Major Gas Phase Products Research Articles

Atmospheric Chemistry of 2-Amino-2-methyl-1-propanol: A Theoretical and Experimental Study of the OH-Initiated Degradation under Simulated Atmospheric Conditions.

The OH-initiated degradation of 2-amino-2-methyl-1-propanol [CH3C(NH2)(CH3)CH2OH, AMP] was investigated in a large atmospheric simulation chamber, employing time-resolved online high-resolution proton-transfer reaction-time-of-flight mass spectrometry (PTR-ToF-MS) and chemical analysis of aerosol online PTR-ToF-MS (CHARON-PTR-ToF-MS) instrumentation, and by theoretical calculations based on M06-2X/aug-cc-pVTZ quantum chemistry results and master equation modeling of the pivotal reaction steps. The quantum chemistry calculations reproduce the experimental rate coefficient of the AMP + OH reaction, aligning k(T) = 5.2 × 10–12 × exp (505/T) cm3 molecule–1 s–1 to the experimental value kexp,300K = 2.8 × 10–11 cm3 molecule–1 s–1. The theoretical calculations predict that the AMP + OH reaction proceeds via hydrogen abstraction from the −CH3 groups (5–10%), −CH2– group, (>70%) and −NH2 group (5–20%), whereas hydrogen abstraction from the −OH group can be disregarded under atmospheric conditions. A detailed mechanism for atmospheric AMP degradation was obtained as part of the theoretical study. The photo-oxidation experiments show 2-amino-2-methylpropanal [CH3C(NH2)(CH3)CHO] as the major gas-phase product and propan-2-imine [(CH3)2C=NH], 2-iminopropanol [(CH3)(CH2OH)C=NH], acetamide [CH3C(O)NH2], formaldehyde (CH2O), and nitramine 2-methyl-2-(nitroamino)-1-propanol [AMPNO2, CH3C(CH3)(NHNO2)CH2OH] as minor primary products; there is no experimental evidence of nitrosamine formation. The branching in the initial H abstraction by OH radicals was derived in analyses of the temporal gas-phase product profiles to be BCH3/BCH2/BNH2 = 6:70:24. Secondary photo-oxidation products and products resulting from particle and surface processing of the primary gas-phase products were also observed and quantified. All the photo-oxidation experiments were accompanied by extensive particle formation that was initiated by the reaction of AMP with nitric acid and that mainly consisted of this salt. Minor amounts of the gas-phase photo-oxidation products, including AMPNO2, were detected in the particles by CHARON-PTR-ToF-MS and GC×GC-NCD. Volatility measurements of laboratory-generated AMP nitrate nanoparticles gave ΔvapH = 80 ± 16 kJ mol–1 and an estimated vapor pressure of (1.3 ± 0.3) × 10–5 Pa at 298 K. The atmospheric chemistry of AMP is evaluated and a validated chemistry model for implementation in dispersion models is presented.

Kinetics and mechanisms of the gas-phase reactions of OH radicals with three C15 alkanes

The reaction constants and products for the reactions of OH radicals with different C15 alkanes, were determined by using Proton-Transfer-Reaction Mass Spectrum (PTR-MS) in the smog chamber. Rate coefficients (in units of 10−11 cm3 molecule−1 s−1) k1(pentadecane) = 1.72 ± 0.18,k2(2,6,10-trimethyldodecane) = 1.90 ± 0.20, k3(nonylcyclohexane) = 2.09 ± 0.22 were first obtained by the relative rate method using toluene and m-xylene as reference compounds at (298 ± 1)K and atmospheric pressure. In addition, the products of these reactions were measured by PTR-MS simultaneously. The results show that the major gas phase products observed in the OH radicals reactions were aldehydes and ketones for C15 alkanes. The possible reaction mechanisms were speculated and the atmospheric implications were also discussed.

Gas and particulate phase products from the ozonolysis of acenaphthylene

Polycyclic aromatic hydrocarbons (PAHs) are recognized as important secondary organic aerosol (SOA) precursors in the urban atmosphere. In this work, the gas-phase ozonolysis of acenaphthylene was investigated in an atmospheric simulation chamber using a proton transfer reaction time-of-flight-mass spectrometer (PTR-TOF-MS) and an aerosol time-of-flight-mass spectrometer (ATOFMS) for on-line characterization of the oxidation products in the gas and particle phases, respectively. SOA samples were also collected on filters and analyzed by ultra performance liquid chromatography/electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-QTOFMS) and gas chromatography/electron impact ionization-mass spectrometry (GC/EI-MS). The major gas-phase products included a range of oxygenated naphthalene derivatives such as 1,8-naphthalic anhydride, naphthalene 1,8-dicarbaldehyde and naphthaldehyde, as well as a secondary ozonide. Possible reaction mechanisms are proposed for the formation of these products and favoured pathways have been suggested. Many of these products were also found in the particle phase along with a range of oligomeric compounds. The same range of gas and particle phase products was observed in the presence and absence of excess cyclohexane, an OH scavenger, indicating that OH radical production from the ozonolysis of acenaphthylene is negligible. SOA yields in the range 23–37% were determined and indicate that acenaphthylene ozonolysis may contribute to part of the SOA observed in urban areas.

Role of stabilized Criegee Intermediate in secondary organic aerosol formation from the ozonolysis of α-cedrene

Atmospheric ozonolysis of sesquiterpenes is an important source of secondary organic aerosols (SOA). The mechanisms by which Criegee Intermediates (CIs) react to form SOA precursors and the influence of environmental conditions, however, remain unclear. On the basis of environmental chamber experiments coupled with detailed characterization of gas-phase and particle-phase products, we present evidence that a significant fraction of CIs from ozonolysis of α-cedrene are stabilized and bimolecular reactions of these stabilized CIs (SCIs) play a key role in the formation of SOA precursors. Ozonolysis experiments were conducted in a 4.5 m3 collapsible fluoropolymer chamber under various conditions in the presence of the OH radical and SCI scavengers. The size and mass of SOA particles produced during ozonolysis were measured directly and used for calculation of particle effective density and mass yield. Gaseous and particulate products were analyzed by several mass spectrometry methods. A total of 14 compounds in gas phase and 17 compounds in particle phase were tentatively identified. The major gas-phase products are secondary ozonides (SOZ) from intramolecular reactions of SCIs. Multifunctional organic acids are dominant particle-phase products. The measured density of aerosol particles is 1.04 ± 0.03 to 1.38 ± 0.03 g/cm3, and the aerosol mass yield is (23.7 ± 0.4)% to (46.4 ± 6.5)%, depending on reaction conditions. The presence of acetic acid, an SCI scavenger, inhibits new particle formation, but leads to increased aerosol mass yield. In contrast, the addition of SO2 dramatically enhances new particle formation and total aerosol yield. The calculated OH formation yield decreases from (62.4 ± 4.9)% to (9.0 ± 1.6)% upon addition of SCI scavengers CH3COOH and SO2, indicating that a large fraction of excited CIs are collisionally stabilized and unimolecular decomposition of SCIs via the hydroperoxide channel can be suppressed by bimolecular reactions. The reaction of SCIs with SO2 leads to the formation of sulfuric acid, an important nucleation precursor. From the consumption of SO2 added as SCI scavenger, a lower-limit yield of SCIs from α-cedrene ozonolysis is estimated at ∼88%. Our work underscores the key role of SCIs in SOA formation and observed composition of gas- and particle-phase products from α-cedrene ozonolysis. Bimolecular reactions of sesquiterpene CIs with atmospherically relevant species (e.g. SO2, H2O) need to be considered when assessing the atmospheric relevance of ozonolysis of sesquiterpenes.

Electrochemical Reduction of CO2 using Copper Oxide Nanoparticles supported on Glassy Carbon Electrodes

ABSTRACTWe have studied the electrochemical reduction of CO2 using Cu2O nanoparticles deposited on planar electrodes. Nanoparticles are prepared in aqueous solution by chemical reduction of CuCl2 using ascorbic acid with polyethylene glycol surfactant. The particles are then re-suspended in ethanol with added Nafion binder and brush-coated onto glassy carbon substrates. The CO2 electroreduction activity is measured in KHCO3 electrolyte under flowing CO2 using a two-compartment electrochemical cell. Product formation rates are determined using gas chromatography; major gas phase products include CO, H2, C2H4, and CH4, while liquid phase products include C2H5OH and 1-C3H5OH. The observed product distribution agrees with results obtained previously using similar Cu2O particles deposited on carbon fiber paper supports, as well as Cu2O catalysts prepared by electrodeposition or thermal oxidation. In particular, the catalysts produce a much higher ratio of C2H4 to CH4 than observed using polycrystalline Cu foil. The potential dependence of the formation rates for hydrocarbon and alcohol products is roughly two times greater than for H2 and CO formation. Both XRD and SEM measurements confirm the Cu2O nanoparticles undergo at least partial reduction to Cu metal under CO2 reduction conditions, accompanied by significant surface morphological changes. Thus the kinetic results are consistent with current models that the increased C2H4/CH4 ratio is due to the presence of a more open atomic structure on the freshly reduced Cu surfaces.

Development of Electrically Controlled Energetic Materials (ECEM)

U.S. Army ARDEC has been examining a novel pyroelectric-type material developed by Digital Solid State Propulsion, LLC for its unique combustion properties and insensitivity. This unique ECEM is formed using an ionic liquid oxidizer and polymer fuel, via sol-gel processes and is quite dense, making it very suitable for energetic applications in both the civilian and military fields. This ECEM is completely insensitive to the application of impact, flame, friction, and shock threats. Once a power of the required voltage is applied, the baseline ECEM rapidly initiates combustion with H2O, CO2, and N2 formed as the only major gas phase products produced. Initial testing of ECEM in perforated pipes appears promising as indicated by uniform ignition along the entire length of the test assembly.

Cl atom initiated oxidation of 1-alkenes under atmospheric conditions

In view of the importance of the oxidation pathways of alkenes in the troposphere, and the significance of Cl atom as an oxidant in marine boundary layer (MBL) and polluted industrial atmosphere, the reactions of four 1-alkenes (C6–C9) with Cl atoms are investigated. The rate coefficients at 298 K are measured to be (4.0 ± 0.5), (4.4 ± 0.7), (5.5 ± 0.9) and (5.9 ± 1.7) × 10−10 cm3 molecule−1 s−1 for 1-hexene, 1-heptene, 1-octene and 1-nonene, respectively. The quoted errors include the experimental 2σ, along with the error in the reference rate coefficients. From the systematic increase in the rate coefficients with the number of carbon atoms, an approximate value for the average rate coefficient for hydrogen abstraction per CH2 group in alkenes is estimated to be (4.9 ± 0.3) × 10−11 cm3 molecule−1 s−1. Based on these rate coefficients, the contribution of Cl atom reactions towards the degradation of these molecules is found to be comparable to that of OH radical reactions, under MBL conditions. The products identified in gas phase indicate that Cl atom addition occurs mainly at the terminal carbon, leading to the formation of 1-chloro-2-ketones and 1-chloro-2-ols. The major gas phase products from the alkenyl radicals (formed by H atom abstraction) are different positional isomers of long chain enols and enones. A preference for dissociation leading to an allyl radical, resulting in aldehydes, lower by three carbon atoms, is indicated. The observed relative yields suggest that in general, the increased contribution of the reactions of Cl atoms towards degradation of 1-alkenes in NOx free air does not result in an increase in the generation of small aldehydes (carbon number < 4), including chloroethanal, as compared to that in the reaction of 1-butene.

Non‐Oxidative Conversion of 1,2‐Dichloroethane in a Non‐Thermal Plasma and Characterisation of the Polymer Formed

AbstractThis paper focuses on the conversion of 1,2‐dichloroethane (EDC) using non‐thermal plasma under reaction conditions that can decompose EDC and yield a value‐added product. A cylindrical double dielectric barrier discharge system has been used to generate non‐thermal plasma and quartz has been used as a dielectric. Our findings show that under non‐oxidative reaction conditions employed in this study, vinyl chloride, a commercially important compound, is the major gas phase product produced. This paper also encompasses a preliminary characterisation of polymer during the reaction. Possible mechanisms for gaseous product formation and polymerisation are presented. magnified image

Heterogeneous Interaction of N2O5 with HCl Doped H2SO4 under Stratospheric Conditions: ClNO2 and Cl2 Yields

The reaction of dinitrogen pentoxide, N2O5, with hydrogen chloride, HCl, in sulfuric acid solutions was studied at temperatures and compositions relevant to the upper troposphere/lower stratosphere. Experiments were performed using a rotating wetted wall flow tube reactor coupled to a chemical ionization mass spectrometer for the gas-phase detection of reactants (N2O5 and HCl) and products (nitryl chloride, ClNO2, and Cl2) using I– as the reagent ion. Uptake coefficients, γ, were measured under stratospheric conditions: 205 < T < 225 K; 50 and 60 wt % H2SO4 solutions; 5.8 × 10(–5) < [HCl]liq < 0.1 M. Uptake coefficients of N2O5 on pure H2SO4/H2O (50 and 60 wt % H2SO4) and HCl-doped H2SO4 were found to be independent of temperature and sulfuric acid composition (weight percent of H2SO4 and HCl concentration) consistent with previous studies. ClNO2 was observed to be a major gas-phase product with its yield strongly dependent on the liquid-phase HCl concentration (5.8 × 10(–5) to 0.1 M HCl) and with a maximum yield of nearly unity at 0.005 M HCl in both 50 and 60 wt % sulfuric acid solutions. The Cl2 yield was <1% under all conditions studied. ClNO2 production was attributed to the heterogeneous reaction of NO2(+)(aq), or H2NO3(+)(aq) (formed in the dissociative ionization of N2O5), with Cl–. The variation of the ClNO2 yield with HCl concentration was attributed to the competition between the reaction of NO2(+)(aq), or H2NO3(+)(aq) with Cl– and H2O. Using our measured yields as a function of HCl concentrations in 50 and 60 wt % H2SO4 solutions at different temperatures, we calculated the variation of the ClNO2 yield under stratospheric conditions. The atmospheric implications of these findings were examined using a 2D atmospheric model. The contribution of this chemistry to ozone depletion was found to be a minor process under nonvolcanic background aerosol levels.

Pyrolytic Decomposition of Ammonia Borane to Boron Nitride

The thermal decomposition of ammonia borane was studied using a variety of methods to qualitatively identify gas and remnant solid phase species after thermal treatments up to 1500 °C. At about 110 °C, ammonia borane begins to decompose yielding H(2) as the major gas phase product. A two step decomposition process leading to a polymeric -[NH═BH](n)- species above 130 °C is generally accepted. In this comprehensive study of decomposition pathways, we confirm the first two decomposition steps and identify a third process initiating at 1170 °C which leads to a semicrystalline hexagonal phase boron nitride. Thermogravimetric analysis (TGA) was used to identify the onset of the third step. Temperature programmed desorption-mass spectroscopy (TPD-MS) and vacuum line methods identify molecular aminoborane (H(2)N═BH(2)) as a species that can be released in appreciable quantities with the other major impurity, borazine. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was used to identify the chemical states present in the solid phase material after each stage of decomposition. The boron nitride product was examined for composition, structure, and morphology using scanning Auger microscopy (SAM), powder X-ray diffraction (XRD), and field emission scanning electron microscopy (FESEM). Thermogravimetric Analysis-Mass Spectroscopy (TGA-MS) and Differential Scanning Calorimetry (DSC) were used to identify the onset temperature of the first two mass loss events.

Product study of oleic acid ozonolysis as function of humidity

The heterogeneous reaction of ozone with oleic acid (OA) aerosol particles was studied as function of humidity and reaction time in an aerosol flow reactor using an off-line gas chromatography mass spectrometry (GC–MS) technique. We report quantitative yields of the major C9 ozonolysis products in both gas and condensed phases and the effect of relative humidity on the product distribution. The measurements were carried out with OA aerosol particles at room temperature. The results indicate that the product yields are increasing with increasing relative humidity during the reaction. Nonanal (NN) was detected as the major gas-phase product (55.6 ± 2.3%), with 94.5 ± 2.4% of the NN yield in the gas, and 5.5 ± 2.7% in the particulate phase, whereas nonanoic, oxononanoic and azelaic acids were detected exclusively in the particulate phase. Using UV-spectrometry, we observed that peroxides make up the largest fraction of products, about half of the product aerosol mass, and their concentration decreased with increasing humidity.

Reactive Uptake of NO3, N2O5, NO2, HNO3, and O3on Three Types of Polycyclic Aromatic Hydrocarbon Surfaces

We investigated the reactive uptake of NO3, N2O5, NO2, HNO3, and O3 on three types of solid polycyclic aromatic hydrocarbons (PAHs) using a coated wall flow tube reactor coupled to a chemical ionization mass spectrometer. The PAH surfaces studied were the 4-ring systems pyrene, benz[a]anthracene, and fluoranthene. Reaction of NO3 radicals with all three PAHs was observed to be very fast with the reactive uptake coefficient, gamma, ranging from 0.059 (+0.11/-0.049) for benz[a]anthracene at 273 K to 0.79 (+0.21/-0.67) for pyrene at room temperature. In contrast to the NO3 reactions, reactions of the different PAHs with the other gas-phase species (N2O5, NO2, HNO3, and O3) were at or below the detection limit (gamma <or= 6.6 x 10(-5)) in all cases, illustrating that these reactions are at best slow. For NO3 we also investigated the time dependence of the reactive uptake to determine if the surface-bound PAH molecules were active participants in the reaction (i.e., reactants). Reaction of NO3 on all three PAH surfaces slowed down at 263 K after long NO3 exposure times, suggesting that the PAH molecules were reactants. Additionally, NO2 and HNO3 were identified as major gas-phase products. Our results show that under certain atmospheric conditions, NO3 radicals can be a more important sink for PAHs than NO2, HNO3, N2O5, or O3 and impact tropospheric lifetimes of surface-bound PAHs.

Decomposition of toluene in a gliding arc discharge plasma reactor

The decomposition of toluene in a gliding arc discharge (glidarc) was performed and studied. Experimental results indicate that the glidarc technology can effectively decompose toluene molecules and has bright prospects of being applied as an alternative tool to decompose volatile organic compounds. It is found that a change in the electrode material had an insignificant effect on the toluene removal efficiency. The toluene removal efficiency increases with increasing inlet gas temperature. The water vapor present in the gas mixture has a favorable effect on the toluene decomposition in the plasma. The energy efficiency is 29.46 g (kWh−1) at a relative humidity of 50% and a specific energy input of 0.26 kWh m−3, which is higher than other types of non-thermal plasmas. Too much or too little oxygen content does not favor toluene decomposition. The major gas phase products detected by FT-IR from the decomposition of toluene with air participation were CO, CO2, H2O and NO2. Some brown depositions were found on the surface of the electrodes, which were polar oxygenous and nitrogenous compounds determined by the GC-MS analysis, such as benzaldehyde, benzoic acid, quinine and nitrophenol from the reaction of toluene with radicals. A possible mechanism for toluene destruction via glidarc technology is proposed and summarized.

The Photodissociation of Perfluoropolyethers by Ultraviolet Light

The 172 nm ultraviolet (UV) photodissociation of perfluoropolyether (PFPE) boundary lubricants is examined experimentally and theoretically. Carbonyl fluoride, COF2, is the major gas phase product that results from the UV irradiation of non-functionalized perfluoropolyethers containing the perfluoromethylene oxide, perfluoroethylene oxide, and the perfluoro-n-propylene oxide monomer units. The energetics for COF2 evolution from the carbon centered radicals R-OCF2OCF2·, R-CF2CF2OCF2·; and from the oxygen centered radicals R-CF2OCF2O·, R-CF2CF2O· are quantified via ab initio calculations. The electronic structure of the transition state geometries leading to COF2 dissociation is characterized. COF2 originating from oxygen-centered radicals, leaving behind a carbon-centered radical on the PFPE main chain, appears to be the most energetically favorable dissociation pathway.

Reaction of phenylperoxy radicals with NO2 at 298 K

In the present work, phenylperoxy radicals were generated by stationary 254 nm photolysis of iodobenzene and nitrosobenzene in the presence of O(2) and NO(2) at 298 K and a total pressure of 1 bar (M = N(2)). Experiments were performed on time scales of seconds or minutes in a temperature controlled photoreactor made of quartz (v = 209 L). Major gas phase products identified and quantified in situ by long-path IR absorption include N(2)O(5), NO, HONO, HNO(3), CO, and o-nitrophenol. In addition, evidence is presented for the formation of an aerosol consisting of p-nitrophenol. The occurrence of N(2)O(5) as a major product in both reaction systems, the strong loss of NO(2) in the iodobenzene system and the comparison of measured product distributions with the results of numerical model calculations suggest that the reaction C(6)H(5)O(2) + NO(2) --> C(6)H(5)O + NO(3), k(5)occurs in both photolysis systems, a major part of the NO(3) being scavenged as N(2)O(5). The results of ab initio calculations imply that proceeds via a short-lived peroxynitrate intermediate. In the photolysis of nitrosobenzene-NO(2)-O(2)-N(2) mixtures, NO and NO(2) compete for C(6)H(5)O(2) radicals. Comparison of measured and modelled product distributions allows to set a lower limit of k(5) > 1 x 10(-12) cm(3) molecule(-1) s(-1) at 298 K. This lower limit is consistent with the assumption that k(5) is equal to the high pressure recombination rate constant of RO(2) + NO(2) --> RO(2)NO(2) reactions, i.e. with k(5) approximately 7 x 10(-12) cm(3) molecule(-1) s(-1) at 298 K, 1bar.

Using phosphorus doping of MoO 3/ZSM-5 to modify performance in methane dehydroaromatisation

The performance of methane dehydroaromatisation catalysts prepared by the impregnation of H-ZSM-5 with ammonium heptamolybdate and phosphomolybdic acid precursors has been found to be different. Particular attention has been paid to hydrogen production which, although the major gas-phase product of dehydroaromatisation, is generally not reported. The use of phosphomolybdate leads to lower activity catalysts. When 5 wt.% of phosphorus is added to the catalyst prepared from ammonium heptamolybdate, the activity is suppressed and H 2 is seen as the sole gas-phase product. The rate of hydrogen formation increases with time on stream passing through a maximum, which is associated with the loss of an inorganic phosphite phase from the catalyst. Bulk MoO 3 and P doped MoO 3 samples are observed to produce H 2 as a result of carburisation. Although the general pattern of hydrogen formation is similar to the 5 wt.% doped zeolite catalyst, in these cases gas-phase carbon oxides are also associated with the hydrogen maximum.

Real-Time Monitoring of the Kinetics and Gas-Phase Products of the Reaction of Ozone with an Unsaturated Phospholipid at the Air−Water Interface

While the kinetics and mechanisms of the reaction of O3 with alkenes in the gas and condensed phases are reasonably well understood, those with unsaturated organics in the intermediate regime at the air−water interface are not. Studies of the reaction of ozone at room temperature with the unsaturated phospholipid, 1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine (OPPC) at the air−water interface, and, for comparison, the fully saturated dipalmitoyl-l-α-phosphatidylcholine (DPPC) were carried out. The phospholipids were exposed at varying surface areas per phosphocholine molecule on a water subphase to a flow of O3 in air (0.25−1 ppm), and atmospheric pressure ionization mass spectrometry (API-MS) was used to monitor the formation of gaseous products in real time. Nonanal was detected as a major gas-phase product of the reaction of ozone with OPPC; no volatile products were observed in the case of DPPC. The yield of nonanal, defined as the nonanal produced per phosphocholine molecule reacted at the air−wat...

Infrared and NMR Spectroscopic Studies of n-Alkanethiols Chemically Grafted on Dimethylzinc-Modified Silica Surfaces

Transmission infrared and solid-state nuclear magnetic resonance (NMR) spectroscopies are used to study the surface species formed by sequential reaction of hydroxylated, high-surface-area silica with dimethylzinc (DMZ) and n-alkanethiols of various chain lengths. Reaction of DMZ forms mainly surface methylzinc species. These convert to Zn-bound ethanethiolate surface species by reaction with ethane thiol, evolving nearly identical amounts of methane at each step. Temperature-dependent infrared spectra of these surface thiolate species reveal that they are stable in air and aqueous environments and up to about 400 K in vacuo. The major gas-phase products identified during annealing are diethyl sulfide and ethylene. Longer-chain n-alkanethiols (up to C16) are reacted in benzene solutions and the modified silica powders are characterized by transmission infrared and solid-state NMR spectroscopies. The spectroscopic data demonstrate that the alkanethiolate chains retain their integrity and bind to the surface via the sulfur atom with their “tails” away from the surface. High thiolate coverages (similar to those of well-characterized self-assembled monolayers of thiols on gold and trichlorosilanes on silicon) are obtained in the case of shorter hydrocarbon chains. Carbon−hydrogen−nitrogen analyses and electron microprobe elemental analyses reveal that the density of the thiolate layer decreases with increasing chain length. This is attributed to the highly irregular surface geometry and may be avoided by using planar substrates, thus providing a novel strategy for self-assembly of organic molecules on various hydroxylated surfaces.

Heterogeneous conversion of NO2 on carbonaceous surfaces

In the present study the heterogeneous conversion of NO2 on commercial soot, freshly prepared flame soot and commercial soot treated with sulfuric acid has been investigated. The experimental results indicate a complex reaction mechanism in which the soot has to be treated as a reactant leading to deactivation of active sites on the surface with increasing NO2 consumption. Mean uptake coefficients γ for the first few minutes of the reaction of ∽10-6 have been determined for a consumption of ∽1013 NO2 cm-2. The γ-values decrease to <10-8 for a consumption of ∽1015 NO2 cm-2, indicating that the influence of soot on the partitioning of NOx in the atmosphere is negligible. Nitrous acid (HONO) and nitric oxide (NO) were found to be the major gas phase products. HONO yields up to 80% are observed, clearly showing that the nature of the reaction is non-catalytic. The humidity dependence of the reaction system was studied and indicates that water is necessary for HONO formation. In the reaction of pure HONO on soot surfaces, high yields of NO were observed, indicating a consecutive process: NO2→HONO→NO. When soot was coated with sulfuric acid the yield of HONO decreased while that of NO increased. This behaviour is caused by an increasing reactivity of the intermediate HONO on the modified surface. The experimental results indicate that without any recycling mechanism HONO formation on soot surfaces is not of major importance in the atmosphere.

Reactive Uptake of ClNO2on Aqueous Bromide Solutions

The reactive uptake of ClNO2 on pure water, NaBr, and mixed NaBr/NaCl solutions was studied using a wetted wall flow tube equipped with a quadrupole mass spectrometer. The measured uptake coefficient, γexpt, varied between approximately 1 × 10-5 and 1 × 10-4, depending on the aqueous salt concentration. Parameters governing the reactive uptake on NaBr solutions were determined interdependently as H2D1 = (0.101 ± 0.015) M cm2 atm-2 s-2. The accommodation coefficient, α, for ClNO2 on aqueous solutions at 275 K was found to be (9 ± 4) × 10-3. The major gas-phase products of the uptake and reaction of ClNO2 and Br- were found to be Br2 and BrNO2. BrNO2 is formed initially and is converted to Br2 in a secondary reaction with Br- in the aqueous phase. BrCl is a minor product. The presence of 1M Cl- had no effect on either the kinetics of uptake or the products formed. By exchanging the halide solutions for 0.1 M OH-, the uptake rate could be increased so that the diffusion limit was reached in the reactor. Vari...