Stability of Molten-Phase Cs–V–O Catalysts for SO<sub>3</sub> Decomposition in Solar Thermochemical Water Splitting

Supported molten cesium vanadate catalysts (Cs–V–O/SiO<sub>2</sub>) showed activities comparable to that of a reference Pt catalyst (1 wt % Pt/TiO<sub>2</sub>) for SO<sub>3</sub> decomposition at moderate temperatures (∼600 °C), which is essential as an O<sub>2</sub> evolution reaction in solar thermochemical water splitting cycles. Stability testing of the catalyst over a 1000 h continuous reaction at 600 °C resulted in deactivation by ∼20% of the initial activity. Kinetic analysis of the activity versus time-on-stream indicated that the observed deactivation behavior can be divided into an induction period (≤100 h) and an acceleration period (>100 h). The deactivation is mainly caused by the vaporization loss of active components (Cs and V) from the molten phase. At the earliest stage, most vapor is generated in the upstream section of the catalyst bed and then redeposits therebelow. Upon repeating these vaporization and deposition cycles, Cs and V move gradually downstream. During this induction period, the deactivation is not obvious because the total Cs and V content of the catalyst bed remains almost unchanged. After this period, however, detachment of Cs and V from the downstream end of the catalyst bed induces accelerated deactivation. The vaporization loss was found to be significantly suppressed by inverting the catalyst bed every 100 h during the stability test. Consequently, this operation reduced the extent of catalyst deactivation from 20% to less than 10% of the initial activity.