Description du sujet de thèse

Ammonia (NH3) is the main ingredient in fertilisers and is also used in the production of basic chemicals (urea, ammonium salts, etc.), with production exceeding 220 Mt per year. Recently, thanks to significant advances in industrial H2 production capacity through electrolysis, NH3 has been proposed as an H2 carrier due to its high energy density of 5.2 kWh kg-1 and its high gravimetric hydrogen content of 17.6% by weight, not to mention the thermodynamically favourable cracking reaction for recovering H2. To date, NH3 has been produced almost exclusively by the Haber-Bosch (HB) process, which has been optimised for over a century and is an energy-intensive process due to its severe operating conditions, namely a temperature above 500 degrees C and a pressure above 150 bar. As a result, 2.4% of the fossil raw materials consumed each year worldwide are used for NH3 synthesis (i.e. H2 production), reflecting the current carbon footprint of the process: 1.2% of the CO2 produced each year worldwide.
Research into catalysts for NH3 synthesis has recently flourished thanks to the use of NH3 as a hydrogen carrier for long-term storage and transport, thanks to existing industrial infrastructure. However, given this application, the traditional HB process is not suitable due to the centralisation of production plants, which is not compatible with the use of intermittent renewable energy sources, as well as the start-up and shutdown regime associated with intermittent green hydrogen production (water electrolysis with surplus electricity). Several alternatives are currently being studied at low technology readiness levels (TRL): chemical looping, photo-assisted catalysis, electro-assisted catalysis and plasma-assisted catalysis. The plasma-assisted process appears to be a realistic solution for a fully electrified process using green hydrogen production thanks to:
- a process operating at low temperature (ambient temperature) and low pressure,
- no thermodynamic limitations under these conditions,
- excellent reported conversion rates, thanks to the ability of plasma to facilitate the dissociation of the N-N triple bond, despite the fact that the origin of the process's efficiency is not yet fully understood.
This thesis, part of the PLASMA-N-ACT project, aims to understand and improve non-thermal plasma-assisted catalysis for ammonia synthesis, and to position this process in relation to the HB process in terms of key performance indicators (economic, energy and environmental criteria). New composite catalysts, based on rare earth nitride - transition metals - alkaline promoters, will be designed with low permittivity SiO2 and Al2O3 supports. The composites have multiple roles, as they: (i) facilitate the adsorption and activation of N2 and H2 reactants at the catalyst-plasma interface; (ii) stabilise the H (metal) and N (vacant sites) species that have formed through plasma excitation in the gas phase on the catalyst surface; (iii) reinforce the electric field around the catalyst surface.
The thesis consists of performing the following tasks:
- using experimental data to develop a kinetic model;
- modelling the pilot-scale reactor for plasma-assisted NH3 synthesis;
- intensifying the reactor and extrapolating to develop the industrial-scale reactor;
- modelling and optimising the entire process, taking into account the reaction kinetics determined previously.

Contexte de travail

The Reactions and Process Engineering Laboratory (LRGP) is dedicated to optimising unit operations used in the chemical, pharmaceutical and agri-food industries. The laboratory mainly studies transfer phenomena (matter, heat, momentum) and chemical reactions and their integration into innovative, more efficient and sustainable processes.
The PhD student will be assigned to the PRIMO (Reaction Process Intensification, Membranes and Optimisation) research area, which develops tools and methodologies for process intensification and synthesis with applications in catalytic reactors and separation processes (membrane).
From an organisational point of view, the student will be supervised on a very regular basis through weekly meetings, or even fortnightly meetings if necessary. An annual thesis monitoring committee will be set up. The PhD student will participate in at least two national or international conferences and will attend Plasma N-ACT project monitoring meetings. If necessary and depending on the needs of the project, funding for training may be arranged.

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.

Contraintes et risques

The PhD student will work exclusively on modelling tasks.
There will be no experimental work during the PhD, so there will be no risk associated with handling chemicals.
The PhD student will be required to produce at least two publications during the funding period for the PhD.