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Effect of microstructures of Pt catalysts supported on carbon nanotubes (CNTs) and activated carbon (AC) for nitrobenzene hydrogenation
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 China b 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
Abstract Pt catalysts supported on carbon nanotubes (CNTs) and activated carbon (AC) were prepared. The structure of the catalysts was characterized using BET, TEM, XPS techniques. The catalytic performance for nitrobenzene hydrogenation was tested. The results show that the mesoporous textures of CNTs make the outer surfaces of CNTs accessible for Pt ion. So Pt particles on CNTs are much smaller, and the proper concentration of surface groups on CNTs makes Pt easy for reducibility. Pt particles on AC are larger due to the microporous texture, and reduction at higher temperature is necessary because of more functional groups on AC. The Pt/CNTs catalysts reduced at lower temperature, exhibit higher activity than the Pt/AC catalysts for nitrobenzene hydrogenation. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Precipitation; TEM; Microstructure
1. Introduction Nowadays heterogeneous catalysis takes a large fraction in industrial processes. Supported catalysts are commonly used. The primary roles of the support are to finely disperse and stabilize small metallic particles and thus provide access to a much larger number of catalytically active atoms than in the corresponding bulk metal [1]. Large surface area of support is favorable for improving the dispersion. Hence, activated carbon (AC) with large specific surface area contributed by developed microporosity texture is widely used as catalyst support. The shortcoming of microporous texture is obvious: active components hardly access the micropores when preparing catalysts, and the reactants scarcely contact with active sites in the micropores due to mass transfer limit under low pressure [2]. Carbon nanotubes (CNTs) have a lower specific surface area compared with AC, but mesoporosity texture formed by entangled CNTs
[3]. Mesoporosity texture can avoid or reduce the disadvantage of microporosity for heterogeneous reaction, thus the metal dispersion and catalytic activity are substantial according to the actual microstructures of supports. Nitrobenzene hydrogenation is important in organic chemistry, as aniline is used as basic raw materials for production of methylene diphenyl diisocyanate (MDI), rubber chemicals, dyes and pharmaceuticals. Noble metal like Pt or Pd supported on AC or like materials, though expensive, are used at all times, considering that the advantages of high activity and longevity in contrast to silica-supported Cu-based catalyst. It was found that CNTs-supported Pt catalyst possesses excellent activity even under mild reaction condition with respect to AC-supported Pt [4]. In this contribution, we focus on the effect of microstructures of CNTs and AC on Pt dispersion and activity in nitrobenzene hydrogenation. 2. Experimental
∗ Corresponding author at: School of Chemical Engineering and the Environment, Beijing Institute of Technology, Beijing 100081, PR China. Tel.: +86 10 6891 2658; fax: +86 10 6891 3293. E-mail address: [email protected] (Y. Zhao).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.02.045
2.1. Preparation of carbonaceous supported Pt catalysts CNTs are made using propane as a carbon source and nickel supported on silica as a catalyst at 600 ◦ C. Then a caustic treatment is used to remove
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Y. Zhao et al. / Materials Chemistry and Physics 103 (2007) 225–229
the support and an acid treatment used to remove the catalyst metal. They are multi-walled carbon nanotubes with length up to several tens of micrometers and external diameters between 15 and 40 nm. Their specific surface area is 175 m2 g−1 . Powdered AC are purchased from Jiangsu Zhuxi Activated Carbon Co., Ltd. Their mean particle size is 70 m and specific surface area is 850 m2 g−1 . CNTs and AC are subjected to acidic oxidation pretreatment. They are refluxed in the mixture of concentrated nitric acid and 60 wt% sulfuric acid for half an hour, followed by watered, filtration and dried. The pretreated supports are dispersed in the aqueous solution of hexachloroplatinic acid, then sodium hydrosulfite (Na2 S2 O4 ) solution (0.32 M) is dropped with vigorous stirring. The slurry is filtrated and dried. The dried catalysts are calcined in nitrogen at 500 ◦ C for 1 h.
2.2. Characterization of supported Pt catalysts The calcined samples are observed using transmission electronic microscopy (JEOL-300CX) operated at 20 kV. The specific surfaces area and pore size distributions of the samples are measured by physical adsorption nitrogen using a Sorptomatic 1990 instrument. Powder XRD patterns are recorded on a Rigaku D/max-RB diffractometer using Cu K␣ radiation (40 kV, 120 mA). X-ray photoelectron spectra (XPS) are obtained by using a KROTAS spectrometer fitted with a Al K␣ source using the Au 4f line (84 eV) for calibration. Raman spectra are taken at 632.8 nm line of a He-Ne laser (RM2000). The Pt loading is analyzed through inductively coupled plasma-atomic emission spectrometry using ICPAES LEEMAN PRODIGY. The chemical analysis of the products is performed by gas chromatography (GC/GC–MS TURBOMASS HP5973).
2.3. Nitrobenzene hydrogenation The catalyst (25 mg) is put in a three-necked bottle, hydrogen is introduced into it to activate the sample at 50 ml min−1 flow rate for 3 h. Then 0.25 ml nitrobenzene and 25 ml alcohol are injected with stirring. The samples are taken intermittently.
Table 1 Characteristics of supports and catalysts
CNTs AC Pt/CNTs Pt/AC
SSA (m2 g−1 )
Vmicro (cm3 g−1 )
Vmeso (cm3 g−1 )
Loading
175 850 165 380
0.00722 0.019964 0.004284 0.006120
0.35247 0.139949 0.27153 0.065846
3.1 2.5
3. Results and discussion 3.1. Physical adsorption According to the data listed in Table 1, the specific surface areas (SSA) of Pt/CNTs and Pt/AC decrease by 5.7 and 55.3%, respectively, compared to the corresponding support. The microporous and mesoporous volumes of two catalysts also drop with respect to that of the supports. The decrease of pore volumes for both Pt/CNTs and Pt/AC samples indicate a fraction of pores are obscured by deposited Pt particles. The blocking of micropores for Pt/AC is especially severe, in accordance with Ref. [5]. Although the surface areas of Pt nanoclusters contribute to the SSA, the blocking of pores caused by deposited Pt particles is dominant and thus leads to the decrease of SSA. Fig. 1 shows the pore-size distributions of catalysts and supports in detail. As shown in Fig. 1, CNTs mainly involves in mesopores formed by entangled CNTs, while AC have developed microporous structure. When Pt nanoclusters are deposited onto CNTs, volume of macropore goes up, as differ from the
Fig. 1. Pore size distributions of (a) CNTs, (b) Pt/CNTs, (c) AC and (d) Pt/AC.
Y. Zhao et al. / Materials Chemistry and Physics 103 (2007) 225–229
227
change of Pt/AC. The macroporosity may result from repulsion of aggregated CNTs by deposited Pt ions during preparation of catalysts. The newly formed macropores further help to reduce the diffusing limit of reactant. 3.2. Transmission electron microscopy (TEM) Supported Pt particles after low reduction temperature (LRT) of 50 ◦ C (Fig. 2) are uniform in size on both CNTs and AC, and Pt clusters are in the range of 3–5 nm over CNTs and 8–10 nm over AC. Pt clusters are homogeneously and separately dispersed on the surfaces of CNTs, while those on AC are collective on exterior surfaces. Fig. 3 presents TEM images of supported Pt catalysts after high reduction temperature (HRT) of 350 ◦ C by hydrogen. Pt particles located on AC redistribute on surfaces and become much finer with 1–3 nm in size, while that on CNTs give no obvious change. Pt dispersion not only associates with SSA of supports, but also connects with their textures. Although SSA of AC is larger by four times than that of CNTs, not all inner surfaces are accessible for Pt ions. Pore-size distribution profiles of catalysts confirm it. The microporosity texture of AC makes deposited Pt aggregated on the outer surfaces of AC, but the mesoporosity texture of CNTs makes exterior surfaces accessible for metallic ions. Therefore, Pt particles upon preparation step are dispersed and small on CNTs and collective on AC. The Pt dispersions
Fig. 3. TEM images of Pt/CNTs (a) and Pt/AC (b) reduced at 350 ◦ C.
after calcination follow the same rule, as it is recognized from the study on carbon black-supported Pt catalysts that sintering resistance is proportional to degree of graphitization [6]. Moreover, the chemical surface composition of AC is the dominant parameter determining the Pt dispersion [7], as connect with the structure characteristics. AC possess a porous network of highly disordered graphitic material, while CNTs are turbostratic graphite structure with basal planes exposed. AC is more reactive than CNTs due to high ratio of prismatic plane to basal plane, so similar oxidizing treatment will bring about denser surface groups on outer surfaces of AC compared with CNTs, causing reduce of sintering resistance and thus conglomeration of Pt particles. Another action of functional groups is that they will hinder the reduction of Pt located on carbonaceous materials [6]. Consequently, Pt/CNTs is easy to be reduced, and LRT is enough for acquiring metallic Pt. When Pt/AC is heated to 350 ◦ C in hydrogen, Pt2+ species are partially reduced to Pt0 , Pt particles recrystallize and the size is controlled by nuclei number and growth rate. Since Pt particles move more easily in reducing atmosphere, Pt particles can diffuse from exterior surfaces to interior surfaces. Because accessible surfaces of AC get larger, more nuclei are formed, thus forming smaller Pt particles. Yet, it is not necessary for Pt/CNTs catalyst to further reduce at high temperature (HT), and reduction at HT would not improve the dispersion of Pt. 3.3. X-ray photoelectron spectroscopy (XPS)
Fig. 2. TEM images of Pt/CNTs (a) and Pt/AC (b) reduced at 50 ◦ C.
Fig. 4 gives XPS spectra of Pt/CNTs and Pt/AC subjected to low temperature (LT) and high temperature (HT) reduction. The
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Fig. 4. XPS spectra of (a) Pt/CNTs after LRT, (b) Pt/CNTs after HRT, (c) Pt/AC after LRT and (d) Pt/AC after HRT.
XPS profiles can be curve fitted to two doublet peaks, one is at 71.5–71.8 eV, the other at 73.6–73.9 eV. The binding energy (BE) of 4f7/2 state of Pt0 species and Pt2+ species is reported to be 70.9 and 73.8 eV [8]. The two doublet Pt 4f7/2 peaks for Pt/CNTs and Pt/AC can be assigned to metallic Pt and oxidized Pt. The slightly higher BE is due to the electronic effects produced by the presence of very small and highly dispersed Pt particles supported on the carbon supports [9]. The XPS results indicate that no significant difference is present in the Pt 4f spectra between Pt/CNTs and Pt/AC subjected to LT or HT treatment, but the atomic percents vary a lot, as shown in Table 2. After reduction at 350 ◦ C, for both samples atomic percents of Pt increase, Table 2 Binding energies and atomic percents of Pt/CNTs and Pt/AC after reduction at 350 ◦ C Binding energy (eV) Pt 4f
Atomic percent (%)
C 1s
O 1s
Pt
C
O
Pt/CNTs LT 71.51 HT 71.58
284.53 284.57
532.2 532.0
1.44 5.03
93.15 90.76
5.41 4.21
Pt/AC LT HT
284.45 284.6
532.3 532.0
1.07 3.87
90.64 90.59
8.29 5.54
71.63 71.85
atomic percents of oxygen decrease. It presents the occurrence of reduction process and of enrichment of Pt onto the outer surfaces. 3.4. Catalytic activity When catalyst is reduced in situ at 50 ◦ C, Pt/CNTs shows excellent activity for nitrobenzene hydrogenation and no activity is shown on Pt/AC catalyst. On reduction at 350 ◦ C, the activity of Pt/CNTs increases by 25%, while that of Pt/AC catalyst improves sensibly (Fig. 5). After reduction at 350 ◦ C for Pt/AC, the reduction degree improves, and Pt particle size also decreases. It follows from reference data that nitrobenzene hydrogenation is a structure insensitive reaction, i.e., the activity does not depend on the crystalline planes of the active metal or the size of it, but only depends on the amount of metal exposed [9,10]. No activity of Pt/AC catalysts after LRT should be assigned to low reduction degree of oxidized Pt species as well as low dispersion. The increase of Pt/CNTs upon HRT can result from the enhancement of Pt surface atomic percent. The main reason why Pt/CNTs after LRT exhibit high activity is small size effect and the easiness of reducibility. It can be drawn that CNTs is superior as catalyst support to AC when Pt is deposited using reduction method.
Y. Zhao et al. / Materials Chemistry and Physics 103 (2007) 225–229
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Fig. 5. Catalytic behaviors of (a) Pt/CNTs and (b) Pt/AC after LRT, (c) Pt/CNTs and (d) Pt/AC after HRT. () Aniline; (䊉) nitrobenzene; () azoxybenzene.
4. Conclusions The homogeneity of surface properties of CNTs ensures uniform distribution of deposited Pt particles. The mesoporous texture of CNTs ensures the whole outer surfaces accessible for Pt ions, which facilitate formation of ultrafine particles. The proper concentration of surface groups is also responsible for the high dispersion and the easiness of reducibility. So even after LRT Pt/CNTs show high catalytic activity in nitrobenzene hydrogenation. Pt particles located on AC are much larger than that on CNTs due to microporosity texture, and HRT is necessary for improvement of amount of metallic Pt and of Pt dispersion. The activity of Pt/AC after HRT is comparable to that of Pt/CNTs after LRT. References [1] E. Auer, A. Freund, J. Pietsch, T. Tacke, Appl. Catal. A: Gen. 173 (1998) 259.
[2] M. Gurrath, T. Kuretzky, H.P. Boehm, L.B. Okhlopkova, A.S. Lisitsyn, V.A. Likholobov, Carbon 38 (2000) 1241. [3] Q.H. Yang, P.X. Hou, S. Bai, M.Z. Wang, H.M. Cheng, Chem. Phys. Lett. 345 (2001) 18. [4] Chun-Hua Li, Zhen-Xing Yu, Ke-Fu Yao, Sheng-Fu Ji, Ji Liang, Nitrobenzene hydrogenation with carbon nanotube-supported platinum catalyst under mild conditions, J. Mol. Catal. A: Chem. 226 (2005) 101. [5] L.B. Okhlopkova, A.S. Lisitsyn, V.A. Likholobov, M. Gurrath, H.P. Boehm, Appl. Catal. A: Gen. 204 (2000) 229. [6] A. Sep´ulveda-Escribano, F. Coloma, F. Rodr´ıguez-Reinoso, Appl. Catal. A: Gen. 173 (1998) 247. [7] A. Erhan Aksoylu, M. Madalena, A. Freitas, M. Fernando, R. Pereira, J.L. Figueiredo, Carbon 39 (2001) 175. [8] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of X-Ray Photoelectron Spectroscopy, Perkin-Elmer Corporation, Minnesota, 1979, p. 152. [9] S.R. de Miguel, O.A. Scelza, M.C. Roman-Martinez, C. Salinas-Martinez de Lecea, D. Cazaola-Amoros, A. Linares-Solano, Appl. Catal. A: Gen. 170 (1998) 93. [10] V.V. Chesnokov, I.P. Prosvirin, N.A. Zaitseva, V.I. Zaikovskii, V.V. Molchanov, Kinet. Catal. 43 (2002) 838.
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 China b 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
Abstract Pt catalysts supported on carbon nanotubes (CNTs) and activated carbon (AC) were prepared. The structure of the catalysts was characterized using BET, TEM, XPS techniques. The catalytic performance for nitrobenzene hydrogenation was tested. The results show that the mesoporous textures of CNTs make the outer surfaces of CNTs accessible for Pt ion. So Pt particles on CNTs are much smaller, and the proper concentration of surface groups on CNTs makes Pt easy for reducibility. Pt particles on AC are larger due to the microporous texture, and reduction at higher temperature is necessary because of more functional groups on AC. The Pt/CNTs catalysts reduced at lower temperature, exhibit higher activity than the Pt/AC catalysts for nitrobenzene hydrogenation. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Precipitation; TEM; Microstructure
1. Introduction Nowadays heterogeneous catalysis takes a large fraction in industrial processes. Supported catalysts are commonly used. The primary roles of the support are to finely disperse and stabilize small metallic particles and thus provide access to a much larger number of catalytically active atoms than in the corresponding bulk metal [1]. Large surface area of support is favorable for improving the dispersion. Hence, activated carbon (AC) with large specific surface area contributed by developed microporosity texture is widely used as catalyst support. The shortcoming of microporous texture is obvious: active components hardly access the micropores when preparing catalysts, and the reactants scarcely contact with active sites in the micropores due to mass transfer limit under low pressure [2]. Carbon nanotubes (CNTs) have a lower specific surface area compared with AC, but mesoporosity texture formed by entangled CNTs
[3]. Mesoporosity texture can avoid or reduce the disadvantage of microporosity for heterogeneous reaction, thus the metal dispersion and catalytic activity are substantial according to the actual microstructures of supports. Nitrobenzene hydrogenation is important in organic chemistry, as aniline is used as basic raw materials for production of methylene diphenyl diisocyanate (MDI), rubber chemicals, dyes and pharmaceuticals. Noble metal like Pt or Pd supported on AC or like materials, though expensive, are used at all times, considering that the advantages of high activity and longevity in contrast to silica-supported Cu-based catalyst. It was found that CNTs-supported Pt catalyst possesses excellent activity even under mild reaction condition with respect to AC-supported Pt [4]. In this contribution, we focus on the effect of microstructures of CNTs and AC on Pt dispersion and activity in nitrobenzene hydrogenation. 2. Experimental
∗ Corresponding author at: School of Chemical Engineering and the Environment, Beijing Institute of Technology, Beijing 100081, PR China. Tel.: +86 10 6891 2658; fax: +86 10 6891 3293. E-mail address: [email protected] (Y. Zhao).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.02.045
2.1. Preparation of carbonaceous supported Pt catalysts CNTs are made using propane as a carbon source and nickel supported on silica as a catalyst at 600 ◦ C. Then a caustic treatment is used to remove
226
Y. Zhao et al. / Materials Chemistry and Physics 103 (2007) 225–229
the support and an acid treatment used to remove the catalyst metal. They are multi-walled carbon nanotubes with length up to several tens of micrometers and external diameters between 15 and 40 nm. Their specific surface area is 175 m2 g−1 . Powdered AC are purchased from Jiangsu Zhuxi Activated Carbon Co., Ltd. Their mean particle size is 70 m and specific surface area is 850 m2 g−1 . CNTs and AC are subjected to acidic oxidation pretreatment. They are refluxed in the mixture of concentrated nitric acid and 60 wt% sulfuric acid for half an hour, followed by watered, filtration and dried. The pretreated supports are dispersed in the aqueous solution of hexachloroplatinic acid, then sodium hydrosulfite (Na2 S2 O4 ) solution (0.32 M) is dropped with vigorous stirring. The slurry is filtrated and dried. The dried catalysts are calcined in nitrogen at 500 ◦ C for 1 h.
2.2. Characterization of supported Pt catalysts The calcined samples are observed using transmission electronic microscopy (JEOL-300CX) operated at 20 kV. The specific surfaces area and pore size distributions of the samples are measured by physical adsorption nitrogen using a Sorptomatic 1990 instrument. Powder XRD patterns are recorded on a Rigaku D/max-RB diffractometer using Cu K␣ radiation (40 kV, 120 mA). X-ray photoelectron spectra (XPS) are obtained by using a KROTAS spectrometer fitted with a Al K␣ source using the Au 4f line (84 eV) for calibration. Raman spectra are taken at 632.8 nm line of a He-Ne laser (RM2000). The Pt loading is analyzed through inductively coupled plasma-atomic emission spectrometry using ICPAES LEEMAN PRODIGY. The chemical analysis of the products is performed by gas chromatography (GC/GC–MS TURBOMASS HP5973).
2.3. Nitrobenzene hydrogenation The catalyst (25 mg) is put in a three-necked bottle, hydrogen is introduced into it to activate the sample at 50 ml min−1 flow rate for 3 h. Then 0.25 ml nitrobenzene and 25 ml alcohol are injected with stirring. The samples are taken intermittently.
Table 1 Characteristics of supports and catalysts
CNTs AC Pt/CNTs Pt/AC
SSA (m2 g−1 )
Vmicro (cm3 g−1 )
Vmeso (cm3 g−1 )
Loading
175 850 165 380
0.00722 0.019964 0.004284 0.006120
0.35247 0.139949 0.27153 0.065846
3.1 2.5
3. Results and discussion 3.1. Physical adsorption According to the data listed in Table 1, the specific surface areas (SSA) of Pt/CNTs and Pt/AC decrease by 5.7 and 55.3%, respectively, compared to the corresponding support. The microporous and mesoporous volumes of two catalysts also drop with respect to that of the supports. The decrease of pore volumes for both Pt/CNTs and Pt/AC samples indicate a fraction of pores are obscured by deposited Pt particles. The blocking of micropores for Pt/AC is especially severe, in accordance with Ref. [5]. Although the surface areas of Pt nanoclusters contribute to the SSA, the blocking of pores caused by deposited Pt particles is dominant and thus leads to the decrease of SSA. Fig. 1 shows the pore-size distributions of catalysts and supports in detail. As shown in Fig. 1, CNTs mainly involves in mesopores formed by entangled CNTs, while AC have developed microporous structure. When Pt nanoclusters are deposited onto CNTs, volume of macropore goes up, as differ from the
Fig. 1. Pore size distributions of (a) CNTs, (b) Pt/CNTs, (c) AC and (d) Pt/AC.
Y. Zhao et al. / Materials Chemistry and Physics 103 (2007) 225–229
227
change of Pt/AC. The macroporosity may result from repulsion of aggregated CNTs by deposited Pt ions during preparation of catalysts. The newly formed macropores further help to reduce the diffusing limit of reactant. 3.2. Transmission electron microscopy (TEM) Supported Pt particles after low reduction temperature (LRT) of 50 ◦ C (Fig. 2) are uniform in size on both CNTs and AC, and Pt clusters are in the range of 3–5 nm over CNTs and 8–10 nm over AC. Pt clusters are homogeneously and separately dispersed on the surfaces of CNTs, while those on AC are collective on exterior surfaces. Fig. 3 presents TEM images of supported Pt catalysts after high reduction temperature (HRT) of 350 ◦ C by hydrogen. Pt particles located on AC redistribute on surfaces and become much finer with 1–3 nm in size, while that on CNTs give no obvious change. Pt dispersion not only associates with SSA of supports, but also connects with their textures. Although SSA of AC is larger by four times than that of CNTs, not all inner surfaces are accessible for Pt ions. Pore-size distribution profiles of catalysts confirm it. The microporosity texture of AC makes deposited Pt aggregated on the outer surfaces of AC, but the mesoporosity texture of CNTs makes exterior surfaces accessible for metallic ions. Therefore, Pt particles upon preparation step are dispersed and small on CNTs and collective on AC. The Pt dispersions
Fig. 3. TEM images of Pt/CNTs (a) and Pt/AC (b) reduced at 350 ◦ C.
after calcination follow the same rule, as it is recognized from the study on carbon black-supported Pt catalysts that sintering resistance is proportional to degree of graphitization [6]. Moreover, the chemical surface composition of AC is the dominant parameter determining the Pt dispersion [7], as connect with the structure characteristics. AC possess a porous network of highly disordered graphitic material, while CNTs are turbostratic graphite structure with basal planes exposed. AC is more reactive than CNTs due to high ratio of prismatic plane to basal plane, so similar oxidizing treatment will bring about denser surface groups on outer surfaces of AC compared with CNTs, causing reduce of sintering resistance and thus conglomeration of Pt particles. Another action of functional groups is that they will hinder the reduction of Pt located on carbonaceous materials [6]. Consequently, Pt/CNTs is easy to be reduced, and LRT is enough for acquiring metallic Pt. When Pt/AC is heated to 350 ◦ C in hydrogen, Pt2+ species are partially reduced to Pt0 , Pt particles recrystallize and the size is controlled by nuclei number and growth rate. Since Pt particles move more easily in reducing atmosphere, Pt particles can diffuse from exterior surfaces to interior surfaces. Because accessible surfaces of AC get larger, more nuclei are formed, thus forming smaller Pt particles. Yet, it is not necessary for Pt/CNTs catalyst to further reduce at high temperature (HT), and reduction at HT would not improve the dispersion of Pt. 3.3. X-ray photoelectron spectroscopy (XPS)
Fig. 2. TEM images of Pt/CNTs (a) and Pt/AC (b) reduced at 50 ◦ C.
Fig. 4 gives XPS spectra of Pt/CNTs and Pt/AC subjected to low temperature (LT) and high temperature (HT) reduction. The
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Y. Zhao et al. / Materials Chemistry and Physics 103 (2007) 225–229
Fig. 4. XPS spectra of (a) Pt/CNTs after LRT, (b) Pt/CNTs after HRT, (c) Pt/AC after LRT and (d) Pt/AC after HRT.
XPS profiles can be curve fitted to two doublet peaks, one is at 71.5–71.8 eV, the other at 73.6–73.9 eV. The binding energy (BE) of 4f7/2 state of Pt0 species and Pt2+ species is reported to be 70.9 and 73.8 eV [8]. The two doublet Pt 4f7/2 peaks for Pt/CNTs and Pt/AC can be assigned to metallic Pt and oxidized Pt. The slightly higher BE is due to the electronic effects produced by the presence of very small and highly dispersed Pt particles supported on the carbon supports [9]. The XPS results indicate that no significant difference is present in the Pt 4f spectra between Pt/CNTs and Pt/AC subjected to LT or HT treatment, but the atomic percents vary a lot, as shown in Table 2. After reduction at 350 ◦ C, for both samples atomic percents of Pt increase, Table 2 Binding energies and atomic percents of Pt/CNTs and Pt/AC after reduction at 350 ◦ C Binding energy (eV) Pt 4f
Atomic percent (%)
C 1s
O 1s
Pt
C
O
Pt/CNTs LT 71.51 HT 71.58
284.53 284.57
532.2 532.0
1.44 5.03
93.15 90.76
5.41 4.21
Pt/AC LT HT
284.45 284.6
532.3 532.0
1.07 3.87
90.64 90.59
8.29 5.54
71.63 71.85
atomic percents of oxygen decrease. It presents the occurrence of reduction process and of enrichment of Pt onto the outer surfaces. 3.4. Catalytic activity When catalyst is reduced in situ at 50 ◦ C, Pt/CNTs shows excellent activity for nitrobenzene hydrogenation and no activity is shown on Pt/AC catalyst. On reduction at 350 ◦ C, the activity of Pt/CNTs increases by 25%, while that of Pt/AC catalyst improves sensibly (Fig. 5). After reduction at 350 ◦ C for Pt/AC, the reduction degree improves, and Pt particle size also decreases. It follows from reference data that nitrobenzene hydrogenation is a structure insensitive reaction, i.e., the activity does not depend on the crystalline planes of the active metal or the size of it, but only depends on the amount of metal exposed [9,10]. No activity of Pt/AC catalysts after LRT should be assigned to low reduction degree of oxidized Pt species as well as low dispersion. The increase of Pt/CNTs upon HRT can result from the enhancement of Pt surface atomic percent. The main reason why Pt/CNTs after LRT exhibit high activity is small size effect and the easiness of reducibility. It can be drawn that CNTs is superior as catalyst support to AC when Pt is deposited using reduction method.
Y. Zhao et al. / Materials Chemistry and Physics 103 (2007) 225–229
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Fig. 5. Catalytic behaviors of (a) Pt/CNTs and (b) Pt/AC after LRT, (c) Pt/CNTs and (d) Pt/AC after HRT. () Aniline; (䊉) nitrobenzene; () azoxybenzene.
4. Conclusions The homogeneity of surface properties of CNTs ensures uniform distribution of deposited Pt particles. The mesoporous texture of CNTs ensures the whole outer surfaces accessible for Pt ions, which facilitate formation of ultrafine particles. The proper concentration of surface groups is also responsible for the high dispersion and the easiness of reducibility. So even after LRT Pt/CNTs show high catalytic activity in nitrobenzene hydrogenation. Pt particles located on AC are much larger than that on CNTs due to microporosity texture, and HRT is necessary for improvement of amount of metallic Pt and of Pt dispersion. The activity of Pt/AC after HRT is comparable to that of Pt/CNTs after LRT. References [1] E. Auer, A. Freund, J. Pietsch, T. Tacke, Appl. Catal. A: Gen. 173 (1998) 259.
[2] M. Gurrath, T. Kuretzky, H.P. Boehm, L.B. Okhlopkova, A.S. Lisitsyn, V.A. Likholobov, Carbon 38 (2000) 1241. [3] Q.H. Yang, P.X. Hou, S. Bai, M.Z. Wang, H.M. Cheng, Chem. Phys. Lett. 345 (2001) 18. [4] Chun-Hua Li, Zhen-Xing Yu, Ke-Fu Yao, Sheng-Fu Ji, Ji Liang, Nitrobenzene hydrogenation with carbon nanotube-supported platinum catalyst under mild conditions, J. Mol. Catal. A: Chem. 226 (2005) 101. [5] L.B. Okhlopkova, A.S. Lisitsyn, V.A. Likholobov, M. Gurrath, H.P. Boehm, Appl. Catal. A: Gen. 204 (2000) 229. [6] A. Sep´ulveda-Escribano, F. Coloma, F. Rodr´ıguez-Reinoso, Appl. Catal. A: Gen. 173 (1998) 247. [7] A. Erhan Aksoylu, M. Madalena, A. Freitas, M. Fernando, R. Pereira, J.L. Figueiredo, Carbon 39 (2001) 175. [8] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of X-Ray Photoelectron Spectroscopy, Perkin-Elmer Corporation, Minnesota, 1979, p. 152. [9] S.R. de Miguel, O.A. Scelza, M.C. Roman-Martinez, C. Salinas-Martinez de Lecea, D. Cazaola-Amoros, A. Linares-Solano, Appl. Catal. A: Gen. 170 (1998) 93. [10] V.V. Chesnokov, I.P. Prosvirin, N.A. Zaitseva, V.I. Zaikovskii, V.V. Molchanov, Kinet. Catal. 43 (2002) 838.