Carbon Alloys part 15 docx

Yoshinaga, Gas permeation properties and characterization of asymmetric carbon membranes prepared by pyrolyzing asymmetric polyimide hollow fiber membrane.. Centeno, Preparation of suppo

However, solid porous supports are not always necessary for asymmetric hollow

carbon fibers [ 14,23,24,30,31,33,34,47] The precursor hollow fiber developed by

Kusuki et al [23] was spun from polyimide and had an O.D of 0.40 mm and an I.D of 0.12 mm It was then heat-treated in air at 673 Kfor 30 min and pyrolyzed at 873-1273

depending on the carbonization temperature The fractured face of the membrane

flexibility on the carbon fiber

3 Permeances of Molecular Sieving Carbon Membranes

3.1 Unary Gases

Gas Separations with Carbon Membranes 475

temperature of 873 IC After correction for the adsorption effect, intrinsic permeances

to single-component gases was correlated with the projected areas of the molecules

than that to hydrogen

3.2 Binary and Ternary Mixtures

Based on the permeation modes shown in Table 1, mixtures of carbon dioxide and nitrogen fall into regime 11 The permeance to the less adsorptive component

The total pressure is not a major factor in the separation of dry gases [30] When gases are condensed in pores, however, a different permeation mode appears As shown in Fig 9, the permeance to carbon dioxide reached a maximum near its critical pressure

BPDA-pp'ODA polyimide procedure gave higher C,HdC,H, and C,H4/C2H, perm- selectivities than those of the corresponding polyimide membrane The C,HdC,H, selectivity was approximately 30 at a permeability coefficient of 50 Barrer, which is

Gas Separations with Carbon Membranes 477

equivalent to a permeance of 3 x lo-' mol m-' s-I Pa-', when the membrane thickness,

6, is 6 pm This suggests that the carbonized membranes possess a micropore structure which is capable of differentiating between alkane and alkene molecules These high separation factors are explained in terms of the minimum size of the molecules The minimum size of C,H, is 0.40 nm, which is smaller than that of C,H,,

C,HdC,H, separation factor was 10-20, while the GH, permeance was larger than

lo4 mol m-'s-' Pa-'

The separation factor for ternary component systems is complicated Figure 11 shows the separation factors for C01-CH4-Hz systems [53], The mole fractions of carbon dioxide and hydrogen were kept equal with a varying mole fraction of CH, The HJCH, separation factor decreased with an increase in the mole fraction of CH, This is in accord with the findings relative to a binary H,-CH, system However, the

CO,/CH, separation factor increased at higher CH, mole fractions

4 Oxidation of Molecular Sieving Carbon Membranes

Heat-treatment in an oxidizing atmosphere improves permeances [21,22,28] Kusakabe et al [28] carbonized BPDA-pp’ODA membranes in an inert atmosphere

at 973 K, followed by oxidization using a mixture of 0,-N, (02 fraction = 0.1) at 573 K for 3 h The oxidation decreased the H/C ratio and increased the O/C ratio, suggesting that peripheral alkyl groups had undergone decomposition and that oxygen had been incorporated into the membrane Figure 12 shows permeances for carbon mem-

an increase in permeance without greatly altering the permselectivities Thus,

size distribution remained relatively unchanged However, treatments in an oxidative atmosphere need to be more extensively studied because results often are not reproducible

In order to examine the long-term stability, Hayashi et al [21] exposed

air at 373 Kfor one month As shown in Fig 13, this exposure caused no change to the

exposure After the long-term oxidation, the membrane was heat-treated in nitrogen

84% of its original value as a result of the long-term oxidation After the heat

Kinetic diameter [nm]

Fig 12 Effect of oxidation at 673 K for 3 h on permeances of membranes carbonized at 973 K Permeation

temperature = 338 K; (0) = as-formed, (0) = oxidized in 0, Permeation temperature = 373 K, (m) = as-formed, (A) = oxizided in 0,-N, mixture (0, fraction = O.l), (0) = oxidized in 0, [28]

Gas Separations with Carbon Membranes

Oxrdation period [day]

Fig 13 Changes in mass and elemental distribution in membranes during the stability test at 373 K i n air

The mass of the initial membrane is assumed to be unity [21]

Fig 14 Effect of long-term exposure to air at 373 K and post heat-treatment in nitrogen at 873 Kfor 4 h on

permeances at 338 K [21]

permeances, for the long-term oxidized and heat-treated carbon membrane, to a

the membrane decreased with increasing exposure time and recovered approximately

selectivity of 80 at 338 K Thus, oxidation in air at 373 K does not appear to have a

permeances Most of the surface oxides are decomposed by post heat-treatment at

873 K

The separation properties of carbon membranes are sensitive to steam, which is

strongly adsorptive Jones and Koros [15] reported that permeances to oxygen and nitrogen for an asymmetric hollow fiber carbon membrane, carbonized at 773-823 K, decreased to 0.4-0.5 of the initial value after the membrane was exposed to air at relative humidities of 2345% at ambient temperature The stability of the carbon

dioxole or tetrafluoroethylene [16] The oxygen flux, after exposure to humidity,

Fuertes [50] prepared carbon membranes by carbonizing cured phenolic resin films at

973 K in vacuum, followed by oxidizing the membranes with air at 537-673 K for 30

min The oxidation decreased the separation factors for single component gases For combinations of adsorptive hydrocarbon and nonadsorptive nitrogen, however, the

permeance to nitrogen decreased as shown in Figure 15, and, as a result, the separa-

tion factor for hydrocarbon increased This effect is pronounced for hydrocarbons, which have high adsorptivity and small molecular size

Carbon membranes with 0.4-1.5 nm diameter pores were also developed and

membrane Carbon membranes of this type have also been applied to the separation

Gas rrixtures:

PertmanUN2 = 50/50

50 90 130 170 210 250 290

Critiwl volume of permeant [dml]

Fig 15 Relationship of nitrogen permeances at 293 K and critical volumes of co-existing gas [50]

Gas Separations with Carbon Membranes 481

6 Conclusions

made by carbonizing precursor membranes When carbonization conditions, such as temperature and time, are properly selected, the permeation properties and stabili- ties of the carbon membranes are greatly improved Permeation through carbon membranes is dependent on molecular size relative to pore size, adsorptivity, diffusiv- ity and pore size distribution Controlled oxidation at elevated temperatures increases permeances of the carbon membranes However, permeances may be decreased by exposing the membranes to an oxidative or humidified atmosphere as the result of formation or the adsorption of oxygen-containing functionalities in the pores Permeances can be recovered by heat-treating the membranes in an inert

hydrocarbons or hydrogen sulfide to the permeate side via surface flow through the pores

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A.B Fuertes and T.A Centeno, Preparation of supported asymmetric carbon molecular sieve membranes J Memb Sci., 144: 105-111, 1998

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Gas Separations with Carbon Membranes 483

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37 N Itoh and K Haraya, A carbon membrane reactor Catal Today, 56: 103-111,2000

38 M.-B Hagg, Membrane purification of C1, gas 11 Permeabilities as function of tempera- ture for CI,, 02, N2, H, and HCl in perfluorinated glass and carbon molecular sieve mem-

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485

Chapter 30

Property Control of Carbon Materials by Fluorination

Hidekazu Touhara

Department of Materials Chemis*, Faculy of Textile Science and Technology, Shinshu

University, Ueda 386-8567, Japan

Abstract: Fluorination is effective in chemically modifying and controlling physicochemical properties of carbon materials over a wide range so creating opportunities to prepare carbon alloys with new functionalities Carbon alloyed nanotubes were prepared by selective fluorination of hidden surfaces of multi-wall carbon nanotubes using a template carbonization technique Carbon-fluorine bonds are formed on the hidden surfaces of the tubes while internal surfaces retained their sp*-hybridization Fluorination (as a form of carbon-alloying) significantly affects such properties of hidden surface extents of nitrogen adsorption, hysteresis loops and pore size distributions Fluorination enhanced the coulomb efficiency of Lilcarbon nanotube rechargeable cells in an aprotic medium (with reversible lithium insertion in hidden surfaces) as studied by galvanostatic discharge-charge experiments

Keywords: Carbon nanotubes, Activated carbon fibers, Fluorination, Hidden surface, Electro- chemical lithium insertion, Adsorption properties

1 Introduction

The interaction of fluorine with carbon materials provides fluorine-carbon materials

carbon and fluorine varies from covalent, through semi-ionic, to ionic, with van der

fibers, and carbon nanotubes Of these, extents of carbon s-p hybridization vary resulting in diverse electronic structures affecting their electrical conductivity, electron ionization potential, electron affinity, etc Physical properties are affected with, for example, electrical conductivity of fluorine-carbon materials changing widely from 2x 10's cm-' (fluorine intercalated HOPG C,F) to 1 x lO-'"S cm-' (graph-

ite fluoride (CF),,) These suggest that fluorination is an effective way to prepare carbon alloys with new functionalities

Of the many types of carbon materials, the chemistry of carbon nanotubes is of

considerable interest in terms of alloying by heteroatom-doping or intercalation and

of side-wall chemical functionality Both single-wall carbon nanotubes (SWNTs) and multi-wall carbon nanotubes (MWNTs) are amphoteric and form donor and acceptor compounds such as graphite intercalation compounds (GICs) [2-61 These properties together with their 1D type host lattice structure, with a central tube (canal), suggest

energy-related applications such as the secondary lithium battery, electric double

layer capacitors and fuel cells In this chapter, control over carbon properties by fluorination is presented with an emphasis on the alloying of carbon nanotubes by selective fluorination of their hidden surfaces of MWNTs [7-lo]

2 Control of Carbon Properties by Fluorination

Binding energies of Cls and Fls bonds of fluorinated carbon materials including

fluorination and fluorinehluoride intercalation procedures modify carbon materials [1,9] Figure 1 shows the progressive change in the C-F bonding from ionic, through

to semi-ionic, to covalent as fluorination temperatures increase from room tempera- ture to 600°C The Cls and F l s binding energies of the graphite fluorides, (CF), and

indicating the binding energy for a completely covalent C-F bond In these fluorinated compounds the graphene (graphite) layers change to arrays of trans-

a) Cl,binding energies for C-F bonds

Fig 1 XPS binding energies (eV) of fluorinated activated carbon fibers (F-ACFs), hidden surface fluorinated multi-wall carbon nanotubes (F-MWNTs) and other fluorinated carbon materials

Propeq Control of Carbon Materials by Fluorination 487

linked cyclohexane-type chairs with sp3-hybridized carbons, and are, consequently,

transparent On the other hand, graphite intercalated by fluorine at room tempera- ture (graphite C,F) is ionic and metallic blue in color In this graphite intercalation compound (GIC), graphene layers with sp*-hybridized carbons are maintained The bonds between fluorine and carbon atoms are not covalent but are ionic The Cls

to that of LiF For activated carbon fibers (ACFs), the Cls binding energy increases from 288.5 to 289.7 eV as the fluorination temperature increases from room temperature to 200°C with the Cls binding energy being between that of (CF),, and

and yellow, to white following fluorination in the temperature range 20-200°C The

conditions The characteristic variations in color of the F-ACFs can be explained in terms of the nature of the C-F bond, hence the electronic structure of F-ACFs The

behave as ACFs in regard to fluorination

3

Fluorination

The Chemistry of Carbon Nanotubes with Fluorine and Carbon Alloying by

3.1 Side Wall Fluorination of Single-wall Carbon Nanotubes

Nanometer-sized carbon tubes (nanotubes), a novel form of carbon, have currently attracted considerable scientific interest and nanotechnological interest as applica- tions to electronic devices and energy-related technologies Progress in bulk

in sufficient quantities are now available for research into the chemical reactivity of SWNTs and MWNts

Mickelson et al [ll] were the first to report the fluorination of purified and

graphite rods in the temperature range 150-600"C for 5 h using fluorine gas diluted with helium Compositions of the products, fluorinated at 150,250,325, and 400°C were CF,,., ,4, CF,,,,,, CF,l,495, and CF,,,, respectively The tube-like morphology of the

the tube

length of the ca 10 mm x 3 mm x 30 km bucky paper samples) Tubes fluorinated at

TEM studies showed that with fluorination at 325”C, the majority of the nanotubes maintained their tube-like morphology

nation, and the information of chemical state in terms of the surface element

distribution/concentration and chemical structures of these materials are of interest

in connection with possible applications

3.2 Muhiwall Carbon Nanotubes

average length, 18.4 nm and 3.6 nm in average external and internal diameters,

fluorine intercalated graphite with semi-ionic C-F bonds In contrast, the inner part

3O0-40O0C, fluorinated buckled layers were observed at the surfaces with distances of

0.65 nm, indicating the formation of graphite fluoride layers (2-3 nm in thickness)

M W N T S

diameter), free from amorphous carbon particles, and prepared by thermal decompo- sition of acetylene over silica-supported cobalt catalysts [ 121 Reaction at room temperature using a mixture of F2, HF, and IF, with the MWNTs yielded inter-

ence of the main reflection at interlayer spacings d (002) of 0.340 nm, with diffractions corresponding to two fluorine-intercalated phases with repeat distances of I, = 0.63

and 0.746 nrn TEM images of graphitized MwNTs showed that the tubular morph- ology had been preserved but that the outer layers of the walls were perturbed

3.3 Temphte-synthesized Carbon Nanotubes

anodic aluminum oxide (alumina) films of uniform and straight channels with nanometer-sized diameters Aligned and monodispersed carbon nanotubes with open

Property Control of Carbon Materials by Fluorination 489

ends were successfully synthesized using chemical vapor deposition (CVD) of carbon

within the pores of the alumina films Such open-ended nanotubes embedded in the alumina template can be filled with selected compounds, and chemical functionality

from the template, can be changed by chemical reaction As fluorination effectively perturbs the x-electron systems of carbon then carbon-alloying of the hidden surfaces

of nanotubes in the Al,O, template via fluorination was attempted experimentally

3.3.1 Hidden Surface Fluorination

with ca 30 nm diameter straight channels The reaction cell was initially evacuated to

propylene in the alumina channels deposited carbon on the channel walls giving

reaction of elemental fluorine with the C-Al,O, films, at 0.1 MPa pressure of fluorine,

50-200°C for 5 days, The resulting compounds were washed with an excess of 46% H F

solution at room temperature to dissolve the N,O, template In this way, the

nanotubes, the hidden surfaces of which were selectively fluorinated, were obtained

in the form of an insoluble fraction (see Fig 2) [9]

Carboncoated Azo3 film Carbon nanotubes

,

m

mm -m

Fig 2 Schematic drawing of the fluorination process of carbon nanotubes (reproduced with permission

from Ref [7])

Fig 3 SEM images of the template-synthesized carbon nanotubes fluorinated at 200°C (reproduced with

permission from Ref [9])

3.3.2 SEM and TEM Observations

Figure 3 shows SEM images of the template-synthesized carbon nanotubes fluori-

nated at 200°C and essentially are a free-standing nanoporous carbon membranes In

nanotubes fluorinated at 200°C These images show that the nanotubes have a wall

thickness of -4 nm and a diameter of -30 nm, the outer diameters remaining

unchanged after fluorination

Figures 4a-q show short and wavy 002 lattice fringes in the cross-section of the tube walls, the lines lying parallel to the tube axis The lattice images of the fluori- nated tubes (Figs 4d-f) show that tubular morphology is preserved and that the hidden surfaces of the tubes may be slightly perturbed The 002-lattice fringe image suggests slight modifications to the hidden surface of the tubes after fluorination, such as buckled fluorinated carbon layers

3.3.3 Characterization by EDX, XPS, and Raman Spectroscopy

The selective fluorination of the hidden surfaces of nanotubes is supported further by

EDX and EELS Figure 5 shows the EDX spectrum for a single tube selected from