ORIGINAL PAPER

A Novel Catalyst Type Containing Noble Metal NanoparticlesSupported on Mesoporous Carbon: Synthesis, Characterizationand Catalytic Properties

Endre Horvath Æ Robert Puskas Æ Robert Remias ÆMelinda Mohl Æ Akos Kukovecz Æ Zoltan Konya ÆImre Kiricsi

Published online: 28 April 2009

� Springer Science+Business Media, LLC 2009

Abstract We report on a method for the controlled syn-

thesis of a new type of high specific surface area mesoporous

carbons denoted as the CMH family. By using mixtures of

colloidal silica particles as templates it was possible to

synthesize samples exhibiting 1,630 m2 g-1 specific surface

area and 4.37 cm3 g-1 pore volume. CMH materials exhibit

high thermal stability in oxygen and can be used as catalyst

supports. This function was demonstrated by synthesizing

Pt/CMH and Rh/CMH catalysts and testing them in the

hydrogenation of cyclohexene. We have found Pt/CMH to be

more stable and easier to regenerate than Rh/CMH.

Keywords Metal nanoparticles � Mesoporous carbons �Catalytic hydrogenation

1 Introduction

Porous materials fall into three different classes according

to the IUPAC nomenclature: (i) microporous materials

with pore diameter less than 2 nm, (ii) mesoporous struc-

tures having pore diameter between 2 and 50 nm and (iii)

macroporous solids possessing pore diameter larger than

50 nm [1]. The well known representatives of microporous

materials are the zeolites and zeotypes having more than

200 different synthetic and natural structures [2, 3]. There

are several mesoporous solids known. They can be

synthesized from silica and alumina as well as from various

oxides and sulfides [4]. Oxides and sulfides as catalyst

support are not neutral, but rather, they are more or less

acidic challenging the preparation of bifunctional catalysts

upon loading metal components [5].

Since carbon as a support material may be regarded as

neutral solid [6, 7], it seemed interesting to test it in the two-

step catalyst preparation procedure recently developed by

Somorjai and coworkers [8–11]. Briefly, the procedure is

based on the preparation of noble metal nanoparticles and

inserting them into mesoporous silicates either by mixing the

nanoparticles into the reactive gel for the crystallization or

inserting them after the preparation of mesoporous silicates

using incipient wetness or ultrasound aided impregnation.

Several papers were published in the last decade on the

synthesis of mesoporous carbon using mesoporous silicates

as template. The mesopores of MCM-48 or SBA-15 type

materials were filled with carbohydrates [12], phenol–

formaldehyde resins [13] or styrene [14] and these carbon

sources were carbonized at high temperature while

excluding oxygen and moisture. Carbon nanotubes were

obtained after dissolving the silicate component with con-

centrated HF or NaOH solutions. Others showed that the

template used for the synthesis of mesoporous silicate can

be carbonized and thus the production of multiwall carbon

nanotubes can be simplified [15]. There are also reports on

the synthesis of single wall carbon nanotubes by the car-

bonization of template molecules in zeolites [16].

Recently, a new trend appeared for the preparation of

mesoporous carbon material with regular pore system. In

these processes silica sol particles are used as template.

The process can be characterized as follows. The solvent of

the silica sol solution is evaporated and the resulting solid

phase is dried. The resulting assembly of spherical particles

features pores of quite homogeneous size distribution.

E. Horvath � R. Puskas � R. Remias � M. Mohl � A. Kukovecz �Z. Konya (&) � I. Kiricsi (&)

Department of Applied and Environmental Chemistry,

University of Szeged, Rerrich Bela ter 1, 6720 Szeged, Hungary

e-mail: konya@chem.u-szeged.hu

I. Kiricsi

e-mail: kiricsi@chem.u-szeged.hu

123

Top Catal (2009) 52:1242–1250

DOI 10.1007/s11244-009-9277-2

These empty spaces are filled with organic compounds as

carbon sources. After the carbonization of the organic

matter at high temperature (generally above 1,000 K), the

silicate phase is dissolved in HF solution and the material

washed and dried. Using such a procedure carbon with high

specific surface area (generally close to 1,000 m2 g-1) can

be obtained. Table 1 summarizes the techniques used for

the synthesis of mesoporous carbon [10–14, 17–27]. We

applied a derivative of the described procedure insofar as

chemical vapor deposition of acetylene and/or ethylene

was used for covering the silicate particles with carbon.

In this article, we report on the systematic synthesis and

characterization of a new type of mesoporous carbon,

CMH, mesoporous carbon from Hungary and its use as

support material for pre-prepared catalytically active metal

nanoparticles. We conclude by demonstrating the catalytic

activity of the synthesized catalysts.

2 Experimental

2.1 Materials

Three different Ludox� silica colloids from Sigma-Aldrich

were used as template material. Colloids SM30 (30 w%

silica contents), AS40 (40 w%) and HS40 (40 w%)

featured 5–8, 13–20 and 8–11 nm average particle size

diameter, respectively.

Acetylene (Messer Hungaria Ltd.) was the carbon source

and was diluted with nitrogen (Messer Hungaria Ltd.).

For dissolution of silica template, 38% HF solution

(Molar Chemicals Ltd., Hungary) was used.

2.2 Synthesis of Mesoporous Carbon

Generally 30 g of Ludox� SM30, AS40 or HS40 colloid

silica was used for the synthesis. We applied these mate-

rials both separately and as bicomponent mixtures with 1:3,

1:1 and 3:1 ratios. All together 12 different compositions

were prepared. The solvent was evaporated in a rotary

evaporator at 363–423 K. The evaporation took 1–1.5 h.

After evaporation the solid matter was dried at 423 K in air

in order to remove the rest of moisture. The obtained dried

silicas were the parent materials for the synthesis of mes-

ostructures. A portion of this dried silica was weighed and

placed in a quartz boat into a tubular quartz reactor. The

reactor was purged with 300 cm3 min-1 nitrogen flow for

10 min in order to remove air and traces of moisture. Then

the reactor was pushed into a preheated (973 K) furnace

and acetylene with a flow rate of 60 cm3 min-1 was

introduced. (The schematic drawing of the reactor system

is presented in Fig. 1).

Table 1 Preparation of mesoporous carbon structures using silicate templates as found in the literature

Name Template Carbon source CVD (K) Carbonization

(K)

Surface area

(m2 g-1)

Pore volume

(cm3 g-1)

Ref.

CMK-1 MCM-48 Sucrose/sulfuric acid 1,073–1,373 1,380 0.3–1.1 [12]

Ludox� HS-40 Resorcinol–formaldehyde gel 1,123 1,512 3.6 [17]

Monodisperse

silica particles

(40–90 nm)

Phenol–formaldehyde/sulfuric

acid

1,073 750 1.68 [13]

MCM-48 Propylene 1,023–1173 850 0.5 [18]

Sucrose/sulfuric acid 1,473 1,500 0.9

CMK-1 MCM-48 Sucrose/sulfuric acid 1,173 1,238 0.75 [19]

CMT-1 Acetylene 1,073 1,076 0.63

CMK-3I SBA-15 Sucrose/sulfuric acid 1,173 1,153 1.21 [20]

CMK3V(950) Styrene 1,123–1,323 1,022 0.89

SBA-15 Acetonitrile 1,173–1,273 779–1,019 0.66–0.83 [14]

SBA-15 Sucrose/sulfuric acid 1,173 724–1,570 0.57–1.23 [21]

Y zeolites Propylene/butylene 993–1,033 1,173 1,344–2,470 0.70–1.44 [22]

Silica S Furfuryl alcohol/oxalic acid 1,173 620 0.87 [23]

Silica T 1,043 1.22

Silica Xerogel Furfuryl alcohol/

p-toluenesulfonic acid

1,073 1,480–2,190 1.53–2.1 [24]

Silica (24 nm) Resorcinol and crotonaldehyde/

oxalic acid

1,173 *1,800 6 [25]

Silica (150–800 nm) Phenol–formaldehyde/sulfuric acid 1,273 706 NA [26]

Ludox� HS-40 Resorcinol–formaldehyde gel 1,123 653–1,228 1.21–5.46 [27]

Top Catal (2009) 52:1242–1250 1243

123

After 60 min reaction time, the acetylene flow was

stopped and the reactor temperature was raised to 1,170 K

in 8 min. The carbonization procedure was continued for

60 min. The reactor was removed from the furnace and

allowed to cool to room temperature. The weight of the

product was measured and the material was characterized

by physical–chemical techniques.

2.3 Synthesis of Pt or Rh Nanoparticles Containing

Mesoporous Carbon

Thirty-five cubic centimeter SM30 silica sol and 1 cm3

5.5 mg cm-3 rhodium sol were combined. The synthesis

procedure of the rhodium sol was described elsewhere [28].

The solvent was evaporated using a rotary evaporator.

The carbonization was performed as described above. The

silica template was removed and the final product was

characterized.

Thirty-five cubic centimeter SM30 silica sol and 40 cm3

1,300 mg dm-3 Pt(NH3)4Cl2 solution was combined and

the solvent evaporated from the mixture. The carbonization

and the silica removal were performed as described above.

The scheme of synthesis procedures can be seen in Fig. 2.

For the catalytic tests, the samples were evacuated and

treated at 573 K for 1 h. The procedure was repeated three

times.

2.4 Separation of Silica

The silica component was dissolved in 150 cm3 40% HF

solution for 12 h. After the treatment, the material was

washed three times with distilled water at 353 K. The

resulting material was duly characterized.

2.5 Characterization

The samples were characterized after each preparation step

to monitor the changes in the specific surface area as

determined by an automatic adsorption equipment

(Quantachrome Nova 2200) at 77 K using nitrogen. Sam-

ples were dehydrated at 423 K for 1 h before the mea-

surement. The specific surface area was calculated by the

BET equation [29] from the adsorption branch. The mes-

opore size distribution was obtained from the desorption

branch of the N2 adsorption isotherm using the Barret–

Joiner–Halenda [30] method.

TEM images were taken to investigate the morphology

of the samples. The TEM measurements were performed in

a Philips CM10 microscope. The samples were sonicated in

an alcoholic solution; a drop of the suspension was put on a

copper mounted holey carbon film and measured after

drying. Enough TEM images were recorded to allow the

statistical characterization of the particle size distribution

of each sample.

In certain cases, XRD profiles were taken and the

TG-DTG diagrams were measured as well, using a Rigaku

Miniflex II XRD equipment and a Derivatograph Q (MOM,

Hungary) thermal analyzer, respectively.

The catalytic activity tests were performed in a contin-

uous flow reactor system equipped with on-line GC-MS

product analysis (Agilent GC 6820).

3 Results and Discussion

3.1 Preparation of Mesoporous Carbon (CMH)

Samples

The Ludox� samples as starting materials were investi-

gated first. The silica particles (average size 13–20 nm)

are stabilized by ammonium ions in Ludox� AS40. The

dried material forms a mesoporous structure characterized

by the adsorption–desorption isotherm shown in Fig. 3.

The calculated pore size distribution generated by the

Fig. 1 Scheme of reactor arrangement

Silica spheres andrhodium nanoparticles

Rhodium dopedsilica template

Rhodium dopedsilica-carbon composite

Rhodium dopedporous carbon

Solvent

Evaporation

Acetylene

973 K

HF-etching

Silica spheres andrhodium nanoparticles

Rhodium dopedsilica template

Rhodium dopedsilica-carbon composite

Rhodium dopedporous carbon

Solvent

Evaporation

Acetylene

973 K

HF-etching

Silica-Spheres

Solvent

Evaporation

Silica-Template Silica-Carbon Composite

Acetylene

973 K

HF-etching

Porous-Carbon

(a)

(b)

Fig. 2 Scheme of the synthesis

procedure of a pure mesoporous

carbon and b mesoporous

carbon containing metal

nanoparticles using SM30

colloid silica as template

1244 Top Catal (2009) 52:1242–1250

123

agglomerated silica particles is presented as an inset in the

corresponding nitrogen adsorption isotherms. The specific

surface area as determined by the BET method was

118 m2 g-1, the average pore size was 5 nm and the spe-

cific pore volume was 0.2 cm3 g-1. Figure 3 shows the

data obtained for Ludox� HS40. This sample had an

Fig. 3 Nitrogen adsorption isotherms (taken at 77 K), pore size distributions (as inset in the isotherms), TEM images and particle size

distribution histograms (as inset in TEM pictures) from starting silica colloid solutions. a SM30, b HS40, c AS40

Top Catal (2009) 52:1242–1250 1245

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average particle size 8–11 nm and the specific surface area

of the dried material was 112 m2 g-1. The maximum of the

pore size distribution was at 4.7 nm and the calculated

specific pore volume was 0.19 cm3 g-1. The characteristics

of Ludox� SM30 are presented in Fig. 3. The primary

silica particles in this colloid solution are stabilized by

sodium ions. The dry material showed the highest specific

surface area (239 m2 g-1) of the three starting materials.

The average pore size was 4 nm and the specific pore

volume 0.35 cm3 g-1.

The samples were placed into the sample holder of the

reactor for chemical vapor infiltration of acetylene. After

finishing this procedure, the solid product was recovered

and the product washed and dried. In Fig. 4, we summarize

the pore system characteristics of the resulting carbona-

ceous materials.

Binary mixtures of silica sols with 1:3, 1:1 and 3:1

concentration ratios were prepared. These samples were

characterized by the same methods that were used for the

pristine components.

When Ludox� colloid solutions AS40 and HS40 were

mixed and treated with acetylene followed by graphitiza-

tion, products with smaller specific surface areas and pore

volumes were obtained as seen in Fig. 5a and Table 2.

These results demonstrate that when using the novel

preparation method developed in our laboratory it is pos-

sible to fine-tune the specific surface area and the pore

volume simply by choosing the appropriate mixture of

starting Ludox� solutions.

The preparation method described here as CMH tech-

nology yields mesoporous carbon products of rather high

specific surface area when using a starting mixture con-

sisting of SM30 and HS40 solutions (Fig. 5b). It demon-

strates the possibility of fine tuning of the surface area and

pore volume.

Figure 5c shows the nitrogen adsorption isotherms of

carbon samples obtained using SM30–AS40 mixtures.

Nitrogen adsorption in the final mesoporous carbon product

increases with increasing amount of SM30 in the binary

silica sol mixtures. The carbon material obtained by using

SM30 silica display the highest specific surface area and

porosity: 1630 m2 g-1 and 4.37 cm3 g-1 pore volume,

respectively. These are rather high values when compared

to the mesoporous carbons found in the literature and listed

in Table 1.

3.2 Preparation of Supported Metal Catalysts

If these mesoporous carbon samples are to be applied as

catalyst supports their thermal stabilities should be known.

The evolution of the weight of the mesoporous carbon

prepared using SM30 (the one possessing the highest spe-

cific surface area) at increasing temperatures is presented in

Fig. 4 Textual characterization

of the intermediate products of

mesoporous carbon preparation:

a Ludox� SM30 colloidal silica

after solvent evaporation

(template), b Ludox� SM30

silica carbon composite, cgrounded Ludox� SM30 silica

carbon composite, and dresulting mesoporous carbon,

CMHSM30

Fig. 5 Nitrogen adsorption isotherms of mesoporous carbon materi-

als prepared using different Ludox� silica templates and their

mixtures

1246 Top Catal (2009) 52:1242–1250

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Fig. 6. Only a very minor gradual weight decrease (less

than 3 wt%) was detected for samples heated up in nitro-

gen atmosphere. When the sample was heated in air the

weight loss started 773 K and ended around 1,123 K. The

sample burned almost completely off; only 2 wt% of ‘‘ash’’

remained. The temperature of 773 K at the onset of the

weight loss can be considered rather high compared to the

thermal stability of other carbonaceous structures. It is also

interesting to note that the temperature corresponding to

the largest weight loss of the reported CMH material is

second only to diamond and is considerably higher than

that of, e.g. multi-wall carbon nanotubes, C60 or amorphous

carbon (see marks on the -50 wt% line in Fig. 6 [31–33]).

This feature is attributed to the higher level of carboniza-

tion or graphitization of the CMH mesocarbon.

The mechanical stability of mesoporous carbon towards

pressure was studied as follows. A portion of mesoporous

carbon sample was pressed into pellets using various

pressures (5, 50, 150 and 300 kp cm-2) for 1 min. The

pellets were crushed. Nitrogen adsorption isotherms were

determined. The BET specific surface areas and the cal-

culated pore volumes are plotted in Fig. 7. It is seen that

both parameters decreased gradually with increasing

pressure.

In Fig. 8, the XRD profiles of the platinum containing

CMHSM30 catalyst are shown. The bottom profile shows

the XRD of the pristine mesoporous carbon sample con-

taining no platinum. The middle profile represents the

catalyst used in the hydrogenation of cyclohexene (the

TEM image of this sample can be seen as inset of Fig. 8).

For comparison, in Fig. 8c, we also present the XRD

profile a sample prepared by the impregnation of CMHSM30

carbon with Pt nanoparticle colloid solution [28].

The reflections at 2h = 39.9� (1), 46.4� (2), 67.7� (3) are

characteristic of the (1 1 1), (2 0 0) and (2 2 0) reflections

of the face centered cubic Pt crystallite, respectively. The

matching of these reflections in the two platinum contain-

ing samples proves the presence of platinum in the catalyst

samples prepared by the method described before. Using

the half-width of the Pt(1 1 1) peak marked (1) in Fig. 8b,

the average size of the Pt crystallite was calculated as

4.7 nm based on the Scherrer formula.

In Fig. 9, the XRD patterns of rhodium nanoparticle

containing samples are depicted. Profile a corresponds to

the dried silica sol treated with rhodium nanoparticles. The

lack of reflections is due to the small concentration of

metal in this sample (2 9 10-5 wt% Rh). After subjecting

this sample to the CVD process in order to develop the

carbonaceous coverage the XRD profile b was obtained.

Table 2 Texture of mesoporous carbon materials prepared using different Ludox� silica templates and their two component mixtures

Used template (weight ratio) SBET (m2 g-1) Vp (cm3 g-1) SBET (m2 g-1) Vp (cm3 g-1) SBET (m2 g-1) Vp (cm3 g-1)

AS40 624 0.99

AS40:HS40 = 3:1 755 2.49

AS40:HS40 = 1:1 745 2.88

AS40:HS40 = 1:3 854 3.17

HS40 1,017 3.55

SM30:HS40 = 1:3 1,018 3.45

SM30:HS40 = 1:1 1,421 4.12

SM30:HS40 = 3:1 1,300 3.85

SM30 1,630 4.37

SM30:AS40 = 3:1 1,439 3.72

SM30:AS40 = 1:1 961 2.64

SM30:AS40 = 1:3 868 2.23

AS40 624 0.99

Fig. 6 Weight loss curves for: a MCHSM30 analyzed in N2

atmosphere, b MCHSM30, in air, c commercial activated carbon

(Fluka), in air, d purified MWCNT (made in our lab) in air. Literature

data at 50% weight loss are added for comparison: d amorphous

carbon [31]; m pure C60 [32]; r diamond [33]; h MWNTs

graphitized at 2,800 �C [33]

Top Catal (2009) 52:1242–1250 1247

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When silica template was dissolved from the sample the

profile c was registered. The XRD pattern of rhodium metal

particles in the mesoporous carbon is observed.

The CMHSM30 carbon sample was impregnated with

pre-prepared rhodium nanoparticle colloidal solution and

the XRD pattern recorded Fig. 9d. Three XRD peaks

characteristic peaks of the rhodium metal particles

(2h = 41.1�, 47.8� and 69.9�) were observed. The reflec-

tion at 2h = 41.1� corresponds to the Rh(1 1 1) plane. This

reflection was used to calculate the average metal particle

size by the Scherrer equation. For the samples (Fig. 9c, d),

the Scherrer equation gave Rh particles sizes of 10.9 and

11 nm, respectively. From these results, it seems that the

rhodium particle size does not change during the CVD

process. The size of the Rh particles in the Rh CMHSM30

composite material was confirmed with TEM (Fig. 10).

These XRD patterns prove that metal particles were

successfully incorporated into the mesoporous carbon

using the procedure developed in our laboratory.

3.3 Catalytic Experiments

The reactor experiments were carried on in a continuous

flow reactor system equipped with on-line GC-MS product

analysis. Catalysts were pretreated as follows. Heat treat-

ment of the catalyst precursor at 773 K either in N2 flow

(100 cm3 min-1) or in 50–50% N2/O2 flow for 1 h was

followed by a reductive treatment at 573 K in H2 flow

(30 cm3 min-1).

After pre-treatment, the reactor temperature was set to

323 K. Reactants were introduced into the reactor by bub-

bling a H2 stream (10 cm3 min-1) through a regulated

temperature (313 K for Pt and 273 K for Rh experiments)

saturator filled with cyclohexene. Cyclohexene feeds

amounted to 5.47 10-3 mol h-1 for Pt, and 7.9 10-4 mol h-1

for Rh containing catalysts. After the reaction, the catalysts

were regenerated by oxidation in 50–50% N2/O2 gas mixture

(100 cm3 min-1) at 773 K followed by reduction in H2

(30 cm3 min-1) at 573 for 1 h.

Since the catalyst support is carbon, we decided to start

with a mild pretreatment, i.e. heat treatment in nitrogen

followed by reduction in hydrogen. However, as it can be

Fig. 7 Mechanical stability of mesoporous carbon, measured for

CMHSM30 sample

Fig. 8 The XRD pattern of a the pristine mesoporous carbon sample

containing no Pt, b Pt-containing MCHSM30 catalyst and c a sample

prepared by impregnation of CMHSM30 carbon with Pt nanoparticle

colloidal solution. Inset shows TEM image of Pt-containing

MCHSM30 catalyst

Fig. 9 The XRD pattern of Rh-containing CMHSM30 catalyst at

different stages of the preparation a dried silica sol containing Rh

nanoparticles, b after subjecting this sample to acetylene CVD

process, c sample b after silica removal and d sample prepared by

impregnation of CMHSM30 carbon with Rh nanoparticle colloidal

solution

1248 Top Catal (2009) 52:1242–1250

123

seen in Fig. 11, fast deactivation was observed in this case.

After activating the used catalysts in 50–50% N2/O2 mix-

ture followed by the reduction step, Pt-containing samples

showed maximum (100%) conversion. The third repeated

experiments using regenerated catalyst samples showed

high reproducibility at maximum conversion for rather

long time on stream as well.

From the results obtained for Pt-containing catalysts, we

decided to use the 50–50% N2/O2 mixture for pretreatment.

The activated rhodium-containing catalyst featured maxi-

mum conversion at the beginning of cyclohexene hydro-

genation. However, after some time (generally after

40 min) the conversion decreased as depicted in Fig. 12. In

2 h reaction time, the conversion dropped to around 50%.

By varying the activation time and the O2 concentration in

the gas mixture, we confirmed that 1 h activation time and

50–50% N2/O2 gas mixture are the most adequate activa-

tion conditions.

4 Conclusions

High surface area and high pore volume mesoporous car-

bon materials—denoted as MCHs—were synthesized by

using mixtures of colloidal silica particles as templates. It

was demonstrated that the prepared materials exhibit high

thermal and mechanical stability making them suitable for

catalyst supports.

Nanostructured metal nanoparticle containing cata-

lysts—Pt/CMH and Rh/CMH—were synthesized by a one

step method. It was shown that the as-prepared catalysts are

active in the hydrogenation reaction of cyclohexene. The

catalyst pretreatment conditions are critical.

Fig. 10 TEM picture of Rh nanoparticles (a) and Rh MCHSM30

composite material (b)

Fig. 11 Influence of pretreatment of Pt-containing catalysts before

the reaction: (m) treatment at 773 K in N2 flow for 1 h, and in H2

stream at 573 K for 1 h; (h,d) treatment in 50% O2/50% N2 stream

at 773 K for 1 h and in H2 stream at 573 K for 1 h

Fig. 12 Influence of pretreatment of Rh-containing catalysts before

the reaction: (m) treatment in 50% O2/50% N2 flow at 773 K for

20 min; (h): treatment in 50% O2/50% N2 stream at 773 K for 1 h;

(d) treatment in 100% O2 stream at 773 K for 1 h

Top Catal (2009) 52:1242–1250 1249

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Acknowledgement The authors gratefully thank Prof. Gabor A.

Somorjai’s helpful discussion and comments. This work was sup-

ported by the Hungarian Research Fund OTKA K73676 and the FP6

STREP ‘‘SANES’’ (017310).

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