Heterogeneous Photochemical Reactions Research Articles

Photoenhanced sulfate formation by the heterogeneous uptake of SO2 on non-photoactive mineral dust

Abstract. Heterogeneous uptake of SO2 on mineral dust is a predominant formation pathway of sulfates, whereas the contribution of photo-induced SO2 oxidation to sulfates on the dust interfaces still remains unclear. Here, we investigated heterogeneous photochemical reactions of SO2 on five mineral oxides (SiO2, kaolinite, Al2O3, MgO, and CaO) without photocatalytic activity. Light enhanced the uptake of SO2, and its enhancement effects negatively depended on the basicity of mineral oxides. The initial uptake coefficient (γ0,BET) and the steady-state uptake coefficient (γs,BET) of SO2 positively relied on light intensity, relative humidity (RH), and O2 content, while they exhibited a negative relationship with the initial SO2 concentration. Rapid sulfate formation during photo-induced heterogeneous reactions of SO2 with all mineral oxides was confirmed to be ubiquitous, and H2O and O2 played key roles in the conversion of SO2 to sulfates. In particular, triplet states of SO2 (3SO2) were suggested to be the trigger for photochemical sulfate formation. Atmospheric implications supported a potential contribution of interfacial SO2 photochemistry on non-photoactive mineral dust to atmospheric sulfate sources.

Heterogeneous photochemical reaction enabled by an ultrasonic microreactor.

The presence of solids as starting reagents/reactants or products in flow photochemical reactions can lead to reactor clogging and yield reduction from side reactions. We address this limitation with a new ultrasonic microreactor for continuous solid-laden photochemical reactions. The ultrasonic photochemical microreactor is characterized by the liquid and solid residence time distribution (RTD) and the absorbed photon flux in the reactor via chemical actinometry. The solid-handling capability of the ultrasonic photochemical microreactor is demonstrated with a silyl radical-mediated metallaphotoredox cross-electrophile coupling with a solid base as a reagent.

Heterogeneous photochemical uptake of NO2 on the soil surface as an important ground-level HONO source.

Nitrous acid (HONO) production from the heterogeneous photochemical reaction of NO2 on several Chinese soils was performed in a cylindrical reactor at atmospheric pressure. The NO2 uptake coefficient (γ) and HONO yield (YHONO) on different soils were (0.42-5.16)×10-5 and 6.3%-69.6%, respectively. Although the photo-enhanced uptake of NO2 on different soils was observed, light could either enhance or inhibit the conversion efficiency of NO2 to HONO, depending on the properties of the soils. Soils with lower pH generally had larger γ and YHONO. Soil organics played a key role in HONO formation through the photochemical uptake of NO2 on soil surfaces. The γ showed a positive correlation with irradiation and temperature, while it exhibited a negative relationship with relative humidity (RH). YHONO inversely depended on the soil mass (0.32-3.25mgcm-2), and it positively relied on the irradiance and RH (7%-22%). There was a maximum value for YHONO at 298K. Based on the experimental results, HONO source strengths from heterogeneous photochemical reaction of NO2 on the soil surfaces were estimated to be 0.2-2.7ppbh-1 for a mixing layer height of 100m, which could account for the missing daytime HONO sources in most areas.

Photoenhanced heterogeneous reaction of O3 with humic acid: Focus on O3 uptake and changes in the composition and optical property.

Heterogeneous photochemical reaction of O3 with humic acid (HA) under simulated sunlight was performed using a flow tube reactor coupled to an O3 analyzer at ambient pressure. It was confirmed that light significantly enhanced the uptake of O3 on HA. The initial uptake coefficient (γi) and the steady-state uptake coefficient (γss) of O3 under irradiation increased by 1.6 and 3.8 times compared to those in the dark, respectively. The γi and γss on HA varied in the range of 0.76-2.77×10-5 and 1.50-9.55×10-6, respectively, which were dependent on various environmental factors including HA mass, total irradiance, initial O3 concentration, O2 content, temperature, relative humidity (RH) and HA solution pH. Both γi and γss showed linear dependence on the total irradiance (0-2.07×1016 photons/(cm2⋅s)) of the light source, and increased with the HA mass (0-3.2μg/cm2), temperature (278-298K) and HA solution pH (4.0-9.6). However, they showed negative correlations with the initial O3 concentration and O2 content. The γi remained constant in the RH range of 7%-60%, while γss exhibited the maximum value at RH=20%. During the ozonization of HA under irradiation, some functional groups were consumed, including CH2, CH3, aromatic CC, OH, CO, COOH and COO-. HA aged by O3 exhibited a decrease in the mass absorption efficiency (MAE) and a small increase in the absorption Ångström exponent between 300 and 600nm wavelength (AAE300,600), which was ascribed to changes in the composition of HA during the photochemical ozonization process.

Photochemical behavior of ferrihydrite-oxalate system: Interfacial reaction mechanism and charge transfer process

Heterogeneous photochemical reactions associated with natural iron (hydr)oxides and oxalic acid have attracted a great deal of scientific attention in the application of organic pollutants degradation. However, the reaction mechanism is still unclear due to the complicated iron cycles and reactive oxygen species (ROS) generation. In this study, the in situ attenuated total reflectance-Fourier transform infrared spectroscopy was implemented to investigate the adsorption process and photochemical behavior of oxalic acid on the surface of ferrihydrite. A comprehensive reaction mechanism from the perspective of charge transfer process, including homogeneous-heterogeneous iron cycling and ROS generation, was illustrated in detail. We found that oxalic acid was first adsorbed on the surface of ferrihydrite with a mononuclear bidentate binding geometry. Interestingly, this mononuclear bidentate complex on the surface of ferrihydrite was stable under visible light irradiation. Subsequently, the whole complex departed from ferrihydrite surface through non-reduction dissolution with the form of Fe(C2O4)+. In the solution, the Fe(C2O4)+ complexes would quickly convert to Fe(C2O4)2− complexes. Under visible light irradiation, the electrons generated from the photolysis of Fe(C2O4)2− complex reacted with O2 to form O2•−/•OOH. Meanwhile, Fe(III) was reduced to Fe(II). Finally, the produced O2•−/•OOH could react with Fe(II) through a one-step way to generate •OH, which possessed higher •OH formation efficiency than that through the two-step way of H2O2 as the intermediates. This study helps us with understanding of in-situ photochemical reaction mechanism of ferrihydrite-oxalic acid system, and also provides guidance to effectively utilize widespread iron (hydr)oxides and organic acids in natural environment to develop engineered systems for water treatment.

Deliquescence behavior of photo-irradiated single NaNO3 droplets

Nitrate-containing particles are ubiquitous in the troposphere because of their secondary production due to anthropogenic emissions of NOx from the combustion of fossil fuels. Nitrate ions are recognized as photoactive species that may contribute to the formation of oxidants in the atmosphere through heterogeneous photochemical reactions. The chemical transformation of aerosol particles in the atmosphere often leads to modification of the particles’ hygroscopic properties. Although the photo-transformation of nitrate ions into nitrite within aerosol particles has been investigated, the influence of the photoproducts formation on the hygroscopic behavior of particles has not been reported. In this study, we examined the hygroscopic properties of single, ultraviolet-irradiated NaNO3 droplets using Raman microspectrometry. We are the first demonstrated that irradiating NaNO3 particles affects their hygroscopic behavior. For short-term exposures, regarding hygroscopic behavior, the irradiated particles exhibited two-stage transitions that were clearly reproduced in the experimental NaNO3-NaNO2 phase diagram. The production of NO2− decreased the deliquescence relative humidity values. For long irradiation times (>5 h), these values are even more affected by the additional production of peroxynitrite and carbonate ions in individual droplets. The NaNO3-NaNO2 deliquescence phase diagram cannot explain the hygroscopic behavior of long-term irradiated particles. Finally, we demonstrated the influence that CO2 has on the photo-transformation process in NaNO3 droplets.

Influences of O2 and O3 on the heterogeneous photochemical reaction of NO2 with humic acids

Oxidizing components in the atmosphere may play competitive roles in the heterogeneous photochemical reaction of NO2 with humic acids (HA). Effects of O2 and O3 on the conversion of NO2 to HONO on HA under simulated sunlight were investigated using a flow tube reactor. The uptake coefficient (γ) of NO2 and the HONO formation rate decreased with the increase of the O2 content (0%–20%) and the O3 concentration (0–100 ppb). The HONO yield was observed to be independent of the O2 content, whereas it inversely depended on the O3 concentration. In addition, the aging process of HA by O2 and O3 under irradiation resulted in the decrease in the reactivity of HA toward NO2, as shown by lower γ and HONO formation rate, while it has little influence on the HONO yield. Finally, the mechanism of role of O2 and O3 in the photochemical reaction of NO2 with HA was discussed in detail.

Key role of pH in the photochemical conversion of NO2 to HONO on humic acid

The heterogeneous photochemical reactions of NO2 with humic acid (HA) were performed using a flow tube reactor coupled to a NOx analyzer. The effects of the pH on the uptake coefficient (γ) of NO2 and HONO and NO yields were investigated in detail. With increasing the pH in the range of 2–12, γ was almost constant with an average value of (4.21 ± 0.46) × 10−6, whereas the HONO yield and NO yield linearly decreased from (81.07 ± 4.07)% and (10.35 ± 3.86)% to (13.87 ± 9.15)% and (1.51 ± 0.94)%, respectively. According to the characterization of HA compositions and possible reaction paths, it can be concluded that the pH may influence the transfer of protons and the equilibrium of HONO with NO2− by varying the contents of carboxyl and phenol groups in HA, which should primarily contribute to the change in the HONO yield with the pH.

Heterogeneous Photochemical Conversion of NO2 to HONO on the Humic Acid Surface under Simulated Sunlight.

The poor understanding of HONO sources in the daytime highlights the importance of the heterogeneous photochemical reaction of NO2 with aerosol or soil surfaces. The conversion of NO2 to HONO on humic acid (HA) under simulated sunlight was investigated using a flow tube reactor at ambient pressure. The uptake coefficient (γ) of NO2 linearly increased with irradiation intensity and HA mass in the range of 0-2.0 μg/cm(2), while it decreased with the NO2 concentration. The HONO yield was found to be independent of irradiation intensity, HA mass, and NO2 concentration. The temperature (278-308 K) had little influence on both γ and HONO yield. Additionally, γ increased continuously with relative humidity (RH, 7-70%), and a maximum HONO yield was observed at 40% RH. The heterogeneous photochemical reaction of NO2 with HA was explained by the Langmuir-Hinshelwood mechanism.

First satellite detection of volcanic OClO after the eruption of Puyehue‐Cordón Caulle

AbstractVolcanoes release large amounts of halogen species such as HCl and HBr, which can be converted into reactive halogens by heterogeneous photochemical reactions that are currently not fully characterized. Here we report on the first satellite detection of volcanic chlorine dioxide (OClO). Measurements were performed using the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography instrument for the ash‐laden plume emitted after the 2011 eruption of Puyehue‐Cordón Caulle in Chile. We also identified volcanic BrO using the Ozone Monitoring Instrument, as well as enhanced HCl in data of the Microwave Limb Sounder instrument. These observations suggest that OClO was formed in the plume by the ClO + BrO reaction in presence of a large excess of ClO. The present satellite data set could help better understand reactive halogen chemistry in volcanic plumes and its impact on atmospheric composition.

Heterogeneous photochemical reaction of ozone with anthracene adsorbed on mineral dust

The heterogeneous reactions of O3 with anthracene adsorbed on TiO2 and on Asian dust storm particles were investigated in the dark and in the presence of light. The reaction rate constants of the heterogeneous reaction between O3 and anthracene adsorbed on TiO2 were increased by a factor of 1.5 in the presence of light compared to the dark conditions. Anthraquinone, which was identified as the main surface product of anthracene reacted with O3 in the dark, can react with O3 quickly in the presence of light. The reactions on Asian dust storm particles exhibited pseudo-first-order kinetics for anthracene loss, and the reactions between O3 and anthracene adsorbed on Asian dust storm particles proceed by the Langmuir–Hinshelwood mechanism in the dark and in the presence of light. At extremely high ozone concentrations, the degradation of anthracene is enhanced by a factor of 3 in the presence of light compared to the dark conditions.

Review of heterogeneous photochemical reactions of NOy on aerosol — A possible daytime source of nitrous acid (HONO) in the atmosphere

As an important precursor of hydroxyl radical, nitrous acid (HONO) plays a key role in the chemistry of the lower atmosphere. Recent atmospheric measurements and model calculations show strong enhancement for HONO formation during daytime, while they are inconsistent with the known sources in the atmosphere, suggesting that current models are lacking important sources for HONO. In this article, heterogeneous photochemical reactions of nitric acid/nitrate anion and nitrogen oxide on various aerosols were reviewed and their potential contribution to HONO formation was also discussed. It is demonstrated that HONO can be formed by photochemical reaction on surfaces with deposited HNO3, by photocatalytic reaction of NO2 on TiO2 or TiO2-containing materials, and by photochemical reaction of NO2 on soot, humic acids or other photosensitized organic surfaces. Although significant uncertainties still exist in the exact mechanisms and the yield of HONO, these additional sources might explain daytime observations in the atmosphere.

Spectroscopic study of organic coatings on fine particles, exposed to ozone and simulated sunlight

This work intends to quantify the variation in optical properties of aerosol by in-situ spectroscopic monitoring the ozonolysis of a mixture of typical biomass burning compounds. The reaction occurs on silica and glass particles in the presence of simulated sunlight. Fused silica particles (Aerosil) were coated with a thin film of a 1:1 mixure of 4-phenoxyphenol with 4-carboxyphenone as a photosensitizer. UV–VIS spectra of dichloromethane extracts from the particles recorded before and after treatment, show development of a new band after prolonged ozone and light exposure. Changes in optical properties are reported, and variations of spectroscopic features are discussed. We show that the ozone-induced heterogeneous photochemical reaction does produce species absorbing light in the solar spectral range. Further, we demonstrate that the heterogeneous photosensitized reactions at 200 ppb ozone (strongly ozone polluted regions) for a time period of 7 h aging process, can increase light absorption of atmospheric aerosols in the tropospheric actinic window (>290 nm) by 0.4 absorption units ng-C−1 O3 ppm−1 in the region 290–358 nm and by 1.0 absorption units ng-C−1 O3 ppm−1 in the region 360–448 nm. Chemical changes of such surface films were identified by diffuse reflectance infrared Fourier transform spectroscopy of coated glass spheres, and we suggest formation of humic-like substances comparable to those reported in continental aerosol.

Spectral dependences of the activity and selectivity of N-doped TiO 2 in photodegradation of phenols

In present study we report the spectral dependencies of the quantum yield of phenol and 4-chlorophenol photodegradation over N-doped TiO 2 and the selectivity of the photocatalyst toward formation the major intermediates. Applying the theoretical model developed earlier [A.V. Emeline, V.K. Ryabchuk, N. Serpone, J. Phys. Chem. B. 103 (1999) 1316; A.V. Emeline, A.V. Frolov, V.K. Ryabchuk, N. Serpone, J. Phys. Chem. B 107 (2003) 7109] for analysis of the experimental spectral dependencies we conclude that extrinsic N-doped induced absorption is formed by overlapping of several single absorption bands, which presumably correspond to different intrinsic defects (color centers) in TiO 2 distinguished by different efficiency of photogeneration of charge carriers. It is also inferred that measurement and corresponding analysis of spectral dependencies of the quantum yield and selectivity of heterogeneous photochemical reactions can be considered as a powerful spectroscopic mean to deconvolve the complex non-resolved absorption spectrum of solids.

Laudatio for Professor Helmut Tributsch

Laudatio for Professor Helmut Tributsch

Heterogeneous photochemical reactions of decabromodiphenyl ether

Brominated diphenyl ethers are a major class of brominated flame retardants, with production of decabromodiphenyl ether (DBDPE) contributing significantly to this total. Although little is known about the mechanisms and rates of DBDPE decay in the natural environment, photochemical transformation is often suggested as a potentially important fate process. In this study, photochemical reactions of DBDPE precipitated onto hydrated surfaces (quartz glass, silica particles, and humic acid-coated silica particles) were measured. Decabromodiphenyl ether was irradiated within a Rayonet photochemical reactor equipped with lamps having maximum emissions (lambdamax) at 300 or 350 nm or with sunlight (West Lafayette, IN, USA at 40 degrees 26'N, 86 degrees 55'W). When DBDPE is plated onto quartz tubes and hydrated with reagent-grade water, phototransformation occurred over a period of days with each light source. With two lamps (lambdamax = 300 nm), about 31% of the initial mass of DBDPE remained after 60 h in the Rayonet reactor. Decabromodiphenyl ether transformed more slowly when irradiated by sunlight, with 30% of the initial compound recovered from tubes after 72 h. In the presence of humic acid (HA) solution, transformation of DBDPE by solar irradiation is slowed even further: approximately 70% remained after 72 h. Solar irradiation of DBDPE adsorbed to humic acid-coated sand particles resulted in the slowest irradiation rates; approximately 88% of the initial mass was recovered after 96 h of exposure. Although the parent compound exhibited slow transformation, analysis by high-performance liquid chromatography (HPLC) indicated some accumulation of transformation products.

Communication between Surfaces by Electron Relay in a Doubly Heterogeneous Photochemical Reaction

Communication between surfaces by electron relay in a doubly heterogeneous photochemical reaction

Modeling of light scattering in photochemical reactors

Heterogeneous photochemical reactions in fluid (water or air) environments have been gaining increasing interests in the past fifteen years, the main application being in pollution abatement by solid semiconductor photocatalyzed systems. One of the unsolved problems is the proper evaluation of the rate of absorbed radiation energy by the participating medium due to the existence of simultaneous absorption and scattering. This is a key property in quantifying kinetic models and designing photoreactors. A model derived from radiative transport fundamentals and chemical reactor engineering concepts is presented. It permits a rigorous quantification of the local volumetric rate of radiant energy absorption (LVREA) inside the reactor. The method was experimentally verified by analyzing the scattering effects (performance changes) produced by addition of transparent, inert particles to a homogeneous photochemical reacting system. With the validated model, and employing radiation absorbing particles (titanium dioxide), the proposed approach was compared with other methods currently in use to evaluate the LVREA. It was shown, in a quantitative way, that the proper application of the radiative transfer equation is a requisite for obtaining accurate results. Finally, the results of changing particle concentration and particle size on the magnitude of the scattered light were explored.

Transformation and decomposition of 1,1,1-trichloroethane on titanium dioxide in the dark and under photoillumination

Abstract 1,1,1-Trichloroethane (CH3CCl3) in dry air was decomposed to carbon dioxide (CO2), chlorine (Cl2) and hydrogen chloride (HCl) at 313–353 K through a heterogeneous photochemical reaction on titinium dioxide (TiO2). The reaction proceeded on TiO2 mainly through the following two steps: (1) thermal catalytic elimination of HCl to produce 1,1-dichloroethene (CH2CCl2), and (2) photochemical decomposition of CH2CCl2 to CO2 and other products. Photo-oxidation of chloride took place on TiO2, which could keep the catalytic activity of TiO2, since inhibitors like HCl could be removed from the surface. A potential influence of the heterogeneous reaction on estimation of the tropospheric lifetime of CH3CCl3 and/or tropospheric OH budget, is suggested.

Composition of Br-containing aerosols and gases related to boundary layer ozone destruction in the arctic

During the third Arctic Gas and Aerosol Sampling Program (March 1989), aircraft measurements of atmospheric gases and aerosols were performed in the European Arctic for the purpose of investigating the phenomenon of boundary layer O 3 destruction. Eight high-volume aerosol filter samples were collected in tropospheric air over the pack ice. In these sampling periods, continuous O 3 measurements were made and trace gases were collected in flasks. For all samples, total elemental bromine collected on the filters in excess of the estimated sea salt component (XSFBr) was found to anticorrelate stronly ( r = −0.90) with the mean ozone concentration observed during the sampling period. These findings are similar to earlier observations at Alert and Barrow. Air samples collected during these periods were analysed for Br-containing gases and hydrocarbons. None of these compounds were well correlated with either O 3 or XSFBr concentration over the course of the experiment. This is probably because variable conditions of local meteorology, atmospheric structure and geographic location influenced the degree to which O 3 was depleted, by affecting the size of the reaction reservoir and the source(s) of the reactants. Samples collected in the surface (∼ 50 m deep) isothermal or slightly stable layer (SSL) over pack ice and with light winds from the direction of the central Arctic showed the highest O 3 depletions. When winds were from the direction of open water, significantly higher O 3 and lower XSFBr values were observed. When the SSL was not present, samples collected below the strong inversion showed less O 3 destruction and lower XSFBr concentrations than similar low altitude samples collected within the SSL. This is consistent with the notion of a larger reservoir volume available for reaction. Gas and aerosol chemistry results were compared for two samples collected close spatially and temporally over ice north of Spitsbergen. Our data indicate that (1) CHBr 3 may be the key organobromine species supplying Br atoms and BrO radicals in a heterogeneous photochemical reaction cycle causing the photolytic destruction of O 3 in the springtime Arctic surface layers, and (2) ambient hydrocarbons (especially C 2H 2) are depleted during O 3 destruction, and may be important in the overall reaction mechanism. This catalytic O 3 depletion process was observed to occur to an extent causing near-total O 3 destruction in the SSL over a 1–2 d period. Thus, relatively rapid photochemical reactions between atmospheric Br, hydrocarbons and aerosols are suggested as driving the photolytic O 3 destruction process.