NCCR-41

Catalysis Today 198 (2012) 85–91

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Synthesis, characterization and electrocatalytic properties of nano-platinum-supported mesoporous carbon molecular sieves, Pt/NCCR-41 Parasuraman Selvam ∗ , Balaiah Kuppan National Centre for Catalysis Research and Department of Chemistry, Indian Institute of Technology-Madras, Chennai 600 036, India

a r t i c l e

i n f o

Article history: Received 14 February 2012 Received in revised form 19 June 2012 Accepted 21 June 2012 Available online 22 August 2012 Dedicated to Dr. Paul Ratnasamy on the occasion of his 70th birthday. Keywords: Mesoporous carbon NCCR-41 Nano-platinum Electrocatalyst Methanol oxidation

a b s t r a c t In this investigation, we report the synthesis and characterization of mesoporous carbon with disordered MCM-41 type structure by carbonization of sucrose within the pores of siliceous MCM-41. The silica template was removed by a standard procedure, viz., etching with HF, followed by filtration, washing and heat treatment, to obtain the mesoporous carbon. The resulting mesoporous carbon, designated as NCCR-41, was then used as a support for the preparation of highly uniform and well-dispersed nano-sized platinum nanoparticles. The supported catalysts with narrow size distribution of the active platinum, referred as Pt/NCCR-41, show excellent electrocatalytic activity for methanol oxidation reaction in comparison with platinum supported activated carbon (Pt/AC) as well as commercial carbon (Pt/E-TEK) catalysts. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Mesoporous carbons having very high surface area are of great technological interest for the development of catalytic, electrocatalytic and hydrogen-storage systems. In particular, porous carbon materials with high surface areas and pore volumes prepared from porous inorganic templates are of current interest for energy storage, gas separation, water purification, catalyst support, electrodes for batteries and fuel cells, and many other applications. In this regard, the controlled fabrication of one-dimensional (1D) porous structured carbon arrays has been extensively exploited in the material research, e.g., carbon nanotubes, which are mostly synthesized by chemical vapor deposition on specific substrates or pyrolysis of carbonaceous materials within the pores of anodic aluminum oxide membranes [1–5]. On the other hand, in recent years, the synthesis of high surface area mesoporous carbons, using silica frameworks as templates, is gaining increased importance [6–12]. This approach is based on a multi-step process via polymerization/carbonization of carbon precursors diffused within porous silica framework structure, viz., MCM-48 [10], SBA-15 [11], IITM-56 [12], etc., followed by the removal of the inorganic matrix by acid or alkali leaching. In this way, several mesoporous carbons were

∗ Corresponding author. Tel.: +91 44 2257 4235; fax: +91 44 2257 4202. E-mail address: [email protected] (P. Selvam). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.06.033

prepared by carbonization of sucrose/furfuryl alcohol precursors within the ordered porous structure of silica [13,14]. Mesoporous silicas with ordered 1D channel have become attractive inorganic templates, e.g., SBA-15 [6–8], for the generation of carbon arrays such as CMK-3 and CMK-5. However, size reduction of the templated carbons employing mesoporous silica is highly difficult to achieve [15]. Thus, it has been reported earlier that the mesoporous MCM-41 silica is not a suitable source for the production of mesoporous carbons plausibly due to fact that, unlike SBA-15, there are no complementary microspores within the silica walls of MCM-41 structure. Therefore, in this investigation, an attempt was made to prepare mesoporous carbon, designated as NCCR-41, employing a modified synthetic approach using hard silica (MCM-41) as template and sucrose as carbon precursor. At this juncture it is of important to note that platinum-supported carbon catalysts are widely used as electrode materials [16,17], e.g., for methanol oxidation reaction at the anodes of direct methanol fuel cells (DMFCs) [18] and for oxygen reduction reaction at the cathodes of proton exchange membrane fuel cells (PEMFCs) [19]. These systems have received considerable attention as clean energy sources for a number of applications [20–24]. However, for promising electrocatalytic applications, the following characteristics are desirable for supported catalysts, viz., high dispersion and excellent stability of the active metal dispersed on the support [25]. In this regard, porous carbons with ordered pore structure, high surface area, nanoscaled morphology, tunable pore characteristics

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with varied surface functionality, and good electrical conductivity are highly desirable [26–28]. Thus, the most commonly used electrocatalyst, both for cathode and anode, is platinum supported on carbon-black [29–34]. On the other hand, it is necessary to obtain a more effective catalyst, both in catalytic performance and electronic conductivity, so as to achieve a higher efficiency. Therefore, in this study, an attempt has also been made to prepare nanoplatinum-supported mesoporous carbon catalyst, viz., Pt/NCCR-41, for the DMFCs. A preliminary account of the work is presented elsewhere [35].

30 min. The mixture was heated to 343 K with string and simultaneously purging N2 to remove soluble O2 . Then 10 ml solution containing paraformaldehyde and sodium carbonate was slowly added drop wise, and the mixture was stirred for 2.5 h. Finally, the sample was filtered, washed with deionized water, and dried in a vacuum oven at 358 K for 20 h. The catalyst obtained was denoted as Pt/NCCR-41. The above same procedure has been adopted to prepare 20 wt% Pt on activated carbon (AC) support, the catalyst obtained has been denoted as Pt/AC. 2.5. Characterization

2. Experimental 2.1. Starting materials All the chemicals used were of analytical grade. Cetyl trimethyl ammonium bromide (CTAB), fumed silica, tetraethoxysilane (TEOS) and hexachloro platonic acid (H2 PtCl6 ) from Sigma–Aldrich. Commercial 20 wt% Pt/carbon (Vulcon XC-72) catalyst, referred as Pt/E-TEK, was procured from E-TEK. Para formaldehyde, sodium carbonate, methanol, hydrofluoric acid and sulphuric acid were obtained from Merck. 2.2. Synthesis of mesoporous silica, MCM-41 Mesoporous silica with MCM-41 structure was synthesized hydrothermally as per the procedure described elsewhere [10,36]. The typical gel (molar) composition was: SiO2 :0.27 CTAB:0.26 TMAOH:0.13 NaOH:68 H2 O. The pH of the gel was adjusted to 11.5 either by adding dilute H2 SO4 or aqueous NaOH and was placed in an air oven at 373 K for 24 h in Teflon-lined stainless steel autoclave. The solid product obtained was washed, filtered and dried at 353 K and were designated as MCM-41. In order to remove the surfactant, water and other gaseous molecules from the mesoporous MCM41, the as-synthesized samples were calcined in a temperature programmed furnace under flowing nitrogen and air. The heating was carried out from room temperature to 823 K with heating rate 1 K/min for 6–12 h. 2.3. Synthesis of mesoporous carbon, NCCR-41 Mesoporous carbon (NCCR-41) was synthesized by adopting the following procedure [7,37], before impregnating sucrose solution the silica template was dried in vacuum at 373 K for 30 min the carbon replica were prepared similar to the procedure reported previously. Typically 1.0 g of ordered mesoporous silica (MCM41 structure with a0 = 4.2 nm; SBET = 966 m2 /g; Vp = 0.86 cm3 /g; D = 2.5 nm) was impregnated with acidic sucrose solution, the mixture was dried at 373 K for 6 h and subsequently at 433 K for 6 h the impregnation step was repeated once again. The obtained dark brown/black sample was carbonized at 1173 K under nitrogen or argon flow for 6 h with heating rate of 5 K/min. The carbon–silica composite obtained after pyrolysis was etched with hydrofluoric (HF) acid at room temperature, to remove the silica template. The template-free carbon product thus obtained was filtered, washed with ethanol, and dried at 393 K. The obtained porous carbon material was designated as NCCR-41 (from MCM-41 silica template). 2.4. Preparation of nano-platinum-supported carbon, Pt/NCCR-41 Platinum nanoparticles with 20 wt% loading was supported on the NCCR-41 by a wet chemical reduction method [38] typically 40 mg carbon and 0.0488 M H2 PtCl6 ·xH2 O aqueous solution were homogenously dispersed in 30 ml water and ultrasonicated for

Powder X-ray diffraction (XRD) measurements were carried out on a Rigaku Miniflex II desktop model advanced powder X-ray ˚ radiation source, operdiffractometer with a Cu K␣ ( = 1.5405 A) ating at 30 kV and 15 mA. Wide and small angles were obtained at scanning rate of 3◦ /min and 0.5◦ /min, respectively. Transition electron microscope (TEM) images were obtained with JEOL 3010 UHR TEM equipped with a Gatan Imaging Filter. Prior to observation, carbon sample was sonicated in absolute ethanol for 10 min and then dropped on to a carbon film supported on a copper grid. Nitrogen adsorption–desorption isotherms were obtained at 77 K on a Micrometrics ASAP 2020 apparatus. The specific surface area of the samples were calculated according to the Brunaur–Emmet–Teller (BET) method, and pore size distribution curves were obtained from the analysis of nitrogen adsorption isotherms using Barrett–Joyner–Halenda (BJH) method. Hydrogen chemisorption measurements for the supported catalysts were performed on a Micrometrics ASAP 2020 system. The catalysts were initially reduced at 473 K in hydrogen, and purged with argon gas at the same temperature for 2 h. The freshly reduced sample is equilibrated at 313 K, and then heated to 773 K to desorb the hydrogen. 2.6. Electrochemical measurements The performance of Pt supported on NCCR-41 nanoporous carbon, activated carbon and E-TEK commercial catalyst for room temperature electro oxidation reaction was evaluated by cyclic voltammetry (CV) using a BAS 100 electrochemical analyzer. A conventional three-electrode cell consisting of the glassy carbon (GC, 0.07 cm2 ) working electrode, Pt wire as counter electrode and SCE reference electrode were used for the cyclic voltammetry studies. The catalyst ink was prepared by ultrasonically dispersing 5.0 mg catalyst in 0.50 ml water and the working electrode was prepared by casting 10 ␮l into a GC disk electrode with 3.0 mm diameter. The CV experiments were performed using 1 M H2 SO4 solution in the absence and presence of 1 M CH3 OH at a scan rate of 25 mV/s. All the solutions were prepared by using double distilled pure water [39]. 3. Results and discussion Powder XRD patterns of ordered mesoporous silica (MCM-41) and disordered mesoporous carbon (NCCR-41) are shown in Fig. 1. As expected, the silica exhibits typical characteristic reflections characteristics of MCM-41 structure indicating the formation of highly ordered 2D-hexagonal mesoporous structure [10,40]. On the other hand, the NCCR-41 carbon exhibits certain mesoscopic order by the presence of a slightly broader reflection, which is systematically shifted towards higher angle suggesting a reduced unit cell size. Table 1 lists the computed cell dimension of both MCM-41 and NCCR-41. Fig. 2 shows XRD patterns of platinum-loaded on various carbon supports, viz., NCCR-41, AC and E-TEK, which exhibit characteristic reflections consistent with face-centered cubic lattice of platinum crystallites. Further, the average particle size of the deposited platinum nanoclusters was calculated, and

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Table 1 Structural and textural properties of various carbons. Carbon

Lattice parameter, a0 (nm)

Surface area, SBET (m2 /g)

Pore volume, Vp (cm3 /g)

Pore size, D (nm)

NCCR-41 AC E-TEKa

3.7 – –

1080 1080 193

0.83 0.79 0.71

2.2 1.3 –

a

VULCAN XC-72.

40

x 10

5

Intensity (a.u.)

4

(c)

6

MCM-41

30

0.10

20 dV/dD (cm /g.Å)

Volume adsorbed (mmol/g)

200

110

100

(c)

10

0.05

(b) 100

2

0.0

0.2

0.4

4 D (nm) 6

0.6

8

0.8

1.0

3

4

(a)

5

6

7

2θ (deg.) Fig. 1. XRD patterns of: (a) mesoporous silica (MCM-41); (b) silica/carbon composite; (c) mesoporous carbon (NCCR-41).

the computed values are summarized in Table 1. The N2 sorption isotherms of mesoporous silica MCM-41 and mesoporous carbon NCCR-41 are shown in Figs. 3 and 4, respectively. It can be seen from these isotherms that both these materials exhibit type IV isotherms with a high N2 uptake at low relative pressures (P/P0 < 0.1) indicating unsaturated adsorption of monomolecules on the mesoporous walls, together with characteristic capillary condensation in the mesopores at P/P0 ∼ 0.3. As depicted in the insets of Figs. 3 and 4, the BJH pore size distribution show a narrow pore size distribution centered at 2.5 nm for MCM-41 and a little broader pore size distribution centered at 2.2 nm for NCCR-41. Further, the pore structure

of NCCR-41 is inevitably disordered because the mesopores of MCM-41 are not interconnected together unlike the case of SBA-15. However, the pore walls are possibly connected by disordered carbon networks in a similar way to the activated carbon prepared by pyrolysis of cellulose [7]. Table 1 lists the textural properties of NCCR-41 along with amorphous carbons, viz., AC and E-TEK. Shown in Figs. 5 and 6 are the HR-TEM images of MCM-41 and NCCR-41, respectively. Both MCM-41 and NCCR-41 show typical of ordered and disordered 2D-hexagonal pore structure and the results are in agreement with XRD data. Further, as expected, the TEM image clearly shows that NCCR-41 is exactly inverse replica of MCM-41, however, with disordered pores owing to the thin wall structure (1.7 nm) of the parent MCM-41. Figs. 7–9 illustrate the high resolution TEM (HR-TEM) images, selected area electron diffraction (SAED) patterns (Figs. 7c–9c), and particle size distribution of Pt/NCCR-41, Pt/AC and Pt/E-TEK catalysts. It can be seen from these figures that the platinum nanoparticles are, although,

222

Volume adsorbed (mmol/g)

Intensity (a.u.)

Pt/NCCR-41

311

220

200

111

NCCR-41

Pt/AC

Pt/E-TEK

20

0.2

dv/dD (cm /g.Å)

2

Fig. 3. N2 -sorption isotherm of MCM-41. Inset shows pore size distribution of MCM41.

200

110

P/P0

10

0.1

0.0 0

5

10

15

20

D (nm)

20

30

40

50

60

70

80

90

2θ (deg.) Fig. 2. XRD patterns of: (a) 20 wt% Pt/E-TEK; (b) 20 wt% Pt/AC; (c) 20 wt% Pt/NCCR41.

0.0

0.2

0.4

0.6

0.8

1.0

P/P0 Fig. 4. N2 -sorption isotherm of NCCR-41. Inset shows pore size distribution of NCCR41.

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Fig. 5. TEM image of mesoporous silica MCM-41.

Fig. 6. TEM image of mesoporous carbon NCCR-41.

Fig. 7. TEM images (a,b) of Pt/NCCR-41, (c) SAED pattern of mesoporous carbon Pt/NCCR-41 and (d) particle size distribution of Pt/NCCR-41.

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Fig. 8. TEM images (a,b) of Pt/AC, (c) SAED pattern of Pt/AC and (d) particle size distribution of Pt/AC.

uniform with a size distribution of about 5 nm (cf. Figs. 7d–9d; Table 2), in agreement with XRD data, but the distribution of the nano-platinum clusters are much more uniform (Fig. 7) on the mesoporous high surface area carbon (NCCR-41) than both the high (AC) and low (E-TEK) surface area carbons. The platinum nanoparticles supported on AC (Fig. 8) and E-TEK (Fig. 9) show predominantly clustering as evidenced from TEM. Fig. 10 presents the electrocatalytic behavior of various platinum supported carbon catalysts under study for methanol oxidation reaction at room temperature. All the samples indicate

the irreversible nature of the methanol electro-oxidation, and the CVs were reproducible. The electrochemical activities of all the catalysts are given in Table 2. The potential of 0.70 V was chosen for comparing the electrocatalytic activities for methanol oxidation because the capacitive current at this potential is negligible compared to that of methanol oxidation current. It is clear from both Fig. 10 and Table 2 that the platinum supported mesoporous carbon exhibited higher current density than the commercial (ETEK) catalyst and the catalyst prepared using activated carbon. These results suggest that in the case of Pt/NCCR-41 a better

Table 2 Microtructural and electrochemical data of Pt/NCCR-41. Catalyst system (20 wt% Pt/carbon)

Pt/NCCR-41a Pt/AC Pt/E-TEKb a b

Pt crystallite size (nm)

XRD

TEM

4.7 ± 0.2 5.0 ± 0.3 5.7 ± 0.5

4.5 ± 1.3 5.5 ± 1.0 4.9 ± 2.0

d1 1 1 (XRD) = 0.226 nm; d1 1 1 (TEM) = 0.230 nm. Commercial catalyst.

Pt dispersion (%)

EAS (m2 /g)

Onset potential (V)

Current, I at 0.7 V (mA/mg Pt)

Activity loss (%)

If /Ib

13.7 11.7 11.8

62 60 64

0.16 0.12 0.12

158.0 139.0 128.0

63 70 93

1.58 1.29 0.72

90

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Fig. 9. TEM images (a,b) of Pt/E-TEK, (c) SAED pattern of Pt/E-TEK and (d) particle size distribution of Pt/E-TEK commercial catalyst.

200

150

Current density (mA/mg Pt)

250

Current density (mA/mg Pt)

dispersion and utilization of active platinum catalyst may possibly play a crucial role in determining the activity. The better dispersion of platinum nanoparticles may be attributed to higher surface area of NCCR-41 as well as the very different surface functionalities of the mesoporous carbon. Further, as can be seen from Table 2, the ratio of the anodic peak current densities in the forward (If ) and reverse (Ib ) scans give another measure of the catalytic activity behavior [41–43]. That is, a higher If /Ib ratio indicates a better oxidation performance of methanol oxidation activities during the anodic scan and low accumulation of carbonaceous species on the electrocatalyst surface, suggesting a better carbon monoxide (CO) tolerance. Further, the higher If /Ib ratio also suggests a more effective removal of the poisoning species, viz., CO, on the electrocatalysts surface. It can be seen from Table 2 that the If /Ib ratio for Pt/NCCR-41 is 1.58 which is much higher than both Pt/AC (1.29) and Pt/E-TEK commercial catalyst (0.72) and therefore the better catalyst tolerance of Pt/NCCR-41. Since the mesoporous structure with high surface area and the large pore size will facilitate the transport of methanol and reaction products more easily. This may be attributed to fine dispersion of metal nanoparticles and elimination of diffusion problem in mesoporous carbon materials. Unlike Pt/AC and Pt/E-TEK catalysts wherein Pt form mostly as clusters as compared to fine dispersion of Pt in Pt/NCCR-41. Further, the metal dispersion values (Table 2) suggest that Pt/NCCR-41 catalyst is better than the other two catalysts,

100

120 90 60 30 0

0

40

80

120

160

Time (min)

50

0 Pt/NCCR-41 Pt/AC Pt/E-TEK

-50 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

E (V) vs SCE, sat. KCl Fig. 10. CVs recorded in mixture of 1 M H2 SO4 and 1 M CH3 OH with a scan rate of 25 mV at 298 K: (a) 20 wt% Pt/NCCR-41, (b) 20 wt% Pt/AC and (b) commercial E-TEK catalyst consisting of 20 wt% Pt. Inset: corresponding chronoamperometric data.

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indicating that Pt/NCCR-41 is more effective against CO poisoning. Choi et al. [44] and Liu et al. [45] have also found that the platinum supported on mesoporous carbon electrocatalysts showed much higher CO tolerance (If /Ib ) compared with that of commercial platinum supported Vulcan XC-72 carbon. 4. Conclusion In summary, mesoporous carbon (NCCR-41) synthesized by hard template method posses a high surface area, large pore volume and enhanced mesoporosity that are desirable for electrocatalyst support. The platinum loaded mesoporous carbon catalyst, Pt/NCCR-41, exhibited higher activity and better stability than that of Pt/E-TEK commercial catalyst. The enhanced activity and the superior performance of Pt/NCCR-41 is due to improved dispersion of uniform platinum nanoparticles as well as better utilization of the catalyst, which may possibly originate from a higher surface area, large pore volume and functional groups present in the newly synthesized carbon. The study also demonstrates that optimized carbon supports can offer significant improvement by way of lowering the noble metal loading. Acknowledgements This work is partially supported by a grant-in-aid for the scientific research by the Ministry of New and Renewable Energy under grant no. 103/140/2008-NT. Department of Science and Technology, New Delhi is gratefully acknowledged for funding NCCR, IIT-Madras. We are also thankful to Professor B. Viswanathan for the fruitful discussion, and Ms. K. Devaki for the help with TEM. References [1] R.C. Bansal, J.B. Donnet, F. Stoeckli, Active Carbon, Marcel Dekker, New York, 1988. [2] R.T. Yang, Adsorbents: Fundamentals and Applications, Wiley Interscience, New York, 2003. [3] A. Corma, Chemical Reviews 97 (1997) 2373. [4] Z. Pan, H. Zhu, Z. Zhang, H.-J. Im, S. Dai, D.B. Beach, D.H. Lowndes, Journal of Physical Chemistry B 107 (2003) 1338. [5] J.P. Tu, L.P. Zhu, K. Hou, S.Y. Guo, Carbon 41 (2003) 1257. [6] R. Ryoo, S.H. Joo, M. Kruk, M. Jaroniec, Advanced Materials 13 (2001) 677. [7] S. Jun, S.H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna, O. Terasaki, Journal of the American Chemical Society 122 (2000) 10712. [8] S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Nature 412 (2001) 169. [9] P. Selvam, P.R. Murthy, K.N. Vamshi, B. Viswanathan, in: L. Albu, V. Deselnicu (Eds.), Proceedings of the 3rd International Conference on Advanced Materials and Systems (ICAMS-2010), Bucuresti, 2010, p. 119.

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