(166f) Decomposition of Hydrogen Iodide in the S-I Thermochemical Cycle Ni Catalyst Systems

One of the most promising routes currently under development for sustainable hydrogen production from water is the Sulphur-Iodine thermochemical cycle, based on three main reactions carried out at three different temperature levels.

Among these reaction, the hydrogen iodine decomposition presents a rather low equilibrium conversion even at high operating temperatures (the equilibrium conversion at 750 K is about 0.22) and subsequently a considerable energy expense for the separation and recirculation of the unreacted species is required. In addition it is well known that the homogeneous gas phase decomposition of hydrogen iodine has a very low rate and the use of a catalytic system is essential for the improvement of the whole cycle thermal efficiency.

In literature different catalytic systems were found effective and in particular the Pt supported alumina showed very good performances with respect to activation energy values and lifetime.

The current focus of this work is the characterization of nickel supported ã-alumina catalysts, which have the unquestionable advantage of a limited cost in comparison with Pt based systems but their catalytic effectiveness and lifetime have to be investigated. In particular three ã-alumina supported nickel catalysts obtained from different preparation methods were investigated, focusing on the relationships between catalytic activity and preparation procedure. Two of them were prepared from ã-alumina by impregnation-calcination method and were obtained from the deposition of two different precursors, such as nickel acetylacetonate and nickel nitrate. The third one is a nickel aluminate catalyst obtained by coprecipitation method. From the experimental data acquired it can be concluded that two of the three catalysts tested demonstrated high catalytic activity, since HI conversion was almost coincident with theoretical equilibrium value. On the other side, for all the catalysts, a gradual but considerable deactivation phenomenon was observed at 500°C, while at a temperature higher than 650°C the catalytic activity was recovered.