Alkyl Chain Propagation by Methylene Insertion on Cu(100)

Abstract

One of the mechanisms proposed for formation of carboncarbon bonds in the Fischer-Tropsch synthesis, the so-called carbide/methylene mechanism, 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 vacuum conditions. Alkyl iodides are used as molecular precursors to generate adsorbed methylene and alkyl groups on a Cu(100) surface. High-resolution electron energy loss spectroscopy and work function change measurements show that CI bond dissociation occurs below 200 K in iodoalkanes to form alkyl groups on the surface. Indirect evidence supports the formation of adsorbed methylene groups via CH2I2 dissociation. Temperature-programmed reaction studies of the CH2 + CD3 reaction show that sequential CH2 insertion followed by β-hydride elimination produces ethylene-d2 and propylene-d3. Similarly, reaction of CH2 with C2D5 produces propylene-d4. All of these reactions are extremely facile, occurring at 230-250 K with activation energies of 12-20 kcal/mol. Similar studies on Cu(110) show that the methylene insertion reaction is structure 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.