Influence of the nature and environment of cobalt on the catalytic activity of Co-BEA zeolites in selective catalytic reduction of NO with ammonia

Abstract

The influence of cobalt environment on the catalytic properties of CoxSiBEA zeolite in selective catalytic reduction of NO with ammonia was studied. Catalysts were prepared by a two-step postsynthesis method which consists, in the first step, of dealumination of parent BEA zeolite to obtain aluminum-free SiBEA support and then, in the second step, of contacting the obtained material with an aqueous solution of cobalt nitrate. DR UV–Vis and XPS results showed that cobalt was successfully incorporated into zeolite beta framework as isolated mononuclear Co(II). The presence of only isolated framework mononuclear Co(II) was evidenced in CoxSiBEA with cobalt content lower than 2 wt % and both isolated framework mononuclear Co(II) and extra-framework octahedral Co(II) for Co3.0SiBEA catalyst. FTIR investigation of pyridine adsorption revealed that the incorporation of cobalt into zeolite framework led to a creation of new Lewis acidic sites which are responsible for high activity in SCR of NO with ammonia. The catalytic activity of CoxSiBEA in selective catalytic reduction of NO with ammonia as reducing agent strongly depends on the nature and environment of cobalt in BEA structure. The single-site Co2.0SiBEA zeolite catalyst was the most active among tested, with maximum NO conversion about 80% at 673 K. In contrast, Co3.0SiBEA catalyst containing a mixture of framework and extra-framework Co(II) had lower activity in SCR of NO process than Co2.0SiBEA at higher temperature due to a competitive reaction of ammonia oxidation to NO.