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H2O and CO2 splitting via the heterogeneous oxidation of zinc vapor


The heterogeneous oxidation of zinc vapor by water or sequestered carbon dioxide eliminates the possibility of reaction passivation associated with the build up of a layer of zinc oxide in the fuel producing step of the Zn/ZnO metal redox cycle. It is therefore possible to achieve high degrees of zinc conversion and also achieve rapid fuel production rates. Although less thermodynamically favored, a chemical equilibrium analysis confirms that high zinc conversions are possible (Fig. 1). Furthermore, the heterogeneous reaction of zinc vapor promotes rapid kinetics, as reaction rates on the order of 10-6 mol cm-2 sec-1 have been observed (Fig. 2), two orders of magnitude greater than the reaction rates observed in TGA studies on the oxidation of solid zinc powder. The heterogeneous oxidation of zinc vapor is thus a promising pathway to rapid production of hydrogen or carbon monoxide concurrent with high zinc conversion. The latter is critical for achieving a high sunlight-to-fuel efficiency.

Fig. 1. The equilibrium extent of the heterogeneous oxidation of zinc vapor by H2O (solid line) and CO2 (dashed line) for stoichiometric conditions at 1 bar.

Fig. 2. The maximum reaction rate observed for the heterogeneous hydrolysis of zinc vapor for temperatures up to 1100 K.

At the Solar Energy Laboratory, we conduct experimental studies to understand the heterogeneous kinetics of Zn vapor oxidation. We employ a tubular flow reactor in which zinc vapor and steam react heterogeneously on the surface of quartz tubes. Hydrogen and carbon monoxide production is measured in-situ with mass spectrometry and gas chromotagrophy, and the ZnO deposits are measured ex-situ. Data is analyzed via the application of differential and integral dispersion flow models.