(83e) Hydrogen Production By Oxidation of Aluminum Nanopowder in Water Under the Action of Laser Pulses

One of the most promising sources of energy for the future is hydrogen, an environmentally friendly fuel with a high calorific value. Currently, up to 96 % of hydrogen is produced from fossil fuels, and another 4 % from water [1, 2]. Several methods are used for producing hydrogen from water: electrolytic [3], photolytic [4, 5], and combinations of these methods [6, 7]. One of the promising methods for producing hydrogen is based on the chemical interaction of water with metals, such as aluminum or magnesium. In particular, the reaction of one gram of aluminum with water can produce 1.24 liters of hydrogen under normal conditions [8, 9]. The disadvantage of this method is that under normal conditions, metal plates and micron-sized aluminum particles have low activity due to the dense oxide film [10].

The interaction of aluminum nanopowder with water was studied in [11, 12]. The authors of [11] heated an aqueous suspension of aluminum nanoparticles to a temperature of 64–66 °C, which led to an acceleration of the reaction, hydrogen evolution, and water heating. The transformation conditions were found under which the residual content of metallic aluminum in the reaction products reached a minimum value of 1.42 wt %.

In this work, instead of heating, it is proposed to use the action of laser pulses on an aqueous suspension of aluminum nanoparticles in order to achieve complete conversion of aluminum and the maximum rate of hydrogen release.

Materials and methods

The experiments were carried out using aluminum powder prepared by gas-phase synthesis at the Institute of Metal Physics, Ural Branch of the Russian Academy of Sciences. The arithmetical mean size of the particles ranges from 100 to 120 nm. The fraction of metallic aluminum in nanoparticle was equal to (65.0 ± 0.6) wt %.

The experiments were carried out by the following method. 35 mL of distilled water was poured into a glass test tube and 10.5 mg of aluminum nanoparticles was added, which corresponded to a concentration of 0.03 wt %. The test tube was placed in an ultrasonic bath and sonicated for 20 min for uniform distribution of aluminum nanoparticles in the volume. Then the suspension was placed in an experimental 57 mL cuvette, which was sealed. In the initial state the cuvette is not transparent to laser radiation. Sedimentation of nanoparticles becomes noticeable in ca. 3 hours after the start of light transmission in the upper part of the cuvette. Hence, the distribution of nanoparticles over the volume during the experiment can be considered as homogeneous.

The functional diagram of the experimental setup is shown in Fig. 1.

A YAG:Nd³⁺ laser (1064 nm, 14 ns, 10 Hz) was used as a source of laser radiation (1). The energy density was controlled using neutral light filters (2). The laser beam through the lens (3) directed perpendicular to the lateral surface of the cuvette. At a lateral surface of the cuvette the beam diameter was 5 mm. On the opposite surface of the cuvette the beam diameter was 4.6 mm. Thus, in the case of weak absorption, laser energy density in the cuvette is nearly uniform. There are two sealed outlets on the cuvette, which were connected to valves (5) and (6) using a gas sampling line. The gas generated as a result of laser action is directed by means of a valve (6) into a measuring cylinder (7) filled with water, placed in a vessel with water (8), or to a gas analyzer (9). The volume of the formed gas was determined by direct measurement of the water displaced from the measuring cylinder (7) with an accuracy of 0.1 mL. The gas composition was determined using an SRS QMS 300 gas analyzer (9). When determining the gas composition, the air in the cell (4) and gas sampling lines was blown with argon through the valve (5).

Experimental

The experiments were carried out at room temperature T₀ = 25 °C. In the absence of radiation, a release of hydrogen from the suspension for 1 hour was not observed.

Under laser irradiation, as a result of the reaction between aluminum and water, water separation occurs with the formation of hydrogen. The dependence of the increase in the volume of gas in the measuring cylinder on the time of irradiation of the suspension by laser pulses with a frequency of 10 Hz was measured. The radiation energy density used was 0.5; 1.0; 2.0; 3.0 and 6.0 J/cm². The energy of laser pulses on a spot 5 mm in diameter was varied within 0.1–1.18 J. In each experiment, we used the same weighed amount of aluminum nanoparticles in water equal to 10.5 mg. The result is shown in Fig. 2.

The maximum volume (V_m) of the produced gas was the same for all laser energy densities used and amounted to V = 9 mL. In this case, the time to reach the maximum value depends on the laser radiation energy density H, varying from 60 min at H = 0.5 J/cm² to 3.5 min at H = 6.0 J/cm². The suspension in the cell, which had initially gray color, during irradiation was bleached and became transparent at attaining the V_m.

The self-heating temperature of the total mass of the suspension during the reaction did not exceed 15 °C.

Experimental points in Fig. 2 are approximated by the formula

V = V_m [1 - exp(-t/Ï„)] (1)

The characteristic times Ï„ of increasing the gas volume to 0.63V_m under the exposure of the suspension at a certain laser energy density are listed in Table 1; correlation coefficients R² are also listed there. Thus, dependences of the volume of nascent gaseous products on the exposure time of suspension at the fixed energy densities are satisfactorily described by formula (1).

With the gas analyzer SRS QMS 300, the composition of the gaseous products of the interaction of aluminum nanoparticles with water was studied after the pulsed-periodic laser exposure for 5 minutes with an energy density of 2.0 J/cm². The result is shown in Table 2.

A control experiment was performed. Water without aluminum nanoparticles was irradiated under similar conditions. Within the measurement error of a gas analyzer, oxygen and hydrogen were not detected. Hence, the hydrolysis of water is absent.

Tables 1 show the characteristic times needed to decrease the concentration of metallic aluminum Ï„ in the suspension by a factor of 2.72 depending on the laser radiation density H. The Ï„ values were used to plot the dependence 1/Ï„ = f(H). The result described by the linear dependence is displayed in Fig. 3. Hence, the characteristic time of metallic aluminum conversion in nanoparticles can be described by the hyperbola formula:

τ = α/H (2)

where α = 4.5 (min∙J/cm²) was estimated from the slope of the line in Fig. 3. The α value indicates the characteristic time required for decreasing the concentration of metallic aluminum by a factor of 2.72 under the action of a unit laser energy density.

Conclusions

It was shown that hydrogen can be produced under the action of laser pulses on the suspension containing 0.03 wt % of aluminum nanoparticles in water (10.5 mg of aluminum in 35 ml of water). Aluminum nanoparticles were found to absorb the energy of laser radiation. The maximum values of the produced hydrogen volume in the laser energy density range of 0.5–6 J/cm² are close and amount to 8.4 mL. The time of complete transformation of metallic aluminum decreases with an increase in the energy density of laser radiation according to the hyperbolic law and reaches 3.5 min at H = 6 J/cm².

References

1. Wang HZ, Leung DYC, Leung MKH, et al. Renew Sust Energ Rev 2009;13:845–53.
2. Balat M. Int J Hydrogen Energy 2008;33:4013–29.
3. Kaya MF, Demir N, Albawabiji MS, et al. Int J Hydrogen Energy 2017;42:17583–92.
4. Juodkazytė J, Seniutinas G, Šebeka B, et al. Int J Hydrogen Energy 2016;41:11941–8.
5. Huang C, Linkous CA, Adebiyi O, et al. Environ Sci Technol 2010;44:5283–8.
6. Geraldo de Melo Furtado J, Gatti GC, Serra ET, et al. Int J Hydrogen Energy 2010;35:9990–5.
7. Chang ACC, Chang H-F, Lin F-J, et al. Int J Hydrogen Energy 2011;36:14252–60.
8. Shevchenko VG, Kononenko VI, Lukin NV, et al. Combust Explos Shock Waves 1999;35:77–9.
9. Tsuchida T, Hasegawa T, Kitagawa T, et al. J Eur Ceram Soc 1997;17:1793–5.
10. Pityulin AN, Shcherbakov VA, Borovinskaya IP, et al. Combust Explos Shock Waves 1979;15:432–7.
11. Godymchuk АYu, Ilyin АP, Astankova АP. Bulletin of the Tomsk Polytechnic University 2007;310:95–7.
12. Ilyin AP, Korshunov AV, Tolbanova LO. Bulletin of the Tomsk Polytechnic University 2007;311:10–4.