N <sub>2</sub> oxidation kinetics in a ns-pulsed discharge above a liquid electrode
Mikhail Gromov, Kseniia Leonova, Nathalie De Geyter, Rino Morent, Rony Snyders, Nikolay Britun, Anton Nikiforov
Abstract
Abstract In this work, the kinetics of nitrogen fixation via plasma-induced N 2 oxidation in a 10 ns pulsed atmospheric pressure water-contacting discharge sustained in air is investigated. Two pulse regimes, a single pulse and a three-pulse burst of 100 kHz, are considered. The densities of relevant radicals (NO, O) are studied by time- and space-resolved laser-induced fluorescence spectroscopy. It is concluded that in a single pulse mode, O atoms are mainly generated by O 2 reacting with electronically excited states of N 2 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi>A</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> <mml:msubsup> <mml:mrow> <mml:mi mathvariant="normal">Σ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>u</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> </mml:mrow> </mml:msubsup> <mml:mo>,</mml:mo> <mml:msup> <mml:mrow> <mml:mi>B</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">Π</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>g</mml:mi> </mml:mrow> </mml:msub> <mml:mo>,</mml:mo> <mml:msup> <mml:mrow> <mml:mi>C</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">Π</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>u</mml:mi> </mml:mrow> </mml:msub> </mml:math> ) and are primarily reduced as a result of O 3 formation. The O density shows a maximum at ∼100 ns after the plasma pulse with number density of ∼10 23 m −3 . NO radicals, on the other hand, are primarily formed by reacting with the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mtext>N</mml:mtext> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> <mml:mfenced close=")" open="("> <mml:mrow> <mml:msup> <mml:mrow> <mml:mi>A</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> <mml:msubsup> <mml:mrow> <mml:mi mathvariant="normal">Σ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>u</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:mfenced> </mml:math> state (up to ∼1 μ s after the pulse) and with OH radicals (up to ∼10’s of μ s), peaking at approximately 60 μ s with a peak density of ∼10 21 m −3 . The NO loss pathway is represented by the reversed Zeldovich mechanism at short time delays (∼10’s μ s), whereas at longer delays (>100’s of μ s) HNO 2 and NO 2 formation causing NO loss start to be dominant. In the burst mode, the energy efficiency of NO formation decreases despite higher N 2 conversion, for which three reasons are suggested: (1) NO removal by the generated <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mtext>O</mml:mtext> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mrow> <mml:mmultiscripts> <mml:mrow> <mml:mi>D</mml:mi> </mml:mrow> <mml:none/> <mml:none/> <mml:mprescripts/> <mml:none/> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> </mml:mmultiscripts> </mml:mrow> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:math> after the discharge pulse through the reverse Zeldovich mechanism, (2) NO oxidation via the accumulated O 3 , (3) pre-ionization induced by high pulse repetition rate (100 kHz) leading to shrinkage of the plasma bulk.