15N Solid State NMR and EPR Characterization of N-Doped TiO2 Photocatalysts

The solid-state nuclear magnetic resonance (SSNMR) analysis and structural characterization of N-doped TiO2 nanoparticle and monolayer materials suitable for visible photocatalysis is reported. The SSNMR analysis of 15N-doped TiO2 synthesized using 15N-urea before calcination indicates formation of various amino functionalities of the type NH, NH2, NH3, and probably NH4+, while the NMR spectrum of the yellow powder that results from high-temperature calcination shows that these nitrogen species oxidize to form nitrate. These and nonisotopically labeled materials were characterized also by X-ray diffraction, UV−vis, and electron paramagnetic resonance spectroscopy. Monitoring the oxidation of 1,2-13C-trichloroethylene (TCE), photochemical activity was confirmed by monitoring the production of CO2 by 13C NMR analysis. The powdered form of N-doped TiO2 proved to be a highly efficient visible light catalyst by oxidizing all of the TCE to chlorinated alkyl and acyl containing molecules along with CO2. The performance of a TiO2−N−TiO2/porous Vycor 7930 borosilicate glass monolayer catalyst was in turn evaluated by monitoring the photochemical oxidation of ethanol and acetone with UV−vis light, and it was found to perform better than TiO2-only monolayers. Direct nitridation of TiO2 powders and monolayers also was conducted to compare the 15N SSNMR and to discern if there is a nitridic bond in these materials. The SSNMR results provide more evidence supporting the hypothesis that the nitrogen atom in N-doped TiO2 is present in interstitial sites when N is in a highly oxidized state.