Hydrogen adsorption on KOH activated carbons from mesophase pitch containing Si, B, Ti or Fe

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Hydrogen adsorption on KOH activated carbons from mesophase pitch containing Si, B, Ti or Fe Mateus Monteiro de Castro, Manuel Martı´nez-Escandell, Miguel Molina-Sabio *, Francisco Rodrı´guez-Reinoso Laboratorio de Materiales Avanzados, Departamento de Quı´mica Inorga´nica, Universidad de Alicante, Apartado 99, E-3080 Alicante, Spain

A R T I C L E I N F O

A B S T R A C T

Article history:

Different activated carbons with large micropore volume (0.78–0.99 cm3/g) have been pre-

Received 22 July 2009

pared by KOH activation of mesophase pitch obtained by co-pyrolysis of a petroleum resi-

Accepted 5 October 2009

due and small amounts of different compounds, triphenylsilane, borane pyridine complex,

Available online 9 October 2009

tetrabutyl orthotitanate or ferrocene. During the preparation, the Ti introduced in the petroleum residue concentrate into the activated carbon, whereas some loss of Si and Fe occurs. The compounds modify the size of mesophase structure formed during the copyrolysis process, as well as the apparent height of lamelae stack, Lc, both having an important effect in the development of the porosity of the activated carbon. However, there is a scarce influence of all heteroatoms in the adsorption capacity of H2 at

196 C and at 25 C,

which seems to be mainly influenced by the volume and size of micropores of the activated carbon. Only the activated carbon containing Fe adsorbs a higher amount of hydrogen at 25 C and 10 MPa than the expected one according to its micropore volume.  2009 Elsevier Ltd. All rights reserved.

1.

Introduction

The interest of hydrogen as a potential substitute for fossil fuels has, in recent years, stimulated the study of a large number of materials such as carbon nanotubes, carbon and graphite fibres, carbon templates, activated carbons, etc., for hydrogen storage [1–4]. The results seem to show that the textural factors (surface area, volume and pore size distribution) of these materials are the most important in determining the amount of adsorbed hydrogen [5,6]. Nevertheless, it seems that whatever the development of porosity, there is a limit to the hydrogen adsorption at room temperature by physical adsorption. This fact has motivated the use of a variety of treatments [7–12] with gases or acid/oxidizing solutions or the deposition of metallic compounds into the activated carbon surface with the purpose of improving hydrogen storage. Furthermore, it was suggested that the factors affecting the electronic density of the graphene layer (by electron withdrawing or donating effects) and the variation of the inter-

laminar space can contribute to enhance the hydrogen adsorption capacity [13,14]. Despite the fact that impregnation is the most common method used to deposit metal and semi-metal compounds on the surface of an activated carbon, an attractive alternative method to this post-treatment consists in the preparation of activated carbon using a mixture of a chemical compound and the carbon precursor. This method is particularly interesting if the metal/semi-metal compound can be dissolved in the carbon precursor, since a homogeneous distribution and a higher dispersion in the activated carbon is expected, although aggregation may occur during the thermal treatment inherent to the activation process. Different activated carbons have been prepared in this work by KOH activation of mesophase pitch obtained by the pyrolysis of a petroleum residue in which a chemical compound has been dissolved. Four different chemical compounds, triphenyl silane (TPS), pyridine borane complex (PyB), tetrabutyl orthotinatate (TBO) or ferrocene (FC), have

* Corresponding author: Fax: +34 965 90 3454. E-mail address: [email protected] (M. Molina-Sabio). 0008-6223/$ - see front matter  2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.10.005

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been dissolved individually into a petroleum residue to obtain mesophase pitches containing Si, B, Ti and Fe, respectively, after pyrolysis. The selected compounds, once dissolved in the petroleum residue, can catalyse or inhibit the reactions taking place during pyrolysis, and, consequently, they affect not only to the amount of mesophase but also its texture [15,16]. It is also possible that these compounds can react with the petroleum residue molecules to form new bonds with carbon and thus incorporate into the structure of the carbon matrix. For instance, the formation of new C–B and C–Si bonds is not excluded, although the main part of the added heteroatoms may incorporate into the carbon matrix as oxide particles [17]. Anyway, if these chemicals modify the structure of pitch, they may also modify the development of porosity during the activation with KOH and, consequently, the adsorptive properties of the activated carbon. KOH is a chemical which is capable of producing microporous activated carbons with an extremely high surface area from a varied range of precursors such as polymers, lignocellulosic materials, coke, coal, etc. [18–21]. Particularly, KOH can develop extensive microporosity when reacts with anisotropic carbon or mesophase based carbon materials [22,23] and this could be of interest for hydrogen storage. The aim of this work is to study the development of porosity produced during the KOH activation of a mesophase pitch prepared by co-pyrolysis of a solution of a chemical compound containing Si, B, Ti or Fe in a petroleum residue. The hydrogen adsorption at 196 and 25 C has been determined in order to establish the influence of the microporosity and the ‘‘heteroatoms’’ in the amount of hydrogen adsorbed.

2.

Experimental

2.1.

Preparation of activated carbons

An aromatic petroleum residue (ethylene tar-R1) [24,25] was mixed, individually, with four different compound, TPS, PyB, TBO, and FC in an ultrasonic bath for an hour, to give mixtures containing 2 wt.% of Si, Fe and Ti or 1 wt.% of B. All compounds are apparently soluble in the petroleum residue. Pyrolysis of the mixtures was performed at 440 C, soak time of 4 h and 1 MPa pressure, thus leading to pitches which contain the metal/semi-metal: PSi, PB, PTi and PFe. A reference pitch P was also prepared. The experimental set up and additional details of the preparation method have been previously described [15,16]. The activated carbons PA, PSiA, PBA, PTiA and PFeA have been prepared from the respective petroleum pitches as follows: KOH and the pitch were mixed in a ball mill during 30 min with a impregnation ratio of KOH/carbon of 3/1 and then thermally treated in a horizontal furnace until 800 C under nitrogen flow of 100 ml/min, soak time of 2 h. Finally, the activated carbon was washed in a Soxhlet apparatus for 24 h with water and dried at 110 C for 24 h in a vacuum stove.

2.2.

Properties of pitches

Metal/semi-metal content (wt.%) was determined by ashing 1 g samples in air at 900 C for 12 h, considering that Si, B,

637

Ti and Fe are completely transformed into SiO2, B2O3 TiO2 and Fe2O3, respectively. To determine the mesophase content, the pitches were mounted in a resin block and were examined by reflected polarised light following the procedure previously published [24]. Aromaticity of the pitches was analysed by FTIR using a BRUKER IFS 66 equipped with a DLaTGS detector in the transmission mode. The areas of absorption corresponding to C–H aromatic vibrations (2990–3150 cm 1) and C–H aliphatic vibrations (2800–2990 cm 1) were obtained and the aromaticity parameter was calculated as the ratio of the two areas (CHar/C-Hal). The X-ray diffraction patterns of the pitches (particle size below 500 lm) were obtained in a Siefert JSO 2002 Debye Flex system. The apparent height of the stack (Lc) of mesophase molecules, mesogens, was calculated from the broadening (FWHM) of the Gaussian for the (002) peak using the Scherrer equation and a shape factor of 0.94. Although Lc values obtained by this procedure do not have a clear meaning [26], they seem to be useful when comparing carbons with similar characteristics [27,28].

2.3.

Properties of activated carbons

Metal/semi-metal content was determined by X-ray fluorescence (XRF) using a Philips PW2400 spectrometer. Metal content in the particle surface was determined by XPS using a VG-Microtech Multilab 3000 spectrometer. The morphology of activated carbons was analysed employing a HITACHI S-3000 N scanning electron microscope (SEM) fitted with a XFlash 3001 dispersive X-ray analyser. The porosity of the activated carbons was deduced from the adsorption isotherms of N2 at 196 C and CO2 at 0 C, using a homemade automatic volumetric equipment, which features two pressure sensors (0–1.3 kPa and 0–0.1 MPa). The samples were degassed at 1 · 10 4 Pa and 250 C. The total volume of micropores, size lower than 2 nm, (VN2 ) was determined by the application of Dubinin–Radushkevich equation to N2 adsorption isotherm. Mesopore volume (Vmeso) was estimated by the difference between the amount of N2 adsorbed at P/ P0 = 0.95 and the total micropore volume (VN2 ). The volume of narrow micropores, size lower than 0.7 nm, (VCO2 ) was also determined by the application of Dubinin–Radushkevich equation to CO2 adsorption isotherm. Therefore, the comparison of both values, VN2 and VCO2 , is an indicator of the homogeneity of the size of the micropores of the activated carbon [29–31]. The hydrogen adsorption isotherms at 196 C were carried out in the same volumetric equipment described above, whereas hydrogen isotherms at 25 C were obtained using a volumetric equipment that reaches 10 MPa. This equipment was also built in our laboratory and has two pressure sensors that cover the pressure ranges 0–0.1 MPa and 0–10 MPa.

3.

Results and discussion

3.1.

Properties of the pitches

Table 1 summarises some properties of the pitches. Metal/ semi-metal content is, generally, high and larger than that

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Table 1 – Characteristics of pitches. Pitch P PSi PB PTi PFe

Metal/semi-metal (%)

Metal/semi-metal theoretical (%)

Mesophase (%)

0 4.5 3.0 6.1 2.9

0 5.7 2.9 5.7 5.7

56 0 90 63 95

introduced into the petroleum residue (2 wt.% Si, Ti or Fe; 1 wt.% B), thus indicating that the heteroatom concentrates in the pitch. Taking into account that the pitch yield after pyrolysis is approximately 35 wt.%, the maximum content expected in the pitch is 5.7 wt.% for Si, Ti or Fe and 2.9 wt.% for B. From the values presented in Table 1 it can be deduced that some part of Fe and, to a lower extent, Si are lost with volatiles along pyrolysis, whereas all the B and Ti introduced into the petroleum residue remain in the pitch. Fig. 1 includes images of the optical texture of the pitches. The differences between the metal/semi-metal containing pitches and the reference pitch are rather significant, indicating that the chemical added to the petroleum residue influences the reactions that take place during pyrolysis. Similarly, Table 1 also shows differences in the characteristics of the pitches. Reference pitch P has a mesophase content of 56% (Table 1) mainly in spherical shape, of size below 60 lm (Fig. 1). The addition of TPS inhibits mesophase formation, producing an almost ‘‘isotropic’’ pitch (Fig. 1) with a rather low apparent height of lamellae stack, Lc. These changes in the characteristics of pitch PSi as compared to pitch P can be justified assuming that TPS reacts with the components of the petroleum residue to yield molecules containing C–Si bonds [15] of lower planarity, restricting the formation of mesophase. PyB leads to a pitch PB constituted by more aliphatic molecules and it contains a larger amount of mesophase than pitch P (Table 1). However, the optical texture is mosaic type. PyB decomposes during pyrolysis to yield borane, which catalyses the polymerization of cracking molecules and increases the reactivity, producing small mesophase structures, mosaic type [16,32]. It is also observed that the Lc value is somewhat lower than in the reference pitch. The optical texture of PTi is more similar to P, although the size of the isochromatic areas of mesophase structures is lower than for P and a larger tendency to coalesce is also observed. The high value of Lc has to be pointed out. TBO decomposes in the first stages of pyrolysis to give TiO2 particles, on which mesophase spheres are formed and coalesced easily [17], due to the low viscosity of the media. The pitch containing Fe exhibits the highest amount of mesophase and the Lc values are very near to those of pitch P. FC decomposes within pyrolysis, yielding iron clusters that catalyse the formation of mesophase [33,34]. The optical texture observed for this pitch, Fig. 1, is of flow domains, typical of low viscosity pyrolysis systems. FTIR contributes to a more detailed characterization of the pitches. Fig. 2 includes FTIR spectra in the wave numbers range of 1900–650 cm 1, region interesting for supplying de-

Hal/Har (FTIR) 9.8 8.3 26.5 8.8 8.1

Lc (nm) 4.37 0.60 2.78 7.08 4.09

tails of the carbon skeleton. The intensity has been normalised to the one of the 1615 cm 1 peak for comparison. P, PFe and PTi pitches (Fig. 2a) show a great similarity, and no peaks due to C–Fe or C–Ti bond were found. Only PTi shows a more intense peak centred at 744 cm 1, but it is due to a larger amount of aromatic rings with four adjacent aromatic hydrogens [35]. In the PSi spectrum, Fig. 2b, a peak not found in P, centred at 1245 cm 1 can be observed, assigned to the balancing and stretching vibrations of Si–CH3 or Si–CH2 bonds and a peak centred at 698 cm 1 corresponding to the stretching vibration of the Si–C bond [35]. There is a wide band (1270– 1350 cm 1) in the spectrum of PB assigned to the bending deformation of B–CH3 bond or to the stretching vibration of B–O bond in borate esters and an intense peak centred at 1396 cm 1 attributed to stretching vibration of B–O bond, thus indicating a possible oxidation of the borane [36]. Hence, the formation of C–Si bonds can easily be inferred, whereas it is not easy to conclude the formation of new C–B bonds.

3.2.

Porosity of activated carbons

All pitches were activated with KOH using the same experimental conditions. The carbon yield obtained after activation was similar in all cases, 36 wt.%, indicating that the metal/ semi-metals, Si, B, Ti or Fe, do not seem to modify the reaction between KOH and the pitch. Table 2 shows the metal/semi-metal content in the activated carbons, deduced from X-ray fluorescence (XRF). Boron, being a very light element, could not be detected by this technique. The maximum amount of heteroatom expected in the activated carbon is 12 wt.% Si, 17 wt.% Ti or 8 wt.% Fe, taking into account the content in the pitch (Table 1) and the activation yield, 36 wt.%. From the comparison of these values with those obtained by XRF, Table 2, it can be deduced that all the Ti and most of the Fe remain in the carbon after activation, whereas part of the Si leaves the carbon during the activation process, possibly during the washing step which is necessary to eliminate the residues of KOH reaction with the pitch. The content of heteroatoms has also been determined by XPS. As this technique provides information about the chemical composition in the solid surface, from the comparison of these values with those of XRF (Table 2) it can be deduced that titanium is distributed homogeneously both inside and outside the activated carbon particles. However, the distribution seems to be more heterogeneous for Si and Fe, since the former seems to be more concentrated in the surface rather than inside the particle, whereas the proportion of superficial atoms of Fe seems to be low in comparison with the total. The low iron content in the particle surface can be attributed to the fact that FC, decomposes during the pyrolysis of the

CARBON

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Abosrbance (a.u.)

a

1900

PTi P PFe

1650

1400

1150

900

650

Wave number (cm-1)

b PSi

Abosrbance (a.u.)

PB

1900

P

1650

1400

1150

900

650

-1

Wave number (cm ) Fig. 2 – FTIR spectra of pitches, 1900–650 cm

Fig. 1 – Optical micrographs of pitches. (a) P; (b) PSi; (c) PB; (d) PTi and (e) PFe.

petroleum residue to yield clusters of Fe(0) of variable size, around which the polyaromatic molecules pilled up [33]. Fig. 3 presents an image of a particle of the activated carbon

1

.

PFeA, on which large size particles of iron, around 0.1–1 microns, can be observed. The high activation temperature very probably favours the aggregation and formation of clusters and, if a significant size is reached, the proportion of atoms in the exterior must be low. Such large aggregates were not observed for the rest of the samples. The oxygen content in the surface of the particle of the activated carbon PA obtained by XPS, is 5 wt.%, which can be attributed to the presence of oxygen surface groups produced during activation. A similar value of oxygen is obtained for the PFeA, which is much lower than those obtained for PSiA, 23 wt.%, and for PTiA, 19 wt.%. Although in these two carbons a proportion of the oxygen can be found forming bonds analogous to those of PA, the presence of Si–O and Ti–O bonds seems to be most probable. The binding energies registered for Si and Ti suggest that these elements can be found in the form of SiO2 and TiO2 particles deposited on the surface of the activated carbon, which must have a nanometric size as they were not detected by SEM. Fig. 4 shows the N2 adsorption isotherms at 196 C for all the activated carbons. The isotherms are of Type I and the more significant differences occur at P/P0 < 0.3, in the

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Table 2 – Properties of activated carbons. Carbon

PA PSiA PBA PTiA PFeA

% Metal/semi-metal XRF

XPS

0 4.0 – 20.9 7.2

0 8.6 – 18.4 1.2

VN2 (cm3/g)

Vmeso (cm3/g)

VCO2 (cm3/g)

0.99 0.90 0.95 0.78 0.98

0.21 0.72 0.56 0.33 0.34

0.81 0.61 0.68 0.52 0.69

1000

PSiA PBA

800

PA PTiA

600

3

-1

V (cm .g STP)

PFeA

400

200

0 0

0.2

0.4

0.6

0.8

1

P/Po Fig. 4 – N2 ( 196 C) adsorption isotherms of activated carbons.

Fig. 3 – SEM micrographs of activated carbons. (a) PFeA (backscatter electrons); (b) PA, (secondary electrons) and (c) PSiA (secondary electrons).

micropores and lower size mesopores. The mean pore size deduced from the application of DFT method is in the range from 2 nm for PA carbon to 2.5 nm for PSiA carbon. It can be observed in Table 2 that the micropore volume values (VN2 ) for all the activated carbons are very high and the differences among them are less important than those observed for Vmeso values. Therefore, it seems that the presence of heteroatoms inside the pitch during the activation of KOH modifies the distribution of size of micropores rather

than the micropore volume, which seems to be dependent mainly on the impregnation ratio or temperature. With the exception of the carbon PTiA, the order in the mesopore volume is: PA < PFeA < PBA < PSiA, contrary to that corresponding to narrow micropores, VCO2 , which is PA > PFeA > PBA > PSiA. Taking into account that the reference carbon contains a larger volume of narrow micropores and lower mesopore volume, it can be deduced that the elements (Si, B or Fe) enhance the widening of micropores during the activation of the pitch, some of them becoming mesopores. In fact, the differences found in mesopore volume seem to be due to pores of a size nearly 2 nm, that is to say, in the frontier with micropores, as curves in Fig. 4 are parallel for P/P0 > 0.3. The isotherm of PTiA also has a more open knee than PA (Fig. 4), indicating that Ti also favours the widening of the microporosity during activation. However, it seems that the high content of TiO2 in this activated carbon makes that both VN2 and VCO2 become the lowest of all the studied activated carbons, since these values are expressed as Vol/g of sample (carbon plus metal oxide). Since the activation method was common for all the activated carbons, the differences in pore size distribution previously discussed are caused by the different characteristics of the pitches. Those which seem to have a larger influence are the size of mesophase structures (Fig. 1) and the apparent height of the stack, Lc (Table 1). Therefore, heteroatoms affect the porosity since they modify both characteristics of the pitches. In general it can be observed that the smaller the size

of mesophase and the lower the height of the stack of polyaromatic molecules constituting the pitch, the more effectively the activating agent can proceed and, consequently, it develops pores of medium size and larger pore volume. Thus, the activated carbon which contains a wider porosity (and a larger total pore volume) comes from the pith PSi, which is ‘‘isotropic’’, with 0% mesophase content (or at least, with mesophase of size which cannot be observed by optical microscopy), exhibiting Lc values rather low, 0.6 nm. The following activated carbon with significant development of porosity comes from pitch containing B. This pitch exhibits a high amount of mesophase, although the important fact is its size, as its optical texture is mosaic type (Fig. 1), and the Lc value is also low. It is also remarkable that the carbon with the lowest development of micropores, PTiA, originates from the pitch with the highest Lc values. An additional evidence of the influence of the structural characteristic of pitch in the development of porosity in the activated carbon is presented in Fig. 5, where a linear relation can be observed between the surface area of the activated carbon, SBET, and the Lc values of the pitch where it comes from. Differences near 1000 m2/g can be found between the activated carbon PTiA (coming from PTi with Lc values of 7.1 nm) and PSiA (coming from PSi with Lc values of only 0.6 nm). Additionally, the aspect of the grains of PSiA is completely different to that of the rest of the activated carbon; PSiA exhibits a continuous matrix (see Fig. 2), the rest of the activated carbons resemble to the reference, PA, which is constituted by agglomeration of small units of size around 1 lm.

3.3.

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Hydrogen adsorption

Hydrogen, due to its low molecular volume and critical temperature, adsorbs weakly on the surface of activated carbon. Consequently, the amount adsorbed under standard pressure and temperature conditions is rather low. The strategies used to enhance the adsorption are based in increasing pressure, decreasing temperature and selecting an activated carbon

where the micropore volume as well as the hydrogen heat of adsorption are as high as possible. In principle, the activated carbons described here are adequate for hydrogen storage applications as they exhibit a high micropore volume, with a majority of narrow micropores, and with ‘‘heteroatoms’’ Si, B, Fe or Ti capable to increase the interaction of the adsorbent with hydrogen. Fig. 6 presents the hydrogen adsorption isotherms at 196 C and 25 C. The shape of the curves is similar to that described in the literature for other activated carbons [8,11,37–39]. It seems to be a type I isotherm, since the amount adsorbed increases fast with pressure at the beginning, and more slowly thereafter. The amount of hydrogen adsorbed is analogous to values found in the literature for porous carbons with high surface area [40] and the hydrogen uptake at 196 C and 0.1 MPa is 2–3 times higher than the uptake at 25 C and 10 MPa, indicating that the temperature has a more notable influence than pressure in hydrogen adsorption [14,37]. From the curves in Fig. 6, data of the amount of hydrogen adsorbed at 196 C, 0.1 MPa and at 25 C and pressures of 3 16

a

PBA PA PFeA PSiA

14

Amount Adsorbed (mmol.g-1)

CARBON

12

PTiA

10 8 6 4 2 0 0

20

40

60

80

100

Pressure (KPa) 3500

PSiA

b PFeA

-1

Amount Adsorbed (mmol.g )

3000

6

PBA

2

S BET (m /g)

2500

PA

PTiA

2000 1500 1000 500 0

0

2

4

6

8

10

Lc(nm) Fig. 5 – Relationship between BET surface area of activated carbons and height of lamelae stack of pitches.

PFeA PA

5

PBA PSiA

4

PTiA 3

2

1

0 0

2

4

6

8

10

Pressure (MPa) Fig. 6 – Hydrogen adsorption isotherms of activated carbons at: (a) 196 C and (b) 25 C.

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and 10 MPa have been obtained, with the aim to relate these values to different parameters of the porous texture of the adsorbents. There are many reports showing a correlation between the amount of hydrogen adsorbed and the characteristics of the porous textures of the materials. When very different materials such as zeolites, MOFs or some carbons are compared, it is preferred the use of BET superficial area [40], whereas in carbons with high micropore volume and a majority of narrow micropores, as described in this study, there is less dispersion of results when the hydrogen uptake is related to the micropore volume. In fact, the best correlations, Fig. 7, can be found when comparing the amount of hydrogen adsorbed at 196 C with the total volume of micropores, VN2 , Fig. 7a, and the amount of hydrogen adsorbed at 25 C with the volume of narrow micropores, VCO2 , Fig. 7b. Fig. 7a shows that the values of the amount of H2 adsorbed at 196 C, 0.1 MPa as a function of VN2 adjust to a straight line that passes through the origin. This indicates both the role of total microporosity (size < 2 nm) in H2 adsorption at 196 C, and that the density of the hydrogen adsorbed in the micropores is similar in all these carbons. Since the amount of hydrogen adsorbed at a specific pressure gives a

limited information of the hydrogen adsorption process, a better correlation should exist when the VN2 values (which were obtained by the application of DR method) are compared with the maximum amount adsorbed obtained by the extrapolation of the isotherms using some theoretical model that adjusts to the hydrogen adsorption isotherm at 196 C. The values of ‘‘monolayer volume Vm’’, deduced from the application of Langmuir equation to the data at 196 C, have been included in Fig. 7a, and a better adjustment of the experimental data to the straight line can be observed. Therefore, it is deduced that the amount of hydrogen adsorbed at 196 C shows a nice correlation with the total micropore volume and also that there is no influence of the heteroatoms introduced in the amount of H2 adsorbed. Fig. 7b shows that both the amount of H2 adsorbed at 25 C (3 MPa and 10 MPa) adjust linearly with VCO2 and both straight lines go near the origin. This fact is indicating the role of narrow microporosity is playing in the adsorption of hydrogen at room temperature in these carbons. In principle, the deviation of data from the straight line should be smaller when the adsorption takes place at a ‘‘low’’ pressure such as 3.0 MPa. Thus, if there are other additional factors influencing the hydrogen uptake, they will be revealed more clearly at 10 MPa rather than at 3 MPa. In this sense, the carbon PFeA is out of the trend shown by the rest of the carbon, exhibiting a value well above the trend line, more marked for 10 MPa. Fig. 7a also includes the values deduced from the application of Langmuir equation to hydrogen adsorption at 25 C. Surprisingly, both values of ‘‘Vm’’ of hydrogen at 196 C and 25 C are similar. This means that the maximum hydrogen amount adsorbed predicted by the Langmuir model is the same and it is related to the total micropore volume, VN2 . However, the carbon PFeA is out of that trend, the Langmuir Vm being higher than expected according to its VN2 value. This fact does not mean that the adsorption capacity of this activated carbon exceeds the volume of micropores. It only indicates that the presence of Fe enhances the amount of hydrogen adsorbed at 25 C in the range of pressures from 0–10 MPa and, consequently, modifies the shape of the adsorption isotherm, increasing the value of ‘‘Vm’’ deduced from Langmuir equation. In any case, it seems that the hydrogen adsorption energy at 25 C of carbon PFeA is higher than for the rest of the activated carbons and, consequently, it shows a higher hydrogen uptake than the expected due to a physisorption process, predominating in the rest of the activated carbons. Further research is necessary to justify the role of iron in the enhancement of the hydrogen adsorption capacity of this activated carbon.

4.

Fig. 7 – Relationship between the amount of adsorbed H2 and micropore volume.

Conclusions

The compounds triphenylsilane, borane pyridine complex, tetrabutyl orthotitanate or ferrocene modify the reactions taking place during the pyrolysis of the petroleum residue yielding pitches with high Si, B, Ti or Fe content and different microstructure. When these pitches are activated with KOH, activated carbons with high micropore volume and some differences in the pore size distribution can be obtained. In

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general, the activation of mesophase pitches with spherical and domain structure and higher Lc values produce activated carbons where micropores are narrower than those obtained from pitches with isotropic or fine mesophase structure (mosaics) and lower Lc values. The activated carbons from these pitches have a wider micropore size distribution, extending to the mesopores. Both the micropore volume and the size of micropores influence the amount of hydrogen adsorbed. A relation between the amount of hydrogen adsorbed at 196 C and the total micropore volume (deduced from N2 adsorption at 196 C) was found. An additional relation between the amount of hydrogen adsorbed at 25 C and the volume of narrow micropore (deduced form CO2 adsorption isotherm at 0 C) was also found, indicating the importance of microporosity and the scarce influence of the metal/semi-metal present in the activated carbon. Only the carbon containing Fe adsorbs a higher amount of hydrogen than expected according to its microporosity, probably due to a stronger interaction with hydrogen than in the rest of the activated carbons.

Acknowledgements Support from the Ministerio de Ciencia e Innovacio´n (Project MAT2007-61734) and the European Union (Network of Excellence Insidepores, NMP3-CT-2004-500895) are acknowledged.

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