Selective Catalytic Reduction of NO by NH3over Vanadium-Containing Zeolites

Abstract Vanadium was introduced into zeolites, predominantly of ZSM-5 type, by isomorphous substitution (vanadium silicalites), by ion exchange with vanadyl sulphate solution (VO zeolites), and by chemical vapor deposition of VOCl 3 with subsequent hydrolysis and calcination ((VO 2.5 ) x –ZSM-5). The catalytic behavior of these materials in the selective catalytic reduction (SCR) of NO by NH 3 was investigated in a flow reactor at temperatures between 450 and 870 K. The speciation of the vanadium component was studied by a combination of spectroscopic techniques (UV–VIS, XPS, ESR, IR) prior to and after catalysis. Vanadyl-exchanged ZSM-5 exhibited high SCR activities, providing reaction rates comparable with that of a reference V 2 O 5 /TiO 2 catalyst though at higher temperatures. Isolated VO 2+ ions could be identified as the active site for the SCR reaction. In particular, any significant contribution to the selective reaction of intrazeolite V(V) oxide aggregates, which may be formed from vanadyl cations at elevated temperatures, and of disruption products thereof, was ruled out. NO could be also reduced over (VO 2.5 ) x –ZSM-5 prepared via chemical vapor deposition of VOCl 3 , which contained vanadium in extra- and intrazeolite oxide clusters and (if prepared from H–ZSM-5) VO 2+ ions. With these preparations, SCR activity was found also with materials void of VO 2+ ions and was assigned to the extra-zeolite vanadium oxide aggregates supported on the external zeolite surface. Conversely, vanadium silicalites were found to show low or no SCR activity at all, which indicates an inferior capability of isomorphously substituted vanadium to catalyze the SCR of NO by NH 3 .