Vacuum-Ultraviolet (147 nm) Photolysis of Carbon Suboxide in the Presence of Methane

Abstract

The vacuum‐ultraviolet photolysis of C₃O₂ in the presence of CH₄ and Ar has been investigated at 25°C using the 147 nm Xe resonance line. The principal gaseous reaction products include CO, C₂H₂, C₂H₄, and some C₂H₆. The yield of acetylene decreases with increasing methane or inert gas pressure while the opposite trend is observed in the case of ethylene. Diagnostic experiments with added H₂ and CO have shown that excess H₂ effectively suppresses C₂H₄ formation, while excess CO virtually eliminates the production of C₂H₂. The results are interpreted in terms of the participation of ¹D and ³P carbon atoms and are consistent with recent flash photolysis‐kinetic spectroscopy work on the C₃O₂/CH₄ system. It is proposed that C₂H₂ formation can be attributed entirely to the reaction $$\mathrm{C}{(}^1\mathrm{D)+}{\mathrm{CH}}_4\to {\mathrm{C}}_2{\mathrm{H}}_2+{\mathrm{H}}_2,$$ whereas C₂H₄ results from reaction of methane with ground state C₂O(³Σ) radicals, producted in a secondary process $$\begin{array}{c}\mathrm{C}{(}^3\mathrm{P)+}\mathrm{CO}+\mathrm{M}\to {\mathrm{C}}_2\mathrm{O}{(}^3\mathrm{\Sigma )+}\mathrm{M},\\ {\mathrm{C}}_2\mathrm{O}{(}^3\mathrm{\Sigma )+}{\mathrm{CH}}_4\to {\mathrm{C}}_2{\mathrm{H}}_4+\mathrm{CO}.\end{array}$$ Both Ar and CO deactivate C(¹D) to C(³P), however, relative to its rate of attack on methane, the deactivation of C(¹D) by argon is an inefficient process.