Nanoparticles of uranium oxide occluded in MCM-41 silica host: Influence of synthesis condition on the size and the chemisorption behavior

Abstract

The paper presents a comparative study on the characteristics of uranium oxide crystallites deposited in the mesopores of MCM-41 by two different methods, one involving the repeated cycles of wet impregnation of template-free MCM-41 with uranyl acetate and the other by way of exchanging the template cations in an as-synthesized host matrix. XPS, DRUV visible and XRD studies revealed that both the preparation procedures resulted in initial binding of uranyl groups with the Si–OH sites of host matrix, a part of which converted to U₃O₈ particles upon calcination. The size of the U₃O₈ crystallites, however, depended upon the synthesis conditions. Thus, more uniform and smaller size (<3 nm) particles of uranium oxide were formed in the case of the impregnation-prepared samples, as compared to the crystallites of 3–15 nm size formed using a template-exchanged procedure. Furthermore, the post-impregnation drying of a sample under the conditions of vacuum and room temperature was vital for the uniformity and for the small size of these crystallites. XRD, TEM and N₂ sorption results showed that the U-loading resulted in a decrease in the unit cell parameter and also in a decrease in the long range ordering of the host matrix while the mesoporosity was preserved in both kinds of samples. At the same time, whereas the crystallites of U₃O₈ were largely dispersed within the pore system of MCM-41 in the case of impregnation-prepared samples , a large fraction existed at the external surface of the samples synthesized through template exchange route. This is attributed to the restriction imposed by template cations to the transport of uranyl species into the pore system. The monitoring by in situ IR spectroscopy revealed that the methanol molecules undergo dehydrogenation as a result of room temperature adsorption over uranium oxide crystallites, giving rise to oxymethylene (–OCH₂) species as primary products which transform subsequently over larger size crystallites to form polyoxymethylene, i.e. (–OCH₂)_n, species. Smaller size crystallites of U₃O₈, on the other hand, promoted the further oxidation of oxymethylene groups resulting thereby in the formation of formate-type complexes.