Does density of cationic sites affect catalytic activity of Co zeolites in selective catalytic reduction of NO with methane?

Selective catalytic reduction of NO with methane (CH4-SCR) in an excess of oxygen over Co ions located in ZSM-5 of various Si/Al composition and in ferrierite, mordenite, chabazite and beta zeolite was investigated. From the comparison of the dependence of the TOF values per Co ion for NO conversion to N-2 and Co ions distribution among the cationic sites on total Cc ion concentration, the catalytic activity of the individual Co ions was estimated. The alpha-type Co2+ ions, located in the main channel of mordenite and ferrierite and coordinated above the rectangle of four framework oxygens of the channel wall exhibit the highest activity in these zeolites. On the other hand, the beta-type Co2+ ions coordinated in the plane of four oxygens of the deformed six-member ring located in the channel intersection of ZSM-5 and in channels of beta zeolite control the activity of these Co zeolites. The sequence of activity of CO2+ ions in CH4-SCR of NO was FER Co-alpha > ZSM-5 Co-beta > BEA Co-beta approximate to ZSM-5 Co-alpha >> FER Co-beta approximate to MOR Co-alpha >> CHA Co approximate to MOR Co-beta. A correlation between the activity of the individual Co ions in CH4-SCR of NO and a distance between the cationic sites was observed.