Redox behaviour of SnO2 nanoparticles encapsulated in the pores of zeolites towards reductive gas atmospheres studied by in situ diffuse reflectance UV/Vis and Mössbauer spectroscopy

Abstract

Nanoparticles of SnO₂ have been embedded in the pores of faujasite-type NaY zeolites by chemical vapour deposition, ion exchange or impregnation followed by hydrolysis. By transmission electron microscopy and from blue-shifts in optical spectra (diffuse reflectance (DR-UV/Vis)) the sizes of the SnO₂ nanoparticles formed by impregnation were determined to be about 3 nm. These samples exhibit the most distinct decreases in the diffuse reflectance and highest rates for changes in the reflectance intensity during reduction with CO. The optical changes can be correlated to the number of oxygen vacancies in the particles, i.e. the deviations from the original stoichiometry SnO₂. In contrast to the reduction with H₂, after reduction with CO all optical changes are completely reversible by addition of oxygen. The different redox behaviour towards CO and H₂ is also confirmed by in situ ¹¹⁹Sn Mössbauer spectroscopy and might be interpreted by the presence of a core of SnO in the nanoparticles prevented against further oxidation by a dense shell of SnO₂.