Kinetics of the NH3 and CO2 solid-state reaction at low temperature

Abstract

Ammonia and carbon dioxide play an important role in both atmospheric and interstellar ice chemistries. This work presents a theoretical and experimental study of the kinetics of the low-temperature NH₃ and CO₂ solid-state reaction in ice films, the product of which is ammonium carbamate (NH₄⁺NH₂COO⁻). It is a first-order reaction with respect to CO₂, with a temperature-dependent rate constant fitted to the Arrhenius law in the temperature range 70 K to 90 K, with an activation energy of 5.1 ± 1.6 kJ mol⁻¹ and a pre-exponential factor of 0.09_−0.08^+1.1 s⁻¹. This work helps to determine the rate of removal of CO₂ and NH₃, via their conversion into ammonium carbamate, from atmospheric and interstellar ices. We also measure first-order desorption energies of 69.0 ± 0.2 kJ mol⁻¹ and 76.1 ± 0.1 kJ mol⁻¹, assuming a pre-exponential factor of 10¹³ s⁻¹, for ammonium carbamate and carbamic acid, respectively.