Activation of CH4, NH3 and

The activation of the bond that is broken occurs via a similar mechanism to that for the oxidative addition over an organometallic substrate. Dissociation leads to the formation of negatively charged adsorbate-fragment orbitals with formally oxidized metal surface atoms. Dissociation typically occurs over the top of a surface atom. The critical point in the activation of the C-H or N-H bond occurs when it is stretched sufficiently such that the empty antibonding bond orbital lowers close enough to the Fermi level to allow for back-donation and electron transfer from metal into the antibonding state of the absorbate. This is illustrated in Fig. 3.44 for the dissociation of H2 over different metal surfaces (see also ref. [4]). In the oxidative addition reaction, the reactant-molecular 

Activation barriers for row 4 metals appear to be the highest, those for the row 5 metals are lower and those for row 6 metals tend to be lowest. Surprising is the low value found for Pd, which may be an artifact of the calculation. 

There is an important difference between the trends found here for methane activation and those reported earlier for CO dissociation. The CO dissociation energy trend is determined by the Oads adsorption energy. Note that whereas CO dissociation on Pt(lll) is more difficult than on the Ni(lll) surface, it is the reverse for CH4 activation. 

Also the activation of C-C bonds and the formation of C-C bonds for partially hydrogenated intermediates do not have to behave similar to that for C-H activation. This follows, for example, from inspection of Fig. 3.51 which will be discussed more extensively later. In this figure, the C-C coupling reactions on Co and Ru are compared. While the barrier for the C-H cleavage of CH4 is higher on Co than on Ru, the barriers for C-C bond formation and C-C bond dissociation are lower on Co than on Ru. On Ru the stronger 

M-C interaction in the product as well as reactant state results in weaker interaction between reacting hydrocarbon fragments, so that barriers for the reaction in both directions increase. This in line with the observed lower rate of hydrocarbon hydrogenolysis observed for Pt as compared with Ni. On Ni the rate of C-H activation is lowered more than that for the CH -CHy bond cleavage reaction. Because of the stronger metal-carbon bond of Pt than Ni, the rate of methane formation by recombination of adsorbed hydrogen with adsorbed CH3 will be lower on the former. 

---