Synthesis of protonated titanate nanotubes tailored by the washing step: Effect upon acid properties and photocatalytic activity

Abstract

"In this work, the acid and photocatalytic properties of titanate nanotubes (NTs) were surveyed. The surface acidity of the NTs was characterized by FTIR with lutidine and pyridine. The photocatalytic degradation of phenol in aqueous suspension was performed to test the photocatalytic properties of the NTs. The results were compared with those obtained from commercial TiO2. NTs were prepared by hydrothermal treatment of TiO2 nanoparticles in a NaOH aqueous solution. During the washing step, three different acid agents (HCl, H2SO4, and HNO3) were used. TiO2 nanoparticles were synthesized previously by the sol-gel method. The photocatalytic materials were characterized by FTIR, XRD, XPS, SBET, UV–vis, and HRTEM.

It was found that the used acid agent significantly affected the amount and type of acid sites. Brönsted acid sites were favored by the use of HNO3. Lewis acid sites were promoted when HCl was employed during the washing step, which is in contrast with the results obtained using the other acids chosen in this work. Besides, the use of HCl promoted the H2Ti3O7 phase. The acid phase H2Ti4O9·H2O was favored when HNO3 was used and H2SO4 formed the H2Ti2O4·(OH)2 acid phase. The presence of Cl, S, and N species on the NTs was not found. Furthermore, Na+ ions were completely removed from the surface of the NTs, which were exchanged by H+ ions.

It was found, in general, that the three catalysts presented a relatively high photocatalytic activity to remove phenol. However, NTs washed with HNO3 (NT-HNO3) displayed the best photocatalytic activity compared to the other NTs. After 200 min, NT-HNO3 reached a phenol degradation yield close to 100%. Commercial TiO2 presented a phenol degradation yield close to 60%. It could be concluded that the acid phase (H2Ti4O9·H2O) and concentration of Brønsted acid sites promoted the photocatalytic activity of NT-HNO3."