Ammonia (NH₃) is emerging as a surprising contender in the quest for sustainable energy sources. Known primarily as a fertilizer ingredient, ammonia’s potential as a hydrogen carrier is being recognized for its practicality and environmental benefits. Unlike pure hydrogen, ammonia is easier to store and transport, and it can be “cracked” back into hydrogen without producing CO₂. Yet, ammonia production through the Haber-Bosch process remains highly energy-intensive, consuming over 1% of global energy and generating considerable CO₂ emissions. Plasma technology, however, could change that by enabling low-energy ammonia synthesis and decomposition.
In a recent study by KAUST and UCL researchers, plasma-driven ammonia processing was modelled using advanced electron collision data. Using QEC (Quantemol Electron Collision) software, the team produced theoretical cross-sections that capture how electrons interact with ammonia molecules and their reactive fragments—NH₂ (amidogen) and NH (imidogen). These cross-sections describe the probability of various electron-molecule interactions, including scattering, excitation, ionization, and dissociation. This data is essential to designing efficient plasma systems for sustainable ammonia production and decomposition.
How Plasma Activation Works
Plasma is a partially ionized gas that contains free electrons and ions. Unlike conventional high-temperature reactions, plasma can drive chemical reactions through electron collisions rather than heat. When high-energy electrons strike ammonia molecules, they can trigger reactions at lower temperatures. These electron-driven processes are quantified through cross-sections, which represent the likelihood of each reaction type occurring. This is especially useful in plasma, where electrons enable reactions even in “cold” gases.
The reaction rate rᵢ for each process depends on factors including electron energy, cross-section data, and molecule properties. A simplified version of the rate equation used in the study is:
rᵢ = nₑ * N * kᵢ
where:
- nₑ = electron density
- N = density of gas molecules
- kᵢ = reaction rate coefficient derived from the cross sections and electron energy distribution.
This equation helps quantify how fast each reaction occurs under specific plasma conditions, critical for optimizing ammonia processing.
Key Findings: Cross Sections for Ammonia and Its Radicals
The study provides cross-sections for several critical processes:
- Elastic Scattering: When electrons collide without causing a chemical change. In polar molecules like NH₃, accounting for forward scattering is essential. Theoretical calculations suggest experimental values often underestimate scattering at low energies due to challenges in measuring forward-scattered electrons.
- Vibrational and Electronic Excitation: Electron impacts can excite ammonia’s vibrational or electronic states, leading to energy redistribution within the molecule or dissociation. These excitations often precede more significant reactions, like breaking NH bonds and creating NH₂ radicals.
- Ionization and Dissociative Electron Attachment (DEA): Ionization occurs when an electron collision removes another electron, forming a positive ion. DEA involves electron attachment that causes the molecule to split, producing a negative ion. Both processes are vital for sustaining plasma, generating ions and free radicals that fuel ongoing reactions.
Using QEC, the team generated cross-section data up to 35 eV (electron volts), relevant for most low-energy electron collisions. For example, DEA showed peaks around 4.97 eV and 10.57 eV, which align with Feshbach resonances observed in ammonia. These resonances—energetic states where electrons briefly stabilize—boost dissociation rates, helping to sustain plasma reactions and produce radicals like NH₂ and NH, essential for efficient ammonia cracking.
Implications for Sustainable Energy and Beyond
This cross-section data enables detailed plasma modelling, paving the way for a cleaner, more efficient ammonia synthesis and decomposition methods. Sustainable, electron-driven ammonia synthesis could make nitrogen fixation feasible without CO₂ emissions, revolutionizing fertilizer production. Similarly, plasma-assisted ammonia decomposition can generate hydrogen on demand without the challenges of hydrogen storage. This makes ammonia a viable, zero-carbon hydrogen carrier for fuel cells and combustion engines.
Beyond energy, ammonia’s plasma chemistry has applications in environmental studies, astrophysics, and planetary science. Ammonia and its radicals, found in various planetary atmospheres and interstellar regions, influence the chemical evolution of these environments. Accurate electron collision data support atmospheric models, enhancing our understanding of ammonia’s role in both Earth-based and extraterrestrial systems.
Future Steps with QEC and Plasma Research
The study showcases QEC’s power to simulate electron collisions with high precision, proving especially useful for low-energy interactions that are difficult to observe experimentally. While this research focused on ammonia, future studies could apply QEC to other nitrogen-based compounds or even refine cross-sections across a broader energy range. As plasma technology advances, tools like QEC will be instrumental in shaping sustainable chemical processes for a cleaner energy landscape.
Conclusion: A Decarbonized Future with Plasma-Driven Ammonia Processing
This research highlights ammonia’s potential to drive us closer to a low-carbon future, leveraging plasma technology for clean, efficient hydrogen production. By combining advanced computational tools with sustainable energy goals, scientists are building the foundation for a decarbonized future where ammonia plays a key role. Plasma technology thus not only transforms our approach to energy but also opens doors to novel applications across various scientific fields, underscoring the importance of robust data and innovative solutions in tackling today’s environmental challenges.
References:
Theoretical cross sections for electron collisions relevant for ammonia discharges part 1: NH3, NH2, and NH – IOPscience Ramses Snoeckx, Jonathan Tennyson, Min Suk Cha, Ramses Snoeckx et al 2023 Plasma Sources Sci. Technol. 32 115020