Poisoning of vanadia based SCR catalysts by potassium: influence of catalyst composition and potassium mobility

Abstract

The deactivation of V₂O₅–(WO₃)/TiO₂ catalysts for selective catalytic reduction (SCR) of NO_x upon exposure to aerosols of KCl or K₂SO₄, at different temperatures, has been studied. All samples exposed for more than 240 hours lost a substantial fraction of their initial activity although lower exposure temperatures slow down the deactivation. K₂SO₄ causes a lower rate of deactivation compared to KCl. This may be related to a faster transfer of potassium from the solid KCl matrix to the catalyst, however, it cannot be ruled out to also be caused by a significantly larger particle size of the K₂SO₄ aerosol (mass based distribution mode: 1.3 μm) compared to that of the KCl aerosol (mass based distribution mode: 0.12 μm). The relative activities of exposed catalysts indicate that promotion with WO₃ accelerates the deactivation, likely due to the enhanced Brønsted acidity which appears to promote the transport of potassium. Using a newly developed experimental protocol consisting of two-layer pellets of SCR catalysts, where one side is impregnated with KCl or K₂SO₄, the potassium transport in such systems, which is assumed to take place through reaction and diffusion over acid sites, was investigated. SEM-WDS measurements on pellets heat treated at 350 °C show that potassium bound in KCl readily leaves its counter ion and thus moves faster into the catalyst compared to potassium from K₂SO₄, which is in agreement with results from the aerosol exposures.