Environ. Sci. Technol. 2008, 42, 4540–4545
AgNO3-Induced Photocatalytic
Degradation of Odorous Methyl
Mercaptan in Gaseous Phase:
Mechanism of Chemisorption and
Photocatalytic Reaction
T O N G - X U L I U , †,‡ X I A N G - Z H O N G L I , * ,†
AND FANG-BAI LI‡
Department of Civil and Structural Engineering, The Hong
Kong Polytechnic University, Kowloon, Hong Kong, and
Guangdong Key Laboratory of Agricultural Environment
Pollution Integrated Control, Guangdong Institute of
Eco-Environment and Soil Science, Guangzhou 510650,
People’s Republic of China
Received December 13, 2007. Revised manuscript received
February 04, 2008. Accepted March 31, 2008.
In this study, AgNO3 films prepared by a simple dip-coating
method were used to remove gaseous methyl mercaptan (CH3SH)
for odor control. The AgNO3 films were characterized by
means of X-ray diffraction (XRD), Fourier transform infrared
spectroscopy (FTIR), scanning electron microscopy/energydispersiveX-rayspectrometry(SEM/EDX),andX-rayphotoelectron
spectroscopy (XPS) before and after the reaction, and asobtained products were identified by means of gas chromatography/mass spectrometry (GC/MS) and ion chromatography.
The experiments demonstrated that the AgNO3 film can induce
a quick chemisorption of gaseous CH3SH to form AgSCH3
and other intermediate products such as R-Ag2S, Ag4S2, and
AgSH on its surface. Under UVA illumination, these sulfur products
can be photocatalytically oxidized to AgSO3CH3 and Ag2SO4.
Then AgSO3CH3 and Ag2SO4 will continue the chemisorption of
gaseous CH3SH, similar to AgNO3, to form AgSCH3 again and
release two final products, HSO3CH3 and H2SO4. Hence it is a
AgNO3-induced photocatalytic reaction for odorous CH3SH
degradation in gaseous phase. This fundamental research about
the mechanism of chemisorption and photocatalytic reaction
provides essential knowledge with potential to further develop
a new process for gaseous CH3SH degradation in odor
control.
Introduction
Mercaptans are highly toxic and corrosive as a group of
offensive odorous compounds with low odor detection
thresholds. Among them, methyl mercaptan (CH3SH) may
be a representative member with a very low odor threshold
of around 0.4 ppb (1). Several techniques have been studied
for removing CH3SH from a gaseous phase such as catalytic
oxidation, adsorption, catalytic incineration, radiolytic decomposition, and biological degradation (2, 3). Recently,
Satokawa et al. (4) reported that Ag-exchanged Y zeolite
* To whom correspondence should be addressed. Tel: +85227666016; fax: +852-23346389; e-mail: [email protected].
†
The Hong Kong Polytechnic University.
‡
Guangdong Institute of Eco-Environment and Soil Science.
4540
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 12, 2008
(Ag-Y) was a favorable adsorbent to remove sulfur compounds from pipeline natural gas fuel under ambient
conditions even in the presence of water vapor, which can
be a new desulfurization process for polymer electrolyte fuel
cells (PEFC). A few reports also demonstrated that some Agexchanged zeolites are effective for removing sulfur compounds such as dimethylsulfide, tetrahydrothiophene, and
thiophenes (5). Shimizu et al. (6) studied the removal of tertbutanethiol under ambient conditions with silver nitrate
supported on silica and silica-alumina, including its reaction
mechanism. Unfortunately, silver nitrate, as an adsorbent,
can be easily saturated and disabled. Alternatively, it has
been confirmed that mercaptans can be strongly adsorbed
by some metals such as gold or silver to form self-assembled
monolayers (SAMs) and undergo quick degradation upon
exposure to UV light irradiation in air to form sulfonate or
sulfate groups eventually (7). However, such a feature
attracted intensive research interests to study some fundamental interfacial interactions among adhesion, wetting,
molecular recognition, and other biological interactions and
also the fundamental research of novel microstructured
organic materials and devices (8). To the best of our
knowledge, there is a lack of research on the photocatalytic
oxidation of thiolates using silver for odor treatment based
on the above-mentioned characters. In this study, a novel
approach to photocatalytic oxidation of the thiolates with
silver under UV illumination was proposed to remove CH3SH
for odor control. The possible mechanism of chemisorption
and photocatalytic reaction in this new process was studied
in detail.
Experimental Section
Preparation of AgNO3 Film Sheets. AgNO3 (analytical grade)
was purchased from Johnson Matthey Chemicals Ltd. and
used to prepare AgNO3 films on Whatman glass fiber sheets
with an area of 18 cm × 26 cm, in which the AgNO3 solution
(0.01 g of AgNO3 in 25 mL of distilled water) was uniformly
dripped on the glass fiber sheet with a catalyst loading of
0.21 g m-2. Then the prepared AgNO3 films were dried in an
oven at 70 °C for 24 h in the dark.
Experimental Setup. All experiments of adsorption and
photocatalytic oxidation of CH3SH in a gaseous phase were
conducted in a batch photoreactor system equipped with
three UVA lamps (light intensity ) 1.24 mW cm-2) and a
CH3SH analyzer (Detcon DM-100-CH3SH) with a detection
limit of 0.1 ppm. The synthetic odorous gas was prepared by
mixing CH3SH and zero air gases in a gas mixing chamber
and its humidity was controlled at 52% ( 2%, before it was
introduced into the photoreactor. A detailed description of
the experimental setup is present in the Supporting Information as Figure S1.
Experimental Procedure. (1) In the experiments of
chemisorption and photocatalytic reaction, the CH3SH gas
with an initial concentration of about 40 ppm (about 55 µmol)
was purged into the photoreactor and was kept in the dark
until it reached a gas-solid adsorption/desorption equilibrium. Then UVA light was turned on and photoreaction took
place. After the CH3SH concentration was reduced to below
0.1 ppm, the light was turned off and the fresh CH3SH gas
was injected into the photoreactor to make the same initial
concentration again. The above experimental procedure was
repeated for five cycles to monitor the variation of CH3SH
concentration inside the photoreactor (2). To characterize
the AgNO3 films affected by the chemisorption and photocatalytic reaction, four AgNO3 films were prepared by equally
coating glass sheets with 0.15 g of AgNO3 and then drying the
10.1021/es7031345 CCC: $40.75
 2008 American Chemical Society
Published on Web 05/14/2008
sheets at 70 °C for 12 h. One of the AgNO3 films was used as
a blank sample, named “AgNO3”. The other three AgNO3
films were exposed to CH3SH for 4 h in the dark to reach a
gas/solid adsorption equilibrium. One of them was named
“Ag-MM-dark” and the other two, which reacted with CH3SH
under UVA illumination for 5 and 165 h, respectively, were
named “Ag-MM-UVA5” and “Ag-MM-UVA165” (3). Since the
adsorbed CH3SH on the AgNO3 film can be oxidized to some
ionic species such as CH3SO3- and SO42- under UVA
illumination, an experiment to separate chemisorption and
photocatalytic reactions was conducted with the following
procedure: (i) A AgNO3 film sheet consisting of seven small
pieces was placed in the photoreactor, in which each piece
of AgNO3 films had a size of 4 cm2 and 0.05 g of AgNO3 equally
and could be individually taken out during the photocatalytic
reaction as required. (ii) The synthetic CH3SH gas was purged
into the photoreactor to make up an initial CH3SH concentration of 100 ppm, and the CH3SH was chemically adsorbed
by the AgNO3 film in the dark. This step was repeated 5 times
until the surface of AgNO3 film was fully covered by adsorbed
CH3SH. According to the CH3SH concentrations before and
after each chemisorption in the photoreactor, the amount
of adsorbed CH3SH on the AgNO3 film was determined to be
96 µmol/piece. (iii) After chemisorption, the photoreactor
was refilled with fresh air to expel all residual CH3SH gas and
then UVA lamps were turned on to conduct the photocatalytic
reaction for up to 165 h. The AgNO3 film samples were taken
out piece by piece at different time intervals for further
analysis. The experiment was repeated twice, and the film
samples were extracted in 20 mL of 1 M nitrate solution at
25 °C for 48 h and then filtered through a 0.22-µm Millipore
filter. The dissolved ions in the nitrate solution were analyzed
by ion chromatography (IC).
Analytical Methods. X-ray diffraction (XRD), Fourier
transform infrared spectroscopy (FTIR), energy-dispersive
X-ray spectrometry (EDX), and X-ray photoelectron spectroscopy (XPS) were employed to fully characterize the AgNO3
films. Gas chromatography/mass spectrometry (GC/MS) and
IC analyses were used to identify the intermediate products.
Details of the analytical conditions are included in the
Supporting Information.
Results and Discussion
Chemisorption and Photocatalytic Reaction of CH3SH on
AgNO3 Film. Experimental results about the reduction of
CH3SH in gaseous phase versus time in five cycles, including
the first cycle in the dark and the following four cycles under
UVA illumination, are shown in Figure 1a. The results showed
that CH3SH after the first injection disappeared quickly in
the dark due to chemisorption, but its adsorption rate during
the dark period gradually reduced in the following four cycles
significantly. The results also revealed that the reduction of
CH3SH under UVA illumination was well maintained in the
following four cycles with a similar pattern. To distinguish
the CH3SH reduction on the AgNO3 film by chemisorption
alone from the chemisorption/photoreaction, the experimental data were further calculated to determine the firstorder rate constants (k) of CH3SH reduction in the dark and
under UVA illumination. The k values for chemisorption alone
under dark conditions varied from k ) 0.3202 min-1 in the
first cycle and k ) 0.0347 min-1 in the second cycle down
to k < 0.01 min-1 in the following cycles. These results indicate
that the chemisorption of CH3SH on the AgNO3 film was
quickly saturated after the first cycle. However, consistent k
values under UVA illumination in the four cycles were
determined to be 0.0567, 0.0588, 0.0615, and 0.0573 min-1,
respectively, which indicated that the saturated AgNO3 film
could be refreshed under UVA illumination due to the
photocatalytic reaction. Hence, a combination of chemisorption and photocatalytic reaction on the AgNO3 film would
FIGURE 1. (a) Time course of the decomposition of CH3SH with
silver in the photoreactor. (b) Odor removal efficiency with
initial CH3SH concentration of 3 ppm for up to 60 min.
be a good approach to reduce CH3SH from gaseous phase
continuously.
To evaluate the efficiency of odor removal at a lower
strength, one more experiment was conducted under the
same experimental conditions with an initial CH3SH concentration of 3 ppm for 60 min. Since the applied CH3SH
analyzer has a detection limit of 0.1 ppm, an olfactometry
analysis using a forced-choice dynamic olfactometer (Olfactomat-n2) with a detection limit of 10 ou/m3 in accordance
with the European Standard Method (EN13725) was employed to determine odor concentration when CH3SH
concentration was below 0.1 ppm. The experimental results
as shown in Figure 1b revealed that the initial odor
concentration of 8228 ou/m3 ([CH3SH]0 ) 3 ppm) was quickly
reduced to 402 ou/m3 at 15 min, 116 ou/m3 at 30 min, and
<10 ou/m3 (equal to [CH3SH] < 4 ppb) at 60 min, respectively.
These results demonstrated a good performance of odor
reduction in such a system.
GC/MS Analysis. The intermediate and final products
were identified by various analyses. Some gaseous products
were first tested by GC/MS. While dimethyl disulfide
(CH3SSCH3, m/z ) 94) was identified in the sample collected
before photoreaction, sulfonate (C2H6SO2, m/z ) 94) with
minor amounts of other sulfonate species including C2H6S2O2
(m/z ) 126) and C2H6S2O4 (m/z ) 158) were detected in the
sample after 250 min of photoreaction.
SO2 Analysis. The gaseous samples (about 3 L each) after
photocatalytic reaction were also analyzed by a photoluminescence-type SO2 analyzer to determine SO2 concentraVOL. 42, NO. 12, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4541
tion. However, the results showed that gaseous SO2 is not a
main product in this process.
IC Analysis. Two main products of methanesulfonic
acid ion (CH3SO3-) and sulfate ion (SO42-) were detected
and their concentrations versus experimental time are shown
in the Supporting Information as Figure S2. The results
showed that as illumination time was extended, the amount
of CH3SO3- increased initially and then decreased gradually
as an intermediate product, while the amount of SO42increased gradually all the time. These results confirmed that
the adsorbed CH3SH on the AgNO3 film can be oxidized under
UVA illumination to form CH3SO3- and then SO42-. However,
it was also noted that the total amount of oxidized sulfur
products (CH3SO3- + SO42-) on the last piece of AgNO3 film
after 165 h was only 23.6 µmol, equivalent to 24.6% of the
total sulfur of 96 µmol as CH3SH. Such a small portion of
sulfur conversion from CH3SH to CH3SO3- and SO42- might
result from two causes: (i) the conversion rate of AgSCH3 to
CH3SO3 or SO42- by photocatalytic reaction on the solid
surface of the AgNO3 film is much slower than the chemisorption of CH3SH; (ii) oxidized products such as Ag2SO4
accumulated on the surface of the AgNO3 film to slow down
or block further photocatalytic reaction.
Color of AgNO3 Film Surface. Four film samples (AgNO3,
Ag-MM-dark, Ag-MM-UVA5, and Ag-MM-UVA165) were
compared to observe the surface color changes due to the
chemisorption and photocatalytic reaction. It was observed
that the color of the AgNO3 film changed from white (AgNO3)
to light yellow (Ag-MM-dark) after the chemisorption in the
dark and further to dark brown (Ag-MM-UVA5 and Ag-MMUVA165) after the photocatalytic reaction under UVA illumination. These color changes indicate that an interactive
reaction between the mercapto group of CH3SH and the Ag+
ion in the interlayer region, rather than simple adsorption,
resulted in a significant change in the structure of silver and/
or sulfur species, possibly via chemical reactions between
CH3SH and Ag+.
SEM/EDX Analysis. Three samples, AgNO3, Ag-MMdark, and Ag-MM-UVA5, were examined by scanning electron
microscopy (SEM)/EDX. Their SEM images are shown in
Figure 2. It can be seen that the surface of the AgNO3 sample
(Figure 2a) was very smooth. After the reaction with CH3SH
in the dark, the surface of Ag-MM-dark sample became rough
and was covered with some S-containing products (Figure
2b). To determine the sulfur content, the Ag-MM-dark sample
was also analyzed by EDX and the ratio of S/Ag was
determined to be 18/82. This result confirmed the existence
of sulfur species on the AgNO3 surface due to the chemisorption reaction. The SEM image of Ag-MM-UVA5 surface
(Figure 2c) showed that, after the photocatalytic reaction
under UVA illumination, its surface was significantly eroded
and densely covered by some deposits with much smaller
particle sizes compared to those on the Ag-MM-dark sample.
In addition, the EDX analysis showed that the Ag-MM-UVA5
sample had a higher S/Ag ratio of 30/70 than the Ag-MMdark sample, which means more sulfur products were formed
on the film surface due to photocatalytic reaction. Furthermore, the surface of the Ag-MM-UVA5 sample was found to
be more porous than the other two samples.
XRD Analysis. The XRD patterns of four samples are
shown in Figure 3 to compare their crystalline structures
during different reactions. The XRD pattern of the AgNO3
sample (curve A) showed a typical AgNO3 structure corresponding to JCPDS 43-0649. The XRD pattern of Ag-MMdark (curve B) showed three diffraction lines at 2θ ) 31.3°,
34.8°, and 37.0°, representing R-Ag2S (JCPDS 14-72), and also
other peaks at 2θ ) 11.7°, 13.5°, 15.1°, 22.5°, 29.7°, 44.3°, and
46.5°. Some similar studies (9) reported that the structure of
silver sulfides in these samples is not the same as R-Ag2S but
Ag2S clusters, because the Ag-S distances of the Ag samples
4542
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 12, 2008
FIGURE 2. SEM images of (A) AgNO3, (B) Ag-MM-dark, and (C)
Ag-MM-UVA5.
FIGURE 3. XRD patterns of (A) AgNO3, (B) Ag-MM-dark, (C)
Ag-MM-UVA5, and (D) Ag-MM-UVA165.
reacted with organic sulfur in the dark are shorter than the
average Ag-S distance of crystalline R-Ag2S. So it can be
speculated that the unknown peaks may be due to other
Ag-S species such as Ag4S2, AgSH, and so on (9). The XRD
pattern of Ag-MM-UVA5 (curve C) showed a similar pattern
to the Ag-MM-dark sample but with lower intensity, indicating that the R-Ag2S and other Ag-S species on the film surface
might disappear and some new products appeared on the
surface. Furthermore, the XRD pattern of Ag-MM-UVA165
(curve D) demonstrated that most peaks related to R-Ag2S
and other Ag-S species almost disappeared, while a new
sharp peak appeared at 2θ ) 31.5°, which may be assigned
FIGURE 4. FTIR spectra of (A) AgNO3, (B) Ag-MM-dark, (C)
Ag-MM-UVA5, and (D) Ag-MM-UVA165.
to β-Ag2SO4 (JCPDS 27-1403). This result confirmed that the
Ag-S species formed by chemisorption can be degraded
under UVA illumination.
FTIR Analysis. The above four samples were further
examined by FTIR and the analytical results are shown in
Figure 4. Compared with the AgNO3 sample as a blank, some
new peaks were found in the FTIR spectra of Ag-MM-dark
and Ag-MM-UVA5. For example, the peaks at 684, 960, and
1305 cm-1 were attributed to the vibration of C-S, S-H, and
C-H, respectively, in CH3SH (10). Hence, it can be concluded
that surface-bound CH3S-Ag can be formed due to the
chemisorption reaction. It was noted that the three peaks of
Ag-MM-UVA5 were slightly stronger than those of the AgMM-dark sample, suggesting that the CH3S-Ag complex can
also be formed under UVA illumination. After longer exposure
times under UVA illumination (curve D), these peaks
decreased obviously, indicating the breakdown of surfacebound CH3S-Ag. Furthermore, the spectrum of Ag-MMUVA5 showed that after the photocatalytic reaction between
AgNO3 and CH3SH under UVA illumination, the new peaks
at 1049 and 1205 cm-1 appeared, attributed to the vibrations
of SdO and S-O relevant to SO42- or CH3SO3- groups (11).
Curve D for Ag-MM-UVA165 also showed that the broad
absorption bands with maxima at 616 and 1107 cm-1 can be
assigned to the stretching mode of O-S-O and S-O bonds
in the Ag2S-Ag2SO4 system (12), which are well matched
with the result of XRD. This result further confirmed that
surface-bound CH3S-Ag formed in the dark can be gradually
oxidized into the sulfonate species under UVA illumination.
XPS Analysis. Figure 5a shows XPS spectra of the S 2p3/2
region of three samples. The AgNO3 sample (curve A)
exhibited no sulfur peaks. After exposure to CH3SH in the
dark, the Ag-MM-dark sample (curve B) showed a peak at
the binding energy of 162.6 eV, representing a characteristic
of the sulfide species on silver. After the photocatalytic
reaction under UVA illumination for 5 h, the Ag-MM-UVA5
sample (curve C) had an increased peak at 162.4 eV. In the
meantime, a new peak at 168.2 eV was also found, attributed
to the sulfonate species formed from the photocatalytic
oxidation reaction. The increase of S 2p peak areas at 162.4
eV before and after photolycatalytic reaction suggests that
the amount of sulfide species increased. It has been commonly assumed that the S-H bond scission occurs even at
room temperature to form Ag-S species in the dark (13),
which was consistent with our result about the formation of
Ag-SCH3. Lewis et al. (14) postulated that the mechanism of
SAMs photooxidation on Ag involved initial C-S bond
cleavage followed by desorption of alkyl fragments and
oxidation of surface-bound sulfur species. In this study, both
reactions of C-S bond cleavage to form Ag-S species and
new sulfonate formation actually occurred under UVA
illumination simultaneously, while the former was faster than
the latter. This phenomenon is well matched with Lewis’s
hypothesis.
So far, the status of the Ag cations was not accurately
determined. Most studies on silver systems indicated an
anomalous negative shift in binding energy (BE) of the Ag
3d peaks depending on the extent of oxidation state, while
a few studies showed no BE shift with the increased oxidation
state (15). From Figure 5b, Ag 3d5/2 (around 368-369 eV) and
Ag 3d3/2 (around 374-375 eV) peaks were compared among
three samples and the relevant data are also listed in Table
S1. Curve B shows that the two peaks shifted toward higher
energy by 0.3 eV for Ag 3d5/2 and 0.1 eV for Ag 3d3/2, which
might indicate the change in the oxidation state of Ag from
Ag+ (AgNO3) to Ag+ (Ag-S species) due to the chemisorption
reaction. Curve C showed that the BE of AgNO3 shifted toward
higher energy by 0.1 eV for Ag 3d5/2 and 0.1 eV for Ag 3d3/2,
which might result from the newly generated Ag2SO4 or
AgSO3CH3 after the photocatalytic reaction.
The XPS results of O 1s and N 1s peaks as shown in Figure
5c,d may be helpful to explain the complicated behavior of
the Ag 3d peaks. The N 1s peak at 406.2 eV for AgNO3 (curve
A) was similar to that for Ag-MM-dark (curve B) at 406.3 eV,
while the peak intensity decreased to a certain extent. For
Ag-MM-UVA5 (curve C), the N 1s peak disappeared completely, which might result from the decomposition of NO3under UVA illumination. The O 1s peak at 532.3 eV for AgNO3
(curve A) was very similar to that for Ag-MM-dark at 532.1
eV (curve B), while the peak intensity decreased to some
degree. These results were in good agreement with those of
N 1s peaks. This was unambiguously assigned to the
decomposition of AgNO3 reacted with CH3SH in accordance
with other studies (6). Furthermore, a new O 1s peak at 531.3
eV for Ag-MM-UVA5 (curve C) should be attributed to the
oxidized sulfur species, since the desorption of sulfone species
and the sulfide-to-sulfone transformation can affect the O
1s XPS spectra as well (16). From the analyses of N and O,
the NO3- in AgNO3 can be replaced by CH3SH to form
Ag-SCH3 complex, which will be oxidized to AgSO3CH3 and
Ag2SO4 during the photocatalytic reaction. After the NO3disappeared, the Ag-SCH3 complex can still be formed by
the replacement of CH3SH on AgSO3CH3 and Ag2SO4. All the
intermediate products detected by different methods are
summarized in Supporting Information Table S2.
Mechanism of Chemisorption. Silver nitrate supported
on silica or silica-alumina is found to be an effective
adsorbent for the vapor-phase adsorptive removal of tertbutanethiol under ambient conditions. The mechanism of
silver sulfides formation was proposed as AgNO3 + C4H9SH
f AgSH f Ag2S f Ag4S2, indicating that most of the silver
species are present as Ag2S-like species (6). The XRD, FTIR,
and XPS results in this study supported the existence of Ag-S
species as the primary products after AgNO3 was exposed to
gaseous CH3SH in the reactor. The investigation on the SAMs
showed that the S-H bond of alkanethiols dissociates on an
Ag or Au surface to form an alkanethiolate species, which
can be strongly attached by chemisorption onto the metal
surface through the S atom as an anchor (17). As a shorterchain molecule with S-H, CH3SH should also have a similar
character. Some studies (18) reported that metal M and M+
(Cu, Ag, or Au) all could react with mercaptans or other sulfur
compounds through the following equation:
M+ + CH3SH f M-CH3 + H+
(1)
In this study the CH3SH-Ag was formed on the surface of
AgNO3 film in the dark according to eq 1, in which the S-H
bond of CH3SH was cleaved due to the activation by Ag+
(19). The results of FTIR and GC/MS suggested that the
CH3S-SCH3 was formed due to scission of the S-H bond.
VOL. 42, NO. 12, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4543
FIGURE 5. XPS spectra of (a) Ag 3d, (b) S 2p, (c) N 1s, and (d) O 1s for the samples (A) AgNO3, (B) Ag-MM-dark, and(C)
Ag-MM-UVA5.
In the meantime, C-S bond cleavage could also occur,
because (i) S-S scission in disulfides and C-S scission in
monosulfides occur generally in organic sulfides adsorbed
on silver under mild conditions (20), and (ii) the Ag-S bond
is more ionic than the C-S bond, based on the electronegativity differences between S and Ag (21). Accordingly, the
organic disulfides and monosulfides are decomposed on
silver, where a C-S bond is replaced by a strong Ag-S bond.
Therefore, the chemisorption reaction in this process should
follow eq 2, involving the cleavage of S-H and C-S bonds
to form Ag-SCH3 and other Ag-S species (Ag2S, Ag4S2, or
AgSH) adsorbed on the surface.
FIGURE 6. Proposed pathway of CH3SH degradation on AgNO3
films by chemisorption and photocatalytic reaction.
AgNO3 + CH3SH f Ag-SCH3 + HNO3 f
Ag-S species (Ag2S, Ag4S2, or AgSH) (2)
Mechanism of Photocatalytic Reaction. In this study, it
was found that the above products formed on the AgNO3
film by chemisorption could be further oxidized under UVA
illumination significantly. To the best of our knowledge, the
mechanism of photocatalytic reaction of desulfurization in
such an Ag-UVA system has not been well studied. The
complex formed by SAMs on the AgNO3 film surface could
be corroded by UVA light. The mechanism of photocatalytic
reaction under UVA illumination involves two competitive
reactions: oxidation, in which the Ag-SCH3 complex was
photocatalytically oxidized to form AgSO3CH3 and further
Ag2SO4, and S-C bond scission, in which the Ag-SCH3
complex was induced by UV light and it led to an increase
in the amount of Ag-S species such as Ag2S, Ag4S2, or AgSH
on the film surface as for chemisorption in the dark. However,
the oxidation of Ag-S species to SO42- is a much slower
reaction than the S-C bond scission, based on the observations of Lewis et al. (14). Hence, a large fraction of Ag-S
4544
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 12, 2008
species accumulated on the film surface, which was obviously
more than the amount of oxidized sulfur. It has been reported
(22) that the alkylsulfonate groups (R-SO3) and sulfate (SO42-)
formed by the photooxidation were only weakly bound to
the Ag substrate and can be readily displaced from the surface
by other thiol molecules. So the oxidized products (Ag2SO4
and AgSO3CH3) could react with CH3SH again through eqs
3 and 4 as follows:
Ag2SO4 + CH3SH f Ag-SCH3 + H2SO4
(3)
AgSO3CH3 + CH3SH f Ag-SCH3 + HSO3CH3
(4)
To have full understanding of the above reactions, the
mechanism of chemisorption and photocatalytic reaction in
this system is proposed as shown in Figure 6. In the dark,
AgNO3 can adsorb CH3SH immediately to form Ag-SCH3
that is then decomposed by the scission of C-S to generate
the Ag-S species (Ag2S, Ag4S2, AgSH, etc). Under UVA
illumination, Ag-SCH3 and other Ag-S species can be
oxidized into AgSO3CH3 and Ag2SO4 that can also react with
CH3SH to form Ag-SCH3 again and release two final products
of HSO3CH3 and H2SO4. It can be seen that with three core
intermediates of Ag-SCH3, AgSO3CH3, and Ag2SO4, the
chemisorption and photocatalytic reaction are continuously
preceded under UVA illumination to degrade gaseous CH3SH
into two aqueous acids as its final products. Therefore, this
AgNO3-UVA light system may provide a new approach to
degrade some thiolates in odorous gases by the AgNO3induced photocatalytic reaction.
Acknowledgments
We thank the Hong Kong Government Research Grant
Committee for financial support of this work (RGC PolyU
5226/06E).
Supporting Information Available
Detailed description of experimental setup, details of analytical conditions, CH3SO3- and SO42- concentrations versus
experimental time, comparison of Ag 3d5/2 and Ag 3d3/2 peaks
among three samples, and summary of intermediate products
detected by different methods. This material is available free
of charge via the Internet at http://pubs.acs.org.
Literature Cited
(1) Hartikainen, T.; Martikainen, P. J.; Olkkonen, M.; Ruuskanen,
J. Peat biofilters in long-term experiments for removing odorous
sulphur compounds. Water, Air, Soil Pollut. 2002, 133, 335–
348.
(2) Buck, T.; Bohlen, H.; Wohele, D. Influence of substituents and
ligands of various cobalt(II)porphyrin derivatives coordinately
bonded to silica on the oxidation of mercaptan. J. Mol. Catal.
1993, 80, 253–267.
(3) Kastner, J. R.; Das, K. C.; Buquoi, Q.; Melear, N. D. Low
temperature catalytic oxidation of hydrogen sulfide and methanethiol using wood and coal fly ash. Environ. Sci. Technol.
2003, 37, 2568–2574.
(4) Satokawa, S.; Kobayashi, Y.; Fujiki, H. Adsorptive removal of
dimethylsulfide and t-butylmercaptan from pipeline natural
gas fuel on Ag zeolites under ambient conditions. Appl. Catal.,
B 2005, 56, 51–56.
(5) Hernandez-Maldonado, A. J.; Yang, R. T. Desulfurization of diesel
fuels by adsorption via π-complexation with vapor-phase
exchanged Cu(I)-Y zeolites. J. Am. Chem. Soc. 2004, 126, 992–
993.
(6) Shimizu, K.; Komai, S.; Kojima, T.; Satokawa, S.; Satsuma, A.
Mechanism of adsorptive removal of tert-butanethiol under
ambient conditions with silver nitrate supported on silica and
silica-alumina. J. Phys. Chem. C 2007, 111, 3480–3485.
(7) Huang, J. Y.; Hemminger, J. C. Photooxidation of thiols in selfassembled monolayers on gold. J. Am. Chem. Soc. 1993, 115,
3342–3343.
(8) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides,
G. M. Self-assembled monolayers of thiolates on metals as a
form of nanotechnology. Chem. Rev. 2005, 105, 1103–1169.
(9) Bruhwiler, D.; Leiggener, C.; Glaus, S.; Calzaferri, G. Luminescent
silver sulfide clusters. J. Phys. Chem. B 2002, 106, 3770–3777.
(10) Tsyganenko, A. A.; Canb, F.; Travert, A.; Maugéb, F. FTIR study
of unsupported molybdenum sulfide: in situ synthesis and
surface properties characterization. Appl. Catal., A 2004, 268,
189–197.
(11) English, R. D.; Stipdonk, M. J. V.; Sabapathy, R. C.; Crooks, R. M.;
Schweikert, E. A. Characterization of photooxidized selfassembled monolayers and bilayers by spontaneous desorption
mass spectrometry. Anal. Chem. 2000, 72, 5973–5980.
(12) Tomaszewicz, E.; Kurzawa, M.; Wachowski, L. J. Study on the
reactivity in the solid state between Ag2S and Ag2SO4. Mater.
Sci. Lett. 2002, 21, 547–549.
(13) Hayashi, A.; Saimen, H.; Watanabe, N.; Kimura, H.; Kobayashi,
A.; Nakayama, H.; Tsuhako, M. Intercalation of gaseous thiols
and sulfides into Ag+ ion-exchanged aluminum dihydrogen
triphosphate. Langmuir 2005, 21, 7238–7242.
(14) Lewis, M.; Tarlov, M. J.; Carron, K. Study of the photooxidation
process of self-assembled alkanethiol monolayers. J. Am. Chem.
Soc. 1995, 117, 9574–9575.
(15) Jayaraman, A.; Yang, R. T.; Munson, C. L.; Chinn, D. Deactivation
of π-complexation adsorbents by hydrogen and rejuvenation
by oxidation. Ind. Eng. Chem. Res. 2001, 40, 4370–4376.
(16) Heister, K.; Frey, S.; Ulman, A.; Grunze, M.; Zharnikov, M.
Irradiation sensitivity of self-assembled monolayers with an
introduced “weak link”. Langmuir 2004, 20, 1222–1227.
(17) Dubois, L. H.; Nuzzo, R. G. Synthesis, structure, and properties
of model organic surfaces. Annu. Rev. Phys. Chem. 1992, 43,
437–463.
(18) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Comparison of
self-assembled monolayers on gold: coadsorption of thiols and
disulfides. Langmuir 1989, 5, 723–727.
(19) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh,
A. N.; Nuzzo, R. G. Comparison of the structures and wetting
properties of self-assembled monolayers of n-alkanethiols on
the coinage metal surfaces, Cu, Ag, Au. J. Am. Chem. Soc. 1991,
113, 7152–7167.
(20) Sandroff, C. J.; Herschbach, D. R. Surface-enhanced Raman study
of organic sulfides adsorbed on silver: facile cleavage of S-S
and C-S bonds. J. Phys. Chem. 1982, 86, 3277–3279.
(21) Bryant, M. A.; Pemberton, J. E. Surface Raman scattering of
self-assembled monolayers formed from 1-alkanethiols: behavior of films at Au and comparison to films at Ag. J. Am.
Chem. Soc. 1991, 113, 8284–8293.
(22) Hutt, D. A.; Cooper, E.; Leggett, G. J. Structure and mechanism
of photooxidation of self-assembled monolayers of alkylthiols
on silver studied by XPS and static SIMS. J. Phys. Chem. B 1998,
102, 174–184.
ES7031345
VOL. 42, NO. 12, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4545