Preparation of mesoporous TiO2/g-C3N4 heterojunction catalyst and visible-light-driven photocatalytic reduction of Cr6+

Combining TiO2 and graphitic carbon nitride (g-C3N4) into TiO2/g-C3N4 heterojunction catalyst is an effective way to narrow band gap of TiO2, inhibit recombination of photoinduced carriers and improve photocatalytic efficiency. Mesoporous TiO2/g-C3N4 heterojunction was synthesized with a sol-gel method and characterized by SEM, TEM, XRD, BET, XPS, UV–Vis DRS, PL spectrum and electrochemical technologies. UV–Vis DRS displayed that band gap of TiO2/g-C3N4 was 2.64 eV which was narrower than that of TiO2 and g-C3N4, suggesting that TiO2/g-C3N4 can produce photoinduced carriers under visible light. BET showed that specific surface area of TiO2/g-C3N4 was 137.06 m2·g−1. Furthermore, I-t curve, Mott-Schottky curve and PL spectrum all proved that photocurrent of TiO2/g-C3N4 was higher than that of TiO2, revealing that introduction of g-C3N4 accelerated the transfer of photoelectrons and restrained the recombination of photo induced carriers. Under the optimal conditions (mass ratio of g-C3N4 to TiO2 was 1:5, TiO2/g-C3N4 dosage was 1.6 g·L−1, pH value was 5, Cr6+ initial concentration was 1 mg·L−1), the removal of Cr6+ reached 98 %. XPS analysis proved that Cr6+ was adsorbed and then reduced to Cr3+on TiO2/g-C3N4 surface in situ, and main reactants for reducing Cr6+ were electrons and ·O2− detected via radical scavenging tests. Moreover, photocatalytic reduction kinetics equation was established based on Langmuir-Hinshelwood (H-L) model and apparent reaction rate (Kapp) constant was calculated. TiO2/g-C3N4 showed a satisfied stability and photocatalytic reduction of Cr6+ still maintained 85 % after seven cycles, demonstrating that TiO2/g-C3N4 had a potential application to remove Cr6+.