Role of tetrahedral Co(II) sites of CoSiBEA zeolite in the selective catalytic reduction of NO: XRD, UV–vis, XAS and catalysis study

The CoSiBEA zeolites are prepared by a two-step postsynthesis method which allows to incorporate cobalt in BEA zeolite as isolated tetrahedral Co(II) species. The incorporation of Co ions in vacant T-sites and their reaction with silanol groups are evidenced by XRD and FTIR respectively. It does not generate acidic Brønsted sites as shown by FTIR of adsorbed pyridine. The state of cobalt species is characterized by DR UV–vis and XAS. These techniques allow to show that for low Co content (0.8 wt%) the cobalt is present as lattice tetrahedral Co(II) species. For much higher Co content (11 wt%) mainly extra-lattice octahedral Co(II) species and/or cobalt oxides are evidenced by DR UV–vis spectroscopy. Zeolite with isolated tetrahedral Co(II) species is active in SCR of NO by ethanol with selectivity towards N2 exceeding 85% for NO conversion from 30 to 70%. When extra-lattice octahedral Co(II) species and/or cobalt oxides appear, the full oxidation of ethanol and NO by dioxygen to CO2 and NO2 respectively, are the main reaction pathways. Possible way of the formation of lattice tetrahedral Co(II) species in the BEA structure is proposed.