Catalytic SO3 Decomposition Activity of SiO2-Supported Alkaline Earth Vanadates for Solar Thermochemical Water Splitting Cycles

Abstract

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).