Nitrogen-doped mixed-phase TiO2 with controllable phase junction as superior visible-light photocatalyst for selective oxidation of cyclohexane

Highlights: • MIL-125/P25 as precursor was used to fabricate nitrogen doped mixed-phase porous TiO₂. • The ratios of anatase and rutile are adjustable by changing P25 amount in precursor. • The catalyst showed enhanced KA oil yield and ketone selectivity under visible light. Nitrogen-doped anatase/rutile mixed-phase TiO₂ photocatalysts with carbonaceous species were successfully fabricated by pyrolyzing MIL-125 (Ti) and P25 composite as the precursor, and subsequently doping nitrogen into the pyrolysis product using urea as the dopant in a sealed reactor. Through varying the addition amount of P25 in the precursor, the ratios of anatase/rutile phases in the photocatalysts were conveniently adjusted. The photocatalysts showed excellent photocatalytic cyclohexane oxidation performances under visible light irradiation. The yield of the KA oil (cyclohexanol and cyclohexanone) reached 112.44 μmol after 5 h illumination for the photocatalyst with the optimal anatase/rutile ratio (N-TiO₂-3), which was 5.66 and 39.87 times that of N-TiO₂-0 (nitrogen-doped anatase TiO₂) and N-P25 (nitrogen-doped P25), respectively. In the meantime, the selectivity toward cyclohexanone could reach 100%. The influences of pyrolysis temperature and atmosphere on the microstructure of catalysts and resulting catalytic performances for cyclohexane oxidation were also investigated. The improved photocatalytic properties were attributed to the combined effects of the high visible-light absorption caused by nitrogen doping, valid separation of photogenerated electron-hole pairs derived from the mixed-phase junction, unique auxiliary function of carbonaceous species and porous structure. A possible photocatalytic mechanism was proposed according to the active species determined by trapping experiments.