Activated Carbons Containing Dispersed Metal Oxide Particles for Removal of Methyl Mercaptan in Air

doi:10.4236/msa.2011.21007

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Materials Sciences and Applicatio ns, 2011, 2, 49-52

doi:10.4236/msa.2011.21007 Published Online January 2011 (http://www.SciRP.org/journal/msa)

Activated Carbons Containing Dispersed Metal

Oxide Particles for Removal of Methyl Mercaptan

in Air

Hisashi Tamai*, Miki Nakamori, Masayoshi Nishikawa, Takeshi Shiono

Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-hiroshima, Japan.

Email: [email protected]

Received October 15th, 2010; revised December 21st, 2010; accepted January 6th, 2011.

ABSTRACT

Activated carbons containing dispersed metal oxide particles were prepared by carbonization of phenol resin contain-

ing metal compounds followed by steam activation. Acetylacetonates of Fe, Mn and V, and Cu nitrate were used as the

sources of metals. The removal of a small amoun t of methyl mercaptan (CH3SH) in air with these activated carbons was

tested in a flow system. Compared with activated carbons without metal oxides, the carbons exhibited high activity for

the removal of CH3SH in air. In particular, activated carbon obtained from Novolac containing 5 wt% Cu showed ex-

cellent behavior over a long time.

Keywords: Activated Carbon, Methyl Mercaptan, Adsorption, Metal Oxide, Oxidation

1. Introduction

Methyl mercaptan (CH3SH) is an odorous gas emitted to

the atmosphere. Even a very small amount of CH3SH in

air makes life uncomfortable, and its removal is impor-

tant. For the removal of CH3SH, various procedures have

been investigated and applied. For instance, the adsorp-

tive removal of CH3SH by activated carbon is one of the

effective methods, because activated carbons with large

specific surface area have high adsorption ability for a

very low concentration of harmful gases. In practice,

activated carbons have been widely used for the removal

of odorous gases [1-6]. However, in most cases, their

adsorption activity is lost due to saturated adsorption.

This limitation suggests th at they are inefficient for prac-

tical use for a long period. On the other hand, it is known

that some metal oxides po ssess high catalytic activity for

oxidative decomposition of organic compounds such as

odorous substances [7-10]. From this point of view, re-

garding the limitation of activated carbons and the cata-

lytic activity of metal oxides, it is expected that metal

oxide particles dispersed in activated carbons may have

improved functions both for adsorption and oxidative

decomposition. As a result, these hybrid materials are

expected to be very effective for the removal of CH3SH

in air for long times.

Activated carbons containing metal oxides are gener-

ally manufactured by impregnation metal compounds

into activated carbons. However, this method decreases

the surface area of the activated carbon, that is, the depo-

sition of metal oxide particles on the pores lowers ad-

sorption capacity. In addition, the supporting of metal

oxide particles on the surface of the activated carbons is

often irregular and the size of metal oxide particles is not

necessarily uniform. Here, the simple preparation of

metal oxides dispersed on activated carbons was investi-

gated. The removal of a small amount of CH3SH in air

by the activated carbons obtained was tested using

CH3SH/air mixed gas in a flow method.

2. Experimental

Metal oxides dispersed in activated carbons were pre-

pared by carbonization of phenol resins containing metal

compounds followed by activation. Acetylacetonates of

Fe, Mn, and V, and Cu nitrate were used. Phenol resins

containing metal compounds were prepared by mixing a

methanol solution of Novolac with metal compounds

dissolved in methanol. After mixing, methanol was re-

moved by flash distillation and then cured at 170˚C for 1

h. The cured mixtures were carbonized in Ar atmosphere

at 800˚C for 1h, and then activated at 900˚C for 1 h. Ac-

tivation was conducted with N2 gas saturated with steam.

The pore characteristics were determined from N2 ad-

Activated Carbons Containing Dispersed Metal Oxide Particles for Re mova l of Met hy l Mercaptan in Air

sorption/desorption isotherms using a Quantachrome

NOVA 3200 apparatus. Metal contents in the activated

carbons were determined by particle induced X-ray

emission analysis (PIXE). The kinds of metal oxides

were determined by X-ray diffraction (XRD). Metal ox-

ides in the activated carbons were observed by transmis-

sion electron microscopy (TEM) and electron probe mi-

croanalysis (EPMA).

The test for the removal of CH3SH in air was con-

ducted by flowing air containing CH3SH through acti-

vated carbon powder in a quartz tube of inner diameter of

4 mm. Typically 10 mg of activated carbon was placed in

the tube. The inlet concentration of CH3SH in the air was

10 ppm. The amounts of CH3SH adsorbed and oxidized

were obtained from the differences between the inlet

concentration of CH 3SH and the outlet one.

3. Results and Discussion

Various activated carbons were prepared from Novolac

containing 1.0 wt% of Fe (AC-Fe-1), 1.0 wt% of V

(AC-V-1), and 1.0 wt% of Mn (AC-Mn-1), and various

wt% of Cu (AC-Cu-1 , -2, -5, and -10). Th e N2 adso rptio n

and desorption isotherms of the activated carbons ob-

tained are shown in Figure 1 and Figure 2, respectively.

Figure 1 also shows the isotherm of the activated carbon

obtained from metal-free phenol resin (AC). As shown in

Figure 1, the major uptake of N2 on AC occurs at rela-

tively low relative pressure (< 0.2). This indicates that

AC is microporous. On the other hand, in the case of

AC-Fe-1, AC-V-1, and AC-Mn-1, the amounts of N2

adsorbed gradually increase with increasing relative

pressure at relative pressure above 0.3, and a hysteresis

was observed between the adsorption and desorption

isotherms of N2. In addition, in the case of AC-Mn-1, a

steep increase in the amount of N2 adsorbed was ob-

served at a high relative pressure of 0.8 ~ 1.0. The for-

mation of macropores is indicated in AC-Mn-1. These

00.2 0.40.60.8

200

400

600

800

1200

1.0

1000

P/ P 0[ -]

(

)

Figure 1. N2 adsorption/desorption isotherms of activated

carbons obtained from phenol resin containing metal com-

pounds. ○: AC, ▲: AC-Fe-1, ■: AC-V-1, ◆: AC-Mn-1.

100

200

300

400

500

600

700

800

900

0.0 0.5 1.

[

]

P /P o [-]

2 volume (cm3/g)

Figure 2. N2 adsorption/desorption isotherms of activated

carbons obtained from phenol resin containing Cu(NO3)2. -:

AC, ▲: AC-Cu-1, ■: AC-Cu-2, □: AC-Cu-10.

results suggest that the activated carbons obtained from

phenol resin containing metal compounds are mesopor-

ous and macroporous. We have earlier reported the

preparation of mesoporous activated carbons by car-

bonization of polymer materials containing metal com-

plexes followed by activation, and metal oxide particles

in carbonaceous materials play an important role in

mesopore formation [11]. Similarly to those results,

mesopores seem to be formed by metal oxide particles in

carbonaceous materials. However, in the case of AC-Cu

shown in Figure 2, an increase in the amounts of N2 ad-

sorbed at a relative pressure above 0.3 and a hysteresis

between the adsorption and desorption isotherms were

not observed, in spite of the increase in Cu content in

phenol resin. AC-Cu are thought to be microporous.

Table 1 shows the pore characteristics. BET specific

surface areas decreased by the addition of metal com-

pounds to phenol resin. This is especially noticeable in

AC-Fe-1. It is supposed that the decrease in BET specific

surface area is attributed to the catalytic effect of metal

oxide particles for steam activation. That is, the catalytic

effect of metal oxides for steam activation excessively

accelerate the activation, and as a result, pores formed in

the carbons are lost by activation. The BET specific sur-

face area of AC-Cu decreased with an increase in Cu

content in Novolac. Table 1 also shows metal contents

and metal species in the activated carbons. 3.8-6.0 wt%

of metal is contained in the activated carbons obtained

from phenol resin containing 1.0 wt% of metal. These

metal contents, determined by PIXE analysis, are com-

parable to the amounts calculated from the carbon yields

2 volume (cm3/g)

after carbonization and activation. Cu contents in AC-Cu

increased with increasing Cu content in the phenol resin.

Activated Carbons Containing Dispersed Metal Oxide Particles for Re mova l of Met hy l Mercaptan in Air 51

Table 1. Characteristics of activated carbons containing dispersed metal oxide particles.

Sample Metal amount

in novolac (wt%) Metal amount

in AC (wt%) Metal species BET-SA

(m2/g) Total porevolume

(cm3/g)

AC - - - 2597 1.20

AC-Fe-1 1.0 3.8 Fe2O3 441 0.58

AC-V-1 1.0 4.5 VO2 1610 1.00

AC-Mn-1 1.0 6.0 MnO, MnO2 1670 1.60

AC-Cu-1 1.0 4.1 CuO, Cu2O 2160 1.00

AC-Cu-2 2.0 4.5 CuO, Cu2O 0.73

AC-Cu-5 5.0 16.4 CuO, Cu2O 0.48

AC-Cu-10 10.0 34.0 Cu, CuO, Cu2O 0.35

These results suggest that the amounts of metal contained

in phenol resin are retain ed in the ACs even after activa-

tion. Metal species formed in the activated carbons were

determined by XRD. The formation of metal oxides was

observed (XRD profiles are not shown). Fe2O3 in

AC-Fe-1, VO2 in AC-V-1, MnO and MnO2 in AC-Mn-1,

CuO and Cu2O in AC-Cu-1. The dispersion of metal ox-

ide particles in activated carbons was observed by TEM

and EPMA. Figure 3 shows the TEM and EPMA images

of AC-Cu-5. The TEM image suggests that large copper

oxide particles, greater than 50 ~ 100 nm, are present.

However, the EPMA image indicates that copper was

uniformly dispersed in the activated carbon. It is sup-

posed that various sizes of copper oxide particles are

dispersed and very fine copper oxide particles which

cannot be resolved by TEM are present in the activated

carbon. Regarding the other metal compounds, similar

results to copper were observed.

The removal of CH3SH in air was tested. Figure 4

shows the removal of CH3SH from air as a function of

flow time of CH3SH/air gas mixture at 200 ml/min flow

speed. Initially, CH3SH was efficiently removed. How-

ever, removal steeply decreased after 200 min flow time.

This seems to be due to saturated adsorption of the AC.

On the other hand, activated carbons with dispersing

metal oxide particles, AC-Fe-1, AC-V-1, and AC-Mn-1,

exhibited higher removal activities than AC over a long

time. Figure 5 shows the removal of CH3SH from air by

AC-Cu, as a function of flow time at 40 ml/min flow

speed. The removal ratios by AC-Cu-1, AC-Cu-2, and

(A) (B)

(C)

Figure 3. TEM, SEM, and EPMA images of AC-Cu-5.

(a): TEM , (b): SEM, (c): EPMA.

0200 400 600 80010001200

Time (min)

(

)

Removal (%)

Figure 4. Removal of methyl mercaptan(CH3SH) in air by

activated carbons containing dispersed metal oxide parti-

cles. Flow rate of CH3SH/Air mixed gas: 200 ml/min. ○: AC,

▲: AC-Fe-1, ◇: AC-V-1, ■: AC-Mn-1.

0500 1000

100

Time (min)

(

)

Removal (%)

Figure 5. Removal of methyl mercaptan(CH3SH) in air by

activated carbons containing dispersed copper oxide parti-

cles. Flow rate of CH3SH/Air mixed gas: 40 ml/min. -: AC,

◆:AC-Cu-1, ●: AC-Cu-2, □: AC-Cu-5, △: AC-Cu-10.

AC-Cu-5 increased with increase in Cu amount. These

removal ratios of CH3SH by AC-Cu are far higher than

that by metal-free AC. Furthermore, it should be noted

that AC-Cu-5, prepared from phenol resin containing 5.0

wt% of Cu, exhibited the highest removal activity and

CH3SH in air was completely removed after 24 h flow

time. On the relationship between surface areas of acti-

vated carbons and Cu contents in phenol resin, BET spe-

cific surface area decreased with increasing content of

Cu, as shown in Table 1. These results suggest that the

removal of CH3SH in air is due to oxidative decomposi-

tion of CH3SH by copper oxide particles, in addition to

(a) (b)

(c)

Activated Carbons Containing Dispersed Metal Oxide Particles for Re mova l of Met hy l Mercaptan in Air

adsorption by the activated carbon. It has been reported

that metal oxides catalyze the oxidation of CH3SH to

dimethyl disulfide [8,9]. Therefore, copper oxide parti-

cles dispersed in the activated carbons prepared in this

work seem to have an extremely high efficiency for the

oxidative decomposition of CH3SH in air. As a result,

activated carbons with dispersed copper oxide particles

are supposed to exhibit high activities for the removal of

a small amount of CH3SH in air.

4. Conclusions

Activated carbons containing a fine dispersion of metal

oxide particles were prepared by carbonization of phenol

resin containing metal compounds followed by steam

activation. These materials exhibited high activity for the

oxidative removal of a small amount of CH3SH in air. In

particular, the activity of activated carbon containing

dispersed copper oxides was very high over a long time.

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