Zinc oxide and zinc have shown to be favorable materials for the water splitting thermochemical cycle due to the moderate disassociation temperature of zinc oxide (2235 K). The cycle is composed of two steps. The first solar step is the endothermic dissociation of zinc oxide.

ZnO = Zn + 0.5O₂

The second is the exothermic hydrolysis of the zinc with steam to produce hydrogen and zinc oxide which is recovered and recycled to the first step.

Zn + H₂O = ZnO + H₂

The net reaction is the splitting of water. Most of the work-to-date on solar production of H₂ via the ZnO/Zn redox reactions has focused on evaluation and demonstration of the solar step (Palumbo et al., 1998; Elorza-Ricart et al., 1999; Weidenkaff et al., 2000a and 2000b; Moeller and Palumbo, 2001; Steinfeld, 2005).  A solar chemical reactor has been experimentally demonstrated at a power level of 10 kW (Haueter et al., 1999).  The present study focuses on the crucial hydrogen production step, which is a major challenge to implementation.  Prior work shows heterogeneous hydrolysis of zinc is constrained because of the formation of a protective coating of ZnO at the reactant surface (e.g., Clarke and Fray, 1979; Weidenkaff et al., 1999; Berman and Epstein, 2000).  An intriguing solution to the passivation problem is to operate at the nanoscale where the large surface to volume ratio favors complete oxidation and the high specific surface area augments the reaction kinetics, and heat and mass transfer processes.  Such a process was originally proposed by researchers at ETH – Swiss Federal Institute of Technology (Weiss et al., 2005).  The intent is to create Zn nanoscale aerosol in a condensation nucleation process with simultaneous hydrolysis.  Entrainment of zinc oxide nanoparticles in a gas flow would allow continuous removal of products.  Exploratory experiments in hot wall flow reactors have been conducted at ETH (Weiss et al., 2005; Wegner et al., 2006; and Ernst at al., 2006) and in the author's laboratory (Abu Hamed et al., 2007).  These experiments have produced hydrogen mass as high as 0.96 of the theoretical maximum but disappointingly low yields of retrievable zinc oxide particles.  Hydrogen production is attributed to hydrolysis of zinc deposited on the walls of the reactor.  Improved yield of nanoscale aerosol is essential for continuous recovery and regeneration of the product ZnO. The present paper presents data obtained in an improved reactor in which Zn-evaporation, steam-quenching, and Zn/H₂O-reaction zones are isolated.  The results of a parametric study to quantify the effects of reactor temperature distribution, residence time, and H₂O:Zn molar ratio on aerosol ZnO and hydrogen yields as well as the evolution of the nano structured metal/metal oxide are reported.

References

Abu Hamed T., Davidson J.H., Stolzenburg M., 2007, Hydrogen production via hydrolysis of Zn in a hot wall flow reactor, To be presented at ASME Energy Sustainability, Long Beach CA.

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Clarke, J.A., and Fray D.J., 1979, “Oxidation of zinc vapour by hydrogen-water vapor mixtures,” Trans. Institute of Mining Metallurgy C., 88, pp. 161-166.

Elorza-Ricart E., Martin P.Y., Ferrer M., and Lédé J., 1999, “Direct thermal splitting of ZnO followed by a quench. Experimental measurements of mass balances,” J. Phys. IV France 9, pp.325-330.

Ernst, F.O., Tricoli, A., Pratsinis, S.E., and Steinfeld, A., 2006, “Co-synthesis of H₂ and ZnO by in-situ Zn aerosol formation and hydrolysis,” American Institute of Chemical Engineers J., 52(9), pp. 3297-3303.

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Wegner, A., K., Ly, H.C., Weiss, R.J., Pratsinis, S.E. and Steinfeld, A., 2006, “In situ formation and hydrolysis of zn nanoparticles for H₂ production by the 2-step ZnO/Zn water-splitting thermochemical cycle,” Int. J. Hydrogen Energy, 31, pp. 55–61

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Weidenkaff A., Reller A., Wokaun A., and Steinfeld A., 2000, “Thermogravimetric analysis of the ZnO/Zn water splitting cycle,” Thermochimica Acta, 359, pp.69-75.

Weidenkaff A., Steinfeld A., Wokaun A., Auer P.O., Eichler B., and Reller A.,1999, “Direct solar thermal dissociation of zinc oxide: condensation and crystallisation of zinc in the presence of oxygen,” Solar Energy, 65, pp. 59-69.

Weiss, R.J., Ly, H.C., Wegner, K., Pratsinis, S.E., and Steinfeld, A., 2005, “H₂ production by Zn hydrolysis in a hot-wall aerosol reactor,” American Institute of Chemical Engineers J., 51, pp. 1966 -1970.