Abstract
Methylidyne (CH) was prepared on Pt(1 1 1) by three methods: thermal decomposition of diiodomethane (CH 2I 2), ethylene decomposition at temperatures above 450 K, and surface carbon hydrogenation. Methylidyne and its precursors are characterized by reflection absorption infrared spectroscopy (RAIRS). The C–I bond of diiodomethane breaks upon adsorption to produce methylene (CH 2), which decomposes to methylidyne at temperatures above 130 K. Above 200 K, methylidyne is the only hydrocarbon species observed with RAIRS, although reaction channels for the formation of methane (CH 4) and ethylene (C 2H 4) are indicated by temperature programmed desorption (TPD). As is well known from numerous previous studies, ethylene decomposes to ethylidyne (CCH 3) upon exposure to Pt(1 1 1) at 410 K. Upon annealing to 450 K, ethylidyne dissociates through two reaction pathways, dehydrogenation to ethynyl (CCH) and C–C bond scission to methylidyne. Ethylene dehydrogenation on the surface at 750 K and under low ethylene exposures produces surface carbon that can be hydrogenated to methylidyne with C–H and C–D stretch frequencies of 2956 and 2206 cm −1, respectively. Hydrogen co-adsorption on the surface causes these frequencies to shift to higher values. Methylidyne is stable on Pt(1 1 1) to temperatures up to 500 K.
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