Investigating the Low Temperature Formation of Cu^II‐(N,O) Species on Cu‐CHA Zeolites for the Selective Catalytic Reduction of NO_x

Abstract

In this work, we show the potentiality of operando FTIR spectroscopy to follow the formation of Cu -(N,O) species on Cu exchanged chabazite zeolites (Cu-CHA), active for the selective catalytic reduction of NO with NH (NH -SCR). In particular, we investigated the reaction of NO and O at low temperature (200 and 50 °C) on a series of Cu-CHA zeolites with different composition (Si/Al and Cu/Al ratios), to investigate the nature of the formed copper nitrates, which have been proposed to be key intermediates in the oxidation part of the SCR cycle. Our results show that chelating bidentate nitrates are the main structures formed at 200 °C. At lower temperature a mixture of chelating and monodentate nitrates are formed, together with the nitrosonium ion NO , whose amount was found to be proportional to the zeolite Brønsted site concentration. Nitrates were found to mainly form with Cu ions stabilized by one negative framework charge (Z), Z-[Cu(OH] or Z-[Cu(O ] , without involvement of Z -Cu ones. This evidence, together with the absence of bridging nitrates in samples with high probability for Cu-Cu pairs, indicate that the nitrate ligands are not able to mobilize copper ions, at variance with what recently reported for NH . Finally, water was found to replace preformed chelating copper nitrates and deplete NO (though with different kinetics) at both temperatures, while favouring the presence of monodentate ones.

Evidence of Mixed‐Ligand Complexes in Cu−CHA by Reaction of Cu Nitrates with NO/NH3 at Low Temperature

“…Figure reports the development of the FTIR spectra obtained when contacting the catalyst with the reactive gas feed. Before the exposure to 1000 ppm of NO and 1200 ppm of NH 3 at 50 °C, the Cu−CHA surface has been saturated with nitrates, formed by exposing the O 2 ‐activated catalyst to 1000 ppm NO and 10 % O 2 at 50 °C . The formation of adsorbed nitrates after this treatment is confirmed by the peaks related to chelating bidentate Cu II structures (1610–1575 cm −1 ) and monodentate ones (1500, 1310 cm −1 , indicated with arrows in Figure , right hand panel) .…”

“…Before the exposure to 1000 ppm of NO and 1200 ppm of NH 3 at 50 °C, the Cu−CHA surface has been saturated with nitrates, formed by exposing the O 2 ‐activated catalyst to 1000 ppm NO and 10 % O 2 at 50 °C . The formation of adsorbed nitrates after this treatment is confirmed by the peaks related to chelating bidentate Cu II structures (1610–1575 cm −1 ) and monodentate ones (1500, 1310 cm −1 , indicated with arrows in Figure , right hand panel) . A small amount of NO + , ascribed to the reaction of NO/O 2 with residual Brønsted sites is also observed (2159 cm −1 ), resulting in the formation of small amounts of water (H 3 O + signals at 1380 cm −1 labelled with ø, see Negri et al .…”

“…In a recent work, the low temperature reactivity of the NO/O 2 mixture on a series of Cu−CHA catalysts with different composition has been investigated by in situ FTIR spectroscopy . On the Cu−CHA catalyst studied in this work (Si/Al=15; Cu/Al=0.5), NO/O 2 exposure at 50 °C results in the formation of Z[Cu II (NO 3 )], i. e .…”

“…A small amount of NO + , ascribed to the reaction of NO/O 2 with residual Brønsted sites is also observed (2159 cm −1 ), resulting in the formation of small amounts of water (H 3 O + signals at 1380 cm −1 labelled with ø, see Negri et al . for more details).…”

“…The role of the nitrate/nitrite species formed in the presence of NO/O 2 in the SCR mechanism is still not well understood . In particular, the equilibrium between Cu‐nitrate and Cu‐nitrite, and their reactivity with NO and NH 3 to release N 2 and H 2 O have been studied.…”