FTIR mechanistic studies on the selective catalytic reduction of NO_x by methane and ethane over supported cobalt catalysts

Abstract

The NO x species formed after NO adsorption and NO + O 2 co-adsorption on Co-ZSM-5, CoY, Co/SiO 2 (high cobalt dispersion) and Co/SiO 2 (low cobalt dispersion) were studied by means of IR spectroscopy. It was found that NO adsorption on all samples leads to the formation of Co 2+ (NO) 2 species, which are stable on Co-ZSM-5 and CoY but are destroyed during evacuation on Co/SiO 2 . The stable species produced upon NO + O 2 co-adsorption are different kinds of surface NO x − anionic species. The principal compounds formed on Co-ZSM-5 and Co/SiO 2 with a high cobalt dispersion are surface monodentate nitrates, characterized by an IR band at ∼1535 cm −1 . These nitrates are thermally stable but easily interact with hydrocarbons which confirms that they are key species in selective catalytic reduction (SCR). No monodentane nitrates are formed on CoY and Co/SiO 2 with a low cobalt dispersion, which accounts for the low SCR activity reported for such samples.

The Role of Active Sites of CoH-ZSM-5 Catalysts for the C2H4-SCR of NO

“…-mononitrosyl ( Fig. 1b-a) [16,17]. By contrast, there was a significant increase in intensity of the Co 2?…”

“…(NO) 2 and Co 3? -NO species, respectively [15][16][17]. The introduction of 10% O 2 led to the disappearance of Co 2?…”

“…The bands at 1,812 and 1,896 cm -1 were assigned to Co 2? -dinitrosyl species [15,16]. A small but distinct band at 1,935 cm -1 observed on fresh CoH-ZSM-5 was assigned to Co 3?…”

In situ DRIFTS study during C2H4-SCR of NO over Co-ZSM-5

“…Among zeolite-based catalysts, Co-ZSM-5 has been found to be effective and thermally stable for HC-SCR of NO x when CH 4 was used as reductant [6][7][8][9][10]. Besides CH 4 , higher hydrocarbons such as C 2 H 2 [11], C 2 H 4 [11,12], C 2 H 6 [13,14], C 3 H 6 [15], C 3 H 8 [16] and i-C 4 H 10 [17] were also used as reducing agents over Co-ZSM-5.…”

“…The general trends of hydrocarbon reducing activities are: (i) alkines ~ alkenes > CH 4 [11], and (ii) larger alkanes > CH 4 [14,16].…”