Effect of temperature on the gas-phase photocatalytic H2 generation using microreactors under UVA and sunlight irradiation

The effect of temperature on the photocatalytic hydrogen generation from a gaseous water-ethanol mixture has been tested in a silicone microreactor containing nine microchannels of 500 mu m (width) x 1 mm (depth) x 47 mm (length) coated with Au/TiO2 photocatalyst under UVA irradiation. Kinetic analyses have indicated that the hydrogen production rate follows the Langmuir-Hinshelwood model. The effect of temperature from 298 to 348 K has been determined by thermodynamic parameters, such as enthalpy (Delta H-not equal.), entropy (Delta S-not equal.) and Gibbs free energy (Delta G(not equal).) of activation, using the transition state theory (TST). The apparent rate constants (k(app)) are higher by increasing the temperature, and the activation energy has been determined to be 24 +/- 1 kJ.mol(-1). In order to evaluate if solar concentration could be used to enhance the photoproduction of hydrogen, the reaction has also been conducted under direct sunlight using a solar concentrator of about 1m in diameter. Finally, the microreactor has been scaled out by a factor of ca. 10 to a device containing thirty-two microchannels of 500 mu m (width) x1mm (depth) x 117.5 mm (length). The specific (i.e. per irradiated area of catalyst) hydrogen production rates of both microreactors using sunlight are very similar suggesting that this technology could lead to viable solar hydrogen production.