multi-wall carbon nanotubes as a support for platinum catalysts for the hydrodechlorination of...
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Multi-wall Carbon Nanotubes as a Support for Platinum Catalysts for the Hydrodechlorination of Carbon Tetrachloride
and Dichlorodifluoromethane Magdalena Bonarowska 1,a, Kuan-Nan Lin 2,b, Marta Legawiec-Jarzyna 1,c,
Leszek Stobinski 1,d, Wojciech Juszczyk 1,e, Zbigniew Kaszkur 1,f, Zbigniew Karpiński 1,g and Hong-Ming Lin 2,h
1 Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52,
01-224 Warsaw, Poland 2 Department of Materials Engineering, Tatung University, 40, Chungshan N. Rd.,
3rd Sec, 104 Taipei, Taiwan e-mail: a [email protected], b [email protected], c [email protected], d [email protected] e [email protected], f [email protected], g [email protected], h [email protected]
Keywords: carbon nanotubes, Pt nanoparticles, catalytic hydrodechlorination, carbon tetrachloride, dichlorodifluoromethane Abstract. Multi-wall carbon nanotubes (MWCNTs) were used as a support for the deposition of highly dispersed platinum. After characterization by several physical techniques, the catalyst was studied in reactions for: hydrodechlorination of carbon tetrachloride and the hydrodechlorination of dichlorodifluoromethane. For the first reaction Pt/MWCNTs were very effective catalysts in terms of both the overall activity and the selectivity to CHCl3; both quantities appeared high and stable. For CCl2F2 hydrodechlorination the catalyst was rather moderate, although very stable, activity and product selectivities were established at a constant level in a relatively short time-on-stream. The MWCNTs-supported Pt particles do not undergo great changes during the reactions, i.e. neither substantial metal sintering occurred nor extensive surface carbonization/chloriding took place. Introduction Chlorine-containing compounds are considered to be very dangerous environmental pollutants. as their noxious effects, toxicity and carcinogenic character have been widely proven. Among these pollutants are, for example, chlorofluorocarbons (CFCs), polychlorobiphenyls (PCBs), volatile organic compounds (VOCs) and other chlorinated solvents. In the past carbon tetrachloride was extensively used as a reagent and a solvent in the chemical and semiconductor industry [1,2]. Its ozone-depleting potential and overall toxicity have caused its abandonment. However, CCl4 is still a substantial by-product of several industrial processes and methods for its safe disposal have to be developed. Freons (chlorofluorocarbons, CFCs) are responsible for the depletion of the ozone layer and they contribute to the greenhouse effect [3-5]. There has been a worldwide ban (following the 1987 Montreal Protocol and its later amendments) on the production and usage of CFCs. However, the existing stocks (estimated at over 2 Mton, about 45% of which is freon CFC-12 (CCl2F2) [6]) need to be disposed of safely. During the past decade, new methods based on catalytic hydrodechlorination of chlorine-containing compounds have been proposed as an alternative to incineration and catalytic combustion. The potential advantage of these new methods relies on the transformation of the compounds into useful chemicals, of which chloroform and methylene chloride (from CCl4) and CH2F2, CHF3, CH3F, CHClF2 (in the case of CCl2F2) are the most important. Most studies of the hydrodechlorination of chlorine-containing compounds were carried out using noble metal catalysts.
Solid State Phenomena Vol. 128 (2007) pp 261-271Online available since 2007/Oct/15 at www.scientific.net© (2007) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/SSP.128.261
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Palladium was found to be a unique catalytic metal because of its activity and, most of all, its selectivity in the catalytic hydrodechlorination of CFCs process. As a consequence most fundamental works [7-13] were carried out with it. Supported platinum catalysts were also tested [6,14,15] but they showed rather low activity and often converted the reactant to less significant products (e.g. alkanes) and a rapid deactivation of the Pt catalysts was usually observed during the time-on stream. As far as gas-phase hydrodechlorination of CCl4 is concerned, platinum appears to be the most effective catalyst compared to other transition metals [16-18]. However, even in the case of Pt, deactivation of the catalyst is still regarded as a major problem [16]. Several factors were proposed to explain Pt deactivation: the metal particle size [19-21], the nature of the support [17,22,23] or the chemical state of platinum [24]. This work was directed towards studying the effect of multi-wall carbon nanotubes (MWCNTs), on the catalytic behavior of platinum. A rather high platinum loading (25 wt.%) was used because it helped to give a more precise characterization of the metal phase (especially because of a considerable overlap of XRD reflections from Pt and carbon material). Therefore, the results refer to the behaviour of a model, high metal loaded, catalyst. In addition, fine Pt particles exhibit high electrocatalytic activity for oxygen reduction [25] and are widely used in fuel cells. The electrocatalytic activity strongly depends on the platinum particle size and distribution. The method of preparation used enables a catalyst with a high concentration of platinum to be produced with, simultaneously, particles of a suitable size for a fuel cell application. Experimental Details Catalyst Preparation and Characterization. The 25 wt.% Pt/MWCNTs catalyst was prepared by the polyol method as described in paper [26]. First, as prepared the MWCNTs, produced by CNT Co. Ltd., Korea (www.carbonnanotube.biz) with outer diameter included in the range of 20-40 nm, were oxidized by refluxing in concentrated HNO3 (60 %) at about 120oC for 6 hours. Then carbon nanotubes, functionalized by carboxyl groups and having got rid of the catalyst (Fe, Ni) and amorphous carbon, were rinsed with large amounts of deionized water (to pH~6) and ethanol. Next, MWCNTs were dried in an air oven at about 50oC for 10 hours. The BET surface area, for MWCNTs after the functionalization, was 460 m2/g. The Pt/MWCNTs were prepared by adding an aqueous solution of H2PtCl6 (Wako, 98.5%) to initially purified and functionalized MWCNTs premixed with ethylene glycol (Wako, 99.0 %). The well-stirred mixture was heated to 190oC while the pH was adjusted to about 9 by dropwise addition of a 0.4 M solution of KOH. Next, the solid was filtered and washed with deionized water and ethanol, and initially dried in an air oven at about 50oC for 2 hours. Finally, the catalyst was heated at 300oC for 3 hours in a flow of argon. The platinum concentration in the catalyst was determined using thermo-gravimetric apparatus SDT 2960 (TA Instruments, USA). TGA analysis was carried out in atmospheric air at a flow rate of 100 ml/min and a heating rate, from room temperature to 1000°C, of 5°C/min. Prior to characterization studies (chemisorption of CO, XRD, TEM) and catalytic screening in CCl4 and CCl2F2 hydrodechlorination, the 25 wt. % Pt/MWCNTs was reduced in a flow of 10% H2/Ar (25 cm3min-1) at 300oC for 1 hour. All gases, H2, Ar and a 10% H2/Ar mixture, all 99.999% pure, were purified by passing through drying and MnO/SiO2 traps. A carbon monoxide chemisorption experiment was carried out in a pulse-flow system described elsewhere [27,28]. The thermal conductivity detector (TCD, Gow–Mac) was kept at 0oC, providing a constant response during the run. Chemisorption of CO pulses was carried out at 25oC, using helium as a carrier gas. XRD experiments were performed with a standard Siemens D5000 diffractometer using Ni-filtered CuKα radiation. The catalyst (freshly reduced and post-reaction)
262 Doped Nanopowders
was scanned by a step-by-step technique, at 2θ intervals of 0.02°. After reactions, the catalyst samples were quickly cooled in a flow of the reaction mixture from the reaction temperatures to ~60oC. The flow of CCl4 (or CCl2F2) and H2 was then stopped and further cooling to room temperature took place in a flow of argon. In the presentation of XRD profiles below the background from the MWCNTs was subtracted. The transmission electron microscope (TEM) images were recorded by a Hitachi-800 TEM system operated at 175 kV and with a resolution of 0.45 nm. The specimens for TEM were prepared by dispersing the powder samples in ethanol using an ultra-sonic bath. Then 1-2 drops of the suspension were deposited onto a copper microscope grid covered with a carbon film.
Catalytic tests and post-reaction characterization. The reactions of the hydrodechlorination of carbon tetrachloride and the hydrodechlorination of dichlorodifluoromethane were carried out at atmospheric pressure, in a glass flow reactor system. The flows of H2 and Ar (both 99.999% pure, further purified by passing through MnO/SiO2 traps) were fixed by mass flow controllers. The reaction was followed by gas chromatography using a HP 5890 series II with FID, equipped with a 5% Fluorcol/Carbopack B packed column (10 ft) from Supelco. In the reaction of carbon tetrachloride (analytical reagent from POCh, Gliwice, Poland, purity >99.6%, provided from a saturator kept at 0oC) with hydrogen, the mass of catalyst was 0.018 g. After reduction, the catalyst was cooled to 90oC, then put into contact with the reaction mixture with a flow of hydrogen (~9 cm3/min) + argon (~20 cm3/min) and CCl4 (~1.3 cm3/min). The partial pressure ratio PH2/PCCl4 was ~7:1. A typical run lasted ~40 hours. In the reaction of dichlorodifluoromethane (CFC-12 from Galco S.A., Belgium; purity 99.9%; the gas flow was metered with a mass flow controller) with hydrogen, the mass of catalyst was 0.080 g. The amount of the catalyst used in the hydrodechlorination of CFC-12 was larger than in the previous reaction because of a significant difference in the activity of the catalyst in the reactions. After reduction, the catalyst was cooled to 180oC, then put into contact with the reaction mixture, with a flow of hydrogen (22.5 cm3/min) + argon (75 cm3/min) and CCl2F2 (2.5 cm3/min). The partial pressure ratio PH2/PCCl2F2 was ~10:1. In order to adequately establish changes in the catalytic behaviour, a typical reaction run lasted ~24 hours. The first stage of the reaction involved a 19-hour period at 180oC. During this time the catalyst performance stabilized in a flow of the reaction mixture, as established by analyzing the reacting gas at 30-min intervals. After 19 hours on-stream, the reaction temperature was lowered, in 10oC steps, and the next experimental points were collected. After catalyst screening at the lowest reaction temperature (160oC) the catalyst performance was tested again at 180oC, giving a good return to the initial behaviour at this temperature. To avoid secondary reactions and limit self-poisoning, the overall conversion was kept low, i.e., below 3%, at the highest temperature of catalyst screening. For both reactions blank experiments with MWCNTs showed negligible activity in the temperature ranges used for screening the Pt/C catalyst, i.e. ≤90oC and ≤180oC for reaction of carbon tetrachloride and dichlorodifluoromethane, respectively. After the kinetic runs the catalyst was investigated by XRD; TEM, to search for changes in the Pt particle size and temperature-programmed hydrogenation (TPH) to detect the species which can be removed by hydrogen (from a 10% H2/Ar flow) from the spent catalysts. Progress of TPH runs, at a 10°C/min temperature ramp, was followed by mass spectrometry (Dycor-Ametek, Pittsburgh). Several masses were monitored during the experiment, but major changes were seen only for m/z 15 (methane evolution; m/z 16 was not selected because of water contribution), m/z 36 (HCl liberation) and m/z 20, which is suggestive of HF liberation (for the hydrodechlorination of dichlorodifluoromethane).
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Results and Discussion Both the TEM image (Fig. 1) as well as XRD line broadenings (based on XRD profiles in Fig. 2 and treated by using Scherrer’s equation) showed that platinum particles supported on MWCNTs are ~8 nm in size.
Fig. 1. TEM image of the Pt/MWCNTs catalyst after preparation and heating in Ar.
Fig. 2. XRD profiles of the Pt/MWCNTs reduced and after reactions with CCl4 and CCl2F2. CO chemisorption was used to determine (i) the dispersion of platinum, expressed as the fraction of metal atoms exposed (FE), and (ii) the size of the platinum crystallites. An average platinum crystallite size (d) was calculated using the equation d (nm) = 1.13/FE; where FE is the fraction exposed [29]. The platinum particle size d assessed from metal dispersion (FE = 0.143, from CO chemisorption) was 7.9 nm. It is in a good conformity with that obtained from the XRD profiles and TEM images. In the investigation, hydrogen chemisorption (not presented) was not a good tool for estimating the dispersion of metal particles, because the uptake of hydrogen on platinum was negligible. Vadlamannati et al. [30] and Kulkarni et al. [31] reported similar observations. They
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Pt/MWCNTs after reaction with CCl2F2
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showed, that for carbon-supported Pt, the equilibrium hydrogen coverage was significantly lower than when this metal was supported on oxides. It was determined from the TEM images, that the platinum particles were deposited on the external surface of the carbon tubes, not inside. This was a good starting point in the investigation. In a different situation, i.e. finding large Pt crystallite inside the MWCNTs (which was possible considering the high metal loading), the samples would be of very limited value for catalytic studies. In the reaction of carbon tetrachloride with hydrogen, the very high conversion level (nearly 100%) indicated that our reactor could not be treated as a differential one. However, additional runs with different charges of the catalyst (not presented) showed that for different conversion levels the selectivity pattern was nearly unchanged. Interestingly, chloroform, a desired product, always appeared as the dominant product, with a selectivity of ~80%, which, like the overall conversion, was stable during a 37-hour screening, (Fig. 3).
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Fig. 3. Time-on-stream performance of the Pt/MWCNTs catalyst in hydrodechlorination of CCl4. Conversion and selectivities to CHCl3 and CH4 (major reaction products). Reaction temperature 90oC.
The other major product was methane (20% or less, depending on the reaction temperature), whilst the rest of the products comprised less than 1% of the total as shown in Table 1.
Solid State Phenomena Vol. 128 265
Product distributiona [%] Reaction temperature
[oC] CH4 C2H6 CH3Cl C3H8 CHCl3 C2Cl4
Conversion [%]
TOF [s-1]
90 22 0.4 trb trb 77 trb 95 0.248
80 19 0.9 0.1 0.1 79 0.6 69 0.180
70 19.5 1.4 0.4 0.2 76.5 1.7 39 0.102 a Very minor products (CH2Cl2, C2Cl6, C2H2Cl4) are not mentioned in the Table. b Trace amounts, typically <<0.1%. Table 1. Hydrodechlorination of CCl4 on 25 wt.% Pt/MWCNTs catalyst: conversion and product
selectivities at 70 - 90oC; EA = 46.2 ± 3.9 kJ/mol
Our recent results with microcrystalline active carbon-supported Pt catalysts [32] of comparable metal dispersion showed rather poor catalytic properties for CCl4 hydrodechlorination. As stated, the overall activity (considered as the conversion level) was found to be very high (nearly 100%) for Pt/MWCNTs, whereas an initially high conversion for Pt-on active carbon decreased considerably during the reaction [32]. Simultaneously, the selectivity to chloroform, comparable for both catalysts at the beginning, remained high and stable only for Pt/MWCNTs (Fig. 3), but decreased for Pt/active carbon [32]. Zhang et al. reported that for a metal particle size range similar to that used in this study (i.e. 5-8 nm), Pt/Al2O3 catalysts showed very stable and high conversion for the hydrodechlorination of CCl4 [20,21]. Smaller Pt clusters tend to be deactivated by surface chloride species. The overall situation may be even more complicated because it is known that surface chloride species contribute to a considerable re-dispersion of platinum over the Al2O3 [33]. Catalytic screening of the Pt/MWCNTs catalyst in CCl2F2 hydrodechlorination showed that stable conversion was achieved in a very short time (below 1 h). It should be emphasized that catalyst deactivation was not observed and that the conversion was stable during a 24-hour screening. This is demonstrated in Fig. 4, where also the product selectivities are shown to be established at a constant level after a relatively period. Methane, trifluoromethane (HFC-23), chlorodifluoromethane (HCFC-22) and difluoromethane (HFC-32) were found to be the only products. The results of a steady state catalytic test are shown in Table 2. The catalytic behaviour of the Pt/MWCNTs catalyst differs markedly from that described in our recent papers for alumina-supported platinum catalysts [15,34]: Pt showed a rather low activity in CCl2F2 hydrodechlorination and a rapid deactivation of the catalysts always accompanied the reaction. Wiersma et al. [6] and Kulkarni et al. [31] observed similar behaviour for activated carbon-supported platinum catalysts. The conversion initially increased with time on stream to a maximum after ~2-8 h and then the catalysts gradually deactivated to attain a steady state after 15-20 h.
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They suggested that two processes, platinum re-dispersion, induced by molecular chlorine, HCl and other chlorine-containing compounds, and coking , which causes deactivation, were responsible for the change in activity. At the beginning of the reaction the rate of metal re-dispersion is higher than the deactivation rate, so conversion increases.
Fig. 4. Time on stream performance of the Pt/MWCNTs catalyst in hydrodechlorination of CCl2F2. Conversion and selectivities to CHF3, CH4, CHClF2 and CH2F2. Reaction temperature 180oC.
Product distribution [%] Reaction temperature
[oC] CH4 CHF3 CHClF2 CH2F2
Conversion [%]
TOF [s-1]
180 37.1 38.2 21.9 2.8 2.1 1.93e-3
170 34.2 40.3 22.2 3.3 1.4 1.28e-3
160 31.5 42.1 22.5 3.9 0.9 8.26e-4 Table 2. Hydrodechlorination of CCl2F2 on 25 wt.% Pt/MWCNTs catalyst: conversion and product selectivities at 160 - 180oC; EA = 69.2 ± 1.1 kJ/mol The re-dispersion stops because of equilibration and only coking, and possibly halogen accumulation on the catalyst surface, governs the conversion changes. In the light of these findings, the results suggest that the very good stability of our Pt/MWCNTs catalysts would follow from an absence of dispersion changes during hydrodechlorination. In order to explain the beneficial and stable catalytic behaviour of the Pt/MWCNTs we checked if any Pt re-dispersion occurred during the hydrodechlorination of carbon tetrachloride and dichlorodifluoromethane. It appeared that, in the course of both reactions, platinum crystallites experience only a small growth, from ~8 nm to ~10 nm (XRD – Fig. 2). A close comparison of the TEM images Fig. 5, Fig. 6 and Fig. 1 confirm the results obtained from the XRD patterns.
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Solid State Phenomena Vol. 128 267
Fig. 5. TEM image of the Pt/MWCNTs catalyst after hydrodechlorination of CCl4.
Figure 6. TEM image of the Pt/MWCNTs catalyst after hydrodechlorination of CCl2F2.
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Our earlier study revealed the presence of a large amount of chlorine and some carbon deposited on Pt/active carbon catalyst after CCl4 hydrodechlorination [32]. A check was undertaken to establish if a similar effect occurred with Pt/MWCNTs. The temperature-programmed hydrogenation study of the spent catalyst (Fig. 7 and 8) showed that only a small amount of chlorine was present on the catalyst after both hydrodechlorination of carbon tetrachloride and the hydrodechlorination of dichlorodifluoromethane. Incidentally, the amount of fluorine present on the catalyst after the latter reaction was considerably less.
Fig. 7. TPH spectra of the Pt/MWCNTs catalyst used in hydrodechlorination of CCl4.
Fig. 8. TPH spectra of the Pt/MWCNTs catalyst used in hydrodechlorination of CCl2F2.
0.00E+00
4.00E-09
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The amount of methane liberated during TPH after both reactions was also rather small, especially when the evolution of methane at temperatures >500oC, characteristic for carbon gasification by hydrogen, is neglected. The results of TPH clearly indicate that the surface of platinum is not blocked by carbon and chlorine-containing species. Therefore, the difference in the behaviour of Pt/MWCNTs and the earlier reported Pt/active carbon catalysts [32] could be explained on the basis that, most probably, the latter samples contained a fraction of very small Pt particles, which due to low metal loading (1 wt.%) could not be easily detected by physical methods and hydrogen chemisorption was also suppressed by the presence of carbon on the metal surfaces [35]. If this is the case, then the small Pt particles (< 5 nm in size) are prone to considerable deactivation, most probably by chloriding with HCl, an inevitable reaction product, and perhaps by CCl4. Such an interpretation is in line with Zhang et al. [16-18], although other workers are of the opinion that smaller Pt clusters are more resistant to chloriding [22]. Another explanation refers to the role of the support. In this respect, recent work on the catalytic hydrodechlorination of chlorobenzene over Pd, supported on amorphous and structured carbons in he form of graphitic carbon nanofibres, showed irreversible losses of activity of both catalysts. However this loss was more extreme in the case of Pd supported on amorphous carbon [36]. It appears that the structured carbon substrate is more effective in stabilizing metal activity.. Another view is that a different degree of interaction between the transition metals, Co, Fe and Ni with carbon materials, including multi-wall carbon nanotubes and activated carbon, was reported by others [37]. Complete wetting was achieved for activated carbon, but only partial wetting for MWCNTs was observed. Possibly, the MWCNTs do not possess, on their surface, sites suitable for the re-dispersion, or spreading, of chlorided Pt species.
Summary Despite the high concentration of platinum in the catalysts, the Pt nanoparticles were uniform in shape and size and were very well dispersed on the MWCNTs surface. MWCNTs proved to be a very useful support for the Pt-active phase in CCl4 and CCl2F2 hydrodechlorination. The Pt particles did not experience noticeable changes during either reaction, i.e. neither did substantial metal sintering, or re-dispersion occur ,nor did extensive surface carbonization and chloriding take place. In contrast to reactions catalyzed by platinum supported on active carbon, in the case of both reactions product selectivities and conversions stabilized at a constant level after a relatively short time and, additionally, deactivation of the catalyst was not observed during the time of screening. In in the reaction of carbon tetrachloride with hydrogen, the conversion level was nearly 100% and chloroform appeared as the dominant product, with a selectivity of ~80%.
Acknowledgements This work was carried out in the framework of the Polish-Taiwanese joint research program under the agreement on scientific cooperation between the Polish Academy of Sciences in Warsaw and the National Science Council in Taipei. Partial support for this study was from the Polish Ministry of Education and Science (Research Project PBZ-KBN-116/T09/2004) and the National Science Council of the Republic of China (Project Number 94-2911-I-036-001).
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Carbon Tetrachloride and Dichlorodifluoromethane 10.4028/www.scientific.net/SSP.128.261
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