Combined in situ FT-IR and TRM analysis of the NO x storage properties of Pt-Ba/Al 2O 3 LNT catalysts

Abstract

The storage of NO 2 and NO/O 2 in the presence and in the absence of CO 2 was investigated at 350 °C on Pt-Ba/Al 2O 3 LNT catalysts and on the reference Ba/Al 2O 3 system, by coupling in situ FT-IR spectroscopy and transient response method (TRM). Experiments were performed by admitting NO 2/CO 2 or NO/O 2/CO 2 mixture (CO 2/NO x ratios in the range 1–7) or, in alternative, by admitting NO 2 or NO/O 2 on catalysts previously saturated by flowing CO 2. In the case of NO 2/CO 2 adsorption, it was found that NO 2 was efficiently stored as nitrate species according to a disproportion reaction, which occurs with evolution of NO, as in the absence of CO 2. In the case of NO/O 2/CO 2 adsorption it was confirmed that two pathways operated simultaneously as in the absence of CO 2: the “nitrite route”, involving the initial formation of surface nitrite and their subsequent evolution to nitrates, and the “nitrate route”, involving NO oxidation to NO 2 over Pt and its subsequent adsorption on Ba in the form of nitrates. The presence of CO 2 inhibits to some extent the “nitrite route”, whereas the “nitrate route” proceeds as in the absence of CO 2.