Sulfuric acid decomposition catalysts and reaction considerations for sulfur-based thermochemical water splitting cycles

The sulfur-based family of thermochemical cycles appears promising for producing hydrogen from water. These cycles employ a high-temperature sulfuric acid decomposition reaction step. The reaction takes place from 700 to 900°C, or higher, and is facilitated by heterogeneous catalysts. The overall energy efficiency of the hydrogen production process is directly correlated to the temperature of the sulfuric acid reaction step. The activity of spinels and perovskites were explored due to their potential to be stable at the high reaction temperatures. This paper presents the temperature dependent activity, from 725 to 900°C, of several complex metal oxide catalysts for the decomposition reaction: two perovskites (FeTiO₃ and MnTiO ₃) and four spinels (NiFe₂O₄, CuFe ₂O₄, NiCr₂O₄, and CuCr ₂O₄). For comparison, a 1 wt% Pt/TiO₂ catalyst and two common metal oxides, CuO and Fe₂O₃, were also studied. The most promising material examined, a CuFe₂O₄ spinel had significantly higher activity than the 1% Pt/TiO₂ catalyst at temperatures of 850°C and above, but much lower activity than the supported platinum catalyst at temperatures below 825°C. These results suggest that to minimize reactor size and maximize energy conversion efficiency for the water splitting cycles, a minimum of two catalyst zones, utilizing both a supported Pt and a spinel catalyst should be employed; a lower temperature section employing a Pt/TiO₂ catalyst followed by a high temperature section utilizing the spinel catalyst.