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Structural Studies of Bulk to Nanosize Niobium Oxides with Correlation to Their Acidity
Hydrated
niobium oxides are used as strong solid acids with a wide
variety of catalytic applications, yet the correlations between structure
and acidity remain unclear. New insights into the structural features
giving rise to Lewis and Brønsted acid sites are presently achieved.
It appears that Lewis acid sites can arise from lower coordinate NbO<sub>5</sub> and in some cases NbO<sub>4</sub> sites, which are due to
the formation of oxygen vacancies in thin and flexible NbO<sub>6</sub> systems. Such structural flexibility of Nb–O systems is particularly
pronounced in high surface area nanostructured materials, including
few-layer to monolayer or mesoporous Nb<sub>2</sub>O<sub>5</sub>·<i>n</i>H<sub>2</sub>O synthesized in the presence of stabilizers.
Bulk materials on the other hand only possess a few acid sites due
to lower surface areas and structural rigidity: small numbers of Brønsted
acid sites on HNb<sub>3</sub>O<sub>8</sub> arise from a protonic structure
due to the water content, whereas no acid sites are detected for anhydrous
crystalline H-Nb<sub>2</sub>O<sub>5</sub>