Increasing Coverage

The right-hand part of Fig. 7.7 corresponds to the second-order desorption of nitrogen atoms from a rhodium surface. As the desorption reaction corresponds to N -I- N —> N2 -I- 2 the rate is indeed expected to vary with A characteristic feature of second-order desorption kinetics is that the peaks shift to lower temperature with increasing coverage, because of the strong dependence of the rate on coverage. 

Figure 13. Relative CO rotor absorbances for increasing coverage.

What STM established first in 1991 for both Cu(110) K and Cu(110)-Cs systems was the localised nature of the reconstruction process and the atom resolved details of the complexity of the structural changes observed with increasing coverage.15 In 1993, Doyen et al.20 provided theoretical support for the experimental observations with both the Cu(110) and Au(110) surfaces. 

In part (a) of Table II is is seen that the first third of a monolayer to adsorb tends to occupy the more strongly acid sites, for each successive increment is less strongly adsorbed. In part (b) the decrease in average heats of adsorption with increasing coverage is quite consistent with the above. 

Molecular modeling work performed by Sasol researchers on fee cobalt (100) shows that increased coverage of 50% atomic carbon will induce a clock type reconstruction (Figure 4.3) similar to that observed for the classic case of Ni (100).28 The adsorption energy of the carbon is stabilized by 15 kJ/mol compared to the unreconstructed surface, resulting in a more stable surface.28 The reconstruction results in a shorter distance between the carbon and cobalt but also an increase in coordination of the cobalt atoms and, thus, fewer broken bonds. The barrier for the carbon-induced clock reconstruction was found to be very small (1 kJ/mol), which suggested that the process is not kinetically hindered. The... 

The change in the nature of the adsorption with increasing coverage (dissociative followed by associative) has been explained by a statistical consideration of the reaction mechanism shown above120). Associative adsorption is expected to occur at vacant sites for which all adjacent olefin binding sites are occupied by earlier dissociation products (or carbon monoxide, as shown by Fig. 6b), because dissociative adsorption (formation of vinyl and hydride species, followed by hydride migration to another alkene) requires two adjacent vacant sites. 

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