Photographs of a bottled mixture of Bincho-tan powder and water (a) before, (b) during, and (c) after irradiation. Credit: ACS, Akimoto et al. Click to enlarge.

This hydrogen generation mechanism is much different than well-known water-splitting with a semiconductive photocatalyst (such as TiO₂ or nano-diamonds), in which optically generated carriers cause the reaction. In an open access paper on the process is published in the ACS Journal of Physical Chemistry C, the researchers speculated that the reactions may be related to those of coal gasification.

Hydrogen is a promising substitute for fossil fuels and usually produced by steam reforming of hydrocarbons and also by electrolysis, thermolysis and photocatalytic reactions. The photocatalytic water splitting opened up by titanium oxide (TiO₂) has attracted much attention because the energy of sunlight 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...In this contribution, we report the optically induced activity of a well-known material, carbon powder, to generate hydrogen from water.

—Akimoto et al.

In their experiments, the researchers used high-grade carbon powder or Bincho-tan charcoal powder of a mean diameter of 5 μm with diameter distribution width 3.5 μm—much smaller than a porous structure in a typical charcoal.

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). Credit: ACS, Akimoto et al. Click to enlarge.

The carbon or charcoal powders were dispersed in distilled water at a ratio of 25.8 mg:9.5 mL or 145 mg:100 mL for small- or large-scale experiments, respectively. 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 for 30 min with magnetic stirring. The generated gas was collected from the full bottle.

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. Water splitting requires an endothermic energy of 237 kJ/mol, corresponding to an energy absorption of 2.5 eV per molecule of water, the authors noted. The irradiation photon energy of 2.3 eV was not sufficient for water splitting by a one-photon process.

Hydrogen was efficiently generated at higher laser power densities; gas yield varied nonlinearly on the laser power density.

The researchers found that the gas volume generated with Bincho-tan charcoal was almost twice that of the carbon powder, a result they partially attributed to the larger surface area.

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).

—Akimoto et al.

Resources

• Ikuko Akimoto, Kousuke Maeda, and Nobuhiko Ozaki (2013) Hydrogen Generation by Laser Irradiation of Carbon Powder in Water, Journal of Physical Chemistry C doi: 10.1021/jp4012558