Characterization of supported Nb catalysts by ToF-SIMS

A series of Nb₂O₅/TiO₂ catalysts were prepared by incipient wetness impregnation of TiO₂ with aqueous solutions of niobium oxalate/oxalic acid. Eight different catalysts with Nb loadings of 0.48-8.5 wt.% Nb were prepared. Analysis of the time-of-flight secondary ion mass spectrometry (ToF-SIMS) spectra of Nb₂O₅/TiO₂ catalysts shows the presence of monomeric, dimeric and trimeric Nb_xO_y clusters at all loadings. This indicates that polymeric Nb oxide species are present even at loadings well below that corresponding to monolayer coverage (approximately 4.5 wt.% Nb). Moreover, it was found that the degree of aggregation of the niobia active phase increases with Nb loading. The tetrameric Nb oxide cluster was the largest fragment detected for amorphous Nb₂O₅. For Nb₂O₅/TiO₂ catalysts the tetrameric Nb cluster was first detected in the 2.8 wt.% Nb catalyst. It is conclude that bulk-like Nb₂O₅ is present in catalysts containing ≥2.8 wt.% Nb (i.e. ≥60% theoretical coverage).