Use of overtones and combination modes for the identification of surface NOx anionic species by IR spectroscopy

Abstract

Adsorption of NO on ZrO2 produces a series of compounds, among them N2O which is characterized by a band at 1232 cm-1 and the respective overtone (12 times less intense) at 2466 cm-1. Co-adsorption of NO and O2 results in the formation of different surface nitrates. Bridging nitrates are characterized by a set of bands at 1640, 1220 and 1004 cm-1 and manifest a combination mode at 2612 cm-1. Bidentate nitrates (1588–1568, 1240 and 1042 cm-1) show a combination mode at 2580 cm-1, which for monodentate nitrates (1515, 1295 and 1019 cm-1) is found at 2522 cm-1. In all these cases the combination mode is by about two orders of magnitude less intense than the fundamental band in the 1650–1500 cm-1 region. The use of overtones and combination modes is proposed to help the assignments of the NOx bands. It is demonstrated that bands in the 1650–1550 cm-1 region, produced after NO + O2 co-adsorption on Fe-ZSM-5, are due to nitrate species.