抄録
Photo-catalysis has been attracting interest of many researchers and intensively studied, increasing expectation of its application. However, only few studies have been reported from the point of view of the reaction engineering, although such studies are necessary for its successful application. The authors studied reaction engineering matters such as the effects of temperature, concentration of reactants and catalysts, and intensity and direction of irradiation on photocatalytic reaction rate of hydrogen generation from H2S. H2S is a waste material from fuel oil desulfurization process, which consumes hydrogen. Hydrogen is produced by reforming methane, emitting CO2 to the atmosphere. Hence hydrogen production from H2S using solar light is expected to reduce CO2 emission. CdS-ZnS photocatalyst, which can be activated by solar light, was co-precipitated by adding Na2S to water solution of Cd(NO3)2 and Zn(NO3)2. The catalyst was spread over the bottom of a beaker containing 100 ml of alkaline water dissolving H2S and irradiated by Xenon lamp. The photocatalytic hydrogen generation rate was independent of catalyst amount as far as the photocatalyst particles cover the bottom surface. The reaction rate increased with but less than in proportion to the irradiation intensity, suggesting that a rate limiting step other than photo-activation exists, for example, adsorption of the reactants at the catalyst surface.
The activation energy of the reaction was estimated at 38 kJ mol-1 which suggested that an elevated reaction temperature would be advantageous for industrial application. The reaction rate is the first order of the reactant concentration, suggesting that the diffusion or adsorption of the reactant could be a rate limiting step. The photo-energy efficiency defined as the heat of combustion of hydrogen per irradiation energy was 8%, which is comparable to that of a-silicon solar cell photovoltaic followed by water electrolysis, and the external quantum yield was 19% at 313K, 0.24 M H2S, and 0.023Wcm-2 irradiation.
