Theoretical Investigation of Two-dimensional (2D) Materials for Nitrogen Activation

As the average CO2 concentration in the air is currently at 417 ppm and rising, the commercial Bosch-Haber’s process of NH3 production being a major contributor of CO2 emissions; it is eminent that sustainable methods of ammonia production have to be explored. While the commercial method of ammonia fixation from N2 gas faces the bottleneck of huge energy demand and carbon footprint; a commercial substitute for Bosch-Haber process is yet to be achieved. Considering the cost and energy needed to explore new materials, computational scanning and analysis will help in reducing efforts needed to develop new materials. N2 being a highly stable molecule, its activation at exothermic condition has always been a challenge. Given a rich electron source that can easily transfer electron to the nitrogen π* orbitals, N-N bond activation can be achieved without supplying large amount of heat. Implementing the idea of two-dimensional (2D) surfaces that can act as electron reservoirs, the reduction of N2 to NH3 on different families of 2D materials have been extensively discussed. The nature of catalysis on Mo-metal single atom catalyst (Mo-SAC) anchored on heteroatom doped graphene substrate of different sizes and edge orientations has been confirmed to be a local site activity (Chapter 3). The scope of implementing several metal SACs has been studied for N2 activation with the N-N bond elongation being triggered by the transfer of electrons from metal d-orbitals to N2 π* orbitals. More importantly, the quantitative catalytic efficiency of one metal over the other is highly influenced by the description of the atomic orbitals (Chapter 4). However, a qualitative trend in the catalytic nature of different metals can be investigated through the use of non-commercial academically licensed computational software. Further investigation on possible catalytic centers on graphene led to incorporating Al-metal clusters, the most earth abundant element, and a trade-off can be made on the catalytic center to be incorporated when graphene is to be used as the 2D support for nitrogen reduction reaction (NRR). A significant reduction in the Gibbs free energy change, ΔGNRR required for NRR can be reduced to 0.78 eV in Al5-cluster as compared to 1.24 eV in vanadium-SAC by modulating the chemical coordination around the metal centers (Chapter 5). We have also studied the catalytic efficiency of two electrically conductive 2D materials – Mo2C and vanadium dichalcogenides (VX2). The conductive nature of these 2D substrates enhances the process of electron transfer from the catalyst the antibonding orbitals of N2. Rather than relying on metal catalyst centers, we have used non-metal atomic centers for NRR on these conducting 2D materials. Here, boron is the atomic center of interest and when introduced as an adatom on defective Mo2C, ΔGNRR reduces to 0.57 eV (Chapter 6) and the same non-metallic boron atom center when introduced as a substitutional dopant on 2H-phase of VS2 shows a further reduction in ΔGNRR to 0.22 eV (Chapter 7). Furthermore, these catalysts can be realised as electrocatalyst due to their conducting nature, and the thesis highlights how the NRR performance can be enhanced with chemical modifications on different 2D substrates. Moving forward with the goal of obtaining commercially viable catalysts that do not involve tedious and sophisticated synthesis protocols that are mandatory for the above-mentioned materials, we have studied porous coordination compounds that can be achieved via reticular synthesis. Two-dimensional metal organic framework (MOF) comprised of porphyrin ligands and early transition metals are investigated for ammonia production at ambient aqueous condition, and Ti-based porphyrin MOF is being proposed as an active electrocatalyst for NRR with ΔGNRR of 0.35 eV (Chapter 8). Besides exploration on different 2D materials for N2 reduction to NH3, we have also put emphasis on studying the correlation between electronic properties of the catalysts and their NRR performance. The notable properties are band-centers, work function, atomic charges, density of states and free energy differences of NRR intermediates. These properties have been found to satisfactorily correspond to ΔGNRR, thereby establishing them as universal parameters for developing novel catalysts. This Ph.D. thesis focuses on the development of experimentally feasible sustainable 2D material with earth abundant and economically viable metal and non-metal active centers. The goal of this research is to develop an active, low cost, stable and efficient electrocatalyst for NRR via an exhaustive computational study.