Effect of H2O on the hydrolysis of UF6 in the gas phase

Abstract In our previous works (Hu et al., 2008, 2009), we found the theoretical evidence that indicates UF 5 OH is an intermediate produced in the first step of UF 6 hydrolysis, UF 6  + H 2 O. When UF 6 /H 2 O ratio is high, UF 5 OH may react with UF 6 or another UF 5 OH, forming species containing U O U bond. In this work, we considered another situation – the hydrolysis of UF 6 in excess of H 2 O, and explored the probable initial reactions of UF 6  + 2H 2 O and UF 6  + 3H 2 O systems using the same relativistic density functional theory calculations. Water molecules may form dimer or trimer and then coordinate with UF 6 , forming complexes UF 6 ·2H 2 O or UF 6 ·3H 2 O, which are more stable than UF 6 ·H 2 O. Compared to the conversion UF 6 ·H 2 O → UF 5 OH·HF, the energy barriers of reactions UF 6 ·2H 2 O → UF 5 OH·H 2 O·HF and UF 6 ·3H 2 O → UF 5 OH·2H 2 O·HF are 4 and 6 kcal/mol lower respectively. The additional H 2 O molecules catalyze the reactions by bridging the hydrogen which transfers from ligand H 2 O to F and stabilizes the transition states. In addition, as water content increases, the reaction step UF 6  + 3H 2 O becomes exothermic and the products-HF, UF 5 OH and two H 2 O molecules-tend to bond tightly into a stable complex.