ALKYL CHAIN PROPAGATION BY METHYLENE INSERTION ON CU(100)

One of the mechanisms proposed for formation of carbon-carbon bonds in the Fischer-Tropsch synthesis, the so-called carbide/methylene mechan ism, involves the propagation of alkyl chains on the catalyst surface by methylene insertion. The studies reported here provide evidence for this reaction on single crystal copper surfaces under ultra-high vacu um conditions. Alkyl iodides are used as molecular precursors to gener ate adsorbed methylene and alkyl groups on a Cu(100) surface. High-res olution electron energy loss spectroscopy and work function change mea surements show that C-1 bond dissociation occurs below 200 K in iodoal kanes to form alkyl groups on the surface. Indirect evidence supports the formation of adsorbed methylene groups via CH2I2 dissociation. Tem perature-programmed reaction studies of the CH2 + CD3 reaction show th at sequential CH2 insertion followed by beta-hydride elimination produ ces ethylene-d2 and propylene-d3. Similarly, reaction of CH2 with C2D5 produces propylene-d4. All of these reactions are extremely facile, o ccurring at 230-250 K with activation energies of 12-20 kcal/mol. Simi lar studies on Cu(110) show that the methylene insertion reaction is s tructure sensitive, being approximately two orders of magnitude faster on Cu(100) than on Cu(110). The source of this difference appears to be slow diffusion of methylene across the corrugated Cu(110) surface. (C) 1994 Academic Press, Inc.