Investigation Of Chemical States Of Iron Species And Nature Of Photocatalytic Property Of ZSM-5 Zeolite

Singleness of the kind and the property of photocatalyst greatly restrict the wide applications of photocatalysis. Developing the novel photocatalysts and exploring the mechanism of photocatalytic process has a real theoretical and practical significance for sustainable development of photocatalytic science.This paper explored the photocatalytic process of the commercial HZSM-5 and FeZSM-5 self-prepared by ion-exchange and hydrothermal synthesis. The photocatalytic activity of these zeolites was evaluated, with the photocatalytic degradation of ethylene and bromomethane in gaseous phase or methyl orange in aqueous solution as model experiments. The experiments were conducted at atmospheric pressure by using a fixed-bed tubular reactor and a batch reactor, respectively. The chemical composition of catalyst was detected in details by XRF, AAS and ICP-AES. The surface configuration, photoresponse and acidity of zeolites were characterized by XRD, XPS, UV-Vis DRS, SPVS, FT-IR and Py-IR. The distribution, coordinated envirnoment and local structure of iron species in HZSM-5 were revealed by XAFS, UV-Raman and ESR.The following results and conclusion were obtained from these studies. (1) Under 254nm of UV irradiation, HZSM-5 zeolite exhibits excellent photocatalytic activity, not only for the gas-phase photocatalytic oxidation of C2H4 and CH3Br but also for the decoloration of methyl orange. The photocatalytic activity depends on the various sources of HZSM-5 and the preparation methods of FeZSM-5. (2) The commercial HZSM-5, which depends on the source and Si/Al ratio, contains 0.02～0.06 wt% iron impurity. The photocatalytic activity has closely correlated with the iron content but not correlated with that of aluminium. The relationship of the conversion of ethylene with the ratio of Fe/(Fe+Si+Al) is linear. With the adding Fe wt%, the FeZSM-5 zeolite via hydrothermal synthesis decreases the conversion of ethylene, whereas the FeZSM-5 zeolite via ion-exchange is almost unchanged. (3) The physicochemical characterization revealed that the iron impurity in HZSM-5 zeolite exists in +3 values, and mainly as isolated tetrahedrally coordinated on the surface of framework. The chemical state of iron species is also related with Si/Al ratio. In NK50 with high ratio...