Hydrogen Production Using Reduced-Iron Nanoparticles by Laser Ablation in Liquids

A recyclable energy cycle using a pulsed laser and base-metal nanoparticles is proposed. In this energy cycle, iron nanoparticles reduced from iron oxides by laser ablation in liquid are used for hydrogen generation. The laser energy can be stored in the base-metal nanoparticles as the difference between the chemical energies of iron oxide and iron. According to the results of an experiment on hydrogen production using the reduced iron nanoparticles, the reaction efficiency of the hydrogen generation at a temperature of 673？K was more than 94% for the ideal amount of generated hydrogen. 1. Introduction It has been expected that the application of hydrogen, produced by using clean, renewable energy, such as solar power, will solve the problem of global warming and depletion of fossil fuels. Many researches on hydrogen production have been conducted around the world [1–6]. Among them, the researches on generating hydrogen on the basis of the reaction of a metal with water are particularly interesting for the following reason: the method used to produce the hydrogen is very simple and low cost. Magnesium [2, 3], aluminum [4], and iron [5, 6] have already been used to generate hydrogen. Iron has the advantage of high safety in regard to handling because it hardly reacts with water at low temperature when compared to magnesium and aluminum. In some researches, metal nanoparticles were used to generate hydrogen [6]. It is important to increase the surface area of the metal particles by “micronizing” the particles to improve the reaction efficiency of metals with water. Moreover, the generated metal oxides should return to metals by reducing to realize an energy cycle. The metal oxides can be reduced to the metal and broken into nanoparticles by using pulsed-laser ablation in liquid [7–13]. This process consists of three steps: first, the metal oxide is heated to a high temperature in a short time by irradiating laser pulses onto a metal-oxide powder in the liquid. Second, after the laser pulse is irradiated, metal oxides are evaporated, and metals are atomized; the separated oxygen is rearranged as the outer shell of the metal particles. Third, after the metals are separated from their oxides near the irradiation point of the laser pulse, the metal atoms are aggregated by cooling, and nanoparticles are produced rapidly. As physical mechanisms, namely, the ablation mechanism of the metal oxide in the liquid, coulomb explosions [14, 15] and thermal ablation have been proposed. With this pulsed-laser ablation in liquid, high reduction efficiency is obtained