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

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 N₂H_x 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 N₂H_x decomposition pathways rather than NH_x 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.