An infrared study of NO decomposition over Cu-ZSM-5
- Lawrence Berkeley National Lab., CA (United States); and others
The interactions of NO with Cu-ZSM-5 have been investigated by means of infrared spectroscopy. Following reduction by CO, most of the copper is present as Cu{sup +} cations. Room-temperature exposure of the reduced catalyst to NO results in the immediate appearance of Cu{sup +}(NO) and Cu{sup +}(NO){sub 2}. With time, these species disappear and are replaced by Cu{sup 2+}(NO) and Cu{sup 2+}(O{sup -})(NO). Evidence of the formation of Cu{sup 2+}(NO{sub 2}) and Cu{sup 2+}(NO{sub 3-}) and of adsorbed N{sub 2}O and N{sub 2}O{sub 3} is also observed. Similar species are observed upon room-temperature exposure of autoreduced and preoxidized Cu-ZSM-5. Above 573 K, the catalyst is active for NO decomposition to N{sub 2} and N{sub 2}O. The selectivity to N{sub 2} increases rapidly with increasing temperature and is essentially 100% at 773 K, the temperature at which the catalyst exhibits maximum activity. Infrared spectra taken under reaction conditions show weak peaks for Cu{sup +}(NO), Cu{sup 2+}(O{sup -})(NO{sub 3}{sup -}), and Cu(NO{sub 3}{sup -}). With increasing temperature, the intensities of the peaks for Cu{sup +}(NO) and Cu{sup 2+}(O)(NO) decrease but the proportion of the former species increases relative to the latter. Based on this evidence and rate data reported in the literature, a mechanism is proposed for the decomposition of NO. The first step in this mechanism is the formation of N{sub 2}O via the decomposition of Cu{sup +}(NO){sub 2}. N{sub 2} is then formed via the reaction of N{sub 2}O with Cu{sup +} sites. O{sub 2} formation is envisioned to proceed via the release of O atoms from Cu{sup 2+}O{sup -} and the subsequent reaction of O atoms with additional Cu{sup 2}O{sup -} to produce Cu{sup 2+}O{sub 2}{sup -}. The variation in the fraction of Cu{sup +} with temperature, deduced from the proposed mechanism, is in qualitative agreement with recent XANES observations. 37 refs., 10 figs.
- OSTI ID:
- 224029
- Journal Information:
- Journal of Catalysis, Journal Name: Journal of Catalysis Journal Issue: 2 Vol. 157; ISSN JCTLA5; ISSN 0021-9517
- Country of Publication:
- United States
- Language:
- English
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