Surface-catalyzed chlorine and nitrogen activation: Mechanisms for the heterogeneous formation of ClNO, NO, NO2, HONO, and N2O from HNO3 and HCl on aluminum oxide particle surfaces

Article abstract of DOI:10.1021/jp301488b

It is well-known that chlorine active species (e.g., Cl₂, ClONO₂, ClONO) can form from heterogeneous reactions between nitrogen oxides and hydrogen chloride on aerosol particle surfaces in the stratosphere. However, less is known about these reactions in the troposphere. In this study, a potential new heterogeneous pathway involving reaction of gaseous HCl and HNO₃ on aluminum oxide particle surfaces, a proxy for mineral dust in the troposphere, is proposed. We combine transmission Fourier transform infrared spectroscopy with X-ray photoelectron spectroscopy to investigate changes in the composition of both gas-phase and surface-bound species during the reaction under different environmental conditions of relative humidity and simulated solar radiation. Exposure of surface nitrate-coated aluminum oxide particles, from prereaction with nitric acid, to gaseous HCl yields several gas-phase products, including ClNO, NO₂, and HNO₃, under dry (RH < 1%) conditions. Under humid more conditions (RH > 20%), NO and N₂O are the only gas products observed. The experimental data suggest that, in the presence of adsorbed water, ClNO is hydrolyzed on the particle surface to yield NO and NO₂, potentially via a HONO intermediate. NO₂ undergoes further hydrolysis via a surface-mediated process, resulting in N₂O as an additional nitrogen-containing product. In the presence of broad-band irradiation (λ > 300 nm) gas-phase products can undergo photochemistry, e.g., ClNO photodissociates to NO and chlorine atoms. The gas-phase product distribution also depends on particle mineralogy (Al₂O₃ vs CaCO₃) and the presence of other coadsorbed gases (e.g., NH₃). These newly identified reaction pathways discussed here involve continuous production of active ozone-depleting chlorine and nitrogen species from stable sinks such as gas-phase HCl and HNO₃ as a result of heterogeneous surface reactions. Given that aluminosilicates represent a major fraction of mineral dust aerosol, aluminum oxide can be used as a model system to begin to understand various aspects of possible reactions on mineral dust aerosol surfaces.

Full text of DOI:10.1021/jp301488b

Article
pubs.acs.org/JPCA
Surface-Catalyzed Chlorine and Nitrogen Activation: Mechanisms
for the Heterogeneous Formation of ClNO, NO, NO2, HONO,
and N2O from HNO3 and HCl on Aluminum Oxide Particle Surfaces
Gayan Rubasinghege and Vicki H. Grassian*
Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States
S
* Supporting Information
ABSTRACT: It is well-known that chlorine active species (e.g., Cl2, ClONO2, ClONO) can form from heterogeneous reactions
between nitrogen oxides and hydrogen chloride on aerosol particle surfaces in the stratosphere. However, less is known about
these reactions in the troposphere. In this study, a potential new heterogeneous pathway involving reaction of gaseous HCl and
HNO3 on aluminum oxide particle surfaces, a proxy for mineral dust in the troposphere, is proposed. We combine transmission
Fourier transform infrared spectroscopy with X-ray photoelectron spectroscopy to investigate changes in the composition of both
gas-phase and surface-bound species during the reaction under different environmental conditions of relative humidity and
simulated solar radiation. Exposure of surface nitrate-coated aluminum oxide particles, from prereaction with nitric acid, to
gaseous HCl yields several gas-phase products, including ClNO, NO2, and HNO3, under dry (RH < 1%) conditions. Under
humid more conditions (RH > 20%), NO and N2O are the only gas products observed. The experimental data suggest that, in
the presence of adsorbed water, ClNO is hydrolyzed on the particle surface to yield NO and NO2, potentially via a HONO
intermediate. NO2 undergoes further hydrolysis via a surface-mediated process, resulting in N2O as an additional nitrogen-
containing product. In the presence of broad-band irradiation (λ > 300 nm) gas-phase products can undergo photochemistry,
e.g., ClNO photodissociates to NO and chlorine atoms. The gas-phase product distribution also depends on particle mineralogy
(Al2O3 vs CaCO3) and the presence of other coadsorbed gases (e.g., NH3). These newly identified reaction pathways discussed
here involve continuous production of active ozone-depleting chlorine and nitrogen species from stable sinks such as gas-phase
HCl and HNO3 as a result of heterogeneous surface reactions. Given that aluminosilicates represent a major fraction of mineral
dust aerosol, aluminum oxide can be used as a model system to begin to understand various aspects of possible reactions on
mineral dust aerosol surfaces.
sources, i.e., chlorofluorocarbon, in the presence of sunlight.12
Numerous studies have shown that heterogeneous reactions
INTRODUCTION
The importance of heterogeneous chemistry of halogen species
■
occurring on PSCs play an important role in the conversion of
and nitrogen oxide on polar stratospheric cloud (PSC) particles
these inactive chlorine compounds into more labile and reactive
has been well established and accounts for the observed large-
forms.13 It has been shown that, in PSCs, both solid and liquid
particles can promote heterogeneous reactions such as14
scale ozone depletion during the Antarctic springtime.1,2
During the polar sunrise, “low ozone events”, i.e., levels of
40 ppb ozone depleted over a period of 5 days to <5 ppb,
have been observed.3−7 Reasons for this decrease have been
attributed to elevated levels of chlorine (Cl2) and bromine
(Br2) that directly or indirectly destroy ozone in the strato-
sphere.7,8 Although bromine was thought to be mostly res-
ponsible for the ozone loss, the presence of chlorine atoms has
been inferred from the distribution of hydrocarbons9,10 and
13C/12C ratio of CO.11
ClONO2 + H2O → HOCl + HNO3
(1)
HOCl + HCl → Cl2 + H2O
(2)
ClONO2 + HCl → Cl2 + HNO3
(3)
Received: February 14, 2012
Revised: April 23, 2012
Published: April 26, 2012
The known mechanism begins with increased concentrations of
atmospheric HCl and ClONO2, by the breakdown of chlorine
© 2012 American Chemical Society
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Figure 1. Cartoon representation of nitrogen and chlorine activation from heterogeneous reactions of nitrogen oxides and hydrogen chloride on
tropospheric particles. Dust particles emitted from the Earth's crust react with atmospheric HNO3 (or NO2) to form an adsorbed nitrate layer on the
particle surface. These “aged” particles can then react with HCl from open oceans leading to active nitrogen and chlorine species. It should be noted
the sun can further initiate reactions for photoactive species on the surface and the gas phase.
Although heterogeneous chlorine activation on PSCs have
been well established, less is known about heterogeneous
chlorine activation on the surface of aerosol particles more
relevant to the troposphere such as mineral dust aerosol. In a
recent field study, Hobe et al. suggest that elevated levels of
reactive chlorine, including ClO, in the upper troposphere and
lower stratosphere near the tropics can be due to heteroge-
neous chlorine activation on cirrus ice clouds/or liquid aerosols
at lower temperatures.15 Therefore, exploring new reaction
pathways involved in the production of active chlorine and
nitrogen species from sources such as gas-phase HCl and
HNO3 may provide additional information on tropospheric
aerosol chemistry and potentially tropospheric ozone concen-
trations. Atmospheric HCl is reported at higher concentrations,
few parts per billion (ppbv), in polluted areas and in some
indoor settings.16−20 The major anthropogenic sources of gas-
phase HCl includes coal burning, waste incineration, chlori-
nated hydrocarbons, automobile exhaust, burning of garbage,
biomass and agricultural products.21−23 Industrial activities, i.e.,
petroleum and semiconductor manufacturing, also contribute to
a significant fraction of atmospheric HCl.24 The existence of gas-
phase HCl in the marine atmosphere is well established as sea
salt arising from wave action, contains a large amount of chlorine
ions, a source of gaseous HCl.20
In addition to reactive chlorine species, nitrogen oxides, in
particular NO, NO2, and N2O, are important in determining
atmospheric ozone concentrations in the lower and upper
atmosphere. These nitrogen oxides participate in a complex
series of chemical and photochemical reactions to produce
tropospheric ozone in a nonlinear relationship with respect to
ozone concentrations. Therefore, peak ozone levels are affected
by the amount of nitrogen oxides present in the gas phase,
which depends on the ground base emissions and subsequent
reactions in the lower atmosphere.25−27 N2O serves as a major
source of stratospheric NOx and thus contributes to catalytic
ozone destruction. Ravishankara et al. indicate that N2O is
currently the single most important ozone depleting substance,
and is anticipated to remain the largest throughout the 21st
century.28
In the atmosphere, nitrogen oxides readily react with parti-
culate matter (e.g., mineral dust and sea salt aerosol) to yield
adsorbed nitrate.29 Valuable information regarding chemical
speciation on the surface has been obtained using FTIR spectro-
scopy.30−35 Several surface species have been proposed follow-
ing the adsorption of nitrogen oxides on mineral oxide sur-
faces, which include nitrate (NO3−), nitrite (NO2−), and nitrosyl
(NO+). Under humid conditions, it has been shown that
coadsorbed water molecules readily solvate adsorbed nitrate ions
on the aluminum oxide surface forming inner and outer sphere
complexes.36
During the past decade, several field and model studies have
reported conversion of inorganic chloride into gaseous chloride
atom precursors, nitryl chloride (ClNO), in marine or costal
atmosphere as well as in midcontinental regions.37,38 On the
basis of field measurements, Tobo et al. shows conversion of
Ca-rich mineral dust particles to more soluble Ca(NO3)2 and
CaCl2 salts in the presence of atmospheric HNO3 and HCl.39
These studies further highlight a possible source of tropo-
spheric nitryl chloride might be the reactions between adsorbed
nitrogen (e.g., N2O5) and chloride species on aerosol dust
particles.40,41 However, the exact role of halogen compounds in
the lower troposphere and the reaction mechanisms involve is
poorly understood. In a recent study by Raff et al., a surface-
mediated coupling of nitrogen oxides and halogen activation
cycles has been shown in which uptake of gaseous NO2 and
N2O5 on solid silica substrate generates adsorbed intermediates,
NO+NO3− and NO2 NO3 , that reacts with HCl to yield ClNO
and ClNO2, respectively.42 Both of these compounds can
readily yield chlorine atoms in the troposphere.43 Given the
importance to this emerging chlorine chemistry in the lower
part of the atmosphere as discussed in a recent article by von
Glasow,44 these types of studies can provide important insights
into the chemistry of these processes.
+
−
In the current study, we explore new heterogeneous
chemistry of dust particles in which gaseous HCl is activated
in the presence of adsorbed nitrate, from reaction of nitric acid,
on aerosol particles yielding gas-phase chlorine and nitrogen
species. Figure 1 shows a cartoon representation that illustrates
possible continued chemistry of aged dust particles. The figure
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shows that aged dust, as a result of heterogeneous reaction with
atmospheric HNO3 to yield adsorbed nitrate, can interact with
gas-phase HCl in the marine environment to yield several
reactive nitrogen and chlorine species, including ClNO and
NOx. Here we combine transmission Fourier transform infrared
(FTIR) spectroscopy with X-ray photoelectron spectroscopy
(XPS) to investigate changes in both gas-phase and surface
adsorbed species during the reaction between adsorbed ni-
trate and gaseous HCl under different relative humidity con-
ditions. Given that aluminum oxides and aluminosilicates
contribute ∼8% by mass to the total dust burden in the atmo-
sphere,36,45,46 aluminum oxides are used as model systems to
begin to understand various aspects of nitrogen oxide and
hydrogen chloride reactions on mineral dust aerosol, with a few
experiments done on calcium carbonate, another reactive com-
ponent of mineral dust for comparison. For the first time we
show that exposure of surface nitrate-coated alumina surfaces to
gaseous HCl yields several gas-phase products, including ClNO,
NO2, and HNO3 under dry (<1 %RH) conditions. At higher
relative humidity, we observe significant production of NO and
N2O only. The results from the current study further highlight
that photodissociation (λ > 300 nm) of the secondary gas-phase
species, ClNO, yields NO and chlorine radicals. The gas-phase
product distribution depends on a number of factors including
particle mineralogy, simulated solar radiation, relative humidity
and the presence of other coadsorbed gases such as NH3. This
work also suggests a possible reaction mechanism for the forma-
tion of chlorine atom precursor nitryl chloride and several other
active nitrogen species under different atmospherically relevant
conditions.
Typically, 250 scans were collected with an instrument resolution
of 4 cm−1 in the spectral range extending from 900 to 4000 cm−1.
Absorbance spectra for gas and adsorbed species were obtained
by referencing single beam spectra of the blank grid and the
oxide-coated grid to single beam spectra collected prior to the
nitric acid exposure.
After 2 h of the reaction, the secondary gas-phase products
were exposed to light (300 < λ < 700 nm) using a broad-band
Hg light source and a broad-band filter, as described
previously.47,48 The experimental setup for these experiments
is described in detail in our previous work.47 Selected experiments
were done with CaCO3 (OMYA, 98% CaCO3).
Ex-situ X-ray Photoelectron Spectroscopy. Reacted
alumina was removed from the tungsten grid and analyzed
using a custom-designed Kratos Axis Ultra X-ray photoelectron
spectrometer with a monochromatic Al Kα X-ray source. The
powder alumna sample was pressed onto an indium foil, on a
copper stub, and introduced into the XPS analysis chamber,
which had a pressure that was maintained in the 10−9 Torr
range during analysis. Wide energy range survey scans were
acquired using the following parameters: energy range from
+1200 to −5 eV, pass energy of 160 eV, step size of 1 eV, dwell
time 200 ms, X-ray spot size 700 × 300 mm2. High resolution
spectra were acquired using the following parameters: energy
range 50−20 eV depending on the peak examined, pass energy
of 20 eV, step size of 0.1 eV, dwell time of 1000 ms. The data
collected were analyzed using CasaXPS data processing
software.
RESULTS AND DISCUSSION
■
As discussed here, laboratory measurements show that surface
reactions of HNO3 and HCl produce active nitrogen and
chlorine species, including ClNO, NO, NO2, and N2O.
EXPERIMENTAL METHODS
■
Source Materials. Commercially available γ-Al2O3 (Degussa,
aluminum oxide C) and CaCO3 (OMYA) with surface areas of
101 ( 4) and 10 ( 0.4) m2 g−1, respectively, were used in these
experiments. Dry gaseous nitric acid was taken from the vapor
of a 1:3 mixture of concentrated HNO3 (70.6% HNO3,
Mallinckrodt) and 95.9% H2SO4 (Mallinckrodt). Dry gas-phase
hydrogen chloride was generated from the vapor of concentrated
HCl (37% HCl, Fisher). Distilled H2O (Fisher, Optima grade)
was degassed prior to use.
Analysis of Gas-Phase Products from the Reaction
between Adsorbed Nitrate and Gas-Phase HCl. Hetero-
geneous uptake of nitric acid on aluminum oxide yields a
chemisorbed nitrate layer on the particle surface as discussed in
our previous work and by several others.25,36,49−51 Vibrational
frequencies of adsorbed nitrate have been assigned and
interpreted using quantum chemical calculations of binuclear
aluminum oxyhydroxy cluster models.36 Under conditions of
lowest relative humidity (<1 %RH ), adsorbed nitrate binds to
the oxide surface in different modes of coordination as mono-
dentate, bidentate, and bridging, based on the assignment of the
peaks in the nitrate ion ν3 spectral region between 1200 and
1650 cm−1.25,36 In the presence of water vapor, coadsorbed
water solvates the nitrate ion forming inner and outer sphere
complexes which are represented by peaks in the region
between 1300 and 1450 cm−1 in the FTIR spectrum.36
Gas-phase product formation upon exposure of the nitrated
particle surface to 100 mTorr of gaseous HCl is shown in
Figure 2. FTIR spectra of gas-phase products were recorded as
a function of reaction time under dry conditions (<1 %RH ) and
wet conditions (45 1 %RH) at 298 K. Under dry conditions
(Figure 2a,b), the data show that several gas-phase products
appear and grow in intensity as the reaction proceeds. On the
basis of the vibrational frequency, the major gas-phase products
as ClNO, HNO3, and NO2 (Table 1) can be identified.52−55 It
should be noted that some fraction of gas-phase HNO3 at t = 0
is formed by the displacement of surface adsorbed nitrate
during the uptake of gas-phase HCl. Quantitative analysis of
instantaneous HCl loss from the gas phase indicates this HNO3
from the displacement can account for ∼8% of the total HNO3
Transmission FTIR Spectroscopy of Adsorbed and
Gas-Phase Species. The experimental setup and the prepa-
ration of saturated surfaces of adsorbed nitrate have been
previously described in detail in earlier work.36,47 In brief,
approximately 5.5 mg of alumina (γ-Al2O3, Degussa) powder, a
model for mineral dust surfaces, was pressed onto half of a
tungsten grid and evacuated for 12 h in the FTIR cell to remove
adsorbed water from the surface. The alumina surface was then
exposed to nitric acid vapor for 30 min at a pressure of
approximately 1.70 Torr at 298 K followed by a 4 h evacuation
of the FTIR cell to remove all weakly adsorbed products. From
previous studies, this process is known to produce a saturated
surface coverage of adsorbed nitrate on alumina.25,47 The cal-
culated nitrate coverage, when normalized to the BET surface
area, is determined to be 5
1 × 1014 molecules cm−2.25
Following introduction of gas-phase HCl and water vapor
(adjusted to different relative humidity values), the valve connect-
ing the FTIR cell to the mixing chamber was closed, letting gas-
phase products accumulate inside the cell as the nitrate-coated
surface was reacting with HCl. During the experiment, infrared
spectra were recorded with a single beam Mattson RS-10000
spectrometer equipped with a narrow band MCT detector.
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Figure 2. Gas-phase product formation from the reaction between gas-phase HCl (100 mTorr) and adsorbed nitrate on aluminum oxide under dry
conditions of <1 %RH (a and b) and wet conditions of 45 1 %RH (c and d) at 298 K. Gas-phase absorption bands are identified based on the
assignments given in Table 1. Gas-phase concentrations are calculated based on the conversion factors given in Table 1.
Table 1. Assignment of Major Infrared Absorption Bands
and Conversion Factors Used To Calculate Gas-Phase
Product Concentrations
shown in Figure 2b. The results presented here are the average
value of triplicate measurements and the reported error
represents one standard deviation. Gas-phase concentrations
of individual species were determined by converting integrated
absorbances to concentrations using calibration factors, given in
Table 1.
gas-
phase
products
integrated
conversion
factor
a
vibrational frequency
absorbance spectral
mode
(cm−1
)
range (cm−1
)
(mol L−1
)
b
b
c
0.87 × 10−4
5.58 × 10−4
0.92 × 10−4
0.31 × 10−4
3.08 × 10−4
Interestingly, under humid conditions, i.e. 45
1 %RH.
HNO3
HCl
ν2
ν1
ν1
ν3
ν1
ν1
1709
2886
1800
1616
1874
2224
1654−1754
2614−3090
1760−1854
1540−1670
1750−1980
2124−2271
(Figure 2c,d), the gas-phase product distribution is quite
different with no ClNO or HNO3 formed. Instead, there is the
growth of NO, NO2, and N2O as a function of reaction
time.53−55 FTIR spectra of gas-phase product formation upon
exposure of nitrated alumina surface to gas-phase HCl under
wet condition (45 1 %RH) at 298 K is shown in Figure 2c.
The variation of gas-phase concentration under these
conditions can be seen in Figure 2d. A comparison of the
gas-phase concentrations reveals a significant change in the gas-
phase product distribution and total gas-phase production
between dry (<1 %RH ) and wet (45 1 %RH) conditions.
This large difference leads to several additional experiments
focused on gas product formation under a wider range of
ClNO
NO2
NO
d
d
d
N2O
0.69 × 10−4
a
b
c
References 52−55. Experimentally determined values. Reference
d
52. References 53−55.
at time, t ∼ zero. Time-course spectra shown in Figure 2a reveal
a significant change in the gas-phase product distribution
during the course of the reaction. Gas-phase concentrations
obtained from FTIR analysis followed by the reaction between
gaseous HCl and adsorbed nitrate under dry conditions is
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Figure 3. Gas-phase product formation from the reaction between gas-phase HCl (2 Torr) and adsorbed nitrate on aluminum oxide under dry
conditions of (<1 %RH) (a and b) and wet conditions of (45 1 %RH) (c and d) at 298 K. Gas-phase absorption bands are identified based on the
assignments given in Table 1. Gas-phase concentrations are calculated based on the conversion factors given in Table 1.
relative humidities ranging from % RH ∼3 to 80 and with
higher pressures of HCl to explore possible reaction pathways
in the reaction between adsorbed nitrate and gas-phase HCl.
Although higher HCl pressures are used to observe possible
reaction products with FTIR spectroscopy, these experiments
are correlated to the results of lower HCl pressure experiments.
Gas-phase product formation upon exposure of surface
nitrate to 2 Torr of gaseous HCl under dry and wet conditions
is shown in Figure 3. Parts a and c of Figure 3 are enlarged
regions of the respective FTIR spectra between 1200 and
2500 cm−1 to illustrate the gas-phase product formation. Clearly,
we observe similar products, under both dry (<1 %RH) and
wet (45 1 %RH) conditions, but with higher ClNO produc-
tion compared with lower HCl pressures, and lower NO2
concentrations as shown in Figure 3b,c. These preliminary
data suggest that NO2 might be participating in one or more
secondary reactions, especially in the presence of higher HCl
pressures.
atmospheric investigations, theoretical56−58 and experimental,59−61
and has clearly proven that in the presence of water along with
HNO3 result in the formation of stable complexes. It is well
established that in PSCs hydrogen chloride binds to the particle
surface, in the presence of HNO3 and H2O (nitric acid hydrates),
leading to the transformation of HCl into more photolabile
Cl2.62−64 A study by Weiser et al. on the Arctic stratosphere under
synoptic conditions showed that HCl dissolved in liquid particles
along with HNO3−H2O, following an unexpected ratio.65
Here the experiments were carried out under different humid
conditions, including % RH 3, 8, 12, 20, 45, and 80.
The relative gas-phase product distributions under different
relative humidity conditions are shown in Figure 4. The plot,
showing the relative gas-phase concentrations of ClNO, NO2,
HNO3, HCl, N2O, and NO under <1 %RH, 3, 8, 12, 20, 45,
and 80 %RH after 120 min of the reaction time, were
obtained by normalizing the gas-phase concentrations to the
largest gas-phase product under all conditions studied,
which is ClNO at % RH < 1. On the basis of the results
observed under different relative humidities, RH greater than
8% appears to yield a different gas-phase product distribution.
Thus, these data will be used in the following discussion to
Effect of Relative Humidity on Gas-Phase Product
Distribution. Water is relatively abundant in the atmosphere and
plays an important role in many processes. The three-component
system, HCl−HNO3−H2O, has been the subject in numerous
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NO2 is highest in % RH < 12 and addition of more water
vapor enhances formation of NO and N2O at higher rates.
The information from these experiments can begin to elucidate
the mechanism involve with the reaction between adsorbed
nitrate and gas-phase HCl on mineral dust particle surfaces.
Mechanism for the Production of Secondary Gas-
Phase Products. Before further discussing the heterogeneous
interactions of gaseous HCl with adsorbed nitrate and the gas-
phase products that form, it is first instructive to consider what
is known about uptake of HNO3 and HCl on metal oxide
surfaces in general and, in particular, on alumina. It is well-
known that heterogeneous uptake of HNO3 by the metal oxide
surface yields adsorbed nitrate via25,31−36
HNO3(g) + OH−(a) → NO3−(a) + H2O
(4)
Upon exposure to gaseous HCl, the FTIR spectra of the
nitrated surface at each relative humidity undergo changes.
Most notably there is a decrease in the intensity of the spectral
bands associated with molecularly adsorbed nitric acid and
chemisorbed nitrate ion as indicated by the analysis of
difference spectra shown in Figure 5. Under dry conditions,
<1 %RH (Figure 5a), peaks at 1265, 1625, and 1558 cm−1
associated with adsorbed bidentate and bridged nitrate species
disappear whereas the peaks at 1517, 1319, and 944 cm−1,
associated with monodentate nitrate species appear to shift
and increase in intensity.36 In addition, molecularly adsorbed
nitric acid, characterized by the peak at 1679 cm−1, also
disappears from the surface. A new absorption band at 1425
cm−1, attributed to partially solvated nitrate, is seen to grow
as the surface is exposed to HCl (g). Under wet conditions,
45 1 %RH (Figure 5b), the peaks at 1408 and 1339 cm−1
associated with solvated nitrate species are observed to
disappear along with the peak at 1631 cm−1 for coadsorbed
water, indicating the participation of surface water in the
reaction at higher relative humidity.
In addition to FTIR data, the presence of adsorbed nitrate
and chloride on the alumina particle surface was further
confirmed by ex-situ XPS analysis in the N 1s and Cl 2p
binding regions, as seen in Figure 6a,b, respectively. In these
experiments, alumina was first analyzed prior to exposure to any
gas-phase reactants and this spectrum is labeled as “Before
reaction” in both binding energy regions. The alumina surface,
exposed to nitric acid, is shown as “After reaction with HNO3”
and this “nitrated” alumina surface is further reacted with gas-
phase HCl and these XPS data are labeled “After reaction with
HCl”. High resolution XPS data, after exposure to HNO3 acid,
showed a peak in the N1s binding energy region at 407 eV,
which is consistent with the presence of adsorbed nitrate as
discussed in detail in Baltrusaitis et al.66 Quantitative analysis of
the nitrate peak area shows ∼33% of the initial surface coverage
of adsorbed nitrate is consumed during the formation of ClNO
and other gas-phase products within 20 min of the reaction. In
addition, a new peak in the Cl 2p binding energy region for
surface adsorbed chlorine was observed, indicating a significant
uptake of HCl during the reaction (Figure 6b).67 Adsorption of
HCl, on metal oxide surfaces such as alumina is thought to
occur via two distinct reaction mechanisms. The first involves
the dissociation of HCl onto aluminum oxygen pairs forming
OH and Al−Cl species.68,69 Several theoretical and laboratory
studies have confirmed the formation of new OH groups and,
thus, existence of this mechanism. In the second pathway, the
reaction likely occurs via analogous mechanism, dissociative
adsorption of HCl, whereupon chloride replaces a surface OH
Figure 4. Relative gas-phase concentrations obtained from FTIR
spectra following the reaction between gas-phase HCl and adsorbed
nitrate on aluminum oxide 298 K under different relative humidities,
ranging from <1 %RH to 80 %RH. The gas-phase concentrations have
been normalized to the largest gas-phase product species under all the
conditions, ClNO at <1 %RH.
understand the variation of gas-phase product formation as a
function of relative humidity. The FTIR spectra of the gas-
phase product formation at % RH 12 clearly indicates early
formation of ClNO and NO2 in higher concentrations followed
by decay as the reaction progress (not shown). Loss of ClNO
and NO2 at longer time scale suggests initiation of secondary
reactions yielding more gas-phase or surface products. In
addition to ClNO and NO2, here we observe formation of NO
and N2O, the major gas-phase products at higher relative
humidities, % RH < 20.
The rate of secondary gas-phase production is greatest
initially (t < 20 min), followed by a slower rate of change over
time. Initial rates of gas-phase product formation under
different relative humidies at 298 K, determined from linear
regression analysis, are given in Table 2. The rates are
Table 2. Surface Area Normalized Initial Rate of Gas-Phase
Product Formation during the Reaction between Gas-Phase
HCl and Adsorbed Nitrate on Aluminum Oxide Particle
a
Surfaces under Different Relative Humidity Conditions
surface area normalized initial rates of gas production × 10−9
b
(molecules m−3 s−1 cm−2
)
% relative
humidity
ClNO(g)
NO2(g)
NO(g)
N2O(g)
<1 (at low HCl 0.48 ( 0.05) 1.8 ( 0.10)
pressure)
<1
12
20
41 ( 3)
38 ( 2)
1.1 ( 0.1)
3.40( 0.14)
2.60( 0.12) 8.0 ( 0.4)
18 ( 1)
0.35 ( 0.021)
1.1 ( 0.010)
45 (at low HCl
pressure)
2.30( 0.15) 1.12 ( 0.06) 0.010( 0.0010)
45
80
31 ( 2)
56 ( 1)
4.8 ( 0.2)
4.8 ( 0.4)
a
Results presented here are the average value of triplicate measure-
ments, determined from linear regression, and the reported error
represents one standard deviation. Gas-phase product formation data
b
from the first 20 min were used in the rate determination. Rates are
normalized to the corresponding BET surface area.
normalized to the BET surface area of γ-alumina. It can be
clearly seen from these data that production of ClNO and
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Figure 5. FTIR difference spectra of surface adsorbed nitrate under (a) dry and (b) wet conditions at 298 K, illustrating the changes of the surface-
bound species during the reaction between adsorbed nitrate and gaseous HCl. Each spectrum was referenced to the surface spectrum prior to
exposure to HCl. Gas-phase absorptions were then subtracted from each spectrum.
groups by chlorine, followed by a loss of water molecule, to
regenerate a more acidic Lewis acid site for further reaction.
Thus, HCl is actively taken up by the alumina surface via the
ionic dissociation mechanism,
HCl(g) → H+(a) + Cl−(a)
(5)
Moreover, the loss of adsorbed nitrate as quantified by XPS
suggests that changes in the mode of coordination of adsorbed
nitrate seen in FTIR (Figure 5a) may be due to a number of
possibilities including rearrangement of adsorbed nitrate
coordination upon uptake of gas-phase HCl, reaction to form
ClNO and adsorption of products and adsorption site
competition due to readsorption processes.
It is well established that nitrate ion is a strong oxidizing
agent in highly acidic solutions that is capable of changing the
oxidation state of halogen species.72−74 Therefore, here we
propose that interaction between adsorbed nitrate and chloride,
on acidic alumina surface, yields NO+NO3− via acid catalysis, as
given in eq 6.
2NO3−(a) + 2H+(a) + 2Cl−(a)
→ NO+NO3−(a) + 2OH−(a) + Cl2(g)
(6)
This surface complex has been proposed to readily react with
further HCl forming gas-phase ClNO and HNO3, as major
products.42 The overall reaction mechanism is given in eq 7.
NO+NO3−(a) + HCl(g) → ClNO(g) + HNO3
(7)
The secondary gas-phase product, HNO3, is readsorbed on the
alumina surface result in more adsorbed nitrate. This agrees
with FTIR data shown in Figure 2 and 3 that indicated an early
production of gas-phase HNO3 followed by a decay as a
function of reaction time.
Gas-phase NO2 is formed via desorption of NO+NO3 in a
−
self-reaction of adsorbed nitrogen species in analogues to the
reaction mechanism between gas-phase NO and NO3,
according to eq 8.75,76
Figure 6. Representative XPS of high resolution (a) N 1s region and
(b) Cl 2p region before and after the reaction between gas-phase HCl
and adsorbed nitrate under dry on aluminum oxide particle surfaces
(<1 %RH) condition at 298 K.
NO+NO3−(a) → 2NO2(g)
(8)
On the basis of the observations under dry conditions, it
appears that gas-phase NO2 is in equilibrium with NO+NO3−(a)
due to the fact that it is not the major gas-phase product. It also
group via an exchange mechanism.70 In a recent study, McInroy
et al.71 highlighted the importance of replacing surface OH
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must be mentioned that this mechanism is solely based on the
observed ClNO and NO2 products species and must therefore
be viewed as one of the many possible reaction schemes, such
as follows.
(H2ONO+ or possibly NO+) have shown to generate hypo-
nitrous acid, HONNOH. The self-reaction of (HON)2 is
known to decompose to N2O over a wide range of pH values,
including highly acidic conditions. The net reaction yields N2O
and HNO3 according to
NO3−(a) + Cl−(a) + 2H+(a) → HOCl(g) + HONO(g)
H+
(9)
8NO2 + 3H2O ⎯⎯→ N2O + 6HNO3
(15)
HONO(g) + Cl−(a) + H+(a) → ClNO(g) + H2O(a)
Thus, a surface-mediated secondary reaction of a primary gas-
phase product, NO2, a product that can be easily seen in the
infrared spectra at lower RHs, may be responsible for the higher
N2O concentrations observed, especially at longer reaction
times.
Role of the Particle Surface in the Reaction Mechanism.
The proposed mechanism above for the formation of sec-
ondary gas-phase products from the reaction between HNO3
and HCl, given by eqs 4−7, occurs via surface adsorbed
intermediates. To better understand and further confirm
the involvement of particle surface, selected control experi-
ments were carried out in the absence of alumina surface.
The gas-phase product formation from homogeneous reaction
of gaseous HCl and HNO3 is shown in Figure 7. According to
(10)
HOCl(g) + Cl−(a) + H+(a) → Cl2(g) + H2O(a)
(11)
The proposed reaction mechanism was further validated using
mass balance calculations of nitrogen-containing species in the
overall reaction for the dry conditions, net reaction of eqs 5−8.
These calculations show that more than 85% of the surface-
bound nitrate, initially present on the surface and reacted with
HCl, can be found in the gas-phase as ClNO, NO2 and HNO3,
indicating that the proposed reaction pathway is the primary
mechanism taking place under these conditions. The
discrepancy can be due to other reaction pathways and uptake
by the reaction chamber walls.
As seen in Figures 2 and 3, under humid conditions, NO and
N2O are formed as major gas-phase products with a con-
comitant loss of ClNO and NO2. Previous studies have shown
that hydrolysis on aqueous solutions may be a loss process for
ClNO in the atmosphere.77,78 In contrast to these observations,
Raff et al.42 discuss an enhancement of ClNO production on
the SiO2 surface in the presence of thin films of adsorbed water
(9−13 %RH), competing with the hydrolysis reaction. This
study further claims the observed enhancement occurs via a
barrierless channel where water acts as a conduit to transfer a
proton from HCl to nitrate, facilitating the formation of ClNO.
In our studies with active alumina surface, we observed higher
production of ClNO at lower relative humidities (<12 %RH),
yet no significant enhancement. In alumina, atmospheric water
vapor readily adsorbs on the particle surface and dissociates,
resulting in a hydroxyl terminated surface.79 Adsorbed water
can hydrogen bond to these hydroxyl groups in equilibrium with
water vapor. It has been shown that coadsorbed water mole-
cules readily solvate adsorbed nitrate ions on the aluminum oxide
surface forming inner and outer sphere complexes.36 For
alumina, it has been shown that the first layer of adsorbed
water forms around 17 %RH and this takes place at slightly lower
relative humidities in the presence of adsorbed nitrate.25,36 Water
uptake on alumina has been discussed in detail in our previous
studies.25 On the basis of all these information and observations,
at higher relative humidities (% RH > 12), formation of 2−4
layers of water on the particle surface may lead to hetero-
geneous hydrolysis of ClNO via eqs 12 and 13 to yield NO and
77,78
NO2.
ClNO(g) + H2O → HONO(g) + HCl(g)
(12)
2HONO(g) → NO(g) + NO2(g) + H2O(a)
(13)
Figure 7. Gas-phase product (ClNO and NO2) formation from the
reaction between gas-phase HCl and HNO3 under dry (<1 %RH)
condition at 298 K. (a) FTIR spectra. (b) Variation of gas-phase
products. In (b) the two dash lines represent corresponding gas-phase
product variations in the presence of γ-Al2O3 surface.
Furthermore, NO+NO3 (a) reacts with water to produce more
−
HONO via52,80
NO+NO3−(a) + H2O(g) → HONO(g) + HNO3(g)
(14)
Figure 7a showing FTIR spectra of the gas phase, significantly
lower production of ClNO and NO2 can be seen compared to
that of in the presence of alumina surface. This is further
highlighted in Figure 7b that shows a comparison of ClNO and
It is well-known that N2O is formed during the heterogeneous
hydrolysis of gas-phase NO2 via HONO on acidic oxide
surfaces.80−82 Reactions of HONO and its protonated forms
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Figure 8. Gas-phase product formation from the reaction between gas-phase HCl and adsorbed nitrate on CaCO3 under dry conditions at 298 K.
Surface of the CaCO3 contains (a) micropuddles of Ca(NO3)2·xH2O (where x = 1, 2, 3, or 4) and (b) a coating of Ca(NO3)2(aq) obtained by
following two different methods described in the text.
NO2 production in the presence (dash line) and absence (solid
line) of the surface. Initial rates of gas-phase product formation
under different relative humidities at 298 K, determined from
linear regression analysis shows that ClNO and NO2 produc-
tion under dry (<1 %RH) conditions is approximately 40-fold
enhanced for ClNO production during the first 20 min and is
4-fold greater for NO2. At longer time scales the enhancement
for ClNO decreases to about 10-fold.
ClNO formation (Net 16). Based on the mechanism propossed
here, the production of ClNO from the surface-mediated
process is similar to the homogeneous pathway, making it
difficult to distinguish between the two. Furthermore, according
to the proposed heterogeneous mechanism, three HCl
molecules are needed for one nitric acid to form one ClNO.
Thus, at lower HCl pressures, HCl becomes the limiting factor
lowering the production of ClNO and yielding more NO2 from
the surface, as seen in the data shown in Figure 2.
The homogeneous reaction between HNO3 and HCl, in the
presence and absence of water is well documented.14,83−85 On
the basis of reactants and identified gas-phase products in
several studies, the proposed overall reaction is similar to the
one observed with aqua regia.14,86 In this reaction pathway, gas-
Effect of Particle Mineralogy on the Proposed
Reaction Mechanism. The new reaction pathway pro-
posed in this study for the formation of ClNO and NO2
highly depends on surface acidity and coadsorbed water.
Aerosol particles with different surface mineralogy are known to
exhibit variations in their surface acidity and the extent of water
uptake. Therefore, further investigation was done using CaCO3
that has shown lower surface acidity and higher water uptake.25
In our previous work, CaCO3 has also been used as a model
system to show that water plays a major role during the uptake
of HNO3 on calcium carbonate due to the fact that coadsorbed
water increases the ionic mobility of adsorbed ions on the
surface, leading to the formation of micropuddles of calcium
nitrate.87 These micropuddles are associated with additional
water even after evacuation forming Ca(NO3)2·xH2O. These
micropuddles are formed during successive cycles of HNO3
exposures followed by H2O vapor exposures. Secondary gas-
phase product formation, under dry (<1 %RH) condition, from
the reaction between gaseous HCl and adsorbed nitrate on
CaCO3, in the presence of additional associated water, is shown
in Figure 8a. As evidence by these data, the gas-phase
production, in particular of ClNO, is less compared to that of
alumina surface. This difference is further confirmed by the
calculated initial rates of gas-phase product formation given in
Table 3. Surprisingly, the initial rate of NO2 formation with
CaCO3 is higher compared to alumina surface under the same
conditions. There was no N2O formation even at higher relative
humidities when CaCO3 was used as the reactive surface (not
shown here). Therefore, it can be proposed that heterogeneous
hydrolysis of NO2 to yield N2O does not occur on the CaCO3
surface due to lower acidity of the carbonate surface leading to
a higher gas-phase concentration of NO2. Additional experi-
ments were carried out using CaCO3 in which adsorbed nitrate
associated with even more water, Ca(NO3)2(aq), was obtained
by exposing the carbonate surface to a gaseous mixture of
HNO3 and H2O (at 80 %RH) followed by a 4 h evacuation.
+
−
phase nitric acid dissociates to yield NO2 and NO3 which
react with Cl− from gaseous HCl to form ClNO2 that readily
dissociates to Cl2 and HONO. HONO reacts with more Cl− to
yield ClNO. The overall reaction of the proposed mechanism
has been described as
3HCl + HNO3 → Cl2 + ClNO + 2H2O
and the individual steps are given as
Thus, ClNO is formed even in the absence of a surface, yet
the product yields are much smaller compared to the
heterogeneous process. Interestingly, the overall reaction for
the heterogeneous process, the net reaction of eqs 4−7, is
similar to the overall reaction obtained for the homogeneous
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Strong overlap between the ClNO absorption cross section and
the solar actinic flux point out greater NO and Cl yields from
the photodissociation under atmospheric conditions, poten-
tially leading to an accelerated ozone depletion. In this study,
photolysis of secondary gas-phase species was monitored by
irradiation of the FTIR cell with broad-band (λ > 300) light,
after completion of the reaction (120 min) between HCl and
adsorbed nitrate on alumina. Gas-phase product formation
upon irradiation at 298 K, under dry (<1 %RH) and wet (45
2 %RH) conditions, are shown in Figure 9a,b, respectively. As
Table 3. Surface Area Normalized Initial Rate of Gas-Phase
Product Formation during the Reaction between Gas-Phase
HCl and Adsorbed Nitrate on CaCO3 under Dry and Wet
a
Conditions
surface area normalized initial rates of gas production ×
b
10−9 (molecules. m−3 s−1cm−2
)
% relative
humidity
ClNO(g)
NO2(g)
NO(g)
1.7 ( 0.069)
N2O(g)
<1
45
6.2 ( 0.25) 6.4 ( 0.35)
a
Results presented here are the average value of triplicate measure-
ments, determined from linear regression, and the reported error
represents one standard deviation. Gas-phase product formation from
b
the first 20 min were used in the rate determination. Rates are
normalized to the corresponding BET surface area.
The gas-phase products formation from this surface upon
exposure to HCl, under dry (<1 %RH) condition, is shown in
Figure 8b. There is even less gas-phase production and formation
of NO. Production of NO can be attributed to heterogeneous
hydrolysis of ClNO, in the presence of extra water on the surface
even under dry conditions. Therefore, additional water associated
with secondary structures on particle surface greatly influence
gas-phase product distribution and the data from these experi-
ments further confirm the propossed reaction mechanism for
the heterogeneous of HCl and adsorbed nitrate.
Surface-mediated acid catalysis is the key step in the pro-
posed reaction mechanism, especially during the formation
of the adsorbed NO+NO3 intermediate. Gas-phase NH3 is
−
also known to titrate surface acidity upon adsorption.88 In
the next set of experiments, the effect of coadsorbed ammonia
was investigated by exposing adsorbed ammonium nitrate on
alumina surface to gas-phase HCl. Exposure of adsorbed nitrate
to gas-phase ammonia (NH3) yields a thin layer of ammonium
nitrate, one or two monolayers of adsorbed NH4NO3, over the
particle surface.89 Under these conditions, lower gas-phase
production was observed. The presence of unreacted HCl, even
after 120 min of reaction time, further verified less reactivity of
the surface. The initial rates of ClNO and NO2 production
were determined to be 1.4 ( 0.05) × 109 and 0.11 ( 0.01) ×
109 molecules m−3 s−1 cm−2, respectively. The lower reactivity,
in the presence of coadsorbed NH3, can be partially attributed
to less availability of surface protons for the acid catalysis. Less
accessibility to reaction sites during HCl uptake can be another
plausible explanation for the low production of ClNO.
Photochemistry of Secondary Gas-Phase Products.
Given that photochemistry dominates the daytime gas-phase
chemistry of the atmosphere, and new light induced pathways
continue to be discovered,90 there is quite a lot of information
available in the literature to realize that the surface and near
surface regions of particles catalyze photochemical reactions
that perturb the composition of the atmosphere. The photo-
chemistry of secondary gas-phase species, in particular ClNO, has
long been a subject of active interest. ClNO absorbs radiation
throughout the visible and near-ultraviolet region.91,92 On the basis
of numerous studies, it is well-known that ClNO photodissociates
to yield gas-phase NO and chlorine atoms via,52,80
Figure 9. Gas-phase product formation upon irradiation of
secondary gas-phase products yielded from the reaction between
gas-phase HCl and adsorbed nitrate on aluminum oxide particle
surfaces at 298 K under (a) dry (<1 %RH) and (b) wet (45 2 %RH)
conditions.
seen in Figure 9a, we observed formation of NO with a
simultaneous decay of the major gas-phase product, ClNO,
upon photolysis under dry conditions. In addition, HNO3 and
NO2 showed a similar loss from the gas phase with broad-band
irradiation. On the basis of mass balance calculations for eq 17,
more than 75% of the photoproduced NO is present in the gas
phase and the rest is adsorbed on the particle surface, as
evidenced by a weak peak in the surface spectrum at 1701 cm−1
for the N−O stretching band associated with adsorbed NO
(not shown here).47 Under humid conditions (Figure 9b), no
additional gas-phase species were observed, but a continuance
production of NO and N2O.
hν
ClNO(g) → Cl• + NO(g)
(17)
From the analysis of ClNO spectrum and theoretical calculations,
the quantum yield of this photochemical reaction is determined to be
roughly equal to 1 in the wavelength region from 365 to 635 nm.93,94
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In good agreement with previous studies, here active Cl and
NO species are generated from photodissciation of ClNO
under dry conditions. Chlorine atoms can then initiate chain
reactions inside the FTIR cell, starting with the reaction with
ClNO that results in even more NO and Cl2.92
the presence of sunlight. During the daytime, ClNO photo-
dissociates to NO and chlorine atoms, initiating a chain of
secondary reactions with other gas-phase species in the atmo-
sphere. These active nitrogen and chlorine species are cap-
able of further participating in nitrogen and chlorine cycles.
Thus, heterogeneous conversion of HNO3 and HCl to labile
nitrogen and chlorine species in the atmosphere could poten-
tially alter the peak concentration and geographical distribu-
tion of ozone.
Cl• + ClNO(g) → NO(g) + Cl2(g)
(18)
Similarly, active Cl can potentially react with gas-phase HNO3
and HCl, yielding more products to the reaction mixture.
Moreover, the reaction mixture becomes even more complex
ASSOCIATED CONTENT
* Supporting Information
Complete listing for refs 15, 20, 24, 37, and 41. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
−
due to photoreduced products of adsorbed nitrate, NO, NO2 ,
and NO2.47,53,95−99
Thus, the daytime chemistry of HCl and adsorbed nitrate is
even more complicated to understand compared to the nighttime
chemistry and influences the chemical balance of the atmosphere
to a greater extent. These laboratory experiments further provide
evidence for the formation of highly reactive chlorine atoms on
aerosol dust particles that is currently not included in atmospheric
chemistry models.
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
■
We thank Dr. Jonas Baltrusaitis for his help in the analysis
and acquisition of XPS data. This material is based on the
work supported by the National Science Foundation under
grant CHE-0952605. Any opinions, findings and conclusions
or recommendations expressed in this material are those of
authors and do not necessarily reflect the views of the National
Science Foundation.
CONCLUSIONS AND ATMOSPHERIC
IMPLICATIONS
■
Conversion of atmospheric nitric acid and hydrogen chloride to
ClNO, NO, NO2, Cl, and Cl2, through an adsorbed nitrate
intermediate under different atmospherically relevant con-
ditions, has been shown. Numerous field studies have shown,
via heterogeneous interactions, HNO3 reacts with mineral
dust and other atmospheric aerosols to yield adsorbed nitrate,
nitrate coatings, and concentrated nitrate solutions.79 Atmo-
spheric mixing of dust particles, having nitrated surfaces,
with sea salt in marine environments is one potential way of
initiating the formation of ClNO and other active gas-phase
species. Besides from sea salt, atmospheric HCl originating
from various natural and anthropogenic sources can react with
adsorbed nitrate on mineral dust particles to result in similar
gas-phase products. The reaction pathway involves a surface-
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