A combined experimental and theoretical study of the catalytic dissociation of methane by the photolysis of acetylene at 185 nm

Experimental irradiations of CH4-C2H2 (1000:1) gas mixtures, at 185 nm, hav e been carried out in order to test the catalytic scheme of dissociation of methane, via the photolysis of acetylene, that is supposed to occur in Tit an's stratosphere. The theoretical evolution of such a system has been inve stigated using three chemical models, and the predictions compared with our experimental data. Our experiments confirm that dissociation of methane ca n be catalysed by the photolysis of acetylene at 185 nm. The quantitative c omparison with theoretical results reveals an experimental effect, which we explain as a heterogeneous process occurring on the walls of the reactor. We propose a mechanism for this wall effect that is in qualitative agreemen t with our observations. We are currently developing an approach that will allow quantification of this effect, with the aim of extrapolating laborato ry results to an open system. We will therefore be able to perform a critic al comparison with existing gas-phase schemes to improve the chemical model ling of Titan's atmosphere. (C) 1999 Elsevier Science Ltd. All rights reser ved.