A review of catalytic sulfur (VI) oxide decomposition experiments

Sulfur (VI) oxide, also known as sulfur trioxide or SO₃, decomposition is an oxygen-generating decomposition reaction that proceeds in the gaseous system SO₃/SO₂/O₂/H₂O at temperatures above 500 K. Maximum decomposition yield of SO₃ to SO₂ and O₂ is best achieved at temperatures of over 1000 K with an appropriate catalyst. According to the literature, noble metals and some transition metal oxides are highly effective catalysts in the laboratory environment. Sulfur (VI) oxide decomposition is the energetic and temperature limiting step of several endothermic hydrogen generating chemical process heat plants. In particular, the General Atomics Sulfur Iodine cycle and the Westinghouse Hybrid Sulfur cycle are candidates for thermal coupling to a high temperature nuclear reactor. Therefore the sulfur (VI) oxide decomposition reaction is a potential heat sink for a high temperature nuclear reactor. Thus, optimization of catalyst selection is required, both for operational efficiency and safety. In this paper, reaction mechanisms and catalyst composition for sulfur (VI) oxide decomposition are reviewed. Chemical kinetics data from previous sulfur (VI) oxide decomposition experiments are extracted from archival journal papers or other open literature. The available experimental database suggests that Pt-based catalysts have the highest stable activity among the noble metals and Fe₂O₃-based catalysts have the highest stable activity among the transition metal oxides. The decomposition temperature of the corresponding metal sulfate dictates the catalytic activity of a given transition metal oxide.