Reactivity of various UO3 modifications in the fluorination to UF4 by Freon-12

Six uranium trioxide modifications, α, β, γ, amorphous, ζ, and δ-UO3, were fluorinated directly to UF4 by reaction with CCl2F2 at 753 K. The first four oxides needed various periods of time to attain stoichiometric conversion to UF4 (99%); these are 4, 4, 2 and 3 h, respectively. The apparent differences are attributed to the degree of disorder in the oxide structure and to their specific surface areas. The latter two oxides (γ and δ-UO3) reacted passively towards fluorination, which was discussed in the light of their surface structure. These two oxides showed neither uranyl bonding character nor residual surface water (molecular or OH-groups). The former effect leads to the presence of an odd oxygen and enhances a covalent character and to a weakening effect on the U-O uranyl bond. The latter effect (surface water) furnishes acid-base centers that catalyze the fluorination process. (orig.)