Florida Solar Energy Center (FSEC) of the University of Central Florida (UCF) and Science Applications International (SAIC) are jointly developing advanced solar thermochemical water splitting cycles for the production of hydrogen. Sulfuric family thermochemical water splitting cycles (TCWSCs) have been widely studied for the hydrogen production. However, decomposition of sulfuric acid in the sulfur-family cycles presents serious materials and catalyst deactivation challenges. Platinum based catalysts are the most active for the H2SO4 decomposition, but they deactivate rapidly. To overcome these difficulties, metal sulfate based TCWSCs have been suggested. However, hydrogen production step of the metal sulfate based cycles (H2O + SO2 + MO = H2 + MSO4) are either thermodynamically unfavorable or kinetically very slow. At FSEC, we have developed a new hybrid photo/thermochemical water splitting cycle represented by the following reactions:

SO2(g) + 2NH3(g) + H2O(l) → (NH4)2SO3(aq) (chemical absorption) 25oC
(NH4)2SO3(aq) + H2O(l) → (NH4)2SO4(aq) + H2 (solar photocatalytic) 80oC
(NH4)2SO4(s) + ZnO(s) → 2NH3(g) + ZnSO4(s) + H2O (solar thermocatalytic) 500oC
ZnSO4(s) → SO2(g) + ZnO(s) + ½O2 (solar thermocatalytic) 1100oC

In this paper, we present the results of thermogravimetric/differential thermal analyses/mass spectrometric (TG/DTA/MS) and gas chromatographic/mass spectrometric (GC/MS) analyses to determine the reaction kinetics and reaction mechanism for the zinc oxide mediated thermal decomposition of (NH4)2SO4. The chemical equilibrium calculations for the reaction between ZnO and (NH4)2SO4 indicate that both ZnSO4 and zinc oxysulfate, ZnO.2ZnSO4, are formed as the stable reaction products.