The Homogeneous Pyrolysis of Acetylene II: The High Temperature Radical Chain Mechanism

Abstract

A comprehensive kinetic/thermochemical model for the high-temperature radical chain decomposition of acetylene and the polyacetylenes is presented. This mechanism is tested against new shock tube data: time-of-flight mass spectra in C₄H₂/H₂, mixtures over 1600-2100K and laser schlieren experiments in C₂H₂; covering 2700-3500 K. Some earlier time-of-flight measurements on C₂H₂, are also modeled. The paper includes a very extensive consideration of species thermochemistry, and the model is in full accord with the current consensus, having Δ_ƒ H⁰(C₂H) = 135·5kcal/mol. Entropies are computed from recent determinations of molecular parameters for C,H and from new high-level ab initio calculations for C₄H reported here.Thetime-of-flight measurementssupport Δ_ƒ H⁰(C₂H₂)= 111 kcal/mol or 28·24kcal/mol for the [C,-(C₁)] group. Rate constants for the key C-H fission reactions of the acetylenes are taken from RRKM calculations calibrated against the direct measurements of dissociation in C₂;H₂, and C₄H₂:. Although there are still some problems with the thermochemistry (in particular, the model probably underestimates the participation of small carbon clusters), it does accurately simulate the early kinetics of the pyrolysis quite generally. The model also resolves some long-standing problems; for example, the current high value for Δ_ƒ ⁰ of C₂H is now fully compensated by an increase in its entropy, and the chosen rate ofC₄H₂, dissociation is completely in line with the direct measurements of this rate.