Novel Nanostructures of Porous Carbon Synthesized with Zeolite LTA-Template andMethanol

Song Lei, Jun-ichi Miyamoto, Tomonori Ohba, Hirofumi Kanoh, and Katsumi Kaneko*Graduate School of Science and Technology, Chiba UniVersity, 1-33, Yayoi, Inage, Chiba 263-8522, Japan

ReceiVed: October 6, 2006; In Final Form: December 6, 2006

Novel nanostructured carbon material was synthesized by applying zeolite LTA as a template and usingmethanol as a carbon source. X-ray diffraction (XRD) revealed that the higher the decomposition temperature,the more graphitic and ordered the structure. The optimum pyrolytic temperature for addition of microporositywas below 1273 K by analysis of N2 adsorption isotherm. The resultant carbons have the long-rangeperiodic structure of a nanoscale curvature according to XRD and Raman spectroscopic examinations.The morphological similarity between zeolite LTA and synthesized carbon was evidenced by scanningelectron microscopic observation. Grand canonical Monte Carlo simulation of N2 adsorption isotherm indicatesthat synthesized nanoporous carbon has a hollow hemispherical structure of which diameter is less than 0.7nm.

Introduction

Porous carbons are multipurpose materials that have beenwidely used in many fields such as air and water purification,catalyst supports, and electrodes for supercapacitors.1-3 Manynovel approaches to control the pore structure have beenproposed by using template-assisted routes. The previousresearches have prepared porous carbon with ordered structureusing a variety of porous template and these resulting carbonsreflect the original template structure. Ryoo et al. preparedmesoporous carbons of regular structures with the templatemethod.4,5 Kyotani et al. succeeded to produce high surface areacarbon by use of zeolite Y as the template; their studiessuggested that those carbons have unique nanostructure andphysical properties.6,7 Therefore, we need to elucidate thenanostructures and physical properties of carbons prepared withthe zeolite template and the growth mechanism of the nano-structured carbon in highly confined nanopore spaces of zeolites.At the same time, it is necessary to develop a convenient methodfor preparation of the nanocarbons with the zeolite-template.To understand the unique structure of nanocarbons prepared inthe zeolite pores, a detailed study on the preparation ofnanocarbons with the different kinds of the zeolite templateshould be carried out. In particular, the growth mechanism ofnanostructured carbons in very small pore spaces of zeolitesmust be studied.

LTA has pores whose aperture is less than 0.7 nm and therebyit can offer the lower limit nanospaces for growth of nanostruc-tured carbons. Methanol is a very small molecule and is oneof the popular chemicals. If methanol is available for thetemplate synthesis of unique nanocarbons, nanocarbons withthe template method should become applicants for an industrialapplication. As one of the hopeful applications of nanoporouscarbons is storage of clean energy gases such as methaneand hydrogen, we are intensely interested in the relationshipbetween the nanostructures and the adsorptivity for supercriticalhydrogen gas.

In this work, nanocarbons are prepared through the chemicalvapor deposition of methanol using zeolite LTA as a template;the unique nanostructures and high adsorption ability are shown.

Experimental

Zeolite LTA from Union Showa K.K Corporation in Japanwas used as the template. Zeolite LTA has a simple cubicstructure with a composition Si/Al ratio of unity, that is, twodifferent-sizedR- andâ-cages (diameters are about 1.1 and 0.7nm, respectively) are arranged in a CsCl type structure. Thepore diameter is defined by an eight-member oxygen ring andit is about 0.7 nm. Methanol was used as a carbon source.Pyrolytic carbon was deposited into zeolite channels bymethanol chemical vapor deposition (CVD). The CVD methodwas performed at 773-973 K for 6 h under nitrogen flow at100 ml min-1. The zeolite/carbon composites were carbonizedat 1073∼1273 K for 4 h under nitrogen flow. The template wasremoved by dissolving in HF solution. Finally, the resultant waswashed by deionized water, and then dried at 337 K for 1 day.The resulting sample was named as Z-PT-CT. Here, Z meanszeolite template, PT and CT indicates pyrolytic temperature andcarbonization temperature in Kelvin, respectively.

The pore structures were evaluated by analysis of nitrogenand hydrogen adsorption isotherms measured at 77 K using aQuantachrome Autosorb-1 instrument. Raman spectra weremeasured with Raman spectrometer (JASCO NRS-3100) equippedwith YAG laser (power 1.5 mW, wavelength 532 nm). Thecrystalline structures of the resultant carbons were examinedwith X-ray diffractometer (XRD) (Miniflex, Rigaku, Japan). Thezeolite template and synthesized carbon materials were observedby means of the scanning electron microscope (SEM) (JEOLJSM-6330).

GCMC Simulation

The N2 adsorption isotherm at 77 K was simulated with grandcanonical Monte Carlo (GCMC) method for the model of ahemispherical single wall structure using the following potentialfunction.

* Corresponding author. E-mail: kaneko@pchem2.s.chiba-s.ac.jp. Tel:+81-43-290-2779. Fax:+81-43-290-2788.

2459J. Phys. Chem. C2007,111,2459-2464

10.1021/jp066564s CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 01/25/2007

An N2 molecule was expressed by a two-centered Lennard-Jones molecule with the quadrupole moment by four Coulombicinteraction centers. The intermolecular interaction is calculatedby the sum of the dispersion interaction and the electrostaticinteraction between partial charges on the atomic sites of anN2 molecule.

Here, εff and σff are the potential well depth of the N2

molecule (εff/kB ) 35.6 K) and the effective diameter (σff )0.3318 nm), respectively. Both nitrogen atoms are situated at0.05047 nm from the N2 molecular center. Two positive andtwo negative charge centers of|qi| ) 0.373 e are distant fromthe N2 molecular center by 0.084 and 0.1044 nm, respectively.Nitrogen-carbon interaction is given also by the Lennard-Jonespotential.

Here,εss/kB ) 30.14 K andσss ) 0.3416 nm were used fora carbon atom. The cross parameters of nitrogen and carbonatoms were given by the Lorentz-Berthelot rule. The randommovement, creation, and removement of a molecule give a newconfiguration whose total potential energy was calculated. Asthe configuration is accepted in the condition of Metropolis’ssampling scheme, the system reaches an equilibrium state.8,9

Results and Discussion

Carbon of Periodicity of Zeolite-Template.Figure 1 showsthe SEM images of zeolite template and synthesized carbonmaterials. The zeolite has the cubic structure as shown in theFigure 1a; the plate-shaped crystals were often observed. Figure1b,c show that synthesized carbon materials have cubic and plateforms that are similar to the template structure, suggesting thatthe resultant carbon copies the template structure.

Figure 2 shows the XRD patterns of zeolite LTA and porouscarbons synthesized of Z-973-CT. The zeolite LTA as thetemplate shows many sharp peaks due to the inherent crystalstructure. Some of sharp peaks were still observable in thepatterns of all the resultant carbons, suggesting that the zeoliteframework of the template is preserved in the structure of thesynthesized carbons. However, a serious restriction of the carbongrowth in the zeolite pores leads to the different peak intensityfrom the template zeolite. The presence of XRD peaks impliesformation of three-dimensional regular structure of synthesizedcarbon6,7,10 irrespective of the growth in highly narrow porespaces. The sharp peak at 7° of nanocarbon indicates theperiodicity of 1.2 nm, whereas peaks at 27, 44, and 54° can beascribed to (002), (101), and (004) diffraction peaks, respec-

tively, which also appear in the zeolite LTA of template,which is an indication of long-range structural ordering, andsuggest that synthesized carbon partly reflect templatestructure.11-14 When the heat-treatment temperature is elevatedfrom 1073 to 1273 K, the XRD peaks become stronger andsharper. Consequently the carbon grown in the pores of zeoliteshould be more crystallized, leading to more stable and regularstructure. Figure 3 shows the XRD patterns of Z-PT-1173resultant carbons. XRD peaks are sharper and more intense,especially for the peaks at 7° and 54°, when the temperature iselevated during the carbon-deposition procedure. Thus, thehigher the decomposition temperature, the more graphitic andordered the structure.

Raman spectra of Z-973-CT samples are shown in Figure4A. G band comes from E2g vibration mode on an orderedgraphitic structure, while the D band is associated with defectivestructures and disorders.15-18 The peaks of the D and G bandsof all samples are observed around 1340 and 1590 cm-1,respectively. The intensity ratios of the G and D band,I(G)/I(D) increased from 0.95 to 1.0 with carbonization temperature

Figure 1. SEM images of zeolite template and carbon synthesized.(A) zeolite template; (B), (C) synthesized carbon materials.

Figure 2. XRD patterns of Z-973-CT. (a) zeolite LTA; (b) CT) 1073;(c) CT ) 1123; and (d) CT) 1273.

Figure 3. XRD patterns of Z-PT-1173. (a) PT) 973; (b) PT) 873;and (c) PT) 773.

æff(r) ) 4εff[(σff

r )12

- (σff

r )6] + ∑i

∑j(*i)

1

4πε0

qiqj

r ij

(1)

æsf (r) ) 4εsf[(σsf

r )12

- (σsf

r )6] (2)

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for Z-973-CT samples. It is noteworthy that the Raman peak isobserved at 310 cm-1 for Z-973-CT samples, as shown in Figure4B. This band is similar to the radial breathing model (RBM)band that is inherent to single-wall carbon nanotubes, double-wall carbon nanotubes, or single-wall carbon onions.19-22 Thus,the band at 310 cm-1 suggests the presence of a single wallcarbon structure of nanoscale curvature, which was suggestedby Kyotani et al.22 and Yoshimura et al.23 However, no peakaround 300 cm-1 was observed for carbon samples prepared at773 and 873 K of the CVD temperature.

TheI(G)/I(D) ratio changed from 0.85 to 0.96 with pyrolytictemperature for Z-PT-1173 samples, which are shown in Figure5. Hence, the fundamental graphitic structure of the nanocarbonis not so sensitive to the carbonization and pyrolytic temperatureexamined in this works

Nanopore Structures and Adsorption Supercritical Hy-drogen. Figure 6 shows the effect of the carbonization tem-perature on the N2 adsorption isotherm at 77 K. The N2

adsorption isotherm at the lowest carbonization temperature(1073 K) is almost of type I, indicating the presence ofmicropores; it has also a slight uptake at aboutP/P0 ) 0.9,showing the contribution by adsorption on the external surfaces.The elevation of the carbonization temperature does notmarkedly increase the adsorption amount belowP/P0 ) 0.4,

indicating only a slight development of micropores by theelevation carbonization temperature. However, remarkablechanges are observed aboveP/P0 ) 0.6 with the elevation ofthe carbonization temperature. In particular, carbonization at1273 K leads to an explicit adsorption hysteresis. Accordingly,elevation of the carbonization temperature above 1123 K shouldgive rise to the carbon deposition on the external surfaces,accompanying with mesopores. Hence, the optimum pyrolyticcondition was examined below 1273 K of the carbonizationtemperature. Figure 7 shows the effect of the pyrolytic tem-perature on the N2 adsorption isotherm of carbon samples at77 K. The adsorption isotherm of Z-773-1173 is close to typeI, but it has a slight uptake aboutP/P0 ) 0.9. The higher thepyrolytic temperature, the greater the adsorption amount. Theelevation of the pyrolytic temperature increases adsorptionamount remarkably, leading to adsorption hysteresis.

Figure 4. Raman spectra of Z-973-CT. (a) CT) 1073; (b) CT)1123; and (c) CT) 1273. (A) High-frequency region. (B) Low-frequency region.

Figure 5. Raman spectra of Z-PT-1173. (a) PT) 773; (b) PT) 873;and (c) PT) 973.

Figure 6. Nitrogen adsorption isotherms of Z-973-CT at 77 K.O, CT) 1073;3, CT ) 1123; and], CT ) 1273.

Figure 7. Nitrogen adsorption isotherms of Z-PT-1173 at 77 K.O,PT ) 773; 4, PT ) 873; and], PT ) 973.

TABLE 1: Pore Structure Parameters of Prepared CarbonMaterials

totalsurface

area/m2 g-1

externalsurface

area/m2 g-1

internalsurface

area/m2 g-1

microporevolume/ml g-1

Z-973-1073 290 50 200 0.09Z-973-1123 360 60 240 0.10Z-973-1273 410 70 260 0.12Z-773-1173 120 20 90 0.03Z-873-1173 230 30 210 0.05Z-973-1173 370 60 250 0.09

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The internal surface area and micropore volume weredetermined by the subtracting pore effect (SPE) method for theRs-plot to evaluate the internal and external surface areas withoutthe overestimation.8 Also, the total surface area was evaluated.The pore structural parameters are summarized in Table 1. Whenthe pyrolytic temperature is 973 K, the contribution by theinternal surface to the total surface area is more than a half.Also, the micropore volume is in the order of 0.1ml g-1. Theseporous carbons exhibit the RBM band, as shown in Figure 4Band thereby these preparation conditions provide microporouscarbon of single-wall carbon of nanoscale curvature.

As the carbon samples are prepared in the very small zeolitepores, there is the possibility of the presence of ultramicropores.Then, we measured the H2 adsorption isotherms at 77 K, becausean H2 molecule is a smaller probe molecule than the N2

molecule. Also, we have a great demand for a better adsorbentfor supercritical H2. The hydrogen uptake of the preparedcarbon materials is correlated to their micropore volume andpore size.24,25 Figure 8 shows hydrogen adsorption isothermsof Z-973-CT samples at 77 K. The H2 adsorption amountsincrease with carbonization temperature, but in the low-pressureregion the uptake does not significantly increase. Therefore,elevation of carbonization temperature increases the micro-

porosity. The saturated adsorption amounts of hydrogen,WL,were evaluated by the Langmuir plot; theWL values per unitpore volume are in the range of 15 to 20 mg ml-1, assummarized in Table 2. AlthoughWL values are smaller thanadsorbed amounts of H2 on single wall carbon nanohorns at 77K and 4 MPa (55 mg ml-1),26 it is useful to analyze thesehydrogen adsorption isotherms with the extended Dubinin-Radushkevich (DR) equation for adsorption of supercriticalgases. The extend DR equation is given by eq 3.27

Here P0q is the quasisaturated vapor pressure in micropores.W is the micropore volume andâE0 is the characteristicadsorption energy. Extended DR plot for hydrogen adsorptionisotherms of Z-973-CT samples at 77 K are shown in Figure 9.TheâE0 andP0q values determined from the extended DR plotsare shown in Table 2.

The hydrogen adsorption isotherms of Z-PT-1173 are shownin Figure 10. When the pyrolytic temperature increases, thehydrogen adsorption amount increases especially in the low-pressure region, indicating an increase in the micropore volume.This result gives a similar tendency observed in the N2

adsorption. The linear extended DR plots of hydrogen adsorptionisotherms of Z-PT-1173 samples were obtained, giving theWL,âE0, andP0q values of Z-PT-1173, which are also summarizedin Table 2. TheP0q andâE0 are almost constant regardless ofdifferent carbonization temperature and pyrolytic temperature.However, theP0q values are 1040-1070 kPa, indicating thatthe pore field of the carbon is sufficiently strong to induce aquasicondensation of supercritical hydrogen. The sum ofâE0

and the enthalpy of vaporization provides the isosteric heat27-29

of adsorption at the fractional filling of e-1, qst. The obtainedqst values are in the range of 31∼33 kJ mol-1, being quite largerthan the isosteric heat of hydrogen adsorption on single wallcarbons (12 kJ mol-1).29 Thus, nanoporous carbon developedin the present study has intensely strong adsorption sites forsupercritical hydrogen.

Single-Wall Hemispherical Carbon Model.The observedRBM band indicates the presence of single wall carbon nanotubelike-structure of 0.7 nm in the diameter. However, the nanotubeof 0.7 nm in diameter cannot be produced in the small porespace of LTA. Literatures suggest that even hemisphericalsingle-wall carbon can give a similar Raman band to the RBMband.21,30Also Kyotani et al. have proposed that their templatedcarbon produced in the pore of zeolite has a chain structure of

Figure 8. Hydrogen adsorption isotherms of Z-973-CT at 77 K.4,CT ) 1073;O, CT ) 1123; and], CT ) 1273.

TABLE 2: Adsorption Parameters Determined fromLangmuir Plot and the Extended DR Plot for HydrogenAdsorption

WL(H2)/mg ml-1 P0q(kPa) âE0 (kJ mol-1)

Z-973-1073 18 1060 31Z-973-1123 20 1060 31Z-973-1273 21 1070 30Z-773-1173 15 1040 32Z-873-1173 16 1040 32Z-973-1173 20 1060 31

Figure 9. The extended DR plots of hydrogen adsorption isothermsof Z-973-CT.4, CT ) 1073;O, CT ) 1123; and], CT ) 1273.

[ln(WL/W)]1/2 ) (RT/âE0)(ln P0q- ln P) (3)

2462 J. Phys. Chem. C, Vol. 111, No. 6, 2007 Lei et al.

hemispherical single-wall carbon caps.22 Then, we simulatedN2 adsorption isotherm of the hemispherical single-wall carboncap of 0.7 nm in the diameter. Figure 11 compares GCMCsimulated isotherms with the experimental isotherm of Z-973-CT at 77 K. Here, simulated isotherms are shown for adsorptionprocesses on the total, internal, and external surfaces of the cap.The simulated adsorption isotherm on the external surface is

far from the experimental one, whereas the simulated isothermon the internal surface can describe well the experimental one.Accordingly, this simulation result supports intensely thepresence of the hemispherical cap structure. Figure 12 showsthe snapshots of adsorption on the internal and external surfacesof the hollow hemispherical cap. Only one N2 molecule can beaccommodated in the inside of the cap. On the other hand, manyN2 molecules are adsorbed on the external walls of the cap,which is far from the real carbon prepared in the restricted spaceof LTA zeolite. As the external surface of the positive curvaturecannot provide enough adsorption sites space in the real sample,the present simulation model overestimates the adsorptionamount on the external surface. Thus, the GCMC simulationcan provide an indirect evidence for formation of the singlewall cap structure in the zeolite pores.

Acknowledgment. The Grant-in-Aid for Scientific Research(S) (No.15101003) by JSPS is acknowledged. S.L. has been inpart supported by 21-COE program: Frontiers of Super-Functionality Organic Devices.

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Figure 10. Hydrogen adsorption isotherms of Z-PT-1173 at 77 K.4,PT ) 773; O, PT ) 873; and], PT ) 973.

Figure 11. Comparison of experimental N2 adsorption isotherm ofZ-973-1273 at 77 K with GCMC simulated adsorption isotherms ontotal, internal, and external surfaces for the hemispherical cap model.Experimental isotherm:], CT ) 1273. Simulated isotherm:b, totalsurface;2, internal surface; and9, external surface.

Figure 12. Snapshots of N2 molecules adsorbed on single-wall carboncap model.

Novel Nanostructures of Porous Carbon J. Phys. Chem. C, Vol. 111, No. 6, 20072463

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