Plasma catalytic ammonia synthesis

November 30th, 2021

R&D Focus Areas:
Ammonia, Computational modelling, Electricity

Lead Organisation:
CSIRO

Partners:
Not applicable

Status:
Completed

Start date:
March 2019

Completion date:
May 2022

Key contacts:
Senior Researcher Anthony B. Murphy – tony.murphy@csiro.au

Funding:
CSIRO ResearchPlus CERC Fellowships

Project total cost:
AUD$350,000

Project summary description:
Ammonia can be used as a storage molecule for the safe storage and transport of hydrogen for the hydrogen economy. It is an excellent choice as a hydrogen storage molecule due to the stoichiometric ratio of H to N in the NH3 molecule. Furthermore, it is cheaper to ship than hydrogen and can be transported using existing natural gas infrastructure. A need therefore exists to produce green ammonia for the hydrogen economy; and non-thermal plasma may provide an answer. This is a completely electrical process, making it easy to combine with renewable energy to prevent the release of CO2. In non-thermal plasma, the energy is transferred to the electrons, which become highly excited with electron energies between 1 and 10 eV, whilst the bulk gas remains near room temperature. As such, thermodynamically unfavourable reactions can occur in the plasma at near ambient conditions.

However, plasma production of ammonia currently lacks efficiency. There is a trade-off between yield and efficiency as yield is increased at higher voltage input whilst energy efficiency decreases. This can be somewhat overcome at the addition of a catalyst. Interactions between plasma and catalyst can create synergy, resulting in a higher ammonia yield than the plasma-alone plus the purely catalytic processes. Changes can occur in the plasma due to the presence of a catalyst, including electric field enhancement and changes in discharge mode; whilst the plasma can cause changes to the physicochemical properties of the catalyst, as well as lowering the activation barrier and changing reaction pathways at the catalyst surface. However, energy efficiencies remain too low to be viable.

This project focus is on increasing the energy efficiency of the plasma process whilst also achieving an acceptable yield. This is being done via four pathways: Reactor design; catalyst design; in-situ studies; and modelling.

  1. Reactor design. An innovative reactor that combines plasma with other technologies was designed. We expected this to increase the energy efficiency of the process through greater control of the energy transferred to different plasma species. Unfortunately, the reactor was never constructed. We also used a standard dielectric barrier discharge (DBD) reactor to investigate different catalysts.
  2. Catalyst design. Several novel catalysts are being tested, some of which are expected to follow the Mars–Van Krevelen mechanism, which has a lower energy barrier than the conventional Langmuir–Hinshelwood mechanism. Initial results are encouraging, showing increased ammonia yield.
  3. In-situ studies. Interactions between the catalyst and plasma are not yet well understood. In-situ techniques, such as in-situ FTIR, are required to detect important surface adsorbed species which can indicate the reaction mechanisms. This data can help us create highly active catalysts. Preliminary studies were completed; however, it was difficult to distinguish between species on the catalyst surface and those on the surrounding glass. Future research would require adapting the IR beam to increase accuracy.
  4. Modelling. Our kinetic model is one of the most extensive models available for non-thermal plasmas interacting with a surface. We used insights obtained from the model to guide reactor and catalyst design. In a collaboration with Columbia University, we used the model to gain deeper insights into the experimental data provided by the US team. (See paper by Winter et al. below). We also collaborated with the University of Sydney to investigate an alternative route that used nitrogen and water as precursors instead of nitrogen and hydrogen (see papers by Hong et al. and Zhang et al. below).

Related publications and key links:
Lea R. Winter, Bryony Ashford, Jungmi Hong, Anthony B. Murphy and Jingguang G. Chen. Identifying surface reaction intermediates in plasma catalytic ammonia synthesis. ACS Catal., 10, 14763-14774, 2020.

Annemie Bogaerts et al. The 2020 plasma catalysis roadmap. Phys. D: Appl. Phys., 53, 443001, 2020

Annemie Bogaerts, Erik C. Neyts, Oliver Guaitella and Anthony B. Murphy, Foundations of plasma catalysis for environmental applications, Plasma Sources Sci. Technol., 31, 053002, 2022

Jungmi Hong, Tianqi Zhang, Renwu Zhou, Liguang Dou, Shuai Zhang, Rusen Zhou, Bryony Ashford, Tao Shao, Anthony B. Murphy, Kostya Ostrikov and Patrick J. Cullen, Green chemical pathway of plasma synthesis of ammonia from nitrogen and water: a comparative kinetic study with a N2-H2 system, Green Chem., 24, 7458–7468, 2022.

Tianqi Zhang, Renwu Zhou, Shuai Zhang, Rusen Zhou, Jia Ding, Fengwang Li, Jungmi Hong, Liguang Dou, Tao Shao, Anthony B. Murphy, Kostya Ostrikov and Patrick J. Cullen, Sustainable ammonia synthesis from nitrogen and water by one-step plasma catalysis, Energy Environ. Mater., 6, e12344, 2023.

Higher degree studies supported:
This project supported a Postdoctoral Fellow (Bryony Ashford) and builds on a previous PhD studentship at CSIRO, undertaken by Jungmi Hong and supervised by Anthony B. Murphy.

 

Reviewed: November 2023