Gaseous Br2 Research Articles

Spontaneous Molecular Bromine Production in Sea-Salt Aerosols.

Bromine chemistry is responsible for the catalytic ozone destruction in the atmosphere. The heterogeneous reactions of sea-salt aerosols are the main abiotic sources of reactive bromine in the atmosphere. Here, we present a novel mechanism for the activation of bromide ions (Br-) by O2 and H2O in the absence of additional oxidants. The laboratory and theoretical calculation results demonstrated that under dark conditions, Br-, O2 and H3O+ could spontaneously generate Br and HO2 radicals through a proton-electron transfer process at the air-water interface and in the liquid phase. Our results also showed that light and acidity could significantly promote the activation of Br- and the production of Br2. The estimated gaseous Br2 production rate was up to 1.55×1010 molecules cm-2 ⋅ s-1 under light and acidic conditions; these results showed a significant contribution to the atmospheric reactive bromine budget. The reactive oxygen species (ROS) generated during Br- activation could promote the multiphase oxidation of SO2 to produce sulfuric acid, while the increase in acidity had a positive feedback effect on Br- activation. Our findings highlight the crucial role of the proton-electron transfer process in Br2 production; here, H3O+ facilitates the activation of Br- by O2, serves as a significant source of atmospheric reactive bromine and exerts a profound impact on the atmospheric oxidation capacity.

Spontaneous Molecular Bromine Production in Sea‐Salt Aerosols

AbstractBromine chemistry is responsible for the catalytic ozone destruction in the atmosphere. The heterogeneous reactions of sea‐salt aerosols are the main abiotic sources of reactive bromine in the atmosphere. Here, we present a novel mechanism for the activation of bromide ions (Br−) by O2 and H2O in the absence of additional oxidants. The laboratory and theoretical calculation results demonstrated that under dark conditions, Br−, O2 and H3O+ could spontaneously generate Br and HO2 radicals through a proton–electron transfer process at the air–water interface and in the liquid phase. Our results also showed that light and acidity could significantly promote the activation of Br− and the production of Br2. The estimated gaseous Br2 production rate was up to 1.55×1010 molecules cm−2 ⋅ s−1 under light and acidic conditions; these results showed a significant contribution to the atmospheric reactive bromine budget. The reactive oxygen species (ROS) generated during Br− activation could promote the multiphase oxidation of SO2 to produce sulfuric acid, while the increase in acidity had a positive feedback effect on Br− activation. Our findings highlight the crucial role of the proton‐electron transfer process in Br2 production; here, H3O+ facilitates the activation of Br− by O2, serves as a significant source of atmospheric reactive bromine and exerts a profound impact on the atmospheric oxidation capacity.

Degradation and Self-Healing of FAPbBr3 Perovskite under Soft-X-Ray Irradiation.

The extensive use of perovskites as light absorbers calls for a deeper understanding of the interaction of these materials with light. Here, the evolution of the chemical and optoelectronic properties of formamidinium lead tri-bromide (FAPbBr3 ) films is tracked under the soft X-ray beam of a high-brilliance synchrotron source by photoemission spectroscopy and micro-photoluminescence. Two contrasting processes are at play during the irradiation. The degradation of the material manifests with the formation of Pb0 metallic clusters, loss of gaseous Br2 , decrease and shift of the photoluminescence emission. The recovery of the photoluminescence signal for prolonged beam exposure times is ascribed to self-healing of FAPbBr3 , thanks to the re-oxidation of Pb0 and migration of FA+ and Br- ions. This scenario is validated on FAPbBr3 films treated by Ar+ ion sputtering. The degradation/self-healing effect, which is previously reported for irradiation up to the ultraviolet regime, has the potential of extending the lifetime of X-ray detectors based onperovskites.

Interaction of ozone and carbon dioxide with polycrystalline potassium bromide and its atmospheric implication

It has been discovered for the first time that gaseous ozone in the presence of carbon dioxide and water vapor interacts with crystalline potassium bromide giving gaseous Br2 and solid salts KHCO3 and KBrO3. Molecular bromine and hydrocarbonate ion are the products of one and the same reaction described by the stoichiometric equation 2KBr(cr.) + O3(gas) + 2CO2(gas) + H2O(gas) → 2KHCO3(cr.) + Br2(gas) + O2(gas). The dependencies of Br2, KHCO3 and KBrO3 formation rates on the concentrations of O3 and CO2, humidity of initial gas mixture, and temperature have been investigated. A kinetic scheme has been proposed that explains the experimental regularities found in this work on the quantitative level. According to the scheme, the formation of molecular bromine and hydrocarbonate is due to the reaction between hypobromite BrO−, the primary product of bromide oxidation by ozone, with carbon dioxide and water; bromate results from consecutive oxidation of bromide ion by ozone Br−→+O3,−O2BrO−→+O3,−O2BrO2−→+O3,−O2BrO3−.

Physical Adsorption and Charge Transfer of Molecular Br2 on Graphene

We present a detailed study of gaseous Br2 adsorption and charge transfer on graphene, combining in situ Raman spectroscopy and density functional theory (DFT). When graphene is encapsulated by hexagonal boron nitride (h-BN) layers on both sides, in a h-BN/graphene/h-BN sandwich structure, it is protected from doping by strongly oxidizing Br2. Graphene supported on only one side by h-BN shows strong hole doping by adsorbed Br2. Using Raman spectroscopy, we determine the graphene charge density as a function of pressure. DFT calculations reveal the variation in charge transfer per adsorbed molecule as a function of coverage. The molecular adsorption isotherm (coverage versus pressure) is obtained by combining Raman spectra with DFT calculations. The Fowler-Guggenheim isotherm fits better than the Langmuir isotherm. The fitting yields the adsorption equilibrium constant (∼0.31 Torr(-1)) and repulsive lateral interaction (∼20 meV) between adsorbed Br2 molecules. The Br2 molecule binding energy is ∼0.35 eV. We estimate that at monolayer coverage each Br2 molecule accepts 0.09 e- from single-layer graphene. If graphene is supported on SiO2 instead of h-BN, a threshold pressure is observed for diffusion of Br2 along the (somewhat rough) SiO2/graphene interface. At high pressure, graphene supported on SiO2 is doped by adsorbed Br2 on both sides.

Surfactant-Promoted Reactions of Cl2 and Br2 with Br– in Glycerol

Gas-liquid scattering experiments are used to explore reactions of gaseous Cl2 and Br2 with a 0.03 M solution of the surfactant tetrahexylammonium bromide (THABr) dissolved in glycerol. At thermal collision energies, 79 ± 2% of incident Cl2 molecules react with Br(-) to form Cl2Br(-) in the interfacial region. This reaction probability is three times greater than the reactivity of Cl2 with 3 M NaBr-glycerol, even though the interfacial Br(-) concentrations are similar in each solution. We attribute the high 79% uptake to the presence of surface THA(+) ions that stabilize the Cl2Br(-) intermediate as it is formed in the charged, hydrophobic pocket created by the hexyl chains. Cl2Br(-) generates the single exchange product BrCl in a 1% yield close to the surface, while the remaining 99% desorbs as the double exchange product Br2 over >0.1 s after diffusing deeply into the bulk. When NaCl is added to the surfactant solution in a 20:1 Cl(-)/Br(-) ratio, the Cl2 reaction probability drops from 79% to 46 ± 1%, indicating that Cl(-) in the interfacial region only partially blocks reaction with Br(-). In parallel, we observe that gaseous Br2 molecules dissolve in 0.03 M THABr for 10(4) times longer than in 3 M NaBr. We attribute this change to formation of stabilizing interfacial and bulk-phase THA(+)Br3(-) ion pairs, in analogy with the capture of Cl2 and formation of THA(+)Cl2Br(-) pairs. The THA(+) ion appears to be a powerful interfacial catalyst for promoting reaction of Cl2 and Br2 with Br(-) and for ferrying the resultant ions into solution.

PS2Br and AsS2X (X = Br, I): similar molecules with completely different shapes – a combined experimental and theoretical study

By the reaction of solid As(4)S(4) with gaseous Br(2) at a temperature of 410 K gaseous AsSBr and AsS(2)Br are formed; the reaction with gaseous I(2) at 468 K leads to the formation of AsSI, AsS(2)I, As(2)S(2)I(2) and As(2)S(3)I(2). Thermodynamic data on these novel species are obtained by mass spectrometry. The experimental findings are extended and confirmed by ab initio quantum chemical calculations. Undoubtedly the molecules AsSX (X = Br, I) and As(2)S(2)I(2) contain As-atoms in the formal oxidation state III. Unexpectedly, in AsS(2)X arsenic is not of formal oxidation state +V but +III: due to the absence of As-S π-bonding, the molecular structure of AsS(2)X is arranged as a dithia-arsirane with an AsS(2) three-membered ring. For comparison with AsS(2)X, PS(2)Br is investigated for the first time. This high temperature species is planar with symmetry C(2v) and phosphorus of the formal oxidation state +V in which the two P=S π-bonds are realized due to their strength over one S-S σ-bond. As(2)S(3)I(2) contains a five-membered As(2)S(3)-ring with one S-S-bond. The ionization energies are determined and compared with those obtained from calculations; they serve as a further indication for the structures realized in these molecules.

Using Bromine Gas To Enhance Mercury Removal from Flue Gas of Coal-Fired Power Plants

Bromine gas was evaluated for converting elemental mercury (Hg0) to oxidized mercury, a form that can readily be captured by the existing air pollution control device. The gas-phase oxidation rates of Hg0 by Br2 decreased with increasing temperatures. SO2, CO, HCl, and H2O had insignificant effect, while NO exhibited a reverse course of effect on the Hg0 oxidation: promotion at low NO concentrations and inhibition at high NO concentrations. A reaction mechanism involving the formation of van der Waals clusters is proposed to accountfor NO's reverse effect. The apparent gas-phase oxidation rate constant, obtained under conditions simulating a flue gas without flyash, was 3.61 x 10(-17) cm3 x molecule(-1) x s(-1) at 410 K corresponding to a 50% Hg0 oxidation using 52 ppm Br2 in a reaction time of 15 s. Flyash in flue gas significantly promoted the oxidation of Hg0 by Br2, and the unburned carbon component played a major role in the promotion primarily through the rapid adsorption of Br2 which effectively removed Hg0 from the gas phase. At a typical flue gas temperature, SO2 slightly inhibited the flyash-induced Hg0 removal. Conversely, NO slightly promoted the flyash induced Hg0 removal by Br2. Norit Darco-Hg-LH and Darco-Hg powder activated carbons, which have been demonstrated in field tests, were inferred for estimating the flyash induced Hg0 oxidation by Br2. Approximately 60% of Hg0 is estimated to be oxidized with the addition of 0.4 ppm of gaseous Br2 into full scale power plant flue gas.

Chemiluminescence of bromine vapor under pulsed lamp pumping

The chemiluminescence kinetics and spectrum of gaseous Br2 under the conditions of broadband optical pumping have been investigated. It has been found that the transition A 3Π1u → X 1Σ g + is the major contributor to the luminescence spectrum.

Gas-Phase Molecular Halogen Formation from NaCl and NaBr Aerosols: When Are Interface Reactions Important?

Unique interface reactions at the surface of sea-salt particles have been suggested as an important source of photolyzable gas-phase halogen species in the troposphere. Many factors influence the relative importance of interface chemistry compared to aqueous-phase chemistry. The Model of Aerosol, Gas, and Interfacial Chemistry (MAGIC 2.0) is used to study the influence of interface reactions on gas-phase molecular halogen production from pure NaCl and NaBr aerosols. The main focus is to identify the relative importance of bulk compared to interface chemistry and to determine when interface chemistry dominates. Results show that the interface process involving Cl-(surf) and OH(g) is the main source of Cl2(g). For the analogous oxidation of bromide by OH, gaseous Br2 is formed mainly in the bulk aqueous phase and transferred across the interface. However, the reaction of Br-(surf) with O3(g) at the interface is the primary source of Br2(g) under dark conditions. The effect of aerosol size is also studied. Potential atmospheric implications and effects of interface processes on aerosol pH are discussed.

Tridimensional imaging of local structure by x-ray absorption spectroscopy

Current x-ray absorption spectroscopy (XAS) data-analysis of ordered and disordered systems is based on a peak decomposition of the n-body distribution functions. The limitation of this approach can be particularly severe for the purpose of modeling the local geometry in liquid and amorphous systems. We have developed a successful method for quantitative analysis of highly disordered structures using the Reverse Monte Carlo (RMC) refinement that can be applied simultaneously to diffraction and XAS data, allowing the construction of realistic tridimensional models. Here we present the results of the application of the method to gaseous Br2 and to liquid Cu. The method incorporates all of the advances related to the application of advanced multiple-scattering (MS) codes and the n-body expansion for XAS data-analysis (GNXAS).

Transition strengths and potential curves for the valence transitions in Br2 from a reanalysis of the ultraviolet-visible absorption at low resolution

The absorption profile of gaseous Br2 in the 320–675 nm region is reanalyzed using a quantum mechanical nonlinear least-squares approach. For the first time the A(1u 3Π)←X(1Σg+) transition is included along with the two stronger transitions, B(0u+ 3Π)←X and C(1u 1Π)←X. The analyzed data include absorption spectra at temperatures between 23 °C and 440 °C along with specific estimates of B←X and (A+C)←X absorption. A new computational device facilitates inclusion of the discrete region of the spectrum (λ>510 nm) in the analysis: The A←X and B←X transitions are treated as entirely bound-free, which is accomplished computationally by just removing the attractive branches of the A and B potentials. The new analysis determines the molar absorptivities of all three transitions within ∼1 L mol−1 cm−1. Although the high precision is partly due to the inclusion of the specific B←X and (A+C)←X data, test computations show that the resolution can be achieved reliably by fitting just the T-dependent spectra. At 23 °C the component peaks are: 10.6 L mol−1 cm−1 at 525 nm (A←X), 82.5 L mol−1 cm−1 at 480 nm (B←X), and 158 L mol−1 cm−1 at 412 nm (C←X).

Bromine activation in the troposphere by the dark reaction of O3 with seawater ice

There is increasing evidence that bromine atoms play a role in tropospheric chemistry in the marine boundary layer. In addition, they are believed to lead to rapid depletion of surface level ozone in the Arctic at polar sunrise. While mechanisms have been proposed for recycling bromine atoms from sea salt particles, the initial reaction(s) leading to the formation of bromine atom precursors is not known. We report here the formation of gaseous Br2 from the reaction of seawater ice with O3 in the dark. These observations suggest that this reaction is a potential source of tropospheric photolyzable bromine in high latitude coastal regions in winter. In addition, it may be the source of the photolyzable bromine gas measured recently in the Arctic by Impey et al. (1997), which is believed to be responsible for the ozone destruction at polar sunrise.

Photolysis of HBr at 77 °K

The photolysis of HBr has been studied at 77 °K. The photodecomposition of solid state HBr is an inefficient process which is strongly inhibited by a reaction product, probably Br atoms, or by added Br2 or trans-butene-2. The results suggest H2 is formed by abstraction from HBr by thermal H atoms, the activation energy for the abstraction being ~350 cal mole−1. The photolysis of both pure HBr and HBr with added Br2 or trans-butene-2 can be described by an absorber complex mechanism similar to that inferred by Hughes and Purnell in the photolysis of HI. The absorption tail of solid HBr extends beyond the gas phase absorption into the wavelength region in which trapped radicals are generated in HBr–olefin mixtures but where there is no H2 evolution. The solid state absorption spectrum of Br2 is contrasted with that of gaseous Br2; an absorption peak is observed at 2725 Å which may be associated with (Br2)n aglomerates.