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A: Environmental, Combustion, and Atmospheric Chemistry; Aerosol Processes, Geochemistry, and Astrochemistry 3
Role of NH in the Heterogeneous Formation of Secondary Inorganic Aerosols on Mineral Oxides Weiwei Yang, Qingxin Ma, Yongchun Liu, Jinzhu Ma, Biwu Chu, Ling Wang, and Hong He J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b05130 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018
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The Journal of Physical Chemistry
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Role of NH3 in the heterogeneous formation of secondary
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inorganic aerosols on mineral oxides
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Weiwei Yanga, b, d, Qingxin Maa, b, c*, Yongchun Liua, b, c, Jinzhu Maa, b, c, Biwu Chua, b, c,
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Ling Wanga, b, Hong Hea, b, c
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a
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Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences,
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Beijing 100085, China
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b
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Beijing 100049, China
State Key Joint Laboratory of Environment Simulation and Pollution Control,
College of Resources and Environment, University of Chinese Academy of Sciences,
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c
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Environment, Chinese Academy of Sciences, Xiamen 361021, China
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d
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Institute of Environmental and Applied Chemistry, College of Chemistry, Central
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China Normal University, Wuhan 430079, China.
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*Correspondence to: Qingxin Ma (
[email protected])
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*Tel.: 86-10-62849337; fax: 86-10-62849337
Center for Excellence in Urban Atmospheric Environment, Institute of Urban
Key Laboratory of Pesticide and Chemical Biology of Ministry of Education,
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ABSTRACT
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In this work, a relationship between the role of NH3 and the properties of mineral
20
oxides (α-Fe2O3, α-Al2O3, CaO and MgO) in the evolution of NO3-, SO42- and NH4+
21
has been established. It was found that the promotion effect of NH3 was more
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favorable for the formation of NO3- (or SO42-) and NH4+ on acidic α-Fe2O3 and
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α-Al2O3 due to acid-base interactions between NO2 with NH3 or between SO2 and
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NH3, while this effect was weaker on basic CaO and MgO possibly due to their basic
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nature. The acid-base interaction (NO2/SO2 with NH3) overpowered the redox
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reaction (SO2 with NO2) on Fe2O3 owing to its unique redox chemistry. However, the
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opposite was found on basic CaO and MgO for the formation of SO42- and NO3-.
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Under equivalent concentration conditions, the two synergistic effects did not further
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strengthen on Fe2O3, CaO and MgO due to a competition effect. However, in
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NH3-rich situations, a synchronous increase of SO42-, NO3- and NH4+ occurred on
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Fe2O3. On acidic Al2O3, the favorable adsorption of NH3 on the surface as well as the
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existence of NO2 with an oxidizing capability synergistically promoted the formation
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of SO42-, NO3- and NH4+.
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1. INTRODUCTION
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Mineral dust with an emission rate of 1000-3000 Tg per year, is the second most
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important type of aerosol particles.1-3 Originating from windblown soil, mineral dusts
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have similar chemical components to crustal rock, in which aluminum, iron, calcium
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and magnesium oxides are the abundant dominant oxides.4, 5 The rich defects, oxygen
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species, hydroxyls and metal atoms on those oxides provide many reactive sites for
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the adsorption and transformation of atmospheric trace gases, participating in the
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formation of new particles and aging existing aerosols.6-9 Therefore, the reactions of
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trace gases on mineral dusts, i.e., heterogeneous reactions, are important processes in
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determining the composition of the gaseous troposphere.10-12
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Nitrogen dioxide (NO2) is one of the most important gaseous pollutants in the
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atmosphere. The high chemical reactivity of NO2 makes it an important processor for
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the formation of acid rain and the depletion of ozone.13, 14 Nitrate coating onto mineral
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components in aerosols has always been observed in field measurements, suggesting a
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non-negligible role of heterogeneous atmospheric chemistry in the contribution of
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nitrate.15,
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reaction of NO2 on mineral oxides indeed results in the production of nitrate species,
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which can become an NO2 sink.17, 18 Moreover, the existence of a mixture of gases
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causes the heterogeneous formation of significant secondary species on the mineral
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dusts.19, 20 For example, many studies have focused on the coexisting reactions of SO2
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and NO2 on mineral dusts, in which a synergistic effect existing in those two gases
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could remarkably promote the buildup of SO42- and NO3-, especially during heavy
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haze periods.21-23
16
Laboratory and model studies have confirmed that the heterogeneous
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Since pre-industrial times, NH3 as an important basic gas, has been injected into the
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atmosphere at an increasing rate of two to five-fold that of pre-industrial times, with
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further increases expected over the next 100 years due to the need in agricultural
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growth.24 Field observations have established a good correlation between NH3 and
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SO42-, NO3- and NH4+ in PM2.5.25 The NH3 is considered to be involved in the ternary
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nucleation process in the NH3-SO2-H2O system.26-29 Notably, environmental chamber
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modeling found that the existing surface of the aerosol played a positive role in the
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degradation of precursor gases (i.e., SO2, NO2 and NH3) and that the degradation rates
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of these precursor gases increased with an increase in the initial ratio of (NH3)/(NO2 +
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SO2).30 Our recent lab study found that the existence of NH3 led to the synergistic
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formation of SO42- and NH4+ on typical mineral dusts.31 Obviously, the NH3 plays an
68
important role in the heterogeneous atmospheric chemistry. While its role in the
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formation of nitrate or its role relative to NO2 in the formation of sulfate on mineral
70
dusts in the multi-gas coexisting system is still paid little attention in lab study.
71
Actually, the heterogeneous reactivity depends greatly on the properties of mineral
72
oxides, such as the acid-base nature, or the redox properties.31-34 For the
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heterogeneous reaction of NO2, basic MgO and CaO were more active compared to
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acidic SiO2.35 A similar phenomenon also occurred with the heterogeneous reaction of
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SO2 in which, among the same category group as Al2O3, the basic Al2O3 was the most
76
active.32 Furthermore, as an ubiquitous oxide in soil as well as in rust coatings and
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bricks, Fe2O3, with its unique Fe2+/Fe3+ redox chemistry, favors the formation of SO42-
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and the heterogeneous conversion of NO2.32,
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focused on the relationship of the oxide nature (including acid-base and redox
80
properties) with its heterogeneous reactivity in multi-gas coexisting system.
33, 36-38
However, few studies have
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In the present study, α-Fe2O3, α-Al2O3 and CaO and MgO particles were chosen as
82
model mineral oxides to investigate the effect of NH3 on the heterogeneous reaction
83
of NO2 as well as on the complex coexisting system of SO2, NO2 and NH3. The
84
surface products were detected in detail and quantitatively via in situ diffuse
85
reflectance infrared Fourier transform spectroscopy (DRIFTS) combined with ion
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chromatography (IC) techniques. Possible mechanisms and atmospheric implications
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were further proposed.
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2. MATERIALS AND METHODS
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2.1. Materials. The α-Fe2O3 was prepared using a precipitation method, as we
90
previously reported, while α-Al2O3 was obtained after the calcination of AlOOH
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(SASOL) at 1200°C.31 CaO and MgO were purchased from Sinopharm Chemical
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Reagent Beijing Co., Ltd. The Brunauer-Emmett-Teller (BET) surface areas of
93
α-Fe2O3, α-Al2O3, CaO and MgO were 24.5, 5.5, 57.8 and 29.1 m2 g-1, respectively, as
94
measured by a Quantachrome Quadrasorb SI-MP. X-ray diffraction (XRD) patterns of
95
those samples were confirmed on a computerized PANalytical X'Pert Pro
96
diffractometer equipped with a Cu Kα radiation source (Figure S1). SO2 standard gas
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(25 ppm in N2, Beijing Huayuan Gases Inc.), NO2 standard gas (25 ppm in N2, Beijing
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Huayuan Gases Inc.), NH3 standard gas (36 ppm in N2, Beijing Huayuan Gases Inc.),
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and high purity N2 and O2 (99.999%, Beijing AP BEIFEN Gases Inc) were used as the ACS Paragon Plus Environment
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The Journal of Physical Chemistry
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gas mixture. 2.2. In situ DRIFTS experiments. The in situ DRIFTS experiments were
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performed on an FTIR spectrometer (Nicolet iS50, ThermoFisher Scientific Co., USA)
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equipped with a high-sensitivity MCT/A detector cooled by liquid nitrogen. Before
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each experiment, the sample was pretreated at 573 K for 120 min in a stream of
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synthetic air (80 % N2 and 20 % O2) of 100 mL min-1 to remove adsorbed species.
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The sample was then cooled down at 303 K until the baseline became stable and
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subsequently exposed to 1 ppmv NO2, 1 ppmv SO2, or/and 1 ppmv NH3 for 8-9 h. All
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the spectra were recorded at a resolution of 4 cm-1 for 30 scans in the spectral range of
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4000 to 600 cm-1.
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2.3. IC measurements. IC was used to measure the surface products formed on the
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samples. The reacted particles were preserved with 1% formaldehyde, which was
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diluted with ultrapure water (after boiling for 20 min to remove dissolved oxygen) to
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10 mL (specific resistance ≥ 18.2 MΩ cm) and then sonicated for 30 min at 298 K.
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The leaching solution was obtained through a 0.22 µm PTFE membrane filter and
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then was analyzed using Wayee IC-6200 ion chromatography equipped with TSKgel
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Super IC-CR cationic or SI-524E anionic analytical column. An eluent of 3.5 mM
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Na2CO3 was used at a flow rate of 0.8 mL·min−1. The nitrate and sulfate ions
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separated by anionic column appeared at 5.344 min and 7.532 min successively; while
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the ammonium ions separated by cationic column was detected at 7.843 min (Figure
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S2).
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3. RESULTS
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3.1. The role of NH3 in the NO2-NH3-mineral dust system. To investigate the
123
effect of NH3 on the heterogeneous reactions of NO2 on mineral dust, in situ DRIFTS
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experiments were performed in a gas flow mixture of 1 ppmv NO2 and 1 ppmv NH3
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balanced with synthetic air at room temperature (303 K). The DRIFTS spectra are
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shown in Figure 1. The absorption bands and their detailed assignments are given in
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Table 1. Figure 1a shows the DRIFTS spectra of an individual reaction of NO2 over
128
α-Fe2O3 as a function of time. Several bands ranging over 1600-1000 cm-1 appeared
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and grew in intensity as time increased. The bands between 1600 and 1200 cm-1 are
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assigned to the degenerate ν3 mode of nitrate species coordinated onto the surface.39
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Generally, ν3 can split into two bands, one at a higher wavenumber (ν3, high) and one at
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a lower wavenumber (ν3, low), varying due to the different bonding configurations of
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monodentate, bidentate and bridging nitrates.40 In the present study, the bands at 1563
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(ν3, high) and 1259 cm-1 (ν3, low) were assigned to bidentate nitrates, and the band at
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1288 cm-1 (ν3, low) was assigned to monodentate nitrate with no observation of its
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higher wavenumber band due to low coverage.19, 41-43 The weak adsorption band at
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1006 cm-1 (ν1) was assigned to nitrate species, possibly in the bidentate state.43 With
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exposure time increasing, solvated nitrate species were observed at 1417, 1379, 1347
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cm-1.19,
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monodentate nitrito products were also detected, as indicated by the presence of the
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bands at 1155 cm-1 (ν3) and 1092 cm-1 (ν1), respectively.38,
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consumption of hydroxyl groups (OH) at the bands of 3689 and 3667 cm-1 indicated
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that the formation of nitrate/nitrito species were due to the interaction of NO2 with
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those OH sites, accompanied by the production of water molecules with the stretching
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adsorption mode and bending adsorption mode at the 3567 and 1632 cm-1 bands.18
44, 45
In addition, a small amount of bridging nitro-nitrito and bridging 42
The obvious
146 147
Table 1. Vibrational assignments of nitrate and nitro-nitrito products formed during
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the reaction of NO2 with mineral dusts. Surface species Monodentate NO3Bidentate NO3
-
Bridging dentate NO3-
Ion-coordinated /solvated NO3nitro-nitrito, NO2N2O4
α-Fe2O3 ν3, low ν3, high ν1 ν3, low
1283-1286 19, 38, 41 1520 1006-1017 39, 41, 46 1259-126519, 38, 40, 41
ν3, high ν1 ν3, low ν3, high ν1 ν2 ν
156319, 38, 40, 41 100643 19, 27 1232 19, 27 1614
ν
1417-1424, 1379, 13418, 19, 41, 44, 45 1215, 1157, 109219, 38,
ν
17348, 19
α-Al2O3
CaO
MgO 152243, 44
1265
39, 49
159039, 49
42
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106141, 47, 48 126644
126444
1596-158539, 41
1632-162339, 41 97241 80741, 47 1333-1328, 84841 120040, 41
1654-163849
1364, 131441 122019 17328, 19
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The Journal of Physical Chemistry
150 151 152
Figure 1. (a) In situ DRIFTS spectra of individual reaction of 1 ppmv NO2 and (b)
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simultaneous reaction of 1 ppmv NO2 and 1 ppmv NH3 on α-Fe2O3 as a function of
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time.
155 156
When NH3 was added into the gas flow with NO2, as shown in Figure 1b, in
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addition to the formation of the nitrate (ca. 1424, 1379, 1347, 1286, 1265, 1017 cm-1)
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and nitrito species (1157, 1092 cm-1) mentioned above, new NH3 absorption bands
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appeared. The bands at 1607 and 1198 cm-1 were assigned to the δas and δs modes of
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NH3 coordinated onto Lewis acid sites, accompanied with the νas mode at 3365 cm-1
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and νs mode at 3258 cm-1 split with the overtone of the asymmetric NH3 deformation
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at 3160 cm-1.50-52 Meanwhile, the deformation vibration mode of NH4+ bound to
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Brønsted acid sites was also present at the band of 1447 cm-1.51 In contrast to the case
164
without NH3 (Figure 1a), the introduction of NH3 resulted in a significant increase in
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the coverage of monodentate nitrate by the growth of the band at 1286 cm-1 (ν3, low)
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and the presence of its ν3,
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adsorption bands of the solvated nitrate species were enhanced, spreading from 1424
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to 1347 cm-1 with a new resolved band at 1399 cm-1, which was possibly due to the
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increased surface hygroscopicity promoted by the addition of NH3.41 Additional bands
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at 1232 and 1215 cm-1 were due to the newly formed bridging nitrate and bidentate
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nitrito species.19, 38 These results indicated that the presence of NH3 promoted the
high
mode at the 1520 cm-1 band.39 Furthermore, the
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conversion of NO2 to nitrate and nitrito species on the surface of α-Fe2O3. Notably,
173
the surface OH still acted as active sites, although the adsorption bands of the
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produced water molecules overlapped partially with those of NH3 at 3546 and 1622
175
cm-1.
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177 178 179
Figure 2. Comparison of the final DRIFTS spectra for (a) α-Fe2O3, (b) α-Al2O3, (c)
180
CaO and (d) MgO after the reaction with 1 ppmv NO2 or/and 1ppmv NH3 for 8.5 h in
181
a gas flow of 100 mL·min-1 at 303 K.
182 183
To clearly illustrate the interactions between NO2 and NH3 on dust particles, the
184
final DRIFTS spectra for the individual and simultaneous reactions are compared in
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Figure 2. On α-Fe2O3 (Figure 2a), NH3 adsorbed predominantly on the Lewis acid
186
sites (3365, 3268, 1604, and 1185 cm-1) after the individual reaction for 8.5 h. When
187
NO2 and NH3 coexisted in the gas flow for 8.5 h, the NH3 absorption band at 1185
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cm-1 grew in intensity and blueshifted to 1198 cm-1 and the bands at 3365, 3258 and
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3160 cm-1 simultaneously increased, indicating the accumulation of NH3 adsorbed
190
species.53 The strengthened adsorption band at 1447 cm-1 indicated that an increasing
191
amount of NH3 preferentially converted into NH4+ with the coexistence of NO2. With
192
respect to the NO2, the addition of NH3 not only promoted the accumulation of nitrate,
193
as mentioned above but also led to the increase of nitro-nitrito species at the bands of
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1215 and 1157 cm-1.19, 38
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The α-Al2O3, CaO and MgO as typical acidic and basic oxides were used further to
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The Journal of Physical Chemistry
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investigate the influence of NH3 on the heterogeneous reaction of NO2 (Figure 2b-d).
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Their detailed reactions with NO2 in the absence and presence of NH3 versus time are
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provided in Figure S3. In the individual reaction (Figure 2b), NO2 interacted weakly
199
with α-Al2O3, with the formation of a relatively small amount of bidentate nitrate at
200
the bands of 1590 (ν3, high) and 1265 cm-1 (ν3, low). 29, 43 The absorption configurations of
201
nitrate on α-Al2O3 showed some similarity with that on α-Fe2O3, although the latter
202
exhibited a much higher activity towards the adsorption of NO2. By comparison, the
203
adsorption of NO2 was favorable on basic CaO and MgO with bridging nitrates (at the
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bands of 1623, 972, 807 cm-1 on CaO, and 1654 cm-1 on MgO) prevailing on the
205
surfaces.40, 41, 47, 48 In addition to solvated nitrate (1333 and 848 cm-1 on CaO, and
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1364 cm-1 on MgO), some bidentate nitrate (at bands of 1596, 1266, 1061 cm-1 on
207
CaO) and monodentate nitrate (at band of 1522 cm-1 on MgO) were also found over
208
those two types of basic oxides. In the case of NH3, almost no adsorption was
209
observed on CaO and MgO and only weak adsorption of NH3 (3356, 3285, 1617 and
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1230 cm-1) on Lewis acid sites was present on α-Al2O3.
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After exposure to both NO2 and NH3 for 8.5 h, no positive influence of NH3 on the
212
formation of nitrate was observed for α-Al2O3, CaO and MgO. For α-Al2O3, it was
213
possibly due to its extremely low surface area while for CaO and MgO, the surface
214
basicity may not be in favor of the adsorption of NH3 and further the interaction of
215
NH3 with NO2. Noting the enhanced disturbance in the OH region ranging from 3731
216
to 3605 cm-1 in the simultaneous reaction on CaO, an intense interaction of OH with
217
the reactants occurred due to the coexistence of NO2 and NH3.
218
To better clarify the effect of NH3 on the heterogeneous reactions of NO2 on
219
mineral dusts, the integrated areas obtained under different conditions versus the
220
reaction time are exhibited in Figure 3. As seen from Figure 3a, a significant
221
synergistic effect between NO2 and NH3 was observed on the surface of α-Fe2O3.
222
Because of the poorly observed bands for the adsorbed NH3 on α-Al2O3, only the
223
integrated area for the NO3- bands is given here (Figure 3b). Almost no obvious
224
promotion effect of NH3 was detected during its interaction with NO2. Similar results
225
were also found over CaO and MgO (Figure 3c-d). These proved that the promotion
226
effect of NH3 for the heterogeneous reaction of NO2 was more obvious on acidic
227
oxides than on basic oxides.
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228 229 230
Figure 3. Comparison of the integrated areas obtained from DRIFTS spectra over (a)
231
α-Fe2O3 (NO3-, 1328-1219 cm-1; NH4+, 1230-1100 cm-1), (b) α-Al2O3 (NO3-,
232
1290-1189 cm-1), (c) CaO (NO3-, 1124-866 cm-1) and (d) MgO (NO3-, 1720-1200 cm-1)
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under individual and simultaneous conditions. The error bar represents the standard
234
deviation from three repeated experiments.
235
3.2. The role of NH3 in the NO2-SO2-NH3-mineral dust coexisting system. The
236
abovementioned results suggested a promotion effect of NH3 for the formation of
237
nitrate species on mineral dusts. Our recent research has also indicated that the
238
presence of NH3 could promote the heterogeneous formation of sulfate/sulfite species.
239
The acid-base interaction was proposed to be responsible for the synergistic effect
240
between SO2 and NH3 on the surface of mineral dusts.31 As reported previously, the
241
coexistence of NO2 with SO2 promotes the formation of sulfate significantly through a
242
redox process, in which the dimer intermediate, N2O4, of the adsorbed NO2 oxidized
243
the adsorbed SO2 to form a sulfate species, while N2O4 was reduced to form a nitrite
244
species,8, 54 as shown below,
245
MN2 O4,ads +MSO3 →MNONO2 +MSO4 (1)
246
MNONO2 +MO→2MNO2 (2)
247
Given the complex evolution relationship among those gases, it is necessary to
248
elucidate the effect of NH3 in the mixed SO2-NO2-NH3 atmospheres. Therefore, SO2
249
was further added into the reaction system, and the heterogeneous formation of SO42-,
250
NH4+ and NO3- versus time is exhibited in detail in Figure S4 and S5. The final
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The Journal of Physical Chemistry
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DRIFTS curves for the mineral oxides after exposure to different atmospheres for 8.5
252
h are thereafter compared in Figure 4.
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254 255 256
Figure 4. Comparison of the final DRIFTS spectra for (a) α-Fe2O3, (b) α-Al2O3, (c)
257
CaO and (d) MgO after the reaction with different gas components for 8.5 h.
258 259
Compared to the individual reaction of SO2, the formation of sulfate was clearly
260
promoted in the presence of NH3 on acidic Fe2O3, which was indicated by the growth
261
of the bands at 1236, 1151 and 1020 cm-1 (Figure 4a, green line).37 In contrast, this
262
promotion effect was weakly observed on Al2O3, CaO and MgO, especially for the
263
latter two basic oxides (Figure 4b-d, green lines). Only sulfite species were detected
264
on Al2O3 and CaO, and the corresponding bands were present at 932 and 940 cm-1,
265
respectively. The adsorption of SO2 on MgO with the coexistence of NH3 was similar
266
to that without NH3 and that the weak formation of sulfate and sulfite appeared
267
separately at the bands of 1213 and 1028 cm-1.8 In the SO2 and NH3 reaction group,
268
the adsorption of NH4+ bound to the Brønsted acid sites was predominant on Fe2O3,
269
which was represented with the bands of 1683 and 1429 cm-1 attributed to δs(NH4+)
270
and δas(NH4+), which were connected to ν(NH4+) in the high frequency region, at 3049
271
and 2858 cm-1. In addition, a single adsorption band for ν(NH3) on Lewis acid site
272
was still observed at 3216 cm-1.31 For Al2O3, the formation of NH4+ on Brønsted acid
273
sites at the band of 1461 cm-1 (δas(NH4+)) and the adsorption of NH3 on Lewis acid
274
sites at the bands of 3360, 3281, 3202 cm-1 (ν(NH3)) and 1230 cm-1 (δs (NH3)) were
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both present.31 However, the bands assigned to adsorbed NH3 were far weaker than
276
the broad adsorption of H2O at the band of 3538 cm-1. The adsorption of NH3 on CaO
277
and MgO was barely observed possibly due to their basic nature.
278
The SO2 reaction in the presence of NO2 was different (Figure 4, blue lines). In
279
contrast to the experiments with SO2 and SO2+NH3, additional sulfate bands at 1340
280
and 1100 cm-1 appeared on the surface of Fe2O3 and at 1308 cm-1 on Al2O3.55, 56 More
281
obviously, the intensities of the bands at 1188, 1150 and 1072 cm-1, which
282
corresponded to the newly formed sulfate species, were very strong on CaO,
283
accompanied by the significant reduction of sulfite species at the band of 940 cm-1.48
284
For MgO, a large amount of sulfate accumulated on the surface, which was
285
characteristic of the remarkably increasing bands at 1213 and 1161 cm-1. In addition,
286
the weak adsorption of sulfite species at the band of 1028 cm-1 vanished at the same
287
time.57
288
With regards to the NO2-adsorbed species, the bands at 1328 cm-1 for CaO and
289
1314 cm-1 for MgO were due to solvated nitrate and ion-coordinated nitrate,
290
respectively.41 Some nitrate bands, such as 1267 cm-1 on Fe2O3, 1260 cm-1 on Al2O3
291
and 982 cm-1 on CaO, partially overlapped with those assigned to sulfate species,
292
while others mostly appeared at the bands ranging from 1683 to 1536 cm-1. The weak
293
adsorption of N2O4 was also observed at approximately 1732 cm-1 on Fe2O3 and
294
MgO.8, 19 These results confirmed that NO2 promoted the oxidation of SO2 to sulfate,
295
which largely proceeded through the abovementioned redox process (eqn. 1). While
296
the promotion effect by NH3 can be driven by the acid-base interaction since no
297
transformation from sulfite to sulfate occurred in this situation. Essentially, those
298
results align with our previous findings.8, 31
299
When SO2, NH3 and NO2 coexisted in the gas flow (Figure 4, red lines), the NH3
300
adsorption was obviously enhanced in the high wavenumber region from 3400 to
301
2800 cm-1, particularly for the basic oxides of CaO and MgO. Meanwhile, the growth
302
of the sulfate bands (1188 and 1072 cm-1) on CaO, and the increase of both the sulfate
303
(1314, 1213 and 1161 cm-1) and nitrate (1638 and 1536 cm-1) bands on MgO were
304
also observed. By comparison, the synchronous growth of SO42-, NO3- and NH4+ was
305
not evident on Fe2O3 and Al2O3. A slight increase in the NH4+ at band of 1429 cm-1
306
and the formation of a new sulfate band at 1298 cm-1 occurred on the former, while
307
only a weak enhancement of the NH4+ band at 1461 cm-1 was shown on the latter.
308
Based on the DRIFTS results, the acid-base interaction between NO2/SO2 with NH3 ACS Paragon Plus Environment
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309
and the redox process between NO2 and SO2 can both occur with the reactions of the
310
three gases. However, of the two mechanisms, the one that is prioritized might be
311
determined by the properties of mineral oxides.
312
3.3. Quantitative analysis of surface products. To elucidate the role of NH3 in the
313
heterogeneous reactions of SO2, NO2 and NH3 on mineral oxides, quantitative
314
analysis of the surface products resulting from those reacted particles in DRIFTS
315
experiments were conducted by IC measurements, as shown in Figure 5. Detailed data
316
that is displayed in Figure 5 has also been summarized in Table S1 and S2. In Figure
317
5a, compared to the individual reactions (SO2/NO2/NH3 reaction), two kinds of gases
318
(SO2 and NH3, NO2 and NH3, SO2 and NO2) coexisting in the system resulted in the
319
synergistic formation of SO42-, NO3- and NH4+ on the mineral oxides. With the
320
coexistence of NH3 (SO2+NH3, NO2+NH3), its promotion function for the formation
321
of SO42- and NO3- was more obvious on acidic Fe2O3 and Al2O3 (especially on Fe2O3)
322
than on basic CaO and MgO. In contrast, the NO2 promotion effect played a more
323
predominant role in the formation of SO42- and NO3- on the basic oxides of CaO and
324
MgO in the SO2+NO2 reaction group.
325 326
Figure 5. The IC results showing the quantity of surface products formed on the
327
reacted particles, which were collected from the DRIFTS cell after reactions under
328
different conditions for 8.5 h. All the concentrations of reactants in (a) were 1 ppmv,
329
and the amounts of SO42- and NO3- formed on MgO follows the right y-axis, while
330
others following the left y-axis. The concentration of NH3 varied from 0.1 ppmv to 1
331
ppmv in (b), while NO2 and SO2 remained at a constant concentration of 0.1 ppmv in
332
this case.
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333
However, when the three kinds of gases were introduced into the system
334
simultaneously, the amount of SO42-, NO3- and NH4+ formed did not further increase,
335
and instead, decreased in comparison with that for the coexistence of SO2 and NH3, or
336
NO2 and NH3 on Fe2O3 and that for the coexistence of SO2 and NO2 on CaO and
337
MgO, but not for Al2O3. These results indicate that the acid-base interaction (between
338
SO2 and NH3, or NO2 and NH3) and the redox process (between SO2 and NO2) may
339
exhibit a competition effect towards the formation of SO42-, NO3- and NH4+.
340
Taking the evolution of SO42- on Fe2O3 as an example, both NH3 and NO2 would
341
compete for the SO2 molecules to induce the transformation of SO2 to SO42-. It
342
seemed that NH3 has a more promotional effect than NO2 on the formation of SO42-
343
on this type of acidic oxide. However, with equal inlet concentrations of the reactants,
344
NH3 would be limited in the interaction with SO2 due to the competition effect of NO2,
345
in that a portion of NH3 can also interact with NO2. Therefore, the amount of SO42-
346
formed under this circumstance would be lower than the amounts formed for the
347
coexisting gases (SO2 and NH3 or SO2 and NO2). To prove this hypothesis, NH3 was
348
increased to up to ten times the concentration of SO2 and NO2, as shown in Figure 5b.
349
All the amounts of SO42-, NO3- and NH4+ increased significantly compared to those in
350
the low concentration situation. These results confirmed that both SO2 and NO2 were
351
favorable for the reaction with NH3 on acidic Fe2O3, further investigation is needed to
352
determine which is more attractive to NH3 or whether there exists an interaction
353
between SO2 and NO2.
354
For other types of basic oxides of CaO and MgO, the interaction between SO2 and
355
NO2 is dominant but can be weakened by the presence of NH3, which may interact
356
with both SO2 and NO2 at the same time. The case of the weakened promotion effect
357
in the three kinds of gas coexisting system did not occur on Al2O3. The results implied
358
that the different roles of NH3 (via an acid-base interaction with SO2 or NO2) and NO2
359
(via a redox process with SO2) are closely related to the properties of the mineral
360
oxides, which will be discussed in detail in a later part.
361
4. DISCUSSION
362
4.1. In the NO2-NH3-mineral dust reaction system. DRIFTS and the IC
363
investigation suggested that the synergistic effect between NO2 and NH3, and that
364
between SO2 and NH3, or SO2 and NO2 varies on mineral oxides with different
365
acidic/basic properties. In the NO2 and NH3 reaction group, the synergistic formation
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The Journal of Physical Chemistry
366
of NO3- and NH4+ was clearly detected on acidic Fe2O3 and Al2O3, especially on
367
Fe2O3, by both DRIFTS and the IC measurements. The DRIFTS spectra (Figures 1,
368
2b) show that the OH groups were the main active sites for the adsorption of NO2 on
369
Fe2O3 and Al2O3, which caused the predominant formation of bidentate nitrate on
370
these two types of oxides. This type of nitrate configuration was due to the adsorption
371
of NO2 on the hydroxyls, followed by a disproportionation reaction of two
372
adsorbed-NO2 molecules through the Langmuir-Hinshelwood (LH) mechanism, with
373
α-Fe2O3 as an example,18, 44 -
-
2FeOOH···NO2,ads →FeO+ NO3 +FeO+ NO2 + H2 O
374
LH (3)
375
In this reaction, the production of nitrate was accompanied by the formation of nitrite
376
species, which were confirmed by the presence of bands at ca. 1215, 1157 and 1092
377
cm-1 on α-Fe2O3. Then, the nitrite species were transformed into nitrate rapidly by
378
excess gaseous NO2 through the Eley-Rideal (ER) mechanism, as expressed in eqn. 4,
FeO+ NO-2 +NO2, g →FeO+ NO-3 +NO g
379
ER (4)
380
No nitrite and adsorbed NO species were observed on α-Al2O3 possibly due to their
381
extreme low concentrations or poor signals. Since the CaO and MgO surfaces were
382
not as hydroxylated as the Fe2O3 and Al2O3 surfaces (Figure 2c, d), no consumption of
383
OH groups was observed when NO2 reacted individually, and the main product was a
384
bridging nitrate. In this case, NO2 may coordinate directly onto the exposed metal
385
atoms (Lewis acid sites), interacting with adjacent adsorbed NO2 (possibly in the form
386
of N2O4) to form nitrite and gaseous NO.42 The reaction equations were listed as
387
follows,
388
O-M+NO2, g →O-M-NO2, ads
389
2O-M-NO2, ads →2M-N2 O4 →2M-NO3 +NO g
(5) (6)
390
In addition, the presence of water on the oxide surface would involve the formation of
391
solvated nitrate (e.g., ca. 1680 cm-1) and solvated nitrite species, as shown in eqns.
392
7-8:18, 41
393
(MO)-NO3, ads +H2 O→MOOH···HNO3
(7)
394
(MO)-NO2, ads +H2 O→MOOH···HNO2
(8)
395
When NH3 was introduced simultaneously with the reaction of NO2, it
396
preferentially adsorbed on Lewis acid sites (Figure 1b). The preferential adsorption of
397
NH3 (as seen in Figure 1b) quickly increases the surface basicity, promoting the
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398
adsorption of NO2. Due to their different adsorption sites, NO2 mainly adsorbed on
399
the OH sites while NH3 coordinated on the exposed metal atoms, and a synergistic
400
effect can be found on Fe2O3.58 In addition, a small amount of NH4+ (1447 cm-1)
401
present on the surface of Fe2O3 suggests that the nitric or nitrous acid formed through
402
eqns. 7-8 participated in the neutralization of adsorbed-NH3:
403
HNO3 +NH3 →NH4 NO3
(9)
404
HNO2 +NH3 →NH4 NO2
(10)
405
Since surface hydroxyls served as the main adsorption sites for water molecules,
406
highly hydroxylated Fe2O3 was favored for the formation of nitric or nitrous acid
407
through reactions 7-8 followed by the formation of NH4+ through reactions 9-10.59-61
408
In contrast, no obvious promotion effect of NH3 occurred with the formation of
409
NO3- for basic CaO and MgO. As postulated above, NO2 mainly coordinated onto
410
exposed metal atoms, meaning that it competed with NH3 for Lewis acid sites. A
411
previous study found that NO3- had a closer connection to the metal atoms in basic
412
oxide, such as CaO and MgO, than in acidic oxides, such as α-Al2O3, TiO2, and
413
γ-Fe2O3.41 The strong bond between NO3- and Ca (or Mg) in the present study may
414
further limit the adsorption of NH3 on the same Ca (or Mg) atoms. However, nitrate
415
connecting to metal atoms showed a strong electron affinity, which would increase the
416
Lewis acid strength of the metal ion and may promote the adsorption of NH3 to
417
certain extent. Furthermore, a small amount of other types of nitrate species including
418
bidentate, monodentate and solvated nitrate species present on CaO and MgO
419
suggests that a small fraction of NO2 did not competed with NH3 for the Lewis acid
420
sites, possibly through the reaction in eqns. 3-4. Therefore, the coexistence with NH3
421
did not reduce the amount of nitrate but promoted its formation slightly, as found from
422
the IC results. The obvious consumption of OH ranging from 3731 to 3605 cm-1
423
(Figure 2c) in the simultaneous reaction confirms the enhanced interactions of NO2
424
and NH3 with the surface, which may account for the additional formation of nitrate
425
and ammonia species on basic oxides.
426
4.2. In the NO2-SO2-NH3-mineral dust reaction system. Since our previous
427
studies have explained the mechanisms regarding the synergistic effect between SO2
428
and NH3 and between SO2 and NO2 in detail, which have been simply mentioned
429
earlier as the acid-base interaction mechanism and redox mechanism, the complex
430
relationship between the NH3 (or NO2) role and the properties of the mineral dusts
431
would be revealed on the basis of these given mechanisms.8, 31 The IC results showed ACS Paragon Plus Environment
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The Journal of Physical Chemistry
432
that the synergistic effect between SO2 and NH3, or NO2 and NH3 prevailed over the
433
synergistic effect between SO2 and NO2 on Fe2O3, especially for SO2 for the
434
formation of SO42-, while the reverse was found over CaO and MgO for the formation
435
of SO42- and NO3-.
436
It has been demonstrated that SO2 mainly interacted with basic O2- sites at the steps
437
and kinks in CaO and MgO to form sulfite species, and only a small amount of sulfate
438
formation can be determined by the four-coordinated oxide anions on the steps and
439
corners of the MgO particles under dry conditions, even in the presence of O2.57 In the
440
individual reaction of SO2 in this work, the amount of sulfate obtained by the IC
441
measurements on CaO and MgO (and Al2O3) can be partially derived from the
442
aqueous oxidation during the measurement process because the dissolved oxygen
443
cannot be excluded completely and the particles might not be preserved very well
444
with formaldehyde. After this correction, the transformation of SO2 and NO2 to SO42-
445
and NO3- still require an oxidation process, in which CaO and MgO had no oxidizing
446
ability but only NO2 can act as an oxidant (Figure 3-4, eqns.1-6). Moreover, given the
447
basic nature, CaO and MgO were more favorable for the adsorption of NO2 than NH3.
448
Therefore, the synergistic effect between SO2 and NO2 has a more promotional effect
449
than that between SO2 (or NO2) and NH3 for the formation of SO42-.
450
The case of Fe2O3 is somewhat different as Fe2O3 itself can transform SO2 into
451
sulfate species, driven by the Fe(III)/Fe(II) redox process by a series of free-radical
452
propagation, termination, and product-formation reactions, or by molecular oxygen
453
activation on oxygen vacancy sites.37,
454
adsorption of NH3 was fast, even with the coexistence of NO2 (Figure 1b), possibly
455
due to the acidic nature of Fe2O3. The quickly enhanced surface basicity would
456
promote the adsorption of SO2 rapidly, which then transformed into SO42- from the
457
Fe(III)/Fe(II) redox process. Furthermore, due to the acidic nature, the adsorption of
458
NO2 as a dimer of N2O4 may be limited, which is a key intermediate promoter for the
459
oxidation of SO2.8 Therefore, the promotional effect of NH3 seemed more obvious
460
than that of NO2 in the formation of SO42- on acidic Fe2O3. For another acidic oxide of
461
Al2O3, the promotion effect of NH3 was basically equal to that of NO2 for the
462
formation of SO42-. However, neither Al2O3 nor NH3 has ability to oxidize SO2 to
463
SO42- in the way Fe2O3 or NO2 does. The remarkably increased amount of SO42- in the
464
presence of NH3 was possibly due to the dramatically enhanced adsorption of water,
465
as indicated by the enhanced 3538 cm-1 band in Figure 4b. which was favorable for
62
The DRIFTS spectra showed that the
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466
the conversion of surface-coordinated sulfite to sulfate species.57
467
It is noted that the formation of nitrate species was promoted significantly by SO2
468
on all mineral oxides. One possible reason can be the re-oxidation of the produced
469
nitrite in eqn. 2.63, 64 Especially with the aid of O2, the NO resulting from the eqns. 4
470
and 6 would combine with nitrite species to form nitrate species, as shown below,54
471
2MNO2 +NOg +O2 →MNO3 (11)
472
IC results (Figure 5a) showed that the addition of a third gas did not further
473
increase the amounts of SO42-, NO3- and NH4+ on Fe2O3, CaO and MgO compared to
474
the amounts in the presence of two gases, which was possibly due to the complex
475
competition effect among those synergistic effects (SO2 and NH3, NO2 and NH3, and
476
SO2 and NO2) with equivalent gas concentrations. On Al2O3, however, the amounts of
477
SO42- and NH4+ were higher in the SO2, NO2 and NH3 reaction group compared to the
478
two-gas reaction groups, indicating that both the acid-base interaction and redox
479
reaction mechanism exhibit a synergistic influence on the formation of SO42- and
480
NH4+. The acidic nature of Al2O3 induced the adsorption of NH3, promoting the
481
adsorption of SO2, which was oxidized into SO42- by NO2 (in the form of N2O4) at the
482
same time. In turn, the formed SO42- increased the Brønsted acid sites on the surface
483
and then promoted the formation of NH4+.31
484
5. CONCLUSION AND ATMOSPHERIC IMPLICATIONS
485
The promotional effect of NH3 on the heterogeneous formation of NO3-, SO42- and
486
NH4+ has been elucidated in detail, which relied greatly on the acid-base and redox
487
properties of mineral oxides. For acid oxides such as α-Fe2O3 and α-Al2O3, the NH3
488
exerts a significant promotion effect on the formation of nitrate species. Moreover, it
489
acted as a counterpart role and was even more important relative to the NO2 in
490
promoting the formation of SO42-. Particularly for α-Fe2O3, its redox chemistry
491
significantly facilitated the promotional effect of NH3. Evidence from the remarkably
492
formed SO42-, NO3- and NH4+ under NH3-rich conditions hinted us that an effective
493
control of the emission of NH3 might be a potential strategy to reduce secondary
494
inorganic aerosols in NH3-rich areas.65 However, the prevailing effect of NO2 over
495
NH3 for the formation of SO42- and NO3- on basic CaO and MgO is indicative of a
496
more effective way to reduce the formation of secondary species in areas containing
497
an abundance of basic aerosols.21, 66 Given the varied minerology of dusts with source
498
location, a differential elaboration of the evolution mechanism of secondary species is
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The Journal of Physical Chemistry
499
necessary.
500
In the system involving the reaction with the coexistence of SO2, NO2 and NH3
501
sharing equal inlet concentrations, however, the synergistic effect initiated separately
502
by NO2 or NH3 will not multiply to obtain further enhanced formation of SO42-, NO3-
503
and NH4+ on the oxides with redox or basic properties. Nevertheless, the IC results of
504
α-Al2O3 illustrate that acidic oxides with non-oxidizing ability are favorable towards
505
the synergistic adsorption of SO2 (or NO2) and NH3 (driven by NH3) and the
506
simultaneously transformation of SO2 (or NO2) and NH3 into SO42- (or NO3-) and
507
NH4+ (oxidized by NO2). Thus, an enhanced synergistic effect is expected on this type
508
of acid oxide in the system of coexisting SO2, NO2 and NH3.
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Page 20 of 26
509
ASSOCIATED CONTENT
510
Supporting Information is available free of charge
511
http://pubs.acs.org.
512
XRD patterns of mineral oxides. The chromatogram of NO3-, SO42- and NH4+. In situ DRIFTS
513
spectra of mineral oxides during exposure to different reaction atmospheres versus reaction time.
514
The tables summarizing the amounts of surface products formed on mineral oxides under different
515
conditions.
516
ACKNOWLEDGMENTS
517
This research was financially supported by the National Key R&D Program of China
518
(2016YFC0202700) and the National Natural Science Foundation of China
519
(91744205), and the Project funded by China Postdoctoral Science Foundation (no.
520
2017M622485).
521
AUTHOR INFORMATION
522
Corresponding Author
523
*E-mail:
524
86-10-62849337
525
Notes
526
The authors declare no competing financial interest.
Qingxin
Ma
(
[email protected]);
Tel.:
527
ACS Paragon Plus Environment
via the Internet at
86-10-62849337;
fax:
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The Journal of Physical Chemistry
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