Design of photocatalysts encapsulated within the zeolite framework and cavities for the decomposition of NO into N 2 and O 2 at normal temperature

Abstract

The design of photocatalysts encapsulated within the zeolite frameworks and cavities is the most promising approach in developing photocatalysts which will operate efficiently and effectively towards the purification of toxic agents such as NO x and SO x in the atmosphere. In the present study, the vanadium silicalite (VS-2) and Ag +/ZSM-5 catalysts were prepared by hydrothermal synthesis and ion-exchange, respectively, and the in situ characterization of these catalysts and their photocatalytic reactivities for the decomposition of NO have been investigated using dynamic photoluminescence, XAFS (XANES, EXAFS), ESR, FT—IR, LTV—VIS, solid-state NMR and XRD techniques along with an analysis of the reaction products. Results obtained with the VS-2 catalyst showed that vanadium oxide moieties are present within the zeolite framework as a 4-fold tetrahedrally coordinated vanadium oxide species having a terminal oxovanadium group (VO). UV irradiation of the VS-2 catalyst in the presence of NO led to the photocatalytic decomposition of NO to form N 2, N 2O and O 2. On the other hand, it was found that the zeolite cavities can stabilize the Ag + ions in an isolated state through their connection with two lattice oxygen anions of the zeolite (2-coordination geometry). These isolated Ag + ions exhibit high photocatalytic reactivities for NO decomposition to form N 2, N 2O and NO 2. Dynamic studies of the excited state of these catalysts showed that the charge transfer from the excited state of the vanadium oxide species or Ag + ions to NO plays a vital role in the initiation of the decomposition of NO into N and O. These findings have demonstrated that metal oxide species and metal ions included within the zeolite frameworks and cavities are strong candidates for new types of environmentally applicable photocatalysts.