Photocatalytic hydrogen production from water-methanol mixtures using N-doped Sr₂Nb₂O₇ under visible light irradiation: Effects of catalyst structure

Nitrogen-doped perovskite type materials, Sr₂Nb ₂O_7-x-N_x (0, 1.5 < x < 2.8), have been studied as visible light-active photocatalysts for hydrogen production from methanol-water mixtures. Nitrogen doping in Sr₂Nb₂O ₇ red-shifted the light absorption edge into the visible light range and induced visible light photocatalytic activity. There existed an optimum amount of nitrogen doping that showed the maximum rate of hydrogen production. Among the potential variables that might cause this activity variation, the crystal structure appeared to be the most important. Thus, as the extent of N-doping increased, the original orthorhombic structure of the layered perovskite was transformed into an unlayered cubic oxynitride structure. The most active catalytic phase was an intermediate phase still maintaining the original layered perovskite structure, but with a part of its oxygen replaced by nitrogen and oxygen vacancy to adjust the charge difference between oxygen and doped nitrogen. These experimental observations were explained by density functional theory calculations. Thus, in Sr₂Nb₂O _7-x-N_x, N 2p orbital was the main contributor to the top of the valence band, causing band gap narrowing while the bottom of conduction band due to Nb 4d orbital remained almost unchanged.