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Abstract

The formation of NO and nitrate adsorbates on supported metal oxides is a valuable tool to gain insight into the coordination geometry and connectivity of surface metal oxide species. In this study, dispersed molybdenum oxide supported on silica SBA-15 with loadings of 5.9 wt % Mo (0.8 Mo/nm2) and 10.2 wt % Mo (2.2 Mo/nm2) has been investigated in detail by transmission IR spectroscopy using NO adsorption and oxidation. The treatment of the reduced 5.9 wt % Mo (0.8 Mo/nm2) MoxOy/SBA-15 with NO leads to the formation of molybdenum dinitrosyl complexes [Mo4+−(NO)2]. The use of isotopically labeled 15NO causes a shift of the 14NO dinitrosyl bands from 1806 and 1702 cm−1 to 1779 and 1682 cm−1, respectively. Higher oxidized nitrogen oxide species, which are formed in the presence of a 14NO/O2 mixture, have been studied on oxidized and reduced 5.9 wt % Mo (0.8 Mo/nm2) MoxOy/SBA-15. Two nitrogen oxide bands are stable toward evacuation and moderate temperature treatment. The presence of 15NO/O2 leads to shifts of these bands from 1618 to 1585 cm−1 and from 1573 to 1534 cm−1. A similar behavior is observed for the higher-loaded sample with 10.2 wt % Mo (2.2 Mo/nm2). As interference with other NO oxidation products and water can be excluded, the bands are assigned to bridging and bidentate surface nitrates, respectively.