Modulation of the acidity of niobic acid by ion-doping: Effects of nature and amount of the dopant ions

The acidity of niobic acid (NBO) has been successfully mitigated and tuned by addition of K+, Ba2+ and Nd3+ dopant species in amounts from 1 to 15 atom nm−2. The characterization of the intrinsic acid properties of the samples was performed by adsorption of NH3 in a volumetric–microcalorimetric coupled line and by temperature programmed desorption (TPD) of 2-phenylethylamine in a thermogravimetric apparatus. The K-dopant was more effective in decreasing the acidity of niobic acid than the Ba- and Nd-dopants. Complementary measurements of the effective acidity of the samples in water by base titrations with 2-phenylethylamine completed the study and revealed a different picture of the effect of the three dopants on the NBO acidity in water. All the results indicated that the K-dopant targeted more selectively the Brønsted acid sites, acting as an ion-exchanger, while Ba- and Nd-species predominantly acted on the Lewis acid sites of the NBO surface.