NO adsorption combined with FTIR spectroscopy as a useful tool for characterization of niobium species in crystalline and amorphous molecular sieves

Amorphous NbMCM-41 and crystalline NbY zeolite were characterized by the adsorption of NO (or oxygen admission followed by NO adsorption) combined with FTIR measurements. XRD, XPS and UV–vis results proved that in both types of materials Nb was present as +5 cation in a tetrahedral coordination involved in Nb–O–Si bonds and no extra framework niobium species were formed. Despite these common features, a substantial difference between the niobium species in these two types of materials was indicated by NO adsorption and FTIR study. In NbMCM-41 activated at 673 K, unsaturated Nbδ+ formed Nb–NO complexes whereas NbOδ− species were responsible for nitrate formation. In NbY zeolite the niobium cations in the framework were saturated and did not form Nb–NO complexes. The interaction of NO with the residual water was documented by the formation of N2O after NO adsorption on NbY. The increase in basicity of Y zeolite after Nb introduction was proved by the formation of nitrite species resulting from electron transfer from the zeolite to NO2.