Uranium oxide nanoparticles dispersed inside the mesopores of MCM-48: synthesis and characterization

Abstract

The nanocrystallites of uranium oxide were dispersed in the mesopores of siliceous MCM-48 by subjecting it to the repeated cycles of incipient wet impregnation followed each time by drying and calcination steps. A loading of up to ~20 wt% of U could be achieved in this manner. The physico-chemical characteristics of the samples were examined using XRD, XPS, IR, DR UV-VIS spectroscopy, Nitrogen sorption and HRTEM techniques. In spite of progressive disordering in the host structure and accompanying decrease in the unit cell parameter after each cycle of impregnation/calcination, the mesoporosity of the host was always preserved to certain extent. Also, the BET area, pore size and pore volume measurements indicated the gradual filling of the pores of MCM-48 as a result of uranium loading. The spectroscopy results revealed that the uranium species were initially bonded to · Si-OH sites of the host matrix in the form of UO₂²⁺ groups which in turn converted to U₃O₈ upon calcination. TEM examination confirmed that the majority (~85%) of U₃O₈ particles were of less than 3 nm size and these were located exclusively within the pore system of host matrix. On the other hand, larger size crystallites were dispersed at the external surfaces. Furthermore, the size of the U₃O₈ particles and the physical characteristics of the U/MCM samples depended considerably on synthesis conditions, the post-impregnation ambient temperature drying under vacuum facilitating the formation of smaller size and more uniformly distributed particles.