Reaction Pathways and Energy Consumption in NH3 Decomposition for H2 Production by Low Temperature, Atmospheric Pressure Plasma

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Abstract

Pathways for NH3 decomposition to N2 and N2H4 by atmospheric pressure nonthermal plasma are analyzed using a combination of molecular beam mass spectrometry measurements and zero-dimensional kinetic modeling. Experimental measurements show that NH3 conversion and selectivity towards N2 formation scale monotonically with the specific energy input into the plasma with ~ 100% selectivity to N2 formation achieved at specific energy inputs above 0.12 J cm−3 (3.1 eV (molecule NH3)−1). The kinetic model recovers these trends, although it underpredicts N2 selectivity at low specific energy input. These discrepancies can be explained by the underestimation of reaction rate coefficients for reactions that consume N2Hx species in collisions with H radicals and/or radial nonuniformities in power deposition, gas temperature, and species concentrations that are not represented by the plug flow approximation used in the model. The kinetic model shows that N2 formation proceeds through N2Hx decomposition pathways rather than NHx decomposition pathways in low temperature, atmospheric pressure plasma. Higher selectivity toward N2 production can be achieved by operating at higher NH3 conversion and with a higher gas temperature. The high energy cost of NH3 decomposition by atmospheric pressure nonthermal plasma found in this work (25–50 eV (molecule NH3 converted)−1; 17–33 eV (molecule H2 formed)−1) is a result of the energy requirement for electron-impact dissociation of NH3 and the significant re-formation of NH3 by three-body recombination reactions between NH2 and H.

Original languageEnglish (US)
Pages (from-to)2101-2118
Number of pages18
JournalPlasma Chemistry and Plasma Processing
Volume44
Issue number6
DOIs
StatePublished - Nov 2024

Bibliographical note

Publisher Copyright:
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2024.

Keywords

  • Ammonia decomposition
  • Kinetic modeling
  • Molecular beam mass spectrometry
  • Nonthermal plasma

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