(476x) Fe-Cu Catalysts for Sulfuric Acid Decomposition in Si Cycle to Thermochemically Produce Hydrogen

Sulfur-Iodine (SI) cycle is one of the most promising thermochemical water splitting cycles to produce hydrogen using high temperature heat transferred from a VHTR (very high temperature gas-cooled reactor). It was originally proposed by General Atomics Co. in 1970s, which was composed of the following three steps.

Bunsen reaction : x I₂ + SO₂ + 2 H₂O = 2 HI_x + H₂SO₄

HIx decomposition : 2 HI = H₂ + I₂

H₂SO₄ decomposition : H₂SO₄ = H₂O + SO₂ + 1/2 O₂

Decomposition of sulfuric acid to SO₂, O₂ and H₂O is the step to absorb the highest heat from a VHTR in the whole SI cycle. This reaction consists of two series reactions: 1) decomposition of sulfuric acid to SO₃ and H₂O at around or above 350^oC and 2) decomposition of SO₃ to SO₂ and O₂ at 750^oC or higher temperature. The second reaction of SO₃ decomposition has been known to take place catalytically while the first reaction of sulfuric acid decomposition can take place with or without catalysts. Noble metal catalysts such as Pt are reported highly active for both sulfuric acid and SO₃ decomposition reactions, but their long-term stability at high temperature over 850^oC is still questionable. In addition, cheap catalysts replaceable to Pt need to be developed from the economic point of view. Non-noble metal oxide catalysts, such as Cr₂O₃, CuO, Fe₂O₃, CeO₂ or NiO, are reported active at or above 800^oC even though their activities and durabilities are comparably lower than those of Pt-based catalysts. However, their activities have been being improved by adopting various supporting materials and preparation methods.

In this study, activity and long-term stability of Fe-Cu combined catalysts supported on alumina or titania have been explored. Catalysts of various Fe-Cu compositions were prepared by coprecipitation. The reactions were conducted at 800 and 850^oC with concentrated H2SO4 of 95 wt% for 20 h. Activity of each catalyst has been compared with those of Pt/alumina and Pt/titania. Effects of metallic species ratio, metal to supporting material ratio, and the third metallic species added on the catalytic activity have been evaluated. Cupric oxides solely supported on alumina or titania have been revealed to have high activities for the SO₃ decomposition at 800 and 850^oC. But their activities slowly decreased as used multiple times on stream, which seemed probably because of the relatively low melting point of CuO. Compared to CuO, ferric oxides solely supported on alumina or titania have showed lower activities but longer life times. Bimetallic catalysts of Fe-Cu supported on alumina or titania showed enhanced life times, while their activities were found placing between the activities of the cupric oxide and the ferric oxide prepared on the same supporting materials. However, the stability enhancment was more remarkable than the activity decrease. This preliminary result has proposed possibility of replacing Pt-based catalysts with cheaper non-noble metal oxide supported catalysts by controlling the combination of metal species, the ratio of metals, supporting materials and preparation methods.