Catalytic SO3 Decomposition Activity and Stability of A-V-O/SiO2 (A = Na, K, Rb, and Cs) for Solar Thermochemical Water-Splitting Cycles

SiO2-supported molten alkaline metal oxides (A–V–O/SiO2) were studied as SO3 decomposition catalysts for solar thermochemical water splitting. Their catalytic activities at moderate temperatures (≤600 °C), which were superior to those of Cu–V–O/SiO2 catalysts, were dependent on A, exhibiting the following sequence: Cs > Rb > K > Na. These activities increased with the A/V ratio. This result is in accordance with the basicity, which favors the adsorption of SO3 to form sulfate. Another important effect of A is to form molten liquid phases, which dissolve the sulfate and facilitate its decomposition to SO2/O2. However, the molten phase with high A/V ratios led to the collapse of the porous SiO2 structure by a corrosion effect. Consequently, the highest catalytic activity was achieved at a composition of A/V ≈ 1.0 for A = K and Cs. The long-term stability test of K–V–O/SiO2 at 550 °C demonstrated no indication of noticeable deactivation during the first 100 h, whereas 20% deactivation occurred during the fol...