photocatalytic degradation of gaseous sulfur compounds by silver-deposited titanium dioxide

www.elsevier.com/locate/apcatb

Applied Catalysis B: Environmental 57 (2005) 109–115

Photocatalytic degradation of gaseous sulfur compounds by

silver-deposited titanium dioxide

Shinji Katoa, Yuji Hiranoa, Misao Iwataa, Taizo Sanob,*,Koji Takeuchib, Sadao Matsuzawab

aNoritake Co., Limited, 3-1-36 Noritake, Nishi-ku, Nagoya 451-8501, JapanbNational Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba West,

16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan

Received 19 December 2003; received in revised form 20 October 2004; accepted 27 October 2004

Available online 8 December 2004

Abstract

Deposition of ultrafine particle of silver on titanium dioxide (TiO2) significantly improved the photocatalytic activity for degradation of

gaseous sulfur compounds, such as hydrogen sulfide (H2S) and methylmercaptan (CH3SH). The silver-deposited photocatalytic filter (Ag-

PCF) was prepared by coating TiO2 powder on the porous ceramics substrate and successively depositing nano-sized silver particles on TiO2

by means of photodeposition method. The photocatalytic degradation rates of hydrogen sulfide and methylmercaptan by Ag-PCF were 7 times

and 14 times larger, respectively, than those by conventional photocatalytic filter. The sulfur atom of hydrogen sulfide was oxidized into

sulfate ion, which was accumulated on the filter. However, the degradation rate did not decline in the experimental period. The deposited silver

seems to act as a co-catalyst that enhances the photocatalytic oxidation of sulfur compounds.

# 2004 Elsevier B.V. All rights reserved.

Keywords: Titanium dioxide; Silver; Photocatalytic degradation; Photodeposition; Sulfur compounds; Hydrogen sulfide; Methylmercaptan; Photocatalytic

filter

1. Introduction

Titanium dioxide (TiO2) is well known as a useful

photocatalyst for elimination of environmental pollutants

[1–6]. TiO2 photocatalyst absorbs photons with wavelengths

below 380 nm and generates electron and hole, which

produce active oxygen species, such as O� and OH� radicals,

and O2� ion, by the reaction with H2O and O2 adsorbed on

the TiO2 surface. The high oxidation activities of the active

oxygen species have been applied for air purification, water

purification, deodorization, self-cleaning, antibacterial coat-

ing, etc. In our field experiments for removing stench, TiO2

photocatalyst exhibited reasonable activity for eliminating

odors of tobacco, cooking exhaust, and volatile organic

* Corresponding author. Tel.: +81 29 861 8166; fax: +81 29 861 8258.

E-mail address: [email protected] (T. Sano).

0926-3373/$ – see front matter # 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcatb.2004.10.015

compounds (VOCs). Especially, the deodorization effect on

short-chain fat acid and aldehydes were quite high.

Reduced sulfur compounds in gas phase, such as

hydrogen sulfide (H2S), methylmercaptan (CH3SH),

dimethyl sulfide (CH3SCH3), dimethyl disulfide

(CH3SSCH3), diethyl sulfide (C2H5SC2H5), etc., were also

degraded by TiO2 photocatalytically [7–12], however, the

degradation rates were not enough for practical applications.

These sulfur compounds are odor compounds emitted from

raw garbage, sewage, feces, etc. The odor thresholds of these

sulfur compounds are extremely low (0.5 ppb for H2S), and

the smells are often unpleasant for human life. The guideline

concentrations of H2S and CH3SH are 20 and 2 ppb in Japan

[13]. The conventional techniques to remove these sulfur

compounds are activated-carbon adsorption method and

ozone oxidation method. The adsorption method needs to

replace and dispose adsorbent, and the ozone oxidation

method involves NOx emission and high initial cost. If the

S. Kato et al. / Applied Catalysis B: Environmental 57 (2005) 109–115110

Fig. 1. Optical micrograph of the photocatalytic filter (PCF) made of TiO2

powder and alumina framework.

degradation rate of sulfur compounds by TiO2 photocatalyst

is improved, the photocatalysis method will be a promising

candidate to remove sulfur compounds from air, since the

photocatalytic method has advantages of low cost, energy

saving, and minimal waste.

Modification of the TiO2 surface will be one of the ways

to improve the degradation rate of sulfur compounds. The

addition of noble metals is a conventional method to modify

surface of catalyst. For example, the SH functional group in

sulfur compounds adsorbs on gold and silver strongly [14–

16]. Additionally, in the photocatalytic reactions, it is

considered that the deposition of metals, such as platinum,

gold, palladium and silver, on TiO2 increases the charge

separation efficiency and inhibits recombination of electron

and hole produced by UV absorption [1,2,17]. The increase

in charge separation efficiency will enhance the formation of

active oxygen species. From these considerations, it is

expected that the deposition of metals on TiO2 enhances the

interaction with sulfur compounds and the formation of

active oxygen species, which result in high degradation rate

of sulfur compounds. Tada et al. reported that the

photocatalytic reduction rate of bis(2-dipyridyl)disulfide

in aqueous phase was improved by deposition of silver on

TiO2 [18]. In the similar scheme, the decomposition rate of

N2O by TiO2 photocatalyst was significantly improved by

deposition of ultrafine particle of silver [19]. However, the

effects of noble metals on photocatalytic degradation of

gaseous sulfur compounds are less well investigated.

In this paper, silver-deposited TiO2 was prepared on an

alumina substrate with three-dimensional framework, and

the characteristics and the photocatalytic activities for

degradation of H2S and CH3SH were analyzed.

Fig. 2. Experimental setup for photodeposition of silver on photocatalytic

filter.

2. Methods

2.1. Preparation of silver-deposited photocatalytic filter

(Ag-PCF)

TiO2 photocatalytic filter (PCF) was prepared by dip-

coating of anatase type TiO2 (relative surface area: 290–

310 m2/g) on alumina substrate with three-dimensional

framework structure. Fig. 1 shows the optical micrograph of

the PCF. The average diameter of frames was 3.0 mm and

the pore size was 18 mm. Because PCF is composed of only

inorganic materials, PCF is tolerant to ultraviolet irradiation

and redox reaction by TiO2. The amount of TiO2 contained

in PCF (50 mm � 50 mm � 13 mm) was 1.50 g.

The ultrafine particle of silver was deposited on PCF by

photodeposition method [19–22]. The experimental setup

for photodeposition is schematically shown in Fig. 2. The

PCF was submerged into 130 cm3 of distilled water in a

quartz reactor. The required amount of silver nitrate solution

(3.71 � 10�4 mol/dm3) was added into the reactor while

stirring with magnetic stirrer, and then the pH of solution

was adjusted to 6.4 by adding KOH solution (0.1 mol/dm3).

UV light from a high pressure Hg lamp (Ushio, 250 W) was

irradiated to the PCF for 5–60 min while N2 gas was passed

through the solution to remove dissolved oxygen. The UV

irradiance at PCF measured by UV detector (Ushio, UIT-

15A with UT-36) was 5.8 mW/cm2. In this process, Ag+ ions

in the solution were reduced to metal silver and were

deposited on PCF. The Ag+ concentration in the solution

after UV irradiation was analyzed by inductively coupled

plasma spectrometry (ICP; Seiko Instruments, SPS-7800),

and the amount of silver deposited on PCF was calculated

from the change in Ag+ concentration. The amount of silver

deposited on PCF was controlled by changing the initial

content of Ag+ in the solution. Two samples of silver-

deposited PCF with 0.20 and 1.0 mg silver were prepared,

and were named Ag-PCF(0.2) and Ag-PCF(1.0), respec-

tively.

The solid phases were identified by X-ray diffractometry

with Cu Ka radiation (XRD; Rigaku, RAD-RVB), X-ray

photoelectron spectroscopy with Al Ka radiation (XPS;

Fisons instruments, Escalab220i-XL), transmission electron

microscopy (TEM; JEOL, JEM-2000EX), and scanning

electron microscopy (SEM; Hitachi, S-3200N) with energy

dispersion X-ray analysis (EDX).

S. Kato et al. / Applied Catalysis B: Environmental 57 (2005) 109–115 111

Fig. 3. Schematic diagram of a testing apparatus for the decomposition of

gaseous sulfur compound.

Fig. 4. SEM photograph of the surface of PCF.

Fig. 5. Relation between apparent UV-transmittance and thickness of PCF.

2.2. Photocatalytic degradation of H2S and CH3SH

The experimental setup for adsorption measurement and

photocatalytic degradation of sulfur compounds is schema-

tically shown in Fig. 3. Ag-PCF was placed at the center of

stainless steel reactor (SUS304, width 50 mm, length

200 mm, depth 15 mm) with a cover window of quartz

(thickness 3 mm). Sample gas was generated by mixing

standard gas of sulfur compound in cylinder (Sumitomo

Seika Chemicals, 50 ppm of H2S or CH3SH diluted in N2

gas), dried air and humidified air using mass flow

controllers. 0.75 ppm of H2S or CH3SH sample gas was

passed through the reactor at a flow rate of 3.0 dm3/min (at

the room temperature). During the photocatalytic degrada-

tion, UV light from a 10 W black light bulb (Toshiba,

FL10BLB) was irradiated to the photocatalyst through the

quartz window. The average UV irradiance at the photo-

catalyst was 2.0 mW/cm2. Gasses eluted from the reactor

were analyzed using a gas chromatograph (Shimadzu, GC-

14B) with flame photometric detector (FPD). The amount of

sulfate ion formed on the photocatalyst was analyzed by an

automated in-line combustion system with ion chromato-

graph (Dia Instruments Co., AQF-100).

Fig. 6. Relation between the amount of silver deposited on PCF and UV

irradiation time. The amount of deposited silver was plotted as (amounts of

silver deposited on PCF)/(amount of silver ion contained in the initial

solution). Initial content of silver: 1.0 mg.

3. Results and discussion

3.1. Characterization of the photocatalysts

The SEM photograph of the PCF is shown in Fig. 4(a).

TiO2 layer with the thickness of about 5–10 mm was formed

on the alumina framework. The apparent UV transmittance

of PCF was analyzed by changing thickness of PCF (Fig. 5).

The transmittance decreased with increasing thickness of

PCF, and became approximately zero at the thickness of

13 mm. Since pressure loss by filter increases with

thickness, we used the PCF with 13 mm thickness, which

is enough to utilize UV light irradiated. The pressure loss of

PCF with 13 mm thickness was 30 Pa at a wind velocity of

1.5 m/s, which is low enough to apply for air purification

filter.


Fig. 7. XRD patterns of the Ag-deposited photocatalytic filter (Ag-

PCF(1.0)). The pattern of Ag-PCF(0.2) was indistinguishable from that

of Ag-PCF(1.0).

The relation between the amounts of silver deposited on

PCF and the UV irradiation time for photodeposition is

shown in Fig. 6. The initial amount of Ag+ in the solution

was 1.0 mg. The amounts of deposited Ag increased during

the first 10 min. In 30 min of UV irradiation, more than 99%

of Ag+ ions (9.3 mmol) in the solution were deposited on

PCF. Similarly, more than 99% of Ag+ ions were deposited

on the PCF when the initial amount of Ag+ in the solution

was 0.20 mg. In the XRD pattern of Ag-PCF (Fig. 7), the

peaks of anatase type TiO2 and corundum type alumina (a-

Al2O3) appeared, and peaks of metal silver and silver-related

compounds were not detected. This indicates that the silver

was deposited in the state of ions or small nondetectable

particles.

Fig. 8(a) shows the SEM photograph of the surface of Ag-

PCF(1.0), and Fig. 8(b) and (c) shows the EDX images

representing concentration profiles of titanium and silver,

respectively. The TiO2 layer did not exfoliate from the

alumina framework during photodeposition process and no

change in texture of the Ag-PCF was observed in this

magnification (Fig. 8(a)). The distribution of titanium on the

Ag-PCF(1.0) (Fig. 8(b)) was similar to that of the PCF, and

significant change was not observed. The silver element was

deposited on only TiO2 layer and did not exist on alumina

Fig. 8. EDX images representing concentration profiles for component ele

substrate (Fig. 8(c)). This indicates that the silver species

were deposited by the aid of photocatalysis of TiO2. Fig. 9(a)

and (b) shows the TEM photographs of the photocatalysts.

The particle size of silver deposited on PCF was 1–10 nm.

The crystallite sizes of TiO2 are 30–50 nm, which is

approximately identical with the values before photodeposi-

tion of silver.

The silver deposited on TiO2 was analyzed by XPS.

Fig. 10(a) shows the spectrum for Ag3d region of Ag-

PCF(1.0) as prepared. The peaks at around 367 and 374 eV

were due to Ag3d5/2 and Ag3d3/2 orbitals, respectively. The

Ag3d peaks are slightly broadened, and are considered to be

the sum of multiple peaks. Since the binding energies of

metal silver (Ag), silver(I) oxide (Ag2O) and silver(II) oxide

(AgO) are 368.2, 367.8 and 367.4 eV [20,21], Ag-PCF(1.0)

seems to contain metal silver and silver(I) oxide. A part of

the silver ion contained in the preparation suspension may be

deposited on TiO2 without being reduced into metal silver

because of shortness of UV-irradiation time.

3.2. Degradation of sulfur compounds by Ag-PCF

The photocatalytic degradation of H2S by Ag-PCF was

analyzed (Fig. 11). At first, the sample gas of H2S (0.75 ppm

in air, relative humidity 50%) was contacted with the

photocatalysts without UV irradiation to evaluate the effect

of silver deposition on the H2S adsorption. Ag-PCF(1.0)

decreased the H2S concentration to 0.19 ppm, and then the

H2S concentration was restored to the initial concentration

in 60 min. The amount of H2S adsorbed on Ag-PCF(1.0) was

2.2 mmol, which was corresponded to 24% of the amount of

silver deposited (9.3 mmol). On the other hand, adsorption of

H2S on Ag-PCF(0.2) and PCF without silver were not

observed. These results suggest that the adsorption of H2S

can be enhanced by silver deposition. The less adsorption

capacity of the sample with low silver content may be due to

the state of silver surface. It is reported that oxidized silver

surface captures a larger amount of sulfur compounds or

CO2 than clean surface of metal silver [15,16,25]. Since Ag-

ments in Ag-PCF(1.0): (a) SEM photograph; (b) titanium; (c) silver.


Fig. 9. TEM photographs of PCFs (a) before photodeposition of silver and (b) after photodeposition (Ag-PCF(1.0)).

PCF(1.0) contained silver(I) oxide (Fig. 10), the adsorption

of H2S on Ag-PCF(1.0) is reasonable. In the preparation of

Ag-PCF(0.2), the UV-irradiation time was same as the case

of Ag-PCF(1.0) although the silver content was 20%. It is

inferred that a large portion of silver ion on Ag-PCF(0.2)

was reduced to metal silver by photoinduced electrons and

the adsorption capacity for H2S was extremely low.

After the adsorption of H2S on Ag-PCF(1.0) was almost

saturated, the UV lamp was turned on. The H2S concentra-

tion of effluent gas rapidly decreased from 0.75 to 0.10 ppm.

The generation of SO and SO2 was not observed by

photoluminescence-type SOx analyzer. These results suggest

that H2S was removed from the gas phase and trapped on the

photocatalyst by photoinduced reaction. The H2S concen-

tration increased slowly up to 0.15 ppm in 1 h of UV

irradiation, and then kept constant. The H2S removal rate

calculated by Eq. (1) was 80%:

H2S removal rateð%Þ ¼ 1 � ½effluentH2S�½influentH2S�

� �� 100 (1)

The removal rates of Ag-PCF(0.2) and PCF without silver

were 70 and 11%, respectively. These results indicate that

Fig. 10. X-ray photoelectron spectra (XPS) of Ag-PCF(1.0); Ag3d region

of the sample as prepared (a) and after H2S removal (b).

the deposition of silver effectively improves the H2S

removal activity of TiO2.

The H2S removal by Ag-PCF was performed repeatedly

in the similar procedure (Fig. 11). By the UV irradiation for

5 h in total, Ag-PCF(1.0) and Ag-PCF(0.2) removed 20.4

and 18.4 mmol of H2S, respectively. The amounts of

deposited silver on Ag-PCF(1.0) and Ag-PCF(0.2) were

9.3 and 1.9 mmol, which were significantly smaller than the

amounts of removed H2S. Furthermore, the H2S removal

rates still kept 80% (Ag-PCF(1.0)) and 70% (Ag-PCF(0.2))

after 12 h. These results indicate that the deposited silver is

not a reactant for H2S removal. Since metal sliver and silver

oxide were not semiconductor photocatalyst, the deposited

silver species seem to act as a co-catalyst that enhances the

photocatalytic activity of TiO2. The XPS spectrum for Ag-

PCF(1.0) after H2S removal test for 5 h contained the peaks

of metal silver and silver(I) oxide (Fig. 10(b)), and no

significant difference between the oxidation states of silver

before and after the H2S removal was observed. This also

implies that the silver was not a reactant but a catalyst.

The Ag-PCF(1.0), which had been used for the H2S

removal test shown in Fig. 11, was placed in 50 ml of

distilled water for 24 h, and the eluate was analyzed by

Fig. 11. Time-course of the concentration of H2S in the presence of PCF

(^), Ag-PCF(0.2) (&) and Ag-PCF(1.0) (~).


Fig. 12. X-ray photoelectron spectrum (XPS) for S2p region of Ag-

PCF(1.0) after the reaction.

Fig. 13. Time-course of the concentration of CH3SH in the presence of PCF

(^), Ag-PCF(0.2) (&) and Ag-PCF(1.0) (~).

inductively coupled plasma spectrometry (ICP) and ion

chromatograph. The amount of silver dissolved was

negligible, indicating that the deposited silver is stable

against washing with water. 0.46 mmol of sulfate ion was

detected, however, this is much smaller than the amount of

H2S removed by adsorption and photocatalytic degradation

(28 mmol). In the previous paper that treated the photo-

catalytic destruction of H2S by TiO2 powder, 95% of sulfur

atoms of H2S destructed were recovered as sulfate ion by

eluting with water [7]. The difference between the previous

paper and this paper may be due to the preparation of

photocatalyst; sintered TiO2 was used in this paper, and this

may make it difficult to recover sulfate ions adsorbed in

small pores. Next, the sulfur content of Ag-PCF(1.0) after

the H2S removal was analyzed by combustion tube method.

The photocatalyst filter was well grinded and was heated up

to 1373 K. The combustion gas was absorbed into H2O2

solution to form sulfate ion, which was analyzed by ion-

chromatograph. The amount of sulfur atom contained in Ag-

PCF(1.0) was 26.4 mmol, which is 94% of removed H2S.

This indicates that most of the removed H2S was trapped on

the photocatalytic filter. However, in this method, the kinds

of sulfur compounds formed were not identified.

Fig. 12 shows the S2p region of XPS spectrum for Ag-

PCF(1.0) used for the H2S removal. The peak at 168.2 eV

and the shoulder at 169.4 eV were due to the sulfate ion

(SO42�). No peaks of hydrogen sulfide (H2S, 170.4 eV),

silver sulfide (AgS, 160.7 eV), elemental sulfur (S,

164.3 eV), and sulfur dioxide (SO2, 174.8 eV) were

observed [23,24]. Additionally, as mentioned above, any

gaseous byproducts were not detected. These results suggest

that the sulfur atom of H2S was mainly oxidized into sulfate

ion and was trapped on the photocatalyst (Eq. (2)):

H2S þ 5

2O2 þ 2e�! SO2�

4ðadsÞ þ H2O (2)

Next, the photocatalytic degradation of CH3SH was

analyzed (Fig. 13). Similarly to the adsorption behavior on

H2S, Ag-PCF(1.0) adsorbed CH3SH without UV irradiation,

while Ag-PCF(0.2) and PCF without silver did not adsorb

CH3SH. The amount of CH3SH adsorbed on Ag-PCF(1.0)

was 1.9 mmol, which was not so smaller than that of H2S

adsorbed on the same sample. After the CH3SH concentra-

tion became identical to the initial concentration, UV light

was irradiated to the photocatalysts. Both Ag-PCF(1.0) and

Ag-PCF(0.2) removed CH3SH and the CH3SH concentra-

tions were reduced to 0.21 ppm. The CH3SH concentrations

slightly increased during first 60 min UV irradiation and

then kept constant at 0.23 ppm (Ag-PCF(1.0)) and 0.28 ppm

(Ag-PCF(0.2)). The CH3SH removal rates of Ag-PCF(1.0)

and Ag-PCF(0.2) were 69 and 63%, respectively, which

were significantly larger than that of PCF without silver

(5%). Thus, the deposition of silver also improved the

photocatalytic activity of TiO2 for CH3SH degradation.

The concentrations of H2S and CH3SH in effluent gases

were rapidly restored to the influent concentrations

(0.75 ppm) after UV irradiation to Ag-PCF(0.2) and PCF

were stopped (Figs. 11 and 13). On the other hand, Ag-

PCF(1.0) removed H2S and CH3SH even after turning the

UV lamp off, and the concentrations of the sulfur

compounds were increased gradually. The amount of H2S

removed was 2.6 mmol, which was similar to that adsorbed

by the photocatalyst as prepared. These results suggest that

the sulfur compounds on the adsorption sites was removed

during the UV irradiation and the adsorption site of Ag-

PCF(1.0) was regenerated.

Even 0.2 mg of silver deposition increased the removal

rate of sulfur compounds under UV irradiation, while Ag-

PCF(0.2) does not adsorb the sulfur compounds in the

absence of UV light. These results suggest that the

adsorption capacity under dark condition was not essential

for the photocatalytic activity on degradation of sulfur

compounds. There are two possibilities: (i) a part of

deposited silver that does not adsorb sulfur compounds

provides active sites for the oxidation of sulfur compounds

to enhance the rate limiting process; (ii) the photoadsorption

of sulfur compounds was enhanced by silver deposition in

the presence of UV light to increase the reaction probability.

The further analyses such as in situ Fourier-transform

infrared spectroscopy (FT-IR) or electron spin resonance

(ESR) will clarify the reaction pathway and the roles of

metal silver and silver(I) oxide.


4. Conclusion

The photocatalytic degradation activities for sulfur

compounds (H2S and CH3SH) were significantly improved

by photodeposition of silver on TiO2. The sulfur atoms were

oxidized into sulfate ion and accumulated on the photo-

catalytic filter. The TEM images and XPS spectra indicated

that the mixture of metal silver and silver(I) oxide were

deposited on TiO2 as small particles with diameters of 1–

10 nm. The oxidation state of deposited silver was not

changed by the H2S degradation. Additionally, the activity

of silver-deposited TiO2 did not decline even after the

amount of decomposed H2S exceeds nine times as much as

deposited silver. These results indicate that the deposited

silver was not a reactant but a co-catalyst. It is expected that

the silver-deposited TiO2 can be used for long-term

applications.

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photocatalytic degradation of gaseous sulfur compounds by silver-deposited titanium dioxide

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