dispersion of pt catalysts supported on activated carbon and their catalytic performance in...

RESEARCH PAPER

CHINESE JOURNAL OF CATALYSIS Volume 29, Issue 3, March 2008 Online English edition of the Chinese language journal

Cite this article as: Chin J Catal, 2008, 29(3): 259–263

Received date: 6 September 2007. * Corresponding author. Tel: +86-411-84686637; Fax: +86-411-84694447; E-mail: [email protected] Foundation items: Supported by the National Natural Science Foundation of China (90206036) and the National Basic Research Program of China (973 Program, 2005CB221405). Copyright © 2008, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.

Dispersion of Pt Catalysts Supported on Activated Carbon and Their Catalytic Performance in Methylcyclohexane Dehydrogenation

LI Xiaoyun, MA Ding, BAO Xinhe* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China

Abstract: Activated carbon-supported Pt catalysts were prepared by the conventional wetness impregnation method, and their catalytic activity for methylcyclohexane dehydrogenation was tested. The impact of different treatments (nitric acid oxidation and H2 heating treatment) of carbon supports on the reaction performance was addressed. The results of nitrogen adsorption and temperature-programmed desorption show that the carbon supports have almost identical porous texture but different types and amounts of surface oxygen groups. This led to different dispersion of Pt particles and thus different activity and stability of the catalysts.

Key words: platinum; activated carbon; surface oxygen group; temperature-programmed desorption; methylcyclohexane; dehydrogena-tion

Carbon materials have been widely used as catalyst supports in industry owing to their low prices, good stability in acidic and basic media, and that the active component (such as pre-cious metals) can be easily recovered by burning away the carbon support [1]. However, how to acquire high dispersion of active phases on the carbon surface is the key point in prepar-ing a highly active catalyst. Researchers found that the surface area and porosity alone cannot explain the metal dispersion over the carbon support. The importance of oxygen functional groups on the carbon surface was discovered. Several studies have been focused on the influences of surface oxygen groups upon the metal dispersion and catalytic performance [2–11]. Although the roles of surface oxygen groups have been re-vealed continually, some conflicting results were reported. For example, Torres et al. [10] compared the dispersion of Pt/C catalysts with carbon support treated by O3, H2O2, and HNO3

oxidants. They concluded that weak acid groups produced by O3 and H2O2 favor the Pt dispersion over the carbon support. However, Coloma et al. [11] acquired a lower Pt dispersion on the H2O2-treated carbon support. Actually, various factors such as the nature of carbon, properties of oxygen groups, and pre-treatment conditions of catalysts can cause the discrepancies.

Methylcyclohexane (MCH) dehydrogenation has long been used as a model reaction to probe catalytic performance of catalysts [12]. As one of the most effective catalysts for this reaction, Pt/Al2O3 has been extensively studied [13]. Recently, this reaction has gained a renaissance because it is considered an efficient method to store and supply H2 through a reversible hydrogenation-dehydrogenation cycle [14]. Meanwhile, a different picture was observed when carbon materials were used as supports for Pt catalysts. Kariya et al. [15] investigated the effect of carbon supports on a series of Pt-containing catalysts under wet-dry multiphase conditions for the reaction. They concluded that activated carbon is superior to alumina because of its larger surface area and wettability. Wang et al. [16] compared the catalytic performance of Pt supported on stacked-cone carbon nanotubes and alumina. The results showed that the Pt on the carbon nanotubes with lower loading (0.25%) has nearly identical activity to the catalyst on alumina with relatively high loading (1%).

In this study, we prepared three Pt catalysts with different dispersions on activated carbon. The effects of surface oxygen groups of the support on the Pt dispersion after reduction and during reaction were investigated using transmission electron

LI Xiaoyun et al. / Chinese Journal of Catalysis, 2008, 29(3): 259–263

microscopy (TEM) and MCH dehydrogenation.

1 Experimental

1.1 Catalyst preparation

The original activated carbon made from coconut shell was designated as AC1. AC1 was oxidized with 5 mol/L HNO3 at 90 °C for 6 h and dried at 60 °C. The sample obtained was designated as AC2. Then, the oxidized sample was treated in H2 at 500 °C to remove the surface oxygen groups. The sample was denoted as AC3. These three samples were used as sup-ports to prepare Pt catalysts by an impregnation method. The Pt loadings were all 1%. During the impregnation, the support was added to an appropriate amount of H2PtCl6 aqueous solu-tion (0.02 mol/L), and the mixture was stirred. Then, the sam-ples were dried at 60 °C overnight and kept in a desiccator. The three catalysts obtained were denoted as Pt/AC1, Pt/AC2, and Pt/AC3.

1.2 Catalyst characterization

The surface area, pore volume, and pore size distribution of the supports were measured by nitrogen adsorption at –196 °C using a Quantachrome Autosorb-1 apparatus. Prior to the N2

adsorption measurement, the sample was degassed at 120 oCfor 15 h.

The TPD experiments of the supports were performed on a TP-MS apparatus constructed in-house. Prior to the TPD ex-periments, 50 mg sample was purged with a He flow (50 ml/min) for 1 h at ambient temperature to obtain the stable baseline. Then, TPD spectra were obtained using He (50 ml/min) as the carrier gas at a heating rate of 10 °C/min from 20 to 900 °C.

The platinum particle size of the catalysts was determined by TEM on a JEOL JEM 2000EX instrument at 120 kV.

1.3 Catalytic tests

MCH dehydrogenation was carried out under atmospheric pressure in a fixed-bed quartz reactor (o.d. = 10 mm). Before entering the reactor, the gases (99.999%) were treated with 5A

zeolite and deoxidation tubes to remove H2O and O2. Prior to the reaction, 50 mg catalyst was reduced in situ by H2 (30 ml/min) at 400 °C for 2 h, and the temperature was then de-creased to 300 °C. At the same time, the system was purged with Ar for 30 min. MCH was vaporized and mixed with Ar through a vaporizer. The rate of liquid MCH was 0.03 ml/min and the Ar/MCH mole ratio was fixed at 1. The effluents were analyzed online using a Shimadzu GC 14B apparatus equipped with a 4 m SE-30 column.

2 Results and discussion

2.1 Activated carbon properties

The pore structure properties of the three supports were characterized by N2 adsorption, and the results are shown in Table 1. The surface area of the three samples is over 1000 m2/g, which is mainly the contribution of micropores as de-termined by the t-plot method. It can be concluded that nitric acid oxidation has very little impact on the texture of original carbon, which is in good agreement with the literature report [17]. On the other hand, although the BET surface area and pore volume increased after H2 treatment at 500 °C, the pore size of AC3 changed very slightly. This indicates that the pore structure of AC3 remained, and less stable carbon species were removed by H2.

The surface oxygen groups of the supports were character-ized by the TPD method, and the CO2- and CO-TPD profiles are shown in Fig. 1. For AC1, two peaks at 90–430 and 550–660 °C were observed in the CO2 release curve, whereas for CO, a single peak at 520–900 °C was detected. This indi-cates that carboxylic acid, lactone, and carbonyl groups exist on the original carbon surface [18]. The amounts of these groups are relatively small. After oxidation by HNO3, several oxygen groups were introduced onto the carbon surface. As seen in Fig. 1, the amount of CO2 evolution from AC2 was an order of magnitude greater than that from AC1. Besides car-boxylic acid and lactone groups centered at 260 and 640 °C, respectively, a new peak at 410 °C appeared, which could be assigned to the decomposition of carboxylic acid anhydride groups. This indicates that after the treatment of HNO3, large amounts of carboxylic acid groups were introduced onto the

Table 1 Pore structure properties of activated carbon supports

Surface area (m2/g) Pore volume (cm3/g) Pore size (nm) Support

Total Micropore a Total Micropore a Micropore b Mesopore c

AC1 1080 853 0.541 0.258 0.52 3.80 AC2 1043 828 0.517 0.249 0.52 3.80 AC3 1236 933 0.628 0.260 0.53 3.81

a Surface area and pore volume of micropores were calculated by the t-plot method. b Pore size of micropores was determined by the HK method. c Pore size of mesopores was determined by the BJH method. AC1: the original carbon; AC2: AC1 oxidized with HNO3; AC3: AC2 heated in H2.


carbon surface, which could be further transformed into car-boxylic acid anhydride groups. The latter is considerably more stable against thermal treatment. The release of CO from AC2 was also considerably higher than that from AC1 though the groups were not completely decomposed at 900 °C. For the AC3 sample, CO2 and CO release were only observed at tem-peratures higher than 500 °C, which demonstrated that car-boxylic acid and anhydride groups could be selectively re-moved, and some thermal stable groups such as lactone, phe-nol, and carbonyl remained on the carbon surface.

2.2 Catalytic tests

The catalytic activity of the three Pt catalysts supported on different activated carbons (Pt/AC1, Pt/AC2, and Pt/AC1) was tested using MCH dehydrogenation as the probe reaction. For all catalysts, the selectivity for toluene was above 99%, whereas the activity and stability were very different. The MCH conversion as a function of time is shown in Fig. 2. ForPt/AC1, the MCH conversions were maintained at about 42%, and no deactivation occurred during the 27 h reaction. Rela-tively low activity and severe deactivation were observed on Pt/AC2. After 4 h reaction, the conversion dropped dramati-cally from the initial value of about 36% to 13%. Instead, when the carboxylic acid and anhydride groups were removed, the resultant Pt/AC3 catalyst showed an increased initial conver-sion of 88%, which was at least two times higher than that of Pt/AC1 and Pt/AC2. However, the conversion decreased gradually to about 70% after 24 h reaction.

2.3 Pt dispersion on activated carbon with different sur-face groups

The fresh and used catalysts were characterized by TEM (Fig. 3). The activity and stability of the catalysts were found to be strongly dependent on the dispersion of Pt. To compare the Pt dispersion of different catalysts, 300 particles were counted for each catalyst, and the particle size distributions are shown in Fig. 4. For fresh Pt/AC1 and Pt/AC2, a relatively wide dis-tribution (ranging from 2 to 12 nm) was observed. After reac-tion, different pictures were observed for the two catalysts. The particle size of Pt/AC1 was greatly reduced, resulting in a narrow distribution centered at 2 nm. Instead, for the Pt/AC2 catalyst, a very slight change was observed after reaction. For Pt/AC3, before the reaction, 2–3 nm particles dominated in the TEM image. However, after 24 h reaction, the nanoparticles aggregated. Some particles as large as 13 nm were observed in the system, which could be correlated to the deactivation of the catalyst during the reaction.

Because these carbon supports have similar porous textures, and the catalysts were prepared by the same experimental procedure, the different types and amounts of surface func-tional groups should be responsible for the different platinum dispersions. During the impregnation, a generally accepted viewpoint is that the presence of surface functional groups can improve the dispersion of the Pt precursor by increasing the hydrophilicity of the carbon surface [19]. However, the dis-persion cannot remain at higher temperature because some groups such as carboxylic acid groups are decomposed during the reduction process. Thus, the thermal resistance of the sur-face functional groups and the interaction between Pt and carbon surface are the decisive factors for the Pt dispersion. For the Pt/AC2 sample, there are several carboxylic acid groups on the surface, which act as the anchoring center to interact with the Pt precursor. When the carboxylic acid groups are de-composed during reduction at 400 °C, Pt particle agglomera-tion takes place. A similar phenomenon was observed on the

Fig. 1 TPD profiles of CO2 (a) and CO (b) from original and modified activated carbon materials

(1) AC1, (2) AC2, (3) AC3

Fig. 2 The relationship between MCH conversion and time for different catalysts at 300 °C

(1) Pt/AC1, (2) Pt/AC2, (3) Pt/AC3


Pt/AC1 sample. However, a better dispersion was obtained for the Pt/AC3 sample. Herein, after less stable oxygen groups were removed at 500 °C, Pt precursor interacted with more stable groups such as carbonyl groups. Pt particles did not agglomerate when the catalysts were reduced at a relatively low temperature (400 °C).

The Pt particle size of the fresh catalyst can be correlated to the initial conversion of MCH. As seen in Fig. 2, Pt/AC3 with the smallest particle size (about 2 nm) has the highest initial

conversion. At the same time, the Pt catalysts with larger par-ticle size (about 5 nm) on AC1 and AC2 have less than half activity on AC3. On the other hand, the three catalysts have different stabilities. Fung et al. [20] observed that the Pt cata-lyst with high dispersion was more resistant to deactivation for MCH dehydrogenation. This is because the carbonaceous deposit is more easily formed on large Pt ensembles. The proposition can explain the different stabilities of Pt on AC2 and AC3 samples. For Pt/AC1, a stable activity was observed

Fig. 3 TEM images of the fresh and used platinum catalysts on activated carbons (a) Fresh Pt/AC1, (b) Fresh Pt/AC2, (c) Fresh Pt/AC3, (d) Used Pt/AC1, (e) Used Pt/AC2, (f) Used Pt/AC3

Fig. 4 Distribution histograms of the fresh and used platinum catalysts on activated carbons (a) Pt/AC1, (b) Pt/AC2, (c) Pt/AC3


during the reaction. Considering the TEM results of the used catalysts, we presume that the redispersion of Pt on AC1 has a compensation effect on the deactivation of the catalyst.

It is known that some surface defects and graphite edges exist on the basal planes of microcrystallites in activated car-bons, where oxygen is apt to be chemisorbed [9]. From the TPD profile of AC1, small amounts of oxygen groups were evolved, which should be attributed to the oxygen chemisorp-tion. Once the oxygen groups are evolved, Pt particles will interact directly with the defects and edges on the carbon sur-face. Being new anchoring sites, these defects and edges can cause a redistribution of Pt particles. Although the interaction between Pt and the basal plane of carbon is not clear, some studies reported that the edges, corners, and sites can influ-ence the Pt particle dispersion [2,3,7,11]. Guerrero-Ruiz et al. [7] concluded that the specific interaction between metal par-ticles and the edges and corners on graphitic layers was the driving force for metal dispersion. Unlike Pt/AC1, the redis-persion phenomenon was not observed on used Pt catalysts on AC2 and AC3. This is because there are still more oxygen groups on the carbon surface.

3 Conclusions

Three activated carbon materials with identical porous tex-ture and different surface oxygen groups were obtained through modification by nitric acid oxidation and H2 heating treatment. Pt catalysts were prepared using these carbons as the support. Surface oxygen groups of the activated carbon strongly influenced the Pt dispersion. Less thermal stable groups, like carboxylic acid groups, were decomposed during the reduction, which caused the Pt agglomeration. However, thermal stable groups can maintain the high dispersion of the catalyst. For the original activated carbon, a small amount of oxygen groups mainly existed on the defects and edges of the surface. After this part of oxygen groups were decomposed, Pt particles directly interacted with the defects and edges, which promoted the redispersion of Pt. In MCH dehydrogenation, the activity and stability of the catalyst were dependent on the Pt particle size. The catalyst with small particle size had a high initial conversion and a slow deactivation rate.

References

[1] Rodriguez-Reinoso F. In: Patrick J W ed. Porosity in Carbon: Characterization and Applications. New York: Halsted Press, 1995. 253

[2] Kang M, Bae Y S, Lee C H. Carbon, 2005, 43(7): 1512 [3] Fraga M A, Jordao E, Mendes M J, Freitas M M A, Faria J L,

Figueiredo J L. J Catal, 2002, 209(2): 355 [4] de Miguel S R, Vilella J I, Jablonski E L, Scelza O A, de Lecea C

S M, Linares-Solano A. Appl Catal A, 2002, 232(1–2): 237 [5] Fraga M A, Mendes M J, Jordao E. J Mol Catal A, 2002,

179(1–2): 243 [6] Okhlopkova L B, Lisitsyn A S, Likholobov V A, Gurrath M,

Boehm H P. Appl Catal A, 2000, 204(2): 229 [7] Guerrero-Ruiz A, Badenes P, Rodriguez-Ramos I. Appl Catal A,

1998, 173(2): 313 [8] de Miguel S R, Scelza O A, Roman-Martinez M C, de Lecea C S

M, Cazorla-Amoros D, Linares-Solano A. Appl Catal A, 1998, 170(1): 93

[9] Rodriguez-Reinoso F. Carbon, 1998, 36(3): 159 [10] Torres G C, Jablonski E L, Baronetti G T, Castro A A, de Miguel

S R, Scelza O A, Blanco M D, Pena Jimenez M A, Fierro J L G. Appl Catal A, 1997, 161(1–2): 213

[11] Coloma F, Sepulveda-Escribano A, Fierro J L G, Rodriguez- Reinoso F. Langmuir, 1994, 10(3): 750

[12] Belskaya O B, Ostrovskii N M, Demanov Y K. React Kinet Catal Lett, 1997, 60(1): 119

[13] Sinfelt J H. J Mol Catal A, 2000, 163(1–2): 123 [14] Okada Y, Sasaki E, Watanabe E, Hyodo S, Nishijima H. Int J

Hydrogen Energy, 2006, 31(10): 1348 [15] Kariya N, Fukuoka A, Ichikawa M. Appl Catal A, 2002,

233(1–2): 91 [16] Wang Y, Shah N, Huffman G P. Energy Fuels, 2004, 18(5): 1429 [17] Figueiredo J L, Pereira M F R, Freitas M M A, Orfao J J M.

Carbon, 1999, 37(9): 1379 [18] Aksoylu A E, Madalena M, Freitas A, Fernando M, Pereira R,

Figueiredo J L. Carbon, 2001, 39(2): 175 [19] Prado-Burguete C, Linares-Solano A, Rodriguez-Reinoso F, de

Lecea C S M. J Catal, 1989, 115(1): 98 [20] Fung A S, Kelley M J, Koningsberger D C, Gates B C. J Am

Chem Soc, 1997, 119(25): 5877

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