Identification of Surface Molecular Hydrates on Solid Sulfuric Acid Films

Article abstract of DOI:10.1021/ja036384l

Infrared spectroscopic and secondary ion mass spectrometric studies reveal the presence of a stable molecular hydrate on the surface of condensed thin films of ionic sulfuric acid hydrates. This surface species is observed to play a role in the interaction of ammonia, reacting rapidly until the material is depleted. A slower, continuous process is also observed, attributed to a diffusion-limited reaction between incoming NH₃ and H₃O⁺ located at or near the surface. Copyright

Full text of DOI:10.1021/ja036384l

Published on Web 10/04/2003
Identification of Surface Molecular Hydrates on Solid Sulfuric Acid Films
Suzanne B. Couling,† Karen L. Nash,† John Fletcher,‡ Alex Henderson,‡ John C. Vickerman,‡ and
Andrew B. Horn*,§
Department of Chemistry, UniVersity of York, Heslington, York, YO1 5DD, UK, Department of Chemistry, UMIST,
SackVille Street, Manchester M60 1QD, UK, and Department of Chemistry, UniVersity of Manchester, Oxford Road,
Manchester M13 9PL, UK
Received May 28, 2003; E-mail: andrew.b.horn@man.ac.uk
In this communication, we report observations of the presence
of a surface molecular hydrate on condensed (solid) sulfuric acid
hydrate films. The composition of this surface layer has been
established through infrared spectroscopy and static secondary ion
mass spectrometry (SSIMS). Such surface layers serve to insulate
the ionic bulk of the hydrate and may play an important role in
atmospheric reactions and nucleation processes.
The reactive uptake of atmospherically relevant species upon
particulate material in the atmosphere is known to show a strong
dependence upon surface composition, bulk mobility, morphology,
and temperature.1 Such experiments identify the availability of water
as one of the main parameters influencing reactivity through both
physical and chemical effects. For condensed aerosol particles such
as those containing sulfuric acid, the availability of water at the
Figure 1. ATR-IR spectra of films of SAM at 200 K. (a) Thin (100 nm);
(b) thick (>250 nm); (c) scaled subtraction: [(a) -0.36(b)].
interface (and the rate at which it can be replaced from the bulk)
is the rate-limiting factor in heterogeneous hydrolysis processes
relevant to the upper troposphere and lower stratosphere. To
parameterize the reactivity of a range of aerosol materials for
inclusion in atmospheric models, it is essential that the mechanisms
of the surface processes involved are understood. For example,
although surface reactivity often relates directly to bulk composition,
there are frequent examples in the literature where reactivity varies
with time as a result of surface depletion or poisoning in a way
that is not reflected in the overall bulk composition. In recent years,
the application of spectroscopic2,3 and surface mass spectrometric4,5
methods have enabled detailed examination of such surface-specific
chemistry, leading to observations such as the saturation reaction
of ice particles with ClONO2, where reactivity ceases once the
surface is fully covered with nitric acid hydrates. Saturation effects
have also been observed in kinetic studies in coated wall flow tubes,
although the surface products or intermediates could not be directly
identified. It is only from the synergy between mechanistic and
kinetic observations and theory that a detailed understanding of
such systems has been obtained.
Figure 2. Infrared spectra of sulfuric acid hydrates and reference spectra.
(a) Absorbance difference spectrum of SAM exposed to 1 × 10-7 mbar
NH3 for 300 s versus clean SAM; (b) bulk SAM; (c) molecular hydrate;
(d) sodium bisulfate reference.
Figure 1 shows IR spectra of thin (100 nm) and thick (250 nm)
films of sulfuric acid monohydrate (SAM) deposited at ca. 210 K
from the reaction between SO3 and H2O, obtained in an attenuated
total internal reflection geometry.6 Given the nature of the optical
configuration, the thick films are largely representative of the bulk
composition, whereas the thin film shows both surface and bulk
features. Scaled subtraction of the bulk spectrum from that of the
bulk + surface (Figure 1c) reveals the presence of a second material
which has a spectrum characteristic of a recently identified
molecular hydrate7 (Figure 2c). The main IR absorption bands of
this material which distinguish it from other sulfuric acid hydrates
are the strong, broad resonances at ca. 1260 and 780 cm-1. Thus
this material, predicted to be stable by theoretical studies,8 is also
clearly present on the surface of SAM films. SSIMS analysis,
including depth profiling, of SAM films deposited in a similar
manner also demonstrates the existence of a surface layer over the
bulk films. Spectra recorded of the film surface indicate a mixture
of acid species, with water availability during deposition represent-
ing the primary influence on the composition of this interface. SIMS
depth profiles of thin films provide evidence that at deposition
temperatures of 200 K the surface layer is not mobile.9 That these
molecular surface layers play a role in the surface chemistry of
condensed sulfate hydrates is confirmed by observations of the
interaction of such films with gaseous NH3. Exposure of a 100 nm
thick SAM film at 220 K to a flux of NH3 results in the formation
of the characteristic absorption bands of NH4+ along with overlap-
† Department of Chemistry, University of York.
‡ Department of Chemistry, UMIST.
ping gains and losses in the sulfate core region (1400-800 cm-1
)
§ Department of Chemistry, University of Manchester.
as shown in Figure 2. By comparison to reference spectra recorded
9
13038
J. AM. CHEM. SOC. 2003, 125, 13038-13039
10.1021/ja036384l CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. Plot of the typical integrated intensity of the δ(NH4+) band vs
time (constant NH3 exposure of 1 × 10-7 mbar to 15 min). Traces are raw
data ()), a linear absorbance vs time fit (dashed line) and the (raw data -
linear uptake) residual due to the saturating surface reaction (solid line).
under similar conditions, these gains and losses are attributed as
follows.10 Besides NH4+, the other main gain is the characteristic
absorption pattern for the bisulfate ion in a salt (as opposed to
SAM). The loss features are attributed to H3O+, SAM-related
bisulfate ions, and the surface molecular hydrate. A plot of the
integrated intensity of the NH4+ deformation mode absorption with
NH3 exposure (Figure 3) reveals two distinct processes, namely an
initially rapid reaction, which appears to saturate, and a continuous
linear uptake. The former can be associated with the reaction
between NH3 and the surface molecular hydrate, while the latter is
attributed to the reaction between NH3 and H3O+ ions in the SAM
structure. The fact that the surface process saturates is due to the
loss of the molecular hydrate during reaction and its replacement
by a capping layer of ammonium bisulfate. The rate of the
continuous reaction is likely to be controlled by the diffusion of
H3O+ from the bulk.
Figure 4. SSIMS spectra of SAM film as deposited (top), and following
exposure to 8 × 10-10 mbar NH3 for 5 min (bottom) both at 200 K.
In summary, although sulfuric acid monohydrate is not believed
to be a major component of atmospheric particulate materials, the
observations described above are relevant to the atmosphere insofar
as they support the theory first expounded by Ianni and Bandy that
molecular hydrates are potentially stable under atmospheric condi-
tions and are likely to play a role in the surface chemistry of sulfuric
acid aerosol. Furthermore, Couling et al. have recently reported
the direct observation of the 1:1 and polyhydrate H2SO4:H2O
molecular complex in sulfuric acid aerosols, which provides further
evidence for the stability of molecular hydrates.11 These species
are not included or described in current models of aerosol reactivity.
A significant amount of water ejection from the surface is
observed by mass spectrometry during exposure to NH3. This is
also accompanied by an increase in the SIMS signal attributable to
surface NH4+HSO4-. Both reactions produce water:
+
H2SO4.(H2O)n + NH3 f NH4 HSO4- +nH2O
(1)
(2)
H3O+ + NH3 f NH4+ + H2O
Much of the H2O released during these reactions is likely to be
retained as water of crystallization by the product ammonium
bisulfate. However, reaction 1 produces significantly more H2O
than reaction 2: moreover, the H2O produced is already at the
surface and thus is readily released into the vacuum.
References
(1) Grassian, V. H. Int. ReV. Phys. Chem. 2001, 20, 467.
(2) Horn, A. B.; Sodeau, J. R.; Roddis, T. B.; Williams, N. A. J. Phys. Chem.
A 1998, 102, 6107.
(3) Koch, T. G.; Banham, S. F.; Sodeau, J. R.; Horn, A. B.; McCoustra, M.
R. S.; Chesters, M. A. J. Geophys. Res. [Atmos.] 1997, 102, 1513.
(4) Donsig, H. A.; Vickerman, J. C. J. Chem. Soc., Faraday Trans. 1997,
93, 2755.
SSIMS analysis of SAM films following exposure to ammonia
provides additional evidence supporting reaction 1 as the dominant
process occurring on the surface. Spectral features are observed
associated with sulfuric acid/ammonium clusters without any
evidence for the formation of ammonium/water clusters (Figure
4). The reaction results in the formation of a layer of ammonium
bisulfate on the surface of the SAM film. Depth profiling by SIMS,
used to monitor the extent of reaction of ammonia with the bulk
material, reveals a very short penetration distance (a few monolayers
only) on the time scale of the experiment. Furthermore, reaction
of the film with ND3 as opposed to NH3 shows extensive proton/
deuterium exchange with new peaks appearing for SD, SOD, SO2D,
and H2SO4D.
(5) Donsig, H. A.; Herridge, D.; Vickerman, J. C. J. Phys. Chem. A 1998,
102, 2302.
(6) Nash, K. L.; Sully, K. J.; Horn, A. B. J. Phys. Chem. A 2001, 105, 9422.
(7) Couling, S. B.; Sully, K. J.; Horn, A. B. J. Am. Chem. Soc. 2003, 125,
1994.
(8) Ianni, J. C.; Bandy, A. R. J. Mol. Struct. (THEOCHEM) 2000, 497, 19.
(9) Fletcher, J.; Henderson, A.; Vickerman, J. C. Manuscript in preparation,
2003.
(10) Nash, K. L.; Sayer, R. M.; Couling, S. B.; Fletcher, J.; Henderson, A.;
Vickerman, J. C.; Horn, A. B. Phys. Chem. Chem. Phys. 2003. Manuscript
submitted.
(11) Couling, S. B.; Fletcher, J.; Horn, A. B.; Newnham, D. A.; McPheat, R.
A.; Williams, R. G. Phys. Chem. Chem. Phys. 2003. In press.
JA036384L
9
J. AM. CHEM. SOC. VOL. 125, NO. 43, 2003 13039