Evolution of High-Energy Electron Beam Irradiation Effects on Zeolite Supported Catalyst: Metal Nanoprecipitation

Abstract

The high-energy electron irradiation effects on Fe-loaded, zeolite-supported catalyst were examined by transmission electron microscopy. In the original sample under little beam irradiation, no nanoparticles larger than 1 nm can be observed, and about half of the loaded Fe is identified as being in a positive ion state (i.e., iron oxides). Metal Fe nanoparticles in neutral state Fe0 were then found to precipitate quickly under beam illumination with an electron dose of ∼2.4 × 107 nm–2 or above at room temperature. Since electron microscopy is widely applied in the characterization of all sorts of catalysts supported on zeolites, the current observations could be treated as a model system to distinguish the metal nanoparticles existing in the original catalyst from those precipitated by electron beam irradiation. It was the ionization effect of electron radiation, other than temperature rise, that played an important role in the formation and growth of the metal precipitates. In the current system, the induced nanoprecipitations were identified as pure Fe metal clusters by electron energy loss spectroscopy (EELS), high-resolution transmission electron microscopy (HRTEM), and electron diffraction. As in current modeling system, although only metal Fe nanoparticles can be observed by EELS if the irradiation effect is ignored at the first place, the functional component in the loaded catalyst is actually a mixture of Fe-oxide and Fe.