NOx Storage Reduction over PtBa/γ-Al2O3 Catalyst

Abstract A transient study on the NO x storage-reduction properties of a PtBaO/γ-Al 2 O 3 catalyst is performed by using a synthetic exhaust gas containing oxygen and nitrogen oxides (storage phase) and a reducing gas containing hydrogen (reduction phase). The influence of water and carbon dioxide is also investigated. It is found that (i) NO x is stored in the form of nitrites and nitrates; (ii) during storage nitrites are oxidized to nitrates, and nitrates are most abundant when the storage process is completed; (iii) 0.3–3% CO 2 has a marked inhibiting effect on the storage of NO x , particularly at low temperature, whereas 1% H 2 O has a promoting effect at low temperature and an inhibiting effect at high temperature. In the presence of 0.3–3% CO 2 +1% H 2 O the process is inhibited at any temperature; (iv) the storage of NO x occurs preferentially in the order at BaO, Ba(OH) 2 , and BaCO 3 . The abundance of the different Ba sites at the catalyst surface depends on the composition of the exhaust gas and of the reducing gas; (v) considerable amounts of NO x are stored up to catalyst saturation and up to the NO x breakthrough in He +3% O 2 atmosphere that correspond to 24% Ba and 13–15% Ba to the best, respectively; (vi) in the presence of 0.3–3% CO 2 and 1% H 2 O in the exhaust these quantities diminish by 20–40% for NO x stored up to catalyst saturation and by 50% for NO x stored up to the NO x breakthrough; (vii) the reduction of the stored NO x is fast and is limited by the concentration of the reducing agent at any temperature in He +2000 ppm H 2 ; (viii) the reduction of the stored NO x is very selective to N 2 (95–100%); (ix) the reduction is slower in the presence of 0.3–3% CO 2 and 0.3–3% CO 2 +1% H 2 O; (x) once all the stored reactive NO x groups have been reduced, in the presence of 0.3–3% CO 2 and at sufficiently high temperature ( T ≥300°C) CO is formed through the reverse WGS reaction. This reaction, however, is of lesser importance when water is present in the exhaust due to thermodynamic constraints. The complete set of reactions involved in the storage-reduction cycle is identified and used to account quantitatively for the bulk of experimental data and to provide a comprehensive chemistry of the process.