Proceedings International Hydrogen Energy Congress and Exhibition IHEC 2005 Istanbul, Turkey, 13-15 July 2005 New evidences on water thermolysis promoted by a ferrite-carbonate system C. Alvani, C. Annunziatini, A. La Barbera, F. Padella, L. Seralessandri, F. Varsano ENEA - CR Casaccia - via Anguillarese 301 - 00060 Roma luca.seralessandri@casaccia.enea.it ABSTRACT Manganese ferrite stoichiometrically mixed with sodium carbonate has proved to be effective in hydrogen production by water thermolysis. A careful observation of phenomena occurring during the heating of the mixture evidences the existence of a new intermediate phase originated by the simple decarbonatation of the manganese ferrite/sodium carbonate mixture. The intermediate phase easily reacts with water producing hydrogen. From experimental results a new three-step process can be defined, so that the carbon dioxide evolution step can be isolated. By using the new three-step cycle a significant increase in terms of energy efficiency of the cycle can be obtained. Keywords: hydrogen production, thermochemical cycle, manganese ferrite, nanostructures. 1. INTRODUCTION Hydrogen production by water thermolysis can be achieved through thermochemical cycles based on metal oxide redox systems. Among the several redox couples that have been proposed, the Tokyo Institute of Technology modified manganese ferrite cycle based on MnFe 2 O 4 /Na(Mn 1/3 Fe 2/3 )O 2 is very promising (Tamaura et al., 1995, Tamaura et al., 1999). Recently, it was demonstrated that a stoichiometric mixture of manganese ferrite/sodium carbonate powders readily reacts with water vapor producing hydrogen in stoichimetric amount at temperature as low as 700°C (Padella et al., 2003, Padella et al., 2005). Water thermolysis occurs according to the following reaction sequence: in the first step the reacting mixture is oxidized by water producing hydrogen and releasing CO 2 2MnFe 2 O 4(s) +3Na 2 CO 3(s) +H 2 O (g) → 6Na(Mn 1/3 Fe 2/3 )O 2 (s) +3CO 2 (g) +H 2 (g) (1) successively, initial reactants are regenerated upon reduction of sodium ferrimanganite in CO 2 atmosphere (oxygen releasing step) 6Na(Mn 1/3 Fe 2/3 )O 2 (s) + 3CO 2 (g) → 2Mn Fe 2 O 4 (s) + 3Na 2 CO 3 (s) +1/2O 2 (g) (2) Hydrogen production reaction (1) needs water excess to go to completion in short time (Kaneko et al., 2002). Not to dramatically limit the process efficiency, it is necessary to reuse unreacted water vapor, avoiding, as much as possible, thermal losses, especially during its recovery in the separation step from hydrogen, and carbon dioxide. In fact, although in principle, hydrogen can be separated by diffusive membranes at reaction (1) temperature, effective CO 2 /H 2 O separation cannot be performed without the introduction of a condensation step. In such a way, the gaseous mixture is cooled down to room temperature and gaseous carbon dioxide can be easily separated from liquid water. Such a separation step is responsible for losses due to unrecoverable water latent evaporation heat. To perform the successive cycle it is necessary to restore the reaction thermal conditions, wasting an amount of energy at least equal to the water latent evaporation heat. The possibility of a CO 2 separation step that avoids water condensation would significantly enhance the process efficiency. 1