Selective catalytic reduction of NO by methane over Co,H-SSZ-13 catalysts: Types and catalytic functions of active Co sites

Abstract

Co-SSZ-13 zeolite catalysts were prepared by solid-state ion exchange with CoCl2 salt for the selective catalytic reduction of NO with methane (CH4/NO–SCR). Fresh catalysts contained hard-to-reduce lattice Co2+/[Co-OH]+ ions and showed activity proportional to the concentration of these species. The NO conversion was accompanied by steam-generating methane oxidation. Under the arising hydrothermal conditions, a fraction of Co ions transformed into Co-oxo species especially in catalysts of high Co content (Co/AlF > 0.3), resulting in boosted activity in NO2-forming catalytic NO oxidation. The increased NO2 concentration brought about higher concentration of surface cobalt nitrate species responsible for the rate-determining methane activation, so the overall rate of the NO–SCR reaction was also accelerated. Steam admixed to the feed decreased the conversion of methane more significantly than that of NO. Its observed high hydrothermal stability renders the Co-SSZ-13 catalyst a very promising candidate for the CH4/NO–SCR reaction.