Combined in situ FT-IR and TRM analysis of the NOx storage properties of Pt-Ba/Al2O3 LNT catalysts

The storage of NO2 and NO/O2 in the presence and in the absence of CO2 was investigated at 350°C on Pt-Ba/Al2O3 LNT catalysts and on the reference Ba/Al2O3 system, by coupling in situ FT-IR spectroscopy and Transient Response Method (TRM). Experiments were performed by admitting NO2/CO2 or NO/O2/CO2 mixture (NOx/CO2 ratios in the range 1 to 7) or, in alternative, by admitting NO2 or NO/O2 on catalysts previously saturated by flowing CO2. In the case of NO2/CO2 adsorption, it was found that NO2 was efficiently stored as nitrate species according to a disproportion reaction, which occurs with evolution of NO, as in the absence of CO2. In the case of NO/O2/CO2 adsorption it was confirmed that two pathways operated simultaneously as in the absence of CO2: the “nitrite route”, involving the initial formation of surface nitrite and their subsequent evolution to nitrates, and the “nitrate route”, involving NO oxidation to NO2 over Pt and its subsequent adsorption on Ba in the form of nitrates. The presence of CO2 inhibits to some extent the “nitrite route”, whereas the “nitrate route” proceeds as in the absence of CO2.