Chemical kinetics of the gas-phase reaction between uranium hexafluoride and hydrogen

Article abstract of DOI:10.1021/j150625a040

A study was made of the chemical kinetics of the homogeneous gas-phase reaction between uranium hexafluoride and hydrogen by measuring the rate of disappearance of UF₆. It has been concluded that the rate-limiting step for which the kinetics have been measured is UF₆ + H₂ = UF₅ + HF + H (2). The reaction has been studied in a steady-state flow system over a temperature range of ～625-825 K. Various surface-to-volume ratios were employed to aid in distinguishing gas-phase reactions from surface reactions. The steady-state concentration of the UF₆ after reaction with H₂ was monitored in a special multipass infrared spectrophotometer at the 626-cm^-1 absorption band of UF₆. The principal problems were corrosion, plugging, surface intrusion, and the deleterious effects of minute traces of water; these problems have greatly slowed progress in this field. Several series of measurements involving different initial species concentrations and residence times, with each series at constant temperature, show that the rate is first order in UF₆. Our measurements yield a dependable Arrhenius curve in terms of a second-order expression for k, the bimolecular specific reaction rate constant for the disappearance of UF₆. It is believed that this overall rate of the disappearance of UF₆ is somewhat less than twice that of the critical reaction step 2, indicated above, so that the specific reaction rate constant k₂ is ～8.7 × 10¹⁴ exp(-34550 kcal/RT) cm³ mol^-1 s^-1. Conclusions have been reached concerning the relative importance of the various elementary reaction steps involved in the chemical mechanism.