Mechanism of Nitric Acid Reduction and Kinetic Modelling
Résumé
In France, the recycling of nuclear waste fuels involves the
use of hot concentrated nitric acid. The understanding and
prediction of the behaviour of the structural materials
(mainly austenitic stainless steels) requires the determination
and modelling of the nitric acid reduction process. Nitric acid
is indirectly reduced by an autocatalytic mechanism depending
on the cathodic overpotential and acid concentration.
This mechanism has been widely studied. All the
authors agree on its autocatalytic nature, characterized by
the predominant role of the reduction products. It is also generally
admitted that neither nitric acid nor the nitrate ion is
the electroactive species. However, the nature of the electroactive
species, the place where the catalytic species regenerates
and the thermodynamic and kinetic behaviour of
the reaction intermediates remain uncertain. The aim of this
study was to clarify some of these uncertainties by performing
an electrochemical investigation of the reduction of
4 M nitric acid at 40 °C at an inert electrode (platinum or
gold). An inert electrode was chosen as the working electrode
in a first step to avoid its oxidation and focus the research on the reduction mechanism. This experimental work
enabled us to suggest a coherent sequence of electrochemical
and chemical reactions. Kinetic modelling of this sequence
was then carried out for a gold rotating disk electrode.
A thermodynamic study at 25 °C allowed the composition
of the liquid and gaseous phases of nitric acid solutions
in the concentration range 0.5–22 M to be evaluated. The kinetics
of the reduction of 4 M nitric acid was investigated by
cyclic voltammetry and chronoamperometry at an inert electrode
at 40 °C. The coupling of chronoamperometry and FTIR
spectroscopy in the gaseous phase led to the identification of
the gaseous reduction products as a function of the cathodic
overpotential. The results showed that the reduction process
is autocatalytic for potentials between 0.6 and 1.15 V/NHE.
The electroactive species may be regenerated at the surface
of the electrode for lower potentials, otherwise this regeneration
process occurs in solution by a homogeneous chemical
reaction. When the potential is less than 0.6 V/NHE, the fast
reduction of nitrogen oxide may lead to rupture of the autocatalytic
cycle.