Thermochemical cycles produce hydrogen through a series of chemical reactions where the net result is the production of hydrogen and oxygen from water at much lower temperatures than from direct thermal decomposition. All chemicals within the cycle are recycled and the heat to drive the reactions, which tend to be endothermic, must be provided by a primary energy source. When the primary energy driver is nuclear or solar heat, hydrogen can be generated without producing greenhouse gasses, and can provide independence from our dwindling supplies of fossil fuels.

Among the high number of thermochemical water-splitting cycles proposed in the literature, the Sulfur-Iodine (S-I) cycle has generated considerable interest. The S-I cycle consists of three simultaneous reactions; the decomposition of hydroiodic acid (HI) to produce hydrogen and generate iodine for recycle, the decomposition of sulfuric acid to produce oxygen and generate SO2 for recycle, and a main reaction where water and the recycle chemicals react to regenerate HI and sulfuric acid. Both the hydroiodic acid decomposition and the sulfuric acid decomposition reactions require a catalyst to achieve significant rates and product yields at expected operating temperatures.

This paper will focus primarily on catalysts that have been examined for the sulfuric acid decomposition reaction. Activities, stabilities, and material properties changes of several metal oxides, perovskites, and supported platinum group metal catalysts will be presented.