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 following 400 h. The deactivation mechanism involves the vaporization loss of active components from the molten phase, which is accelerated in the presence of SO3.

Supported molten cesium vanadate catalysts (Cs–V–O/SiO2) showed activities comparable to that of a reference Pt catalyst (1 wt % Pt/TiO2) for SO3 decomposition at moderate temperatures (∼600 °C), which is essential as an O2 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.

SO 3 decompositionThermochemical water splitting Sulfureiodine cycle a b s t r a c t CeeV oxide catalysts supported on bimodal mesoporous SiO 2 were prepared by a wet impregnation method to study their catalytic activity for SO 3 decomposition, which is a key step in thermochemical water splitting cycles based on the sulfureiodine process. Asprepared CeeV/SiO 2 catalysts exhibited more than 50-fold higher turnover frequency at 600 C compared to unsupported CeVO 4 . This is in accordance with the smaller size (13 nm) of CeVO 4 particles highly dispersed in SiO 2 mesopores, compared to approximately 600 nm for unsupported CeVO 4 particles. The CeeV/SiO 2 catalysts exhibited a higher activity than supported V 2 O 5 or CeO 2 catalysts, which coexisted depending on the Ce/V ratio. Among the studied catalysts with different Ce/V molar ratios, the highest activity was achieved at Ce/ V ¼ 0.9, at which the greatest specific surface area of CeVO 4 was attained. The catalyst demonstrated SO 3 decomposition rates comparable to those of Pt/Al 2 O 3 in a wide range of WHSV (3.6e110 g-H 2 SO 4 g À1 h À1 ) and no indication of noticeable deactivation during 40 h of the catalytic reaction at 650 C.

Alkaline earth vanadates (Ae–V: Ae = Ca, Sr, and Ba) were supported on mesoporous SiO2 by a wet impregnation method. The catalytic activity of the prepared materials for the decomposition of SO3 into SO2 and O2, which is a key step in solar thermochemical water splitting cycles, was investigated. In the temperature range 700–800 °C, the Ae–V/SiO2 catalysts exhibited remarkably high activities, which were superior to those of supported Pt catalysts in a wide range of weight hourly space velocities (55–220 g-H2SO4 g–1 h–1). Despite the melting points of the materials exceeding 1000 °C, the high activity was determined to be closely related to the unusual melting behavior of Ae–V. Under the reaction atmosphere, the Ae–V phase was converted to AeSO4 and molten V2O5 (melting point = 690 °C) via facile solid–gas reactions between SO3 and alkaline earth elements displaying high basicity. Notably, upon contact with the molten V2O5 phase, the as-deposited AeSO4 was immediately decomposed into SO2 and O2 to regenerate the Ae–V phase. The catalyst, which solidified at lower temperatures (<690 °C), could not decompose the sulfate and was therefore unable to drive the catalytic cycles. Consequently, the SO3 decomposition rate at <690 °C was lower than that of an alkaline vanadate (Cs–V) with a melting point as low as 500 °C but higher than that of a rare earth vanadate (La–V) with the highest melting point (>1800 °C).

Molten-state potassium metavanadate (KVO 3 ) supported on mesoporous SiO 2 materials have emerged as active catalysts for SO 3 decomposition over a moderate temperature range (¯650 °C), which is a potential O 2 evolution reaction useful for solar thermochemical water splitting. The molten phase formed at ²520 °C contained tetrahedral VO 4 2¹ , which plays a vital role in accelerating the SO 3 uptake and conversion to SO 2 /O 2 . The present study aimed to reveal how the SO 3 decomposition activity is affected by adding other oxoanions such as borate (BO 3 3¹), carbonate (CO 3 2¹ ), and phosphate (PO 4 3¹ ) into the melt. Although borate showed a deteriorating effect, phosphate tended to improve the catalytic activity when the P/V molar ratio was equal to or less than 0.5. The addition of phosphate produced a mixed phosphate vanadate with a composition of KV 2 PO 8 , which consists of infinite tetrahedral PO 4 and pyramidal VO 5 linked by vertex sharing. Because of the congruent melting at temperature as low as ³530 °C, KV 2 PO 8 may be expected as another candidate of active molten phase catalyst for SO 3 decomposition.