Jointly published by React.Kinet.Catal.Lett.Akadémiai Kiadó, Budapest Vol. 76, No. 2, 207-212and Kluwer Academic Publishers, Dordrecht (2002)

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RKCL4076

PALLADIUM CATALYSTS SUPPORTED ON ACTIVATED CARBONWITH DIFFERENT TEXTURAL AND SURFACE CHEMICAL

PROPERTIES

Min Kang*, Min Woo Song and Kyung Lim KimDepartment of Chemical Engineering, Yonsei University, Seoul 120-749, Korea

Received February 4, 2002Accepted April 3 2002

Abstract

A commercial activated carbon was used as catalyst support in Pd/AC catalysts.The effects of the different surface oxygen groups and textural properties of thecarbon supports on the metal dispersion of the supported catalysts were analyzed.

Keywords: Activated carbon, surface oxygen group, dispersion

INTRODUCTION

The design of carbon-supported catalysts is based on the knowledge ofinteraction and location of the active species. The main role of the carbonsupport is to facilitate the formation and the stability of well-dispersed activesites since the metal particles interact weakly with the carbon support. Theinteraction degree between activated carbon and metal depends on the texturalproperties and the nature and concentration of surface oxygen groups. Thesurface oxygen groups may be modified by suitable thermal or chemicaltreatment. Oxidation can increase the concentration of surface oxygen groups,while heating under inert atmosphere may be used to selectively remove someof these functions. Carboxyl, carbonyl, phenol, quinone and lactone groupshave been identified on carbon surfaces [1].

208 MIN KANG et al.: ACTIVATED CARBON

Some authors [2] have found that the interaction of metal precursormolecules with carbonaceous supports, by means of surface oxygen groups,leads to a high dispersion. However, in another work [3], it is shown that theoxidation of an activated carbon used as support has a negative influence ongetting a catalyst with a high dispersion. Also, the properties of carbon-supported catalyst are influenced by other factors, i.e., the preparation method,the textural and chemical properties of supports, the nature of the metalprecursor used [4]. This paper describes the Pd/AC to investigate the influence of the supportsurface chemistry on the properties of the catalysts prepared by impregnation.Supports after different treatments have been used to prepare palladiumcatalysts. Supports and catalysts were characterized by BET, TPD and H2

chemisorption and tested on the gas phase benzene hydrogenation.

EXPERIMENTAL

The activated carbon used as support was obtained from the Norit Corp.(Norit ROX 0.8). This sample was subjected to different pretreatments tomodify its porous structure and surface chemistry: With 5% O2 (in N2) at 698 Kfor different times in order to achieve the desired burn-off (B.O). The followingsamples were prepared: AC1-original carbon; AC2-AC1 oxidized with 5% O2

for 6 h at 698 K (B.O.=10.4%); AC3-AC1 oxidized with 5% O2 for 10 h at698 K (B.O.=18.5%); AC4-AC2 treated under nitrogen at 873 K for 1 h. The 2 wt.% Pd/AC catalysts were prepared by wet impregnation of eachactivated carbon with solutions of metal nitrate. The impregnated catalysts weredried in an oven for 24 h at 393 K and then calcined at 723 K in air. Thecalcined catalysts were subsequently reduced in a H2 at 673 K for 2 h. The textural characterization of the samples was based on the N2 adsorption-desorption isotherm, determined at 77 K with a Micromeritics ASAP 2000system. The micropore volumes and mesopore surface areas were determinedby the t-method [5]. To determine the amount of surface oxygen groups, TPD process wasperformed on all supports. The temperature increased at a rate of 5 K/min up to1273 K. The decomposition products were measured by on-line massspectrometry. The pH of the aqueous slurry of the supports was measured as follows: theslurries were prepared in a ratio of 10 mL of water per 1 gram of carbon; thismixture was stirred and the pH was measured several times until a constantvalue was reached.

MIN KANG et al.: ACTIVATED CARBON 209

H2 chemisorption was measured in a static volumetric apparatus equippedwith a high-vacuum system and a Setra pressure transducer (Model 204D).Amounts of irreversibly chemisorbed hydrogen, estimated from hydrogendesorption isotherm by a back-extrapolation method, constituted the basis forcalculating metal dispersions. Catalytic activity measurements were carried out in a fixed bed reactor using200 mg of catalyst. The reaction was performed at 373 K with a reactionmixture (50 s cm3) containing hydrogen and benzene in a H2/C6H6 ratio of 11.

RESULTS AND DISCUSSION

The textural properties of all supports are collected in Table 1, whilenitrogen adsorption isotherms are shown in Fig. 1. It may be concluded that theoxidation process increases the micropore volume and the mesopore surfacearea. The support submitted to thermal treatments under inert atmosphere(AC4) shows a similar micropore volume with respect to the parent material,AC2 but a highest mesopore surface area.

Table 1

Textural properties of activated carbon supports

Sample Vmicro (cm3/g) Smeso (m2/g)

AC1 0.392 125AC2 0.442 150AC3 0.537 167AC4 0.457 208

In oxidations the pore volume increases with burn-off, but it may beexpected that, for higher degrees of oxidation, there will be a decrease in themicropore volume and subsequently in the mesopore volume, due to thecollapse of the pores. By TPD it can be seen that surface oxygen groups are converted to CO2 andCO. There are many publications dealing with these groups [1,6]. They areclassified as acidic, basic or neutral groups depending on the pH of the carbonsupport. These groups decompose upon heating in an inert atmosphere, themost acidic evolving as CO2 and the less acidic, and more stable, evolving asCO [6]. The amounts of CO2 and CO released, total oxygen and pH value aregiven in Table 2. Oxidation processes lead to an increase in all type of surface

210 MIN KANG et al.: ACTIVATED CARBON

oxygen groups. Oxidation results in an increase in concentration of COreleasing surface groups where the increase in CO2 releasing groups is ratherlimited. Also, oxidation treatment leads to a more acidic surface confirmed bypH values.

0.0 0.2 0.4 0.6 0.8 1.0

200

300

400

AC4

AC2

AC1

AC3

Vad

s (cm

3 at

STP

)

P/P0

Fig. 1. Nitrogen adsorption isotherms at 77 K on the activated carbon before andafter different oxidative treatments

All reduced Pd/AC catalysts were studied by H2 chemisorption to determinethe dispersion as shown in Fig. 2. A linear relationship has been found betweenthe metal dispersion and the total amount of surface oxygen groups. Since theoxygen groups on activated carbon support are the anchoring sites for themetallic precursors as well as the metallic phase after reduction [7], the H2

chemisorption results are in accordance with the results obtained in the surfacechemical analysis of the activated supports. Also, the surface charge presentedas pH (acidity) arises from the interaction between the carbon surface and theaqueous solution, and will determine the strength of the interaction with themetal precursors. This changes the dispersion.

MIN KANG et al.: ACTIVATED CARBON 211

Table 2

CO2 and CO evolution in the TPD experiments of supports and their pH values

Support CO2 (µmol/g)a CO (µmol/g)a pH Total O2b (µmol/g)

AC1 62 604 7.2 364AC2 274 1299 5.3 923.5AC3 362 1613 4.8 1168.5AC4 131 987 6.9 624.5

a Amount of gas evolved in the TPD analyses up to 1273 Kb Calculated from the amounts of CO and CO2 emitted [0.5(CO)+(CO2)]

0

100

200

300

400

500

Hyd

roge

n up

take

(µm

ol/g

cat)

Act

ivit

y (µ

mol

gPt-1

s-1)

0

10

20

30

40

50

Pd/AC4Pd/AC3Pd/AC2Pd/AC1

Catalysts

Fig. 2. Amounts of irreversible hydrogen adsorption and the activity in benzenehydrogenation on all catalysts

212 MIN KANG et al.: ACTIVATED CARBON

An increase in metal dispersion was observed with the increase in the totalamount of oxygen and acidity. The only exception to this trend occurs withPd/AC4, showing the highest mesopore surface area. This result can beexplained by textural properties. It is known that the pore walls of the mesoporeare the best places for the metallic species to adsorb. Though the chemicalsurface composition of activated carbon is the dominant factor determining thedispersion of palladium, the mesopore surface area can also affect on thedispersion. The gas phase benzene hydrogenation is considered to be a structure-insensitive reaction on platinum group metals, in such a way that its rate isproportional to the amount of surface metal atoms, not being affected by theparticle size or crystallographic plane exposed. The results for all catalysts arepresented in Fig. 2. The results indicate that the activity of the catalystsupported on the original activated carbon is lower than those of the catalystssupported on the oxidized and heat-treated supports. Therefore, benzenehydrogenation results are in agreement with the H2 chemisorption results.

REFERENCES

1. J.L. Figueiredo, M.F.R. Pereira, M.M.A. Freitas, J.J.M. Orfao: Carbon, 37, 1379 (1999).2. J.S. Noh, J.A. Schwartz: Carbon, 28, 675 (1990).3. H.E. Van Dam, H. Van Bekken: J. Catal., 131, 335 (1991).4. N. Krishnankutty, J. Li, M.A. Vannice: Appl. Catal., 173, 137 (1998).5. K.S.W. Sing: Carbon, 27, 5 (1989).6. U. Zielke, K.J. Hüttinger, W.P. Hoffman: Carbon, 34, 983 (1996).7. F. Rodriguez-Reinoso: Carbon, 36, 159 (1998).