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

Kumar, Dharmesh ; Dey, G. K. ; Gupta, N. M. (2003) Nanoparticles of uranium oxide occluded in MCM-41 silica host: influence of synthesis condition on the size and the chemisorption behavior PCCP: Physical Chemistry Chemical Physics, 5 (24). pp. 5477-5484. ISSN 1463-9076

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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.

Item Type:Article
Source:Copyright of this article belongs to Royal Society of Chemistry.
ID Code:82467
Deposited On:10 Feb 2012 15:33
Last Modified:10 Feb 2012 15:33

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