FTIR Studies and Kinetic Modelling of NOX Reduction and NOX Storage

Abstract: FTIR spectroscopy and kinetic modelling have been used to study catalysts for NOX reduction and NOX storage.

For Pt/Al2O3 exposed to NO, oxygen and propene IR-bands associated with nitrates, acetate, formate, carbonates and isocyanate were observed. Experiments with NO, oxygen and HNCO showed that isocyanate is an intermediate for reduction of NOX.

For Pt/BaO/Al2O3 exposed to NO, propene and excess oxygen mainly IR-bands associated with nitrates and acetate were observed. During hydrocarbon excess conditions the sizes of these bands decreased and a band associated with isocyanate became apparent.

The adsorption and desorption of oxygen and NO or NO2 at different temperatures on Al2O3, BaO/Al2O3, Pt/Al2O3, and Pt/BaO/Al2O3 were studied with FTIR spectroscopy. For Al2O3 and BaO/Al2O3 exposed to NO and oxygen small nitrite and hyponitrite bands were observed. When exposed to NO2 and oxygen large nitrate-bands were observed. For Pt/Al2O3 and Pt/BaO/Al2O3 exposed to NO and oxygen small nitrite and hyponitrite-bands were observed at 100 and 150ºC. At 200ºC and at higher temperatures large nitrate-bands were formed. It was found that Pt influences the stability of the nitrates via the NO2 decomposition reaction.

The deactivation of Pt/BaO/Al2O3 by SO2 was studied with FTIR spectroscopy. After exposures to SO2 and oxygen at 350ºC bands associated with surface and bulk sulphates were observed. Treatment in hydrogen reduced the size of these bands and the bands associated with surface sulphates were more readily reduced.

The oxidation of NO on Pt/Al2O3 was modelled with an Eley-Rideal mechanism. A good agreement between model and experiments was obtained.

The storage of NOX on Al2O3 was modelled. A mechanism with gas phase N2O3, N2O4 and N2O5 as important intermediates could accurately describe the storage and release of NO2 in presence and in absence of NO.

A high temperature catalyst for diesel exhaust after-treatment was modelled. A mechanism with nitrate, partially oxidised hydrocarbon and isocyanate as important intermediates was used. The model showed reasonable agreement with experimental data for CO, NOX and NO2. There was however a lack of fit for hydrocarbon, which was suspected to be related to the heterogeneity of diesel fuel.