Heterogeneous Reactions between Toluene and NO2 on Mineral

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Heterogeneous reactions between toluene and NO on mineral particles under simulated atmospheric conditions Hejingying Niu, Kezhi Li, Biwu Chu, Wenkang Su, and Junhua Li Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00194 • Publication Date (Web): 21 Jul 2017 Downloaded from http://pubs.acs.org on July 21, 2017

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Environmental Science & Technology

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Heterogeneous reactions between toluene and NO2 on

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mineral particles under simulated atmospheric conditions

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Hejingying Niua, Kezhi Lia, Biwu Chub Wenkang Sua and Junhua Lia*

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a

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Environment, Tsinghua University, Beijing, 100084, China

State Key Joint Laboratory of Environment Simulation and Pollution Control, School of

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b

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for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China

State Key Joint Laboratory of Environment Simulation and Pollution Control, Research Center

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1

ACS Paragon Plus Environment *Corresponding author: Tel.: +86 10 62771093, E-mail address: [email protected] (J. Li)


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Abstract: Heterogeneous reactions between organic and inorganic gases with aerosols are

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important to the study of smog occurrence and development. In this study, heterogeneous

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reactions between toluene and NO2 with three atmospheric mineral particles in the presence or

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absence of UV light were investigated. The three mineral particles were SiO2, α-Fe2O3 and BS

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(butlerite and szmolnokite). In a dark environment, benzaldehyde was produced on α-Fe2O3. For

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BS, nitrotoluene and benzaldehyde were obtained. No aromatic products were produced in the

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absence of NO2 in the system. In the presence of UV irradiation, benzaldehyde was detected on

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the SiO2 surface. Identical products were produced in the presence and absence of UV light over

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α-Fe2O3 and BS. UV light promoted nitrite to nitrate on mineral particles surface. Based on the

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X-ray photoelectron spectroscopy (XPS) results, a portion of BS was reduced from Fe3+ to Fe2+

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with the adsorption of toluene or the reaction with toluene and NO2. Sulfate may play a key role in

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the generation of nitrotoluene on BS particles. From this research, the heterogeneous reactions

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between organic and inorganic gases with aerosols that occur during smog events will be better

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understood.

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Keywords: toluene, NO2, benzaldehyde, nitrotoluene, heterogeneous reaction

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ACS Paragon Plus Environment

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Graphical Abstract

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INTRODUCTION

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Heterogeneous reactions in the atmosphere play a major role in transforming gases and

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determining the compositions of the atmosphere.1 During smog formation, heterogeneous and

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homogeneous reactions among particles, as well as natural and anthropogenic gases, are the main

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contributors to the polluted air.2 Recent studies of heterogeneous reactions have focused on single

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organic or inorganic gases with aerosol particles.3-5 However, the atmospheric environment is a

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mixed gas system, and irradiation effects need to be considered for heterogeneous reactions.

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Toluene is a ubiquitous environmental pollutant that can be both biogenic and anthropogenic.

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Sources of anthropogenic toluene include industrial emissions, indoor air and exhaust from motor

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vehicles.6 Toluene is the most abundant aromatic compound in urban atmospheres7 and accounts

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for approximately 20-40% of total aromatic volatile organic compounds (VOCs).8 Toluene and its

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derivatives, such as benzaldehyde and nitrotoluene, have received considerable attention due to

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their adverse effects on both the environment and human health.9-14 Some of these compounds are

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mutagenic15, 16, carcinogenic17, 18 or otherwise toxic to aquatic19, 20 and terrestrial organisms.21, 22

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Therefore, the formation of nitrotoluene and benzaldehyde in the atmosphere must be investigated.

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The emission of benzaldehyde in atmosphere has been attributed to biogenic emissions,

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photochemical reactions and anthropogenic emissions.23-25 Benzaldehyde is produced in the

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atmosphere via complex reactions between toluene, hydroxyl radicals and NOx involving both

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heterogeneous and homogeneous reactions.26, 27 In contrast, the emission of nitrotoluene in the

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atmosphere is almost exclusively anthropogenic, primarily from the incomplete combustion of

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fossil fuels and industrial processes.28 Nitrotoluene can also be formed from natural atmospheric 4


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homogeneous reactions; toluene reacts with the OH radical to form an OH-monocyclic aromatic

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adduct that reacts with O2 and NO2 to form 3- nitrotoluene.26 However, studies of the formation of

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nitrotoluene via atmospheric heterogeneous reactions have been limited.

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Mineral aerosols originate from soil particles that have been entrained by strong wind currents and

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enter the atmosphere.29 The chemical composition of mineral aerosols is similar to that of crustal

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rocks, which are dominated by SiO2, Al2O3 and Fe2O3.30 Furthermore, iron and sulfate ions are the

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most prevalent transition metal and ion, respectively, in atmospheric aerosols.2 NO2 is one of the

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major gaseous pollutants in vehicle exhaust. Therefore, mineral aerosols, NO2 and toluene are

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ubiquitous in ambient air. Recent studies have examined heterogeneous reactions of NO2 and

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mineral particles.31-34 These studies have focused on different products, such as nitrite and nitrate,

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on the mineral particle surface. However, few studies were carried out to investigate mixtures of

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gases, especially mixtures of inorganic and organic gases. 35, 36 In these few studies, the effects of

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NO2 and toluene on magnetite have indicated that NO2 and water decrease the adsorption of

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toluene. In this study, SiO2, α-Fe2O3 and iron sulfate were selected as model mineral particles to

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investigate the effects of heterogeneous reactions of toluene and NO2 on these substrates. This

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research will facilitate the elucidation of the chemical behavior and heterogeneous reactions of

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toluene, as well as the sources of nitrotoluene and benzaldehyde in the atmosphere, under dark or

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UV light conditions. In this research, several in situ experiments were designed to investigate

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the chemical reactions between toluene and NO2 with mineral particles. UV light was used to

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simulate daytime sunlight. The mineral particles were examined by XPS to detect the changes on

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particle surfaces. 5



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MATERIALS AND METHODS

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The three mineral particles physical characterization, chemicals used in this study and particles

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pretreated procedure were shown in supporting information.

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Reaction procedure and analytical methods. All experiments were performed at

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305-306 K. Infrared spectra of solid samples were recorded on a Fourier transform infrared (FTIR)

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Spectrometer (Nicolet 6700; Thermo Fisher) equipped with a mercury cadmium telluride (MCT)

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detector. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were

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recorded in the spectral range from 4000 to 650 cm-1 at a resolution of 4 cm-1, and in general, 32

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scans were performed during the experiments. The mass of each particle was around 0.05g. The

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powder particle was placed in the Harrick IR reactor. The reactor was sealed with a dome. All

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gases were fully mixed before directly introduction into the reactor. The details of the procedure

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are similar to those used in a previous study.

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ranges of toluene and NO2 were 160±3 ppm and 40±3 ppm, respectively. When toluene or NO2 is

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mixed with air, the ranges of toluene and NO2 were 166±3 ppm and 47±3 ppm, respectively.

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GC-MS was not coupled with DRIFTS. After DRIFTS experiment, each sample was transferred to

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a headspace bottle with an aluminum lid with a rubber pad. The lid was sealed by a capper. Then,

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the sealed sample was analyzed by gas chromatography-mass spectrometry (GC-MS; QP2010Plus,

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Shimadzu) on an instrument equipped with an automated headspace sampler (PE HS 40, Perkin

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Elmer). A 2-meter pathlength gas cell (Thermo Fisher) was used to investigate the homogeneous

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reactions of toluene and NO2 on dark and UV conditions.

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When toluene and NO2 were mixed with air, the

For the experiments with UV light, the irradiation was supplied by a xenon lamp (500 W) with 6


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continuous emission in the 300-400 nm range through the dome window. The spectrum of Xe

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lamp is offered in Fig. S1. An optical fiber was used to transmit the light from the xenon lamp

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(Aulight, Beiijng).

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RESULTS AND DISCUSSION

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Characteristics of mineral particles. The pretreated α-Fe2O3 and FeSO4∙7H2O samples

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were characterized by XRD. Based on the XRD patterns (Fig. S2), FeSO4∙7H2O was identified as

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Fe(SO4)(OH)∙2(H2O) (butlerite, PDF 71-2397) and FeSO4∙H2O (szmolnokite, PDF 81-0019). In

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this study, BS was used to represent the pretreated products of FeSO4 ∙ 7H2O. α-Fe2O3 was

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confirmed as hematite. The physical properties for all mineral particles are listed in Table S1.

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From SEM observation results (Fig. S3), hematite was nearly spherical, BS and SiO2 were

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amorphous.

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In situ DRIFTS study. To investigate the heterogeneous reactions of toluene and NO2 with

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mineral particles, in situ DRIFTS experiments were performed in a selective flow system. The

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detailed assignment of the vibrational bands of the surface species that formed when samples were

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exposed to NO2, toluene or both is provided in Table 1. The geometries of surface species were

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demonstrated in Fig. S4. The detailed infrared spectra of the SiO2, α-Fe2O3 and BS particles in

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flows of NO2, toluene and both gases with synthetic air under dark or UV light conditions are

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shown in Fig. 1-2 and Fig. S5. For the homogeneous reactions, based on the 2-meter pathlength

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gas cell, it was different to detect new aromatic products (Fig. S5). The DRIFTS spectra results for

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SiO2, α-Fe2O3 and BS exposed to toluene are shown in Fig. S6. At room temperature, toluene 7



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interacted weakly with SiO2, as indicated by the disappearance of the peak at 3741 cm-1 and the

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appearance of a peak at 3614 cm-1. The latter band indicated the preferential adsorption of toluene

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on isolated hydroxyl groups present on the surface of the material. Furthermore, the peaks at 3276

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and 1621 cm-1 weakened with increasing exposure time, indicating that toluene displaced some

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water molecules on the surface of SiO2. This phenomenon was also observed on the surface of

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α-Fe2O3 and BS (Fig. S6b and c). In the C-H stretching region (3100-2850 cm-1), peaks at 3087,

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3063, 3031, 2929 and 2880 cm-1 were observed (Fig. S6a). The first three peaks were assigned to

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the C-H stretching vibrational band of the aromatic rings. The latter two peaks were assigned to

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the C-H asymmetric stretching of methylene and symmetric stretching of methyl.38-40 This result

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indicates that hydrogen-π bonding of toluene is preferred over H bonding between its methyl

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group and -OH surface groups. The peaks at 1602 and 1496 cm-1 correspond to the stretching

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vibrational and out-of-plane bending vibrational bands of C-H bonds, respectively.38 However, no

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new aromatic product was observed (Fig. S6a).

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The infrared spectra of SiO2 particles with NO2 in the absence and presence of UV are shown in

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Fig. 1a1 and Fig. 1a2. In a flow of NO2 without UV light, the peaks at 1676 and 1620 cm-1 were

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predominant, indicating as adsorbed nitric acid and bridging nitrate or water, respectively (Fig.

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1a1). In previous studies, the peaks at 1628 and 1602 cm-1 were assigned to gas-phase NO2.41 The

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broad band at 1620 cm-1 may overlap with the adsorbed gaseous NO2 band. Therefore, the peak at

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1620 cm-1 did not have a definite conclusion. Compared with the results observed under dark

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conditions, several peaks appeared upon UV irradiation (Fig. 1a2). The peaks at 1731 and 1716

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cm-1 can be attributed to adsorbed N2O4.42 The peak at 1804 cm-1 may be from a surface dinitrosyl 8


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complex.42 The peak at 1669 cm-1 was assigned to nitric acid.42 For SiO2, the UV irradiation

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promoted N2O4 generation on particles (Fig. 1a2).

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When toluene was added to the flow with NO2 in the dark environment, no new peaks were

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observed (Fig. 2a1). In contrast, with UV irradiation, new peaks were observed at 1700 and 1685

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cm-1, indicating the formation of benzaldehyde (Fig. 2a2), which was confirmed by the GC-MS

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results. The peaks observed at 1808 and 1790 cm-1 corresponded to adsorbed NO (Fig. 2a2).43

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The heterogeneous reactions of α-Fe2O3 with NO2 in either the presence or absence of UV light

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were similar (Fig. 1b1 and Fig. 1b2). During the dark experiment, the peaks at 1080 and 1221 cm-1

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were assigned to nitrite products. These peaks appeared first and decreased with exposure time. In

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contrast, the peaks at approximately 1622 cm-1, which were assigned to bridging nitrate,

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strengthened with exposure time. Then, monodentate nitrate bands were observed as peaks at 1270

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cm-1.32 The peaks at 1297 and 1337 cm-1 rapidly increased and acted as the dominant nitrate on the

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α-Fe2O3 surface. These two peaks were due to nitric acid.44 Based on these results, the reaction

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between α-Fe2O3 and NO2 was attributed to the conversion of the nitrite to nitrate, and the final

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stable product was nitric acid (Fig. 1b1). The peaks at 1809 and 1790 cm-1 assigned to adsorbed

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NO were observed both in the presence and absence of UV irradiation (not demonstrated in the

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figures). With respect to the NO2 and toluene gaseous mixture group, the peak at 1758 cm-1 was

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assigned to adsorbed N2O4 because NO2 molecules tend to dimerize at low temperatures (Fig.

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2b1).

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adsorbed N2O4.45, 46 The peaks at 1699, 1678 and 1654 cm-1, which are relatively large, indicate

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the presence of different types of adsorbed benzaldehyde with carbonyl groups that interact with

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Previous studies have confirmed that the peaks ca. 1745 and 1710 cm-1 correspond to

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surface Lewis acid sites on the α-Fe2O3 surface (Fig. 2b1).47-49 The peaks at 1699, 1687 and 1658

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cm-1 were assigned to the carbonyl stretching vibration of benzaldehyde for the UV irradiation

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experiments (Fig. 2b2).

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The peaks resulting from the heterogeneous reaction of NO2 and BS are listed in Table 1. The peak

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at 2098 cm-1 corresponds to NO+ or NO2+, which is not yet confirmed (Fig. 1c1). 50 This band was

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only observed for the BS group. This result indicated that the sulfate ion may enhance the yield of

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NO+ or NO2+. Bidentate nitrate and bridging nitrate were detected, and their intensities increased

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during the process. The peak at 1888 cm-1 (Fig. 1c1) was assigned to the Fen+-NO band or

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attributed to adsorbed N2O3.51 However, the peak was observed during the first 5 min, and its

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intensity then weakened and ultimately disappeared. Therefore, the peak at 1888 cm-1 may be due

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to adsorbed N2O3. In contrast, the intensities of the peaks at 1809 and 1790 cm-1, which

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correspond to adsorbed NO complexes on the Fe3+ site, increased during the experiment.43 The

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two peaks immediately disappeared when the supply of NO2 was cut off and air purged the BS

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particle for 5 min (Fig. 1c1). However, the peak at 2098 cm-1 did not disappear within 20 min after

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ceasing the injection of NO2 into the system (data not shown). The peak at 1880 cm-1 appeared

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after more than 10 min under UV light irradiation (Fig. 1c2), in contrast to its appearance after

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only 2 min under dark conditions (Fig. 1c1). These results suggest that irradiation with 300-400

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nm light favors the formation of N2O3, which is consistent with a previous report.52 In the NO2

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and toluene group, the band at 1671 cm-1 corresponded to the carbonyl (C=O) stretching vibration

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of benzaldehyde (Fig. 2c1). The peak observed at 1521 cm-1 was assigned to the asymmetric

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stretching vibrational band of C-N. This result was confirmed by the following GC-MS analysis. 10


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As shown in Fig. 3a, peaks were observed in the total ion chromatogram (TIC) of the products for

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the heterogeneous reaction between toluene and NO2 on BS particles with UV irradiation. The

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four peaks were identified as benzaldehyde, 2- nitrotoluene, 3-nitrotoluene and 4-nitrotoluene, and

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these results were confirmed based on the spectra of standard samples. The results indicated all the

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nitrotoluene isomers were detected.

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The peaks at 1809 and 1790 cm-1 were not observed in the α-Fe2O3 (Fig. 2b1) and BS (Fig. 2c1)

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experiments, in contrast to the experiment with only NO2 (Fig. 1b1 and Fig. 1c1), possibly due to

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toluene competing with NO2 for the Fe3+ site. This result indicates that toluene is more reactive

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than NO2 in this study. Other studies have suggested that toluene is more reactive and competes

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with NO2 on oxidized magnetite particles, whereas NO2 is more reactive on reduced magnetite.36

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This result implied that nitrotoluene could be produced in the atmosphere under suitable

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conditions. Sulfur dioxide (SO2) and α-Fe2O3 are a common air pollution gas and a common

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mineral particle, respectively. Sulfuric acid can be produced from the reaction of SO2 with water.

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Therefore, heterogeneous reactions among SO2, NOx, water and iron particles (BS in this study)

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could lead to the formation of more toxic nitrotoluene products. The results suggest that the source

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of nitrotoluene in the atmosphere is from both direct anthropogenic emissions and secondary

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organic aerosols (SOAs) generated from heterogeneous reactions between aromatic compounds,

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NO2 and particulate matter. These SOAs that contribute to smog are less volatile than the gas

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(toluene) originally emitted.

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nitrotoluene and benzaldehyde can be generated during the night on BS particles. It should be

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noted that toluene and NO2 do not react under dark conditions on SiO2 particles (Fig. 2a1), i.e.,

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Experiments conducted under dark conditions confirmed that

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both toluene and NO2 are stable when mixed in the gas phase in dark conditions. Therefore, in this

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study, heterogeneous reactions promote the oxidation of toluene to benzaldehyde. Importantly,

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exposure of toluene to the three types of particles under irradiation did not result in the formation

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of new products in the absence of NO2. This result indicates that NO2 significantly influences the

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heterogeneous reaction for the generation of benzaldehyde and nitrotoluene during the day and

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night.

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Effects of UV irradiation. From the DRIFTS results (Fig. 1b2), the peaks at 1082 and

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1219 cm-1 vanished quickly when UV light was applied to the reaction between α-Fe2O3 and NO2.

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This result suggests that UV light promotes the transformation from nitrite to nitrate. This

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conclusion was also verified by XPS results (Fig. S7). The binding energy of the peak at

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approximately 407 eV was assigned to NO3-.54 The intensity of nitrate generated from the UV

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experiment is 48 times larger than that from the dark experiment through a normalization

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calculation. UV light increases the number of defect sites carrying negative charges on the

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α-Fe2O3 surface by increasing the amount of nitrate species on the surface.

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that for BS group, nitrate was only observed in the UV experiment. Note that the peaks at

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approximately 401.8 eV and 400.0 eV have yet to receive a definite assignment. Some of the

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previous studies considered these peaks as two different NO+ species.55 Other studies assigned

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them to NO- and nitrogen species at defects.56 In addition, for the BS particle exposed to NO2, the

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integrated area ratio of bidentate nitrate to bridging nitrate under UV light conditions was always

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higher (20-44%) than the ratio under dark conditions (Fig. S8). With irradiation, bidentate nitrate,

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which is more stable than bridging nitrate, was promoted on the BS surface. Therefore, the

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It should be noted

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irradiation of UV promoted the reaction on BS particles.

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To analyze the chemical composition of the surface of α-Fe2O3 and BS particles exposed to

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toluene or toluene and NO2, the high resolution spectra XPS of O1s, Fe2p and Fe 3p were

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collected. For α-Fe2O3, the O1s spectra were fitted with 529.4 eV, 531.1-531.2 eV, 532.6±0.3 eV,

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which were assigned to the main oxide O-Fe, lattice FeOH and/or O=C, and O-C and/or adsorbed

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water, respectively.57-59 The peak around 532.5 eV, could be the O-N (nitrate).60 For BS particle,

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O1s spectra were fitted with one more peak at 532.2 eV (O=S).61 In Tab. S1, the ratio of adsorbed

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oxygen/lattice oxygen (AO/LO) was calculated based on the fitted areas of different O1s species.

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With respect to the XPS O1s spectra, an increase in the ratio of adsorbed oxygen/lattice oxygen

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(AO/LO) was observed for both α-Fe2O3 and BS samples exposed to toluene alone under dark

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conditions, as shown in Tab. S1. For α-Fe2O3, the ratio change is close to the uncertainty limits, so

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a definite mechanism cannot be confirmed at this stage. For BS particles, the increase of the

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AO/LO ratio appears to occur at the expense of the oxide, suggesting that toluene interacts with

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the oxygen from the oxide.36 In contrast, the ratio of AO/LO increased dramatically for toluene

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adsorbed on α-Fe2O3 in the UV group (Tab. S2). However, the ratio was similar when comparing

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the results in the absence and presence of UV for the BS group, which indicates that UV

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irradiation promotes the formation of adsorbed oxygen on the α-Fe2O3 surface instead of that on

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the BS particles. The increase in adsorbed oxygen may arise from surface hydroxyl and not from

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C=O (Fig. S9d). GC-MS analysis did not indicate the formation of new products from the reaction

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of toluene and α-Fe2O3 under UV irradiation.

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From the DRIFTS results, the integrated areas of the peaks at 3099-2991 cm-1 and 2991-2846 or 13



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3099-2993 cm-1 and 2989-2784 cm-1 are shown in Fig. S10. The former represents the aromatic

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ring (AR) C-H, and the latter represents the C-H symmetric and asymmetric vibration of methyl

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and methylene (MM). These two parameters suggest that toluene has been adsorbed on the

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substrates. For the dark experiment, the changes in AR and MM were similar, and the adsorption

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of toluene reached a stable state after 40 min. However, in the irradiation experiment, the

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intensities of the AR and MM bands reached equilibrium in 200 min. This result may indicate that

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the adsorption pattern of toluene on the α-Fe2O3 surface is different when in the presence of UV

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light.

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The integrated areas of TICs for different products on the three mineral particles with NO2 and

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toluene under dark and UV light conditions are shown in Fig. 3b. UV light dramatically stimulated

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the formation of benzaldehyde on iron particles. Approximately 2.5 and 7.7 times more

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benzaldehyde was produced under UV light compared to the dark experiment on α-Fe2O3 and BS

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particles, respectively. For nitrotoluene, the promotive effect of UV was reasonable. In previous

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studies, 3-nitrotoluene has been detected due to homogeneous reactions between OH radicals,

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aromatic compounds and NOx.26 It should be noted that the benzaldehyde quantity on SiO2 is

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slightly greater than that on BS, which indicates that part of the benzaldehyde may be produced in

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the gas phase. In the gas phase, benzyl was generated by the UV irradiation of toluene.

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Benzaldehyde was produced from the reactions between benzyl and NxOy. Benzaldehyde was then

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adsorbed on particle surfaces and detected by FTIR. This requires further investigation. The

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benzaldehyde amount on α-Fe2O3 was less compared to that on BS particles under UV light. This

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probably occurs because UV light promotes the production of nitrate from NO2 on α-Fe2O3 14


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surfaces and nitrate did not participate in benzaldehyde formation (Fig. S11).

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The temporal changes in the integrated areas of benzaldehyde are presented in Fig. S12. For the

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dark experiment, the peaks at 1699, 1678 and 1654 cm-1 were assigned to C=O stretching of

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benzaldehyde (Fig. S12a). For the irradiation experiment, the peaks at 1699, 1687 and 1658 cm-1

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correspond to the carbonyl stretching vibration of benzaldehyde (Fig. S12b). The integrated area

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continuously increased with time under dark conditions. The amount reached a maximum after 90

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min and desorbed gradually with time. In contrast, in the irradiated groups, the integrated area and

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the benzaldehyde amount increased during irradiation time.

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Discussion of the heterogeneous reactions. In the absence of UV irradiation, no new

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aromatic products were detected on SiO2, even in the presence of NO2 and toluene. This suggested

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that toluene and NO2 could not react in the gas phase during nighttime. However, with the

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participation of iron compounds, heterogeneous reaction products were observed. Toluene reacts

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with NO2 to produce benzaldehyde and nitrotoluene, which are low volatility pollutants. Based on

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previous results, iron chemical catalysis of NO2 and toluene to generate benzaldehyde is a

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synergetic process. To analyze the chemical composition of the surface of α-Fe2O3 and BS

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particles in the presence of toluene and/or NO2, high resolution XPS spectra of Fe 2p and Fe 3p

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were collected. As shown in Fig. 4b1, the binding energy decreased with toluene adsorption on BS.

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The process was proposed to occur via C-H activation of methyl groups followed by hydrogen

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abstraction via the basic oxygen in the oxide,36 which would result in reduction of the nearby Fe3+

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to Fe2+ (Fig. 4). However, the binding energy did not change obviously for the α-Fe2O3 group (Fig.

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4a1). This phenomenon was similar when UV irradiation was available in the system. 15



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Based on the in situ DRIFTS measurements in different flow systems, two different mechanisms

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can be proposed for the formation of benzaldehyde and 2-nitrotoluene from the heterogeneous

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reaction of toluene and NO2 with iron particles. For benzaldehyde generation, no product was

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observed when only nitrate was present on α-Fe2O3 and toluene was injected (Fig. S11). In this

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case, the hypothetical reaction between toluene and NO2 on iron compound surfaces to form

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benzaldehyde could be as shown in reactions 1-3 below.62

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(1)

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(2)

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(3)

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The reaction of toluene and NO2 to form nitrotoluene on BS is an electrophilic nitration reaction.58

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This reaction is used in industrial applications to produce mononitrotoluenes, which are

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recognizable intermediates of many compounds, including pharmaceuticals, dyes, explosives,

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pesticides, etc.63 It is noted that the industrial technology for the synthesis of nitroaromatic

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compounds use mixtures of nitric acids and sulfuric acids. In this study, the BS particle could

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supply solid acid (sulfate), which provides proton to promote the production of NO2+ from nitric

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acid.64 In this reaction, NO2+ is considered to be a key reactant in toluene nitration. Based on the

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DRIFTS results, the peak at approximately 2098 cm-1 can be assigned to NO+ or NO2+.

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In summary, benzaldehyde and nitrotoluene could be generated both in the absence and presence

308

of UV irritation during heterogeneous reactions. The mineral aerosols and NO2 play important 16


Page 17 of 30


309

roles during the toluene oxidation and nitration processes. The iron particles could catalyze the

310

oxidation of toluene to from benzaldehyde with NO2. Both benzaldehyde and nitrotoluene are

311

recognized as the SOAs during atmosphere process. During smog pollution weather, the

312

contribution of SOAs and secondary inorganic aerosol are found to be of similar importance.65

313

The heterogeneous reactions could be a contributor to SOAs. During the oxidation of toluene with

314

NO2, some by-products (HONO and NO) was formed. Both the two gases contribute to

315

photochemical smog formation. This phenomenon suggests heterogeneous reactions are important

316

contributors to smog formation.

317

NOTES

318

The authors declare no competing financial interest.

319

ACKNOWLEDGMENTS

320

This work was supported by the National Natural Science Foundations of China (Grant No.

321

21325731& 21221004) and the “Strategic Priority Research Program” of the Chinese Academy of

322

Sciences (XDB05010102).

323

Supporting Information Available

324

The characterization of three mineral particles. Assignments O 1s XPS results of α-Fe2O3 and BS

325

particles in the and absence and presence of UV light. The homogeneous reaction results and the

326

DRIFTS of three particles to toluene. Integrated areas of nitrate, AR, MM and benzaldehyde at

327

different conditions.

328

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329

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Table and Figure Captions

510 511

Table 1. Assignment of vibrational bands (cm-1) of surface species formed when mineral particles

512

were exposed to NO2, toluene or NO2 and toluene. 24


Page 25 of 30


Surface species

Gas

In this work SiO2 dark

Bidentate nitrito* Monodentate nitrate*

UV

UV

NO2

1221

1219

NO2+Toluene

1210

1216

1556, 1270

1554, 1271

1559

1557

1558

NO2+Toluene Bidentate nitrate* Bridging nitrate*

Nitric acid (HNO3)

NO2 NO2+Toluene

1585

BS dark

UV 1210 32 ,1220-120532 1556 32, 1275 32 1547 43 1545 67,1276 67

1563

1575

1575

1561

1580

1578

NO2

1616, 1603

1613, 1606

1621, 1608

1621, 1607

NO2+Toluene

1616, 1603

1620, 1603

1621, 1603

1623

1669

1337, 1296

1339, 1297

1658

1336, 1290

1336, 1297

1809, 1790

1809, 1790

NO2 NO2+Toluene

Adsorbed NO

α-Fe2O3 dark

NO2

In literature

1676

NO2 NO2+Toluene

1808, 1790

1582 32 1588 43 1581 67 1615 32 1628 43 1603 67 1677 41,1399 41, 1315 41, 1338 43 1710 66, 1320 66

1809, 1790

1809, 1790

1809, 1790

1807 51,1788 51

1809, 1790

513 514

Table 1. Continued Surface species

Gas

In this work SiO2 dark

Adsorbed N2O3 or n+ Fe -NO Adsorbed N2O4 NO NO2+ aromatic

or

dark

UV

BS dark

UV

1888

1880

NO2+Toluene

1884

1885

1731, 1716

NO2+Toluene +

α-Fe2O3

NO2

NO2

UV

In literature

1748-1740 42 , 1710 42 1749 3

1758

NO2

2098

2094

NO2+Toluene

2098

2099

3086,

3091,

Toluene

3087,

3089,

3082,

188050

3085,

2290-2102 42 3075 38, 3033 25



ring, methyl and methylene ν(C-H) NO2+Toluene

Page 26 of 30

3063, 3031, 2929, 2880

3064, 3032, 2930, 2883

3031, 2927, 2880, 2835

3057, 3028, 2953, 2930, 2877

3060, 3032, 2931, 2880

3066, 3033, 2927, 2880

3085, 3068, 3029, 2931, 2880

3088, 3071, 3038, 2929, 2876, 2837, 2761

3085, 3065, 3028, 2996, 2949, 2922, 2883

3075, 3031, 2983, 2949, 2878

3069, 3030, 2967, 2926, 2880

3047, 3035, 2994, 2938, 2885

skeletal ν

Toluene

1602, 1496

1603, 1496

1602, 1496

1602, 1496

1602, 1496

1602, 1496

(C-C)

NO2+Toluene

1603, 1496

1600, 1496

1603, 1496

1603, 1496

1603, 1496

1598, 1496

ν(C=O)

NO2+Toluene

1700, 1685

1699, 1678, 1654

1699, 1687, 1658

1671

1687

38

, 2932 38, 2877 38 3072 68, 2740 68 , 2956 68 3086 6, 3062 6 , 3031 6, 2928 6, 2880 6

1596 38 ,150138

1694-1629 38 , 1689 6, 1682 11, 1680 69

ν(C-N)

NO2+Toluene

1521

1523

515

26


Page 27 of 30


516 517

Figure 1. In situ DRIFTS spectra of SiO2 (a1 and a2), α-Fe2O3 (b1and b2) and BS (c1and c2) as a

518

function of time in a flow of 47 ppm NO2 in the absence and presence of UV irradiation,

519

respectively.

520

Note: Each group of spectra was collected at its respective background baseline. The baseline

521

intensity did not change with time. The offset was applied only for a clear view both for Figure

522

1&2.

523

27



Page 28 of 30

524 525

Figure 2. In situ DRIFTS spectra of SiO2 (a1 and a2), α-Fe2O3 (b1 and b2) and BS (c1 and c2) as a

526

function of time in a flow of 40 ppm NO2 and 160 ppm toluene in the absence and presence of UV

527

irradiation, respectively.

28


Page 29 of 30


528 529 530

Figure 3. Products of the heterogeneous reaction of toluene with NO2 on the surface of BS with

531

UV irradiation (a) and the integrated areas of TICs for different products on the three particles

532

under 40 ppm NO2 and 160 ppm toluene in the presence or absence of UV light (b).

533

29



Page 30 of 30

534

535 536 537

Figure 4. Fe2p and Fe3p XPS spectra of fractions from α-Fe2O3 (a1 and a2) and BS (b1 and b2) in

538

dark experiments.

30


Heterogeneous Reactions between Toluene and NO2 on Mineral

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