Surface chemistry of C[sub 1] hydrocarbon fragments on copper: C-H bond activation and C-C bond formation

Thesis/Dissertation · Tue Dec 31 23:00:00 EST 1991

OSTI ID:7302464

Chiang, C M

This thesis focuses on the synthesis, bonding, reaction mechanisms, kinetics, and thermodynamics of methyl (CH[sub 3]) and methylene (CH[sub 2]) bound to copper surfaces. These C[sub 1] hydrocarbon fragments have been studied on single-crystal surfaces under ultra-high vacuum conditions using mass spectrometry and electron spectroscopies. Methyl groups have been isolated on copper surfaces by: (1) trapping gas phase methyl radicals generated in the pyrolysis of azomethane (CH[sub 3]N[sub 2]CH[sub 3]), and (2) dissociatively adsorbing iodomethane (CH[sub 3]I). Surface vibrational analysis has identified the adsorbed methyl fragments, which disproportionate above 400 K to produce methane and ethylene in a 2:1 ratio. At higher coverages, methyl groups also couple to form ethane. A direct comparison of the bonding and chemistry of adsorbed methyl groups in the presence and absence of coadsorbed iodine indicates that the main effect of iodine is to block surface sites. Mechanistic studies in which H, CH[sub 3], and CH[sub 2] (formed by dissociative adsorption of CH[sub 2]I[sub 2]) were coadsorbed and/or isotopically-labeled show that C-H bond scission (CH[sub 3] [yields] CH[sub 2] + H) is the rate-determining step. Methane is subsequently formed by reductive elimination (CH[sub 3] + H [yields] CH[sub 4]) while ethylene is formed primarily by methylene insertion coupled with [beta]-hydride elimination (CH[sub 2] + CH[sub 3] [yields] C[sub 2]H[sub 4] + H). This methyl/methylene coupling reaction represents the first observation of the chain-growth step proposed in the Fischer-Tropsch synthesis for long-chain hydrocarbons. Numerical integration of the rate equations using experimentally-determined kinetic parameters shows that H recombination to form H[sub 2] and CH[sub 2] coupling to form C[sub 2]H[sub 4] can also occur. Copper-carbon bond strengths and heats of formation for the adsorbed hydrocarbon fragments have been determined.

• Research Organization: Columbia Univ., New York, NY (United States)
• OSTI ID: 7302464
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201* -- Chemical & Physicochemical Properties
ADSORPTION
ALKYL RADICALS
CHEMICAL ACTIVATION
CHEMICAL BONDS
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
COPPER
ELEMENTS
FISCHER-TROPSCH SYNTHESIS
KINETICS
METALS
METHYL RADICALS
METHYLENE RADICALS
RADICALS
REACTION KINETICS
SORPTION
SURFACES
TRANSITION ELEMENTS