NOx storage reduction over Pt-Ba/γ-Al2O3 catalyst

A transient study on the NOx storage-reduction properties of a Pt-BaO/gamma -Al2O3 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) NOx 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% CO2 has a marked inhibiting effect on the storage of NOx, particularly at low temperature, whereas 1% H2O has a promoting effect at low temperature and an inhibiting effect at high temperature. In the presence of 0.3-3% CO2 + 1% H2O the process is inhibited at any temperature; (iv) the storage of NOx occurs preferentially in the order at BaO, Ba(OH)(2), and BaCO3. 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 NOx are stored up to catalyst saturation and up to the NOx, 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% CO2 and 1% H2O in the exhaust these quantities diminish by 20-40% for NOx stored up to catalyst saturation and by 50% for NOx stored up to the NOx breakthrough; (vii) the reduction of the stored NOx 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 NOx is very selective to N-2 (95-100%); (ix) the reduction is slower in the presence of 0.3-3% CO2 and 0.3-3% CO2 + 1% H2O; (x) once all the stored reactive NOx groups have been reduced, in the presence of 0.3-3% CO2 and at sufficiently high temperature (T greater than or equal to 300 degreesC) 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. (C) 2001 Academic Press.