Surface Nitrite Research Articles

The reaction of NO2 with solid anthrarobin (1,2,10-trihydroxy-anthracene)

Reactions of atmospheric oxidants with condensed organic materials are implicated in secondary processes of relevance to the radiation budget of the atmosphere and to its oxidation capacity. A solid film of anthrarobin (1,2,10-trihydroxyanthracene) was exposed to gaseous NO2 at concentrations in the ppb range at atmospheric pressure, temperatures between 283 and 313 K and relative humidity between 20 and 80%. Gaseous HONO was the main product evolving from the anthrarobin surface. The reaction probability based on gas-kinetic collisions ranging between 2 × 10−6 and 7 × 10−7 decreased with time due to consumption of reactive surface molecules. It also decreased as a function of the NO2 concentration, which is in agreement with a Langmuir–Hinshelwood type surface reaction. A Langmuir constant of 5 × 10−3 ppb−1 for the reversible adsorption and a surface reaction rate constant of about 10−20 cm2 s−1 was derived. The reaction rate increased with increasing temperature, and the corresponding activation energy for the overall process was +39 kJ mol−1. The effect of humidity was to increase the reaction rate but also to increase the apparent HONO output, which was clearly not due to a bulk effect. The results suggest that the primary reaction is an electron transfer process between a deprotonated hydroxy group (equivalent to a phenoxide ion) hydrolyzed on the surface in the presence of humidity and NO2 leading to a surface nitrite. The results provide evidence that the system investigated may be a model system for the reaction of NO2 with soot.

NOx Storage on BaO(100) Surface from First Principles: a Two Channel Scenario

NO2 adsorption at a BaO(100) surface is investigated by means of spin polarized GGA density functional theory. A periodic supercell procedure is employed, and two redox reaction channels are mapped out, involving two chemisorbed NO2 molecules per supercell. The chemisorption is studied in two subsequent steps. The reaction paths are initiated by NO2 adsorption in the form of a nitrite over a Ba2+ site. This generates an electron hole among the surrounding surface oxygen atoms. A reaction path branching occurs as the second NO2 either (a) acts as surface oxidant, forming a surface nitrite−peroxide pair by releasing NO(g), or (b) binds to an O-surf site to form a formal surface nitrate. A redox reaction involving surface nitrite−nitrate interconversion is also addressed. The computed results are employed to interpret experimental observations of surface nitrites, peroxides, NO(g) desorption, and surface Ba(NO3)2 formation. The understandings are discussed in the context of the NOx storage concept of lean-bu...

Supported CuO+Ag/Partially Stabilized Zirconia Catalysts for the Selective Catalytic Reduction of NOx under Lean Burn Conditions: 1. Bulk and Surface Properties of the Catalysts

Thermally stable cubic mesoporous zirconia samples stabilized by the alkaline-earth cations (Ca, Sr, Ba) were synthesized via the coprecipitation route followed by refluxing in the presence of surfactants. These systems were used as supports for copper cations and then modified by the addition of silver nanoparticles using impregnation or photoassisted deposition techniques. The structural, textural, and surface features of these nanosystems were studied by using TEM, X-ray diffraction, EXAFS, nitrogen adsorption isotherms, SAXS, FTIRS of adsorbed CO, and TPD of adsorbed NOx species. Partially stabilized zirconia samples were found to possess a disordered cubic structure. A higher tendency of bulky Ba cation to segregate in the surface layer is reflected in a higher degree of surface disordering, higher concentration of hydroxyls, and greater coordination unsaturation of isolated copper cations. In contrast to such traditional supports as γ-alumina, stabilized zirconia supports appear to favor formation of small reactive (probably, three-dimensional) clusters of copper cations possessing an increased reactivity and decreased strength of oxygen bonding with these cations. It is reflected in decreased thermal stability of surface nitrite and nitrate species located at these centers as compared with such species on the surface of CuO/alumina catalysts. This feature seems to be primarily determined by the specificity of the surface structure of fluorite-like supports (ceria, zirconia). Silver incorporation into copper oxidic clusters decreases the strength of copper–oxygen bonds as well as the thermal stability of adsorbed nitrite–nitrate species. For samples prepared via the photodeposition route, the clustering degree of copper cations is usually lower than in the case of samples obtained by traditional impregnation procedure.

Isothermal “light-off” during catalytic N2O decomposition over Fe/ZSM-5

The catalytic decomposition of N2O over Fe/ZSM-5 was studied at pressures up to 1 atm. As the partial pressure of N2O in the feed to a packed bed reactor was varied, the catalytic activity was observed to abruptly increase at inlet partial pressures above some critical value. This abrupt increase in activity was not due to the reaction exotherm, but is believed to be a kinetic phenomenon. In similar experiments where the temperature was varied the activity did not jump to a higher level. The dual levels of activity were observed for several Fe/ZSM-5 catalysts that spanned a range of SiO2/Al2O3 ratios from 30 to 280, irrespective of whether the iron was introduced into the zeolite by ion exchange or by sublimation of iron chloride. When the inlet partial pressure of N2O to the packed bed reactor was sufficiently high to cause the catalyst to operate in the high activity regime, the high activity state was sustained throughout the catalyst bed, even though the N2O partial pressure in the latter part of the bed had dropped below the critical level. Microkinetic modeling shows that this kind of behavior is possible in an isothermal catalytic system. The microkinetic model includes two redox cycles. In one cycle the cation oxidation state alternates between Fe2+ and Fe3+. In the second, more active redox cycle there is alternation between a surface nitrite and nitrate. The former redox cycle predominates at low partial pressures and the latter at high N2O partial pressures.

Transmission FT-IR and Knudsen Cell Study of the Heterogeneous Reactivity of Gaseous Nitrogen Dioxide on Mineral Oxide Particles

The heterogeneous reactivity of gaseous nitrogen dioxide on mineral oxide particles was investigated. In particular, spectroscopic and kinetic measurements have been made to investigate surface reactions of NO2 on Al2O3, Fe2O3, and TiO2 at 298 K. Both gas-phase and surface-bound products are formed from the reaction of NO2 with these mineral oxide particles. At low coverages, FT-IR spectra of the mineral oxide surface exposed to gaseous NO2 show absorptions due to surface nitrite, specifically a chelating nitrito species. As the coverage increases, the surface becomes populated with surface nitrate bonded in several different bonding coordinations (monodentate, bidentate, and bridging). The predominant gas-phase product is NO, although there is a small amount (<1%) of detectable N2O. A Knudsen cell reactor coupled to a quadrupole mass spectrometer was used to measure the uptake coefficient, γ, for NO2 on these oxide particles and to characterize gas-phase product formation. The Knudsen cell data showed NO...

Heterogeneous chemistry of NO2 on mineral oxide particles: Spectroscopic evidence for oxide‐coordinated and water‐solvated surface nitrate

In this study, transmission FT‐IR and diffuse reflectance UV‐vis spectroscopies were used to probe the heterogeneous chemistry of gaseous nitrogen dioxide on mineral oxide particles. In particular, the role of pre‐ and post‐adsorbed water on the heterogeneous chemistry of NO2 on Al2O3 and TiO2 was investigated. At 298 K, NO2 reacts with the surface of these mineral oxides to form surface nitrite and nitrate. The spectroscopic data show that two distinct types of surface nitrate exist, oxide‐coordinated and water‐solvated nitrate. The relative amounts of oxide‐coordinated and water‐solvated nitrate are dependent on the coverage of adsorbed water on the surface of the mineral oxide particle. The importance of this water layer in the heterogeneous atmospheric chemistry of mineral oxide aerosol is discussed.

Propane and oxygen action on NOx adspecies on low-exchanged Cu-ZSM-5

A surface intermediate with a C/N ratio close to 3 has been shown by TPD to form at co-adsorption of NO and propane as well as NO, propane and O2 on low-exchanged Cu-ZSM-5. The adsorption of NO, propane and oxygen has been studied to evaluate their effect on the formation of this complex. Its formation is accompanied by a decrease in the concentration of surface nitrite–nitrate. The kinetics of nitrite–nitrate adspecies formation as a function of the reagents concentration and temperature has been investigated. Some NO adspecies have been found to decompose yielding N2O.

Electronic Structure of Sodium Nitrate: Investigations of Laser Desorption Mechanisms

This theoretical study uses ab initio quantum mechanical methods to investigate the electronic properties of ground and excited state sodium nitrate. We calculated electronic properties of the crystalline material for bulk, clean, and defected surfaces. The results of these calculations are used to explain the photoexcitation/desorption mechanism and support the conclusions of an earlier experimental investigation of the laser desorption of NO from single-crystal sodium nitrate. Ab initio periodic Hartree−Fock (PHF) theory was used to investigate the “molecular−ionic” character of crystalline sodium nitrate. The calculations indicate that the electronic structure of the bulk and cleavage surface are virtually identical (i.e., no shift in the resonant absorption profile is expected). This finding is consistent with the experimental results on the wavelength dependence of NO desorption yields for crystalline NaNO3. However, changes in the absorption manifold are found to accompany the removal of external nitrate oxygens (producing surface nitrite groups). The presence of these chemical defects causes states to appear in the band gap producing a red shift in the absorption band. Complete active space self-consistent field (CASSCF) calculations on NO3-, NO3, and NO2-, support the “local excitation model” of the π* ← π2 transition. These calculations also indicate that the transition energies of the nitrate ion are unaffected by the presence of the surrounding ions. The photoexcitation/dissociation mechanism of the nitrate ions in the crystal, however, differs from gas-phase processes in that the neutral photodetachment channel, observed in the gas phase, is energetically inaccessible in the solid due to the stabilization of ionic species (relative to neutrals) by the crystalline field.

A role of surface nitrite and nitrate complexes in NOx selective catalytic reduction by hydrocarbons under oxygen excess

NO adsorption over three types of catalytic systems, such as cation-exchanged zeolites, transition metal oxides supported on γ-Al2O3, and partially stabilised tetragonal ZrO2 (PSZ) was studied by using the TPD method. NO forms several surface complexes having different desorption temperatures. TPD results compared with catalytic properties of these systems in the selective reduction of NO x by propane under oxygen excess showed that strongly bound nitrites and nitrates appeared to be true intermediates in this reaction.

Chemical and electronic characterization of pure SnO 2 and Cr-doped SnO 2 pellets through their different response to NO

The interaction of NO at room temperature (RT) with pure and Cr-doped SnO 2 self-supporting pellets, pretreated in dry oxygen at 673-873 K, has been studied by FT-IR spectroscopy. With both pure and doped pellets treated at 673 K, NO at pressures ⩽ 10 N m −2 causes a reaction (NO⇌NO + + e −) that is responsible for the increase of a broad electronic absorption, which is irreversible to evacuation at RT, but no peaks due to surface species appear. For pressures > 10 N m −2, peaks due to nitrites, nitrosyls and N 2O appear, without further increase of the electronic absorption, which actually becomes completely reversible at RT, while surface nitrites and nitrosyls are irreversible under evacuation. The formation of these species is responsible for the shift of the equilibrium involving electrons. The electronic absorption is due to the overlap of two bands: one at 0.1–0.18 eV, assigned to the electron transition from monoionized oxygen vacancies to the conduction band; the other, at 0.45–0.6 eV, assigned to a similar electronic transition involving oxygen divacancies. Thermal treatment at 873 K affects the electronic response of both the pellets. On Cr-doped pellets the electronic response is completely killed. SnO 2 pellets show the behaviour described for pretreatment at 673 K, except for the shape of the electronic absorption: the relative intensities of the two overalapping bands at 0.1–0.18 and 0.45–0.6 eV bands change in favour of the latter.

Surface reactions of NO 2 with machined Zn, Cd, Cu and Ag surfaces studied by He I/II photoelectron spectroscopy

He I/II ultraviolet photoelectron spectra are reported for the interaction between the machined surfaces of the metals zinc, cadmium, copper and silver and molecular nitrogen dioxide. There is a complex variety of ways in which nitrogen dioxide can react with metal surfaces. Simple and dissociative adsorption have been reported and surface reaction can also occur between adsorbed species. Although ultraviolet photoelectron spectra reflect the valence shell density of states of a surface they do not always give information which can be interpreted in atomic terms or used to give unambiguous identifications of surface species. However using chemical shift information and to some extent, a fingerprint approach, we infer from our ultraviolet photoelectron spectroscopy (UPS) study that for zinc and cadmium the likely result of the nitrogen dioxide interaction is the formation of a surface nitrite and possibly nitrate for large nitrogen dioxide doses. For copper and silver surfaces it appears that the majority species is oxygen, probably in the form of surface oxide.

Biological studies in the vicinity of a shallow-sea tidal mixing front. I. Physical and chemical background

A study has been made of the distribution and activities of bacteria and zooplankton as they varied seasonally in 1980 and 1981 in the vicinity of a shallow-sea tidal mixing front in the western Irish Sea (approximate position 53° 20' N, 5° 45' W to 53° 50' N, 5° 0' W ). This paper presents the physical and chemical background to these studies as shown by the variations in temperature and salinity and concentrations of chlorophyll a , phaeopigments, cellular adenosine triphosphate (ATP), nitrate, nitrite and ammonium nitrogen, in sections normal to the front. Observations at drogue stations were made to establish the extent of diurnal variations in these properties but these appeared to be small relative to other variations. As the front developed, higher chlorophyll a concentrations appeared in the surface stratified water, in contrast to the bottom stratified water and mixed water, with highest concentrations at the surface at the stratified side of the front and in subsurface patches in the vicinity of the pycnocline. As the phytoplankton populations increased nitrate became depleted in the surface stratified water but nitrite and ammonium nitrogen concentrations remained at about the same levels. Cellular ATP concentration did not appear to be a useful measure of total biomass but indicated high biological activity in the surface stratified water.

Nitric oxide adsorption onto NiO/MgO and CoO/MgO solid solutions: an IR and ESR study

When nitric oxide is adsorbed onto NiO/MgO and CoO/MgO solid solutions, disproportionation occurs giving gaseous N 2O and surface nitrites. In the case of CoO/MgO, particularly exposed Co +2O −2 ion pairs react to form Co + and nitrites of a new kind. NO also coordinates to surface metal ions, giving weakly bound nitrosyls which, in the case of Ni +2 and Co +, are paramagnetic.

Mechanism of NO adsorption and decomposition on ruthenium catalysts

Thermal desorption and IR spectroscopic studies of NO adsorption on disperse Ru-black and Ru/Al2O3 catalysts have revealed that NO is adsorbed as a surface nitrosyl, nitrite and nitrate complex, the stability of which depends on the temperature. A mechanism for the interaction of adsorbed NO with Ru catalysts is suggested.