Hydrogen Generation by Laser Irradiation of Carbon Powder in Water

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Hydrogen Generation by Laser Irradiation of Carbon Powder in Water Ikuko Akimoto, Kousuke Maeda, and Nobuhiko Ozaki J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp4012558 • Publication Date (Web): 19 Aug 2013 Downloaded from http://pubs.acs.org on August 29, 2013

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The Journal of Physical Chemistry

Hydrogen Generation by Laser Irradiation of Carbon Powder in Water Ikuko Akimoto*, Kousuke Maeda, and Nobuhiko Ozaki Department of Material Science and Chemistry, Faculty of Systems Engineering, Wakayama University

ABSTRACT: We report the photochemical activity of carbon powder in the generation of hydrogen from water by laser irradiation, without any electrodes and photocatalysts. The gas was obtained by laser irradiation in the VIS-NIR range and was dependent nonlinearly on the laser power density. From a gas component analysis and a repeated irradiation experiment, it was found that the carbon powder was oxidized and acted as a sacrificial reagent in the photochemical hydrogen generation. In addition, a highly carbonized charcoal, known as Bincho-tan, was found to effectively work for the hydrogen generation.

KEYWORDS: charcoal, water dissociation, sacrificial reagent, carbon oxidation, carbon neutral

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1. Introduction Hydrogen is a promising substitute for fossil fuels and usually produced by steam reforming of hydrocarbons and also by electrolysis, thermolysis and photocalalytic reactions1. The photocatalytic water splitting opened up by titanium oxide (TiO2)2 has attracted much attention because the energy of sun-light can be converted into clean gaseous fuel. To date, various materials, including rare and heavy elements, which are able to response to visible light, have been developed as photocatalysts for water splitting, as reviewed in Ref. 3. In this contribution, we report the optically induced activity of a well-known material, carbon powder, to generate hydrogen from water. The carbon allotropes—graphene, graphite, nanotubes, fullerenes, and diamond— are notable materials for electronic or spin devices.4 Carbon powder is amorphous carbon of unspecified structure, which, however, is characterized as a type of graphite. It has been used as an electrode, daub, or tribological material. There are some reports in which solid carbon assists reactions to generate hydrogen from water. Kawai and Sakata in 19795 and Sato and White in 19816, 7 reported that efficient hydrogen generation could be achieved by the photocatalytic decomposition of water on TiO2 or Pt/TiO2 accompanied by solid carbon under the illumination of a high-pressure mercury lamp. In those reactions, the solid carbon was oxidized by the generated oxygen and assisted photocatalytic hydrogen generation by avoiding the reverse reaction leading to water. It is also reported in electrochemical gasification that coal8,9 or activated carbon10, 11 assisted water electrolysis by reducing required electric energy than conventional water electrolysis. However, optical activity of carbon powder itself has not known thus far. It is worth studying the optically activated functions of carbon powder, because the material is inexpensive and an effective absorber of the sun-light as its black color indicates.


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Carbon is one of the most important elements in the biological cycle on earth. Charcoal, which is a carbon material, is made of wood, and therefore, is an intermediate in the earth’s carbon cycle. From the viewpoint of sustainable development, charcoal is a promising carbon material. A high-quality charcoal, known as Bincho-tan in Japan, is made by steaming the stems of the ubame oak (Quercus phillyraeoides) at extremely high temperature, ~1000 °C. 12 It removes pyroligneous acid and results in Bincho-tan charcoal with 93 % high carbon content than normal charcoal.S1 Although many practical applications have been found for Bincho-tan charcoal besides the common fuel usage, any light-responsive activities have not yet been identified. In this study, we focus on the optical functions of a commercial high-grade carbon powder and the Bincho-tan charcoal powder. We found that hydrogen was generated when a mixture of carbon powder and distilled water was irradiated by intense nanosecond laser pulses at room temperature. Electrodes or any other photocatalysts were not necessary for such hydrogen generation. Therefore, the present hydrogen generation mechanism is much different than the well-known water splitting with a semiconductive photocatalyst, such as TiO22 or nano-diamonds13, in which optically generated carriers cause the reaction. We discuss the present reaction in comparison to those of coal gasification.14-16 2. Experimental We used high-grade carbon powder (A) (SEC, SCN-5, 99.5%) or Bincho-tan charcoal powder (B) (Latest Coop., Wakayama, Japan) of a mean diameter 5 µm with diameter distribution width 3.5 µm, which is much smaller than a porous structure in a typical charcoal. From BET surface analysis, surface area of the Bincho-tan charcoal powder (B) (25 m2/g) was twice of the high-grade carbon (A) (13 m2/g). Image of powder samples was observed by scanning electron microscopy (SEM) (JEOL, JSM-6300), where the powders were


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prepared by drying under nitrogen gas atmosphere. We could not find other differences in relative density or elemental analysis by EDX between (A) and (B).S2 In a bottle with a small Teflon®-coated stirring bar, the carbon powder (A) or the charcoal powder (B) was dispersed in distilled water at a ratio of 25.8 mg:9.5 mL or 145 mg:100 mL for small- or large-scale experiment. The mixture was irradiated by an unfocused beam (6.2 mm in diameter) of 5 ns laser pulses from a tunable optical parametric oscillator excited by a Q-switched YAG laser operated at 10 Hz repetition rate (Spectra Physics, MOPO or Continuum, Surelite) for 30 min with magnetic stirring. The generated gas was collected from the full bottle into a glass tube through a sealed Teflon® or silicon tube by the waterdisplacement method under air. Remaining bubbles in the bottle were displaced by water pressure applied by syringe or peristaltic pump. The collected gas volume was measured with a scale on a tube at a resolution of 0.05 mL. The wavelengths of the laser pulses were selected in the VIS-NIR region. Gas components, which were generated by irradiating with 532 nm radiations at 166 mJ/cm2 (50 mJ) for 60 min, were analyzed offsite by quadrupole mass spectrometry after two days of transport (Nuclear Engineering Co.,Ltd., Ibaraki, Japan). We selected a sodium glass tube as a container because it is known that small molecules such as hydrogen or helium penetrate less into sodium glass than into the commonly used borosilicate glass (Pyrex®).17, S3 The portion of molecules N2, CO, and C2H4 of the same molecular mass at 28 was determined by filtered partial presser measurements and mass fragments at N and C in quadrupole mass spectrometer. We compared the components of two gas samples generated under different circumstances. One was generated under air using distilled water, which normally contains a certain amount of dissolved air. The other was generated under argon atmosphere inside a


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large bag with an optical window using degassed water, which was prepared by freeze/pump/thaw cycles, and finally placed under argon. 3. Results Figure 1 shows the typical appearance of a reaction: photographs of the bottled mixture before (a), during (b), and after (c) irradiation. Bubbles of the generated gas can be observed in (b) just above the irradiation site. Interestingly, as shown in (c), the powder after irradiation showed some aggregation, and color-less, transparent water appeared soon after stirring was stopped, in contrast to the behavior of the suspension before irradiation (a). In this demonstration, ca. 80% of the container was filled with water to better observe gas generation in the closed bottle. The gas sounded explosive when it was ignited immediately after opening the cap. The water temperature rose from 22 to 29 °C during the 30-min irradiation at 182 mJ/cm2 (55mJ) for 9.5 mL volume of water. By comparison, no gas was generated from pure water itself under the same irradiation conditions. This is understandable because ordinary water splitting requires an endothermic energy of 237 kJ/mol,2 which corresponds to an energy absorption of 2.5 eV per molecule of water. The irradiation photon energy of 2.3 eV was not sufficient for water splitting by a one-photon process.

gas bubbles laser irradiation

(a)

(b)

(c)


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Figure 1. Photographs of a bottled mixture of Bincho-tan powder and water (a) before, (b) during, and (c) after irradiation at 532 nm. Pictures (a) and (c) were taken 30 min after stopping the stirring.

The gas was efficiently generated at higher laser power densities. Figure 2 shows the laser power dependence of a generated gas volume after 30 min of irradiation at a laser wavelength of 532 nm for both carbon powders (A) and (B). Gas volumes of >0.05 mL were detectable above a power density of ca. 50 mJ/cm2. No gas evolution was observed at a weak laser power density of 1.7 mJ/cm2 (0.5 mJ with a diameter of 6.2 mm). When we used a pristine stem block of Bincho-tan charcoal and focused the beam to 100 mJ/cm2 (0.5 mJ with 0.8 mm diameter) on the block, gas bubbles were visible. Therefore, the gas yield depended nonlinearly on the laser power density. The gas volume generated with Bincho-tan charcoal powder (B) was almost twice that of the high-grade carbon powder (A) under the same irradiation conditions, while the threshold power densities were nearly coincident. We considered that the higher efficiency in (B) was partially attributable to the twice larger surface area described in previous section than that of (A). In addition, we also measured that generated gas volume increased by 1.6 times when a smaller Bincho-tan powder (mean diameter 1 µm, SA=120 m2/g) was irradiated with the same condition. The increase ratio of gas volume (1.6) was less than that of surface area (4.8). As the BET surface area of the Bincho-tan (B) was only twice that of the high-grade carbon powder (A), therefore, there must be additional reasons of the twice efficiency in the Bincho-tan (B). For example, rarely included ions absorbed from soil possibly act as catalysts in Bincho-tan. Hereafter, we present the results obtained using Bincho-tan charcoal powder (B).


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(A) carbon (B) charcoal

Gas Volume (mL)

1.5

λexc=532 nm

1.0

exc. time 30 min 0.5

0.0 0 50 100 150 2 Incident Laser Power Density (mJ/cm )

Figure 2. Laser power dependence of generated gas volume with irradiation at 532 nm for 30 min, with high-grade carbon powder (A) (open circles) and Bincho-tan charcoal powder (B) (solid circles).

Figure 3 shows the dependence of the generated gas volume on the irradiated wavelength for 30-min at a laser power density of 112 mJ/cm2 (34 mJ), except for a point at 850 nm (enclosed by a bracket), which was obtained at a lower power density of 73 mJ/cm2 (22 mJ) due to used apparatus limit. Optical reflectivity of a polished Bincho-tan block is also shown in the upper panel. Gas yields were obtained at irradiated wavelengths in the VIS-NIR range, although the obtained volume at 850 nm was underestimated. A tendency for a reduction in gas yield was observed at longer wavelengths, anti-correlating to the reflectivity.

R (%)

4

Gas Volume ( mL )

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2 0

0.4

0.2

( ) 0.0 400

500

600

700

800

900

Irradiated Wavelength (nm)


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Figure 3. Irradiated wavelength dependence of the generated gas volume (solid circles) by pulse irradiation of 112 mJ/cm2 (34 mJ) for 30 min, except for a point (enclosed by a bracket) at 850 nm by 73 mJ/cm2 (22 mJ). In upper panel, optical reflectivity spectrum of a polished Bincho-tan block in the same wavelength range is shown.

We found the degradation of the carbon powder’s function in the gas generation as shown in Fig. 4. At the first 60-min irradiation, almost twice amount of that shown in Fig.2 at 144 mJ/cm2 was generated corresponding to the twice prolonged irradiation time. We checked linear increase of gas generation by every 10-min irradiation up to 60 min. However, the amount of the generated gas decreased upon repeated irradiation for 60 min, where the same powder was repeatedly irradiated by laser but the water was freshly replaced each time. The carbon powder became less effective for gas generation, indicating that the carbon powder was modified by the photochemical reaction.

Gas Volume (mL)

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2

1

0

1

2

3

4

5

6

Repeated Number

Figure 4. Generated gas volumes obtained by repeated irradiation at 532 nm for 60 min at 144 mJ/cm2 (45 mJ), on the same powder (B), replacing the water with fresh one each time. The error bars indicate deviations by several experiments.


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The surface morphology of the carbon particles was also observed. Figure 5 shows typical SEM images of a Bincho-tan powder before (a, b) and after (c, d) irradiation. As the particles were grained to be smaller than a typical porous structure in a charcoal, we observed solid particles with mean size of 5 µm (a, c). The surface of the carbon particle was changed by the irradiation: the sample showed smooth surfaces and sharp edges before irradiation (b), while it showed rough surfaces and dull edges after irradiation (d). The rough surface might induce the postirradiation aggregation of particles, as shown in Fig. 1(c).

(a)

(c)

(b)

(d)

Figure 5. Typical SEM images of pieces of the Bincho-tan powder before (a,b) and after (c, d) irradiation, with wide (a, c) and focusing (b, d) views.

Table 1 shows the components of the two gas samples generated under different conditions, in comparison to those of the standard air. The gas generated under argon atmosphere consisted of 48.7% hydrogen, 20.5% carbon monoxide, 0.513% carbon dioxide, and others,


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excluding the argon portion of 61.0% that was exuded from the argon-purged water. The amount of oxygen (1.28%) was lower than the expected value for ideal water splitting, i.e., half that of hydrogen. On the other hand, the gas generated under air consisted of 32.5% hydrogen, 11.0% carbon monoxide, 0.100% carbon dioxide, 0.200 % methane, and others. The nitrogen content of 46.0% indicated that air was emitted from normal distilled water; therefore, oxygen of at least one-fourth the amount of nitrogen should be included. However, the oxygen content was low at 7.10%, which indicated that the oxygen dissolved in water was consumed in the photochemical reaction. Therefore, instead of ideal water splitting, carbon reacted with water to produce carbon monoxide, carbon dioxide and methane. Consequently, it was concluded that the carbon powder acts as a sacrificial reagent in the photochemical hydrogen-generation reaction. Table 1. Ratio of components included in the gases generated with carbon powder (B) at 1 atm in argon-purged conditions or in ambient air. The upper (lower) values in the second row exclude (include) the argon component. H-C abbreviates hydrocarbon except methane. For comparison, the components of standard air (G1) are also listed, where the measured hydrogen amount 0.0500% was higher than a well-known standard one due to residue gas and moisture in a vacuumed chamber attached by a mass spectrometer.

(%)

H

O

CO

CO

CH

N

Gene. gas

48.7

1.28

20.5

0.513

0.260

(in argon)

(19.0)

(0.500)

(8.00)

(0.200)

32.5

7.10

11.0

0.0500

19.0

0.000

2

2

H-C

HO

Ar

5.13

23.1

0.513

-

(0.100)

(2.00)

(9.00)

(0.200)

(61.0)

0.100

0.200

46.0

2.60

0.100

0.600

0.170

0.0100

79.6

0.0100

0.360

0.830

2

4

2

2

Gene. gas (in air) Std. air


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4. Discussion Oxidation of carbon has been investigated as carbon combustion, gas generation, the oxidation of graphite, and the etching of graphite under extreme conditions, i.e., at high temperature, high pressure, or with a hyperthermal atomic oxygen beam.16–19 In the present study, the reaction between carbon and water proceeded by intense light irradiation, generating macroscopic amounts of hydrogen. A clue to understand what reactions emit hydrogen generation from water might be found in the classic technique of coal gasification, that is, the steaming of coal at high temperature (~800 °C) and high pressure (a few MPa) (HTHP).14-16 In this process, carbon reacts with water via the following:

Cs H Og → H g COg,

(1)

Cs 2H Og → 2H g CO g,

(2)

3 Cs O g H Og → H g 3COg,

(3)

2 Cs O g 2H Og → 2H g 2CO g,

(4)

2 Cs 2H Og → CH g CO g.

(5)

Let us speculate possible reactions based on the analyzed ratio of products. Although it is inconclusive discussion based on rough estimation, the consideration help us to understand the reaction mechanism. Notably, hydrogen-to-carbon monoxide ratio was 2.4 : 1.0 under argon atmosphere and 3.0 : 1.0 under air. The carbon dioxide content also increased under argon atmosphere. It was difficult to quantify the evolved amount of carbon dioxide, because it is soluble in water. We observed decreases of pH of water by one reproducibly after irradiation at several times, indicating dissolution of carbon dioxide into water. From the law of mass action, assuming 100% of carbon dioxide ionized to HCO3-/CO32-, we calculated that


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carbon dioxide evolved only one four-hundredth of generated hydrogen. Therefore, we suppose that the reaction (1), modified for liquid-phase water, mainly progresses under argon atmosphere accompanying with less rate of the reaction (2), while reactions (1) and (3), modified for liquid-phase water, mainly progress under air in a certain mutual ratio accompanying with less rate of the reactions (2) and (4). Reactions in which carbon monoxide is consumed, for example

COg H Og → H g CO g,

(6)

possibly proceeded under laser irradiation and changed the hydrogen-to-carbon monoxide ratio, as it happened in coal gasification.15 From the amount of methane and other hydrocarbon in Table 1, the reaction (5) and another reactions producing hydrocarbon also occur at certain rate. From the nonlinear laser-power dependence of gas generation with the threshold behavior shown in Fig. 2, a multiple-photon-absorption process is expected to realize such reactions. We estimated that 64% of the irradiated laser power was used in the reaction, because 10% of the incident laser power was lost as reflection by a glass container while 26% was converted to heat energy, raising the temperature by 7 °C. Here, we calculated the gas yield by assuming that the detected gas volume is the same as the generated gas volume. As a result, an ideal gas amount of 1.8 × 1019 molecules was generated by incident pulses of 55 mJ during the first 30min irradiation under air. This corresponded to a single molecule of ideal gas being generated by 90 photons. If only a multi-photon process is involved in the reaction, a much higher yield would be expected by a shorter pulse with higher peak power. We performed an irradiation experiment with pulses of extreme peak height and short width (800 nm, 2.5 W, 1 kHz, 30 fs, 2 mm in diameter), affording a laser power density of 80 mJ/cm2, which is of sufficient density to


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generate gas in the case of nanosecond pulses. However, no bubbles were detectable and only the water temperature rose by more than 20 °C. This fact indicated that a pulse duration greater than sub-pico-second is necessary for observable gas generation. Therefore, it is implied that gas generation occurs through a sequence of several excitations, one of which requires multiple-photon absorptions while some of which occur after lattice relaxation of a few pico- or nano-seconds. Certain excited states, which are similar to those achieved in coal during gasification under the HTHP conditions, are possibly stepped in the present carbon material upon the laser irradiation, although the details of the excited states are still unsolved. The possibility of the achievement of local HTHP conditions by laser pulses still could not be eliminated. Finally, we mention the efficiency of hydrogen generation in the present study in comparison with two carrier assisted reactions. 11, 13 We obtained 2.1 mL of gas after 1 h of irradiation with a 45 mJ/pulse at 532 nm under air atmosphere, as shown in Fig. 4. According to Table 1, hydrogen should be present at a level of 33%, so that the amount of hydrogen generated could be calculated as 0.027 mmol. This is corresponding to 1.5 L hydrogen generation by 1 kWh laser pulses powder, which is much less than 450 L that was estimated in the carbon-assisted electrochemical hydrogen generation by 1 kWh electric power.11 On the other hand, our amount is comparable to the production by a photocatalytic water reduction with hydrogenterminated nano-diamonds,13 in which 0.06 mmol hydrogen was generated for the first hour of irradiation with 80 mJ/pulse at 532 nm. Taking into account the difference in the irradiated pulse power and the similar threshold phenomenon of laser-power dependence, the obtained hydrogen amount is comparable to present study, although the generation mechanism is so much different.


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5. Conclusion We generated hydrogen gas from a mixture of pure carbon powder and water via irradiation with intense nanosecond laser pulses without any electrodes or photocatalysts. The generated gas mainly included hydrogen and carbon monoxide. Consequently, it was shown that the carbon powder worked as a sacrificial reagent rather than a reusable photocatalyst. A highly carbonized charcoal, Bincho-tan, was found to work effectively for hydrogen generation. The admixture of carbon monoxide in the generated hydrogen is a disadvantage of our method; nevertheless, the ready availability and low cost of carbon material are significant advantages. The present method is expected to be useful in certain circumstances, e.g., the demand for hydrogen gas in a tiny container such as a lab-on-tip device without an electrode (wireless).

AUTHOR INFORMATION *Corresponding Author Sakaedani 930, Wakayama 640-8510, Japan. Phone Number: +81-73-457-8295 E-mail: [email protected]

ACKNOWLEDGMENT This work was supported by the Original Research Support Project of Wakayama University 2011–2012. The authors greatly acknowledge Mr. M. Hamazaki of the Nuclear Engineering Co., Ltd. for gas analysis by quadrupole mass spectrometry. The authors thank Mr. M. Shirai of iCeMS, Kyoto University, for femto-second laser irradiation and Prof. M. Hashimoto, Wakayama University, for discussion about pH calculation.


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SUPPORTING INFORMATION AVAILABLE Information regarding to carbonation degree of Bincho-tan powder; practical usage of Bincho-tan; or retention capacity of hydrogen in several kind of glasses. This material is available free of charge via the Internet at http://pubs.acs.org.

ABBREVIATIONS HTHP, high temperature and high pressure; SEM, scanning electron microscopy; TiO2, titanium dioxide; XPS.

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Soc. Rev. 2009, 38, 253. 4. Krueger, A. Carbon Materials and Nanotechnology; Wiley-VCH: 2010. 5. Kawai, T.; Sakata, T.; Hydrogen Evolution from Water Using Solid Carbon and Light Energy. Nature 1979, 282, 283. 6. Sato, S.; White, J.M.; Photocatalytic Reaction of Water with Carbon over Platinized Titania. J. Phys. Chem. 1981, 85, 336. 7. Sato, S.; White, J.M.; Photodecomposition of Water over Pt/TiO2 Crystals. Chem. Phys.

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37, 9514-9523. 12. Tatsumoto, H., Ed.; A Book of Charcoal, B&T books; Tokyo: 2002 (in Japanese). 13. Jang, D.M.; Myung, Y.; Im, H. S.; Seo, Y. S.; Cho, Y. J.; Lee, C. W.; Park, J.; Jee, A.-Y.; Lee, M.; Nanodiamonds as Photocatalysts for Reduction of Water and Graphene Oxide.

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Society 168th National Meeting, p.85-93. Atlantic City, September 1974. 17. Norton, F. J.; Helium Diffusion through Glass. J. Am. Ceram. Soc. 1953, 36, 90. 18. Nicholson, K. T.; Minton, T. K.; Sibener, S. J.; Spatially Anisotropic Etching of Graphite by Hyperthermal Atomic Oxygen. J. Phys. Chem. B 2005, 109, 8476−8480. 19. Larciprete, R.; Lacovig, P.; Gardonio, S.; Baraldi, A.; Lizzit, S.; Atomic Oxygen on Graphite: Chemical Characterization and Thermal Reduction. J. Phys. Chem. C 2012, 116, 9900−9908.


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Hydrogen Generation by Laser Irradiation of Carbon Powder in Water

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