(145c) DFT-Informed Energetics of Plasma-Enabled Reactions Pathways and Microkinetic Modeling for Ammonia Synthesis on Transition and Low-Melting Point Metals

Ammonia is a crucial chemical (> 200 millions tons a year) due to its widespread use in fertilizer production, and its potential as a hydrogen carrier in a future "hydrogen economy." Currently synthesized using thermal catalysis, ammonia synthesis is currently a fossil energy-powered, high temperature-high pressure, energy-intensive process that is responsible for ~2% of the world's CO2 emissions. Ideally, as the world transitions to cleaner, renewable energy, the net CO2 emissions with ammonia synthesis would decrease. However, the harsh conditions required for thermally catalyzed ammonia synthesis demands a highly centralized, continuously-on process that is incompatible with renewable energy. Thus, currently, alternative methods to synthesize ammonia under milder conditions are actively sought, including plasma catalysis in plasma reactors.

Optimizing plasma-assisted ammonia synthesis, including optimizing the catalyst to be loaded on the reactor, is currently challenging due to an incomplete understanding of the most relevant reaction mechanisms involved in the process. One snippet of this debate include the role of vibrational versus radical species on ammonia formation. Some first principles-informed models have attempted to shed light on the relevant mechanism, but centered on vibrational species and only the typical Langmuir-Hinshelwood reactions associated with thermal catalysis were considered [1]. More comprehensive models including plasma radicals also attempted similar insights, but used generic, metal-agnostic parameters, and while including Eley-Rideal reactions, it only considered NH_X species [2]. These models thus may be at odds with experiments reporting the appearance of N2Hx species on the metal [3].

To enable first principles-informed comprehensive modeling of all plausible Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) reactions involving N_YH_X surface species and plasma radicals we performed density functional theory (DFT) to obtain reaction energies for 50+ reaction events across 9 metal surfaces of different degrees of nitrophilicity (Fe, Co, Ni, Pd, Cu, Au, Ag, Ga, Sn), and used scaling relationships (both from the literature and new ones generated from our own select DFT calculations) to provide corresponding activation energies. Hydrogen dissolution events were also included in our calculations, motivated by previous observation of a hydrogen sink effect in experiments by some of us [4]. Ultimately, the obtained data was input into a microkinetic model, which we used to explore the effect of plasma composition on catalyst state and dominant reaction pathways. Our results indicate that plasma radical species such as N(g) and H(g) only need minimal concentration in the plasma phase (even several orders of magnitude lower than that of vibrational species) to make ER pathways dominant and the formation of N_YH_X species plausible. However, N_YH_X seem more likely to hydrogenate until saturation and spontaneous dissociation via ER reactions with H(g) radicals than to dissociate early via LH reactions. The dominant role of ER reactions makes for a crucial factor blurring differences in activities across different metals. Finally, we present relationships between energetic descriptors and experimentally measured catalyst activities in DBD and RF reactors.

1. Metha et al. Nature Catal. 2018, 1, 269-265

2. Van't Veer et al. J. Phys. Chem. C 2020, 124, 42, 22871–22883

3. Lea et al. ACS Catal. 2020, 10, 24, 14763–14774

4. Shah et al. ChemCatChem 2020, 12, 1200-1211