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Pd nanoparticles introduced inside multi-walled carbon nanotubes for selective hydrogenation of cinnamaldehyde into hydrocinnamaldehyde
Applied Catalysis A: General 288 (2005) 203–210 www.elsevier.com/locate/apcata
Pd nanoparticles introduced inside multi-walled carbon nanotubes for selective hydrogenation of cinnamaldehyde into hydrocinnamaldehyde Jean-Philippe Tessonnier a,1,*, Laurie Pesant a,1, Gabrielle Ehret b, Marc J. Ledoux a,1, Cuong Pham-Huu a,1 a
Laboratoire des Mate´riaux, Surfaces et Proce´de´s pour la Catalyse, UMR 7515 du CNRS, ECPM, Universite´ Louis Pasteur, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France b Groupe Surfaces et Interfaces, Institut de Physique et Chimie des Mate´riaux de Strasbourg, UMR 7504 du CNRS, 23 rue du Loess, BP 43, 67034 Strasbourg Cedex, France Received 25 January 2005; received in revised form 13 April 2005; accepted 26 April 2005 Available online 1 June 2005
Abstract Palladium nanoparticles (4–6 nm) were deposited inside multi-walled carbon nanotubes (MWNTs) via a simple impregnation using an aqueous solution containing a palladium salt. The low surface tension of the solvent allows a complete filling of the tube, leading, after thermal treatments, to the formation of small and homogeneous palladium particles decorating the inner cavity of the support. The impregnation method was extremely efficient as no palladium particle located on the outer surface of the tubes was observed. The catalyst was tested for the selective hydrogenation of cinnamaldehyde which contains both a C C and a C O bond. The nanotubes based catalyst exhibits along with a high catalytic activity an extremely high selectivity towards the C C bond hydrogenation when compared to a commercial catalyst supported on a high surface area activated charcoal. A peculiar metal-support interaction and the absence of micropores and of oxygenated surface groups on the carbon nanotubes support are proposed to explain these results. # 2005 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Palladium nanoparticules; Cinnamaldehyde hydrogenation
1. Introduction The understanding of the macroscopic phenomena seems to be nowadays relatively well under control in catalysis but the comprehension of what happens on a nanoscopic scale remains to be improved. The best way to get access to this knowledge is to reduce the size of the active site in order to provide a detail understanding of such a phase at a chemical level. Unfortunately, it is difficult to avoid aggregation. One of the most common ways to avoid it is to deposit the active phase into nanocages or nanochannels like zeolites. * Corresponding author. Tel.: +33 3 90 24 26 75; fax: +33 3 90 24 26 74. E-mail address: [email protected] (J.-P. Tessonnier). 1 Member of European Laboratory for Catalysis and Surface Science (ELCASS). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.04.034
However, the diffusion and the accessibility of the reactants to the catalytic sites can be lowered, thus rendering such system unable to operate under severe conditions [1,2]. Recently, carbon nanotubes were discovered and extensively studied due to their unique chemical and mechanical properties and for different potential applications [3–6]. The composites area was the first field of application developed for these carbon nanostructures along with the heterogeneous catalysis field where recent works have reported peculiar catalytic properties observed on carbon nanofibers when used as support [6–10]. However, as far as the catalytic application is concerned, almost no report dealing with the use of metal particles encapsulated inside the carbon nanotubes has been published in the literature, while a large range of research has been devoted to the synthesis of such encapsulated materials [11–13].
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Recently, it was reported that metal particles with an average diameter centred on 5 nm can be easily obtained inside carbon nanotubes, by classical impregnation from an aqueous solution [14]. Similar tube filling processes have also been recently reported by Rossi et al. [15] using a combination of environmental scanning electron microscopy (ESEM) and transmission electron microscopy (TEM). The results obtained revealed that the CNT wall inside the channel was disordered and hydrophilic which allows the easy filling of the tube with a liquid, thanks to the action of capillary forces. Recently, Kim et al. [16] have reported the filling of CNTs with different solvents, i.e. glycerine, ethylene glycol and distilled water, using optical microscopy operated at room temperature. Work carried out in the laboratory has shown that CNTs with an inner diameter of ca. 50 nm can be rapidly and easily filled with an aqueous solution of mixed iron and cobalt nitrates by pore filling process at room temperature and atmospheric pressure, conducing after thermal treatments to the formation of CoFe2O4 nanowires with uniform diameter [17]. It is expected that specific properties will be observed with these encapsulated nanomaterials, especially in the heterogeneous catalysis field, compared to those usually encountered with traditional catalysts, i.e. powders, grains and extrudates. The introduction of foreign elements inside the nanotube may modify the physical and chemical properties of the encapsulated material and also of the support itself, thanks to the electron transfer between the graphite structure and the metal particle [18–20]. Besides, it was reported by Dravid [21] that nickel nanoparticles encapsulated in carbon nanotubes were not damaged when immersed for several months in an aqua regia medium. The peculiar tubular morphology of the nanotube support combined with the exceptional high mechanical strength of the tube wall could also play an important role as a confinement medium during thermal treatments and during the catalytic process. Recent results obtained in the laboratory have shown that active phase located inside the carbon nanotube cavity exhibit an extremely high activity when compared to that observed on traditional grain size catalysts [22–24]. NiS2 nanoparticles, i.e. 3–5 nm size, located inside multi-walled carbon nanotubes exhibit at low-temperature an extremely high and stable desulfurisation activity when compared to that observed on the high surface area activated charcoal or SiC catalysts. The high catalytic activity was attributed to the existence of a confinement effect, which significantly alters the reactant partial pressure inside the tube cavity, thus inducing the subsequent modification of the reaction rate despite the fact that the macroscopic reaction parameters remain totally identical. Future progresses in catalysis are closely dependent on the development of new catalytic materials with smart characteristics. Technology is constantly pushing towards smaller materials with increased properties, i.e. activity and
selectivity in order to minimize waste. The aim of the present article is to report a simple method for preparing palladium nanoparticles encapsulated inside carbon nanotubes, based on the rapid filling of this material with an aqueous solution containing the active phase precursor. The microstructure of the material will be extensively investigated by means of high-resolution transmission electron microscopy, while its catalytic performance will be evaluated using the selective hydrogenation of cinnamaldehyde as a probe reaction. The results will be compared with those obtained on a commercial catalyst supported on a high surface area activated charcoal.
2. Experimental section 2.1. MWNT characteristics The carbon nanotubes (Pyrograff III) were supplied by Applied Science Ltd., USA, in a purified form. Typical TEM image of this starting support is presented in Fig. 1A. The carbon nanotubes display an average external diameter ranging between 60 and 100 nm and length up to several hundred micrometers. Some remaining iron catalyst encapsulated inside the MWNT tubule was also observed (Fig. 1B). However, these iron particles were completely encapsulated by a layer of carbon, which renders them inaccessible to the reactants. High-resolution TEM micrograph reveals the presence of an amorphous layer of carbon on the outer surface of the nanotube (Fig. 1C). It is worth noting that carbon nanotubes synthesised by CVD are usually covered on their outer surface with a layer of amorphous carbon, which is probably formed during the cooling step of the synthesis [25]. The specific surface area measured by mean of the BET method using N2 as adsorbant at the liquid nitrogen temperature, was about 20 m2 g 1. The solid was essentially mesoporous with an average pore size distribution centred at around 40 nm. No trace of micropores has ever been observed. The used CNTs were previously treated using an acidic solution in order to remove iron particles remaining accessible. The tubes were open on both ends owing to their relatively large diameter, which prevents the formation of caps on the tube tip. 2.2. Pd deposition The palladium salt (1.25 g of PdNO36H2O, Strem Chemicals) was dissolved in an appropriate amount of distilled water (40 ml) and dropped on the carbon nanotubes (10 g) under vigorous stirring until a paste-like solid was obtained. The theoretical Pd loading was set to be around 5 wt.%. The wet solid was allowed to dry at room temperature for overnight before thermal treatments. It was then calcinated in air at 350 8C for 2 h in order to decompose the palladium salt into its corresponding oxide.
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2.3. Characterisation techniques Low and high-resolution TEM observations were conducted using a Topcon 002B microscope working at 200 kV accelerating voltage and with a point-to-point resolution of 0.17 nm. Samples were dispersed in ethanol in an ultrasonic bath, and a drop of the suspension was deposited on a holey carbon-coated copper TEM grid. The metal dispersion was determined from measurement of over 400 particles from a number of regions of the observed sample. The specific surface area was measured using a Coulter SA3100 sorptometer. The sample was outgassed at 100 8C for 2 h in order to desorb moisture from its surface. The pore size distribution was determined from the adsorption isotherm. 2.4. Catalytic test The hydrogenation of cinnamaldehyde was carried out in a slurry reactor under atmospheric pressure at 80 8C. The 5 wt.% Pd catalyst (0.1 g) was added to 10 ml of cinnamaldehyde dissolved in 50 ml of dioxane. Hydrogen (20 ml min 1) was continuously fed into the reactor via a mass flow controller. The reaction was carried out under stirring at 500 rpm. A small amount (0.2 ml) of the reacting medium was withdrawn at regular times and analyzed by gas chromatography (Varian 3800) equipped with a capillary column (Carbobond with 50 m in length) and a FID detector.
3. Results and discussion 3.1. Microstructure investigation
Fig. 1. (A) Low-magnification transmission electron micrograph of the carbon nanotubes. (B) TEM micrograph showing the presence of iron phase encapsulated inside the MWNT tubule. (C) The topmost surface of the carbon nanotube was covered by an amorphous layer of carbon which could be attributed to the self-pyrolysis of the reactants during the carbon nanotubes synthesis.
The oxide was subsequently reduced in flowing hydrogen at 400 8C for 2 h with a heating rate of 2 8C min 1. The sample was allowed to cool down under argon and then discharged and stored in a flask under argon in order to avoid excessive superficial oxidation by air. The palladium loading measured by elemental analysis carried out after thermal treatments was 4.4 wt.%.
The TEM images of the reduced sample are presented in Fig. 2A and B. Low magnification TEM image clearly shows that all the palladium particles were located inside the carbon nanotubes (Fig. 2A). This can be explained by the peculiar tubular morphology of the support, which can induce capillary forces during the impregnation process. Previous studies reported in the literature [26] have shown that elements and compounds with surface tension lower than 190 mN m 1 are efficient for wetting and filling the carbon nanotubes. Water has a surface tension value of 72 mN m 1 [27], and thus it was expected to strongly wet and fill the nanotubes. However, using pure oxide, i.e. V2O5, Ajayan et al. [27] have observed along with the filling by capillarity some covering, also by capillarity, of the outer surface of the tube by a thin layer of oxide. Apparently, pure melting oxide was not selectively filling the tube. Such phenomenon could also be related to the balance between the filling by capillarity and the optimal size of the tube. It has been reported that capillarity was reduced when the inner cavity of the tube was smaller. According to the work by Ugarte et al. [19] the wide nanotube cavities appeared to be more preferentially filled compared to the narrower cavities. In small cavities, the van
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Fig. 2. TEM images of the palladium particles introduced inside the MWNT channel. The filling was relatively high according to the absence of the palladium particles located on the outer surface of the tubes. Some Pd particles could be seen at the tip end of the tubes, near the mouth, probably because they were expelled during the drying phase.
der Waals repulsion forces are significantly higher than that of capillarity, and thus inhibit the penetration of the solution inside the tube channel. In the present work, the hydrophobic character of the outer surface of the nanotubes, the relatively large inner cavity and the relatively low metal precursor concentration, i.e. 5 wt.%, in the solution were probably at the origin of the almost absence of any palladium particles on the outer surface of the tube excepted on the region near the tube tip where some palladium particles were observed (Fig. 2B). The rapid vaporisation of the solution located next to the tube tip was expected to be responsible for the observed result. During the thermal treatment water was slowly evaporated leaving behind palladium oxide particles, which were subsequently reduced into the metallic phase under the hydrogen flow. It should be noted that the percent of tubes which were filled was relatively high, >30%, compared to that usually observed in previous works reported in the literature (<10%) [19]. Careful examination of the TEM image indicates that the palladium particles shape was faceted which could be due to a strong metal-support interaction with the inner wall surface of the carbon nanotubes. Such a relatively high interaction could give rise to the relatively high dispersion of the palladium particles inside the catalyst and avoid sintering during the different thermal treatments steps. Similar results have also been observed with NiS2 introduced inside the MWNT cavity for use as catalyst in the selective oxidation of H2S into elemental sulfur at low reaction temperature [23]. The relatively high interaction between the deposited palladium and the inner wall of the carbon nanotubes could come from the electronic modification of the graphene planes due to the presence of curvature. The presence of structural defects on the tube wall could also induced anchorage of the active phase particles on the wall
surface as evidenced by the high-resolution TEM image presented in Fig. 3. The figure shows the flat interface between the metal particle and the inner tube wall surface with disordered graphene planes. The nature of the palladium precursor salt, i.e. palladium nitrate and palladium acetylacetonate, has almost no detected effect on the final dispersion state of the metallic particles. Statistical measurements performed on 400 metal particles lead to an average particle size distribution (PSD) centred at around 5 1 nm (Fig. 4) despite of the presence in some areas of the catalyst of relatively large palladium particles with a size close to 30 nm (Fig. 2B). The palladium particle size dispersion in the carbon nanotubes was slightly higher compared to what is usually observed on traditional supports such as alumina or activated charcoal, which are expected to provide a stronger metal-support
Fig. 3. High-resolution TEM image of the metal particle deposited on the inner surface of the carbon nanotube.
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Fig. 4. Average palladium particle size deduced from statistical TEM analysis over the (A) Pd/MWNT and (B) Pd/AC catalysts.
interaction (Fig. 4B). However, the palladium particles dispersion observed in the MWNT was relatively high in regard of the low surface area of the support compared to that of the activated charcoal, i.e. 10 m2 g 1 instead of 900 m2 g 1. Fig. 5 shows representative TEM images of the palladium particles introduced inside the MWNT (A) and deposited on the high surface area activated charcoal (B). The palladium particles deposited on the activated charcoal were aggregated leading to a highly inhomogeneous dispersion of the metal on the support surface. It was expected that the concave surface inside the nanotube could induce a peculiar electronic interaction with the metal precursor, which in turn could significantly modify its dispersion. The tubular morphology of the nanotube could also induce a local pressure increase during the thermal treatment, leading to a possible positive influence on the final metallic phase dispersion. It has been reported by Baker’s group [8,10] that when nickel is supported on carbon nanofibers, the metal crystallites adopt morphology very different to that usually existing on traditional support materials, i.e. flat-like morphology versus round-shaped
one. The faceted metal particles formed were attributed by the authors to a specific interaction between the surface of the nanostructure and the metal particle during its crystallization, providing new specific sites of reaction absent on conventional metal particles grown on conventional supports. It has been reported previously that NiS2 particles located inside the carbon nanotubes channel exhibit unusual high catalytic activity when compared to that was observed on the NiS2 supported on traditional supports such as SiC or silica in a grain form [24]. The curvature of the channel could also induce modification on the molecular adsorption on the metallic phase, which in turn modifies the catalytic activity or selectivity of the reaction products. 3.2. Hydrogenation catalytic activity The catalytic performance of the as-prepared catalyst was evaluated and compared with that of the commercial palladium (5 wt.%) supported on high surface area (800 m2 g 1) activated charcoal catalyst (Aldrich) in the hydrogenation of cinnamaldehyde in liquid phase [28].
Fig. 5. TEM images of the palladium particles introduced inside the MWNT channel (A) and of a commercial palladium on activated charcoal catalyst (B).
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Fig. 6. Reaction pathways in the hydrogenation of cinnamaldehyde.
Selective hydrogenation of organic compounds is a crucial process in the manufacturing of petrochemicals and fine chemicals. The metals generally employed in such reactions are palladium and platinum. The hydrogenation of cinnamaldehyde is a parallel and consecutive reduction of different functional groups present in the same starting substrate, i.e. C C and C O bonds (Fig. 6). The catalytic results obtained on the two catalysts are reported in Fig. 7A and B. The conversion was similar over the two catalysts, even if the full conversion was faster obtained on the carbon nanotubes, i.e. 100% of conversion after 25 h instead of 27 h for the commercial catalyst (Fig. 7B). Blank tests carried out over the supports alone showed no catalytic activity. It is worth noting that at higher hydrogen flow rate the hydrogenation activity on the activated charcoal was significantly increased leading to a higher rate as that observed on the carbon nanotubes based catalyst despite a high difference was still observed on the product selectivity. Previous report [29] dealing with the cinnamaldehyde hydrogenation have shown that at low hydrogen flow rate the Pd supported on the MWNTs exhibit a much faster hydrogenation rate which could be linked to the diffusion rate of hydrogen towards the active site located inside the activated charcoal microporosity. A more significant difference was observed between the two catalysts in terms of product selectivity. On the carbon nanotubes based catalyst (Fig. 7A), almost only hydrocinnamaldehyde (HCALD), i.e. the selective C C bond hydrogenation, was observed. The fully hydrogenated
product (PP) contributes to about 10%. On the commercial catalyst, both HCALD and PP were observed in an almost equal proportion (Fig. 7B). Cinnamyl alcohol was never observed in any case. These results clearly pointed out the superiority of the carbon nanotubes supported palladium catalyst versus the high surface area activated charcoal catalyst in terms of selectivity. The slightly higher catalytic performance was attributed to the high external surface area of the carbon nanotubes compared to that of the classical powder support catalyst where a large part of the surface was due to micropores, not easily accessible by the reactants, especially in the liquid phase where high diffusional limitations are predominant. The high external surface area of the carbon nanotubes allowed a significant decrease in the mass transfer limitation, which is predominant in liquid-phase reactions, since the transport of the reactants from the bulk of the liquid to the external surface of the catalyst is strongly dependent in the size of the pores of the catalyst. Similar observation has been reported by Chambers et al. [8] during the hydrogenation of light olefins over nickel supported on carbon nanofiber. The difference in the selectivity could be attributed to several factors: (i) the existence of a peculiar metal-support interaction between the palladium metal crystallites and the carbon nanotubes surface in the inner cavity, i.e. electronic modification through the electron transfer between the metal and the support, which in turn modifies the adsorption and selectivity of the products [30,31], (ii) the complete absence of any microporosity as found on the activated charcoal, which could modify the residence time of the reactants and products and their adsorption properties and (iii) the low concentration of oxygenated groups on the MWNT surface compared to their high concentration on the activated charcoal, could also influence in a significant manner the catalytic selectivity by modifying the adsorption mode of the reactant. Toebes et al. have recently reported that the hydrocinnamaldehyde formation can be highly enhanced by removing the oxygenated groups present on the Pt/CNF catalyst [32]. However, in our case the palladium particles were located inside the carbon nanotubes channel, which could be less affected by the presence of oxygenated groups as that
Fig. 7. Catalytic activity for the selective hydrogenation of cinnamaldehyde into hydrocinnamaldehyde over (A) Pd/MWNT and (B) Pd/AC catalysts at 80 8C in a liquid-phase medium: (*) cinnamaldehyde, (&) hydrocinnamaldehyde, and (~) 3-phenylpropanol (PP).
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Fig. 8. Catalytic activity for the hydrogenation of hydrocinnamaldehyde over (A) Pd/MWNT and (B) Pd/AC catalysts at 80 8C in a liquid-phase medium: (*) cinnamaldehyde, (&) hydrocinnamaldehyde, and (~) 3-phenylpropanol (PP).
observed by Toebes et al. [32]. Pore size distribution determined from the adsorption branch of the isotherm has shown that on the activated charcoal the micropores (<3 nm) contribution amounted to 50%, whereas no micropores where observed on the carbon nanotubes sample. Finally, it should be stressed that the presence of residual acidity on the activated charcoal could also modify the hydrogenation pathway leading to a less selective catalyst. It should also be noted that an interaction would be also possible between the C C bond of the substrate and the exposed basal plane associated with a cloud of delocalized p-electrons of the support. Elemental analysis carried out on the filtered solution reveals no trace of palladium species, which indicate that leaching phenomenon if present was negligible. Catalytic experiments were also conducted on the two catalysts using the hydrocinnamaldehyde, which is the C C hydrogenation product as reactant (Fig. 8). The hydrocinnamaldehyde was not further reduced on both catalysts under the reaction conditions employed, which indicates that the re-adsorption of the molecule through the C O bond did not occur on the palladium site regardless of the nature of the support. The observed result indicates that on the Pd/ MWNTs catalyst the C C hydrogenated product was rapidly desorbed from the active site whereas on the Pd/ activated charcoal the cinnamaldehyde was adsorbed in a different way leading to the almost simultaneous hydrogenation of the C O and C C bonds without desorption of the C O hydrogenated intermediate as observed on the Pd/ MWNTs catalyst. Such adsorption mode could be directly linked to the electronic state of the metallic active site.
4. Conclusion In summary, palladium metal nanoparticles can be easily introduced in the carbon nanotubes by a simple wetness impregnation followed by classical thermal treatments. The presence of a confinement effect induced by the carbon nanotube wall during the thermal treatment has probably a strong influence on the formation of these small metal particles. This catalyst exhibits a high selectivity towards the
C C bond hydrogenation, which could be attributed to the peculiar morphology of the support, which in turn, significantly modified the adsorption properties of the metal surface itself. Such catalysts can be readily used in several reactions, and will probably open a new era in the field of nanosized catalytic materials with high selectivity, especially in liquid phase medium where a high external surface area is needed in order to avoid mass transfer limitation. Recent work from the laboratory has shown that the catalytic performance can be significantly improved by using silicon carbide nanotubes as support instead of classical forms such as powders or grains [31]. Work is in progress in order to explore in more details the potential applications of this new family of material support in the catalysis field and especially in liquid-phase reaction by introducing a magnetic separator inside the MWNT cavity [17].
Acknowledgment R. Vieira is gratefully acknowledged for helpful discussion during the building of the micropilot test.
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Pd nanoparticles introduced inside multi-walled carbon nanotubes for selective hydrogenation of cinnamaldehyde into hydrocinnamaldehyde Jean-Philippe Tessonnier a,1,*, Laurie Pesant a,1, Gabrielle Ehret b, Marc J. Ledoux a,1, Cuong Pham-Huu a,1 a
Laboratoire des Mate´riaux, Surfaces et Proce´de´s pour la Catalyse, UMR 7515 du CNRS, ECPM, Universite´ Louis Pasteur, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France b Groupe Surfaces et Interfaces, Institut de Physique et Chimie des Mate´riaux de Strasbourg, UMR 7504 du CNRS, 23 rue du Loess, BP 43, 67034 Strasbourg Cedex, France Received 25 January 2005; received in revised form 13 April 2005; accepted 26 April 2005 Available online 1 June 2005
Abstract Palladium nanoparticles (4–6 nm) were deposited inside multi-walled carbon nanotubes (MWNTs) via a simple impregnation using an aqueous solution containing a palladium salt. The low surface tension of the solvent allows a complete filling of the tube, leading, after thermal treatments, to the formation of small and homogeneous palladium particles decorating the inner cavity of the support. The impregnation method was extremely efficient as no palladium particle located on the outer surface of the tubes was observed. The catalyst was tested for the selective hydrogenation of cinnamaldehyde which contains both a C C and a C O bond. The nanotubes based catalyst exhibits along with a high catalytic activity an extremely high selectivity towards the C C bond hydrogenation when compared to a commercial catalyst supported on a high surface area activated charcoal. A peculiar metal-support interaction and the absence of micropores and of oxygenated surface groups on the carbon nanotubes support are proposed to explain these results. # 2005 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Palladium nanoparticules; Cinnamaldehyde hydrogenation
1. Introduction The understanding of the macroscopic phenomena seems to be nowadays relatively well under control in catalysis but the comprehension of what happens on a nanoscopic scale remains to be improved. The best way to get access to this knowledge is to reduce the size of the active site in order to provide a detail understanding of such a phase at a chemical level. Unfortunately, it is difficult to avoid aggregation. One of the most common ways to avoid it is to deposit the active phase into nanocages or nanochannels like zeolites. * Corresponding author. Tel.: +33 3 90 24 26 75; fax: +33 3 90 24 26 74. E-mail address: [email protected] (J.-P. Tessonnier). 1 Member of European Laboratory for Catalysis and Surface Science (ELCASS). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.04.034
However, the diffusion and the accessibility of the reactants to the catalytic sites can be lowered, thus rendering such system unable to operate under severe conditions [1,2]. Recently, carbon nanotubes were discovered and extensively studied due to their unique chemical and mechanical properties and for different potential applications [3–6]. The composites area was the first field of application developed for these carbon nanostructures along with the heterogeneous catalysis field where recent works have reported peculiar catalytic properties observed on carbon nanofibers when used as support [6–10]. However, as far as the catalytic application is concerned, almost no report dealing with the use of metal particles encapsulated inside the carbon nanotubes has been published in the literature, while a large range of research has been devoted to the synthesis of such encapsulated materials [11–13].
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Recently, it was reported that metal particles with an average diameter centred on 5 nm can be easily obtained inside carbon nanotubes, by classical impregnation from an aqueous solution [14]. Similar tube filling processes have also been recently reported by Rossi et al. [15] using a combination of environmental scanning electron microscopy (ESEM) and transmission electron microscopy (TEM). The results obtained revealed that the CNT wall inside the channel was disordered and hydrophilic which allows the easy filling of the tube with a liquid, thanks to the action of capillary forces. Recently, Kim et al. [16] have reported the filling of CNTs with different solvents, i.e. glycerine, ethylene glycol and distilled water, using optical microscopy operated at room temperature. Work carried out in the laboratory has shown that CNTs with an inner diameter of ca. 50 nm can be rapidly and easily filled with an aqueous solution of mixed iron and cobalt nitrates by pore filling process at room temperature and atmospheric pressure, conducing after thermal treatments to the formation of CoFe2O4 nanowires with uniform diameter [17]. It is expected that specific properties will be observed with these encapsulated nanomaterials, especially in the heterogeneous catalysis field, compared to those usually encountered with traditional catalysts, i.e. powders, grains and extrudates. The introduction of foreign elements inside the nanotube may modify the physical and chemical properties of the encapsulated material and also of the support itself, thanks to the electron transfer between the graphite structure and the metal particle [18–20]. Besides, it was reported by Dravid [21] that nickel nanoparticles encapsulated in carbon nanotubes were not damaged when immersed for several months in an aqua regia medium. The peculiar tubular morphology of the nanotube support combined with the exceptional high mechanical strength of the tube wall could also play an important role as a confinement medium during thermal treatments and during the catalytic process. Recent results obtained in the laboratory have shown that active phase located inside the carbon nanotube cavity exhibit an extremely high activity when compared to that observed on traditional grain size catalysts [22–24]. NiS2 nanoparticles, i.e. 3–5 nm size, located inside multi-walled carbon nanotubes exhibit at low-temperature an extremely high and stable desulfurisation activity when compared to that observed on the high surface area activated charcoal or SiC catalysts. The high catalytic activity was attributed to the existence of a confinement effect, which significantly alters the reactant partial pressure inside the tube cavity, thus inducing the subsequent modification of the reaction rate despite the fact that the macroscopic reaction parameters remain totally identical. Future progresses in catalysis are closely dependent on the development of new catalytic materials with smart characteristics. Technology is constantly pushing towards smaller materials with increased properties, i.e. activity and
selectivity in order to minimize waste. The aim of the present article is to report a simple method for preparing palladium nanoparticles encapsulated inside carbon nanotubes, based on the rapid filling of this material with an aqueous solution containing the active phase precursor. The microstructure of the material will be extensively investigated by means of high-resolution transmission electron microscopy, while its catalytic performance will be evaluated using the selective hydrogenation of cinnamaldehyde as a probe reaction. The results will be compared with those obtained on a commercial catalyst supported on a high surface area activated charcoal.
2. Experimental section 2.1. MWNT characteristics The carbon nanotubes (Pyrograff III) were supplied by Applied Science Ltd., USA, in a purified form. Typical TEM image of this starting support is presented in Fig. 1A. The carbon nanotubes display an average external diameter ranging between 60 and 100 nm and length up to several hundred micrometers. Some remaining iron catalyst encapsulated inside the MWNT tubule was also observed (Fig. 1B). However, these iron particles were completely encapsulated by a layer of carbon, which renders them inaccessible to the reactants. High-resolution TEM micrograph reveals the presence of an amorphous layer of carbon on the outer surface of the nanotube (Fig. 1C). It is worth noting that carbon nanotubes synthesised by CVD are usually covered on their outer surface with a layer of amorphous carbon, which is probably formed during the cooling step of the synthesis [25]. The specific surface area measured by mean of the BET method using N2 as adsorbant at the liquid nitrogen temperature, was about 20 m2 g 1. The solid was essentially mesoporous with an average pore size distribution centred at around 40 nm. No trace of micropores has ever been observed. The used CNTs were previously treated using an acidic solution in order to remove iron particles remaining accessible. The tubes were open on both ends owing to their relatively large diameter, which prevents the formation of caps on the tube tip. 2.2. Pd deposition The palladium salt (1.25 g of PdNO36H2O, Strem Chemicals) was dissolved in an appropriate amount of distilled water (40 ml) and dropped on the carbon nanotubes (10 g) under vigorous stirring until a paste-like solid was obtained. The theoretical Pd loading was set to be around 5 wt.%. The wet solid was allowed to dry at room temperature for overnight before thermal treatments. It was then calcinated in air at 350 8C for 2 h in order to decompose the palladium salt into its corresponding oxide.
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2.3. Characterisation techniques Low and high-resolution TEM observations were conducted using a Topcon 002B microscope working at 200 kV accelerating voltage and with a point-to-point resolution of 0.17 nm. Samples were dispersed in ethanol in an ultrasonic bath, and a drop of the suspension was deposited on a holey carbon-coated copper TEM grid. The metal dispersion was determined from measurement of over 400 particles from a number of regions of the observed sample. The specific surface area was measured using a Coulter SA3100 sorptometer. The sample was outgassed at 100 8C for 2 h in order to desorb moisture from its surface. The pore size distribution was determined from the adsorption isotherm. 2.4. Catalytic test The hydrogenation of cinnamaldehyde was carried out in a slurry reactor under atmospheric pressure at 80 8C. The 5 wt.% Pd catalyst (0.1 g) was added to 10 ml of cinnamaldehyde dissolved in 50 ml of dioxane. Hydrogen (20 ml min 1) was continuously fed into the reactor via a mass flow controller. The reaction was carried out under stirring at 500 rpm. A small amount (0.2 ml) of the reacting medium was withdrawn at regular times and analyzed by gas chromatography (Varian 3800) equipped with a capillary column (Carbobond with 50 m in length) and a FID detector.
3. Results and discussion 3.1. Microstructure investigation
Fig. 1. (A) Low-magnification transmission electron micrograph of the carbon nanotubes. (B) TEM micrograph showing the presence of iron phase encapsulated inside the MWNT tubule. (C) The topmost surface of the carbon nanotube was covered by an amorphous layer of carbon which could be attributed to the self-pyrolysis of the reactants during the carbon nanotubes synthesis.
The oxide was subsequently reduced in flowing hydrogen at 400 8C for 2 h with a heating rate of 2 8C min 1. The sample was allowed to cool down under argon and then discharged and stored in a flask under argon in order to avoid excessive superficial oxidation by air. The palladium loading measured by elemental analysis carried out after thermal treatments was 4.4 wt.%.
The TEM images of the reduced sample are presented in Fig. 2A and B. Low magnification TEM image clearly shows that all the palladium particles were located inside the carbon nanotubes (Fig. 2A). This can be explained by the peculiar tubular morphology of the support, which can induce capillary forces during the impregnation process. Previous studies reported in the literature [26] have shown that elements and compounds with surface tension lower than 190 mN m 1 are efficient for wetting and filling the carbon nanotubes. Water has a surface tension value of 72 mN m 1 [27], and thus it was expected to strongly wet and fill the nanotubes. However, using pure oxide, i.e. V2O5, Ajayan et al. [27] have observed along with the filling by capillarity some covering, also by capillarity, of the outer surface of the tube by a thin layer of oxide. Apparently, pure melting oxide was not selectively filling the tube. Such phenomenon could also be related to the balance between the filling by capillarity and the optimal size of the tube. It has been reported that capillarity was reduced when the inner cavity of the tube was smaller. According to the work by Ugarte et al. [19] the wide nanotube cavities appeared to be more preferentially filled compared to the narrower cavities. In small cavities, the van
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Fig. 2. TEM images of the palladium particles introduced inside the MWNT channel. The filling was relatively high according to the absence of the palladium particles located on the outer surface of the tubes. Some Pd particles could be seen at the tip end of the tubes, near the mouth, probably because they were expelled during the drying phase.
der Waals repulsion forces are significantly higher than that of capillarity, and thus inhibit the penetration of the solution inside the tube channel. In the present work, the hydrophobic character of the outer surface of the nanotubes, the relatively large inner cavity and the relatively low metal precursor concentration, i.e. 5 wt.%, in the solution were probably at the origin of the almost absence of any palladium particles on the outer surface of the tube excepted on the region near the tube tip where some palladium particles were observed (Fig. 2B). The rapid vaporisation of the solution located next to the tube tip was expected to be responsible for the observed result. During the thermal treatment water was slowly evaporated leaving behind palladium oxide particles, which were subsequently reduced into the metallic phase under the hydrogen flow. It should be noted that the percent of tubes which were filled was relatively high, >30%, compared to that usually observed in previous works reported in the literature (<10%) [19]. Careful examination of the TEM image indicates that the palladium particles shape was faceted which could be due to a strong metal-support interaction with the inner wall surface of the carbon nanotubes. Such a relatively high interaction could give rise to the relatively high dispersion of the palladium particles inside the catalyst and avoid sintering during the different thermal treatments steps. Similar results have also been observed with NiS2 introduced inside the MWNT cavity for use as catalyst in the selective oxidation of H2S into elemental sulfur at low reaction temperature [23]. The relatively high interaction between the deposited palladium and the inner wall of the carbon nanotubes could come from the electronic modification of the graphene planes due to the presence of curvature. The presence of structural defects on the tube wall could also induced anchorage of the active phase particles on the wall
surface as evidenced by the high-resolution TEM image presented in Fig. 3. The figure shows the flat interface between the metal particle and the inner tube wall surface with disordered graphene planes. The nature of the palladium precursor salt, i.e. palladium nitrate and palladium acetylacetonate, has almost no detected effect on the final dispersion state of the metallic particles. Statistical measurements performed on 400 metal particles lead to an average particle size distribution (PSD) centred at around 5 1 nm (Fig. 4) despite of the presence in some areas of the catalyst of relatively large palladium particles with a size close to 30 nm (Fig. 2B). The palladium particle size dispersion in the carbon nanotubes was slightly higher compared to what is usually observed on traditional supports such as alumina or activated charcoal, which are expected to provide a stronger metal-support
Fig. 3. High-resolution TEM image of the metal particle deposited on the inner surface of the carbon nanotube.
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Fig. 4. Average palladium particle size deduced from statistical TEM analysis over the (A) Pd/MWNT and (B) Pd/AC catalysts.
interaction (Fig. 4B). However, the palladium particles dispersion observed in the MWNT was relatively high in regard of the low surface area of the support compared to that of the activated charcoal, i.e. 10 m2 g 1 instead of 900 m2 g 1. Fig. 5 shows representative TEM images of the palladium particles introduced inside the MWNT (A) and deposited on the high surface area activated charcoal (B). The palladium particles deposited on the activated charcoal were aggregated leading to a highly inhomogeneous dispersion of the metal on the support surface. It was expected that the concave surface inside the nanotube could induce a peculiar electronic interaction with the metal precursor, which in turn could significantly modify its dispersion. The tubular morphology of the nanotube could also induce a local pressure increase during the thermal treatment, leading to a possible positive influence on the final metallic phase dispersion. It has been reported by Baker’s group [8,10] that when nickel is supported on carbon nanofibers, the metal crystallites adopt morphology very different to that usually existing on traditional support materials, i.e. flat-like morphology versus round-shaped
one. The faceted metal particles formed were attributed by the authors to a specific interaction between the surface of the nanostructure and the metal particle during its crystallization, providing new specific sites of reaction absent on conventional metal particles grown on conventional supports. It has been reported previously that NiS2 particles located inside the carbon nanotubes channel exhibit unusual high catalytic activity when compared to that was observed on the NiS2 supported on traditional supports such as SiC or silica in a grain form [24]. The curvature of the channel could also induce modification on the molecular adsorption on the metallic phase, which in turn modifies the catalytic activity or selectivity of the reaction products. 3.2. Hydrogenation catalytic activity The catalytic performance of the as-prepared catalyst was evaluated and compared with that of the commercial palladium (5 wt.%) supported on high surface area (800 m2 g 1) activated charcoal catalyst (Aldrich) in the hydrogenation of cinnamaldehyde in liquid phase [28].
Fig. 5. TEM images of the palladium particles introduced inside the MWNT channel (A) and of a commercial palladium on activated charcoal catalyst (B).
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Fig. 6. Reaction pathways in the hydrogenation of cinnamaldehyde.
Selective hydrogenation of organic compounds is a crucial process in the manufacturing of petrochemicals and fine chemicals. The metals generally employed in such reactions are palladium and platinum. The hydrogenation of cinnamaldehyde is a parallel and consecutive reduction of different functional groups present in the same starting substrate, i.e. C C and C O bonds (Fig. 6). The catalytic results obtained on the two catalysts are reported in Fig. 7A and B. The conversion was similar over the two catalysts, even if the full conversion was faster obtained on the carbon nanotubes, i.e. 100% of conversion after 25 h instead of 27 h for the commercial catalyst (Fig. 7B). Blank tests carried out over the supports alone showed no catalytic activity. It is worth noting that at higher hydrogen flow rate the hydrogenation activity on the activated charcoal was significantly increased leading to a higher rate as that observed on the carbon nanotubes based catalyst despite a high difference was still observed on the product selectivity. Previous report [29] dealing with the cinnamaldehyde hydrogenation have shown that at low hydrogen flow rate the Pd supported on the MWNTs exhibit a much faster hydrogenation rate which could be linked to the diffusion rate of hydrogen towards the active site located inside the activated charcoal microporosity. A more significant difference was observed between the two catalysts in terms of product selectivity. On the carbon nanotubes based catalyst (Fig. 7A), almost only hydrocinnamaldehyde (HCALD), i.e. the selective C C bond hydrogenation, was observed. The fully hydrogenated
product (PP) contributes to about 10%. On the commercial catalyst, both HCALD and PP were observed in an almost equal proportion (Fig. 7B). Cinnamyl alcohol was never observed in any case. These results clearly pointed out the superiority of the carbon nanotubes supported palladium catalyst versus the high surface area activated charcoal catalyst in terms of selectivity. The slightly higher catalytic performance was attributed to the high external surface area of the carbon nanotubes compared to that of the classical powder support catalyst where a large part of the surface was due to micropores, not easily accessible by the reactants, especially in the liquid phase where high diffusional limitations are predominant. The high external surface area of the carbon nanotubes allowed a significant decrease in the mass transfer limitation, which is predominant in liquid-phase reactions, since the transport of the reactants from the bulk of the liquid to the external surface of the catalyst is strongly dependent in the size of the pores of the catalyst. Similar observation has been reported by Chambers et al. [8] during the hydrogenation of light olefins over nickel supported on carbon nanofiber. The difference in the selectivity could be attributed to several factors: (i) the existence of a peculiar metal-support interaction between the palladium metal crystallites and the carbon nanotubes surface in the inner cavity, i.e. electronic modification through the electron transfer between the metal and the support, which in turn modifies the adsorption and selectivity of the products [30,31], (ii) the complete absence of any microporosity as found on the activated charcoal, which could modify the residence time of the reactants and products and their adsorption properties and (iii) the low concentration of oxygenated groups on the MWNT surface compared to their high concentration on the activated charcoal, could also influence in a significant manner the catalytic selectivity by modifying the adsorption mode of the reactant. Toebes et al. have recently reported that the hydrocinnamaldehyde formation can be highly enhanced by removing the oxygenated groups present on the Pt/CNF catalyst [32]. However, in our case the palladium particles were located inside the carbon nanotubes channel, which could be less affected by the presence of oxygenated groups as that
Fig. 7. Catalytic activity for the selective hydrogenation of cinnamaldehyde into hydrocinnamaldehyde over (A) Pd/MWNT and (B) Pd/AC catalysts at 80 8C in a liquid-phase medium: (*) cinnamaldehyde, (&) hydrocinnamaldehyde, and (~) 3-phenylpropanol (PP).
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Fig. 8. Catalytic activity for the hydrogenation of hydrocinnamaldehyde over (A) Pd/MWNT and (B) Pd/AC catalysts at 80 8C in a liquid-phase medium: (*) cinnamaldehyde, (&) hydrocinnamaldehyde, and (~) 3-phenylpropanol (PP).
observed by Toebes et al. [32]. Pore size distribution determined from the adsorption branch of the isotherm has shown that on the activated charcoal the micropores (<3 nm) contribution amounted to 50%, whereas no micropores where observed on the carbon nanotubes sample. Finally, it should be stressed that the presence of residual acidity on the activated charcoal could also modify the hydrogenation pathway leading to a less selective catalyst. It should also be noted that an interaction would be also possible between the C C bond of the substrate and the exposed basal plane associated with a cloud of delocalized p-electrons of the support. Elemental analysis carried out on the filtered solution reveals no trace of palladium species, which indicate that leaching phenomenon if present was negligible. Catalytic experiments were also conducted on the two catalysts using the hydrocinnamaldehyde, which is the C C hydrogenation product as reactant (Fig. 8). The hydrocinnamaldehyde was not further reduced on both catalysts under the reaction conditions employed, which indicates that the re-adsorption of the molecule through the C O bond did not occur on the palladium site regardless of the nature of the support. The observed result indicates that on the Pd/ MWNTs catalyst the C C hydrogenated product was rapidly desorbed from the active site whereas on the Pd/ activated charcoal the cinnamaldehyde was adsorbed in a different way leading to the almost simultaneous hydrogenation of the C O and C C bonds without desorption of the C O hydrogenated intermediate as observed on the Pd/ MWNTs catalyst. Such adsorption mode could be directly linked to the electronic state of the metallic active site.
4. Conclusion In summary, palladium metal nanoparticles can be easily introduced in the carbon nanotubes by a simple wetness impregnation followed by classical thermal treatments. The presence of a confinement effect induced by the carbon nanotube wall during the thermal treatment has probably a strong influence on the formation of these small metal particles. This catalyst exhibits a high selectivity towards the
C C bond hydrogenation, which could be attributed to the peculiar morphology of the support, which in turn, significantly modified the adsorption properties of the metal surface itself. Such catalysts can be readily used in several reactions, and will probably open a new era in the field of nanosized catalytic materials with high selectivity, especially in liquid phase medium where a high external surface area is needed in order to avoid mass transfer limitation. Recent work from the laboratory has shown that the catalytic performance can be significantly improved by using silicon carbide nanotubes as support instead of classical forms such as powders or grains [31]. Work is in progress in order to explore in more details the potential applications of this new family of material support in the catalysis field and especially in liquid-phase reaction by introducing a magnetic separator inside the MWNT cavity [17].
Acknowledgment R. Vieira is gratefully acknowledged for helpful discussion during the building of the micropilot test.
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