Hydrogen uptake by carbon nanofibers catalyzed by palladium

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International Journal of Hydrogen Energy 29 (2004) 97 – 102

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Hydrogen uptake by carbon nano'bers catalyzed by palladium Dan Lupua;∗ , Alexandru Radu Biri0sa , Ioan Mi0sana , Adrian Jianub , Gerd Holzh3uterb , Eberhard Burkelb a National

Institute for Research and Development of Isotopic and Molecular Technologies, P.O. Box 700, Cluj-Napoca R-3400, Romania b University of Rostock, August-Bebel Str. 55, Rostock 18055, Germany Accepted 4 March 2003

Abstract Carbon nano'bers of herringbone conformation were obtained by chemical vapor deposition on Pd=La2 O3 catalyst, from ethylene—hydrogen mixture. After the removal of La2 O3 , samples with various Pd/C ratios were obtained by oxidation in air. The hydrogen sorption capacities measured gravimetrically at 10 MPa in pure hydrogen, for six di:erent batches of samples, show a good correlation with the Pd/C ratio reveling a catalytic e:ect of Pd, which supplies atomic H. A possible charge transfer might lead to the increasing of the H uptake with the increasing Pd/C ratio. A saturation value of 1.5% H (mass) per carbon is reached at rather high Pd/C mole ratio (∼ 1) for nano'bers with 425 –455 m2 g−1 BET surface area. ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Hydrogen uptake; Carbon nano'bers

1. Introduction The great interest in carbon nanostructures as hydrogen storage media resulted in substantial theoretical [1–3] and experimental research work, some of them reporting 5 –10% H (mass per carbon) H uptake [4–7]. The controversy [5] related to the ability of carbon nanotubes to store signi'cant amounts of hydrogen is considered to consist in the quality of materials used by various authors, the method used for measurements or the activation procedure for H uptake. Some of the critical reviews of the existing data [8–13] emphasized the need for reproducible experiments performed in di:erent laboratories [10,12]. A careful examination of the adsorption based on the model of condensation of as a monolayer [14] leads to a maximum storage capacity of 3.3% H at the surface while, inside the cavity, it starts from 1.5% H and increases with the tube diameter. The experimental results [14] for gas phase experiments show a typical adsorption curve with maximum 0.6% H and the electrochemical discharge capacity gives higher values (2% H), in ∗ Corresponding author. Tel.: +40-264-184-037; fax: +40-264-42042. E-mail address: [email protected] (D. Lupu).

good correlation with the BET speci'c surface area for different samples. The model was further con'rmed by other experiments [15] showing 5.5% H hydrogen physisorption at 77 K, which drops to 0.6% H at room temperature (6 MPa). Although other authors [9] report an increase of hydrogen adsorption (maximum 1.5% H) with the increasing BET surface area (measured with nitrogen) for activated carbon samples, they point out that nano'ber samples do not 't into this characteristic. The lack of such a correlation for carbon nano'bers (maximum 0.7% H adsorption at room temperature and 10:5 MPa) was also noted [16] and assigned to the presence of relatively narrow pores which are unseen by nitrogen used to determine the BET surface area [9,16]. Other works [11] reported very little hydrogen sorption in carbon materials; less than 0.1% H at room temperature and 3:5 MPa, casting serious doubts on any claims for values larger than 1% H. An interesting route to facilitate the hydrogen sorption in carbon nanostructures (CNS) has been to use hydrogen absorbing alloys as a “by-pass” for hydrogen transfer [7,17,18]. However, the hydrogen sorption of on alloy may be altered by surface oxidation, causing major e:ects in the hydrogen transfer rate to CNS and a less sensitive metal seems preferable.

0360-3199/03/$ 30.00 ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S0360-3199(03)00055-7

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The palladium—hydrogen system is the oldest and most well characterized [19]. The equilibrium pressure of the hydride phase at room temperature is below 0:01 atm (∼ 0:001 MPa) much below the 1–10 MPa range (where the H uptake by CNS is of interest) in which the H content of Pd can be accurately measured. On the other hand, among the catalyst used in CVD technique, Pd proved also to be active [20] for carbon nanotube synthesis. Based on the idea that palladium could be a good by-pass for hydrogen transfer to carbon nanostructures, allowing even H di:usion through the Pd catalyst particles at the tube end (if present) and supplying atomic H at the Pd—carbon interface, we started experiments for synthesis of carbon nanostructures on Pd-based catalysts and H uptake. In this paper, we report the results for the carbon nanostructures prepared from ethylene/hydrogen (4:1) on Pd=La2 O3 catalyst, in the same conditions as prepared by authors [6] using Ni–Cu catalyst. 2. Experimental 2.1. Synthesis of carbon nanostructures An aqueous solution containing 1 ml PdCl2 0:18 M and 4 ml lanthanum nitrate 0:08 M was homogeneously distributed on the inner side of a quartz tube of 2 cm inner diameter and 12 cm long in successive steps, slowly evaporating the solvent under gentle heating. After calcination in air at 420◦ C 1 h, the quartz tube was introduced in the isothermal region (40 cm) of a furnace, into a quartz tube of 2:6 cm inner diameter (120 cm long) for the chemical vapor deposition technique (CVD). The catalyst was activated at 350◦ C in pure hydrogen Now (from a metal hydride storage container) at 20 ml min−1 rate after which the system was vacuumed for 1 h, raising the temperature to 680◦ C. At this stage a mixture of ethylene/hydrogen (4:1 volume ratio) was introduced at on overall pressure of 0:3 bar (0:03 MPa) for CVD synthesis similar to that reported in literature for Ni–Cu catalysts in stationary mode [6]. A slight increase of the pressure was observed during the 'rst 15 min. After 1 h, the furnace was cooled at the rate of 5◦ C min−1 , allowing pure argon in the tube when cooled to room temperature. The yield of carbon deposit evenly distributed on the walls of the inner quartz tube was ∼ 1:6 mg=mgPd=La2 O3 . The lanthanum oxide from the sample was removed by dissolution in HCl 37%, 48 h, 'ltering on a porous sintered ceramic plate, washed with distilled water and dried 1 h at 160◦ C. For the dissolution of Pd from some samples, they were immersed in HNO3 65% for 20 h followed by 'ltration, washing with distilled water and drying at 160◦ C. 2.2. Characterization of the carbon nanostructures The morphology of the carbon nanostructures was examined by transmission electron microscopy using a Zeiss EM912, operated at 120 kV.

2.3. Hydrogen sorption measurements A Sartorius high pressure microbalance type 4436 for hydrogen was used for the measurements of H uptake at 10 MPa. The accuracy was checked with hydrogen absorption in reference samples of Pd, and MmNi4:3 Mn0:7 (Mm = mischmetall, mixture of rare earth metals) and by comparison with the values obtained in a volumetric Sievert type installation, used in 25 years of experience of the group in characterizing hydrogen absorption/desorption isotherms for various alloy. The gravimetric and volumetric results were in a good agreement: 0:74±0:01% H for Pd (0.78 H/Pd mole ratio), both for pure Pd and for Pd resulting after complete oxidation of C from our samples. Because Pd from the samples also absorb hydrogen (0.74% H at 10 MPa H2 as measured), the value corresponding to the Pd content in each sample was subtracted from the equilibrium overall weight gain (after ∼ 3 h) under hydrogen pressure. The correction for the buoyancy e:ect was made by measuring the buoyancy with helium: the data were in agreement with the calculated buoyancy using 2:26 g cm−3 as speci'c weight for the carbon content. With these two corrections, the H uptake normalized to the carbon content has been evaluated for each measurement (20 –200 mg of sample, equilibrated with quartz in the balance). All the measurements for hydrogen uptake were carried out after pumping to vacuum (10−2 Torr) for 5 h, heating the sample cell at 135◦ C and continuous pumping during cooling the sample to the room temperature, until the sample weight is stable for 2 h. At this stage, 10 MPa pure hydrogen was admitted and the weight increase measured after the recorded signal becomes constant for 2 h. 3. Results and discussion 3.1. Characterization of the carbon nanostructures The TEM examination of the samples from which only the lanthanum oxide was removed, Fig. 1, show that they consist of carbon nano'bers, of 10 –100 nm outer diameter several m long. They contain Pd particles at the tip, some of them are multibranched. Fig. 1b shows a conical shaped Pd particle at the tip of a CNF, consistent with the mechanism of growth of a 'lament with a 'sh-bone structure [21]. An internal hollow channel of a few nm can be seen only for some of the CNFs, at the left top of Fig. 1b. There is also rather much irregular pyrolytic deposition on the CNF surface. In order to be able to study the dependence of the H uptake on the Pd content, oxidation treatments in air were performed in a quartz boat at 520◦ C followed by cooling in pure argon Now to stop the oxidation at the desired stage. This step is useful not only for removing the amorphous carbon and puri'cation [20] but in our case it eventually

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Fig. 2. TEM images of the carbon nano'bers/Pd after oxidation in air at 520◦ C for: (a) 60 min; (b) 90 min. Fig. 1. TEM images of carbon nano'bers produced on Pd=La2 O3 from ethylene/hydrogen mixture, after removal of La2 O3 (a), a conical-shaped Pd particle is present at the tip of a nano'ber (b).

increases the Pd/C ratio due to partial oxidation of the carbon. The TEM images in Fig. 2 show that the oxidation results in the cleaning of the 'bers by removing most of the pyrolytic deposit (Fig. 2a), their fragmentation and agglomeration with the Pd particles (Fig. 2b). The presence of the metal particles exerts a catalytic e:ect on the oxidation process [22,23], lowering the oxidation temperature. For the

samples with Pd removed by dissolution in HNO3 , the TEM images at higher magni'cation reveal an angle of 40 –45◦ between the fringes and 'ber axis (Fig. 3), a characteristic of the “herringbone” arrangement which, however, cannot be distinguished very well on the whole 'ber. 3.2. Hydrogen uptake measurements The experiments have been carried out at the room temperature on samples free of lanthanum oxide, after dissolution in HCl 37%, on six di:erent batches of CNFs. Usually

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Fig. 3. Higher magni'cation TEM micrographs of carbon nano'ber samples from which Pd has been removed by dissolution in HNO3 , showing (a) the lattice fringe images of the graphene sheets within the structure and (b) details of their “herringbone” arrangement.

starting with 100 –200 mg of CNF/Pd, 2–3 oxidation steps were performed (30 –60 min at 520◦ C) in order to obtain di:erent Pd/C concentrations in the sample, the hydrogen uptake was measured in the initial state and after each oxidation step. The Pd content was 'nally measured after prolonged oxidation (3 h), when all the carbon content is lost, checked by no solid carbon residue left after dissolution of Pd in HNO3 . The Pd content in each stage of a given set was thus known. In order to evaluate the H uptake by CNFs, two corrections were needed: the amount of hydrogen absorbed by the

Pd content of the sample and the buoyancy e:ect. Pure Pd samples resulted after prolonged oxidation absorbed 0.74% H (0.78 H/Pd mole ratio) at 10 MPa, in good agreement with measurements on Pd foil and with the literature [19]. This value has been taken into account for all the samples, with the Pd content known in each stage. The H uptake per carbon was calculated by subtracting the contributions of H absorption by Pd and the buoyancy from the overall weight gain. After the admission of pure H2 at 10 MPa equilibrium is attained in 3–5 h with no further weight change in 20 –40 h but the most part of hydrogen sorbed by carbon requires less than 30 min. This should be taken only as a qualitative observation because the method used here does not allow kinetic measurements. Desorption experiments at room temperature showed that only 70 –80% of the H uptake by CNFs is released at 0:1 MPa but it can be reversibly sorbed. Initially it was observed that a decrease of the carbon content to about one half, by oxidation, results in an increase by 7–8 times of the H uptake—obviously a catalytic e:ect of Pd. Although the uptake show an increase with the decreasing of C content in a series, a plot of the results for di:erent batches vs. the fraction of C left after the oxidation steps spread so widely that this correlation was ruled out. This leads to the hypothesis that not the modi'cations of the CNFs themselves are important but rather the multiplication of their contact areas with palladium (Fig. 2b). When the correlation with the Pd/C ratio was envisaged, all the results from the six di:erent batches laid on a single, well-de'ned curve, Fig. 4a. The H uptake increases linearly with the ln(Pd/C) as shown in Fig. 4b. At the highest Pd/C ratios obtained here, the uptake at 10 MPa is in agreement with the results of moderate uptake (1.5 –2% H) reported by other groups [9,14]. The supply of H atoms, by dissociation of H2 on Pd surface and di:usion, at all Pd/CNF interfaces seems to be of major importance for favoring the hydrogen sorption by CNFs, the contact areas increasing with the Pd/C ratio. However, the results show that the saturation value of ∼ 1:5% H is reached rather diTcult, at high Pd/C ratios, suggesting that the inNuence is not e:ective too far from the Pd sites. The rapid hydrogen/deuterium isotopic exchange on graphitized carbon black surfaces [24] proves the mobility of H atoms on such surfaces, suggesting a possible spillover. The use of hydrogen absorbing alloys or metals (all of them able to dissociate H2 ) as catalysts, reported also by other authors [5,7,17,18], might be an important way of improving the hydrogen storage capacity of the carbon nanostructures. Recently it was also reported an H uptake increase by multiwalled carbon nanotubes [25] prepared in situ on Ni/MgO, explained in the terms of a chemisorption process favored by the hydrogen spillover from the catalyst onto the carbon nanotubes. The BET surface areas of the CNFs alone were measured with krypton, after the dissolution of Pd in 6N HNO3 . The values obtained are: 139, 425 and 455 m2 g−1 for the CNFs

D. Lupu et al. / International Journal of Hydrogen Energy 29 (2004) 97 – 102 1.6

H uptake [% weight H per Carbon]

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by the CNF/Pd composites, compared to pure Pd, may be assigned to the CNFs. Moreover, on desorption experiments showed that 70 –80% of the H uptake assigned to carbon is desorbed by lowering the equilibrium pressure to 0:1 MPa, in agreement with other authors, which is not a characteristic of the Pd–H2 system. In our opinion, the relationship reported in Fig. 4 cannot be explained only by the catalytic hydrogen transfer from Pd to CNF. There is also possible a charge transfer e:ect, taking into account the work function of Pd (5.1–5:6 eV) [29] and carbon nanotubes (4.95 –5:05 eV) [30], which might change the surface characteristics of the CNFs. However, it is premature to discuss this hypothesis at this stage because no systematic experiments with other dopants (electron donors or acceptors) are available. Finally, it is worth noting that the catalytic route is similar to the electrochemical charging with hydrogen both of them involving atomic H. Electrochemically, ca. 2% H can be sorbed by activated carbons, noticeably higher compared to 0.4% H for gaseous hydrogen, interpreted as an easier penetration of atomic H into the nanopores [31]. 4. Conclusions

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Fig. 4. The dependence of the hydrogen uptake by the carbon nano'bers studied here on the Pd/C ratio. The di:erent symbols correspond to di:erent batches of CNFs.

exhibiting 0.08, 1.46 and 1.43% H, respectively. The linear relationship between the H uptake and surface area, reported for carbon nanotubes and high surface area graphite [14] does not hold for nano'bers, as reported already by other authors [9,16]. High hydrogen storage capacity was reported for “herringbone” type carbon nano'bers for which the pre-heating in helium Now is emphasized as an important step [26] but other authors reported recently [27] only 1.1% H storage capacity for herringbone type carbon nano'bers, slightly increased to 1.5% after heat treatment in nitrogen at 1200◦ C. However, our samples, checked by volumetric method with activation at much higher temperatures (400◦ C) on unoxidized samples resulted in the same uptake value as with the gravimetric method: 0:08 ± 0:01% H. Measurements in our experimental conditions on samples containing only Pd, obtained after prolonged oxidation (no C detectable), resulted in 0.78 H/Pd at 10 MPa and 0:71 H=Pd at 0:1 MPa, in very good agreement with the data on Pd–H2 system using the pure Pd [28]. This shows that the behavior of Pd is not a:ected by C and that additional H uptake

Carbon nano'bers of herringbone type structure obtained on Pd catalyst exhibit catalyzed H uptake. At constant pressure (10 MPa), starting from 0.08% H, the H uptake increases with the increasing Pd/C ratio in the sample, up to a saturation value of 1.5% H. Measurements on Pd samples after the complete removal of C by oxidation show similar absorption capacity with pure Pd. Because 70 –80% of the uptake is desorbed at 0:1 MPa, the increase of H/CNF with Pd/C ratio might be due to a possible modi'cation of the CNF surface by charge transfer between the carbon nano'bers and Pd. The enhanced H uptake in the presence of atomic H supplied by Pd is similar to the electrochemical charging, also involving atomic H. Reliable literature data on the electrochemical discharge capacity of carbon nanostructures reporting storage capacities not higher than 2% H [14,31], comparable to our results, points out for prudence towards expecting much higher values. However, studies with other dopants on CNFs with more uniform heringbone type structure are required to establish if the charge transfer could inNuence signi'cantly the H uptake. References [1] [2] [3] [4]

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