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

The mitigation and tuning of the niobic acid (NBO) acidity has been successful pursued 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 effective acidity of the samples in water by base titrations with 2-phenylethylamine completed the study. A different picture of the effect of the three dopants on the NBO acidity was observed in water. All the results indicated that the K-species doped more selectively the Brønsted acid sites, acting as an ion-exchanger, while Ba- and Nd-species had prevalent action on the Lewis acid sites of the NBO surface.