Detailed ion chemistry in methaneoxygen flames. II. Negative ions

Abstract

The detailed negative ion chemistry is discussed for two premixed methaneoxygen flames of fuel-lean and fuel-rich compostion burning at atmospheric pressure. As in the preceding paper on positive ions (Part I), a complete family of concentration profiles for natural, negative flame ions has been measured mass spectrometrically below 100 amu for each flame, sampled along the axis through the flame front. The ion chemistry is explained with reference to published data on neutral concentration profiles, rate constants for ion-molecule reactions, and thermochemical values when available. Total positive and negative ion profiles show the relative location of the negative ions. The negative ions initiate upstream as O 2 − by electron attachment to O 2 in both flames. A group of profiles peaking in the reaction zone arises by charge-transfer and proton-transfer reactions with a variety of netural intermediates where OH − and O −, being strong bases, play a major role; in the fuel-lean flame the ion pairs CO 3 − CHO 3 − and CO 4 − CHO 4 − are exceptional, however. All profiles decrease rapidly near the downstream edge of the reaction zone by collisional detachment processes and associative detachment with hydrogen, hydroxyl, and oxygen radicals. In the fuel-lean flame only, several oxygenated ionic species persist through the burnt gas. The outstanding difference in the fuel-rich flame is the presence of carbonaceous anions C n H x −( n ⩾ 2, x = 0–3). Proton abstraction from stable acetylenic compounds yields broad single-peaked profiles. In contrast, carbonaceous radicals give rise to sharp double-peaked profiles where the downstream peaks appear to involve nondissociative electron-attachment processes. Compared with the positive ion discussion (Part I), these negative ion studies provide both corroborative and additional information about the underlying neutral chemistry of combustion.