N Atom Radicals and N 2 (A 3 ∑ u + ) Found To Be Responsible for Nitrogen Oxides Conversion in Nonthermal Nitrogen Plasma Gui-Bing Zhao, Xudong Hu, Morris D. Argyle, and Maciej Radosz* Department of Chemical & Petroleum Engineering, University of Wyoming, Laramie, Wyoming 82071-3295 All species that are likely to be responsible for nitrogen oxides (N 2 O, NO, and NO 2 ) conversion in nitrogen plasma are analyzed in detail through carefully designed systematic experiments and theoretical analysis. The effect of ppm-level CO 2 , CO, and 1% CO on N 2 O conversion reveals that the N 2 O conversion occurs mainly by interaction with N 2 (A 3 ∑ u + ) excited species. The effect of 1% CO on the NO conversion suggests that only N atom radicals are predominantly involved in NO conversion. NO 2 conversion, on the other hand, occurs by interaction with both N 2 (A 3 ∑ u + ) and N atom radicals. Therefore, only two active species, N 2 (A 3 ∑ u + ) and N atom radicals, are found to be responsible for nitrogen oxides conversion in nitrogen plasma. Introduction Application of short-duration high-voltage pulses to a wire-cylinder reactor producing nonthermal plasma induced by highly nonhomogeneous electric fields (co- rona discharge processes) has been extensively investi- gated 1-5 and used for conversion of nitrogen oxides (NO x ). 6-16 When the wire electrode is positively charged, induced plasma channels (positive streamers) propagate from the wire anode to the cylinder cathode, and the discharge pulse itself is then called a positive corona discharge. The energetic electrons in the streamer can excite molecular nitrogen and produce many kinds of chemically active species, including metastable excited states (like N 2 (A 3 ∑ u + )), radicals (like N atoms), and cations (like N 2 + ), depending on the electron energy. These active species may contribute to the conversion of nitrogen oxides. Despite the extensive previous research, substantial uncertainty remains about the mechanism of nitrogen oxides conversion in nonthermal nitrogen plasma. In particular, the contribution of the electronic excited states of N 2 to N 2 O, NO, and NO 2 conversion has not been fully recognized. (1) N 2 O conversion: Three mechanisms for N 2 O conversion have been proposed. First, N 2 O conversion may occur through interaction with the electronic excited states of the N radical, N( 2 D), as suggested by Hill et al. 17 via the following reaction: Second, N 2 O conversion may occur through interaction with the lowest energy electronic excited state of N 2 , N 2 (A 3 ∑ u + ), as suggested by Thomas et al. 18 and Golde: 19 Third, N 2 O conversion may occur through interaction with the cation N 2 + , as suggested by Willis et al. 20 and Hu et al. 21 because the charge-transfer reaction of N 2 + and N 2 O has a large rate constant: 22 The subsequent electron-ion recombination dissociation reaction has an even larger rate constant: 23 (2) NO conversion: The conversion mechanism of ni- tric oxide (NO) in nonthermal plasmas has been more extensively investigated than N 2 O. Two mechanisms for NO conversion have been presented. Most investiga- tors 10,12,13,15,24-27 have proposed that N atom radicals are responsible for NO conversion in a balance gas of nitrogen. By contrast, Fresnet et al. 28,29 proposed that an excited electronic state of N 2 ,N 2 (a′ 1 ∑ u - ), plays the main role in NO conversion kinetics, due to the large rate constant for the reaction of N 2 (a′ 1 ∑ u - ) and NO: 30 (3) NO 2 conversion: Hu et al. 7 proposed that NO 2 conversion occurs by reaction with N radicals: A single positive streamer event is an ionization wave which propagates against the direction of the electron drift, with a typical velocity of 10 7 -10 8 cm/s. The plasma channel of the streamer has a radius of ∼10 -1 -10 -2 cm and contains ∼10 14 electrons/cm 3 , 31,32 with the result that the discharge lasts less than 100 ns in a cylindrical reactor that has a diameter on the order of 1 cm, as reported by Hu et al. 21 The postdischarge period (the interval period between the discharges, typically, larger * Corresponding author. E-mail: radosz@uwyo.edu. Tel: 307-766-2500. Fax: 307-766-6777. N( 2 D) + N 2 O f N 2 + NO k ) 1.32 × 10 12 cm 3 ‚mol -1 ‚s -1 N 2 (A 3 Σ u + ) + N 2 O f 2N 2 + O k ) 3.73 × 10 12 cm 3 ‚mol -1 ‚s -1 N 2 + + N 2 O f N 2 + N 2 O + k ) 3.61 × 10 14 cm 3 ‚mol -1 ‚s -1 N 2 O + + e f N 2 + O k ) 1.20 × 10 17 cm 3 ‚mol -1 ‚s -1 N 2 (a′ 1 Σ u - ) + NO f N 2 + N + O k ) 2.17 × 10 14 cm 3 ‚mol -1 ‚s -1 N + NO 2 f N 2 O + O k ) 1.81 × 10 12 cm 3 ‚mol -1 ‚s -1 N + NO 2 f NO + NO k ) 1.38 × 10 12 cm 3 ‚mol -1 ‚s -1 5077 Ind. Eng. Chem. Res. 2004, 43, 5077-5088 10.1021/ie049795z CCC: $27.50 © 2004 American Chemical Society Published on Web 07/16/2004