Surface carbon atom migration

Transport may occur over the surface or through the atmosphere, with ihe latter most likely to involve molecular intermediates. This is true with platinum-alumina, for example, where oxidizing atmospheres during regeneration produce volatile PtO molecules. In nickel-silica catalysts exposed to carbon monoxide, nickel carbonyl serves the same purpose. For surface transport, atomic migration is favored, but depends on the substrate composition. 

The simplest example of oxygen spillover is found in the adsorption of oxygen on carbon. The spillover oxygen migrates from the basal carbon (donor) to carbon atoms exposed at steps between layers of the graphite surface, where it reacts with the edge carbons (acceptor).71 In this case the donor and acceptor phase consist of the same material with different surface properties. 

In agreement with Balandin s theory it was assumed by Eidus (79,80) that methylene radicals adsorbed on two adjacent centers of the catalyst are dimerized to ethylene which remains adsorbed on a doublet subsequently a new methylene group is added to one of the carbon atoms with a hydrogen atom migrating to the carbon atoms of the ethylene which is then desorbed further growth with formation of 1-olefins proceeds similarly a shift of the double bond inside of the molecule may occur. The view of Craxford and co-workers that all polymerizing methylene radicals remain adsorbed to the surface was contradicted by Eidus as inconsistent with experimental evidence (84b). 

One mechanism for surface area loss is crystallite migration, for which Kinoshita et al.66 concluded that the mechanism of surface area loss was two-dimensional Ostwald ripening by means of ad-atom migration on the carbon surface. Nevertheless, trap sites for the migrating ad-atoms on the surface of the carbon can produce nucleation points for generation of... 

We propose mechanisms for carbon formation in the presence of hydrogen on nickel and iron surfaces. Benzene is assumed to adsorb on the metal surface and to be hydrogenated to intermediates. These intermediates decompose to form atomic carbon which migrates through a metal crystallite to form carbon filaments. 

The analysis by XPS of the fresh catalysts shows the presence of the chloride ion peak (Cl 2p3/Jt - 198.4 eV). The presence of chlorine species in metal catalysts prepared from metal chlorides has already been reported, indicating the difficulty of eliminating all the Cl by Hs treatment, despite the fact that TPR showed practically complete reduction of the metal (ref. 12). Actually, only a small amount of this Cl modifies the electronic density of metal atoms, leading to the formation of electron-defficient metal species (MT ) on the surface (refs. 11-13), which we have detected by XPS. The rest of the chlorine may remain as HCl or can substitute OH groups attached to the carbon support. These findings can be explained if one accepts that, in the presence of H2, the HCl is held by the carbon carrier, but it car back-migrate to the vicinity of the metal after H2 removal. Table 2 also summarizes the metal/carbon atomic ratios (M/C). As this ratio is an estimate of metal dispersion, a parallelism exists between the experimental M/C XPS ratios and the corresponding values obtained by H2 chemisorption. 

---