarized and the unpaired electron is almost completely localized on the oxygen atom. At high degrees of covering, for an appreciable proportion of the NO molecules, the bond with the surface is weaker. In this case, the EPR spectra show a hyperfine structure (HFS) with a constant which changes with change in the cation in the order Li⁺ Na⁺ K⁺. The replacement of the singly charged Na⁺ by the doubly charged Ca²⁺ produces a marked change in the adsorption properties of the zeolite. The adsorption of NO on CaA leads not only to polarization of the adsorbed molecule but also to transfer of the electron from the nitrogen atom to the atoms of the adsorbent; this is recorded in the EPR spectrum in the form of an F-center. On further adsorption, the NO molecules are adsorbed both on the nitrogen atom and on the oxygen atom of the first molecule; thus, NO₂ and N₂O are formed.