Activity, selectivity and durability of VO–ZSM-5 catalysts for the selective catalytic reduction of NO by ammonia

Abstract ZSM-5 zeolite was loaded with vanadyl ions (VO 2+ ) by treatment of Na–ZSM-5 with aqueous VOSO 4 solution at pH 1.5–2. The catalytic material was tested for the selective catalytic reduction of NO with ammonia at temperatures between 473 and 823 K and normal pressure using a feed of 1000 ppm NO, 1000 (or 1100) ppm NH 3 and 2% O 2 in He. The catalyst proved to be highly active, providing, e.g. initial NO conversions of >90% at 620 l g −1  h −1 (≈400 000 h −1 ) and 723 K, and selective, providing nitrogen yields equal to NO conversion at equimolar feed in a wide temperature range and only minor N 2 O formation at NH 3 excess. Admixture of SO 2 (200 ppm) resulted in an upward shift of the useful temperature range, but did not affect the catalytic behaviour at temperatures ≥623 K. No SO 2 conversion was noted at T  ≤ 723 K and 450 l g −1  h −1 . The poisoning effect of water (up to 4.5 vol%) was weak at temperatures between 623 and 773 K. VO-ZSM-5 catalysts are gradually deactivated already under dry conditions, probably by oxidation of the vanadyl ions into pentavalent V species. This deactivation is considerably accelerated in the presence of water.