Evidence for oxygen vacancy formation in HZSM-5 at high temperature
The properties of HZSM-5 zeolite (Si/Al=25) were investigated, at high temperature (623–973 K), by ac conductivity and 129Xe-NMR. The 129Xe-NMR and conductivity results suggest, for the first time, that HZSM-5 dehydroxylation at temperatures higher than 673 K leads to the formation of oxygen vacancies. It was found that the concentration of oxygen vacancies is proportional to the degree of dehydroxylation (temperature of treatment). Gas-phase O2 as well as H2O are incorporated into oxygen vacancies. Different conduction mechanisms were established for HZSM-5 in the temperature range investigated. Defect chemistry equations are used to describe the formation and evolution of lattice defects of HZSM-5 with temperature. Furthermore, the role of the HZSM-5 lattice defects in molecule activation at high temperatures is discussed. It is suggested that the active sites are either oxygen vacancies (in the absence of gas phase oxygen) or paramagnetic O- species formed by gas-phase oxygen incorporation into oxygen vacancies.