Development of catalysts for sulfuric acid decomposition in the sulfur–iodine cycle: a review

To achieve carbon-neutral energy vectors, researchers have investigated various sulfur-based thermochemical cycles. The sulfur–iodine cycle has emerged as a cost-effective global process with massive hydrogen production potentials. However, all sulfur-based thermochemical cycles involve sulfuric acid decomposition reaction, which is highly corrosive and energy intensive. The activation energy of this reaction can be reduced using catalysts that decrease the onset temperature of the reaction. Renewable heat sources such as solar and waste nuclear heat demand high stability to operate within a wide temperature window (650°C–900°C). Several metal/metal oxide systems based on noble and transition metals have been investigated over the last twenty years. In the literature, supported Pt-based catalysts are regarded as the prime choice for stable operations. However, during catalytic operations, noble metals are degraded owing to sintering, oxidation, leaching, and other processes. Transition metal oxides such as Fe, Cu, Cr, and Ni exhibit promising catalytic activity at high temperatures; however, at low temperatures (>600°C), their activation is reduced owing to poisoning and the formation of stable sulfate species. The catalytic activity of transition metal oxides is determined by the decomposition temperature of its corresponding metal sulfate; thus, the metal sulfate formation is considered as the rate-limiting step. Herein, the catalytic systems studied over the last decade are summarized, and recommendations for designing robust catalysts for commercial applications are presented.