The conventional ammonia synthesis method - the Haber–Bosch process – converts atmospheric N2 into ammonia using H2 and metal catalysts. This process employs high pressures (100 - 200 atm) and temperatures (700−800 K). Non-equilibrium plasmas have been shown to synthesize ammonia using N2 and H2 at relatively lower temperatures and pressures (~473 K and 1 atm) in the presence of various catalysts. This plasma catalysis process has the potential to become an option for ammonia synthesis in remote applications. However, an atomistic understanding of non-equilibrium plasma-assisted catalytic processes is lacking. This is largely due to the complex nature of plasma catalysis. Free electrons in the plasma collide with heavier gas particles to generate a rich mix of reactive species, including vibrationally or electronically excited molecules, radicals, atoms, and ions. Additionally, the plasma may alter the rates of surface reactions as well. Interaction with plasma could also lead to the accumulation of surface charge on the catalyst surface. It is often useful to isolate these effects in order to study their effect on overall catalytic activity. We employ Density Functional Theory (DFT) calculations using the Gaussian and plane wave method to understand the effect of surface charge on surface reactivity. This method is applied to investigate plasma-catalytic ammonia synthesis on supported M/Al2O3 single-atom catalysts (M=Fe, Ru, Rh, Co, Ni, Pt). We investigate the effect of negative surface charge on the adsorption of nitrogen. We find that the surface adsorption characteristics of nitrogen are altered by the surface charge accumulation. The results from this modeling study could help explain the synergistic effects in plasma catalysis.

Understanding the significance of plasma-induced surface charging on ammonia synthesis through DFT modelling

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ACS Fall 2022

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