Adsorption and decomposition of methyl halides on Ni(100): A TPD, SIMS and AES study

Abstract

The adsorption and decomposition of CH 3X (X = Cl, Br and I) on Ni(100) has been studied using TPD, SIMS and AES. CH 3Cl adsorbed at 100 K desorbs with almost no decomposition. Low coverages of CH 3Br, however, dissociate readily to surface CH 3(a) and Br(a) at 100 K. At higher coverages of CH 3Br, adsorption is molecular and dissociative. The maximum amount of CH 3Br dissociated is 0.16 ML; 0.12 ML decomposes to surface hydrogen, H(a), and carbon, C(a) and 0.04 ML forms CH 4. The adsorption of CH 3I is almost completely dissociative at 100 K, forming mainly CH 3(a) and I(a). Upon heating, low coverages of CH 3(a) decompose completely to H(a) and C(a) below 260 K and there is no evidence for significant accumulation of either CH(a) or CH 2(a). For high coverages of CH 3(a), CH 4 is formed from CH 3(a) and H(a) with a reaction activation energy of 10.0 ± 1.0 kcal/mol. The maximum amount of CH 3I dissociated is 0.17 ML; 0.09 ML decomposes to H(a) and C(a) and 0.08 ML forms CH 4. On deuterium precovered Ni(100), methane formation is enhanced, peaks at 18 K lower temperature, is dominated by CH 3D and occurs at low coverages of CH 3(a). The effective activation energy for methane formation on D/Ni(100) is 7.5 ± 1.0 kcal/mol. There is no evidence for the formation of CH 2D 2. No C 2 and higher hydrocarbons are detected in TPD for any of the methyl halides.