Gas-phase NO Research Articles

Nitrate Ion Photolysis in Thin Water Films in the Presence of Bromide Ions

Nitrate ions commonly coexist with halide ions in aged sea salt particles, as well as in the Arctic snowpack, where NO(3)(-) photochemistry is believed to be an important source of NO(y) (NO + NO(2) + HONO + ...). The effects of bromide ions on nitrate ion photochemistry were investigated at 298 ± 2 K in air using 311 nm photolysis lamps. Reactions were carried out using NaBr/NaNO(3) and KBr/KNO(3) deposited on the walls of a Teflon chamber. Gas phase halogen products and NO(2) were measured as a function of photolysis time using long path FTIR, NO(y) chemiluminescence and atmospheric pressure ionization mass spectrometry (API-MS). Irradiated NaBr/NaNO(3) mixtures show an enhancement in the rates of production of NO(2) and Br(2) as the bromide mole fraction (χ(NaBr)) increased. However, this was not the case for KBr/KNO(3) mixtures where the rates of production of NO(2) and Br(2) remained constant over all values of χ(KBr). Molecular dynamics (MD) simulations show that the presence of bromide in the NaBr solutions pulls sodium toward the solution surface, which in turn attracts nitrate to the interfacial region, allowing for more efficient escape of NO(2) than in the absence of halides. However, in the case of KBr/KNO(3), bromide ions do not appreciably affect the distribution of nitrate ions at the interface. Clustering of Br(-) with NO(3)(-) and H(2)O predicted by MD simulations for sodium salts may facilitate a direct intermolecular reaction, which could also contribute to higher rates of NO(2) production. Enhanced photochemistry in the presence of halide ions may be important for oxides of nitrogen production in field studies such as in polar snowpacks where the use of quantum yields from laboratory studies in the absence of halide ions would lead to a significant underestimate of the photolysis rates of nitrate ions.

Effect of soot on the storage-reduction performances of PtBa/Al 2O 3 LNT catalyst

The effect of soot on the storage-reduction performances of a PtBa/Al 2O 3 catalyst is investigated in this work. It is found that the presence of soot reduces the NO x storage capacity of the catalyst, evaluated in presence of water and CO 2 in the feed stream in the range 200–350 °C and with different values of the NO inlet concentration. Besides the presence of soot favors the decomposition and the reduction of the stored nitrates, while soot is being oxidized. A direct reaction between the stored nitrates and soot is suggested, that has been explained on the basis of the surface mobility of the adsorbed nitrates. This soot oxidation pathway involves surface species and parallels the NO 2-soot oxidation that occurs in the presence of gas-phase NO 2. Finally, the presence of soot does not influence appreciably the behavior of the PtBa/Al 2O 3 catalyst in the reduction by H 2 of the stored nitrates: in all cases N 2 is the main reaction product and ammonia is observed in minor amounts.

Deactivation mechanism of PtOx/TiO 2 photocatalyst towards the oxidation of NO in gas phase

This study has been undertaken to investigate the roles of PtO and PtO 2 deposits in photocatalytic oxidation of NO over Pt-modified TiO 2 catalysts. These photocatalysts were prepared by neutralization method and characterized by XRD, BET, XPS, TEM and FTIR. It was found that Pt dopant existed as PtO and PtO 2 particles in as-prepared photocatalysts. And these Pt dopants would change their oxidation states during the photocatalytic oxidation reaction. An in situ XPS study indicated that a portion of PtO 2 on the surface of Pt/TiO 2 was reduced to PtO under UV irradiation. The migration of electrons to PtO 2 particles could separate the electrons and holes, resulting in the improvement of photocatalytic activity. And the depletion of PtO 2 by electrons could lead to the deactivation of Pt/TiO 2 catalyst. Moreover, PtO particles could be corroded to form Pt 2+ ions by HNO 3, which was one of the products of photocatalytic oxidation of NO. NO would adsorb on Pt 2+ related sites to form Pt n+ –NO nitrosyls, retarding photocatalytic oxidation of NO to NO 2.

Photoenhanced NO2 Loss on Simulated Urban Grime

The present study focuses on the heterogeneous reaction between gaseous NO(2) and solid pyrene/KNO(3) films, used as a simplified proxy of urban grime. This reaction is investigated under simulated atmospheric conditions with respect to relative humidity, NO(2) concentration and irradiation in a coated-wall flow-tube reactor. The geometric steady-state uptake coefficients γ(geo) for pyrene/KNO(3) films exposed to 50 ppbv of NO(2) ranged from 1.12×10(-7) in the dark to 2.67×10(-6) under near-UV irradiation (300-420 nm) and decreased with increasing NO(2) concentration in the range 30-120 ppbv. NO(2) removal is linearly dependent on light intensity, with release of gas-phase NO and HONO. Analysis of the solid film by ion chromatography and GC-MS showed the formation of nitrite ions and traces of 1-nitropyrene. A light-induced reaction mechanism is proposed. The results discussed herein suggest that PAH-containing urban grime on windows and buildings may be a key player in urban air pollution.

Comment on “On the Electron Affinity of Nitromethane (CH3NO2)”

Nitrogen dioxide, NO2, being a stable free radical is a reactive gaseous species that is well-known as an anthropogenic pollutant; it is the major initiator of photochemical smog in the troposphere and is an ozone-depleting agent in the stratosphere.(1) Recently, following the successful multireference ab initio determination of an accurate value for the adiabatic electron affinity (AEA) of nitromethane, CH3NO2,(2) it has come to our attention that a similar high-level multiconfigurational convergence of theory and experiment has not been reported for NO2. The previous CH3NO2 study utilizing the N-state multiconfigurational quasi-degenerate perturbation theory of second order method, NS-MCQDPT,(3) showed that for CH3NO2 electron capture is dominated by the -NO2 group, and the level of theory in that study should also apply to gas-phase NO2.

Mechanism and micro-kinetics of direct N2O decomposition over BaFeAl11O19 hexaaluminate and comparison with Fe-MFI zeolites

Mechanistic and kinetic aspects of direct N2O decomposition over BaFeAl11O19 hexaaluminate were investigated in the Temporal Analysis of Products (TAP) reactor and compared with those previously determined for Fe-MFI zeolites. The catalysts were chosen due to their de-N2O operation in significantly different temperature regimes. Several micro-kinetic models were evaluated for describing the transient responses of N2O, N2, and O2 obtained in N2O pulse experiments at 823–973K. Thorough discrimination between these models enabled us to conclude that the preferred models of N2O decomposition over BaFeAl11O19 and Fe-MFI zeolites differ in the reaction pathways leading to O2 and N2. Gas-phase N2 and O2 are simultaneously formed over BaFeAl11O19 upon interaction of gas-phase N2O with a bi-atomic surface oxygen (*–O2) species. Contrarily, the formation of O2 over Fe-MFI occurs via a sequence of three elementary heterogeneous steps and limits the overall rate of N2O decomposition. Despite the easy O2 formation, BaFeAl11O19 is less active for N2O decomposition below 973K than the Fe-MFI zeolites due to the low coverage by *–O2. According to our quantitative micro-kinetic analysis, this species is formed when gas-phase N2O reacts with a mono-atomic oxygen (*–O) species. This reaction pathway is strongly influenced by the degree of isolation of iron species. The higher the degree of iron isolation in the catalyst, the lower the de-N2O activity.

ChemInform Abstract: Enhancement of N2O4 on Porous Glass at Room Temperature: A Key Intermediate in the Heterogeneous Hydrolysis of NO2?

The heterogeneous hydrolysis of NO{sub 2} at surfaces in the atmosphere is believed to be a significant source of HONO, a key OH precursor in urban areas. However, the mechanism of this reaction is not known. The uptake of 2.9 Torr of NO{sub 2} in N{sub 2} at a total pressure of 508 Torr on a porous glass surface with varying amounts of surface-adsorbed water was studied using FTIR at 294 K. The ratio of N{sub 2}O{sub 4} to NO{sub 2} was enhanced on the glass surface relative to the gas phase. On a relatively dry surface, the formation of surface-adsorbed HNO{sub 3} was observed over a period of {approximately}20 h, likely due to the reaction with small amounts of water on the surface. Gas-phase NO and N{sub 2}O were also generated. When larger amounts of water were initially present on the surface, surface-adsorbed HNO{sub 3} was formed immediately, as well as gas-phase NO, N{sub 2}O, and HONO. Although the NO{sub 2} concentrations used in the present studies are much larger than those found in the atmosphere, this work suggests that N{sub 2}O{sub 4} should be considered as a key intermediate in the heterogeneous hydrolysis of NO{sub 2} to formmore » HONO.« less

Release of Gas-Phase Halogens by Photolytic Generation of OH in Frozen Halide−Nitrate Solutions: An Active Halogen Formation Mechanism?

To better define the mechanisms by which condensed-phase halides may be oxidized to form gas-phase halogens under polar conditions, experiments have been conducted whereby frozen solutions containing chloride (1 M), bromide (1.6 x 10(-3) to 5 x 10(-2) M), iodide (<1 x 10(-5) M), and nitrate (0.01 to 1 M) have been illuminated by ultraviolet light in a continually flushed cell. Gas-phase products are quantified using chemical ionization mass spectrometry, and experiments were conducted at both 248 and 263 K. Br(2) was the dominant product, along with smaller yields of IBr and trace BrCl and I(2). The Br(2) yields were largely independent of the Br(-)/Cl(-) ratio of the frozen solution, down to seawater composition. However, the yields of halogens were strongly dependent on the levels of NO(3)(-) and acidity in solution, consistent with a mechanism whereby NO(3)(-) photolysis yields OH that oxidizes the condensed-phase halides. In support, we observed the formation of gas-phase NO(2), formed simultaneously with OH. Gas-phase HONO was also observed, suggesting that halide oxidation by HONO in the condensed phase may also occur to some degree. By measuring the production rate of condensed-phase OH, using benzoic acid as a radical trap, we determine that the molar yield of Br(2) formation relative to OH generation is 0.6, consistent with each OH being involved in halide oxidation. These studies suggest that gas-phase halogen formation should occur simultaneously with NO(x) release from frozen sea ice and snow surfaces that contain sufficient halides and deposited nitrate.

Study of DPNR catalysts for combined soot oxidation and NO x reduction

The behavior of a model PtBa/Al 2O 3 model NSR catalyst in both the NO x storage/reduction and soot oxidation is investigated in this work. It is found that the presence of soot negatively influences the NO x storage capacity of the catalyst, evaluated at 623 K in presence of water and CO 2 in the feed stream: in fact the amounts of NO x stored in presence of soot decrease by nearly 30% in the presence of roughly 10% (w/w) of soot. The presence of soot has a destabilizing effect on the NO x adsorbed species, which decompose to a large extent in the absence of gas phase NO and oxygen. This has also been confirmed by dedicated TPD and TPO experiments. However the presence of soot does not appreciably affect the behavior of the PtBa/Al 2O 3 catalyst in the reduction by H 2 of the stored nitrates, being in all cases N 2 the major reaction product along with minor amounts of ammonia. During the storage of NO x , soot oxidation takes place. Notably, the stored NO x participate in the soot oxidation upon release of NO 2 and O 2 which actively oxidize soot. However a direct participation of the adsorbed NO x species in the oxidation of soot cannot be excluded.

Effect of primary amine hydrocarbon chain length for the selective catalytic reduction of NO x from diesel engine exhaust

The effect of increasing primary amine hydrocarbon chain length on the SCR of NO x from diesel engine exhaust was investigated and compared to ammonia. Methylamine (CH 3NH 2), ethylamine (CH 3CH 2NH 2), propylamine (CH 3CH 2CH 2NH 2) and butylamine (CH 3CH 2CH 2CH 2NH 2) were tested using a 12 cell mini core NH 3 – SCR catalyst cut from a 400 cpsi block. There is a steady decrease in NO x conversion as the length of the hydrocarbon chain increases (from 50% for methylamine to 26% for butylamine). For the same number of carbons in the amine, primary amines are more active reductants than methyl substituted secondary or tertiary amines. For example, ethylamine (NO x conversion of 45%) is more active than dimethylamine (NO x conversion of 34%). Since the amines are reactive in the gas phase in the temperature range of diesel engine exhaust, gas phase conversions were estimated by replacing the mini core SCR catalyst with an equivalent length of quartz beads. There was no smooth transition in gas phase NO and NO x conversions with increasing hydrocarbon chain length. The results suggest a different mechanism for gas phase reactions depending on the nature of the amine.

Low-temperature selective catalytic reduction of NO with NH 3 over Mn [sbnd]Ce oxides supported on TiO 2 and Al 2O 3: A comparative study

Mn Ce oxides were supported on TiO 2 and Al 2O 3 by an ultrasonic impregnation method and used for selective catalytic reduction (SCR) of NO with NH 3 at low-temperature (80–220 °C). Mn Ce/TiO 2 showed a relatively higher SCR activity than Mn Ce/Al 2O 3 at the temperature range of 80–150 °C. When the reaction temperature was higher than 150 °C, Mn Ce/Al 2O 3 exhibited superior SCR activity to Mn Ce/TiO 2. NH 3 temperature programmed desorption study proved that Mn Ce/TiO 2 was mainly Lewis acidic, while Mn Ce/Al 2O 3 could provide more Brönsted acid sites. These acid sites play an important role in SCR according to in situ diffuse reflectance infrared transform spectroscopy (DRIFT) analysis. The main SCR reaction was a typical Eley–Rideal mechanism on Mn Ce/TiO 2, which took place between coordinated NH 3 / NH 4 + and gas-phase NO. For Mn Ce/Al 2O 3, the reaction mainly occurred via another pathway when the temperature exceeded 150 °C, which commenced with the adsorption and oxidation of NO and was followed by reaction between NO 2 or NO 2-containing compounds and NH 3 adspecies. This reaction pathway makes a significant contribution to the improved NO conversion for Mn Ce/Al 2O 3 at higher temperature.

Influence of NO x adsorbed species on component permeation through ZSM-5 membranes

A thin ZSM-5 film was grown on an α-alumina support, resulting in a composite membrane. The membranes were characterized by SEM and adsorption branch n-hexane/helium permporometry. In addition, the permeation of gas mixtures containing NO 2, NO, N 2 and argon was evaluated. The effect of temperature and gas mixture composition on the component permeation and selectivity was investigated. It was found that NO x permeation through the ZSM-5 membrane was partially surface concentration dependent and was thermally activated. However, transport by gas translational diffusion seemed to dominate at the conditions studied. The presence of various NO x adsorbed species appeared to influence diffusion of NO 2 in ZSM-5 and reduced transport of other inert and weakly adsorbed components over a wide temperature range (20–400 °C). Strongly adsorbed surface nitrate species formed in the presence of gas phase NO x should be responsible for the reduced transport of these components at the elevated temperature. The findings are of interest for possible applications of ZSM-5 membranes for component separation at high temperature.

CO/NO and CO/NO/O 2 reactions over a Au–Pd single crystal catalyst

NO reduction by CO was investigated over a AuPd(1 0 0) model catalyst at near atmospheric pressures. The alloy catalyst exhibits higher CO 2 formation rates below ∼550 K than does pure Pd, although the binding energy of NO on the alloy surface is substantially less, i.e., its dissociation tendency is much less, compared with pure Pd. This behavior is rationalized by the fact that the low CO/NO-binding energies with the alloy surface provide a substantial population of empty ensembles for NO dissociation at relatively low temperatures. Moreover, AuPd(1 0 0) catalyzes the CO + NO reaction with much higher N 2 selectivity than does pure Pd. Reaction kinetics data reveal that contiguous Pd sites are essential for NO dissociation. The reaction orders in CO and NO pressures, vastly different from Pd and Rh, are also a consequence of the low-binding energies of CO and NO on the alloy surface. It is also found that low-pressure NO promotes the CO + O 2 reaction via gas-phase NO 2 formation; the latter dissociates to form O (ads) more efficiently than does O 2 below ∼600 K. However, when the NO pressure exceeds a critical value, gas-phase NO 2 causes surface oxidation and thus inhibits CO 2 formation. The current study suggests that AuPd alloys should be superior catalysts compared to traditional catalysts with respect to the “cold start” problem in catalytic automobile pollutant removal.

Relationship between Pd oxidation states on TiO 2 and the photocatalytic oxidation behaviors of nitric oxide

This study has been undertaken to investigate the relationship between Pd oxidation states on TiO 2 photocatalysts and their photocatalytic oxidation behaviors of NO. Three types of Pd-modified TiO 2 with different Pd oxidation states were prepared by wet impregnation method, neutralization method and photodeposition method, respectively. And these Pd-modified photocatalysts were characterized by X-ray diffraction analysis, X-ray photoelectron spectrum analysis (XPS), UV–Vis diffuse reflectance spectra and temperature programmed desorption (TPD). It was found from XPS results that the dominant oxidation states of Pd on these Pd-modified TiO 2 catalysts were Pd 2+, PdO, and Pd 0, respectively. NO-TPD results showed that the NO adsorption capacity was improved greatly by the modification of Pd 2+ ions. The activity tests showed that Pd-modified TiO 2 by a wet impregnation method increased photocatalytic activity compared to pure TiO 2 (Degussa P25). It was concluded that Pd 2+ ions on as-prepared TiO 2 catalysts provided key contributions to the improvement of photocatalytic activity. However, Pd 0 and PdO deposits on TiO 2 almost had no positive effect on NO oxidation. The mechanism of photocatalytic oxidation of NO in gas phase over Pd-modified TiO 2 was also proposed.

Efficient Photocatalytic Removal of NO in Indoor Air with Hierarchical Bismuth Oxybromide Nanoplate Microspheres under Visible Light

In this study, hierarchical bismuth oxybromide (BiOBr) nanoplate microspheres were used to remove NO in indoor air under visible light irradiation. The BiOBr microspheres were synthesized with a nonaqueous sol-gel method by using bismuth nitrate and cetyltrimethyl ammonium bromide as the precursors. On degradation of NO under visible light irradiation (lambda > 420 nm) at 400 part-per-billion level, which is typical concentration for indoor air quality, these nonaqueous sol-gel synthesized hierarchical BiOBr microspheres exhibited superior photocatalytic activity to the chemical precipitation synthesized counterpart BiOBr bulk powder and Degussa TiO2 P25 as well as C doped TiO2. The excellent catalytic activity and the long-term activity of nonaqueous sol-gel synthesized BiOBr microspheres were attributed to their special hierarchical structure, which was favorable for the diffusion of intermediates and final products of NO oxidation. Ion chromatograph results confirmed that nitric acid was produced on the surface of BiOBr microspheres during the photooxidation of NO in gas phase. This work suggests that the nonaqueous sol-gel synthesized BiOBr nanoplate microspheres are promising photocatalytic materials for indoor air purification.

Effect of NO2 Concentration on Product Yields of the Gas-Phase NO3 Radical-Initiated Reaction of Ethyl- and Dimethyl-naphthalenes

Alkylnaphthalenes are minor constituents of vehicle fuels and are emitted into the atmosphere in vehicle exhaust and other sources of incomplete combustion. In the lower atmosphere, alkylnaphthalenes react with OH radicals during daylight hours and with NO3 radicals during evening and nighttime, and both radical-initiated reactions have been postulated to explain ambient alkylnitronaphthalene profiles. To obtain insight into the NO3 radical reaction mechanism, we have investigated the formation of potentially genotoxic ethyl- and dimethyl-nitronaphthalenes and quinones, as well as aromatic carbonyls, from the NO3 radical-initiated reactions of ethylnaphthalenes (ENs) and dimethylnaphthalenes (DMNs) over the NO2 concentration range approximately 0.2-2 ppmV. Our results for the formation of alkylnitronaphthalenes are consistent with gas-phase NO3 radical-initiated reactions being a source of alkylnitronaphthalenes in ambient air, and it appears that quinone formation from the gas-phase nighttime reactions of NO3 radicals with 2,6- and 2,7-DMN may be important in the atmosphere and warrants further investigation.

In situ FT-IR Study on the Transient States of Adsorbed Species during CO Oxidation by O2 and NO over Pt/.GAMMA.-Al2O3 Catalyst

The transient state of adsorbed species during the CO–NO reaction on Pt/γ-Al2O3 catalyst was monitored using in situ FT-IR. Experiments were performed at the low temperature region for elevating adsorption rate. The amount of adsorbed species on the surface was calculated from band-area of absorbance. The change in the CO consumption rate in the CO–O2 reaction was viewed by relating to that in the relative area of linear-CO on the catalyst. The experiments where catalyst was exposed to gaseous NO showed that the increase in the concentration of NO in the feed stream resulted in the increase in the relative area of nitrate species as bridge-NO and in a concomitant decrease in that of linear-NO. An argument is made in that bridge-NO was formed by binding gas phase NO onto the chemisorbed atomic oxygen that was formed in the dissociation of linear-NO. The participation of chemisorbed atomic oxygen in CO oxidation by NO over Pt/γ-Al2O3 at low temperature was viewed in the results of experiment where the catalyst was exposed to gases containing a fixed amount of CO and varied amount of NO.

BaO/Al2O3/NiAl(110) Model NOx Storage Materials: The Effect of BaO Film Thickness on the Amorphous-to-Crystalline Ba(NO3)2 Phase Transition

The reaction of NO2 with BaO (0.15−2 ML and >30 ML)/Al2O3(12 ML)/NiAl(110) model NOx storage materials was studied. A thick (∼12 ML), ordered Al2O3 film was prepared as the support oxide on a NiAl(110) substrate to minimize the effect of the intermixing between the two oxide phases (BaO and Al2O3) on the NOx chemistry of BaO. The growth of a thick alumina film, prepared by atomic oxygen deposition onto NiAl(110), follows a layer-by-layer growth mode, and the resulting film is much more stable when exposed to NO2 than the ultrathin alumina films studied before. The interaction of NO2 with the model NOx storage systems at low coverages of BaO shows behaviors fundamentally different from a thick BaO film, as nitrite species form at low exposures of NO2, followed by nitrate formation at high NO2 exposures. In contrast, on the thick BaO layer, nitrite−nitrate ion pairs form at 300 K under UHV conditions (PNO2 ∼ 1 × 10−9 Torr). However, at elevated NO2 pressures (≥1 × 10−5 Torr), the thick BaO film is gradually converted into amorphous Ba(NO3)2 at 300 K. Raising the temperature of the samples with ΘBaO > 1 ML after NO2 exposure (in the absence of gas phase NO2) leads to the phase transformation of the amorphous Ba(NO3)2 layer into crystalline Ba(NO3)2 particles in the temperature range of 500−600 K. No phase transformation is observed in samples with ΘBaO < 1 ML.

Influences of various Pt dopants over surface platinized TiO 2 on the photocatalytic oxidation of nitric oxide

Various surface platinized TiO 2 were prepared by four different preparation methods and investigated with respect to their behaviors in UV photocatalytic oxidation of nitric oxide. The physicochemical properties of the Pt modified TiO 2 were investigated by X-ray diffraction analysis, X-ray photoelectron spectrum analysis, transmission electron microscopy, and photoluminescence spectra. From the experimental results, it was found that new electronic states were observed above the valence bands of PtOx-TiO 2 and PtClx-TiO 2. And the lifetime of electrons and holes was found prolonged in the PtOx-TiO 2 catalysts. The activity tests showed that the dopants existed as metallic Pt and platinum chloride had little contribution to the photocatalytic oxidation of NO in gas phase. However, the dopant which existed as PtOx could improve the NO photocatalytic oxidation efficiency and the reaction rate. The photocatalytic activity of the 0.05 at% PtOx-TiO 2 was nearly three times higher than that of the pure Degussa P25 with an inlet NO concentration of 200 ppm.

Nitrogen Monoxide Interaction with Cu(I) Sites in Zeolites X and Y: Quantum Chemical Calculations and IR Studies

The NO molecules adsorbed on Cu(I) sites in FAU lattice show strong red shift of stretching bond frequencies in comparison to the gas-phase NO. Three IR mononitrosyl bands can be found in Cu(I)Y (1815, 1793, and 1780 cm−1) and two in Cu(I)X (1772 and 1757 cm−1), respectively. Theoretical models allow association of all these bands with the species adsorbed on site II. The highest band in Cu(I)Y (1815 cm−1), not observed in this work, is assigned to mononitrosyl species inside 6T rings with 1Al and Cu(I) coordinated to a single tetrahedron; the middle one (1793 cm−1) derives both from 6T/1Al rings with 2(2) coordination and from 6T/2Al rings with 2(1) coordination, while the lowest band (1780 cm−1) is due to the 6T/2Al rings with copper bound with two framework tetrahedrons. As for Cu(I)X, the higher (1772 cm−1) and the lower (1757 cm−1) bands are assigned to the adsorption complexes inside 6T/3Al rings with 2(1) and 3(3) coordination, respectively. No experimental mononitrosyl band is assigned to the spec...