Effect of the Cr6+ concentration in Cr-incorporated TiO2-loaded MCM-41 catalysts for visible light photocatalysis

Abstract

The activity of 25% TiO2-loaded various amounts of Cr-incorporated MCM-41 (25%TiO2/Cr-MCM-41) was studied as a catalyst for photodegradation of 4-chlorophenol under visible light (400–800nm). Cr-MCM-41 with different ratios of Si to Cr was synthesized by hydrothermal method and loaded with 25wt.% TiO2 utilizing sol–gel method. The surface areas of Cr-MCM-41 and 25% TiO2/Cr-MCM-41 samples decreased with increasing the amount of Cr incorporated. XRD analysis showed that the MCM-41 structure was formed only in the samples with Si/Cr ratio larger than 20. Temperature program reduction (TPR) and UV–vis spectra results clearly indicated that only Cr6+ was present in the samples with the ratio of Si to Cr more than 20. Whereas the material with the ratio of Si to Cr less than 20 contained chromium with mixed oxidation states. It was found that the photodegradation ability of 25% TiO2/Cr-MCM-41 was highly related to the amount of Cr6+ which strongly interacted with TiO2; the optimum atomic ratio of Si to Cr was 20.This optimum catalyst was compared with two kinds of 25wt.% TiO2-loaded CrSi20Ox bulk oxides. The overall photodegradation ability of these titania loaded bulk oxide catalysts under visible light was about 45% less active than 25% TiO2/Cr-MCM-41 (Si/Cr=20). TPR results clearly showed that the interaction of Cr6+ and TiO2 in the bulk oxide catalysts was weaker than that in TiO2/Cr-MCM-41. A good correlation was demonstrated between the Cr6+–TiO2 interaction (CrOTi) and catalyst's photoactivity. As a result, the MCM-41 structure can enhance the interaction between TiO2 and Cr6+.All the TiO2/Cr-MCM-41 catalysts deactivate gradually under visible light due to the reduction of Cr6+ in the catalysts. TPR and temperature program oxidation (TPO) results clearly indicated that the average oxidation state of chromium in TiO2/Cr-MCM-41 (Si/Cr=20) was +2 after 23h of photoreaction. The reduced chromium in the photocatalyst can be completely reoxidized by heating the catalyst at 450°C for 3h.