Francesco Castellino

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

The deactivation of V2O5–(WO3)/TiO2 catalysts for selective catalytic reduction (SCR) of NOx 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 Bronsted 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.