Effects of coexisting oxoanions on SO₃ decomposition activity of molten-phase potassium metavanadate catalysts

Abstract

Molten-state potassium metavanadate (KVO₃) supported on mesoporous SiO₂ materials have emerged as active catalysts for SO₃ decomposition over a moderate temperature range (≤650 °C), which is a potential O₂ evolution reaction useful for solar thermochemical water splitting. The molten phase formed at ≥520 °C contained tetrahedral VO₄²⁻, which plays a vital role in accelerating the SO₃ uptake and conversion to SO₂/O₂. The present study aimed to reveal how the SO₃ decomposition activity is affected by adding other oxoanions such as borate (BO₃³⁻), carbonate (CO₃²⁻), and phosphate (PO₄³⁻) 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 KV₂PO₈, which consists of infinite tetrahedral PO₄ and pyramidal VO₅ linked by vertex sharing. Because of the congruent melting at temperature as low as ∼530 °C, KV₂PO₈ may be expected as another candidate of active molten phase catalyst for SO₃ decomposition.