Methane Dissociative Adsorption on the Pt(111) Surface over the 300−500 K Temperature and 1−10 Torr Pressure Ranges

Abstract

The dissociative adsorption of methane on the Pt(111) surface has been investigated and characterized over the 1-10 Torr pressure and 300-500 K temperature ranges using sum frequency generation (SFG) vibrational spectroscopy and Auger electron spectroscopy (AES). At a reaction temperature of 300 K and a pressure of 1 Torr, C-H bond dissociation occurs in methane on the Pt(111) surface to produce adsorbed methyl (CH(3)) groups, carbon, and hydrogen. SFG results suggest that C-C coupling occurs at higher reaction temperatures and pressures. At 400 K, methyl groups react with adsorbed C to form ethylidyne (C(2)H(3)), which dehydrogenates at 500 K to form ethynyl (C(2)H) and methylidyne (CH) species, as shown by SFG. By 600 K, all of the ethylidyne has reacted to form the dissociation products ethynyl and methylidyne. Calculated C-H bond dissociation probabilities for methane, determined by carbon deposition measured by AES, are in the 10(-8) range and increase with increasing reaction temperature. A mechanism has been developed and is compared with conclusions from other experimental and theoretical studies using single crystals.

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“…When a defect-free platinum (111) crystal face was prepared, the dissociation probability of molecular hydrogen was below the detection limit of 10 -3 , 4 . When methane was incident on the Pt(111) surface the dissociation probability to produce carbon and hydrogen, which ultimately deposited a layer of carbon on the platinum surface, was below 10 -8 (27). This type of C-H bond breaking reaction cannot be detected at low pressures or in ultra-high vacuum.…”

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“…This is shown for the C-H dissociation of methane on the platinum (111) surface, which has a very low reaction probability that is one in a hundred million methane molecule upon incidence on the platinum surface would dissociate ( Figure 11a) [39] . It is immeasurable in vacuum but at one Torr of methane pressure, after 60 seconds vibrational spectroscopy can detect CH and other fragments that form on the metal surface ( Figure 11b).…”

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“…For example, the C-H dissociation of methane on Pt (111) has a probability as low as * 1 9 10 -8 at 300 K. After reaction of 1 Torr of methane with Pt(111) for 60 s at 300 K, the sum-frequency generation spectrum shows adsorbed methyl (CH 3 ) as a broad peak at 2880 cm -1 . If the same reaction is performed at 10 Torr of methane, a sharper peak at 2880 cm -1 indicates the formation of ethylidyne by coupling adsorbed CH 3 groups and C atoms on the surface [19]. The dramatic effect of reaction pressure on the selectivity have been observed in the cyclohexene hydrogenation and dehydrogenation reaction over the stepped Pt(223) surface with cyclohexene and hydrogen partial pressure ratio fixed at 1/10 [20].…”