Synthesis and characterization of composite catalysts Cr/ZSM-5 and their effects toward photocatalytic degradation of p-nitrophenol

Raw rice husks (white particles) were used to produce silica through submission to consecutive chemical treatment using NaOH and HCl solutions. The prepared silica was well incorporated with other components under hydrothermal conditions to synthesize ZSM-5. Cr framework-substituted ZSM-5 with constant Si/(Cr + Al) ratios have been synthesized hydrothermally and characterized by physicochemical methods, e.g. X-ray diffraction (XRD), FT-IR, UV–vis spectroscopy and N2 adsorption. FT-IR spectroscopy of Cr-substituted ZSM-5 shows new bands at 688 and 627 cm−1 due to extra framework chromium oxide. The replacement of Al3+ by Cr ion causes a shift of Si–O–T vibration to lower wave numbers with the appearence of a new band at 990 cm−1 in Cr-ZSM-5 samples which can be directly correlated with the entrance of chromium ions into the framework of the ZSM-5 lattice and the presence of Cr–O–Si linkage in the structures. UV spectroscopy showed absorption bands at 263 and 381 nm related to the tetrahedrally coordinated environment in the (Cr5+–O−1)∗ ← (O2−double bondCr6+) charge transfer transition state of isolated Cr(VI) atoms inside the ZSM-5 matrix.
The photocatalytic activity of Cr incorporated ZSM-5 zeolite toward the degradation of p-nitrophenol (PNP) was well investigated at atmospheric pressure, 25 °C, with H2O2 as an oxidizing agent. The enhanced photocatalytic activity of 0.4CrZ is attributed to charge-transfer excited complex between Cr in zeolite along with PNP ligand in addition to higher surface area and high dispersion Cr in the framework comparatively. More information on local structures of metal oxides inside zeolites and their photocatalytic activities toward PNP were deduced, correlated and discussed.