Flash Decomposition of Acetylene, Ethylene, Ethane, and Methane on Tungsten

The decomposition of acetylene, ethylene, ethane, and methane initially adsorbed on tungsten wires at 95°K was investigated by flash-filament spectroscopy, with products identified by mass spectroscopy; the major gas-phase product was hydrogen in all four cases. The flash-filament spectrum of ethylene consists of two equal hydrogen peaks resulting from a two-step dehydrogenation of chemisorbed ethylene: $$*\mathrm{CH}\text{2–}\mathrm{CH}\text{2*→}{\displaystyle {lim}^{\text{>200°K}}}*\mathrm{CH}=\mathrm{CH}\text{*+2}\mathrm{H}\mathrm{*,}$$ $$\mathrm{CH}\mathrm{*=}\mathrm{CH}\mathrm{*\to }{\displaystyle {lim}^{\text{>̃300°K}}}(\mathrm{C}\text{2)+2}\mathrm{H}\mathrm{*,}$$ $$2\mathrm{H}\mathrm{*\to }{\displaystyle {lim}^{\mathrm{fast}}}\mathrm{H}\text{2(}\mathrm{g})$$ . Acetylene dehydrogenation follows Reactions (2) and (3). When compared to previous results obtained on iridium, ethylene is seen to be more unstable on tungsten than on iridium. [Reaction (1) proceeds at an appreciable rate above 200°K on tungsten but only above 400°K on iridium.] This difference in stability of surface ethylene is discussed in terms of the possible surface structures of ethylene imposed by the topography of the metal surface. Adsorption of ethane and methane at 95°K is interpreted in terms of the following steps: $$\mathrm{CH}\text{4(}\mathrm{g}\mathrm{)\to *}\mathrm{CH}\text{3+}\mathrm{H}\mathrm{*,}$$ $$\mathrm{C}2\mathrm{H}\text{6(}\mathrm{g}\mathrm{)\to *}\mathrm{CH}\text{2–}\mathrm{CH}\text{3+}\mathrm{H}\mathrm{*,}$$ and, where adjacent sites are available, $$*\mathrm{CH}\text{2–}\mathrm{CH}\text{3→*}\mathrm{CH}\text{2–}\mathrm{CH}\text{2*+}\mathrm{H}\mathrm{*.}$$ Detailis found in the flash-filament spectra resulting from each of these surface species. The major peak in the spectra of both ethane and methane occurs at 570°K and is attributed to the decomposition of the surface methyl group (*CH₃) in the case of methane, and the surface ethyl group (*CH₂–CH₃) in the case of ethane. A mechanism is proposed for the decomposition of these species which is consistent with the present results and with the hydrogenolysis activity of tungsten.