Design of Materials for Electroreduction of Nitrogen to Ammonia: A Computational Perspective

Abstract

Theoretical studies and computational modeling play a key role in predicting various properties of catalytic materials, including their activity and selectivity towards a desired electro-catalytic pathway. In the last decade, density functional theory calculations have been used to study the activity and selectivity of various materials for the nitrogen electroreduction, an extremely important aspect in the development of novel technologies for the ammonia synthesis. However, the design of an ideal electro-catalyst that exhibits high activity for nitrogen reduction to ammonia while at the same time suppresses the competing hydrogen evolution reaction is still an ongoing pursuit. Here we will use the example of molybdenum nitride and molybdenum carbide to illustrate different aspects in the design of materials for nitrogen electroreduction to ammonia and discuss different challenges associated with this effort [1, 2]. We will first present calculations of free energy diagrams and kinetic volcano plots aimed at understanding the activity of different surfaces of the molybdenum nitride (Mo2N) and molybdenum carbides (MoC and Mo2C) towards hydrogen evolution reaction and nitrogen electroreduction to ammonia. Secondly, we will discuss the thermodynamics calculations directed towards understanding the stability of these materials at different cell potentials and pH. Namely, our results show that at the nitrogen reduction potentials the catalytic surface can be covered with the H-adatoms rather than the N-adatoms. In addition, the carbides and nitrides can be converted to oxynitrides or oxides, which can lead to the decrease in the activity and durability of the catalytic material. Molybdenum carbides and nitrides will be used to illustrate how computational modeling can direct the design of the active sites with highest efficiency for nitrogen reduction to ammonia, but also support the optimization of reaction conditions to ensure the catalyst durability. [1] I. Matanovic, F. H. Garzon, and N. J. Henson, Electro-reduction of Nitrogen on Molybdenum Nitride: Structure, Energetics, and Vibrational Spectra from DFT, Phys. Chem. Chem. Phys. 2014, 16: 3014-3026. [2] I. Matanovic and F. H. Garzon, Nitrogen Electroreduction and Hydrogen Evolution on Cubic Molybdenum Carbide: A Density Functional Study, Phys. Chem. Chem. Phys. 2018, 20: 14679-14687.