Article
pubs.acs.org/JPCC
Reaction of Nitrogen Dioxide with Ice Surface at Low Temperature
(≤170 K)
Jaehyeock Bang, Du Hyeong Lee, Sun-Kyung Kim,† and Heon Kang*
Department of Chemistry, Seoul National University, 1 Gwanak-ro, Seoul 151-747, Republic of Korea
S Supporting Information
*
ABSTRACT: We studied the adsorption and reaction of nitrogen dioxide gas
on the surface of an ice film at temperatures of 100−170 K under ultrahigh
vacuum (UHV) conditions. Cs+ reactive ion scattering (RIS) and low-energy
sputtering (LES) techniques were used to identify and quantify the reactants
and products on the surface of the ice film, in conjunction with the use of
temperature-programmed desorption (TPD) to monitor the species desorbed.
Temperature-ramping experiments were performed to examine the changes in
the populations of these species as a function of temperature. Adsorption of
NO2 gas on the ice film at <110 K produced physisorbed species that may
possibly possess negative charge character (NO2δ‑), as deduced from the NO2
and NO2− signals in the RIS and LES experiments. At 110−130 K, NO2δ‑
species were either desorbed as NO2 gas or converted to nitrous acid
(HONO), NO3−, and H3O+ on the surface. Nitrous acid gas was desorbed at
140−160 K. The efficiency of conversion of NO2 to surface nitrous acid was
about 40%, and that to nitrous acid gas was about 7%. The efficiency of the reaction of NO2 on the ice surface may be higher
than that at the gas/liquid water interface. The reaction efficiency increased with a decrease of the NO2 coverage and was
inversely correlated with the N2O4 coverage, which favors the mechanistic interpretation that an isolated NO2 molecule reacts
with water. However, NO2 can diffuse on the ice surface to form clusters at ≥120 K. Under these conditions, the possibility that
dimerization of NO2 contributes to the hydrolysis reaction of NO2 may not be excluded.
1. INTRODUCTION
The interaction between nitrogen oxide gases and the surface of
ice is an interesting research topic from the perspective of
fundamental chemistry as well as environmental atmospheric
chemistry. For example, nitrogen oxide gases react with ice
particles in cold atmosphere to produce nitric acids, and this
reaction may be of importance for stratospheric ozone
chemistry.1 The heterogeneous reactions of nitrogen oxide
gases with ice surfaces under conditions similar to the
atmospheric environment have been extensively studied in
laboratory model experiments.2−7
Surface science studies under ultrahigh vacuum (UHV)
conditions have also been undertaken to gain fundamental
insight into the interaction of nitrogen oxides with ice
surfaces.8−15 In these studies, ice samples were prepared on
well-defined substrate surfaces under UHV conditions at low
temperature in the form of a thin ice film or a codeposited film
of nitrogen oxides and water, and various surface spectroscopic
tools were used to analyze the samples. These spectroscopic
analytical tools include temperature-programmed desorption
(TPD) mass spectrometry, reflection absorption IR spectroscopy, photoelectron spectroscopy, molecular beam scattering,
reactive ion scattering (RIS), and low-energy sputtering
(LES).8−15 For evaluation of the reactions of nitrogen oxides,
techniques that facilitate identification of the chemical states of
surface species as well as desorbed gases are required. However,
© 2015 American Chemical Society
there are not many surface spectroscopic methods suitable for
this purpose at present. IR spectroscopy can be used to
determine the structure of various nitrogen oxides with the
assistance of theoretical calculation for analysis of the
spectrum.8−11,13 It is often difficult, however, with IR
spectroscopy to unambiguously identify the molecular
structures of adsorbates on the ice surface that have subtle
chemical differences. For this reason, the reactions of nitrogen
oxides on the ice surface and especially the mechanisms of
these reactions at the molecular level are still understood only
to a limited extent. What makes it more challenging is that the
mechanisms of reaction of nitrogen oxides are not fully
understood, even in the aqueous solution phase.16
The present study focuses on the reaction of nitrogen dioxide
(NO2), with special attention to identification of the chemical
states of the reactants and products formed on the ice surface
and of the gaseous products. In an earlier study, Kim et al.15
reported the conversion of nitrogen dioxide to nitrous acid on a
thin [ca. four bilayer (BL)] ice film grown on a Ru(0001)
substrate. The study could be performed only at temperatures
below ∼140 K, because thin ice films are unstable at a higher
temperature and undergo a roughening transition.17,18 Herein,
Received: June 9, 2015
Revised: September 3, 2015
Published: September 3, 2015
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DOI: 10.1021/acs.jpcc.5b05497
J. Phys. Chem. C 2015, 119, 22016−22024
Article
The Journal of Physical Chemistry C
we extend the temperature range of the investigation to ∼170 K
by preparing a thick (∼100 BL) ice film on a Pt(111) substrate.
By using a thick ice sample, the occurrence of the roughening
transition of the ice film can be significantly delayed, and the
possible interfering effects of a metal substrate to the reaction
on the ice film surface can be removed. Neutral molecules and
ions formed on the surface are identified using mass
spectrometry by performing RIS and LES experiments,
respectively. These results, combined with the detection of
gaseous species by TPD, enable us to elucidate the reaction
paths of NO2 on the ice surface.
temperature-ramping experiments. Whenever necessary, fresh
ice films were prepared for the measurements so as to reduce
the accumulated Cs+ beam dosage.
3. RESULTS
3-A. Identification of Surface Species Using RIS and
LES. NO2 gas was adsorbed onto an ice film sample and the
surface was analyzed using RIS and LES. Figure 1a shows the
2. EXPERIMENTAL SECTION
The experiments were performed in an UHV chamber
equipped with the instruments required to carry out surface
analysis by RIS, LES, and TPD.19,20 Ice films were grown on
the (111) face of a Pt single crystal by backfilling the chamber
with H2O or D2O vapor at a partial pressure of ∼1 × 10−7 Torr.
A crystalline ice film was grown by deposition of water vapor
on Pt(111) at 140 K to achieve a thickness of ∼100 BL, and the
sample was flash-annealed at 150 K to ensure formation of a
crystalline ice phase. The thickness of the ice film was
determined by TPD measurement. It was observed that ice
films of this thickness could maintain a flat surface with nearly
uniform film thickness without undergoing a roughening
transition until the sample temperature reached 165−170 K
during heating at a rate of 0.5 K s−1.18 NO2 gas was used after
purification by going through several freeze−pump−thaw
cycles, because NO2 slowly decomposed during storage in a
glass−stainless steel cylinder. NO2 gas was introduced into the
chamber through a leak valve and guided close to the sample
through a tube doser. The mass spectrum of purified NO2 gas
showed NO+ and NO2+ peaks with an intensity ratio of 10:1,
which confirmed the high purity of the gas with an insignificant
amount of NO impurity. During the exposure of the sample to
NO2 gas, the nominal gas pressure inside the chamber read by
an ionization gauge was 1 × 10−9 Torr, but the actual gas
pressure near the sample surface was significantly higher than
this value. TPD was used for accurate quantification of the
amount of adsorbed NO2 gas. NO2 surface coverage was
calculated by comparing TPD intensities of NO2 desorption
signals with the reference signal of water desorption from H2O
monolayer on Pt(111) and using the electron impact ionization
cross sections of NO2 and H2O.
RIS and LES experiments were conducted by collision of a
Cs+ ion beam from a low-energy ion gun (Kimball Physics)
with the sample surface at a kinetic energy of 35 eV, unless
otherwise specified. This procedure results in the ejection of
positive and negative ions from the sample surface as a result of
Cs+ collisions; the ejected ions are detected by a quadrupole
mass spectrometer (ABB Extrel) with the ionizer switched off.
In RIS experiments, neutral species at the surface are picked up
by the scattering Cs+ projectiles to form Cs+−neutral clusters.
In LES experiments, positive or negative ions that are
preformed on the surface are ejected by impact with lowenergy Cs+ ions. Secondary ionization of neutral species on the
ice surface is suppressed at the collision energy of 35 eV.
Therefore, RIS and LES signals respectively reveal the identities
of neutral and ionic species present on the sample surface.19,20
To minimize charging21 or surface contamination of the ice film
sample by Cs+ beams, the conditions of low Cs+ exposure were
employed, which was typically an incident Cs+ flux of 1 × 1011
ions cm−2 s−1 and a spectral acquisition time <100 s for
Figure 1. (a) RIS spectrum of D2O-ice film surface with adsorbed
NO2 at a coverage of about 0.04 MLE at 95 K [NO2 (0.04 MLE)/D2O
(∼100 BL)/Pt(111)]. (b) RIS spectrum for the same sample,
measured during a gradual increase of the temperature from 120 to
125 K. The D2O film was grown on Pt(111) to yield a thickness of
about 100 BL at 140 K, and the sample was briefly annealed at 150 K
to form a crystalline ice film with prominent Ih(0001) surfaces. NO2
gas was adsorbed on the ice film surface at 95 K.
RIS spectrum of the neutral species present on the surface of a
crystalline D2O film with adsorbed NO2 at a coverage of 0.04
monolayer equivalents (MLE) at 95 K. The NO2 coverage is
expressed in units of MLE, where 1 MLE is equivalent to the
water molecule density of Ih(0001), 1.05 × 1015 molecules
cm−2. The strongest signal in the spectrum corresponded to
Cs(D2O)+ at m/z = 153 amu/charge, where these species are
formed by capture of D2O molecules from the surface by Cs+
ions. The second most intense signal corresponded to
Cs(NO2)+ at m/z = 179, as shown in the magnified view
(inset), which indicates the presence of NO2 adsorbates on the
surface. In addition, a peak derived from Cs(N2O4)+ was
observed at m/z = 225 for which the intensity was much less
than that of Cs(NO2)+. The Cs(N2O4)+ signal indicates the
presence of a minute quantity of N2O4 on the ice surface, which
may be formed by dimerization of NO2 adsorbates.
The RIS spectrum presented in Figure 1b was acquired while
the sample was gradually heated from 120 to 125 K. Therefore,
the spectrum displays all the species formed in the temperature
range of 120−125 K. Essentially all of the species observed in
Figure 1a were detected in Figure 1b. Notably, the intensity of
the originally weak signal at m/z = 181 increased; the relative
intensity of this peak versus the peak at m/z = 179 increased
from 4% at 95 K to 12% at 120−125 K. The signal at m/z =
181 can be interpreted as arising from two species: a nitrogen
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dioxide isotopolog, Cs(N16O18O)+, which has a natural
abundance of 0.41%, and nitrous acid Cs(DNO2)+.
Figure 2 shows LES signals of the negative ions present on
the surface of a crystalline ice film (H2O) with adsorbed NO2 at
Figure 2. LES spectrum of H2O-ice film surface with adsorbed NO2 at
a coverage of ∼0.04 MLE [NO2 (0.04 MLE)/H2O (∼100 BL)/
Pt(111)]; the spectrum was acquired while the temperature increased
from 105 to 114 K. The H2O sample was prepared in the same way
used for the D2O sample in Figure 1. NO2 gas was adsorbed at 95 K.
Figure 3. (a) Graphical raw data for TPRIS evaluation of D2O-ice film
with adsorbed NO2 at low coverage [NO2 (0.04 MLE)/D2O (∼100
BL)/Pt(111)]. (b) RIS yield curves for NO2, DNO2, and N2O4
(converted from the data presented in part a). (c) TPRIS
measurement data for the signals of interest from a D2O-ice film
with high NO2 coverage [NO2 (0.5 MLE)/D2O (∼100 BL)/Pt(111)].
Cs(N2O3)+ curve is omitted for simplicity. (d) RIS yield curves for
NO2, DNO2, N2O3, and N2O4 converted from the data shown in part
c. The D2O films were prepared in a crystalline phase as described in
the caption of Figure 1. The temperature-ramping rate was 0.5 K/s.
The experimental error bars are shown in light colors for the RIS yield
curves.
a coverage of 0.04 MLE at 95 K. The LES spectrum was
accumulated from data acquired while the temperature was
increased from 105 to 114 K. The sample was prepared in
essentially the same manner as that used for the sample in
Figure 1a, except that H2O was used instead of D2O. The
choice of H2O or D2O was made only for the purpose of easy
distinction of the mass spectrometric signals, i.e., to avoid
overlapping of various signals of hydrolyzed NO2 species. In
terms of the reactivity of NO2, no difference was detected with
the use of H2O or D2O. LES signals were observed for NO2−
(m/z = 46) and NO3− (m/z = 62). These LES signals, along
with the signal of Cs(DNO2)+ in the RIS analysis, provide clear
evidence that NO2 reacts with water on the ice surface at these
low temperatures.
Ice films with higher NO2 coverage (0.5 MLE) were also
examined, and their RIS and LES spectra are provided in the
Supporting Information. The surface species present in the
high-coverage samples were similar to those observed at low
coverage, as shown in Figures 1 and 2, viz., NO2, DNO2, NO2−,
and NO3−. Some differences were also observed. For the
samples with high NO2 coverage, a new signal corresponding to
Cs(N2O3)+ (m/z = 209) appeared, and the intensity of the
Cs(N2O4)+ signal increased. These changes indicate the
occurrence of NO2 dimerization to form N2O3 and N2O4, in
agreement with the previous reports.9−11,15 No drastic changes
were observed for the NO2− and NO3− signals compared to the
spectrum of the low-coverage samples (Figure 2), except that
the intensity of the NO3− peak was stronger than that of the
NO2− peak at high NO2 coverage.
3-B. Temperature-Dependent Variation of Surface
Species. Temperature-programmed RIS and LES (TPRIS and
TPLES) experiments were performed to evaluate the
appearance and disappearance of various surface species as a
function of temperature. TPRIS and TPLES are useful
techniques for monitoring the progress of the reaction on the
ice surface as a function of temperature by detecting the
generated reaction intermediates and products.22 Figure 3a
shows the results of TPRIS measurement for the signals of Cs+,
Cs(NO2)+, Cs(DNO2)+, and Cs(N2O4)+ on the D2O-ice
sample with low NO2 coverage (0.04 MLE). The intensity of
all the RIS signals declined sharply above ∼125 K [Cs(D2O)+
signal also; not shown]. These intensity drops are thought to be
due to changes in the ice surface morphology that affect the
scattering efficiency of Cs+ ions, rather than due to changes in
the population of surface species.19,23 Structural transformation
of the ice surface may possibly cause this change, as has been
observed for an amorphous solid water (ASW) film at a similar
temperature (∼120 K) in previous studies.23 Also, the reaction
of NO2 may disrupt the original crystallinity of the ice surface,
thereby reducing the scattering efficiency of Cs+ ions. We have
checked that the intensity change is not due to charging of the
ice film by incident Cs+ beams.21 The intensity of the RIS peaks
increased again sharply near 170 K (cutoff region). This change
is due to the roughening transition of the thinning ice film as a
result of evaporation of water, which exposes patches of
monolayer water on the Pt(111) surface.17,18 It is necessary to
discriminate the variations in the RIS intensity due to the
surface condition changes from those due to changes in the
surface species. For this, the absolute RIS intensity was
converted to the RIS yield.24 The RIS yield represents the
normalized RIS intensity and is defined as Y(x) = Ix/∑Ii, where
Ix is the RIS intensity of species x and ∑Ii is the summation
over all RIS signals, including Cs+. The RIS yield, Y(x), is
proportional to the surface population of species x and the
specific detection efficiency of species x by RIS.24
The RIS yields [Y(x)] of NO2, DNO2, and N2O4 were
calculated from the RIS intensities (shown in Figure 3a) and
are plotted as a function of temperature in Figure 3b. Y(NO2)
decreased sharply in the range of 120−130 K and became
undetectable above ∼132 K. Y(DNO2) increased monotonically up to ∼130 K and then gradually decreased at higher
temperatures. Y(N2O4) remained at a low concentration level
and became undetectable above ∼130 K. Notably, DNO2
species, a product of NO2 hydrolysis, were formed even at
low temperature (≤100 K), although the population was small.
Another notable feature is that the temperature dependences of
Y(DNO2) and Y(N2O4) exhibited inverse correlation, whereby
the DNO2 curve rises with temperature in the region of 110−
130 K, whereas the N2O4 curve falls.
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Parts c and d of Figure 3 show the RIS intensities and the
RIS yield curves, respectively, acquired for a D2O-ice sample
with high (0.5 MLE) NO2 coverage. The experiment was
performed to compare the results obtained at low and high
NO2 coverages and to evaluate the correlation between DNO2
formation and the formation of dimeric NO2 species, such as
N2O3 and N2O4. The results obtained at high coverage were
qualitatively similar to those obtained at low coverage, i.e.,
Y(NO2) decreased sharply in the region of 130−135 K.
Y(DNO2) increased monotonically up to ∼140 K and then
decreased at higher temperatures. Y(N2O3) and Y(N2O4)
dropped to zero above ∼135 K, although Y(DNO2) increased
at this temperature. The absolute populations of N2O3 and
N2O4 species increased at high NO2 coverage, but the relative
abundance of these samples compared to the other species did
not increase noticeably.
The inverse correlation between the populations of DNO2
and N2O4 as a function of temperature at low as well as high
NO2 coverage suggests that N2O4 is not a precursor to DNO2
formation, and vice versa. This interpretation arises because the
RIS yield curves of the TPRIS measurement represent the
populations of various surface species present at the given
temperature of the sample, rather than representing the
temporal change of the populations of species involved in
sequential reactions.22 The observation agrees with those of a
previous study conducted for thin (∼4 BL) ice films at higher
NO2 coverages (0.15−6.5 MLE).15 These results suggest that
hydrolysis of NO2 occurs as a result of the interaction between
a single molecule of NO2 adsorbed on the surface and water
molecules, rather than through the formation of N 2 O4
intermediates. At an NO2 coverage of 0.04 MLE in the present
study, the NO2 adsorbates are separated by four water
molecules on average, and the NO 2−water interaction
dominates over the NO2−NO2 interaction. This argument
will be valid only when the temperature is low enough to
suppress the diffusion of NO2 on the surface to form NO2
clusters.
Figure 4 shows the results of TPLES analysis of the NO2−
and NO3− anion signals. The intensity of LES signals varies
with changes in the morphology of the ice surface, in addition
to changes in ion population. To measure the LES intensity
variation due to the surface morphology change, we used Br−
ions as an internal reference (Supporting Information). As
such, Br− ions were added to the NO2-adsorbed ice film by
depositing a small amount (∼0.01 MLE) of NaBr vapor
generated from a thermal evaporation source. NaBr is
spontaneously ionized to Na+ and Br− ions on the ice surface,
and Br− has a thermodynamic tendency to reside on the ice
surface, whereas Na+ migrates from the ice surface to the
interior.25 These ions did not affect the reaction of NO2 with
ice surface based on RIS and LES analysis. The intensities of
the NO2− and NO3− peaks for this sample were divided by the
intensity of the Br− signals. The thus-calibrated intensities of
the NO2− and NO3− signals may discriminate the effects of
changes in the surface morphology. However, even the
calibrated LES intensities may not be quantitatively proportional to the population of the ions because the sputtering
efficiency can be quite different for NO2− and NO3−.
Figure 4a shows the temperature-dependent variation of the
calibrated intensities of the NO2− and NO3− peaks for the
H2O-ice sample with low (0.04 MLE) NO2 coverage. The
results obtained at high (0.5 MLE) NO2 coverage are shown in
Figure 4b. For both samples, NO2− and NO3− signals were
apparent even at low temperature, where the intensity of the
NO2− peak was higher than that of the NO3− peak. The NO3−
signal became stronger than the NO2− signal at high
temperature. However, for detailed features, the two samples
have quite different TPLES profiles. In general, the shapes of
the TPLES curves indicate that NO2− is formed prior to NO3−
during a temperature scan. However, this does not mean that
NO2− is a precursor to NO3− formation, and NO2− and NO3−
may be produced through independent routes.
Given that NO2− and NO3− were formed in the surface
reaction, it is natural to expect that countercations should also
be produced by the reaction. The hydronium ion is the most
likely candidate as a positive ion product. However, positive ion
signals were either undetected or were of extremely low
intensity, in contrast with the strong intensity of the negative
ion signals, unless the extent of NO2 exposure was large. The
undetectability of the hydronium ion signal may either be due
to its absence from the surface or its low sputtering efficiency.
To detect the hydronium ions, the amount of NO2 adsorption
had to be increased substantially. Figure 5 presents the results
Figure 5. TPLES curves of positive ions generated from H2O-ice film
with a large amount (1.2 MLE) of adsorbed NO2 [NO2 (1.2 MLE)/
H2O (∼100 BL)/Pt(111)]. The intensity of the TPLES peaks was not
calibrated. The temperature-ramping rate was 0.5 K/s.
Figure 4. (a) TPLES curve of negative ions measured for H2O-ice film
with low NO2 coverage [NO2 (0.04 MLE)/H2O (∼100 BL)/
Pt(111)]. (b) Corresponding results for sample with high NO2
coverage [NO2 (0.5 MLE)/H2O (∼100 BL)/Pt(111)]. The samples
were prepared using the same procedures described in the caption of
the previous figures. The curves for NO2− and NO3− show the
intensities calibrated relative to the Br− reference signal (see the text).
The temperature-ramping rate was 0.5 K/s.
of positive ion TPLES measurement for the sample with a NO2
coverage of ∼1.2 MLE. Signals of H3O+ and its hydrated
(H5O2+) species appeared above ∼110 K, increased in intensity
up to ∼125 K, and then decreased at higher temperatures. The
intensity of the signals was not calibrated against a reference
signal because of the lack of appropriate internal standards for
the positive ions. The changes in the TPLES curve therefore
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DOI: 10.1021/acs.jpcc.5b05497
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Comparison of the intensity of the peaks for NO+ and NO2+
desorption at 125 K (Figure 6a) shows that the ratio of the
NO+:NO2+ peak areas is 11:1. This ratio agrees with the
fragmentation pattern of NO2 gas from mass spectrometry,7
which further confirms that the 125 K peak corresponds to
NO2 desorption. Likewise, for the 150 K peak, the intensity
ratio of NO+ to DNO2+ is 80:1. The mass spectrometric
fragmentation pattern of DNO2 gas is unknown and thus could
not be used for comparison. Nevertheless, there appears to be
no alternative possibility other than that the 150 K peak of NO+
is produced by ionization of DNO2 gas, for the reasons
mentioned above. Therefore, the area ratio of the 125 and 150
K peaks in the NO+ curve (which is 12:1) may represent the
relative amounts of desorbed NO2 and DNO2 gas. More
accurate estimation of this ratio can be made by summing the
intensities at m/z = 30, 46, and 48 for the corresponding peaks.
The calculation shows that the ratio of NO2 to DNO2 gas in the
total desorption flux is 13:1.
Figure 6b shows the results of TPD analysis of the high NO2
coverage (0.5 MLE) D2O-ice film. The same species were
desorbed from the high coverage sample as those observed at
low NO2 coverage. However, the temperatures at which
desorption occurred were significantly different. The NO2
desorption peak shifted from ∼125 K at low NO2 coverage
to a higher temperature (∼133 K) at high NO2 coverage. An
exponential increase was observed for the low-temperature side
of the peak. In the temperature region of 120−125 K, the TPD
curves for the samples with low and high NO2 coverage (Figure
6a,b) are almost superimposable on each other when their
absolute heights are compared. These features indicate zeroorder desorption of NO2 gas. The peak corresponding to
DNO2 desorption, which appeared for the m/z = 30 and 48
signals, was broadened and shifted toward a lower temperature
(130−150 K) at high NO2 coverage. Analysis of the TPD peak
areas indicates that the desorption fluxes of NO2 and DNO2
occurred in a ratio of 15:1 from this sample.
The observation that the NO2 desorption kinetics follows the
zeroth order even at submonolayer NO2 coverage is quite
surprising. Such behavior can be explained if the NO2
molecules are sufficiently mobile during TPD such that they
can diffuse on the surface and aggregate into NO2 clusters.
Desorption of NO2 from the aggregated clusters would follow
zeroth-order kinetics, rather than the first-order kinetics
expected for the desorption of dispersed NO2 adsorbates. If
NO2 on the ice surface is highly diffusive and exhibits a
tendency toward dimerization, the possibility that NO2
dimerization contributes to NO2 hydrolysis cannot be complete
excluded, even when the NO2 coverage is low.
reflect the changes in the population of ions on the surface as
well as changes in the surface morphology.
3-C. Temperature-Programmed Desorption Study.
Three mass spectrometric signals related to the reaction of
NO2 were detected at m/z = 30, 46, and 48 in the temperatureprogrammed desorption experiments. The TPD curves of these
signals at low (0.04 MLE) and high (0.5 MLE) NO2 coverages
are displayed in parts a and b of Figure 6, respectively. The
Figure 6. (a) TPD curves for D2O-ice film sample with low NO2
coverage [NO2 (0.04 MLE)/D2O (∼100 BL)/Pt(111)]. (b) TPD
curves for sample with high NO2 coverage [NO2 (0.5 MLE)/D2O
(∼100 BL)/Pt(111)]. The temperature-ramping rate was 0.1 K/s for
all scans, except for the scan detecting DNO2 desorption at low
coverage in part a, for which the ramping rate was 0.5 K/s and the
intensity of the DNO2 desorption peak was thus 5 times larger. The
curves are magnified by the factors indicated. The m/z = 48 curve
(blue) indicates the intensity of DNO2+ only (see the text).
NO+ signal (m/z = 30) can be produced by electron impact
ionization of NO, NO2, and DNO2 gases inside the mass
spectrometer ionizer. The NO2+ signal (m/z = 46) can likewise
be produced from NO2 and DNO2 gases. The signal at m/z =
48 may either be the parent ion signal of DNO2 or an
isotopolog of nitrogen dioxide (N16O18O), or both. The
portion of this signal arising from DNO2 was estimated by
subtracting the intensity of the N16O18O+ peak from the m/z =
48 signal, which was calculated from the intensity of the N16O2+
peak at m/z = 46 and the isotopolog ratio N16O2:N16O18O =
1:0.0041. The signal at m/z = 48 displayed in Figure 6
represents the DNO2+ component only, which was extracted in
this way.
The profile for desorption of NO+ from the low NO2
coverage (Figure 6a) sample showed an intense peak at ∼125
K and a weak peak at ∼150 K. On the basis of the RIS analysis
(Figures 1 and 3), the sample surface was free of NO
adsorbates at these temperatures. Therefore, the NO+ signal
must arise from the desorption of NO2 and/or DNO2
adsorbates. Comparison of the profiles of the NO+, NO2+,
and DNO2+ curves shows that the peak at 125 K was common
to the NO+ and NO2+ curves but was absent from the DNO2+
curve. On the other hand, the peak at 150 K appeared in the
NO+ and DNO2+ curves but not in the NO2+ curve. These
features indicate that the peak at 125 K is due to desorption of
NO2 gas, whereas the peak at 150 K is due to desorption of
DNO2 gas. The result also indicates that the electron impact
ionization of DNO2 gas preferentially produces NO+ rather
than NO2+. This is consistent with the interpretation that
DNO2 gas is nitrous acid (DO−NO), indeed, rather than D−
NO2.
4. DISCUSSION
4-A. Reaction Path and Efficiency. TPRIS, TPLES, and
TPD measurements provide information about changes in the
surface and gaseous products induced by increasing temperature. Here, we discuss the temperature-dependent variation of
these products and their mutual correlation and complementarity, which is useful for understanding the pathway for
reaction of NO2 on the ice surface. For easy comparison, the
TPRIS, TPLES, and TPD results of several key species
measured at low NO2 coverage are combined into one graph
in Figure 7. First, we consider the NO2 species. The decrease of
the RIS yield for NO2 at 120−130 K correlates with the
appearance of a NO2 TPD peak in the same temperature
region. This indicates that the decrease in the population of
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DOI: 10.1021/acs.jpcc.5b05497
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enough to deduce the quantitative correlation between the
various products.
The efficiency of conversion of adsorbed NO2 to DNO2 may
be estimated from the TPRIS and TPD data. First, the amount
of DNO2 on the surface can be estimated from the TPRIS data.
Figure 7 shows that the highest value of Y(DNO2) at ∼130 K is
about 40% of the Y(NO2) value at ∼110 K, which corresponds
to the initial population of NO2 adsorbates (0.04 MLE) prior to
thermal desorption. Thus, the surface population of DNO2 at
∼130 K was determined to be ∼0.016 MLE. In this estimation,
we assume that the RIS efficiencies are the same for DNO2 and
NO2 and are independent of the condition of the ice surface,
which may be a reasonable approximation for temperatures of
100−130 K to provide an estimation at the order-of-magnitude
level of accuracy. NO2 and DNO2 gases are desorbed in a ratio
of 13:1 for this sample on the basis of the TPD results
presented in Section 3-C. Thus, the amount of DNO2 gas
desorbed is estimated to be 0.04 MLE × (1/14) ≈ 0.003 MLE.
Here, we assume that all NO2 adsorbates are detected by TPD,
irrespective of the form of the desorbing species, and none
remain on the surface after evaporation of the ice film; this was
checked by TPD scans up to 300 K (not shown). The efficiency
of conversion of surface DNO2 to nitrous acid gas is 0.003
MLE/0.016 MLE ≈ 20%. These analyses demonstrate the
efficient conversion of NO2 to nitrous acid on the cold ice
surface: about 40% of the adsorbed NO2 molecules is converted
to surface nitrous acid, and eventually about 7% is desorbed as
nitrous acid gas.
It is difficult to quantitatively measure the populations of
NO2− and NO3− ions from their LES intensities alone, even
after their intensity calibration with Br−, as mentioned above.
This is because these ions may be ejected from the surface via
collisional fragmentation of larger molecular structures that are
hydrogen-bonded to the ice surface, rather than existing as
isolated NO2− and NO3− units on the surface and being
sputtered intact during LES. This can make the sputtering
efficiency differ significantly for NO2− and NO3−, and also for
Br−. The NO2− and NO3− signals persist up to the temperature
of ice film sublimation (160−175 K). TPD experiments show
that the amount of NO 2 and DNO 2 gases that are
simultaneously desorbed upon sublimation of the ice film at
160−175 K is on the order of 0.001 MLE (Figure 7). We may
consider that these desorbed gases are formed partly as a result
of desolvation of NO2− and NO3− species incorporated in the
ice film during the evaporation of multilayer water. This means
that the amount of ionic products formed in the sample may be
less than ∼0.001 MLE. In this estimation, we have neglected
the ions that possibly remain on the Pt surface after water
evaporation, which are indicated by the fluctuating NO2− and
NO3− intensities above ∼172 K in Figure 7. The populations of
these ions may be substantially smaller than the ion populations
created at the surface of ice film at 140−170 K, if we consider
that LES efficiency of ions is usually very high on a bare metal
surface compared to that on an ice surface.
On the basis of the TPRIS, TPLES, and TPD data, a pathway
for reaction of NO2 is proposed, as shown in Scheme 1. First,
the adsorption of NO2 on the ice surface at ≤100 K may
initially produce physisorbed species in which NO2 is hydrogen
bonded to the ice surface and exists as a partially negatively
charged NO2δ‑ moiety in certain adsorption structures. Upon
heating the surface at 110−130 K, the physisorbed NO2δ‑
species are either desorbed as NO2 gas or react with water to
form surface HONO and other species that plausibly contribute
Figure 7. Comparison of TPRIS, TPLES, and TPD results. RIS yield
curves (NO2, DNO2, and N2O4; left ordinate scale) are presented as
solid lines. LES curves (NO2− and NO3−; right ordinate scale) are
presented as dashed lines. TPD curves are shaded: NO2 (red), DNO2
(blue), and D2O (pink). The heights of the TPD curves are arbitrarily
rescaled for easy comparison at the different desorption temperatures.
Data are provided for temperatures up to 175 K, higher than the cutoff
temperature (170 K) of the other figures. The ice film has desorbed
almost completely at 175 K, leaving only water monolayer on Pt(111).
The second peak of DNO2 TPD signal centered around 172 K
coincides with evaporation of multilayer water, which may cause
desolvation of DNO2 and other dissolved species in the ice film.
NO2 species on the ice surface at these temperatures is mainly
due to NO2 desorption. Furthermore, the NO2− signal in the
LES profile shows a similar variation to the NO2 signal in the
RIS profile at temperatures below ∼115 K. The similar
variation of NO2 and NO2− signals at low temperature
appeared for differently prepared samples also, including
those with different NO2 coverage, shown in Figure S3 of
Supporting Information. This behavior may indicate that the
NO2 and NO2− signals originate from the same surface species.
If we give credence to this interpretation, then the NO2
adsorbate is considered to be partially negatively charged
(NO2δ‑) at low temperature (<115 K). At higher temperatures,
other surface species may also contribute to the NO2− signal.
The increase in the intensity of the DNO2 peak in the RIS
curve in the region of 100−130 K occurs in conjunction with
the increase in the intensity of the NO3− peak in the LES
profile. This correlation suggests the possibility that a single
reaction channel produces both DNO2 and NO3−. At >130 K,
the population of DNO2 is higher than that of NO2, and DNO2
becomes the most abundant neutral species on the surface. The
surface population of DNO2 decreases in the temperature
region of 130−160 K, which correlates with the TPD peak of
DNO2 at 140−160 K. This shows that desorption of DNO2 gas
contributes to the decrease of the population of DNO2 on the
ice surface. It is demonstrated below that the contribution of
DNO2 desorption is about 20%. Therefore, additional channels
must operate to deplete the surface population of DNO2 at
130−160 K. One possibility is the conversion of DNO2 to ionic
species such as NO2− and NO3−. However, no distinct
correlation was observed between the temperature dependence
of DNO2 and these ions at the corresponding temperatures.
The LES and RIS techniques can detect only the species
forming at the outermost surface, whereas the products below
the surface cannot be detected. At high temperature (>130 K),
molecular diffusion is activated in the ice film, and certain
species may migrate from the surface to the subsurface region,
depending on their thermodynamic preference for residing on
the ice surface or in the interior. Under such circumstances, the
LES and RIS measurements of surface populations may not be
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or on ice surfaces even at 100 K. Finlayson-Pitts and coworkers35−37 examined the heterogeneous reaction of adsorbed
NO2 with water films by using IR spectroscopy, and they
proposed a mechanism in which NO2 reacts with water through
NO2 dimer intermediates that can autoionize to NO+NO3− and
facilitate further reaction. On the ice surface, NO2 can diffuse
and aggregate into clusters at temperatures above ∼120 K, as
indicated by the TPD observations. For this reason, one cannot
rule out the possibility that dimerization of NO2 plays a role in
the reaction of NO2 on the ice surface, even at low NO2
coverage (0.04 MLE). On the other hand, the inverse
correlation between the NO2 reaction efficiency and the
population of N2O4 does not support the occurrence of the
dimeric reaction.15 To fit together these observations, a few
scenarios may be possible. One possibility is that the energy
barrier for the reaction of isolated NO2 molecules on ice surface
is much lower than that predicted by the theoretical
calculations with small water clusters (∼120 kJ mol−1).34
Recent studies38 indicate that theoretical modeling of reactions
on ice surface requires proper inclusion of long-range
electrostatics and surface heterogeneity, which have been
neglected in the water cluster calculations. Another possibility
is that dimerization of NO2 occurs extremely fast such that the
rate of dimerization does not appear in the overall rate
expression for NO2 hydrolysis. Apparently, the mechanism for
the reaction of NO2 with ice or water surface still remains an
interesting problem that requires further study.
NO2 is efficiently converted to nitrous acid on the cold ice
surface. About 40% of the adsorbed NO2 molecules are
converted to surface nitrous acid, and eventually, about 7% of
the NO2 molecules is desorbed as nitrous acid gas. The facile
occurrence of the reaction at low temperature indicates that the
energy barrier of the reaction is indeed very low. Because the
probability of NO2 gas sticking on the ice surface is close to
unity at 100 K, the observed efficiency for conversion of surface
NO2 to nitrous acid can be regarded as the reaction efficiency
of NO2 gas colliding with the surface. For comparison, the mass
accommodation coefficient of NO2 gas on the surface of liquid
water is 1.5 × 10−3 at room temperature.39 Apparently, the
efficiency of the reaction of NO2 gas on the ice surface at low
temperature is greater than that of the reaction on the surface
of liquid water at room temperature. Owing to such a low
energy barrier and the inverted temperature effect, one may
expect that the heterogeneous reaction of NO2 will be efficient
on the surfaces of ice particles in the cold atmosphere.
Scheme 1. Proposed Pathway for Reaction of NO2 on Ice
Surface
to the NO2− and NO3− signals. The concomitant detection of
hydronium ions above ∼110 K suggests that the ionic products
formed on the surface may be similar to nitric acid. Nitrous acid
may also partially ionize and contribute to the H3O+ and NO2−
signals. The surface HONO species are desorbed as HONO gas
at 140−160 K. Finally, evaporation of the ice film at >160 K
reconverts the dissolved ionic species into NO2 and HONO
gas.
Interestingly, the products and intermediates detected in the
present experiment include all the products of NO2 hydrolysis
in aqueous solution (reaction 1):
2NO2 (g) + H 2O(l)
→ HONO(g) + H+(aq) + NO3−(aq)
(1)
Owing to the similarity of products, it may be claimed that NO2
is as completely hydrolyzed on the ice surface as in aqueous
solution. As a counter argument, it may be possible that the
observed reactions on the ice surface only correspond to certain
intermediate stages of NO2 hydrolysis given that the mobility of
water molecules is low and the solvation of adsorbed species
must be incomplete. Another notable feature is the formation
of diverse hydrolysis products, even at the lowest temperature
investigated (≤100 K), although their formation yield is
relatively small. The diversity of reaction channels may be a
unique characteristic of the ice surface, which may be related to
the fact that the ice surface provides a wide variety of binding
sites and water-coordinating environments for adsorbing
molecules owing to different dangling bond arrangements of
the surface and different dipole directions of neighboring water
molecules.22,26
4-B. Comparison of NO2 Reactions on Ice and in
Liquid Water Films. It is well-known27 that the heterogeneous reactions of NO2 on wet surfaces produce HONO gas,
which may be important as a precursor for hydroxyl radicals in
polluted urban environments. Kinetic studies of this reaction in
the laboratory model experiments show that the rate of
formation of nitrous acid is first-order with respect to the
concentrations of NO2 and water vapor.28−33 These observations refute the possibility that reaction 1 occurs in a single-step
as a termolecular process or involves NO2−NO2 encounter as
the rate-determining step. This mechanistic interpretation of
the reaction on water films is consistent with the observations
on ice films, whereby the efficiency of the reaction of NO2
decreases with increasing NO2 coverage and is inversely
correlated with the surface population of N2O4.15 The simplest
mechanism that can be deduced from these observations is that
NO2 hydrolysis on ice occurs through the reaction between an
isolated NO2 molecule and water.
Quantum chemical calculations indicate that the reaction of
NO2 with small water clusters requires a substantially high
activation energy (∼120 kJ mol−1), and the energy barrier is
not significantly reduced by the effect of water solvation.34
There is an apparent discrepancy between this high energy
barrier for the NO2 reaction with water cluster and the facile
occurrence of the reaction on water films at room temperature
5. SUMMARY AND CONCLUSION
In the present study, the interaction of NO2 gas with ice surface
was investigated by preparing a thick ice film that could be
maintained under UHV up to ∼170 K, which is close to the
stratospheric temperature (>180 K). The use of a thick ice film
eliminated the possibility that the reaction on the sample
surface is affected by the metal substrate or a roughening
transition of the ice film; these effects could interfere with the
measurement of the reaction on a thin (∼4 BL) ice film in the
previous work.15 Also, TPRIS and TPLES measurements were
performed under the conditions of low NO2 coverage to
investigate the reaction mechanism more closely. The following
deductions about the reaction of NO2 on the ice surface could
thus be drawn.
(i) Adsorption of NO2 gas on the ice surface produces
various surface species, including NO2δ‑, HONO, NO3−, H3O+,
and N2O4. Among these, NO2δ‑ is the major species present on
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the surface at low temperature (<110 K), and it may be present
as an NO2 moiety with partial negative charge character in the
hydrogen-bonded adsorption structures. Upon heating the
sample at 110−130 K, the NO2δ‑ species are either desorbed as
NO2 gas or react with the ice surface to form HONO, NO3−,
and H3O+. Nitrous acid gas is desorbed from the surface at
140−160 K. Interestingly, many of these species are formed on
the surface even at ∼100 K, albeit in low populations.
(ii) The formation of nitrous acid shows no correlation with
the presence of N2O4 on the ice surface. The conversion
efficiency of NO2 to nitrous acid gas increases with a decrease
in the NO2 coverage. 15 These observations favor the
mechanistic interpretation that hydrolysis of NO2 occurs via
the interaction between an isolated NO2 molecule and water
rather than through the formation of N2O4 species. However,
NO2 molecules can diffuse on the ice surface and form clusters
at ≥120 K. For this reason, the possibility that NO2 clusters
play a role in the hydrolysis of NO2 cannot be ruled out
completely.
(iii) The efficiency of conversion of NO2 to surface nitrous
acid is about 40%, and the efficiency for conversion to nitrous
acid gas is about 7%. The efficiency of the reaction of NO2 on
the ice surface is higher than the mass accommodation
coefficient of NO2 gas at the gas/liquid water interface.
(iv) The observed hydrolysis of NO2 on the ice surface at
low temperature indicates that the reaction has a low energy
barrier. This observation suggests that the heterogeneous
reaction of NO2 may be efficient on ice particles in cold
atmospheric environment.
■
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpcc.5b05497.
RIS and LES spectra of ice samples with high NO2
coverage (0.5 MLE) (Figures S1−S3) (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: surfi[email protected]. Fax: +82-2-875-7471.
Present Address
†
Chemistry Track, Natural Science Department, Korea Air
Force Academy, Cheongwon, Chungbuk 363-849, Republic of
Korea.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was supported by Samsung Science and Technology
Foundation (SSTF-BA1301-04).
■
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