Sistema Nanotubos de Carbono

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Simultaneous Production of Hydrogen and Carbon Nanotubesin a Conventional Microwave Oven

S. Nomura1, H. Yamashita2, H.Toyota1, S.Mukasa1, Y. Okamura2

1 Department of Mechanical Engineering, Ehime University, Ehime, Japan

2 Department of Applied Chemistry, Ehime University, Ehime, Japan

Abstract: Hydrogen and carbon nanotubes (CNTs) were produced by generating plasma in an

organic solvent. The 2.45GHz microwave in-liquid plasma is generated by using a conven-

tional microwave oven. The hydrogen generation efficiency using the in-plasma method is

approximately 34% that of the electrolysis of water. When cyclohexane is used as the source

liquid, multi-walled carbon nanotubes (MWCNTs) having a diameter of approximately 30 nm

can be grown on porous silica supported by Mo and Co dip coating technique. This process

enables hydrogen and MWCNTs to be simultaneously manufactured by the in-liquid plasma

method.

Keywords: in-liquid plasma, plasma in liquid, hydrogen, CNT, microwave, bubble

1. Introduction

In-liquid plasma by irradiating High-frequency (HF) or

microwave (MW) is generated in the bubbles in the liquid

[1,2]. The temperature of the gas inside the bubbles can

reach several thousand K, however, since the liquid tem-

perature does not rise above the saturation temperature [3,

4], the insides of the bubbles in the liquid become a

plasma field that can be used in a low-temperature envi-

ronment. It is theorized that large amounts of activated

species exist inside the bubbles due to the raw material

being supplied to the insides of the bubbles by the evapo-ration of the liquid itself. If the in-liquid plasma can be

used as a chemical reactor, we expect that it will become

possible to attain much higher reaction rates than in con-

ventional gas-phase plasma. The authors have been pro-

posed applying in-liquid plasma as a replacement for

gas-phase plasma.

Up to now the authors have revealed that when 2.45

GHz microwave plasma is generated in an organic solvent

hydrogen gas with a purity of 70 to 80% can produced [5].

Since the in-liquid plasma method can directly decom-

pose the liquid itself, hydrogen can be extracted from

waste oils, such as engine oil or cooking oil. If flammable

gases such as hydrogen can be synthesized while simul-

taneously solidifying the carbon, it will be possible to

create flammable gases without emitting CO2. With fur-

ther advancements, if the residual carbon is transformed

into nanotech materials at the same time the flammable

gases such as hydrogen from waste liquids are produced,

it can be expected an in-liquid plasma zero emission sys-

tem.

On the other hand, the catalyst chemical vapor deposi-

tion (CCVD) method is one method for synthesis of CNT

[6, 7]. With this method, methane, ethylene, acetylene,

benzene and other such chemicals serve as the source of

carbon supply. Since these are originally liquids, they can

easily be applied to the in-liquid plasma method for use

organic solvent source material. The authors applied the

CCVD method in a liquid, CNTs can be synthesized in

benzene solution dispersed a metal-supported catalyst [9].

Large-volume CNT creation can be expected by using the

in-liquid plasma method.

The purpose of this research is to develop a process that

will produce gas by using the in-liquid plasma method

while simultaneously solidifying the carbon and synthe-

sizing useful carbonized materials, such as CNTs or acti-

vated charcoal, at high speed and low cost. In this study,the simultaneous production of hydrogen and CNT inside

a conventional microwave oven is carried out.

2. Experimental apparatus and procedure

Figure 1 shows the experimental apparatus. With the

exception of the piping used for extracting exhaust gases,

which is mounted on the top of the device, this reactor has

nearly the same design as a commercially available mi-

crowave oven (with a frequency of 2.45.GHz). The mag-

netron, which is the device on the right side, irradiated the

inside of the reactor with 2.45 GHz microwaves. These

microwaves were received by an antenna positioned in the

reactor vessel, generating plasma at the tip of the antenna.

In order to avoid the microwave energy from being ab-

sorbed by items inside the reactor, such as the reaction

vessel, reactor platform and piping were made using

heat-resistant gas and silicone rubber. The power con-

sumption of the reactor when the device was in operation

was 1,260 W, of which 750 W was microwave output.

A schematic of the receiving antenna is shown in Fig. 2.

Seven antennas were positioned on a copper sheet. One

copper rod having a diameter of 1.5 mm was placed at the

center of the copper plate and the six antennas were posi-

tioned at even spacing perpendicularly around a circum-

ference that is 1/4 the wavelength of the microwave pass-



ing through the liquid. The length of the antenna is 21 mm,

which is 1/4 the wavelength of the microwave in consid-

eration of the dielectric constant of the liquid

(n-dodecane: 2.01), so that the electric field will be the

strongest at the tip of the electrode. The antenna unit is

installed on a Teflon ® base so that the reactor vessel willnot be damaged by the heat when the plasma is being

generated.

When the tip of the electrode is in the liquid, plasma

will be generated inside the bubbles inside the liquid,

however, since the surface of liquid will go down as the

liquid is decomposed, the electrode will protrude beyond

the surface of the liquid and plasma will be generated at

the gas phase side above the surface of the liquid. Hence,

the experiment is conducted with the electrode completely

submerged in the liquid.

The production gas is collected by the water substitu-

tion method for the gas that is expelled from the pipeconnected to the reactor vessel.

n-dodecane was used as the experimental liquid in this

experiment to hydrogen production. The receiving an-

tenna for the microwaves is placed on the bottom of the

reactor, then, 500 ml n-dodecane is poured into the reactor.

The inside of the reactor and the piping used for extract-

ing the exhaust gas are filled with argon gas. In this con-

dition, microwaves are irradiated and plasma was gener-

ated. 1000 ml of generated gas was recovered using the

water substitution method.

3. Production of Hydrogen Using a Microwave Oven

Table 1 shows the results of the gas chromatography

the 1000 ml of gas. When the plasma is generated in

analysis of the recovered gas. It took 28 seconds to collect

n-dodecane, low-grade flammable hydrocarbon gases,

such as methane, ethylene and acetylene are generated in

addition to the 74% hydrogen. When only four types of

generated gases are examined, the ratio of hydrogen at-oms and carbon atoms comprising the gas is 1:0.225.

The ratio of these atoms in the n-dodecane is 1：0.462.

The ratio of the number of carbon atoms declined after the

plasma is generated. There are carbon atoms that have not

been included in the generated gas. The following shows

the chemical reaction formula for what is assumed to be

when these carbon atoms are all changed into graphite.

aC12H26(l)

→ nH2H2 + nCH4CH4 + nC2H4C2H4 + nC2H2C2H2 + bC(s)

(1)

where,

a = (2nH2 + 4nCH4 + 4nC2H4 + 2nC2H2)/26 (2)

b = 12a (nCH4 + 2nC2H4 + 2nC2H2) (3)

Here, it is supposed that nH2 :nCH4 : nC2H4 : nC2H2 are the

molecular ratio as shown in Table 1. The molarity of these

mixed gases is expressed as ngas and become

nH12 ＝（0.74/0.98）・ngas (4)

nCH4 ＝（0.02/0.98）・ngas (5)

n C2H4 ＝（0.74/0.98）・ngas (6)

n C2H2 ＝（0.20/0.98）・ngas (7)

The enthalpy of formation per 1 mol of gas in the reac-

tion in eq.(1) is shown below.n-dodecane:12C(s) + 13H2 → C12H26(l)

Δ H （C12H26）= －350.9 kJ/mol (8)

Methane: C(s) + 2H2 → CH4(g)

Δ H （CH4） = －74.87 kJ/mol (9)

Ethylene: 2C(s) + 2H2 → C2H4(g)

Δ H （C2H4） = 52.47 kJ/mol (10)

Acetylene:2C(s) + H2 → C2H2(g)

Δ H （C2H2） = 226.73 kJ/mol (11)

If these four chemical reaction formulas are substituted

in eq. (1),

aΔ H （C12H26）+ nCH4 Δ H （CH4）+ nC2H4Δ H （C2H4）+ nC2H2Δ H （C2H2）＝73.9ngas (12)

is obtained. The enthalpy of formation per 1 mol of gas in

the reaction in eq. (1) is 73.9 kJ/mol.

Present experiment that was performed by consuming

1260 W of electricity, it took 28 seconds to produce the

1000ml of gas. When the reactor enthalpy in eq. (1) is

Table 1 Contents of gas generated by MW in-liquid plasma in

n-dodecane

Product H2 CH4 C2H4 C2H2

Vol. % 74 2 2 20

Fig.1 In-liquid plasma reactor using a

conventional microwave oven.

Fig.2 Microwave receiving antenna.



converted to a per liter of gas ratio, it is 3.3 kJ/L. Hence,

the maximum 9.4% of electric power consumption is the

amount of energy consumed in the chemical reaction of

eq. (1).

When ratio of generated hydrogen is estimated, we see

that the amount of energy needed to generate 1 mol ofhydrogen converts to 1070 kJ/mol. On the other hand,

from eq. (8) we found that the enthalpy for creating 1 mol

of hydrogen in the reaction where all of the n-dodecane,

which is the liquid that gives the most ideal chemical re-

action, breaks down into hydrogen and graphite, is 27

kJ/mol. From this experiment it was found that the crea-

tion of acetylene is the main cause for the increase in en-

ergy.

The standard enthalpy of formation of acetylene is 227

kJ/mol, which is high in comparison to others, hence, in

the in-liquid production of hydrogen process, there is a

need to find reaction conditions that suppress the forma-tion of acetylene.

Here, by comparing this method with the electrolysis of

alkaline water, the actual production efficiency of hydro-

gen is calculated. Since the required enthalpy for creating

1 mol of hydrogen from water is 286 kJ/mol (H2O→H2 +

1/2O2), if it is supposed that the common value of 80%

for the energy conversion ratio for creating hydrogen

from the electrolysis is used, the required amount of en-

ergy to create 1 mol of hydrogen is approximately 360

kJ/mol. This means that the energy conversion ratio of

this equipment is 34% that of the electrolysis of alkaline

water. Even when the energy consumption of the magne-

tron (750W) is converted, it is 54%, which makes it ex-

pensive at the present time when it is used only for creat-

ing hydrogen. Also, the actual production of hydrogen by

in-liquid plasma method corresponds to approximately

1% of that by natural gas steam reforming method.

At the present time, the cost is high if the process is

limited to only creating hydrogen. However, we can use

the in-liquid plasma method to reduce CO2 emissions to

zero and use it on a wide variety of waste liquids, such as

cyclohexane, benzene, engine oil and vegetable oil.

Moreover, since the internal gas temperatures of the

in-plasma method reach approximately 2,000 to 5,000 K

[9], this process offers the merit of being able to createhydrogen by processing chlorinated lubricants and other

such liquids that are otherwise difficult to break down.

4. Experiments in the synthesis of CNTs

Carbons remain in the liquid after the hydrogen has

been generated. In our previous work [8], a metal sup-

ported catalyst was dispersed into a benzene solution and

synthesis of carbon nanotubes in liquid was successful,

showing that is possible to recover the residual carbons as

CNT. Based on this knowledge, in this research we used

cyclohexane (dielectric constant = 2.02), which has the

same basic structure as CNT. This is because CNT could

easily be formed by decomposing and then reconnecting

the cyclohexane when the plasma is generated. Porous

silica that is supported by metallic catalyst (Mo and Co)

dip coating technique on the antenna was mounted as a

catalyst for growing the CNT. The porous silica has an

average pore diameter of 1.12μm, that is tube shapedwith an interior diameter of 0.86 cm and length of 10 cm.

This tube is cut in half. As shown in Fig. 1, the silica of

this half cylinder was placed in a reactor vessel.

Fig. 3 (a) shows a SEM image. It is confirmed the

presence of CNT on the porous silica that had been coated

with the metal catalyst. The CNT had a diameter of ap-

proximately 30nm and a length of approximately 1μm. It

believes that CNT growth is dependent on the diameter of

the supported metal catalyst inside the narrow pores.

From the TEM image shown in Fig. 3(b), several vertical

stripes on the CNT can be observed. Based on these ver-

tical stripes it means that the tube shaped over-lappingMWCNT were created.

Figure 4 shows the results of the Raman spectrum

measurement. The ratio of the D band and G band is ap-

proximately 2.5. This value is small compared to other

SWCNTs because the present nanotubes are layered sev-

eral times.

The amount of hydrocarbons remaining in the liquid

was measured after the generation of plasma. The time

was recorded until 1000 ml of gas was generated and

performed a quantitative analysis of the carbides remain

(a) SEM image (b) TEM image

Fig. 3 CNTs synthesized by plasma in cyclohexane

500 1000 1500 2000

Raman Shift / cm-1

I n t e n s i t y

/ a r b . u n i t

Fig. 4 Raman spectrum of CNTs grown on porous

silica



ing in the liquid after the generation of plasma. The liquid

was passed through a filter and the mass of the carbides

was measured. These results are shown in Fig.5 .The

maximum amount of carbides produced was at 30 s, after

which the values become approximately 0.1 g.

The amount of carbide experimentally produced is approximately 15% of the production amount that could be

theoretically estimated in chemical reaction process.

However, the recovering carbide is the mass remaining

inside the reactor and excludes the materials that adhered

to the piping and the container for the water substitution

method.

5. Simultaneous production and CNTs

The gas analysis were performed by gas chromatogra-

phy when the plasma was generated in cyclohexane. Just

as with n-dodecane, when the plasma is generated in

cyclohexane, flammable hydrocarbon gases of low mo-lecular weights such as hydrogen, methane, ethylene and

acetylene are generated. The purity of the hydrogen gas

falls to approximately 40 % with the simultaneously pro-

ducing CNTs by using metallic catalyst. The reason for

this difference between hydrogen purity of n-dodecane

and that of simulations production of cyclohexane is not

clear at this moment. The effect of catalyst on hydrogen

production will be the subject to further study. As a result,

the type and amount of the catalyst plays a major role in

the production of not only the CNTs, but in the production

of hydrogen.

While only a slight amount of CNTs were attained, the

simultaneous production of CNTs and hydrogen is suc-

cessful in a simple apparatus that uses a conventional mi-

crowave oven. The residual carbon from the simultaneous

production was not only in the reactor vessel, but also

attached to other pieces of equipment, such as the piping

and the container for the water substitution. In considera-

tion of the results of this experiment, we would like to

conduct further research, including research into the de-

sign of the apparatus that will enable methods that will

solidify a considerable amount of the carbon and collect it

for use as nanotech materials.

5. Conclusion

The experiments in producing hydrogen and CNTs us-

ing a conventional microwave oven were conducted andthe following results were obtained.

(1) The generation of 74% pure hydrogen gas in

n-dodecane is confirmed through in-liquid plasma

generation inside a commercially available micro-

wave oven.

(2) In this experiment, the ratio of hydrogen generation is

approximately 34% that of electrolysis of water.

(3) In cyclohexane, CNT can be grown on porous silica

supported by Mo and Co dip coating technique.

(4) When cyclohexane is used as a solvent, the CNTs and

hydrogen could be simultaneously produced. The use

of in-liquid plasma to simultaneously create hydro-gen and form CNTs by using a conventional micro-

wave offers the new usefulness in technologies for

disposing of waste oils.

cknowledgements

This work was partially supported by Grants-in Aid

from the Ministry of Education, Culture, Sports, Science

and Technology of Japan (No.20360098).

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25 30 35 40 45 500

0.05

0.1

0.15

Plasma outbreak tim e [s]

Q u a n t i t y o f c a r b i d e

[ g ]

Fig. 5 Quantity of carbide remaining in liquid

after plasma generation producing 1000 ml gas.

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Sistema Nanotubos de Carbono