Mechanism of the Pyrolysis of Acetylene

Abstract

A free radical chain mechanism is presented to describe the kinetics of the pyrolysis of acetylene over the temperature range 700-2400 O K . The mechanism is based on the following initiation reaction and subsequent addition, abstraction, and decomposition reactions of the radicals involved are shown to account for the products observed and for the kinetic characteristics of the reaction.Pour decrire la cindtique de la pyrolyse de l'acdtylene entre 700 et 2400 OK, on propose un mdcanisme en chaine impliquant des radicaux libres et qui serait bask sur la rkaction d'initiation suivante 2 C z H z -+'C2H +*CzH3 des rkactions subsdquentes d'addition, d'abstraction et de ddcomposition des radicaux impliquks permettent de rendre compte des produits obtenus et de la cinktique caractdristique de la rdaction.

Radical processes in the pyrolysis of acetylene

“…These features of the reaction unambiguously pointed to a free radical character for the pyrolysis with the induction period and subsequent auto acceleration produced by the initial build-up of one or more reactive auto catalytic intermediates. Thus all early efforts to provide a mechanism assumed it was a relatively simple free radical mechanism [1, 3,9,10]. These efforts were limited by the lack of accurate thermochemical data on both molecules and radicals and frequently used what are now considered, grossly incorrect heats of formation, rate constants or both.…”

“…The main features of the pyrolysis which have emerged have been the following: (1) Below 1000°K the reaction is predominantly a polymerization producing very few species smaller than C4H4 [l,2]; (2) The overall reaction rate appears to be close to second order in C2H2 concentration [l-41; (3) There is a distinct and reproducible induction period which decreases with increasing temperature and increasing acetylene concentration [ 1, 2,4]; (4) Small amounts of NO (0.1-20%) can almost totally inhibit the reaction [1, 2,6,7]; (5) During the NO inhibited reaction, the NO is very slowly consumed together with some acetylene after which the reaction proceeds with its normal rate [1,2]; (6) The pyrolysis is surface catalyzed in quartz vessels; (7) NO inhibits the gas phase reaction up to 1400°K [5], even at 2% NO concentrations; (8) 1% of HBr reduces the rate 18 fold at 700°K [8]; (9) At 500"C, 1.5% of styrene [20] or vinyl acetylene [19] accelerate C2H2 polymerization 3-fold and 5-fold, respectively, with no induction period evident.…”

“…Thus all early efforts to provide a mechanism assumed it was a relatively simple free radical mechanism [1, 3,9,10]. These efforts were limited by the lack of accurate thermochemical data on both molecules and radicals and frequently used what are now considered, grossly incorrect heats of formation, rate constants or both.…”

Direct synthesis of hydrogenated graphene via hydrocarbon decomposition in plasmas

“…Earlier, we investigated the possibility of synthesizing free graphene by means of high-power plasma torch at the pressure of 150 -710 Torr [11,12]. Analysis of the processes in the carbon nucleation zone of the plasma jet generated by the plasma torch indicates high concentration of the atomic hydrogen and of the C 6 H 6 -like compounds; these substances, according to [13][14][15][16], might promote the graphene formation. Additionally, it is common knowledge that application of the DC plasma torches makes it possible to synthesize high-quality pure carbon nanomaterials from inexpensive precursors [17], thus reducing the requirements to the carbon source, its cost and the final product cost.…”

Modelingof Acetylene Pyrolysis under Steel Vacuum Carburizing Conditions in a Tubular Flow Reactor

“…Detailed kinetic mechanisms proposed in literature may consist of hundreds of species and reactions. Typically, these investigations include shock tube pyrolysis of acetylene [1][2][3][4][5][6][7][8][9][10][11][12] and pyrolysis in flow systems [13][14][15]. The deposition of pyrolytic carbon from various hydrocarbons has also been described by some researchers [16][17][18][19][20].…”