Mechanism of heterogeneous oxidation of carbonyl sulfide on Al 2O3: An in Situ diffuse reflectance infrared fourier transform spectroscopy investigation

Article abstract of DOI:10.1021/jp055646y

Heterogeneous reaction of carbonyl sulfide (OCS) on the surface of different types of alumina (Al₂O₃) at 298 K was investigated in a closed system and a flowed system using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The effects of calcination temperature of the Al₂O₃ on its catalyzed reactivity were studied. The crystal structure and surface area of the Al ₂O₃ were characterized using X-ray diffraction (XRD) and the Brunauer - Emmett - Teller (BET) method. This paper revealed that adsorbed OCS could be catalytically oxidized on the surface of Al₂O ₃ to form gas-phase CO₂ and surface hydrogen carbonate (HCO₃^-) and sulfate (SO₄^2-) species at 298 K. The surface hydroxyl (OH) species on the Al₂O₃ had been found to be the key reactant for the heterogeneous oxidation of OCS. Furthermore, the surface hydrogen thiocarbonate (HSCO₂^-) species, an intermediate formed in the reaction of OCS with OH, can be observed on the thermal-treated Al₂O₃. On the basis of these results, the reaction mechanism of heterogeneous oxidation of OCS on Al ₂O₃ is discussed.

Full text of DOI:10.1021/jp055646y

J. Phys. Chem. B 2006, 110, 3225-3230
3225
Mechanism of Heterogeneous Oxidation of Carbonyl Sulfide on Al2O3: An in Situ Diffuse
Reflectance Infrared Fourier Transform Spectroscopy Investigation
Junfeng Liu, Yunbo Yu, Yujing Mu, and Hong He*
State Key Laboratory of EnVironmental Chemistry and Ecotoxicology, Research Center for Eco-EnVironmental
Sciences, Chinese Academy of Sciences, Beijing 100085, China
ReceiVed: October 4, 2005; In Final Form: December 17, 2005
Heterogeneous reaction of carbonyl sulfide (OCS) on the surface of different types of alumina (Al2O3) at 298
K was investigated in a closed system and a flowed system using in situ diffuse reflectance infrared Fourier
transform spectroscopy (DRIFTS). The effects of calcination temperature of the Al2O3 on its catalyzed reactivity
were studied. The crystal structure and surface area of the Al2O3 were characterized using X-ray diffraction
(XRD) and the Brunauer-Emmett-Teller (BET) method. This paper revealed that adsorbed OCS could be
-
catalytically oxidized on the surface of Al2O3 to form gas-phase CO2 and surface hydrogen carbonate (HCO3 )
and sulfate (SO42-) species at 298 K. The surface hydroxyl (OH) species on the Al2O3 had been found to be
the key reactant for the heterogeneous oxidation of OCS. Furthermore, the surface hydrogen thiocarbonate
-
(HSCO2 ) species, an intermediate formed in the reaction of OCS with OH, can be observed on the thermal-
treated Al2O3. On the basis of these results, the reaction mechanism of heterogeneous oxidation of OCS on
Al2O3 is discussed.
1. Introduction
on the surfaces of four types of Al2O3. The surface hydrogen
carbonate (HCO3-), hydrogen thiocarbonate (HSCO2-), and
sulfate (SO42-) species were found to be the major products
formed from heterogeneous oxidation of OCS on the surface
of Al2O3. On the basis of the experimental results, a composite
reaction mechanism of heterogeneous oxidation of OCS on
Al2O3 is proposed.
Carbonyl sulfide (OCS) is the most abundant atmospheric
sulfur containing gas in the remote troposphere.1,2 It is relatively
inert in the troposphere, and it can be transported into the
stratosphere where its photooxidation is considered to be an
important source of stratospheric sulfate during volcanically
quiescent periods.1-5 From the point of view of the environment,
it is important to study the global OCS cycle. Most studies focus
on the homogeneous reaction of OCS with OH in the gas-phase
and the consumption of OCS by the plants, soil, etc.5 Hetero-
geneous interactions between gaseous molecules with wet or
dry aerosol particles have gained considerable interest since they
have the potential to alter the process of atmospheric chemistry
significantly.4,6 The surface of oxide particles in the atmosphere
can adsorb and catalyze reactions of trace gases and, thus,
change the chemical balance of the atmosphere.7,8
Aluminum is one of the most abundant elements in atmo-
spheric particles. It has been reported that alumina (Al2O3)
surface catalyzes the oxidation of H2S or CS2.9,10 However, to
our knowledge, very few studies have examined the possibility
of heterogeneous reaction and the conversion pathway of OCS
on the surface of atmospheric particles. Therefore, we studied
the reaction mechanism of OCS on the surface of Al2O3 as a
simplified model. To understand the mechanism of heteroge-
neous reaction of OCS on the Al2O3 surface, diffuse reflectance
infrared Fourier transform spectroscopy (DRIFTS) was used in
this study. DRIFTS can be used to observe the nature of the
interaction of gas with a solid surface. In addition, in situ
DRIFTS is especially useful to explore mechanisms of hetero-
geneous reactions of gases on a solid surface by providing
information on reactive intermediates formed on the surface.
The present paper is devoted to a systematic in situ DRIFTS
study on the mechanism of the heterogeneous reaction of OCS
2. Experimental Section
2.1. Materials. Four types of Al2O3 used in this experiment
were prepared from boehmite (AlOOH, Shandong aluminum
Corporation) by calcining at different temperatures. The samples
of Al2O3-A, Al2O3-B, Al2O3-C, and Al2O3-D were obtained by
calcining AlOOH at 573, 873, 1273, and 1473 K for 3 h,
respectively. Before DRIFTS measurement, all Al2O3 samples
were pretreated in an in situ infrared cell by heating in 100
mL/min of O2 at 873 K (except the Al2O3-A sample which was
heated at 573 K) for 3 h.
All reactant gases were used without further purification as
follows: Carbonyl sulfide (OCS, 2%, OCS/N2, Scott Specialty
Gases Inc.); O2 (99.99% purity, Beijing AP BEIFEN Gases Inc.).
2.2. Techniques of Characterization. BET Experiment. The
nitrogen adsorption-desorption isotherms were obtained at 77
K over the whole range of relative pressures, using a Micromer-
itics ASAP 2000 automatic equipment. Specific areas were
computed from these isotherms by applying the Brunauer-
Emmett-Teller (BET) method.
X-ray Diffraction Experiment. The samples were characterized
by X-ray diffractometry using a computerized Rigaku D/max-
RB diffractometer (Japan, Cu Ka radiation, 1.54056 nm). The
step scans were taken over a 2θ range of 10-90° in steps of
0.02°/s.
In Situ DRIFTS Experiment. In Situ DRIFTS spectra were
recorded on a NEXUS 670 (Thermo Nicolet Instrument
Corporation) FT-IR, equipped with an in situ diffuse reflection
* Corresponding author. Phone: +86-10-62849123. Fax: +86-10-
62849123. E-mail: honghe@ rcees.ac.cn.
10.1021/jp055646y CCC: $33.50 © 2006 American Chemical Society
Published on Web 02/02/2006

3226 J. Phys. Chem. B, Vol. 110, No. 7, 2006
Liu et al.
Figure 1. X-ray diffraction patterns of Al2O3 samples.
TABLE 1: Specific Area of the Al2O3 and AlOOH Samples
sample
BET area (m2/g) 318
AlOOH Al2O3-A Al2O3-B Al2O3-C Al2O3-D
277 257 121 12
Figure 2. Dynamic changes of in situ DRIFTS spectra of the Al2O3-A
sample as a function of time after exposure to 1000 ppm OCS + 95%
O2 in a closed system at 298 K.
chamber and a high-sensitivity mercury cadium telluride (MCT)
detector cooled by liquid N2. The sample (about 11 mg) for the
in situ DRIFTS studies was finely ground and placed into a
ceramic crucible in the in situ chamber. The total flow rate was
100 mL/min in all the flow systems, and the Volume of the closed
system was 30 mL. The reference spectrum was measured after
the pretreated sample was cooled to 298 K in a purified O2
stream. The infrared spectra were collected and analyzed using
a data acquisition computer with OMNIC 6.0 software (Nicolet
Corp.) installed. All spectra reported here were recorded at a
resolution of 4 cm-1 for 100 scans.
2.3. Calibration Curve of the Gas-Phase OCS Concentra-
tion. A series of in situ DRIFTS spectra at a steady state of the
flow system with various concentrations of OCS (40-2000
ppm) were recorded at 298 K. The integrated areas of the
absorption peak of gaseous OCS in the range of 1980-2120
cm-1 have a linear correlation with the concentration of OCS
gas (R2 > 0.99). The concentration of gas-phase OCS was
determined by measuring the in situ DRIFTS spectra peak areas
of gaseous OCS.
Figure 3. Heterogeneous reaction of 1000 ppm OCS + 95% O2 on
the Al2O3-A, Al2O3-B, Al2O3-C, and Al2O3-D samples or a gold mirror
at 298 K in a closed system.
3. Results and Discussion
3.1. Characterizations. BET. BET results are shown in Table
1. The surface areas of the AlOOH, Al2O3-A, Al2O3-B, Al2O3-
C, and Al2O3-D samples were decreasing with the increasing
calcination temperature.
closed. The in situ DRIFTS spectra on the Al2O3-A sample were
recorded as a function of time and are shown in Figure 2. Strong
peaks of gas-phase OCS appeared at 2071 and 2052 cm-1 15,16
.
A pair of peaks of gaseous carbon dioxide (CO2) was observed
X-ray Diffraction. The XRD patterns of all samples of AlOOH
and AlOOH calcined at 573, 873, 1273, and 1473 K are
presented in Figure 1. The Al2O3-A sample shows a character-
istic of an amorphous structure and still exists mainly as AlOOH.
Signals due to the γ-Al2O3 (2θ ) 67°, 46°, and 37°) can be
seen in the type Al2O3-B sample.11 The Al2O3-C sample exists
mainly as the crystal of θ-Al2O3, and the crystal of γ-Al2O3 is
also observed in Al2O3-C.12,13 The Al2O3-D sample exists mainly
as the crystal of R-Al2O3 evidence in line at 2θ ) 43°, 35°,
and 57°.12,13 With rising of the calcination temperature, the
crystal structure of Al2O3 changes from γ-Al2O3 to θ-Al2O3 and
then to R-Al2O3.14
at 2337 and 2361 cm-1 17,18
.
The bands at 1647 and 1412 cm-1
-
are due to νas(OCO) and νs(OCO) of surface HCO3 species,
respectively,15,17-19 and the very weak band at 1354 cm-1 is
assigned to surface SO42- species.20-24 It is also found that the
peaks for gas-phase CO2 and surface SO42- species increased
in intensity with time, while the peaks of gaseous OCS
-
diminished. The bands for HCO3 were not grown apparently
from 2 to 20 min, because the surface HCO3 species is the
-
intermediate of OCS oxidation, and exist in a dynamic balance
process of formation and consumption. These results suggest
that OCS in O2 can be finally conVerted into gas-phase CO2
2-
and surface SO4 species on the Al2O3-A sample surface at
3.2. Heterogeneous Reaction of OCS on the Al2O3 samples
in a Closed System. The Al2O3-A sample (AlOOH calclined
at 573 K for 3 h) was exposed to a flow of 1000 ppm OCS +
95% O2 at 298 K for 5 min, and then, the inlet and outlet were
298 K.
The same set experiments were also carried out using the
Al2O3-B (γ-Al2O3), Al2O3-C (θ-Al2O3), and Al2O3-D (R-Al2O3)
samples, respectively. The gas-phase OCS concentrations shown

Mechanism of Oxidation of OCS on Al2O3
J. Phys. Chem. B, Vol. 110, No. 7, 2006 3227
Figure 4. Dynamic changes of in situ DRIFTS spectra of the Al2O3-A sample as a function of time in a flow of 1000 ppm OCS + 95% O2 at
298 K.
Figure 5. Dynamic changes of in situ DRIFTS spectra of the Al2O3-A sample as a function of time in a flow of 1000 ppm CO2 + 95% O2 at
298 K.
in Figure 3 were determined by comparing the OCS absorbance
peak area with the calibration curve of the gas-phase OCS
concentration. To distinguish the effect of the surface of the
system on the loss of OCS, a control experiment was performed
under the same conditions by replacing Al2O3 sample with a
gold mirror. In this case, only a very weak change of the gas-
phase OCS concentration was observed during the experiment,
indicating that the system may consume a small quantity of OCS
through the adsorption or the heterogeneous reaction catalyzed
by the surface of the system. The concentration of gaseous OCS
drastically decreases with the Al2O3-A sample, whereas it
decreases mildly with the Al2O3-B, Al2O3-C, and Al2O3-D
samples. It is thought that the surface area has an important
influence on the absorption and reaction of OCS; however, the
Al2O3-A and Al2O3-B samples have similar surface areas, but
they exhibit different reaction rates. Therefore, there must be
other factor affecting the decrease of OCS.
3.3. Dynamic State in Situ DRIFTS Study of OCS + 95%
O2 on the Al2O3-A Samples in a Flow System. To gain further
information about the reaction of OCS on the Al2O3-A sample,
in situ DRIFTS spectra of the Al2O3-A sample as a function of
time were measured in a flow of 1000 ppm OCS + 95% O2 at
298 K (Figure 4). Similar to Figure 2, the peaks due to surface
HCO3 (1639 and 1412 cm-1) and SO4 (1333 cm-1) were
observed, and the intensities of these peaks increased with time
during 120 min (Figure 4B). On the basis of the previous studies,
the peak located at 1242 cm-1 could be assigned as the vibration
-
2-
of surface HCO3 species15,17-19 or be assigned to surface
-
HSO3 species.25,26 To confirm our assignments about the
-
-
-
surface HCO3 and HSO3 species, the Al2O3-A sample was
exposed to a flow of 1000 ppm CO2 + 95% O2 at 298 K. As
-
can be seen in Figure 5B, the vibration peaks of surface HCO3
species were located at 1655 and 1404 cm-1 on the Al2O3-A
sample. While the Al2O3-A sample was exposed to a flow of

3228 J. Phys. Chem. B, Vol. 110, No. 7, 2006
Liu et al.
Figure 6. Dynamic changes of in situ DRIFTS spectra of the Al2O3-A sample as a function of time in a flow of 200 ppm SO2 + 95% O2 at
298 K.
Figure 7. Dynamic changes of in situ DRIFTS spectra of the Al2O3-B (γ-Al2O3) sample as a function of time in a flow of 1000 ppm OCS + 95%
O2 at 298 K.
200 ppm SO2 + 95% O2 at 298 K, as shown in Figure 6B, the
vibration peaks of surface HSO3- species were located at 1242
cm-1. Therefore, we finally assigned the peak located at 1242
measured in this rapid reaction. Considering that the thermal
pretreatment of Al2O3 was expected to reduce the surface OH
groups, further experiments were performed on the thermal
pretreatment of Al2O3 samples to observe these intermediates.
cm-1 (in Figure 4B) to the vibration mode of surface HSO3
-
-
species instead of surface HCO3 species. The broad band
Figure 7 shows the in situ DRIFTS spectra of the Al2O3-B
sample in a flow of OCS + 95% O2 at 298 K. The bands at
1653 and 1423 cm-1 provided similar evidence for the formation
centered at 3543 cm-1 was assigned to the ν(OH) stretching of
adsorbed H2O.13,27 The peak at 1639 cm-1 was assigned to a
-
combination band produced from νas(OCO) of surface HCO3
-
of surface HCO3 species, but the intensity ratio of 1653 and
species and δ(HOH) of adsorbed H2O, and the peak at 1412
cm-1 was assigned to νs(OCO) of surface HCO3- species.15,17-19
Meanwhile, it should be noted that introduction of 1000 ppm
OCS led to a drastic increase in the intensity of negative peaks
at 3740 and 3666 cm-1 in Figure 4A. In the model proposed
by Peri,28,29 these bands were attributed to the vibrations of
surface hydroxyl (OH) species. The consumption of surface OH
species meant that the reaction between OCS and surface OH
must occur. However, other intermediates, favorable for un-
derstanding the mechanism of OCS conversion, were not
-
1423 cm-1 indicates a low surface concentration of HCO3
species and a lot of adsorbed H2O. It should be noted that a
new peak at 1579 cm-1 was observed, which is assignable to
surface hydrogen thiocarbonate (HSCO2-) species as an
intermediate of the hydrolysis of OCS.15,30,31 Meanwhile, the
peaks located at 1996 and 1938 cm-1 due to the physically
adsorbed OCS were observed at the beginning of the reaction
and then diminished.30,31 This diminishing of the physically
adsorbed OCS may be deriVed from the competition adsorption

Mechanism of Oxidation of OCS on Al2O3
J. Phys. Chem. B, Vol. 110, No. 7, 2006 3229
Figure 8. Dynamic changes of in situ DRIFTS spectra of the Al2O3-C sample as a function of time in a flow of 1000 ppm OCS + 95% O2 at
298 K.
Figure 9. Dynamic changes of in situ DRIFTS spectra of the Al2O3-D sample as a function of time in a flow of 1000 ppm OCS + 95% O2 at
298 K.
-
between the chemically adsorbed surface species (e.g. HCO3
and the physically adsorbed OCS.
)
surface HSO3- and HCO3- species also need the participation
of the surface OH species.
To confirm the presence of surface HSCO2 species as an
-
It is apparent that the thermal treatment of Al2O3 could slow
OCS oxidation and enable the observation of the reaction
intermediate. In the region of OH groups, a drastic increase in
the intensity of negative peaks at 3772, 3732, and 3682 cm-1
attributed to the vibrations of surface OH groups (Figure 7A)
was observed. These results strongly suggest that the formation
of surface HSCO2- species is derived from the reaction between
surface OH species and OCS. Although the thermal treatment
of Al2O3 could reduce the surface OH species and the formation
intermediate of OCS oxidation, the same experiment was
performed on the Al2O3-C sample (AlOOH calclined at 1273
K for 3 h). As shown in Figure 8, exposure of the sample to
-
OCS resulted in the appearance of surface HCO3 (1653 and
-
2-
1435 cm-1), HSCO2 (1579 cm-1), and SO4 species (1333
cm-1) (Figure 8B), which was also accompanied by the
formation of negative peaks at 3755 and 3678 cm-1 due to
surface OH groups (Figure 8A). Obviously, the oxidation of
OCS shows the same mechanism on the Al2O3-B and Al2O3-C
samples. However, high-temperature calcination of AlOOH
resulted in an activity loss of catalytic oxidation for OCS, as
evidenced by the lower reaction rate (Figure 3).
-
of surface HSCO2 species, the conversion rate of surface
HSCO2- species into surface HSO3- species under this condition
might be decreased more greatly; thus, surface HSCO2- species
were clearly detected. This indicates that the surface OH species
-
play an important role in the formation of surface HSCO2
To further investigate the role of surface OH groups in OCS
oxidation on Al2O3, the same experiment was performed on the
-
species and that the surface HSCO2 species converted into

3230 J. Phys. Chem. B, Vol. 110, No. 7, 2006
Liu et al.
the heterogeneous oxidation of OCS through reducing the
surface OH species, which is implied as the key reactant of the
reaction.
Acknowledgment. This research was financially supported
by the National Natural Science Foundation of China (40275038).
Figure 10. Mechanism of heterogeneous oxidation of OCS on the
AlOOH and Al2O3 samples.
References and Notes
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comparison with the γ-Al2O3 sample. As shown in Figure 9A,
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cm-1) was detected, and no peaks attributable to surface
HSCO2-, HCO3-, and SO42- species were observed (Figure 9B).
In addition, the peak due to surface adsorbed H2O (3398 and
1645 cm-1) was detected. This result confirms our suggestion
about the role of surface OH groups in OCS oxidation on the
γ-Al2O3 surface.
3.4. Proposed Mechanism of Heterogeneous Oxidation of
OCS on Al2O3. On the basis of the above results, we propose
the following possible mechanism of the heterogeneous oxida-
tion of OCS on Al2O3, as shown in Figure 10.
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OCS first reacted with the surface OH species to form the
-
surface HSCO2 species on AlOOH and γ-Al2O3 at room
-
temperature. The oxidation of surface HSCO2 species by
oxygen containing surface species and the surface OH species
proceeded readily, followed by the formation of the surface
-
2-
HCO3 and SO4 species at room temperature. When the
oxygen containing surface species were consumed, O2 in the
gas-phase can supplement it so that the oxidation reaction can
-
continue until the surface is fully covered by surface HCO3
and SO42- species. Both the oxygen containing surface species
and surface OH species can contribute to this reaction. When
the heat-treated Al2O3 samples were exposed to OCS + 95%
-
O2, the reaction between surface OH with surface HSCO2
species was slowed. At the same time, the conversion rate of
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-
HSCO2 species became the dominant surface species. This
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This study reveals that OCS can be catalytically oxidized on
2-
Al2O3 surface to produce gas-phase CO2 and surface SO4
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species as final products at 298 K. Surface HSCO2 species
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-
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