Carbon 43 (2005) 1512–1516

www.elsevier.com/locate/carbon

Effect of heat treatment of activated carbon supportson the loading and activity of Pt catalyst

Min Kang a, Youn-Sang Bae b, Chang-Ha Lee b,*

a LCD R&D Center, Samsung Electronics Co., Ltd., Yongin 449-711, Koreab Department of Chemical Engineering, Yonsei University, Seoul 120-749, Korea

Received 10 April 2003; accepted 26 January 2005

Available online 5 March 2005

Abstract

The study has been done on the effect of heat treatment of activated carbon at 1573–1773 K on its structural and electronic prop-

erties as a catalyst support. The X-ray diffraction result indicated that a partly graphitized structure was formed when the activated

carbon was heated to a high temperature (1673 K). From the X-ray photoelectron spectroscopy result, it was found that Pt0 con-

centration was increased, but PtO and PtO2 concentrations were decreased with an increase in the heat treatment temperature. From

the van Dam�s model applied to this result, it might be concluded that more ‘‘p-sites’’ are created as the heat treatment temperaturebecomes higher. From the CO-chemisorption result, the highest loading was observed in case of Pt/AC1673 sample. This improved

loading ability could be explained by the special interaction of the graphitic planes (p-sites) with the metal particles. Based on thecatalytic activity, CO-chemisorption and XPS results, it is concluded that the well-loaded Pt0 species mainly contribute to the cat-

alytic activity. Moreover, it was found that different degrees of graphitization of heat treated activated carbon could cause different

surface Pt0 and improve the resistance of carbon support against gasification under air oxidation.

� 2005 Elsevier Ltd. All rights reserved.

Keywords: Activated carbon; Catalyst support; Heat treatment; X-ray photoelectron spectroscopy

1. Introduction

Catalysts made of noble metal supported by activated

carbon are of considerable interest due to their applica-

tions in various processes such as purification of tere-

phthalic acid and destruction of volatile organic

compound (VOC) [1–6]. The porous structure and sur-face chemistry of activated carbon are important prop-

erties in connection with its adsorbent behavior [7–15].

Since most carbon supported catalysts suffer from car-

bon burn-off in the case of gas phase oxidation (>500

K), it is important to provide long-term stability by

0008-6223/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2005.01.035

* Corresponding author. Tel.: +82 2 2123 2762; fax: +82 2 312 6401.

E-mail address: leech@yonsei.ac.kr (C.-H. Lee).

keeping them from gasifying to carbon dioxide. In order

to improve the resistance of carbon supports to gasifica-

tion under air oxidation conditions, many studies have

been carried out [9,16–18]. Coloma et al. [16] reported

that the formation of partial graphitization made the

surface of carbon more resistant to air oxidation due

to the p-complex structures (p-sites) in the carbon basalplanes. These p-sites are developed when oxygen surfacecomplexes are removed from the surface of activated

carbon, e.g., by heat treatment in an inert atmosphere

[16–18]. These delocalized p electrons have been usedto account for the basicity of carbons without (or with

a small amount) of oxygen surface groups [16].

This work investigated the activity and stability of

various Pt/activated carbon catalysts in CO oxidation.The carbon supports and catalysts were character-

ized by BET, XPS, and CO-chemisorption. From these

M. Kang et al. / Carbon 43 (2005) 1512–1516 1513

results, the effects of heat treatment at different temper-

atures on the pore structure, the surface concentration

of Pt0, and the catalyst loading (or dispersion) of acti-

vated carbon were studied. In particular, the formation

of p sites under various heat treatment conditions wasinvestigated.

2. Experimental

2.1. Preparation of activated carbon supports and

catalysts

The activated carbon (AC) used as parental materialwas obtained from the Norit Corp. (Norit ROX 0.8).

The AC was washed with 2 M HCl solution for 12 h

under reflux for the purpose of removing sulfur and

some ash, and then washed with distilled water under

reflux for 6 h. This treated AC was dried at 383 K over-

night. The AC was evacuated at 573 K for 10 h, and

then heated to a final temperature (1573, 1673 and

1773 K, respectively) in the condition of oxygen-freedried nitrogen. After the sample was maintained at the

final temperature for 1 h, it was cooled to room temper-

ature. In this study, the samples prepared at three differ-

ent final temperatures were referred to as AC1573,

AC1673 and AC1773, respectively.

Catalysts were prepared via the incipient wetness

method aiming for 2 wt.% Pt on the AC using purchased

precursor salt, H2PtCl6 Æ xH2O. The impregnated cata-lysts were dried in the nitrogen condition at 393 K for

24 h to remove water and then calcined at 633 K for

12 h in the nitrogen condition to prevent the carbon

support from burning off.

2.2. Characterization

Nitrogen adsorption isotherms at the temperature ofliquid nitrogen were obtained by a standard method.

The samples were preliminarily outgassed at 573 K for

3 h in a stream of helium. The range of relative pressures

(P/P0) varied from 0.0009 to 0.95; the equilibration time

was 10 s. The overall volume of micropores (WODR) and

the standard free energy of adsorption (E0) were calcu-

lated from the adsorption isotherms using the Dubi-

nin–Radushkevich equation [19]. The microporesurface area (SDR) and average pore diameters (LDR)

were calculated according to the following semi-empiri-

cal relations (Eqs. (1) and (2)) proposed by Stoeckli [20].

SDR ðm2=gÞ ¼ 2000W 0 ðcm3=gÞ=LDR ðnmÞ ð1Þ

LDR ðnmÞ ¼ 10:8=ðE0 � 11:4ÞðE0 ½kJ=mol�Þ ð2ÞX-ray diffraction experiments were performed on a

standard diffractometer using CuKa radiation. The cal-cined samples were scanned in the 2h range of 10–80�.

XPS was carried out with an aluminum anode as

unmonochromatized X-ray source (1486.6 eV run at

10 kV and 15 mA, fixed analyzer transmission).

Metal loading was probed in a conventional flow sys-

tem with a thermal conductivity detector, using hydro-

gen as a carrier gas. A short-time reduction of thecatalysts was made in situ (heating the sample to

373 K in flowing hydrogen for 5–10 min, followed by

cooling to ambient temperature). Several pulses of CO

(4.0 lmol) were then injected in 1-min interval. For themeasurements, 30–80 mg of catalyst was taken and 2–4

injections of CO were proved to be sufficient for satura-

tion. The stoichiometric coefficient was assumed to be

unity.

2.3. Activity test

The carbon monoxide oxidation tests were performed

under a stoichiometric condition in 1% CO/N2 and 0.5%

O2/N2 at a fixed space velocity of 3 · 103 h�1. Each runutilized approximately 1 g of catalyst with 80–120 mesh

particles. The analyses of effluent gases were performedwith an on-line gas chromatography with a TCD and

Carboxen-1000 column (60/80 mesh, 15 ft · 1/8 in. stain-less steel). At each reaction temperature, the reaction

continued for about 2 h to achieve steady-state activity.

The activity measurement was conducted in an ascend-

ing temperature manner so that the light-off behavior

was recorded. The catalytic activity was ranked based

on the light-off temperature (T50) at which 50% conver-sion occurred. In case of the stability test, a long-term

time on the stream (TOS) experiment using carbon mon-

oxide oxidation test was performed under the stoichio-

metric condition of 1% CO/N2 and 0.5% O2/N2 at a

fixed space velocity of 3 · 103 h�1 and 450 K.

3. Results and discussion

Fig. 1 shows the adsorption isotherms of the raw AC

and those treated at 1573 (AC1573), 1673 (AC1673) and

1773 K (AC1773), respectively. The microstructure

parameters of these materials obtained by analyzing

the shape of the isotherm are presented in Table 1.

The treatment of the carbon at 1573 K resulted in the

increase in the mean size of the micropores (0.77 nm forAC and 1.21 nm for AC1573) while its volume

(0.30 cm3/g) remained close to that of the untreated car-

bon (0.32 cm3/g). The inner surface area (SDR) of

AC1573 was decreased by about 40% compared with

that of the AC sample.

The carbon treated at the highest temperature

(1773 K) showed a dramatic decrease in the volume of

micropores by about 90%, from 0.32 cm3/g (AC) to0.037 cm3/g (AC1773). Therefore, the internal surface

area (SDR) of AC1773 (64 m2/g) was less than 10% of

0.0 0.2 0.4 0.6 0.8 1.00

100

200

300

AC1673

AC1773

AC1573

AC

Vad

s (cm

3 at

STP

)

P/P0

Fig. 1. Nitrogen adsorption isotherms (77 K) of the heat-treated ACs.

Table 1

Characterization of the pore structure of the ACs investigated

Carbon materials

AC AC1573 AC1673 AC1773

WODR (cm3/g) 0.32 0.30 0.19 0.037

SDR (m2/g) 854 540 237 64

LDR (nm) 0.77 1.21 1.22 1.22

E0 (kJ/mol) 26.0 19.8 19.6 19.5

0 10 20 30 40 50 60 70

C(002)

AC1673

AC1573

AC

Inte

nsity

(a.

u.)

2θ

Fig. 2. XRD profiles of the heat-treated ACs at different temperatures.

Table 2

Peak position of Pt4f and relative abundance of different Pt species

determined from XPS measurements on the Pt/AC systems

Pt4f7/2(eV)

Pt4f/C1sa Pt4f5/2(eV)

Pt0

(%)

PtO

(%)

PtO2(%)

Pt/AC 71.55 1.34 74.65 58.4 27.8 13.8

Pt/AC1573 71.55 1.45 74.8 64.7 23.8 11.5

Pt/AC1673 71.55 1.42 74.75 75.2 18.3 6.5

Pt/AC1773 71.55 0.56 74.75 78.0 12.4 9.6

a Determined by XPS corrected for atomic sensitivity factors (area)

[22].

1514 M. Kang et al. / Carbon 43 (2005) 1512–1516

that of the AC sample. The average size of the microp-

ores (LDR = 1.22 nm) was identical to that of other sam-

ples, AC1573 and AC1673.

The results of the XRD studies of raw and thermally

treated carbon supports are presented in Fig. 2. The

XRD patterns show that the structures of the AC and

AC1573 materials are nearly amorphous. In the

AC1673, a distinct ordering of the atomic structurecould be observed. The result indicates that the partly

graphitized structure was formed when the activated

carbon was heated to high temperature (1673 K).

A model for describing the chemistry of the impreg-

nation of hexachloroplatinic acid on a carbon support

has been proposed, in which two types of ligand sites

for the Pt precursor were assumed: (1) oxygen-contain-

ing functional groups on the basal plane edges and (2)p-complex structures in the carbon basal plane [16,21].In this study, this model was applied to predict the

impregnation of Pt precursors on the AC samples.

The XPS peak position and the area obtained from

the fit are listed in Table 2. The corresponding individ-

ual doublets and their calculated superposition are in-

cluded in Fig. 3. The oxidation state of active metal is

at least in part responsible for the catalytic behavior.

The broad Pt4f5/2/4f7/2 doublets apparently represent a

superposition of contribution from several oxidation

states. For the analysis of the oxidation of active Pt,

each composite peak shape was deconvoluted into

three-doublet binding energies. The deconvolution pro-

cedure should follow the requirement that both the

Pt4f band position and the multiplet splitting must beconsistent with the characteristic ranges of each species.

As a test, the intensity ratio for the doublet components

of a given species was not fixed during the fit.

The oxidation state of Pt appears to be important in

determining the catalytic activity. Thus, the concentra-

tion of Pt0 was calculated by the XPS deconvolution

procedure; 58.4% for Pt/AC, 64.7% for Pt/AC1573,

75.2% for Pt/AC1673, and 78% for Pt/AC1773. This re-sult indicates that Pt0 concentration increases with an in-

crease in the heat treatment temperature. On the other

hand, as shown in Table 2, the concentrations of PtO

and PtO2 decreased as the heat treatment temperature

Inte

nsit

y (a

.u.)

68 70 72 74 76 78 80

AC1673

AC

AC1573

Pt0

Binding energy (eV)

Fig. 3. Curve fit of the XPS 4f spectra of the Pt/AC systems.

40

60

80

100 Pt/AC1673 Pt/AC1573 Pt/AC Pt/AC1773 Conventional catalyst

Con

vers

ion

to C

O2 (

%)

M. Kang et al. / Carbon 43 (2005) 1512–1516 1515

increased. From the van Dam�s model, two types of li-gand sites for the Pt precursor were presented on a car-

bon support: oxygen-containing functional groups and

p-complex structures (p-sites) [21]. PtO and PtO2 mayhave been formed on the oxygen-containing functionalgroups while Pt0 on the p-sites. When the heat treatmenttemperature was increased, the Pt0 concentration in-

creased while the PtO and PtO2 concentrations de-

creased. From this result, it may be concluded that

more p-sites were created as the heat treatment temper-ature became higher.

Table 3 presents the loading and the average particle

size of Pt catalysts on the AC determined by CO-chemi-sorption. The highest loading was obtained from the Pt/

AC1673 sample. This increased loading ability can be

explained by the special interaction of the graphitic

planes (p-sites) with the metal particles [16]. Since thedensity of p-electrons on the metal was increased bythe heat treatment, such high loading of Pt catalysts

may be observed on the Pt/AC1673 sample. The lower

loading of Pt/AC1773 is closely related to the textureof AC1773 (SDR = 64 m

2/g, WODR = 0.037 cm3/g) as

mentioned in Table 1. Since most of the Pt particles

Table 3

CO-chemisorption results of the Pt/AC systems

CO-chemisorption

(ml CO/g catalyst)

Loading

(%)

dp(nm)

Pt/AC 1.43 13.6 8.3 (10.3)a

Pt/AC1573 1.07 10.5 10.7

Pt/AC1673 1.82 15.4 7.3

Pt/AC1773 0.98 9.3 12.0

a Before calcination (633 K).

are located in the pore region, moderate surface area

and porosity are essential for better metal loading. In

the case of the Pt/AC catalyst, the surface oxygen groups

can participate in the genesis of the catalysts favoring

the sintering of metal particles. However, it appears that

when the thermally unstable surface oxygen groups (e.g.,carboxyl group) decompose during the calcination step

(633 K), the re-loading of metal particles takes place.

H2-chemisorption was attempted to measure the loading

of Pt. However, no reliable data could be obtained, pos-

sibly due to the strong hydrogen adsorption of activated

carbon.

Fig. 4 shows the light-off curves of the Pt/AC systems.

Their activity increased with the heat treatment from Pt/AC to Pt/AC1673. In addition, the activity of Pt/

AC1673 was higher than that of the Pt-based conven-

tional catalysts. The order of carbon monoxide oxida-

tion activity was consistent with the loading and

surface concentration of Pt as well as with the fraction

of Pt0. The Pt/AC1773 gives the lowest activity due to

the lowest surface area and the surface concentration

of Pt. Based on the catalytic activity, CO-chemisorptionand XPS results, it is concluded that the well-loaded Pt0

species mainly contribute to catalytic activity.

The stability of Pt/AC is critical in applications be-

cause carbon can be burned-off in the air. The total car-

bon monoxide oxidation of the Pt/AC1673, which

showed highest activity and CO-chemisorption in this

study, was carried out continuously with 1% CO gas

at 450 K for more than 80 h. The degree of oxidationwas measured by the weight change of the Pt/AC1673

[5]. The weight of the Pt/AC1673 was almost unchanged

300 350 400 450 500

0

20

Temperature (K)

Fig. 4. CO conversion light-off curves of the Pt/AC systems. The

reaction gas mixture consisted of 1% CO/N2 and 0.5% O2/N2 at

GHSV = 3000 h�1.

1516 M. Kang et al. / Carbon 43 (2005) 1512–1516

after the reaction. It implies that the Pt/AC1673, which

has the best physical and chemical properties in this

study, is considerably stable for a long time under the

oxidizing condition. It seems that the partially graphi-

tized surface of AC1673 contributes to enhancing the

resistance to air oxidation.

Acknowledgment

This work was supported by Korea Research Foun-

dation Grant (KRF-2001-005-E00031).

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