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Materials Chemistry and Physics 103 (2007) 225–229

Effect of microstructures of Pt catalysts supported on carbon nanotubes(CNTs) and activated carbon (AC) for nitrobenzene hydrogenation

Yun Zhao a,b,∗, Chun-Hua Li b, Zhen-Xing Yu b,Ke-Fu Yao b, Sheng-Fu Ji c, Ji Liang b

a School of Chemical Engineering and the Environment, Beijing Institute of Technology, Beijing 100081, PR Chinab Department of Mechanical Engineering, Tsinghua University, Beijing 100084, PR China

c The Key Laboratory of Science and Technology of Controllable Chemical Reactions, Ministry of Education,Beijing University of Chemical Technology, Beijing 100029, PR China

Received 15 December 2005; received in revised form 6 August 2006; accepted 18 February 2007

bstract

Pt catalysts supported on carbon nanotubes (CNTs) and activated carbon (AC) were prepared. The structure of the catalysts was characterizedsing BET, TEM, XPS techniques. The catalytic performance for nitrobenzene hydrogenation was tested. The results show that the mesoporous

extures of CNTs make the outer surfaces of CNTs accessible for Pt ion. So Pt particles on CNTs are much smaller, and the proper concentrationf surface groups on CNTs makes Pt easy for reducibility. Pt particles on AC are larger due to the microporous texture, and reduction at higheremperature is necessary because of more functional groups on AC. The Pt/CNTs catalysts reduced at lower temperature, exhibit higher activityhan the Pt/AC catalysts for nitrobenzene hydrogenation. 2007 Elsevier B.V. All rights reserved.

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eywords: Nanostructures; Precipitation; TEM; Microstructure

. Introduction

Nowadays heterogeneous catalysis takes a large fraction inndustrial processes. Supported catalysts are commonly used.he primary roles of the support are to finely disperse andtabilize small metallic particles and thus provide access to auch larger number of catalytically active atoms than in the

orresponding bulk metal [1]. Large surface area of support isavorable for improving the dispersion. Hence, activated carbonAC) with large specific surface area contributed by developedicroporosity texture is widely used as catalyst support. The

hortcoming of microporous texture is obvious: active compo-ents hardly access the micropores when preparing catalysts,nd the reactants scarcely contact with active sites in the micro-

ores due to mass transfer limit under low pressure [2]. Carbonanotubes (CNTs) have a lower specific surface area comparedith AC, but mesoporosity texture formed by entangled CNTs

∗ Corresponding author at: School of Chemical Engineering and the Environ-ent, Beijing Institute of Technology, Beijing 100081, PR China.el.: +86 10 6891 2658; fax: +86 10 6891 3293.

E-mail address: zhaoyun@bit.edu.cn (Y. Zhao).

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254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2007.02.045

3]. Mesoporosity texture can avoid or reduce the disadvantagef microporosity for heterogeneous reaction, thus the metal dis-ersion and catalytic activity are substantial according to thectual microstructures of supports.

Nitrobenzene hydrogenation is important in organic chem-stry, as aniline is used as basic raw materials for productionf methylene diphenyl diisocyanate (MDI), rubber chemicals,yes and pharmaceuticals. Noble metal like Pt or Pd supportedn AC or like materials, though expensive, are used at all times,onsidering that the advantages of high activity and longevity inontrast to silica-supported Cu-based catalyst. It was found thatNTs-supported Pt catalyst possesses excellent activity evennder mild reaction condition with respect to AC-supported Pt4]. In this contribution, we focus on the effect of microstructuresf CNTs and AC on Pt dispersion and activity in nitrobenzeneydrogenation.

. Experimental

.1. Preparation of carbonaceous supported Pt catalysts

CNTs are made using propane as a carbon source and nickel supportedn silica as a catalyst at 600 ◦C. Then a caustic treatment is used to remove

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Table 1Characteristics of supports and catalysts

SSA (m2 g−1) Vmicro (cm3 g−1) Vmeso (cm3 g−1) Loading

CNTs 175 0.00722 0.35247AC 850 0.019964 0.139949Pt/CNTs 165 0.004284 0.27153 3.1P

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26 Y. Zhao et al. / Materials Chemis

he support and an acid treatment used to remove the catalyst metal. They areulti-walled carbon nanotubes with length up to several tens of micrometers

nd external diameters between 15 and 40 nm. Their specific surface area is75 m2 g−1. Powdered AC are purchased from Jiangsu Zhuxi Activated Car-on Co., Ltd. Their mean particle size is 70 �m and specific surface area is50 m2 g−1.

CNTs and AC are subjected to acidic oxidation pretreatment. They areefluxed in the mixture of concentrated nitric acid and 60 wt% sulfuric acid foralf an hour, followed by watered, filtration and dried. The pretreated supportsre dispersed in the aqueous solution of hexachloroplatinic acid, then sodiumydrosulfite (Na2S2O4) solution (0.32 M) is dropped with vigorous stirring. Thelurry is filtrated and dried. The dried catalysts are calcined in nitrogen at 500 ◦Cor 1 h.

.2. Characterization of supported Pt catalysts

The calcined samples are observed using transmission electronic microscopyJEOL-300CX) operated at 20 kV. The specific surfaces area and pore size dis-ributions of the samples are measured by physical adsorption nitrogen using aorptomatic 1990 instrument. Powder XRD patterns are recorded on a Rigaku/max-RB diffractometer using Cu K� radiation (40 kV, 120 mA). X-ray pho-

oelectron spectra (XPS) are obtained by using a KROTAS spectrometer fittedith a Al K� source using the Au 4f line (84 eV) for calibration. Raman spectra

re taken at 632.8 nm line of a He-Ne laser (RM2000). The Pt loading is analyzedhrough inductively coupled plasma-atomic emission spectrometry using ICP-ES LEEMAN PRODIGY. The chemical analysis of the products is performedy gas chromatography (GC/GC–MS TURBOMASS HP5973).

.3. Nitrobenzene hydrogenation

The catalyst (25 mg) is put in a three-necked bottle, hydrogen is introducednto it to activate the sample at 50 ml min−1 flow rate for 3 h. Then 0.25 mlitrobenzene and 25 ml alcohol are injected with stirring. The samples are takenntermittently.

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Fig. 1. Pore size distributions of (a) CNTs,

t/AC 380 0.006120 0.065846 2.5

. Results and discussion

.1. Physical adsorption

According to the data listed in Table 1, the specific surfacereas (SSA) of Pt/CNTs and Pt/AC decrease by 5.7 and 55.3%,espectively, compared to the corresponding support. The micro-orous and mesoporous volumes of two catalysts also drop withespect to that of the supports. The decrease of pore volumes foroth Pt/CNTs and Pt/AC samples indicate a fraction of poresre obscured by deposited Pt particles. The blocking of microp-res for Pt/AC is especially severe, in accordance with Ref. [5].lthough the surface areas of Pt nanoclusters contribute to theSA, the blocking of pores caused by deposited Pt particles isominant and thus leads to the decrease of SSA.

Fig. 1 shows the pore-size distributions of catalysts and sup-orts in detail. As shown in Fig. 1, CNTs mainly involves in

esopores formed by entangled CNTs, while AC have devel-

ped microporous structure. When Pt nanoclusters are depositednto CNTs, volume of macropore goes up, as differ from the

(b) Pt/CNTs, (c) AC and (d) Pt/AC.

istry and Physics 103 (2007) 225–229 227

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Y. Zhao et al. / Materials Chem

hange of Pt/AC. The macroporosity may result from repulsionf aggregated CNTs by deposited Pt ions during preparation ofatalysts. The newly formed macropores further help to reducehe diffusing limit of reactant.

.2. Transmission electron microscopy (TEM)

Supported Pt particles after low reduction temperature (LRT)f 50 ◦C (Fig. 2) are uniform in size on both CNTs and AC, andt clusters are in the range of 3–5 nm over CNTs and 8–10 nmver AC. Pt clusters are homogeneously and separately dispersedn the surfaces of CNTs, while those on AC are collective onxterior surfaces.

Fig. 3 presents TEM images of supported Pt catalysts afterigh reduction temperature (HRT) of 350 ◦C by hydrogen. Ptarticles located on AC redistribute on surfaces and becomeuch finer with 1–3 nm in size, while that on CNTs give no

bvious change.Pt dispersion not only associates with SSA of supports, but

lso connects with their textures. Although SSA of AC is largery four times than that of CNTs, not all inner surfaces areccessible for Pt ions. Pore-size distribution profiles of catalystsonfirm it. The microporosity texture of AC makes deposited Pt

ggregated on the outer surfaces of AC, but the mesoporosityexture of CNTs makes exterior surfaces accessible for metallicons. Therefore, Pt particles upon preparation step are dispersednd small on CNTs and collective on AC. The Pt dispersions

Fig. 2. TEM images of Pt/CNTs (a) and Pt/AC (b) reduced at 50 ◦C.

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Fig. 3. TEM images of Pt/CNTs (a) and Pt/AC (b) reduced at 350 ◦C.

fter calcination follow the same rule, as it is recognized fromhe study on carbon black-supported Pt catalysts that sinter-ng resistance is proportional to degree of graphitization [6].

oreover, the chemical surface composition of AC is the dom-nant parameter determining the Pt dispersion [7], as connectith the structure characteristics. AC possess a porous networkf highly disordered graphitic material, while CNTs are tur-ostratic graphite structure with basal planes exposed. AC isore reactive than CNTs due to high ratio of prismatic plane

o basal plane, so similar oxidizing treatment will bring aboutenser surface groups on outer surfaces of AC compared withNTs, causing reduce of sintering resistance and thus conglom-ration of Pt particles. Another action of functional groups ishat they will hinder the reduction of Pt located on carbonaceous

aterials [6]. Consequently, Pt/CNTs is easy to be reduced, andRT is enough for acquiring metallic Pt.

When Pt/AC is heated to 350 ◦C in hydrogen, Pt2+ speciesre partially reduced to Pt0, Pt particles recrystallize and theize is controlled by nuclei number and growth rate. Since Ptarticles move more easily in reducing atmosphere, Pt particlesan diffuse from exterior surfaces to interior surfaces. Becauseccessible surfaces of AC get larger, more nuclei are formed,hus forming smaller Pt particles. Yet, it is not necessary fort/CNTs catalyst to further reduce at high temperature (HT),nd reduction at HT would not improve the dispersion of Pt.

.3. X-ray photoelectron spectroscopy (XPS)

Fig. 4 gives XPS spectra of Pt/CNTs and Pt/AC subjected toow temperature (LT) and high temperature (HT) reduction. The

228 Y. Zhao et al. / Materials Chemistry and Physics 103 (2007) 225–229

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Fig. 4. XPS spectra of (a) Pt/CNTs after LRT, (b) Pt/CN

PS profiles can be curve fitted to two doublet peaks, one ist 71.5–71.8 eV, the other at 73.6–73.9 eV. The binding energyBE) of 4f7/2 state of Pt0 species and Pt2+ species is reported to be0.9 and 73.8 eV [8]. The two doublet Pt 4f7/2 peaks for Pt/CNTsnd Pt/AC can be assigned to metallic Pt and oxidized Pt. Thelightly higher BE is due to the electronic effects produced byhe presence of very small and highly dispersed Pt particles sup-orted on the carbon supports [9]. The XPS results indicate that

o significant difference is present in the Pt 4f spectra betweent/CNTs and Pt/AC subjected to LT or HT treatment, but thetomic percents vary a lot, as shown in Table 2. After reduc-ion at 350 ◦C, for both samples atomic percents of Pt increase,

able 2inding energies and atomic percents of Pt/CNTs and Pt/AC after reduction at50 ◦C

Binding energy (eV) Atomic percent (%)

Pt 4f C 1s O 1s Pt C O

t/CNTsLT 71.51 284.53 532.2 1.44 93.15 5.41HT 71.58 284.57 532.0 5.03 90.76 4.21

t/ACLT 71.63 284.45 532.3 1.07 90.64 8.29HT 71.85 284.6 532.0 3.87 90.59 5.54

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ter HRT, (c) Pt/AC after LRT and (d) Pt/AC after HRT.

tomic percents of oxygen decrease. It presents the occurrencef reduction process and of enrichment of Pt onto the outerurfaces.

.4. Catalytic activity

When catalyst is reduced in situ at 50 ◦C, Pt/CNTs showsxcellent activity for nitrobenzene hydrogenation and no activitys shown on Pt/AC catalyst. On reduction at 350 ◦C, the activ-ty of Pt/CNTs increases by 25%, while that of Pt/AC catalystmproves sensibly (Fig. 5).

After reduction at 350 ◦C for Pt/AC, the reduction degreemproves, and Pt particle size also decreases. It follows fromeference data that nitrobenzene hydrogenation is a structurensensitive reaction, i.e., the activity does not depend on therystalline planes of the active metal or the size of it, but onlyepends on the amount of metal exposed [9,10]. No activity oft/AC catalysts after LRT should be assigned to low reductionegree of oxidized Pt species as well as low dispersion. Thencrease of Pt/CNTs upon HRT can result from the enhancement

f Pt surface atomic percent.

The main reason why Pt/CNTs after LRT exhibit high activitys small size effect and the easiness of reducibility. It can berawn that CNTs is superior as catalyst support to AC when Pts deposited using reduction method.

Y. Zhao et al. / Materials Chemistry and Physics 103 (2007) 225–229 229

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ig. 5. Catalytic behaviors of (a) Pt/CNTs and (b) Pt/AC after LRT, (c) Pt/CNT

. Conclusions

The homogeneity of surface properties of CNTs ensures uni-orm distribution of deposited Pt particles. The mesoporousexture of CNTs ensures the whole outer surfaces accessibleor Pt ions, which facilitate formation of ultrafine particles. Theroper concentration of surface groups is also responsible forhe high dispersion and the easiness of reducibility. So evenfter LRT Pt/CNTs show high catalytic activity in nitrobenzeneydrogenation. Pt particles located on AC are much larger thanhat on CNTs due to microporosity texture, and HRT is necessaryor improvement of amount of metallic Pt and of Pt dispersion.he activity of Pt/AC after HRT is comparable to that of Pt/CNTsfter LRT.

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