Activity of Co Ion Sites in ZSM-5, Ferrierite, and Mordenite in Selective Catalytic Reduction of NO with Methane

Abstract

Selective catalytic reduction of NO with methane (CH4-SCR) in an excess of oxygen was investigated over CoH–, CoNa–, CoNaK–, CoBa–, and CoxOy–Co-zeolites of ZSM-5, ferrierite, and mordenite structures. The exchanged Co ions have been shown to exhibit activity in CH4-SCR of NO, while protons or oxide-like Co species in Co–zeolites contribute to the CH4-SCR activity mostly by enhancement of the oxidation of NO to NO2. The sequence of Co–zeolites activity in CH4-SCR of NO (per zeolite weight) was Co–ZSM-5≅Co–ferrierite⪢Co–mordenite for the highest Co loadings at Si/Al of 12, 8.4, and 9.2, but for the average Co ion site in the zeolite (in terms of turnover frequency, TOF, values) it was Co–ZSM-5>Co–ferrierite⪢Co–mordenite. From a comparison of the dependence of TOF values for NO conversion to N2 per Co ion and of the relative concentrations of the exchanged α-, β-, and γ-type Co ions (determined from quantitative analysis of the characteristic Co(II) Vis spectra) on the total Co concentration, the most active Co sites were estimated. The α-type Co ions, bound to framework oxygens of the wall of the main channel of mordenite and ferrierite, are the most active among the Co ions in these zeolites. On the other hand, in ZSM-5 the β-type Co ions, coordinated to the deformed six-membered ring at the intersection of straight and sinusoidal channels possess the highest activity. The main factors contributing to the individual Co ions' activity are suggested to be cation location in the inner zeolite volume, coordination to the framework, and distances between the Co ions.