Bunsen reaction : x I2 + SO2 + 2 H2O = 2 HIx + 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 350oC and 2) decomposition of SO3 to SO2 and O2 at 750oC 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 850oC 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 800oC 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 850oC 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 850oC. 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.