KEY REACTIONS IN THE OXIDATION OF ACETYLENE BY ATOMIC OXYGEN

An overview is presented on kinetic and mechanistic investigations, du ring the last 8 years, of key elementary reactions in the oxygen-atom initiated oxidation of acetylene, an important intermediate in the com bustion of all hydrocarbons. The reactions reported on involve the hig hly reactive species HCCO, CH2((XB1)-B-3), CH2(a(1)A(1)), CH(X-2 Pi), CH(A(2) Delta), C3H2, and C2H(X-2 Sigma(+)) which arise in this oxidat ion process and which play a crucial role in a number of important fla me phenomena, such as prompt NO formation, NO reburning, the chemistry of soot precursors, chemiluminescence and chemi-ionization. Most of t he studies were carried out on low-pressure, helium-diluted C2H2/O/H ' 'atomic flames'' in an isothermal flow reactor, at temperatures in the 290 - 1000 K range, using the Discharge - Flow technique combined wit h Molecular Beam sampling -Threshold Ionization Mass Spectrometry. Som e elementary reactions of C2H were investigated by means of the Pulse Laser Photolysis /Chemiluminescence technique. Several of the reaction s discussed have been newly identified, such as: a minor channel of th e HCCO + O reaction forming CH(X-2 Pi and a (4) Sigma) + CO2; a domina nt channel of the fast C3H2 + O reaction forming C2H + CO and being th e main source of C2H in the C2H2/O/H systems; and the C2H(X-2 Sigma(+) ) + O --> CH(A(2) Delta) + CO reaction, which was unambiguously identi fied as the source of the strong CH chemiluminescence of the investig ated atomic flames and of which the (channel) rate coefficient was fou nd to be somewhat above 10(-11) cm(3)s(-1). Rate coefficient and produ ct distribution measurements of various other fast reactions of HCCO, CH2(X), CH(X), C3H2 and C2H(X) are discussed. Finally, a detailed reac tion mechanism of C2H2/O/H systems, based in part on the reported work , is discussed; this mechanism was shown to reproduce observed concent ration-vs.-time profiles of the important reactive C-1-C-4 species in atomic flames to a high degree of accuracy.