THE REACTION-PATH FOR RECOMBINATION OF SURFACE CHX SPECIES

Using the semi-empirical ASED method, the reaction path for carbon-carbon bond formation was studied on a Rh40 cluster simulating a fcc (111) surface. Highly hydrogenated surface carbon fragments are found to have high activation energies for recombination. This is due to steric hindrance by the hydrogen atoms. The lowest activation energy was found for the recombination of a three-fold-bonded carbon atom with a CH2 species, to form a vinylidene intermediate. By comparing this reaction step on clusters simulating ruthenium and palladium, it appeared that the activation energy for carbon-carbon bond formation is rather insensitive to the metal-carbon bond strength. Experimental results are presented, obtained on silica-supported Rh, Co and Ru catalysts, that confirm that carbon-carbon bond formation is favourable from CH(x) fragments in which the average value of x is approximately 1.