Ab Initio Study of the Role of Entropy in the Kinetics of Acetylene Production in Filament-Assisted Diamond Growth Environments

Abstract

We present a new theoretical strategy, ab initio rate constants plus integration of rate equations, that is used to characterize the role of entropy in driving high-temperature/low-pressure hydrocarbon chemical kinetics typical of filament-assisted diamond growth environments. Twelve elementary processes were analyzed that produce a viable pathway for converting methane in a feed gas to acetylene. These calculations clearly relate the kinetics of this conversion to the properties of individual species, demonstrating that (1) loss of translational entropy restricts addition of hydrogen (and other radical species) to unsaturated carbon-carbon bonds, (2) rotational entropy determines the direction of the rate-limiting abstraction reactions, and (3) the overall pathway is enhanced by high beta-scission reaction rates driven by translational entropy. These results suggest that the proposed strategy is likely applicable to understand gas-phase chemistry occurring in the systems of combustion and other chemical vapor depositions.

Direct Measurement of Radical-Catalyzed C6H6 Formation from Acetylene and Validation of Theoretical Rate Coefficients for C2H3 + C2H2 and C4H5 + C2H2 Reactions

“…For the reactions of C 2 H 3 and C 4 H 5 radicals with the vinyl iodide precursor, the rate for each was taken as equal to the reported rate of C 2 H 4 + CH 3 . 75 First-order loss processes are difficult to quantify in this experiment, but the rate of decomposition of C 2 H 3 is well known to be negligible at low pressures and temperatures below 1000 K, 76,77 and other first-order loss processes (wall-catalyzed recombination reactions, diffusion out of the sampling region) were minimized using sufficient C 2 H 2 concentrations to ensure a bimolecular loss rate 5−10 times faster than that expected for first-order loss (typically several hundred s −1 ). 52 Nevertheless, experiments on other reaction systems using the same apparatus indicate that there is likely some impact from wall reactions.…”