Efficient NO adsorption and release at Fe3+ sites in Fe/TiO2nanoparticles

Abstract

The adsorption and release of nitrogen oxide (NO) molecules at the surface of Fe-doped TiO₂ nanoparticles, made by a template-free sol–gel process, has been studied. Fe³⁺ ions are found to be highly effective NO adsorption sites in Fe/TiO₂. Up to ∼89 μmol g⁻¹ of the adsorbed NO can be released by exposure to trace amounts of H₂O, which exceeds the water-induced release capacity of other NO storage materials by at least a factor of 2. The surprising efficiency of water as a trigger molecule for NO release is due to its large dipole moment, which causes a much stronger coordination to the Fe³⁺ species than is the case for NO. In addition, a new IR band at 1840 cm⁻¹ has been found for Fe-doped TiO₂ nanoparticles exposed to NO gas. This band is assigned to the stretch vibration of an NO species coordinatively bonded to an Fe³⁺ site. In contrast to Fe²⁺–NO vibrations in e.g.zeolites, Fe³⁺–NO vibrations are seldom observed, and this is the first time that convincing evidence is reported for the presence of Fe³⁺–NO in Fe/TiO₂. The implications of these findings for the application of Fe-doped TiO₂ as a novel NO storage and release material are briefly discussed.