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Abstract

The nitrogen oxides (NO(x)) decomposition on illuminated TiO(2) surfaces has been widely studied, but little is known about the subsequent formation of non-nitrogen containing products. In this study, TiO(2) coated glass surfaces and TiO(2) films with nitrate anions (either premixed with TiO(2) as KNO(3) or deposited from gaseous NO(x)) are irradiated with broad-band light. Upon irradiation, detected gas phase products include NO(2), HNO(2), and O(3). To the best of our knowledge, this is the first study that reveals the production of O(3) from TiO(2) surfaces. By surface charge transfer reactions, nitrate anions are oxidized into nitrate radicals and their photochemistry (almost in the visible) leads to O(3) formation, enhancing the oxidizing power of these surfaces.

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... 4,5 The photochemistry of nitrate on TiO 2 has been shown to even lead to emission of ozone. 6 In related terms, photocatalytic processes are used in TiO 2 -based appliances for air pollution remediation or for self-cleaning purposes. 7,8 The origin of the photocatalytic activity of TiO 2 lies in UV-radiation-induced electron/hole pairs, which can, directly or indirectly via reactive oxygen species formed therefrom, enter into the redox cycles of atmospheric gases, such as volatile organic compounds (VOC), nitrogen oxides, H 2 O, and O 2 . ...
... 16 Furthermore, the nitrate anion has been observed to act as a reactive intermediate in the photooxidative production of a nitrate radical, which might be important for the fate of adsorbed organic compounds. 17,6 X-ray photoelectron spectroscopy (XPS) as a classical ultrahigh vacuum (UHV) surface analysis technique provides a valuable tool to study adsorption of nitrogen oxide species and the effect of UV-radiation on metal oxide surfaces. XPS has been employed under dark conditions to study adsorption of NO 2 , NO, and other nitrogen oxide species on single-crystal surfaces and on thin-film supported TiO 2 and SiO 2 nanoparticles 18−20 and to accompany experiments on the effect of NO 2 on PbO particle solubility under atmospheric conditions. ...
... Very similar adventitious carbon signals have been observed in earlier high-pressure XPS studies, 25 and they originate likely from the chamber walls due to displacement by water vapor and O 2 added to the chamber. 6 After measurements at #2 (the sequence shown in Figure 5a) the C−H/CC component has clearly reduced in relation to the more oxidized −COOH compounds. The small peak observed at ∼294.5 eV during the elevated pressure experiments may be attributed to potassium contamination (K 2p) due to diffusion from the heater or the bulk of the single crystal to the surface as in a previous AP-XPS study. ...
Article
Heterogeneous reactions of nitrogen oxides on metal oxide surfaces have been suggested to play a significant role in environmental chemistry, physics and engineering. Many of the metal oxide compounds found among atmospheric mineral dust particles are inherently semiconducting substrates. Due to their low band gap, they are effective photoactive materials in the environmentally relevant ultra violet (UVA) range of solar radiation. Here, we have studied nitrogen oxide species evolution and photochemistry on TiO2(110) surfaces in the context of atmospheric chemistry by means of near ambient pressure X-ray photoelectron spectroscopy (AP-XPS) coupled with a 375 nm UV-laser module. In the presence of molecular O2 only, changes in TiO2 surface potential under UV irradiation were observed, attributed to band flattening. Under humid conditions, a significant increase in the BE-range attributed to surface hydroxyl groups was observed, which may be the basis for the light-induced super-hydrophilicity observed elsewhere with titania based nanomaterials. The formation of surface nitrite and nitrate was observed after exposure to NO2 in the dark. Core-level metal cation, O and N XPS-spectra were measured at elevated pressures of O2, NO2 and H2O. By selective UV irradiation of only the XPS measurement spot on the sample, we obtained differential information of the surface chemical state on the UV-irradiated compared to dark reference spots. Upon UV irradiation, increased oxidation of NO2 was observed, while in turn a substantial increase of a reduced nitrate species possibly from electron transfer to nitrate and of a further reduced nitrogen species was observed during exposure to UV-radiation. The effect of surface hydroxylation and the involvement of carbon containing surface compounds in the formation of nitrogenated organic species are emphasised.
... Of some concern, however, are studies that demonstrate the formation of harmful intermediates (e.g., nitrous acid, HONO), which are far more harmful to human health than either NO or NO2 during the disposal of NOx [22,23]. Not least is the potential that nitrates (NO3 − ) produced and deposited on the TiO2 particulate surface in the disposal of NOx may be implicated in reNOxification reactions; that is, back to NOx [24][25][26] and formation of ozone [26] that would forestall the application of TiO2-based photocatalytic surfaces to improve the quality of urban air environments. ...
... Of some concern, however, are studies that demonstrate the formation of harmful intermediates (e.g., nitrous acid, HONO), which are far more harmful to human health than either NO or NO2 during the disposal of NOx [22,23]. Not least is the potential that nitrates (NO3 − ) produced and deposited on the TiO2 particulate surface in the disposal of NOx may be implicated in reNOxification reactions; that is, back to NOx [24][25][26] and formation of ozone [26] that would forestall the application of TiO2-based photocatalytic surfaces to improve the quality of urban air environments. ...
... Regrettably, the rate of absorption of photons and the number of absorbed photons remain inaccessible quantities in heterogeneous photocatalysis because of experimental limitations. Consequently, the photonic yield (ξ) was introduced and defined in a manner similar to the quantum yield (Φ) with the main difference being that reference is made to the number of photons of a given wavelength of the actinic light (N°hν) incident on the photoreactor, and not on the number of photons absorbed by the photocatalyst under stationary conditions, as expressed by Equation (26) or in differential form by Equation (27) = (26) = (27) Both Φ and ξ are defined as in photochemistry [37]; the former is more appropriate, however, since only absorbed photons can initiate interfacial chemical reactions, thus Φ is an intrinsic characteristic of a photocatalyst. Nonetheless, ξ may be a more practical parameter to assess experimentally as it depends on the number of incident photons and not on the number of photons actually absorbed by the photocatalyst; ξ is also an intrinsic characteristic of a photocatalyst. ...
Article
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This article reviews the efforts of the last two decades to deNOxify the atmospheric environment with TiO2-based photocatalytic materials supported on various cementitious-like substrates. Prior to undertaking this important aspect of applied photocatalysis with metal-oxide emiconductor photocatalysts, however, it is pertinent to describe and understand the fundamentals of Heterogeneous Photocatalysis. The many attempts done in a laboratory setting to degrade (deNOxify) the major components that make up the NOx, namely nitric oxide (NO) and nitrogen dioxide (NO2), but most importantly the efforts expended in deNOxifying the real environment upon depositing titania-based coatings on various model and authentic infrastructures, such as urban roads, highway noise barriers, tunnels, and building external walls among others, are examined. Both laboratory and outdoor experimentations have been performed toward NOx being oxidized to form nitrates (NO3−) that remain adsorbed on the TiO2-based photocatalytic surfaces (except in tunnels—indoor walls) but get subsequently dislodged by rain or by periodic washings of the infrastructures. However, no serious considerations have been given to the possible conversion of NOx via photocatalytic reduction back to N2 and O2 gases that would restore the atmospheric environment, as the adsorbed nitrates block the surface-active sites of the photocatalyst and when washed-off ultimately cause unduly damages to the environment.
... Furthermore, in recent studies TiO 2 exhibits antiviral and cancer inactivation capabilities, and targeted TiO 2 can be a safe alternative to harmful chemical treatments in the biomedical field (Ziental et al., 2020). However, since the inactivation using TiO 2 is through ROS, the generation of ozone must be addressed upon installation (Monge et al., 2010). Given the current state of the technologies and the current trends in research and development, the nanoparticle market may be described as available and growing (Skyspring, 2021; Sigma-Aldrich, 2021). ...
... Filter Coatings (Joe et al., 2014;Park and Hwang, 2014;Heo et al., 2020;Pazokifard et al., 2012;Woo et al., 2012;Pyankov et al., 2012;Chen and Liang, 2020;Ziental et al., 2020;Monge et al., 2010;Skyspring, 2021;Sigma-Aldrich, 2021) • Relatively easy to incorporate coated filters into an existing HVAC system. • The nanoparticle field is currently growing and will continue to expand in applications and available technology. ...
Article
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COVID-19 forced the human population to rethink its way of living. The threat posed by the potential spread of the virus via an airborne transmission mode through ventilation systems in buildings and enclosed spaces has been recognized as a major concern. To mitigate this threat, researchers have explored different technologies and methods that can remove or decrease the concentration of the virus in ventilation systems and enclosed spaces. Although many technologies and methods have already been researched, some are currently available on the market, but their effectiveness and safety concerns have not been fully investigated. To acquire a broader view and collective perspective of the current research and development status, this paper discusses a comprehensive review of various workable technologies and methods to combat airborne viruses, e.g., COVID-19, in ventilation systems and enclosed spaces. These technologies and methods include an increase in ventilation, high-efficiency air filtration, ionization of the air, environmental condition control, ultraviolet germicidal irradiation, non-thermal plasma and reactive oxygen species, filter coatings, chemical disinfectants, and heat inactivation. Research gaps have been identified and discussed, and recommendations for applying such technologies and methods have also been provided in this article.
... Thus, O 3 formation was possibly a combination of both NO x and VOC photodissociation plus reactions with OH radicals that were formed in the 365 nm/TiO 2 system [47]. Monge et al. (2010) [49] reported the possibility of O 3 formation from illuminated TiO 2 surfaces. If this is the case, then O 3 becomes an inevitable byproduct in the 365 nm/TiO 2 system treating barn emissions, and it would be difficult to elucidate the specific contribution of UV/TiO 2 and that of O 3 to the overall odor elimination process. ...
... Thus, O 3 formation was possibly a combination of both NO x and VOC photodissociation plus reactions with OH radicals that were formed in the 365 nm/TiO 2 system [47]. Monge et al. (2010) [49] reported the possibility of O 3 formation from illuminated TiO 2 surfaces. If this is the case, then O 3 becomes an inevitable byproduct in the 365 nm/TiO 2 system treating barn emissions, and it would be difficult to elucidate the specific contribution of UV/TiO 2 and that of O 3 to the overall odor elimination process. ...
Article
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Control of gaseous emissions from livestock operations is needed to ensure compliance with environmental regulations and sustainability of the industry. The focus of this research was to mitigate livestock odor emissions with UV light. Effects of the UV dose, wavelength, TiO2 catalyst, air temperature, and relative humidity were tested at lab scale on a synthetic mixture of nine odorous volatile organic compounds (VOCs) and real poultry manure offgas. Results show that it was feasible to control odorous VOCs with both photolysis and photocatalysis (synthetic VOCs mixture) and with photocatalysis (manure offgas). The treatment effectiveness R (defined as % conversion), was proportional to the light intensity for synthetic VOCs mixtures and followed an order of UV185+254 +TiO2 > UV254 + TiO2 > UV185+254 ; no catalyst > UV254 ; no catalyst. VOC conversion R > 80% was achieved when light energy was >~60 J L−1. The use of deep UV (UV185+254) improved the R, particularly when photolysis was the primary treatment. Odor removal up to~80% was also observed for a synthetic VOCs mixture, and actual poultry manure offgas. Scale-up studies are warranted.
... However, some recent studies on pure photocatalysts and on self-cleaning window glass also observed the formation of harmful intermediates, for example nitrous acid (HONO) [6,38,39], which is even more harmful than the primary reactants NO and NO 2 . In addition, renoxification originating from adsorbed nitrate, proposed by the photocatalytic formation of the nitrate radical (NO 3 ) on TiO 2 , was recently observed in laboratory experiments [40][41][42][43][44]. Both results are dependent on the material used and the applied experimental conditions, but would potentially contradict the application of photocatalytic surfaces to improve urban air quality. ...
... However, gas phase renoxification through photocatalytic reaction of surface NO 3 − and subsequently formation of O 3 was recently proposed for photocatalytic active surfaces in chamber studies [43,44]. Thus, studies on this possible renoxification process were another focus of the present laboratory photoreactor experiments using the cementbased materials A and B. Therefore, O 3 was measured in parallel to the photocatalytic NO xexperiments. ...
Article
Small scale bed flow photoreactor experiments were performed to assess the photocatalytic performance of cement-based TiO2-containing materials for NOx reduction through the determination of kinetic parameters under variation of the experimental conditions (relative humidity, flow rate, mixing ratio and light intensity) and monitoring of potential reaction products in the gas phase and the aqueous extract of the surface. The results clearly demonstrated the general potential of the tested material to photocatalytically remediate gas phase NOx by conversion into nitrite and nitrate as identified reaction products at the surface. The measured uptake coefficients for NO and NO2 under atmospheric relevant conditions were in the range of 5 × 10⁻⁵ with a corresponding surface deposition velocity of about 0.5 cm s⁻¹. However, it became also clear that the photocatalytic activity is in part significantly dependent on the experimental conditions. The relative humidity and the mixing ratio of the air pollutant were identified as the most important parameters. In addition, under certain conditions, a renoxification process can occur. The comprehensive results of the present study are discussed in detail to develop recommendations for a possible future application of this technique to improve urban air quality.
... Several studies have assessed the heterogeneous chemistry of atmospherically relevant oxidants with PAHs (Donaldson et al., 2009;Monge et al., 2010;Styler et al., 2011;Zhou et al., 2019). Recently, Mekic et al. (2020a) and Jiang et al. (2021) have shown that the photosensitized degradation of DMSO by the excited triplet state of typical PAH compounds (fluorene ( 3 FL * ), 3 PHE * , 3 FLA * and 3 PYR * ) leads to the formation of OSs, among others, in both the gas and aqueous phases. ...
Article
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The presence of organic sulfur compounds (OS) at the water surface acting as organic surfactants, may influence the air-water interaction and contribute to new particle formation in the atmosphere. However, the impact of ubiquitous anthropogenic pollutant emissions, such as SO2 and polycyclic aromatic hydrocarbons (PAHs) on the formation of OS at the air-water interface still remains unknown. Here, we observe large amounts of OS formation in the presence of SO2, upon irradiation of aqueous solutions containing typical PAHs, such as pyrene (PYR), fluoranthene (FLA), and phenanthrene (PHE) as well as dimethylsulfoxide (DMSO). We observe rapid formation of several gaseous OSs from light-induced heterogeneous reactions of SO2 with either DMSO or a mixture of PAHs and DMSO (PAHs/DMSO), and some of these OSs (e.g. methanesulfonic acid) are well established secondary organic aerosol (SOA) precursors. A myriad of OSs and unsaturated compounds are produced and detected in the aqueous phase. The tentative reaction pathways are supported by theoretical calculations of the Gibbs energy of reactions. Our findings provide new insights into potential sources and formation pathways of OSs occurring at the water (sea, lake, river) surface, that should be considered in future model studies for a better representation of the air-water interaction and SOA formation processes.
... Indeed, the reduction of NO 2 by TiO 2 leads to the production of HNO 3 , HONO, NO and nitrate [8e10]. The UV-induced formation of ozone has also been evidenced [11]. The photolysis of HNO 3 involving the formation of NO 2 , OH and possibly HONO has been underlined on proxies and on real urban grime films [12]. ...
... For example, both field and model studies have reported significant reductions of acidic gases and O 3 in dust plumes 14,[16][17][18][19] . On the other hand, recent studies indicate that mineral dust can induce chemical reactions in the presence of sunlight, defined as heterogeneous photochemistry 20,21 . Such observations suggest that dust has the potential to participate in and promote the atmospheric photochemical processes directly as a reactant or catalyst 22 . ...
Article
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Understanding new particle formation and their subsequent growth in the troposphere has a critical impact on our ability to predict atmospheric composition and global climate change. High pre-existing particle loadings have been thought to suppress the formation of new atmospheric aerosol particles due to high condensation and coagulation sinks. Here, based on field measurements at a mountain site in South China, we report, for the first time, in situ observational evidence on new particle formation and growth in remote ambient atmosphere during heavy dust episodes mixed with anthropogenic pollution. Both the formation and growth rates of particles in the diameter range 15-50 nm were enhanced during the dust episodes, indicating the influence of photo-induced, dust surface-mediated reactions and resulting condensable vapor production. This study provides unique in situ observations of heterogeneous photochemical processes inducing new particle formation and growth in the real atmosphere, and suggests an unexpected impact of mineral dust on climate and atmospheric chemistry.
... However, some recent studies on pure photocatalysts and on self-cleaning window glass also observed the formation of harmful intermediates, for example nitrous acid (HONO) [13e17], which is even more harmful than the primary reactants NO and NO 2 [18]. In addition, re-noxification and ozone formation originating from adsorbed nitrate, attributed to the photocatalytic formation of the nitrate radical (NO 3 ) on TiO 2 , was recently observed in laboratory experiments [16] [19]. Both results depended on the material used and are still a subject of controversial discussion . ...
... Photocatalysis on TiO 2 is a major topic of research due to its straightforward implications for depolluting appliances [33, 34] and atmospheric chemistry [35]. For instance, TiO 2 is a component of natural mineral dust that represents an important reactive aerosol in the atmosphere affecting the ozone budget and the climate [36]. In this context, adsorption of water and hydroxylation of the surface, which are key aspects to understand TiO 2 photocatalysis in the environment, offer still major open questions. ...
Article
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Recent years have witnessed fast advancements in near ambient pressure X-ray photoelectron (NAPP) spectroscopy, which is emerging as a powerful tool for the investigation of surfaces in presence of vapors and liquids. In this paper we present a new chamber for the investigation of solid/vapor interfaces relevant to environmental and atmospheric chemistry that fits to the NAPP endstation at the Swiss Light Source. The new chamber allows for performing X-ray photoelectron spectroscopy (XPS) and electron yield near-edge X-ray absorption fine structure spectroscopy (NEXAFS) using soft, tender and hard X-ray in vacuum and in near-ambient pressures up to 20 mbar at environmentally relevant conditions of temperature and relative humidity. In addition, the flow tube design of the chamber enables the dosing of sticky reactive gases with short pressure equilibration time. The accessible photoelectron kinetic energy ranges from 2 to 7000 eV. This range allows the determination of surface and bulk electronic properties of ice and other environmental materials, such as metal oxides and frozen solutions, which are relevant to understanding atmospheric chemistry. The design of this instrument and first results on systems of great interest to the environmental and atmospheric chemistry community are presented. In particular, near-ambient pressure XPS and NEXAFS, coupled to a UV-laser setup, were used to study the adsorption of water on a TiO2 powder sample. The results are in line with previously proposed adsorption models of water on TiO2, and, furthermore, indicate that the concentration of water molecules tends to increase upon UV irradiation. In a second example we illustrate how NEXAFS spectroscopy measurements at the chlorine K-edge can provide new insight on the structures of eutectic and sub-eutectic frozen NaCl solutions at high and low relative humidity, respectively, indicating the formation of solution and solid NaCl phases, respectively. Finally, we demonstrate the assets of this new chamber for the dosing of sticky acidic gases and, in particular, for the investigation of formic acid uptake on ice surfaces.
... To enhance the HONO production, a glass surface of 120 cm × 40 cm coated with TiO 2 and exposed to ambient air for several days was introduced in the chamber. The detailed procedure can be found in Monge et al. (2010). ...
Article
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A NitroMAC (French acronym for continuous atmospheric measurements of nitrogenous compounds) is an instrument which has been developed for the semi-continuous measurement of atmospheric nitrous acid (HONO). This instrument relies on wet chemical sampling and detection using high performance liquid chromatography (HPLC)-visible absorption at 540 nm. Sampling proceeds by dissolution of gaseous HONO in a phosphate buffer solution followed by derivatization with sulfanilamide/N-(1-naphthyl)-ethylenediamine. The performance of this instrument was found to be as follows: a detection limit of around 3 ppt with measurement uncertainty of 10% over an analysis time of 10 min. Intercomparison was made between the instrument and a long-path absorption photometer (LOPAP) during two experiments in different environments. First, air was sampled in a smog chamber with concentrations up to 18 ppb of nitrous acid. NitroMAC and LOPAP measurements showed very good agreement. Then, in a second experiment, ambient air with HONO concentrations below 250 ppt was sampled. While a NitroMAC showed its capability of measuring HONO in moderate and highly polluted environments, the intercomparison results in ambient air highlighted that corrections must be made for minor interferences when low concentrations are measured.
... The conclusion, that the presence of NO x is necessary for the formation of O 3 was confirmed by the two clean air experiments, where no photocatalytically driven formation of O 3 using the boosted material at 50 % RH was observed. Such a formation of O 3 was identified the first time in chamber experiments with NO x on a TiO 2 coated glass surface by Monge et al. 2010b. The obtained results are very similar to the data shown in Fig. 2. They proposed a new surface reaction occurring on photocatalytically active surfaces. ...
Article
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Chamber studies were performed to investigate the efficiency of a photocatalytically active cementitious coating material to depollute contaminated air. The results showed a photocatalytic effect on ozone (O3), proven by an increase of the geometric uptake coefficient from 5.2 × 10−6 for the inactive to 7.7 × 10−6 for the active material under irradiation. Measured first-order rate constants for nitrogen oxides (NOx) under irradiation are in the range of 2.6–5.9 × 10−4 s−1, which is significantly higher compared to the inactive material (7.3–9.7 × 10−5 s−1) demonstrating the photocatalytic effect. However, no significant photocatalytic degradation was observed for the studied volatile organic compounds (VOCs) toluene and isoprene resulting in only an upper limit uptake coefficient of 5.0 × 10−7 for both VOCs. In all experiments using the photocatalytically active material, a clear formation of small carbonyl (C1–C5) gas phase compounds was identified which is suggested to result from the photocatalytic degradation of organic additives. In contrast to the uptake observed for pure O3, during the experiments with NOx (≥50 % relative humidity), a clear photocatalytic formation of O3 was observed. For the material investigated, an empirically derived overall zero-order rate constant of k 0 (O3) ≈ 5 × 107 molecules cm−3 s−1 was determined. The results demonstrate the necessity of detailed studies of heterogeneous reactions on such surfaces under more complex simulated atmospheric conditions as enabled by simulation chambers.
... Many laboratory studies have been conducted to explore the photocatalytic features of TiO 2 in heterogeneous reactions. The results show that UV-illuminated pure TiO 2 or mineral dust greatly enhanced the uptake of O 3 , NO x (NO + NO 2 ), SO 2 , volatile organic compounds (VOCs), compared to that under dark conditions [27][28][29][30][31][32][33][34][35][36] . Under a typical flux of 10 3 -10 4 photons·cm −2 · nm −1 between 300-390 nm in the solar radiation 37 , excited TiO 2 is theoretically able to generate photoactive species that drive rapid redox reactions of HCHO on mineral dust aerosols and photocatalytic anthropogenic surfaces. ...
Article
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The uptake of formaldehyde (HCHO) on mineral dust affects its budget as well as particle properties, yet the process has not yet been fully investigate. Here, TiO2 and nitrate-doped TiO2 aerosols were used as proxies for mineral dust, and the uptake of HCHO was explored in a chamber under both dark and illuminated conditions. The uptake loss of HCHO on UV-illuminated aerosols is 2–9 times faster than its gaseous photolysis in our experimental system. The uptake coefficient in the range of 0.43–1.68 × 10⁻⁷ is 1–2 orders of magnitude higher than previous reports on model mineral dust particles. The reaction rate exhibits a Langmuir-Hinshelwood-type dependence on nitrate content and relative humidity, suggesting the competitive role of nitrate salts, water vapor and HCHO on the TiO2 surface. The reaction produces carbon dioxide as the main product and gaseous formic acid as an important intermediate. The hydroxyl radical produced on illuminated TiO2 primarily drives the fast oxidation of HCHO. The nitrate radical arising from the TiO2-catalyzed photoreaction of nitrate synergistically promotes the oxidation process. This study suggests a novel oxidation route for HCHO in the atmosphere, taking into account high abundance of both mineral dust and anthropogenic TiO2 aerosols.
... Another issue, which should be addressed, is the photolysis of NO 2 and the subsequent gas-phase chemistry and formation of ozone [13]. This possibility was considered in the present study and a long-term photocatalytic test with a very low space velocity (in the range of 0.1 min −1 ) was carried out, showing that this reaction did not occur in any significant extent. ...
Article
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Titanium dioxide is the most important photocatalyst used for purifying applications. The purpose of this study was to investigate the photocatalytic activity of the commercial product Protectam FN2, containing Evonik P25 titanium dioxide nanopowder, with regard to NO and NO2 abatement. Photocatalytic experiments on the photocatalytic coatings on concrete and plaster substrates were carried out in two types of flow reactors, namely one with laminar flow and another with ideally-mixed flow, under “real world conditions” of temperature, relative humidity, irradiation intensity and pollutant concentrations. The results showed that the photocatalytic process significantly reduced the concentration of both nitrogen oxides in the air. The reaction rate, i.e., the decrease in the concentration of NOx achieved in the steady-state for the inlet concentration of NO and NO2 of 0.1 ppmv which corresponds to highly polluted urban air, was up to 75 and 50 μmol m⁻² h⁻¹, respectively, at the flow rate of 3000 cm³ min⁻¹ and relative humidity of 50%. Further, even two years after their application to the surface of concrete walls along a busy thoroughfare, the photocatalytic coating maintained high effectivity.
... However, TiO 2 has also been observed to produce NO x by photo-oxidation of Ammonia under atmospheric conditions [137]. Additionally, Monge et al. [138] revealed the production of O 3 to be induced by TiO 2 upon exposure to NO x under illumination, and the deactivation of TiO 2 surfaces by nitrate coating formation was observed ( [130] and references therein) resulting in a more complex role of TiO 2 in atmospheric chemistry than previously assumed. Photo-catalytic decomposition of O 3 on TiO 2 under atmospheric conditions has been observed by various authors (e.g., [139]) and might represent an important sink of ozone in the atmosphere. ...
Article
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In spite of the still increasing number of engineered nanomaterial (ENM) applications, large knowledge gaps exist with respect to their environmental fate, especially after release into air. This review aims to summarize the current knowledge of emissions and behavior of airborne engineered nanomaterials. The whole ENM lifecycle is considered from the perspective of possible releases into the atmosphere. Although in general, emissions during use phase and end-of-life seem to play a minor role compared to entry into soil and water, accidental and continuous emissions into air can occur especially during production and some use cases such as spray application. Implications of ENMs on the atmosphere as e.g.; photo-catalytic properties or the production of reactive oxygen species are reviewed as well as the influence of physical processes and chemical reactions on the ENMs. Experimental studies and different modeling approaches regarding atmospheric transformation and removal are summarized. Some information exists especially for ENMs, but many issues can only be addressed by using data from ultrafine particles as a substitute and research on the specific implications of ENMs in the atmosphere is still needed.
... Illuminated nitrate ions on a TiO 2 surface can combine with holes (h vb + ) produced by the semiconductor and become nitrate radicals as shown by Reaction 4. These radicals absorb strongly in the visible region, producing NO 2 and NO with the application of lower energy light than would otherwise be required (Reactions 5 and 6). 21,38,39 Painted Samples. ...
Article
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The indoor environment provides unique surfaces and lighting conditions which affect the photochemistry taking place there. As indoor illumination sources typically output wavelengths too long to affect gas phase photochemistry, the potential for surface photochemistry induced by indoor light sources has been mostly unexplored. In this proof of concept study, we report the emission of gas phase nitrogen oxides as a product of the illumination of glass surfaces coated with nitrate-doped TiO2 and nitrate-deposited indoor paint, using a variety of common indoor light sources. Fluorescent, incandescent, halogen, and LED lights were studied, and a Xenon lamp was used for baseline measurements. NOx was emitted from all samples, thus establishing that renoxification can occur in indoor environments. NO2 (g) was the predominant species emitted from samples coated with nitrate-doped TiO2 and NO (g) was the predominant species emitted from nitrate-deposited painted glass surfaces. It was also found that heating from the light sources had no effect on the production of NOx. This preliminary study establishes the potential for heterogeneous photochemistry to occur on real indoor surfaces and opens the way for further research to be conducted under realistic indoor conditions.
... The reactions of ozone (O 3 ) and hydroxyl radicals (OH) with aromatic compounds at the air-water interface can be a competitive chemical process to the photosensitized reactions of aromatic compounds (Alvarez et al., 2012;Jammoul et al., 2009;Rana & Guzman, 2020;Reeser et al., 2009;Wang et al., 2021). The heterogeneous oxidation of PAHs of atmospheric relevance has been extensively investigated in past years Monge et al., 2010;Styler et al., 2011;Zhou et al., 2019). However, the structure of gas-phase compounds formed during PAH photodegradation and the reaction pathways leading to their formation are still unclear. ...
Article
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The photochemical reactions of organic compounds at the water surface lead to the formation of gas‐phase molecules that may contribute to new particle formation in the atmosphere. Here, we observe the formation of organic sulfur (OS) compounds that are known secondary organic aerosol (SOA) precursors, upon irradiation of aqueous solutions containing typical polycyclic aromatic hydrocarbons (PAHs) such as pyrene, fluoranthene, and phenanthrene as well as dimethylsulfoxide (DMSO). The reactivity between the excited triplet states of PAHs (³PAHs*) and DMSO was determined by transient absorption spectroscopy and a tentative reaction mechanism for DMSO degradation was proposed, supported by theoretical calculations of the reaction Gibbs energies. In all cases, we observe rapid formation of methanesulfonic acid, methanesulfinic acid, methylsulfonylmethane, ethyl methanesulfonate, hydroxymethanesulfonic acid, and 2‐hydroxyethanesulfonic acid by use of novel membrane inlet‐single photon ionization‐time of flight mass spectrometry. These results suggest that ubiquitous PAHs and DMSO at the sea surface may represent an alternative source of OS compounds during daytime, through photochemical processes that should be considered in future models to better represent the SOA formation processes in the atmosphere.
... There is potential for the formation of side products other than NO 2 as a result of NO oxidation in humid air, including HONO, H 2 O 2 , N 2 O and O 3 [12,24]. These by-products are evidenced in a number of studies on pure TiO 2 [25][26][27][28][29] and are either detrimental to air quality directly, do not result in net reduction of NOx, and/or contribute to ozone production. The three recent reviews on this topic highlight the need for further analysis of by-product formation [24,30]. ...
Article
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NOx is a pervasive pollutant in urban environments. This review assesses the current state of the art of photocatalytic oxidation materials, designed for the abatement of nitrogen oxides (NOx) in the urban environment, and typically, but not exclusively based on titanium dioxide (TiO2). Field trials with existing commercial materials, such as paints, asphalt and concrete, in a range of environments including street canyons, car parks, tunnels, highways and open streets, are considered in-depth. Lab studies containing the most recent developments in the photocatalytic materials are also summarised, as well as studies investigating the impact of physical parameters on their efficiency. It is concluded that this technology may be useful as a part of the measures used to lower urban air pollution levels, yielding ∼2% NOx removal in the immediate area around the surface, for optimised TiO2, in some cases, but is not capable of the reported high NOx removal efficiencies >20% in outdoor urban environments, and can in some cases lower air quality by releasing hazardous by-products. However, research into new material is ongoing. The reason for the mixed results in the studies reviewed, and massive range of removal efficiencies reported (from negligible and up to >80%) is mainly the large range of testing practices used. Before deployment in individual environments site-specific testing should be performed, and new standards for lab and field testing should be developed. The longevity of the materials and their potential for producing hazardous by-products should also be considered.
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Nitrogen oxides (NO x ), especially nitric oxide (NO) and nitrogen dioxide (NO2), play a major role in atmospheric chemistry and air pollution. They constitute a major environmental and health concern as they are toxic compounds and also facilitate the formation of ozone and acid rain. As a consequence, increasingly stronger regulations and policies are in place enforcing actions to reduce emissions and to lower the overall pollutant levels. Recent studies and events have shown that emission standards are frequently exceeded as well as emission treatment systems not being as efficient as they are claimed to be. Apart from reducing the emissions directly at the emission source, which appears to be more difficult than anticipated, semiconductor photocatalysis presents an appealing alternative capable of removing NO x and other air pollutants from the air once they have already been released and dispersed. Additionally, photocatalysis needs neither maintenance nor external reagents because the only requirements are sunlight, water, and molecular oxygen, which are already present in outdoor conditions. This chapter will summarize the basic principle and mechanism of photocatalytic NO x abatement, including studies on the reaction pathways and intermediate products, highlighting the principal challenges of the technique. Additionally, recently reported strategies of improving the activity, spectral response, and selectivity of the photocatalysts are reviewed. As the intended applications are functionalized building materials, the specific challenges of coating the photocatalysts onto or integrating them into the matrices of these building materials are addressed. Finally, a collection of results from field tests and simulations are reported to estimate the efficacy of the technology in real-world scenarios.
Article
The reaction of new dinuclear gold(I) organometallic complexes containing mesityl ligands and bridging bidentate phosphanes [Au(2)(mes)(2)(μ-LL)] (LL=dppe: 1,2-bis(diphenylphosphano)ethane 1a, and water-soluble dppy: 1,2-bis(di-3-pyridylphosphano)ethane 1b) with Ag(+) and Cu(+) lead to the formation of a family of heterometallic clusters with mesityl bridging ligands of the general formula [Au(2)M(μ-mes)(2) (μ-LL)][A] (M=Ag, A=ClO(4)(-), LL=dppe 2a, dppy 2b; M=Ag, A=SO(3)CF(3)(-), LL=dppe 3a, dppy 3b; M=Cu, A=PF(6)(-), LL=dppe 4a, dppy 4b). The new compounds were characterized by different spectroscopic techniques and mass spectrometry The crystal structures of [Au(2)(mes)(2)(μ-dppy)] (1b) and [Au(2)Ag(μ-mes)(2)(μ-dppe)][SO(3)CF(3)] (3a) were determined by a single-crystal X-ray diffraction study. 3a in solid state is not a cyclic trinuclear Au(2)Ag derivative but it gives an open polymeric structure instead, with the {Au(2)(μ-dppe)} fragments "linked" by {Ag(μ-mes)(2)} units. The very short distances of 2.7559(6) Å (Au-Ag) and 2.9229(8) Å (Au-Au) are indicative of gold-silver (metallophilic) and aurophilic interactions. A systematic study of their luminescence properties revealed that all compounds are brightly luminescent in solid state, at room temperature (RT) and at 77 K, or in frozen DMSO solutions with lifetimes in the microsecond range and probably due to the self-aggregation of [Au(2)M(μ-mes)(2)(μ-LL)](+) units (M=Ag or Cu; LL=dppe or dppy) into an extended chain structure, through Au-Au and/or Au-M metallophilic interactions, as that observed for 3a. In solid state the heterometallic Au(2)M complexes with dppe (2a-4a) show a shift of emission maxima (from ca. 430 to the range of 520-540 nm) as compared to the parent dinuclear organometallic product 1a while the complexes with dppy (2b-4b) display a more moderate shift (505 for 1b to a max of 563 nm for 4b). More importantly, compound [Au(2)Ag(μ-mes)(2)(μ-dppy)]ClO(4) (2b) resulted luminescent in diluted DMSO solution at room temperature. Previously reported compound [Au(2)Cl(2)(μ-LL)] (LL dppy 5b) was also studied for comparative purposes. The antimicrobial activity of 1-5 and Ag[A] (A=ClO(4)(-), SO(3)CF(3)(-)) against gram-positive and gram-negative bacteria and yeast was evaluated. Most tested compounds displayed moderate to high antibacterial activity while heteronuclear Au(2)M derivatives with dppe (2a-4a) were the more active (minimum inhibitory concentration 10 to 1 μg mL(-1)). Compounds containing silver were ten times more active to gram-negative bacteria than the parent dinuclear compound 1a or silver salts. Au(2)Ag compounds with dppy (2b, 3b) were also potent against fungi.
Article
Teragram quantities of crustal and volcanic aerosol are released into the atmosphere on an annual basis. Although these substrates contain photoactive metal oxides, little is known about the role that they may play in catalyzing the heterogeneous phototransformation of semivolatile organic species. In the present study, we have investigated oxalic acid photochemistry at the surface of Fe(2)O(3), TiO(2), Mauritanian sand, and Icelandic volcanic ash in the presence and absence of oxygen using a photochemical Knudsen cell reactor. Illumination of all sample types resulted in the production of gas-phase CO(2). In the case of Mauritanian sand, the production of gas-phase CO(2) scaled with the loss of surface oxalic acid. In the absence of oxygen, the production of CO(2) by the sand and ash films scaled with the absorption spectrum of iron oxalate, which suggests that the reaction is at least in part iron-mediated. The presence of oxygen suppressed CO(2) production at the Fe(2)O(3) surface, enhanced CO(2) production at the Mauritanian sand surface, and did not have a net effect upon CO(2) production at the Icelandic ash surface. These different oxygen dependencies imply that oxalic acid photochemistry at the authentic surfaces under study was not solely iron-mediated. Experiments at the TiO(2) surface, which showed enhanced CO(2) production from oxalic acid in the presence of oxygen, suggest that Ti-mediated photochemistry played an important role. In summary, these results provide evidence that solid-phase aerosol photochemistry may influence the atmospheric lifetime of oxalic acid in arid regions, where its removal via wet deposition is insignificant.
Article
Titanium dioxide is a relatively inexpensive, nontoxic, and easy to handle material, which is of great interest, as evidenced by the wide number of applications as well as the number of publications that have appeared in the past 20 years. Although anatase is often noted as being the most photoactive polymorph, there are numerous studies showing that rutile has comparable or even greater photoactivities than anatase. When excited with light of wavelength equal to or greater than the band gap energy, there is the generation of electronhole pairs in the conduction and valence bands, respectively. It is noteworthy that, due to the limitation of techniques for aerosol particle characterization, there are few studies that have simultaneously detected the size, composition, and phase of small atmospheric particles. It is generally assumed that metals detected in the aerosols are attributed to their corresponding metal oxides with no or limited knowledge of the exact phase.
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Self-cleaning surfaces containing TiO2 nanoparticles have been postulated to efficiently remove NOx from the atmosphere. However, UV irradiation of NOx adsorbed on TiO2 also was shown to form harmful gas-phase byproducts such as HONO and N2O that may limit their depolluting potential. Ambient pressure XPS was used to study surface and gas-phase species formed during adsorption of NO2 on TiO2 and subsequent UV irradiation at lambda = 365 nm. It is shown here that NO3-, adsorbed on TiO2 as a byproduct of NO2 disproportionation, was quantitatively converted to surface NO2 and other reduced nitrogenated species under UV irradiation in the absence of moisture. When water vapor was present, a faster NO3- conversion occurred, leading to a net loss of surface-bound nitrogenated species. Strongly adsorbed NO3- in the vicinity of coadsorbed K+ cations was stable under UV light, leading to an efficient capture of nitrogenated compounds.
Article
TiO2 coated glasses and TiO2/KNO3 films exposed to NOx produce O3 under both artificial and natural illumination.
Article
At the laboratory scale, photocatalysis is a promising method to convert many air pollutants such as nitrogen oxides and volatile organic compounds, to safer products for human health but also environmentally more acceptable, such as nitrate and carbon dioxide.Indoor and industrial applications of photocatalysis to remove local air pollutants from the atmosphere are now numerous. Large scale outdoor applications of photocatalysis started with self-cleaning glass, coatings and paints for buildings, and several outdoor experiments have been documented regarding the photocatalytic reduction of NOx levels in urban environment, such as tunnels, streets and highways.The potential applications of photocatalysis, to remove or mitigate a wide range of global warming contributors from the atmosphere, seem an attractive method to help fighting climate change. By harnessing solar energy, photocatalytic processes consume less energy than conventional methods. This review article shows that photocatalysis may be applied successfully to eliminate or transform of all major long-lived well mixed greenhouse gases, but also soot and tropospheric ozone and other short-lived climate forcers. The cases of sulphur hexafluoride and nitrogen trifluoride are also discussed.
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This chapter reviews literature reported during 2009 on titanium, zirconium and hafnium.
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Air pollution by nitrogen oxides represents a serious environmental problem in urban areas where numerous sources of these pollutants are concentrated. One approach to reduce the concentration of these air pollutants is their light-induced oxidation in the presence of molecular oxygen and a photocatalytically active building material which uses titanium dioxide as the photocatalyst. Herein, results of an investigation concerning the influence of the photon flux and the pollutant concentration on the rate of the photocatalytic oxidation of nitrogen(ii) oxide in the presence of molecular oxygen and UV(A) irradiated titanium dioxide powder are presented. A Langmuir-Hinshelwood-type rate law for the photocatalytic NO oxidation inside the photoreactor comprising four kinetic parameters is derived being suitable to describe the influence of the pollutant concentration and the photon flux on the rate of the photocatalytic oxidation of nitrogen(ii) oxide.
Article
We made intensive measurements of wavelength-resolved spectral irradiance in a test house during the HOMEChem campaign and report diurnal profiles and two-dimensional spatial distribution of photolysis rate constants (J) of several important indoor photolabile gases. Results show that sunlight entering through windows, which was the dominant source of ultraviolet (UV) light in this house, led to clear diurnal cycles, and large time- and location-dependent variations in local gas-phase photochemical activity. Local J values of several key indoor gases under direct solar illumination were 1.8–7.4 times larger—and more strongly dependent on time, solar zenith angle, and incident angle of sunlight relative to the window—than under diffuse sunlight. Photolysis rate constants were highly spatially heterogeneous and fast photochemical reactions in the gas phase were generally confined to within tens of cm of the region that were directly sunlit. Opening windows increased UV photon fluxes by 3 times and increased predicted local hydroxyl radical (OH) concentrations in the sunlit region by 4.5 times to 3.2 × 10⁷ molec cm⁻³ due to higher J values and increased contribution from O3 photolysis. These results can be used to improve the treatment of photochemistry in indoor chemistry models and are a valuable resource for future studies that use the publicly available HOMEChem measurements.
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During the European Life+ project PhotoPAQ (Demonstration of Photocatalytic remediation Processes on Air Quality), photocatalytic remediation of nitrogen oxides (NOx), ozone (O3), volatile organic compounds (VOCs), and airborne particles on photocatalytic cementitious coating materials was studied in an artificial street canyon setup by comparing with a colocated nonactive reference canyon of the same dimension (5 × 5 × 53 m). Although the photocatalytic material showed reasonably high activity in laboratory studies, no significant reduction of NOx, O3, and VOCs and no impact on particle mass, size distribution, and chemical composition were observed in the field campaign. When comparing nighttime and daytime correlation plots of the two canyons, an average upper limit NOx remediation of ≤2 % was derived. This result is consistent only with three recent field studies on photocatalytic NOx remediation in the urban atmosphere, whereas much higher reductions were obtained in most other field investigations. Reasons for the controversial results are discussed, and a more consistent picture of the quantitative remediation is obtained after extrapolation of the results from the various field campaigns to realistic main urban street canyon conditions.
Article
The photocatalytic activities of two commercial TiO2 particles (P25 and R996) and their Fe-modified counterparts (Fe-P25 and Fe-R996) towards NO oxidation were examined under various light and humidity conditions (dry or humid conditions under UV or visible light irradiation). The photocatalytic activity of R996 increased upon the modification under all four experimental conditions but the Fe-R996 particles still exhibited lower activity than the P25-based photocatalytic particles (P25 and Fe-P25) under all of the conditions. However, when the particles were mixed with a surface hardening agent (SHA) and spread onto a cement block, Fe-R996 exhibited much higher photocatalytic activity than Fe-P25 under visible light irradiation and humid conditions in terms of NO3– evolution and NO removal. We highlight that not only the intrinsic structure of the photocatalyst powder but also the synergy between the medium embedding the photocatalytic particles and the photocatalyst should be considered in applications for air purification.
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Despite the increased awareness of heterogeneous reaction on mineral dust, the knowledge of how the intensity of solar irradiation influences the photochemistry activity remains a crucially important part in atmospheric research. Relevant studies have not seriously discussed the photochemistry under weak sunlight during haze, and thus ignored some underlying pollution and toxicity. Here, we investigated the heterogeneous formation of nitrate and nitrite under various illumination conditions by laboratory experiments and field observations. Observed by in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), water-solvated nitrate was the main surface product, followed by other species varying with illumination condition. The growth of nitrate formation rate tends to be slow after the initial fast with increasing light intensity. For example, the geometric uptake coefficient (γgeo) under 30.5 mW/cm² (5.72 × 10−6) has exceeded the 50 % of that under 160 mW/cm² (1.13 × 10−5). This case can be explained by the excess NO2 adsorption under weak illumination while the excess photoinduced active species under strong irradiation. Being negatively associated with nitrate (R² = 0.748, P 2 = 0.834, P 2 = 0.632, P
Article
The photocatalytic decomposition of nitrogen oxides (NOx) has attracted significant interest as a potential measure of reducing NOx levels in the urban atmosphere. Since photocatalytic activity is highly variable depending on atmospheric conditions, the uptake of NO, NO2 and HONO was studied on a commercial photocatalytic dispersion paint in a flow photoreactor as a function of the relative humidity and temperature. Since the relative humidity is a function of the surface's temperature, here both dependencies were carefully decoupled for the first time. In addition, for the first time the temperature dependence of the whole NOx reaction system including the important intermediate HONO was investigated. While for NO and NO2 strong negative humidity dependencies were observed, the photocatalytic uptake of HONO increased with humidity. For constant relative humidity no temperature dependence of the photocatalytic oxidation of NO was observed, whereas the photocatalytic NO2 uptake decreased with increasing temperature, which is explained by a temperature dependent adsorption equilibrium of the surface active NO2. HONO uptake showed a positive temperature dependence confirming the proposed photocatalysis of nitrite in a layer of adsorbed water on the surface of the photocatalyst. The missing/negative temperature dependencies of the photocatalysis of NO/NO2 are overcompensated by their strong negative relative humidity dependencies, leading to increasing uptake for both pollutants when photocatalytic surfaces are heated by solar irradiation in the atmosphere.
Article
Photocatalytically active building materials can contribute to better air quality by mineralizing the hazardous nitrogen oxides to nitrate. Evaluation of these materials is very important for optimization and implementation at target sites. However, the currently widely used ISO 22917-1:2016 test is unsuitable to estimate the effect on real scenarios since it employs too high concentrations and only evaluates the NO oxidation potential, completely ignoring the more relevant NO2. Herein, we present a methodology to better simulate the real-world performance of these materials based on its effect on the ternary mixture NO/NO2/O3 under environmentally relevant concentrations, which can also be simulated by means of kinetic analysis of experimentally less complex single component measurements. From this, a new figure of merit - the trend value - is derived to rate the materials' performance.
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The widespread applications of ozone technologies are established on the basis of large-scale manufacture of ozone generator and chemical reactivity of ozone. It is hence necessary to summarize the principles of ozone generation and to analyze the physicochemical properties of ozone, which are of fundamental significance to indicate its technical developments and practical applications. This review presents a summary concerning ozone generation mechanisms, the physicochemical properties of ozone, as well as the applications of ozone in water treatment. Ozone can be produced by phosphorus contact, silent discharge, photochemical reactions, and electrochemical reactions, principally proceeding by the reaction of oxygen atom with oxygen molecule. There are side reactions to the generation of ozone, however, which are responsible for ozone depletion including thermal decomposition and quenching reactions by reactive species. The solubility of ozone in water is much higher than that of oxygen, suggesting that it may be reliably applied in water and wastewater treatment. Based on the resonance structures of ozone, one oxygen atom in ozone molecule is electron-deficient displaying electrophilic property, whereas one oxygen atom is electron-rich holding nucleophilic property. The superior chemical reactivity of ozone can also be indirectly revealed by radical-mediated reactions initiated from homogenous and heterogeneous catalytic decomposition of ozone. Owing to the reliable generation of ozone and its robust reactive properties, it is worthy to thoroughly elaborate the applications of ozone reaction in drinking water disinfection and pre- or post-treatment of industrial wastewater including cyanide wastewater, coking wastewater, dyeing wastewater, and municipal wastewater. The structural characteristics of ozone reactors and energy requirement of applied technologies are evaluated. In addition, future directions concerning the development of ozone generation, ozone reactivity, and industrial wastewater ozonation have been proposed.
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The presence of organic sulfur compounds (OSs) at the water surface, acting as organic surfactants, may influence the air-water interaction and contribute to new particle formation in the atmosphere. However, the impact of ubiquitous anthropogenic pollutant emissions, such as SO2 and polycyclic aromatic hydrocarbons (PAHs) on the formation of OSs at the air-water interface still remains unknown. Here, we observe large amounts of OSs formation in presence of SO2, upon irradiation of aqueous solutions containing typical PAHs such as pyrene (PYR), fluoranthene (FLA), and phenanthrene (PHE), as well as dimethylsulfoxide (DMSO). We observe rapid formation of several gaseous OSs from light-induced heterogeneous reactions of SO2 with either DMSO or a mixture of PAHs/DMSO, and some of these OSs (e.g. methanesulfonic acid) are well established secondary organic aerosol (SOA) precursors. A myriad of OSs and unsaturated compounds are produced and detected in the aqueous phase. The tentative reaction pathways are supported by theoretical calculations of the reaction Gibbs energies. Our findings provide new insights into potential sources and formation pathways of OSs occurring at the water (sea, lake, river) surface, that should be considered in future model studies to better represent the air-water interaction and SOA formation processes.
Article
Semiconductor photocatalysis could be an effective means to combat air pollution, especially nitrogen oxides, which can be mineralized to nitrate. However, the reaction typically shows poor selectivity, releasing a number of unwanted and possibly toxic intermediates such as nitrogen dioxide. Up to now, the underlying principles that lead to this poor selectivity were not understood so a knowledge-based catalyst design for more selective materials was impossible. Herein, we present strong evidence for the slow oxygen reduction being one the causes, as the competing back-reduction of nitrate leads to the release of nitrogen dioxide. Consequently, engineering the photocatalyst for a better oxygen reduction efficiency should also increase the nitrate selectivity.
Article
Photocatalytic materials are potentially an effective remediation technology for indoor air purification. Here, we assess the impact of photocatalytic paint’s porosity on indoor nitrogen oxides (NOx) and nitrous acid (HONO) levels. We observed that the porosity of photocatalytic paints plays a paramount role for the NO2 removal. The increase of porosity from PVC 53 % to PVC 80 % leads to an increase of the geometric NO2 uptake coefficient from (3.3±0.5)·10-6 to (2.7±0.1)·10-5. At the same time, high quantity of HONO formed by NO2 conversion on photocatalytic paint is emitted in the air. The formation of HONO which is considered as a harmful compound and a major player in the oxidative capacity of indoor air is reduced as the paint porosity increases. Based on these results, further optimizations should be considered for future commercialization of photocatalytic paints aimed for indoor applications.
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Semiconductor photocatalysis could be an effective means to combat nitrogen oxides (NOx) based air pollution through mineralisation of NOx to nitrate. However, most of the typically TiO2-based catalysts employed show a much higher reactivity towards NO than NO2, leading to an accumulation of this unwanted and toxic intermediate. By grafting the photocatalyst with small amounts (≤0.1 at%) of isolated iron(III) ions, the reactivity towards NO2 is increased by the factor of 9, bringing it up to par with the NO-reactivity and alleviating the problem with intermediate accumulation. Consequently, the observed selectivity of the reaction is dramatically increased from less than 40% to more than 90%. The paper also discusses possible mechanisms for this very beneficial behavior.
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Secondary organic aerosol (SOA), a dominant air pollutant in many countries, threatens the lives of millions of people. Extensive efforts have been invested in studying the formation mechanisms and influence factors of SOA. As promising materials in eliminating air pollutants, the role of photocatalytic materials in SOA formation is unclear. In this study, TiO2 was employed to explore its impact on SOA formation during the photooxidation of m-xylene with NOx in a smog chamber. We found that the presence of TiO2 strongly suppressed SOA formation. The yields of SOA in the photooxidation experiments of m-xylene with NOx were 0.3-4%, while negligible SOA was formed when TiO2 was added. When ((NH4)2SO4) was introduced as seed, the presence of TiO2 decreased the yields of SOA from 0.3-6% to 0.3-1.6%. The sharply decreased concentrations of reactive carbonyl compounds were the direct cause of the suppression effect of TiO2 on SOA formation. However, the suppression effect was influenced by the addition of seed and the initial concentration of NOx. Reaction mechanisms of the photocatalysis of m-xylene with and without NOx were proposed.
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The field of surface science provides a unique approach to understanding bulk, surface and interfacial phenomena occurring during TiO2 photocatalysis. This review highlights, from a surface science perspective, recent literature that provides molecular-level insights into photon-initiated events occurring at TiO2 surfaces. Seven key scientific issues are identified in the organization of this review. These are: (1)Â photon absorption, (2)Â charge transport and trapping, (3)Â electron transfer dynamics, (4)Â the adsorbed state, (5)Â mechanisms, (6)Â poisons and promoters, and (7)Â phase and form. This review ends with a brief examination of several chemical processes (such as water splitting) in which TiO2 photocatalysis has made significant contributions in the literature.
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[1] The nitrate formation on dust particles is considered as a sink for atmospheric NOy (such as HNO3). However mineral dust is shown here to be an effective photocatalyst for transformation of nitrate anions into NO and NO2, without involving its photolysis. The photodecomposition of NO3− at the surface of synthetic mineral dust samples of SiO2, TiO2, mixed TiO2-SiO2 and authentic sand doped with 6% NO3− was studied by means of a flow-tube at 298 K with UV-illumination in the 340–420 nm range at relative humidities between 5 and 80%. Both NO and NO2 are observed during irradiation of films composed of either mixed TiO2-SiO2, pure TiO2 and authentic minerals from the Sahara. The relative humidity strongly affects the concentration of NOx released into the gas phase. The photoinduced nitrate conversion into NOx is discussed as being a potential renoxification process of the atmosphere.
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1] Mineral dust contains material such as TiO 2 that is well known to have photocatalytic activity. In this laboratory study, mixed TiO 2 -SiO 2 , Saharan dust and Arizona Test Dust were exposed to NO 2 in a coated wall flow tube reactor. While uptake in the dark was negligible, photoenhanced uptake of NO 2 was observed on all samples. For the mixed TiO 2 -SiO 2 , the uptake coefficients increased with increasing TiO 2 mass fraction, with BET uptake coefficients ranging from 0.12 to 1.9 Â 10 À6 . HONO was observed from all samples, with varying yields, e.g., 80% for Saharan dust. Three-dimensional modeling indicates that photochemistry of dust may reduce the NO 2 level up to 37% and ozone up to 5% during a dust event in the free troposphere. Citation: Ndour,
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Nitrogen oxides, including nitrogen dioxide and nitric acid, react with mineral dust particles in the atmosphere to yield adsorbed nitrate. Although nitrate ion is a well-known chromophore in natural waters, little is known about the surface photochemistry of nitrate adsorbed on mineral particles. In this study, nitrate adsorbed on aluminum oxide, a model system for mineral dust aerosol, is irradiated with broadband light (lambda > 300 nm) as a function of relative humidity (RH) in the presence of molecular oxygen. Upon irradiation, the nitrate ion readily undergoes photolysis to yield nitrogen-containing gas-phase products including NO(2), NO, and N(2)O, with NO being the major product. The relative ratio and product yields of these gas-phase products change with RH, with N(2)O production being highest at the higher relative humidities. Furthermore, an efficient dark reaction readily converts the major NO product into NO(2) during post-irradiation. Photochemical processes on mineral dust aerosol surfaces have the potential to impact the chemical balance of the atmosphere, yet little is known about these processes. In this study, the impact that adsorbed nitrate photochemistry may have on the renoxification of the atmosphere is discussed.
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Titanium dioxide is the most important photocatalysts used for purifying applications. If a TiO2- containing material is left outdoors as a form of flat panels, it is activated by sunlight to remove harmful NOx gases during the day. The photocatalytic efficiency of a TiO2-treated mortar for removal of NOx was investigated in the frame of this work. For this purpose a fully equipped monitoring system was designed at a pilot site. This system allows the in situ evaluation of the de-polluting properties of a photocatalytic material by taking into account the climatologic phenomena in street canyons, accurate measurements of pollution level and full registration of meteorological data The pilot site involved three artificial canyon streets, a pollution source, continuous NOx measurements inside the canyons and the source as well as background and meteorological measurements. Significant differences on the NOx concentration level were observed between the TiO2 treated and the reference canyon. NOx values in TiO2 canyon were 36.7 to 82.0% lower than the ones observed in the reference one. Data arising from this study could be used to assess the impact of the photocatalytic material on the purification of the urban environment.
Article
The phenomenon of facade soiling and staining is probably as old as the cities themselves. Unfortunately, industrial pollution has exacerbated the problems with impacts on both the quality of the urban environment and the life cycle cost of the buildings. The development of new materials that can be easily applied on facades, with both self-cleaning and de-polluting properties would be a significant step towards improvements in urban quality of cities. The aim of the European Project PICADA, as Photocatalytic Innovative Coverings Applications for De-pollution Assessment, is actually to develop a range of such novel materials and to evaluate large scale effect (typically in a canyon street). The self-cleaning and de-polluting abilities arise from the photocatalytic properties of titanium dioxide which can be introduced either in the building matrix or surface coatings. Such materials, when exposed to solar UV rays, will act as a catalyst for the photoinduced decomposition of organic molecules adsorbed or occluded on its surface. Two binders were considered in the objectives of this project. These were cement-based materials and organic-based coatings. This paper presents results obtained on the cementitious formulations containing nanoparticle composites of titanium dioxide. Investigations were undertaken in the laboratory to evaluate the self-cleaning and de-polluting properties of these impregnated materials on organic molecules as well as volatile organic compounds (VOCs) and nitrogen oxides (NOx). Analysis of the data indicates that the products exhibit significant activity with facile decomposition kinetics.
Article
The role of heterogeneous reactions of trace atmospheric gases on solid aerosol surfaces in the troposphere is not well understood. In this review, the heterogeneous chemistry of nitrogen oxides and volatile organic compounds on the surface of atmospherically relevant particles such as oxides, soot, carbonates and mineral dust is discussed. The approach to unravelling both the molecular level details and the global significance of these reactions is one that combines laboratory investigations and atmospheric chemistry models. The laboratory investigation utilizes spectroscopic measurements to provide an understanding of the details of the chemistry that occurs in the gas phase and on the surface of the particle together with kinetic measurements in order to quantify the rates of these reactions. These fundamental measurements provide a basis for understanding the detailed molecular level physical chemistry of atmospheric processes. The kinetic data, together with the details provided by the spectroscopic studies, are incorporated into atmospheric chemistry models in order to gain a greater understanding of the role that heterogeneous chemistry plays in the chemical balance of the troposphere. Several examples from the present author's laboratory are discussed in this review. These examples include: (i) the heterogeneous uptake and reactivity of nitrogen oxides (nitrogen dioxide and nitric acid) on oxide and dust particles; (ii) the role of water in the heterogeneous uptake of nitric acid on oxide, carbonate and dust particles; (iii) the heterogeneous production of nitrous acid on silica and soot particles; and (iv) the heterogeneous uptake and reactivity of acetone and other carbonyl compounds on oxide and mineral dust particles.
Article
Titanium dioxide (TiO2) oxidized nitric oxide (NO) to nitric acid (HNO3) very rapidly under ultraviolet light illumination, but some NO was oxidized to nitrogen dioxide (NO2), while activated carbon (AC) adsorbed NO2 well. A mixture of TiO2 and AC was thus confirmed to be an excellent photoassisted catalyst for removal of low concentration (sub-ppm) NOx from air. Addition of 1–3 wt.% of ferric oxide (Fe2O3) to the mixture could markedly increase the catalytic activity. Even though the catalytic activity gradually declined with the reaction time, it could be completely recovered only by washing the catalyst with water, suggesting that HNO3 accumulated on the catalyst was removed. It is inferred that the photoilluminated TiO2 generates reactive oxygen species which oxidize (NO and NO2) to (NO2 and HNO3). AC can trap NO2 and give TiO2 enough time to oxidize NO2 to HNO3. Fe2O3 seems to be a promoter which can attract more NO/NO2 to the surface of the catalyst.
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This review summarises the key topics in the field of self-cleaning coatings, concentrating on the materials that have been used in commercial applications and recent research that aims to improve these materials. Hydrophobic and hydrophilic coatings are discussed, and the various mechanisms of self-cleaning are described and related to the material properties of the coatings. Although several multinational companies have released products incorporating self-cleaning coatings, there remains much potential in this field.
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Reactions on/in condensed media play important roles in the chemistry of the troposphere. These reactions can be classified as heterogeneous reactions, those occurring on the surfaces of solids, and multiphase reactions, those occurring inside a liquid droplet. A few general considerations in the treatment of heterogeneous and multiphase reactions are discussed. Based on the knowledge and experience obtained from the study of stratospheric heterogeneous and multiphase reactions, possible characteristics of such processes in the troposphere are described and illustrated using a few examples. It is noted that the variety of condensed media in the troposphere makes this a rich field for investigation. The possible importance of reduction reactions in the oxidizing atmosphere is examined. The reactions on soot are highlighted and the potential importance of reactions on organic aerosol and other media is discussed. Finally, the outlook for progress in this area is very briefly discussed.
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The TiO2-based DeNOx catalyst which is now commercially implemented world-wide is described, emphasizing the importance of the resistance to SOx-poisoning and plugging by particulates. Various NOx reduction technologies are compared and recent trends in catalytic DeNOx installations in Japan are reviewed. For wider applications and to meet the stringent emission requirements, newly developed DeNOx catalysts such as for high-temperature gas turbine exhaust and for simultaneous removal of NOx and CO are also introduced.
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The feasibility of applying photocatalytic technology for removal of nitrogen oxide (NO) and volatile organic compounds (VOCs) at typical indoor air level using TiO2 (Degussa P-25) were investigated. Benzene, toluene, ethylbenzene and o-xylene (BTEX) were chosen because they are the most abundant in the indoor environments. The concentrations conducted for this study is 200 parts per billion (ppb) NO and less than 100ppb for BTEX respectively. Sensitive analyses were conducted for NO, BTEX and a mixture of NO and BTEX under different residence time, levels of humidity and initial concentrations. Under the experimental conditions, it was found that the presence of NO promoted the photodegradation of BTEX under low and high humidity levels. However, at moderate humidity levels, the presence of NO reduced the photodegradation of BTEX. The promotion effect decreased with decreasing residence time. The overall photodegradation of NO and BTEX decreased acutely in relation to increasing humidity. In addition, NO2 was found to react with BTEX as well.
Article
The effects of initial NO concentration, gas-residence time, reaction temperature and ultraviolet (UV) light intensity on the photocatalytic decomposition of NO have been determined in an annular flow-type and a modified two-dimensional fluidized-bed photoreactors. The decomposition of NO by photocatalysis increases with decreasing initial NO concentration and increasing gas-residence time. The reaction rate increases with increasing UV light intensity. The light transmission increases exponentially with the bed voidage at superficial gas velocity above 1.3 times the minimum fluidizing velocity (Umf) in the two-dimensional fluidized-bed photoreactor. In the two-dimensional fluidized-bed photoreactor, NO decomposition reaches >70% at the gas velocity of 2.5Umf. A two-dimensional fluidized-bed photoreactor is an effective tool for high NO decomposition with efficient utilization of photon energy.
Article
The reactions of nitrate radical (photochemically generated by the flash photolysis of a mixture of ceric ammonium nitrate and nitric acid) with amino acids are reported. An electron transfer reaction pathway is proposed for phenylalanine and histidine, whereas a hydrogen abstraction pathway is invoked for the other amino acids, based on the magnitudes of the second- order rate constants and teh observation of cation radicals. The reactivity of the nitrate radical is compared with that of other oxidizing radicals, such as Cl2−CO3−and SO4−. The nitrte radical is expected to show a higher reactivity than SO4− based on their redox couples, but an inverse reactivity relationship is observed and is suggested to be due to the difference in solvation in the transition state.
Article
The photoenhanced uptake of nitrogen dioxide (NO2) to the surface of commercially available self-cleaning window glass has been studied under controlled laboratory conditions. This material is one of an array of modern building products which incorporate titanium dioxide (TiO2) nanoparticles and are finding increasing use in populated urban areas. Amongst the principal drivers for the use of these materials is that they are thought to facilitate the irreversible removal of pollutants such as NO2 and organic molecules from the atmosphere and thus act to remediate air quality. While it appears that TiO2 materials do indeed remove organic molecules from built environments, in this study we show that the photoenhanced uptake of NO2 to one example material, self-cleaning window glass, is in fact accompanied by the substantial formation (50–70%) of gaseous nitrous acid (HONO). This finding has direct and serious implications for the use of these materials in urban areas. Not only is HONO a harmful respiratory irritant, it is also readily photolysed by solar radiation leading to the formation of hydroxyl radicals (OH) together with the re-release of NOx as NO. The net effect of subsequent OH initiated chemistry can then be the further degradation of air quality through the formation of secondary pollutants such as ozone and VOC oxidation products. In summary, we suggest that a scientifically conceived technical strategy for air quality remediation based on this technology, while widely perceived as universally beneficial, could in fact have effects precisely opposite to those intended.
Article
TiO2, a component of atmospheric mineral aerosol, catalyses the reduction of NO2 to nitrous acid (HONO) when present as an aerosol and illuminated with near UV light under conditions pertinent to the troposphere.
Article
The action spectra of the colloidal TiO2-photosensitized oxidations of bifunctional aromatics in 1 mM phosphate colloidal media provide firm evidence that electron transfer from outer-sphere donors can compete with excited hole relaxation at nanoparticle interfaces. The possibility and extent of the competition are largely determined by the dependence of Marcus nuclear factors on the donors' reversible redox potentials E°D/D+• relative to the valence band edge. Good electron donors are degraded by OH radicals produced in the oxidation of water by thermal holes, whereas direct electron transfer into excited holes (the pathway favored by less oxidizable substrates) leads to enhanced quantum efficiencies at short wavelengths. The ultimate decline of the quantum efficiency for the oxidation of phthalate (the most endoergic donor of the set) by λ ≤ 320 nm photons indicates that the relaxation of highly excited carriers takes place in discrete steps commensurate with electron transfer reorganization energies. The latter observation is consistent with the opening of low order multiphonon channels for the disposal of kinetic energy quanta larger than the depth of surface ν̄O-H 3700 cm-1 vibrational sinks.
Article
A deactivating behavior of TiO2 photocatalysts in NO2 oxidation to HNO3 was studied with use of TiO2 nanoparticulate thin films (0.5−1.5 μm thick) under UV light illumination over 10 h. The photocatalytic activity was decreased with accumulation of HNO3 on the TiO2 surface. For thicker TiO2 films, the deactivation rate was found to be slower. The amount of HNO3 trapped on the TiO2 surface was increased and finally saturated, at which the largest amount of HNO3 was proportional to the thickness of the film. On the basis of the results, we concluded that the produced HNO3, inhibiting the reaction as a physical barrier, must be able to diffuse on the TiO2 surface at a rate of at least more than 1.5 μm h−1, and finally distributes homogeneously on the whole film. The maximum density of HNO3 accumulated on the TiO2 surface was estimated to be 2 molecules nm−2 under standard conditions. Finally, when the steady state is reached, the photocatalytic activity remained 8% of the initial one.
Article
Bleached out: The detection of an oxidized co-product, hydrogen peroxide, during an important tropospheric reaction, combined with the detection of photoaccelerated surface nitrate formation (a competing pathway), allows a likely mechanism to be proposed (see figure) which explains previously observed experimental stoichiometries.
Article
The exchange of NO3 radicals with the aqueous-phase was investigated at room temperature (293 K) in a series of wetted denuders. From these experiments, the uptake coefficient of NO3 was determined on 0.1 M NaCl solutions and was found to be (NO3) 2 10-3 in good agreement with recent studies. The Henry coefficient of NO3 was estimated to be KH(NO3) = 1.8 M atm-1, with a (2) uncertainty of 3 M atm-1. From the upper limit for the Henry coefficient (KH = 5 M atm-1) and available thermodynamic data, the redox potential of dissolved NO3/NO 3 – is estimated to be in the range of 2.3 to 2.5 V. This range is at the lower boundary of earlier estimates. The results are discussed in the light of a recent publication. Based on our data and a model of the transport and chemistry in the liquid film, an upper limit is derived for the product of the Henry coefficient KH and the rate coefficient k 10 of the potential reaction NO3 + H2O HNO3 + OH. For KH = 0.6 M atm-1, we find k 10 < 0.05="">-1 atm-1, i.e., about 100 times smaller than what was suggested by Rudich and co-workers. Because of its small solubility, heterogeneous removal of NO3 is only important under conditions where the dissolved NO3 is removed quickly from equilibrium, for example by reactions with Cl– or HSO 3 – ions in the liquid-phase. Otherwise, heterogenous removal should mainly proceed via N2O5.
Article
Towards the aim of improving the air quality in the urban environment via the application of innovative TiO2 based photocatalytic coverings, a field campaign took place within the frame of the EU PICADA project (http://www.picada-project.com) to asses the expected depollution efficiency of such materials under realistic conditions. Furthermore, extensive numerical modeling was performed via the application of the RANS CFD code for microscale applications MIMO, in an effort to asses the sensitivity of the developing flow field and the corresponding dispersion mechanism and hence of the depollution efficiency of the PICADA products on a wide range of factors, with most notably the length of the street canyon, the thermal exchange between the heated street canyon walls and the air and the approaching wind direction. For the needs of the PICADA project a new, simple module had to be implemented into MIMO to be able to model the removal of NOx from a street canyon whose walls have been treated with a photocatalytic product. The model simulations results presented in this paper, show that MIMO is indeed capable of predicting the effectiveness of the photocatalytic products in question. At the same time, they reveal a strong dependence of the developing flow and concentration fields inside the field site street canyon configuration on most of the aforementioned factors with most notably the direction of the approaching wind.
Article
In recent years, there has been a tremendous amount of research and development in the area of photocatalysis (heterogeneous and homogeneous), a process included in a special class of oxidation techniques defined as Advanced Oxidation Processes (AOPs), all characterized by the same chemical feature, production of OH radicals. This paper reviews the use of sunlight to produce the OH radicals by TiO2 photocatalysis and photo-Fenton process. The reacting systems necessary for performing solar photocatalysis are described. The paper also summarizes most of the research carried out related to solar photocatalytic degradation of water contaminants, and how it could significantly contribute to the treatment of persistent toxic compounds. It outlines how to enhance the process efficiency by integration with biotreatment. Various solar reactors for photocatalytic water treatment mainly based on non-concentrating collectors built during the last few years are also described in detail in this review, as well as the use of the solar photocatalytic processes to inactivate microorganisms present in water, placing special emphasis on experimental systems made to optimize this disinfection technique.
Article
Scientific studies on photocatalysis started about two and a half decades ago. Titanium dioxide (TiO2), which is one of the most basic materials in our daily life, has emerged as an excellent photocatalyst material for environmental purification. In this review, current progress in the area of TiO2 photocatalysis, mainly photocatalytic air purification, sterilization and cancer therapy are discussed together with some fundamental aspects. A novel photoinduced superhydrophilic phenomenon involving TiO2 and its applications are presented.
Article
Photocatalytic oxidation (PCO) of nitric oxide (NO) over TiO2 catalyst was studied at source levels (5–60 ppm). The PCO process involves a series of oxidation steps by the OH radical: NO→HNO2→NO2→HNO3. The product NO2 can be collected in an adsorbent bed and either recycled back to the combustion chamber or recovered as nitric acid. The ratio of NO2− to NO3− from spent catalyst liquor drops with irradiation time until a steady state is reached. The reactions are limited by thermodynamic equilibrium after ∼12 s space time. The steady-state experimental data from space time and inlet concentration effects can be described with the Langmuir–Hinshelwood (L–H) kinetic model with R2=0.9208.
Article
The ozone decomposition onto mineral surfaces prepared with traces of solid TiO2 in a matrix of SiO2 in order to mimic mineral dust particles has been investigated using a coated-wall flow-tube system at room temperature and atmospheric pressure. The ozone uptake coefficients were measured both under dark conditions and irradiation using near UV-light. While uptake in the dark was negligible, a large photoenhanced ozone uptake was observed. For TiO2/SiO2 mixtures under irradiation, the uptake coefficients increased with increasing TiO2 mass fraction (from 1 to 3 wt %), and the corresponding uptake coefficient based on the geometric surfaces ranged from 3 x 10(-6) to 3 x 10(-5). The uptake kinetics was also observed to increase with decreasing ozone concentration between 290 and 50 ppbv. Relative humidity influenced the ozone uptake on the film, and a reduced ozone loss was observed for relative humidity above 30%. The experimental results suggest that under atmospherically relevant conditions the photochemistry of dust can represent an important sink of ozone inside the dust plume.
Article
The heterogeneous reaction of NO(2) on Saharan sand collected from different locations has been studied at 298 K and 25% relative humidity using a horizontal coated-wall flow tube. The sand samples originated from Mauritania, Algeria, Morocco and Tunisia and were taken as simplified proxies for mineral dust. While the uptake in the dark was always very small, a photo-enhanced uptake of NO(2) was observed on all four samples showing that natural minerals do have a photochemical activity. The uptake coefficient gamma(BET) was measured for all sands. In the dark, the gamma(BET) values are (1.60 +/- 0.24) x 10(-8), (0.43 +/- 0.06) x 10(-8), (0.94 +/- 0.14) x 10(-8) and (0.59 +/- 0.09) x 10(-8) for the samples from Mauritania, Algeria, Morocco and Tunisia, respectively. Under realistic atmospheric conditions, the observed photo-enhancement leads to uptake coefficients of (1.46 +/- 0.21) x 10(-7), (0.35 + 0.05) x 10(-7), (1.30 +/- 0.19) x 10(-7) and (0.89 +/- 0.13) x 10(-7), respectively, i.e. an enhancement factor ranging from 8 to 15. This study shows that the photochemistry of natural minerals will impact significantly on the heterogeneous chemistry of NO(2).
Article
In the atmosphere, gas-phase nitrogen oxides including nitric acid react with particle surfaces (e.g., mineral dust and sea salt aerosol) to yield adsorbed nitrate, yet little is known about the photochemistry of nitrate on the surface of these particles. In this study, nitrate adsorbed on alumina surfaces, a surrogate for mineral dust aerosol, is irradiated with broadband light (lambda > 300 nm) in the absence and presence of coadsorbed water, at <1% and 45 +/- 2% relative humidity (%RH), respectively, and molecular oxygen. Upon irradiation, the nitrate ion readily undergoes photolysis to yield nitrogen-containing gas-phase products, NO2, NO, and N2O. Although NO2, NO, and N2O form under the different conditions investigated, both coadsorbed water and molecular oxygen change the gas-phase product distribution, with NO being the major product under dry and humid conditions in the absence of molecular oxygen and NO2 the major product in the presence of molecular oxygen. To the best of our knowledge, this is the first study to investigate the role of solvation by coadsorbed water in the photochemistry of adsorbates at solid interfaces and the roles that molecular oxygen, adsorbed water, and relative humidity may have in photochemical processes on aerosol surfaces that have the potential to alter the chemical balance of the atmosphere.
Article
The bandgap of solid-state TiO2 (3.2 eV) enables it to be a useful photocatalyst in the ultraviolet (lambda < 380 nm) region of the spectrum. A clean TiO2 surface in the presence of sunlight therefore enables the removal of harmful NOx gases from the atmosphere by oxidation to nitrates. These properties, in addition to the whiteness, relative cheapness and non-toxicity, make TiO2 ideal for the many de-NOX catalysts that are currently being commercially exploited both in the UK and Japan for concrete paving materials in inner cities. There is need, however, for further academic understanding of the surface reactions involved. Hence, we have used surface specific techniques, including X-ray photoelectron spectroscopy and Raman spectroscopy, to investigate the NOx adsorbate reaction at the TiO2 substrate surface.
Article
This study investigates photocatalytic degradation of nitrogen oxides overtitania-based photocatalysts illuminated by ultraviolet and visible light. The TiO2 photocatalyst was synthesized in a sol-gel process using titanium butoxide as the precursor. After calcination between 150 and 300 degrees C, the synthesized TiO2 responded strongly to visible light photocatalytically degrading NO(x), probably because of the existence of carbonaceous species that act as sensitizers. The optimum calcination temperature was found to be around 200 degrees C. Additionally, platinum ion-doped TiO2 was prepared by impregnation using Pt(NH3)4(NO3)2 as a dopant, which improved the photocatalytic activity that degraded NO(x) in the visible light region. The Pt ion was doped in oxide form at the surface of TiO2 and was expected to be responsible for sensitization. At an optimum calcination temperature of around 200 degrees C, the Pt ion-doped TiO2 exhibited higher activity in the further oxidation of NO2 to NO3- clearly reducing NO2 selectivity. The TiO2 catalysts chemically prepared by either the sol-gel process or impregnation exhibited stronger activity than conventional TiO2 when illuminated under a fluorescent lamp. Rinsing with water was responsible for the restored reactivity of prepared TiO2 catalysts for NO(x) degradation.
Article
TiO2 loading on woven glass fabric is applied to treat nitrogen oxides (NOx) by photocatalytic oxidation (PCO). In this paper, the PCO behavior of NO at high concentrations was studied by PCO of NOx at source levels (20-168 ppm). The PCO efficiency reached 27% in this experiment, while the inlet NOx concentration was 168 ppm (147 ppm NO). The dependency of the reaction rate on several key influencing factors (relative humidity, space time, inlet concentration, oxygen percentage) was also studied. The results illustrate that the resulting hydroxyl radical and active oxide play an important role in the oxidation of NOx. The reactions are limited by the thermodynamic equilibrium after ca. 15s space time. A possible explanation for the catalyst deactivation is the accumulation of nitric acid and nitrous acid on the TiO2 surface during the PCO of NOx. However, the photocatalytic activity can be recovered with a simple heat treatment. The results from the study of the effect of the inlet concentration were described with the Langmuir-Hinshelwood model.
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