(631a) Carbon Catalysts for Hydrogen Iodide (HI) Decompostion Inside the S-I Thermochemical Cycle

Sulfur-iodine cycle is today one of the most widely investigated thermochemical cycles for hydrogen production. It consists of three reactions, whose sum is exactly water decomposition to give oxigen and hydrogen. Hydrogen iodide decomposition is one of these three reactions, the one, indeed, where hydrogen production takes place.

This gas phase reaction, rather insensitive from a thermodynamic point of view to variations of temperature, can anyway proceed faster if temperature is raised and using a proper catalyst. Various catalysts have been already considered for this application, particularly Nickel and Platinum, but also other transition metals, supporting them over alumina, ceria, carbon. Carbon alone has been recently investigated not only as support, but as a catalyst itself.

Our work is focused on the study of hydrogen iodide decomposition using commercial carbon catalysts in pellets (from NORIT) having different particle diameter and surface properties.

These catalysts gave us good results, providing conversion values, at 400-500°C, very close to thermodynamical expected ones, even at high spatial velocity. Also, they did not show any deactivation after more than several days of works. A kinetic mechanism was considered for them and the experimental results fitted on the basis of the related equations, allowing us to get the activation energy and the adsorption parameters.

Since the reaction was studied both feeding in the reactor the azeotropic hydriodic acid solution (57% by weigth of HI in water) ? which is the usual solution coming from the other sections of S-I cycle ? and feeding the pure hydrogen iodide gas, it was also possible to evaluate the effect of water on the efficiency of the reaction. Interference of little amounts of iodine or sulphur compounds (which could follow HI in the reactor as derived from the previous reactions of the cycle) was also considered.

The carbon catalysts investigated, due to the good results of our experimentation and to their low cost and availability, can therefore be considered as an interesting solution for HI decomposition, also for large-scale plant.